Gaining Light into High-Pressure Carbon Dioxide Hydrogenation to Chemical Energy Carriers

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Doctoral Thesis

Gaining Light into High-Pressure Carbon Dioxide Hydrogenationto Chemical Energy Carriers

Author(s): Reymond, Helena

Publication Date: 2017

Permanent Link: https://doi.org/10.3929/ethz-b-000243467

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DISS. ETH NO. 24792

Gaining Light into High-PressureCarbon Dioxide Hydrogenation

to Chemical Energy Carriers

A thesis submitted to attain the degree of

DOCTOR OF SCIENCES of ETH ZURICH

(Dr. sc. ETH Zurich)

presented by

Helena Catherine ReymondMSc ing. chim. dipl., EPF Lausanne

born on 18.04.1987citizen of Geneva (Ge)

Switzerland

accepted on the recommendation of

Prof. Dr. Dr. h.c. Philipp Rudolf von Rohr (ETH Zurich), examinerProf. Dr. Christophe Coperet (ETH Zurich), co-examiner

2017

© Helena Reymond, 2017

Finding the balance.

Acknowledgements I

Acknowledgements

The five years I have spent at LTR have been not only incredibly for-mative and captivating, but also highly eventful. I would like to expressmy deepest appreciation to all who have contributed to the successfuloutcome of this thesis with their precious advice, energy and support.

First and foremost, I would like to express my gratitude to Philipp Rudolfvon Rohr for giving me the opportunity to carry out my PhD thesisunder his supervision. I am grateful for the freedom he granted me in thedefinition and management of my project, while offering his continuedsupport and trust. I wish to thank him for the insightful discussions fromscientific matters to other topics beyond my research, and also on theskilful art of negotiating the purchase of a Raman spectrometer.

I would like to thank Christophe Coperet for accepting the task of co-referee as well as for his unwavering and contagious enthusiasm, andthe wise choice of wines occasionally closing Sinergia meetings. I wishto extend my gratitude to the whole Sinergia team for the enlighteningbrainstorming meetings from Zurich to Tarragona and for the friendlycooperative approach which contributed so much to the advancementof my project. Sinergia meetings were always a source of many ideasand motivation ! I thank Atsushi Urakawa for helping me quick off theSinergia project and his knowledgeable inputs, and Joost VandeVondelefor showing me a new facet of chemistry. Special thanks go as well toHung-Kun, Juan Jose, Kim, Indre, Praveen and Atul for their invaluableparticipation in the Sinergia project.

I would like to thank those whose contribution is not readily visible inmy thesis but without which the experiments would probably not haveeven started. I would like to acknowledge the help of Bruno Kramer in

II Acknowledgements

the design and built of high pressure components, Peter Feusi for hismechanical assistance, and Daniel Trottmann for teaching me the basicsof electronics.

I would like to thank my former and present colleagues of the LTRgroup for the convivial working atmosphere. Special thanks go to my of-fice mates Michi and Thomas for sweetening the days in ML H15, I verymuch enjoyed the pranks and sugar/caffeine-high laughs, to Gina for hel-ping me focus my mind through sport, to Agnieszka for the comradeshipshared in the Assistenz, at conferences and rowing outings, to Sergio forcatalysis discussions, to Roger for the latest gossips and Martin for themultitude of nicknames I will probably never get free of ! Thank you aswell to the students who contributed to my project bringing motivationand new perspectives, Andreas, Selin, Victor, Simon, Carmen, Giovanniand Patrice.

Last but not least, a thought to my family and friends for their inde-fectible support over the years, and most specially to Loıc, whose lovingcare knows no time zone, reaching across continents.

Zurich, January 2018 - Helena Reymond

Abstract III

Abstract

CO2 hydrogenation to chemical energy carriers is of theoretical and in-dustrial relevance to the joint challenges of sustainable energy produc-tion and greenhouse gas emission mitigation. Governmental policies andresearch investments to stimulate markets will mature new energy indus-tries and foster the next generation of low-carbon technologies. Althoughno panacea, the valorisation of CO2 unlocks crucial innovation opportu-nities, materially supported by growing carbon capture and sequestra-tion, and economically supported by the sinking cost of renewable windand solar energy. As both types of power are intermittent, they promotethe development of storage technologies like water electrolysis for thegeneration of H2. However, because H2 is ill-suited as storage and distri-bution medium as such, its high reducing power can be used to supplythe energy to activate the thermodynamically stable CO2 molecule forits subsequent transformation into clean synthetic fuels such as methanoland formic acid.

Besides their importance in the chemical sector, the utilisation of theseproducts as energy carriers is widely recognised. Nevertheless their pro-duction efficiency via direct CO2 hydrogenation lags behind due to contro-versy around fundamental mechanistic issues and to ineluctable ther-modynamic barriers. The aim of the present thesis is to advance theunderstanding of high-pressure CO2 reduction mechanisms by in situspectroscopic studies, and apply the thus gained knowledge to design analternative multistep reactive pathway for the continuous production offormic acid and methanol via methyl formate.

To complement the established efficiency of microreactors in reactionscreening and bridge the operating and optical gaps, a micro view-cell is

IV Abstract

developed for Raman microscopy at extreme conditions with minimumflow interference for genuine reaction analysis. Based on a flat sapphirewindow unit sealed in a plug flow-type enclosure holding the sample, thecell features unique 14 mm working distance and 0.36 numerical aper-ture and resists 400 ◦C and 500 bar. The applicability of the cell as insitu device for the collection of physical and chemical data marks tech-nical advancement towards rational heterogeneous catalyst and processdesigns.A parametric study of the hydrogenation of CO2 to methanol on a Cucatalyst asserts the benefits of high pressure, temperature and prolongedcontact times on conversion. The characterisation of the reaction bulkby Raman spectroscopy evidences in situ condensation as a function ofoperating conditions, as well as of extent of reaction. The promoting ef-fect of phase separation on reaction behaviour demonstrates that processconditions should combine kinetic optimum with phase optimum for theperformance to profit from a synergistic effect.Catalyst surface characterisation under CO2 hydrogenation conditionsestablishes structure-performance relationships for coinage metals sup-ported on SiO2 and CeO2. Surface methoxy and formate species arepinpointed as reactive intermediates on CeO2-supported catalysts in thesynthesis of methyl formate. Plausible reactive pathways are advancedsubject to validation due to the complexity stemming from the highoxygen-mobility of the support. Activity in formate synthesis is also de-monstrated by unprecedented use in flow of an immobilised homogeneousRu catalyst.The performance of a cation-exchange resin in the hydrolysis of methylformate to formic acid and methanol exposes the link between kineticsand phase behaviour in the liquid-liquid regime. The strong affinity ofthe resin for water and the fast solubilisation of methyl formate supportpseudo-first order homogeneous kinetics and justify the weak influenceof gas-phase methyl formate on performance.The physical and chemical knowledge gained in methanol and methylformate synthesis and hydrolysis are successfully merged into a three-reactor system to convert CO2 and H2 into formic acid in a conti-nuous fashion. Methanol and dimethyl ether are produced as valuableco-products. An admixture of Ag/Al2O3 and the resin achieves the es-terification of excess methanol with short-lived formic acid derivatives

Abstract V

into methyl formate, and to subsequently transform it into formic acidand methanol by hydrolysis. The acidic resin also catalyses the dehydra-tion of methanol into dimethyl ether, thereby shifting the equilibriumfavourably and facilitating downstream purification. Parameters key tothe improvement of the overall process are identified relative to the dy-namic equilibrium nature of the reactive system and to the complexityin analysing formic acid.

This thesis underlines the importance of in situ studies to unravel theintricate factors governing CO2 reduction efficiency. The results achieveddemonstrate that the synthesis of formic acid from CO2 hydrogenationis indeed possible over heterogeneous catalysts in a multi-step reactiveroute, and prove that alternative processes offer avenues for technicaladvances.

Resume VII

Resume

L’hydrogenation du CO2 en composes porteurs d’energie chimique estd’interet a la fois theorique et industriel face aux defis correles de produc-tion d’energie durable et de mitigation d’emissions de gaz a effet de serre.Les politiques gouvernementales et les investissements dans la recherchepour dynamiser les marches vont favoriser l’emergence d’industries lieesaux nouvelles energies et promouvoir la prochaine generation de techno-logies a faibles emissions carbone. Bien que la valorisation du CO2 nesoit pas une panacee, elle presente des opportunites decisives en termed’innovations, tant sur le plan technique des progres quand a la captureet la sequestration de carbone, que sur le plan economique du fait de ladiminution des couts des energies renouvelables (solaire et eolienne). Cessources d’energie etant par nature intermittentes, elles promeuvent le de-veloppement de technologies de stockage, telle que l’electrolyse de l’eaupour la production d’hydrogene. En raison de sa pauvre densite energe-tique, H2 est inadequat au stockage et a la distribution d’energie. Ce-pendant sa forte energie reductrice peut etre exploitee pour l’activationdes molecules thermodynamiquement stables de CO2 et sa transforma-tion consecutive en carburants de synthese propres comme le methanolet l’acide formique.

Outre leur importance dans l’industrie chimique, l’utilisation de ces pro-duits comme vecteurs energetiques est globalement acceptee. Toutefois,l’efficacite de leur production via l’hydrogenation directe du CO2 suscitedes controverses autour d’aspects mecanistiques fondamentaux, de memequ’elle accuse les coups de barrieres thermodynamiques ineluctables. Parconsequent, la presente these a pour but premierement d’approfondir lacomprehension des mecanismes de reduction de CO2 a haute pression, etdeuxiemement compte tenu des connaissances acquises de conceptuali-

VIII Resume

ser un chemin reactif alternatif en plusieurs etapes visant la synthese encontinu d’acide formique et de methanol par l’intermediare de formiatede methyle.

Dans le but de complementer les atouts des microreacteurs pour le cri-blage de reactions chimiques et d’en combler les ecarts entre les condi-tions operatoires de reaction et celles d’analyse optique, une cellule op-tique est developpee pour l’application de microscopie Raman sous condi-tions reactionnelles extremes. Elle permet une etude authentique de re-actions a ecoulement continu en garantissant des interferences hydro-dynamiques minimes. Basee sur une fenetre en saphir vissee dans uneenceinte qui contient l’echantillon a analyser dans un canal la traversantpour simuler un reacteur tubulaire, elle presente une unique combinai-son d’une distance de travail de 14 mm avec une ouverture angulaire de0.36 et resiste a des conditions d’operations atteignant 400 ◦C et 500 bar.L’utilisation de la cellule pour l’acquisition in situ de donnees chimiqueset physiques marque un avancement technologique vers un design ration-nel de catalyseurs et de procedes.Une etude parametrique de l’hydrogenation du CO2 en methanol surCu/ZnO/Al2O3 demontre les avantages reactionnels d’operer a hautepression et temperature avec de long temps de residence. La caracterisa-tion du fluide effluent par spectroscopie Raman demontre un phenomenede condensation in situ qui advient dependamment de la pression, de latemperature de meme que de l’avancement reactionnel. L’effet promo-tionnel sur la dynamique de reaction induit par la separation physiquedes phases prouve qu’un procede efficace doit combiner un optimumcinetique avec un optimum de phases afin de beneficier d’un effet syner-gique.Les surfaces de divers catalyseurs a metaux supportes sont caracteri-sees sous des conditions pertinentes a la reduction du CO2, pour etablirdes liens entre structure catalytique et reactivite chimique. Des groupesmethoxy et formiates peuplant la surface sont identifies comme especesreactives intermediaires lors de la synthese de methyl formiate sur des ca-talyseurs a base de CeO2. De possibles chemins reactionnels sont avancessous reserve de confirmation en raison de l’extreme versatilite structuralede l’oxyde de metal en question. Bien que specifiquement developpe pourune operation en continu, pour la premiere fois un catalyseur immobilise

Resume IX

de Ru fait preuve d’une certaine activite dans la formation de derives deformiate.La performance d’une resine echangeuse de cations lors de l’hydrolyse dumethyle de formiate en acide formique et methanol expose les liens entrele comportement de la reaction et celui des phases liquide-liquide. Laforte affinite de la resine pour l’eau, combinee a la rapide solubilisationdu formiate de methyle, legitiment une cinetique de catalyse homogenede pseudo premier ordre et justifient la faible influence de l’evaporationde formiate de methyle sur la performance reactionnelle.Les informations acquises lors de la synthese et de l’hydrolyse de me-thanol et de formiate de methyle sont par la suite integrees avec succesen un ensemble de trois reacteurs pour convertir en continu le CO2 etl’H2 en acide formique et methanol. Le methanol et du dimethyle ethersont obtenus comme co-produits de valeur utile. Un melange physiqued’Ag/Al2O3 et de la resine acide permet l’esterification de methanol enexces avec des especes ephemeres derivees d’acide formique pour formerdu formiate de methyle. Ce dernier est apres coup rapidement transformeen acide formique et en methanol par hydrolyse. Il s’avere que la resineacide catalyse egalement la deshydratation du methanol en dimethyleether, faisant ainsi basculer l’equilibre reactionnel favorablement et fa-cilitant d’autant la purification des produits. La maıtrise des equilibresdu systeme reactionnel et la complexite de l’analyse de l’acide formiquesont etroitement impliquees dans l’optimisation du procede multi-etapes.

Cette these met en evidence l’importance d’etudes in situ pour delierles facteurs gouvernant l’efficacite de la reduction du CO2. Les resultatsatteints demontrent que la synthese d’acide formique par hydrogenationdu CO2 par le biais de catalyseurs heterogenes est en effet realisableselon un chemin reactionnel en plusieurs etapes, prouvant ainsi de memecoup que le design de procedes alternatifs ouvrent des avenues sur desavancees technologiques a ne pas manquer.

Contents XI

Contents

Acknowledgements I

Abstract III

Resume VII

Nomenclature XV

1 Introduction 11.1 Carbon capture, storage and utilisation . . . . . . . . . . 11.2 Hydrogenation of CO2 to liquid fuels . . . . . . . . . . . . 2

1.2.1 Formic acid and intrinsic instability . . . . . . . . 31.2.2 Methanol and mechanistic intricacy . . . . . . . . 4

1.3 Objectives and overview of the thesis . . . . . . . . . . . . 5

2 Essentials of process intensification and theoretical back-ground 92.1 CO2 hydrogenation . . . . . . . . . . . . . . . . . . . . . . 10

2.1.1 Reaction network . . . . . . . . . . . . . . . . . . . 102.1.2 Phase behaviour . . . . . . . . . . . . . . . . . . . 11

2.2 Flow chemistry and novel process windows . . . . . . . . . 122.2.1 Microreaction technology . . . . . . . . . . . . . . 122.2.2 Heterogeneous catalysis . . . . . . . . . . . . . . . 132.2.3 High-pressure chemistry . . . . . . . . . . . . . . . 13

2.3 In situ spectroscopy . . . . . . . . . . . . . . . . . . . . . 142.3.1 Opportunities in catalysis and phase behaviour . . 142.3.2 Raman spectroscopy . . . . . . . . . . . . . . . . . 15

2.4 Equilibrium thermodynamics . . . . . . . . . . . . . . . . 192.4.1 Phase equilibrium . . . . . . . . . . . . . . . . . . 20

XII Contents

2.4.2 Reaction equilibrium . . . . . . . . . . . . . . . . . 22

3 Micro view-cell for in situ Raman microspectroscopy 253.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 263.2 Continuous CO2 hydrogenation . . . . . . . . . . . . . . . 27

3.2.1 Microreaction setup . . . . . . . . . . . . . . . . . 273.2.2 Online GC analysis . . . . . . . . . . . . . . . . . . 28

3.3 Raman microscope . . . . . . . . . . . . . . . . . . . . . . 283.4 View-cell for phase behaviour and reaction analysis . . . . 31

3.4.1 View-cell design . . . . . . . . . . . . . . . . . . . 313.4.2 Light collection efficiency . . . . . . . . . . . . . . 343.4.3 Sample focusing and heating . . . . . . . . . . . . 37

3.5 View-cell applicability for in situ measurements . . . . . . 383.5.1 Plug flow hydrodynamics . . . . . . . . . . . . . . 383.5.2 Detection of physical phase transition . . . . . . . 393.5.3 Monitoring of chemical composition . . . . . . . . 443.5.4 Characterisation of catalyst surfaces . . . . . . . . 47

3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4 Methanol synthesis over Cu/ZnO/Al2O3 514.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 524.2 Materials and methods . . . . . . . . . . . . . . . . . . . . 53

4.2.1 Thermodynamic modelling . . . . . . . . . . . . . 534.2.2 Catalyst . . . . . . . . . . . . . . . . . . . . . . . . 544.2.3 CO2 hydrogenation to methanol . . . . . . . . . . 554.2.4 Dew point determination . . . . . . . . . . . . . . 55

4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . 574.3.1 Parametric study . . . . . . . . . . . . . . . . . . . 574.3.2 Interplay between reaction and phase behaviour . 634.3.3 In situ condensation . . . . . . . . . . . . . . . . . 664.3.4 Critical temperature estimation . . . . . . . . . . . 704.3.5 Catalyst surface species and reaction mechanism . 72

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5 Methyl formate synthesis over heterogeneous catalysts 755.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Contents XIII

5.2 Materials and methods . . . . . . . . . . . . . . . . . . . . 795.2.1 Catalysts . . . . . . . . . . . . . . . . . . . . . . . 795.2.2 CO2 hydrogenation over air-sensitive catalysts . . 805.2.3 In situ Raman analysis of heterogeneous catalysts 81

5.3 Results and discussion . . . . . . . . . . . . . . . . . . . . 825.3.1 In flow performance of an immobilised Ru NHC-

pincer catalyst . . . . . . . . . . . . . . . . . . . . 825.3.2 Surface characterisation of metal supported catalysts 85

5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 98

6 Methyl formate hydrolysis by ion-exchange resin 1016.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 1026.2 Materials and methods . . . . . . . . . . . . . . . . . . . . 103

6.2.1 Thermodynamic modelling . . . . . . . . . . . . . 1036.2.2 Kinetic modelling . . . . . . . . . . . . . . . . . . . 1046.2.3 Catalyst and chemicals . . . . . . . . . . . . . . . 1056.2.4 Swelling and sorption experiments . . . . . . . . . 1056.2.5 Hydrolysis of methyl formate . . . . . . . . . . . . 1066.2.6 Analytical method . . . . . . . . . . . . . . . . . . 107

6.3 Results and discussion . . . . . . . . . . . . . . . . . . . . 1086.3.1 Methyl formate hydrolysis phase equilibrium . . . 1086.3.2 Parametric study . . . . . . . . . . . . . . . . . . . 1106.3.3 Reaction locus . . . . . . . . . . . . . . . . . . . . 1176.3.4 Kinetic modelling . . . . . . . . . . . . . . . . . . . 119

6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 121

7 Formic acid synthesis via methyl formate intermediary 1237.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 1247.2 Materials and methods . . . . . . . . . . . . . . . . . . . . 125

7.2.1 Catalysts . . . . . . . . . . . . . . . . . . . . . . . 1257.2.2 Continuous multistage reduction of CO2 . . . . . . 125

7.3 Results and discussion . . . . . . . . . . . . . . . . . . . . 1267.3.1 Synthetic feasibility . . . . . . . . . . . . . . . . . 1267.3.2 Effluent purification . . . . . . . . . . . . . . . . . 1317.3.3 Energetic and economic feasibility . . . . . . . . . 132

7.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 133

XIV Contents

8 Conclusion and perspectives 1358.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368.2 Conclusion and perspectives . . . . . . . . . . . . . . . . . 138

A Appendix 147

Bibliography 169

List of publications 197

Curriculum Vitae 198

Nomenclature XV

Nomenclature

Abbreviations

BPR back pressure regulatorC carboncat catalystCCD charge couple deviceCCS carbon capture and sequestrationDFAFC direct formic acid fuel cellDME dimethyl etherDOE department of energyEOS equation of stateFID flame ionisation detectorFA formic acidGC gas chromatographyGHSV gas hourly space velocityHB hydrogen bondIR infraredL-L liquid-liquidLCA life cycle assessmentLTR Laboratory of Transport Processes and ReactionsMeOH methanolMF methyl formateN reference state, normal conditions (273.15 K, 1 atm)NA numerical apertureNMR nuclear magnetic resonanceNPW novel process windowsPID proportional-integral-derivative

XVI Nomenclature

QH quasi homogeneous(R)WGS (reverse) water gas shiftsc supercriticalSNF Swiss National Science FoundationSRK Soave-Redlich-KwongSS stainless steelTCD thermal conductivity detectorTON turn over numberTRL technology readiness levelUV ultravioletWD working distanceWHSV weight hourly space velocity

Roman letters

A µm/min GC peak aread mm diameterEa kJ mol−1 activation energy

f′

He - helium factorF - objective functionG kJ mol−1 Gibbs free energyH kJ mol−1 enthalpyI arbitrary units Raman intensityk various units reaction rate constantK - reaction equilibrium constantn mol mole numberp bar pressureR - molar ratio H2:CO2

R J mol−1 K ideal gas constantRI - Raman intensity ratioRn - molar ratio water:MFS % selectivityt s, min, h time

Nomenclature XVII

T ◦C temperaturex mol % molar fractionX % conversionY mmol g−1 h space time yield

Greek letters

α - molecular polarisability∆ν cm−1 Raman shiftλ nm wavelengthν s−1 frequencyνi - stoichiometric coefficient{ν} kJ mol−1 virtual energy levelσ % standard deviation

Sub- and superscripts

◦ reference state, standard conditions (298.15 K, 1 atm)0 initial valuec criticalf formationg gasi component indexlq liquidp particler reaction

1

Chapter 1

Introduction

Driven by environmental and economic incentives, the use of carbon di-oxide (CO2) as alternative carbon source opens up new venues to addressthe interrelated issues of climate change and fossil feedstock deficiency.This thesis targets efficient and reliable energy storage by investiga-ting the synthesis of CO2-derived chemical energy carriers as buildingblocks towards an innovating heterogeneous catalytic pathway for theproduction of formic acid.

1.1 Carbon capture, storage and utilisation

The connection between climate change and fossil feedstock deficiencyis broadly accepted imputing over 65 % of the inflation in atmosphericCO2 to alarming concentrations to fossil fuel combustion for energy pro-duction and transportation.1 In its position of leading greenhouse gas,the implication of these anthropogenic emissions on the environmentspurred the deployment of global mitigation policies, as most recentlyadopted in the Paris agreement.2 More specifically, this agreement aimsat holding this century the rise in global average temperature to below2 ◦C.

Mitigation strategies aim at lowering CO2 build-up either by cuttingback on emissions with energy-efficient processes and switching to less

2 1. Introduction

carbon-intensive energy sources such as hydrogen3 (H2) or renewableenergy; else strategies can also do so by carbon capture and sequestration(CCS) from the atmosphere and point-sources, like power plants andindustries.4,5 In May 2017, the world’s first commercial plant to capturecarbon dioxide directly from air opened in Zurich, Switzerland.6 Oncefiltered and purified, the CO2 gas is sold as product for agricultural use,but could be extended to other markets like food or chemical and energyindustries. Particularly suited for large-scale storage of waste CO2, CCSsecures, with the forecasted growth rate of emissions, an inexhaustiblesupply of non-toxic carbon feedstock available for recycling into a rangeof industrial products.7–9

In the context of global warming, the utilisation of CO2 for the pro-duction of chemical energy carriers is of both, theoretical and industrial,relevance to close the gap between the commitments made in the In-tended Nationally Determined Contributions submitted by the involvedcountries and the required reduction of effects attributed to greenhousegas emission.10 Indeed, although such strategy may not offer perma-nent fixation, nor commensurate with the ever increasing energy de-mand,11,12 it promises clean energy innovations and major technologicalbreakthroughs essential to accelerate the transition away from the cur-rent fossil fuel-dominated to a low-carbon economy.

1.2 Hydrogenation of CO2 to liquid fuels

The thermodynamic stability of CO2 and its relative chemical inert-ness render its activation energy-intensive. Substantial energy input canbe provided from intermittent energy sources such as wind and solar,or from geothermal and hydraulic power converted to electric energy.While electricity can be used for direct conversion of CO2, poor effi-ciency is reported and there are presently no practical solutions availa-ble for large-scale operation in either electrocatalytic or photocatalyticCO2 conversion.13 In contrast, using renewable electricity to generategreen H2 as chemical energy from water electrolysis offers advantages toproduce clean synthetic fuels.14,15

1.2 Hydrogenation of CO2 to liquid fuels 3

Currently, hydrogen storage is achieved via compression, liquefaction,physisorption, or metallic and complex hydrides.16 However, the hand-ling of potentially explosive hydrogen gas, its low gravimetric and volu-metric densities, hydrogen loss, and the extensive energy consumptionassociated with these processes limit its use as energy carrier. Therefore,efficient means to store and distribute energy are of utmost necessity toreach targets set by the Department of Energy (DOE).17 The couplingof renewable H2 with the recycled carbon dioxide waste hence unlocksCO2 as a renewable building block of commodity chemicals and fuels,such as alcohols and hydrocarbons. Liquid hydrocarbon fuels remain anideal transportable energy storage, and the production of high energy-density liquids like methanol and formic acid calls for particular researchinterest as the efficiency of the hydrogenation can be improved by theuse of appropriate catalysts.

1.2.1 Formic acid and intrinsic instability

Formic acid (HCOOH) can be synthesised by CO2 hydrogenation as inReaction (1).

CO2 hydrogenation to formic acid

(1) CO2 + H2 HCOOH

The reversible equilibrium between formic acid and CO2/H2 broadenshorizons using CO2 as a practical hydrogen storage medium. Indeed, with4.3 wt.% of H2,18 formic acid and its derivatives are high density hyd-rogen carriers with promising energy storage capacity (53 g L−1) whichexceeds by far pressurised H2 tanks (39.4 g L−1 at 700 bar, energy re-quired for compression not considered). In the liquid state at ambientconditions, formic acid streamlines storage and transportation and canproduce high purity H2 (and CO2) when decomposed. Furthermore, for-mic acid itself can be injected as fuel in direct formic acid fuel cells(DFAFCs).19

4 1. Introduction

However, the reduction reaction is energetically hampered and seriouspractical difficulty lies in the low thermodynamic stability of formic acidand its fast decomposition kinetics. The use of homogeneous Ru catalystsachieved substantial yields in aqueous media stabilising the product byesterification with alcohols or addition of bases.20 For a large-scale synt-hesis of formic acid by CO2 hydrogenation, a major challenge remainsin the development of heterogeneous catalysts, which are economicallyadvantageous in product separation and offer potential continuous ope-ration. The use of an external stabilising agent in such operation causingcostly downstream purification then becomes counterproductive and ra-pid product isolation on stream calls for a completely new strategy fora continuous formic acid synthesis from CO2 hydrogenation.

1.2.2 Methanol and mechanistic intricacy

Methanol (CH3OH) is one of the most important commodities of the che-mical industry with a worldwide demand of approximately 70 Mt y−1.21

Beside its applications in the chemical industry for a large variety of enduses like plastics, resins, pharmaceuticals, chemical fibres, paint and pes-ticides, the utilisation of methanol as chemical energy carrier is widelyrecognised. Blended with gasoline or used directly in a fuel cell,22 met-hanol is further seen as bridging technology to replace fuels as a meansof energy storage, ground transportation fuel, and platform chemical forsynthetic hydrocarbons. Dimethyl ether (CH3OCH3), a potential sub-stitute of diesel oil due to its superior combustion performance, can besynthesised from methanol via dehydration. In addition, fundamentalolefinic building blocks can be produced via the methanol-to-olefin pro-cess. Incidentally, it has been proposed to shift the current oil and gaseconomy towards a “Methanol Economy” in light of the versatility andgood combustion performance of methanol and dimethyl ether.23,24

Methanol can be produced from the exothermic reduction of CO2 withthree molar equivalents of H2 (Reaction (2)). However, this process isaccompanied by the competitive formation of untargeted carbon monox-ide (CO) via the endothermic reverse water gas shift reaction ((R)WGS,

1.3 Objectives and overview of the thesis 5

Reaction (3)). The so-formed CO can subsequently also undergo hydro-genation to yield methanol (Reaction (4)).

CO2 hydrogenation to methanol

(2) CO2 + 3H2 CH3OH + H2O

(3) CO2 + H2 CO + H2O

(4) CO + 2H2 CH3OH

The catalytic synthesis of methanol from syngas (CO/H2) is a robustcommercial process which can be readily adapted to CO2/H2 feeds offe-ring currently the only possibility of utilising CO2 emissions on a largescale as demonstrated, amongst other companies, by Mitusui Chemi-cals and Carbon Recycling International.25,26 Despite a rapidly growingnumber of scientific papers on the subject, key mechanistic aspects re-main contentious, such as the main carbon oxide source in methanol.Methanol was assumed to be synthesised from CO hydrogenation untilthe isotope labelling work of Chinchen et al.27 which suggested CO2 asprominent carbon oxide source over Cu/ZnO/Al2O3 industrial catalystsunder industrial conditions (i.e. 50 bar, 250 ◦C). However, since copperis also a good catalyst for the WGS reaction, the conversion from CO toCO2 is facilitated and the controversy persists.28–30 Other critical spe-culations debating the nature of the catalytically active sites and thenumber and nature of reacting phases prevent fundamental mechanisticunderstanding for a rational process optimisation.

1.3 Objectives and overview of the thesis

A high-pressure microreaction plant was previously built at the Labo-ratory of Transport Process and Reactions (LTR)31 for the continuoushydrogenation of carbon dioxide.32 This thesis makes use of the lab plantto explore innovating reaction concepts and opportunities for heteroge-neous catalytic development.

6 1. Introduction

The thesis is part of a broader interdisciplinary project funded by theSwiss National Science Foundation (SNF, CRSII2-154448)33 which aimsat achieving efficient CO2 reduction to specific hydrocarbons by tighte-ning the gaps between complementary and essential disciplines from pro-cess engineering and computational chemistry via catalyst engineering.The common overall goal targets the development of a continuous synt-hetic path to simultaneously form formic acid and methanol from CO2

and H2, exploiting methyl formate as an intermediate. The aforementi-oned major thermodynamic obstacle in the direct formic acid synthesiscan be solved, or at the very least mitigated, by inclusion of a secondaryreaction to transform formic acid sooner than its fatidic decompositionback into reagents. The new pathway shall bypass thermodynamic con-straints by reacting short-lived formic acid, or precursors of it transientlyadsorbed at the catalyst surface, with methanol to yield methyl formateby esterification. If methanol can also be produced in situ by CO2 hyd-rogenation in the same reactor as formates, then methyl formate may besynthesised in one operational step. Even though methyl formate showsgood chemical properties as a fuel,34 it is not economically viable and itis more advantageous to produce formic acid and methanol rather thanmethyl formate. Since formic acid is commercially produced by hydro-lysis of methyl formate with methanol as a by-product, methyl formateshall act as a stable intermediate allowing an optimal CO2 hydrogena-tion step before converting it in a second operation step into formic acidand methanol via hydrolysis.

The reactions composing the two-step approach are depicted in Figure1.1, neglecting the RWGS reaction and the production of water for thesake of brevity.

1.3 Objectives and overview of the thesis 7

HCOO(H)

CH3OH* Catalyst 1

Catalyst 2

HCOOH

CH3OH

REACTION STEP 2

Catalyst 3 HCOOCH3

*

INTERMEDIATE

REACTION STEP 1

CO2 + H2

* H2O not shown for brevity

Figure 1.1 – Scheme of the targeted two-step synthetic approach for the continuousconversion of CO2 and H2 into formic acid and methanol via formate intermediate.

In the first reaction step, CO2 is hydrogenated to methanol and pre-cursors of formic acid prior to their transformation to methyl formate.Catalyst 1 consists in an industrial Cu-based methanol synthesis cata-lyst, while Catalyst 2 represents two different types of heterogeneouscatalysts selective for formates, developed in the frame of the Sinergiaproject in an iterative procedure based on feedback from experimentaland computational results. The second reaction step involving Catalyst 3makes use of a commercially available acidic catalyst for the hydrolysisof methyl formate into formic acid and methanol.

Within the frame of the Sinergia project, the objectives of this thesisare threefold, focusing on the process engineering for an improved un-derstanding of CO2 hydrogenation to condensable products.

I. Develop the microreaction plant to gain visual and spectroscopicaccess into stainless steel reactors and house in situ monitoring ofhigh-pressure CO2 hydrogenation for a genuine study of dynamicbehaviours.

II. Study CO2 reduction to methanol and formate derivatives to pro-vide physical and chemical insight to back up atomistic simulati-ons and converge towards an optimal catalyst in a holistic appro-ach.

III. Evaluate the operation parameters of each individual reaction in-volved to determine a strategy validating the feasibility of thetargeted two-step approach for the hydrogenation of CO2 to for-mic acid and methanol in continuous flow.

8 1. Introduction

The thesis begins with a theoretical chapter, providing the fundamentalscientific background on CO2 hydrogenation network, and thermodyna-mics of chemical and physical equilibria for process simulation. The be-nefits of flow chemistry and high pressure in gas-phase chemistry are alsointroduced together with the working principle of Raman spectroscopyto set the context for the next chapter validating a high-pressure opticalcell developed for the in situ analysis of phase behaviour and detection ofcatalyst surface species. The three following chapters report experimen-tal results on the individual reactions of CO2 hydrogenation to methanol,formate derivatives, and methyl formate hydrolysis, respectively. Thesechapters lay the grounds for the last experimental chapter proving thepotential of the multi-step concept towards formic acid and methanolproduction. The thesis closes with considerations on the viability of theprocess, a general conclusion and perspective for future work.

9

Chapter 2

Essentials of processintensification and theoreticalbackground

This Chapter sets the context of the thesis in a state-of-the-art engi-neering perspective and provides the theoretical tools necessary for itsunderstanding. At first, serving as central thread running through thewhole work, the reaction framework generated from the targeted two-step reactive pathway outlined in Chapter 1 is presented mired in thecomplexity of its inherent equilibrium nature. The benefits of continu-ous processing and high-pressure chemistry are then broadly reviewedin terms of process intensification to ground the experimental methodsapplied for the fulfilment of the thesis objectives. The working principleof Raman spectroscopy is also introduced as a means to characterise ca-talyst surface chemistry and study the dynamic behaviour of multiphasereactive systems. At last, fundamentals of phase and chemical equili-brium thermodynamics are covered for process simulation.

10 2. Essentials of process intensification and theoretical background

2.1 CO2 hydrogenation

2.1.1 Reaction network

Table 2.1 lists the main reactions and thermodynamics involved in thetwo-step process introduced in Chapter 1. Because the complexity ofthe network strongly contributes to the speculative dimension surroun-ding CO2 reduction mechanisms, the tabulated list arbitrarily sequencesthe reactions according to the likelihood of their occurrence over eachindividual catalyst as tackled in the experimental chapters: ReactionsR.1-R.3 for methanol synthesis (Chapter 4), Reactions R.4-R.5 methylformate synthesis (Chapter 5), and Reaction R.6 for methyl formate hyd-rolysis (Chapter 6). Reaction R.7 sums up the overall multi-step process(Chapter 7). Apart from Reaction R.6 which was studied in liquid phase,thermodynamic reaction values were calculated on the basis of forma-tion entities in gas phase in reason of the predominance of the latterphase under the studied CO2 reduction conditions. Standard enthalpiesand Gibbs free energies of formation used for the calculation of standardenergies of reaction are reported in Appendix A (Table A.1).35,36

As proof to the statement made in the Introduction, the strongly ender-gonic Reaction R.4 confirms the impossibility of an efficient direct con-version of CO2 to formic acid. Albeit the targeted overall Reaction R.7does not predict more optimistic energetics either, much to the contraryfrom its highest Gibbs free energy, carrying out the series of reactions ina consecutive manner has the power to weaken the endergonic hindrance.The alternative reactive pathway shall do so by breaking the process intoenergetically more favourable steps defined by smaller ∆G◦

r(g), and byacting on the thermodynamics of single reactions using target-specificcatalysts. The high linear-dependency between reactions of the networkprovides a means to tune the overall process efficiency by acting locallyon each equilibrium-limited step and induce a succession of responses fora broader impact.

2.1 CO2 hydrogenation 11

Table 2.1 – Reaction network and gas phase standard enthalpy ∆H◦r(g)

and Gibbs

free energy ∆G◦r(g)

of reaction.

∆H◦r(g)

∆G◦r(g)

Reaction kJ mol−1 kJ mol−1

(R.1) CO2 + 3H2 CH3OH + H2O −49.3 +3.5

(R.2) CO2 + H2 CO + H2O +41.2 +28.6

(R.3) CO + 2H2 CH3OH −90.5 −25.1

(R.4) CO2 + H2 HCOO(H) +15.5 +43.4

(R.5) CH3OH + HCOO(H) HCOOCH3 + H2O −19.8 −12.5

(R.6) HCOOCH3 + H2O CH3OH + HCOOH +8.1(lq) +4.7(lq)

(R.7) 3CO2 + 7H2 2CH3OH + 2H2O + HCOOH −83.1 +50.4

2.1.2 Phase behaviour

In the present system dealing with gaseous reagents and less volatileproducts, a multiphase system may ensue. Phase equilibrium then dicta-tes partition concentrations of reactive components, directly affecting theprocess outcome. While the phase diagram of a pure fluid is fairly straig-htforward, the identification of the supercritical region in catalytic reacti-ons involving multicomponent systems becomes complex: critical pointsmay exist over a range of pressures, temperatures, and compositions,resulting in a critical curve. Coexisting phases of different compositionand additional effects such as gas-liquid immiscibility, discontinuity ofcritical lines or retrograde condensation may occur preventing the deter-mination of the location of phase boarder curves in the p,T ,x-space.37–39

Moreover, the composition of reactive systems varies in function of theextent of reaction, thus the critical point may change along the axis ofthe tubular reactor. Knowledge of the phase behaviour of the system isnecessary to make beneficial use of the number and nature of reactingphases and to interpret their effect on the reaction rate and selectivity.


2.2 Flow chemistry and novel processwindows

Flow chemistry and Novel Process Windows (NPW) are major chemicalengineering concepts in the field of process intensification.40 Both con-cepts contribute in developing technologies which can lead to compact,safe, energy-efficient, economic and sustainable processes to increase pro-ductivity. The former concept achieves design intensification by minimi-sing separation expenditure, while the latter reaches chemical intensifi-cation through the deliberate use of harsh chemistries at unusually highpressure, temperature or concentration.41,42 Microreaction technologyenables the safe application of both concepts, whereby flow chemistry ismade possible by the use of heterogeneous catalysis and high-pressurechemistry in gas-phase reaction follows the principle of NPW. Microre-action technology, heterogeneous catalysis and high-pressure chemistryare introduced in the next paragraphs to establish the basis of the experi-mental work of this thesis. The reasons for their utilisation are explainedthrough the engineering benefits they bring to efficiently explore newsynthetic perspectives.

2.2.1 Microreaction technology

Microprocess technology has initiated the shift from batch to flow proces-ses, where reactors of extremely small dimensions integrate fundamentalfunctions particularly important for a reliable study of chemical reacti-ons with screening of catalytic materials in little amount, optimisationof reaction conditions, inline monitoring of reaction progress, and ex-traction of kinetic parameters.43–45 Their tubular geometry (≤ 1 mm indiameter) and small reaction volume allow for continuous processing withminimal holdup of chemicals. These features not only increase process sa-fety for NPW with easier containment in the event of leakage or breakupwhen dealing with potentially explosive H2 and harsh operating condi-tions; they also grant high surface-to-volume ratio for intensified trans-port processes. High heat transfer rates reduce thermal gradients, ideal

2.2 Flow chemistry and novel process windows 13

for operating highly exothermic processes isothermally, while the shor-ter diffusion lengths improve mixing efficiency for reduced mass transferhindrance on reaction rates. The level of technical control is improvedto an extent which allows to drift away from classical paths towards in-novative chemical transformations. Further detail on the engineering ofmicro-process technologies can be found in [46,47].

2.2.2 Heterogeneous catalysis

Heterogeneous catalysis refers to catalytic systems where the phase ofthe catalyst differs from the one of the reactants. In this case reactionthus requires mass and heat transfer through interfaces, in oppositionto homogeneous catalysis where the catalyst occupies the same phaseas the reaction system. In addition to the general benefits of catalysisenhancing reaction rates with lower by-product and waste production,heterogeneous catalysis simplifies the recovery of the catalyst from efflu-ent rendering separation units obsolete and allows continuous processingfor improved productivity with minimum downtime. It has helped inreducing energy consumption and improving atom economy of existingprocesses, and will continue to be primordial for the development ofmore efficient catalytic processes.45,48,49 The principles of heterogeneouscatalysis can be found in [50,51].

2.2.3 High-pressure chemistry

The application of NPW in the hydrogenation of CO2 to liquid fuelspursues dual kinetic and thermodynamic goals for its chemical activationand subsequent transformation into products (p ≥ 200 bar, T ≥ 200 ◦C).Indeed, the mild critical properties of carbon dioxide (pc = 74 bar,Tc = 31 ◦C) opens engineering possibilities by using it simultaneouslyas reagent and as supercritical solvent.52 Thermodynamically, pressureimpacts the reaction volume on which depends the mole fraction-basedequilibrium constant.53 As can be seen from the reactions listed in Ta-ble 2.1, CO2 hydrogenation is generally accompanied by a decrease in


the number of moles, which an increase in pressure would counteractfollowing Le Chatelier’s principle by shifting the equilibrium in favourof product synthesis. Kinetically, the effect of pressure in gas-phase re-actions lies in the higher concentrations which increases the likelihood ofencounter between reactants. Furthermore, the tunable physical proper-ties (density, diffusivity, viscosity, surface tension, thermal conductivity,heat capacity) of supercritical fluids between those of gas and liquid sta-tes offer unique combination of dissolution power and transport qualitiesfor enhanced reaction rates. The high miscibility of H2 in supercriticalCO2 (scCO2) compared to most other organic solvents allows the for-mation of a single homogeneous feed and high hydrogen concentrationsat the catalyst surface, leading to reduced diffusion resistances throughthe absence of phase boundaries. High reaction rates were confirmed bythe improved product yields in the homogeneous hydrogenation of CO2

to formate derivatives.54,55

2.3 In situ spectroscopy

2.3.1 Opportunities in catalysis and phase behaviour

Online analytical methods like chromatography or other methods re-quiring sampling of the effluent are adequate for the evaluation of theoverall efficiency of a catalytic process. However, in addition to beingoften lengthy, such end-of-pipe analysis techniques follow a black boxapproach preventing fast reaction monitoring and the rationalisation ofcatalyst development and reaction engineering. As a matter of fact, phasetransitions caused by changes in conditions between the reactor and theanalytical device affect the composition of the sample and transient spe-cies or unstable intermediate are overlooked, prejudicing the understan-ding of the genuine reaction mechanism.

In situ spectroscopic studies have permitted major advances in cataly-sis by providing in-depth molecular information about catalyst structureand surface species to improve existing processes as well as to develop

2.3 In situ spectroscopy 15

new ones.56 Because the surface state of a catalyst is strongly affectedby its environment, they developed tools to gain insight at various scalesinto catalyst operation through spectroscopic observation under realisticoperation conditions and elucidate structure-activity relationships.49,57

Spectroscopic studies have also been applied to the study of dynamicphase behaviour in multiphase reactive systems, which can only be de-termined in situ as a function of operating conditions and effluent com-position, to benefit from the tunability of physical properties.39,58

With the apparition of NPW praising the advantages of extreme con-ditions for new chemistries, the constraints for such in situ technologyhave become ever more stringent.41,59 Bridging the pressure and tempe-rature gaps thus becomes a necessity for the fundamental understandingof these dynamics and pave the way for the knowledge-based design ofCO2-reduction catalyst materials and processes.

2.3.2 Raman spectroscopy

Raman spectroscopy is particularly well-suited to in situ analysis sinceit is non-intrusive and requires no sample preparation. Sensitive to mo-lecular vibrations, it delivers molecular information on the catalyst, onthe reactants, and on their interactions over a wide range of conditi-ons.60,61 Moreover, it works in back-scattering mode necessitating onlyone window on the sample rendering the optical arrangement for in situanalysis more convenient than for infrared (IR) spectroscopy, which isbased on the transmission of light following the Beer-Lambert equation.In contrast to IR obscured by intense activity, the presence of water doesnot hinder the applicability of Raman spectroscopy, neither do metal ox-ide supports. The latter do not exhibit intense Raman bands in the Ra-man shift window where supported metals are expected (800-1100 cm−1)which eases the detection of adsorbates.


The Raman effect

The classical description of the Raman effect presented in this paragraphis inspired from [62]. For a quantum mechanical treatment of vibrationalRaman scattering explaining the selection rules, the reader is referred to[63].

The Raman effect is a light scattering phenomenon, which can be con-sidered in a qualitative way as a collision between a photon and a mo-lecule. When light of the incident frequency ν0 impinges on a molecule,it is scattered either at the same frequency as the incident light in anelastic encounter, or at some shifted frequency in an inelastic one. Con-ceptually, the process describes a transition from some initial state tosome intermediate or virtual state {ν} followed by a transition back toone of the real states of the molecule.

In a more quantitative description of the light-matter interaction, the Ra-man effect arises form the interaction of an oscillating electromagneticradiation with a molecular charge distribution resulting in the transferof energy from the radiation field to the internal (vibrational and rota-tional) motions of the molecule or vice versa. The applied electric fieldpolarises the distribution of charges of the molecule thereby inducing adipole moment as described in Equation 2.1. The external electric fieldE is itself characterised by a vector amplitude E0 and an oscillationfrequency ω0 as in Equation 2.2.

−→µ = α−→E (2.1)

−→E =

−→E0 · cos (ω0t) (2.2)

where α stands for the molecular polarisability, a proportionality con-stant which expresses the extent to which the electric field disturbs theelectron density from its equilibrium nuclear geometry. However, α isnot a static quantity and changes as the molecule oscillates. It can be

2.3 In situ spectroscopy 17

expressed as a Taylor series in the normal coordinates Q about its equi-librium configuration Q0, where Q groups the individual normal modesq. Neglecting the higher order terms of the power series,

α(Q) = α0 +

N∑q=1

(∂α

∂q

)q0

· q +1

2

N∑q=1

(∂2α

∂q∂q′

)q0q′0

· q · q′

0 +O(q3) (2.3)

Considering each individual normal mode q oscillates with a frequencyωq:

q = q0 cos (ωqt) (2.4)

The conceptual core of Raman scattering is then obtained by combi-ning Equations 2.1-2.3 and making use of trigonometric formula to yieldthe time-dependent induced dipole moment, which acts as a secondarysource of scattered radiation, emitting light at three distinct frequenciesof oscillation (Figure 2.1).

µ(t) = α0 · E0 cos (ω0t) +1

2

(∂α

∂q

)q0

· q0 · E0 · cos ([ω0 − ωq] t)

+1

2

(∂α

∂q

)q0

· q0 · E0 · cos ([ω0 + ωq] t)

(2.5)

The first term in Equation 2.5 corresponds to the unshifted frequency ofRayleigh (elastic) scattering in which the molecule is retuned to its origi-nal state and does not contain information on nuclear molecular degreesof freedom. The two other terms describe Raman scattering, in whichinformation is contained in the difference in frequency between the inci-dent and scattered photons, for this difference corresponds to vibrationaland rotational energy level spacings of the molecule. This difference, la-belled ∆ν, is called the Raman shift. The second term corresponds tooscillations between the laser and a molecular normal mode at frequen-cies red-shifted of (ω0 − ωk) from the incident light. It represents the


inelastic scattering event, called Stokes scattering, in which energy istransferred from the radiation field to the molecule resulting in a lowerenergy photon. The third term is characterised by a source of radiationblue-shifted by (ω0 + ωk) describing the anti-Stokes scattering in whichenergy is given up from the molecule to the field.

ν0 νq-νq

Inte

nsity

Ene

rgy

Anti-StokesRaman Scattering

StokesRaman Scattering

Rayleigh Scattering

WavenumberRaman shift∆ν = |ν0 – νq| = νvib

ν = 0

ν = 1

{ν}

ν = 0

ν = 1

{ν}

Figure 2.1 – Schematic illustration of Rayleigh scattering, Stokes and anti-StokesRaman scattering for a diatomic molecule (one vibrational mode). The laser frequencyν0 is represented by the upward arrows, and the frequency of the scattered photon νqby downward arrows. The dashed lines indicate the virtual state {ν}. Adapted from[63].

Advantage of Raman scattering

Visible Raman is the most common configuration, but heterogeneouscatalyst samples often exhibit strong fluorescence when the Raman scat-tering is excited by wavelengths in the visible spectral window. The fluo-rescence intensity causes an extremely intense background overwhelmingthe weak Raman bands.

Since the virtual level need not correspond to an actual molecular ei-

2.4 Equilibrium thermodynamics 19

genstate, the sample may be irradiated with a laser beam wavelengthanywhere from ultraviolet (UV) to IR (λ = 200 − 1300 nm). The useof different excitation radiations affords some differences which mainlyaffect Raman signal intensity and fluorescence interference. The inten-sity of light scattering is proportional to the 4th power of the excitationfrequency. Exciting the sample with longer wavelengths near-IR can cir-cumvent fluorescence, but at the cost of Raman intensity. Alternatively,UV Raman spectroscopy offers lower spectral resolution but affords hig-her Raman signal without fluorescence limitations.56 The absence ofrestriction on excitation wavelengths for Raman scattering provides aversatile method suitable for the characterisation of a variety of cataly-tic materials under realistic conditions.

2.4 Equilibrium thermodynamics

As previously stated, reaction behaviour is tributary of the number andnature of phases present in the reactor. Therefore, the outcome of thetargeted process depends on chemical reaction equilibrium as well as onphysical phase equilibrium, the basics of both are presented hereafter.

The equilibrium-limited reactions presented in Section 2.1 follow thegeneral form of reversible reactions.

A + Bk+k−

C + D (2.6)

where A and B stand for reactants, and C and D for products.

In situ phase transitions or a limited solubility of components, e.g. met-hyl formate in water, may force the reactive system to partition intoseveral phases. The resulting multiphase system can then be decoupledinto simultaneously occurring chemical and phase equilibria, characte-rised by a minimum energy function at given conditions of pressure p,temperature T and composition. The Gibbs free energy G (Equation 2.7)


dictates the direction toward which the system evolves as a measure ofthe energy available for phase transition and chemical reaction, whereH stands for enthalpy, S for entropy, U for internal energy, and V forvolume.

G = H − TS = U + pV − TS (2.7)

Following the second law of thermodynamics, equilibrium conditions maythus be found by direct minimisation of Equation 2.7, expressed in termsof the experimentally measurable entities T , p, and mole number ni,where µi is the chemical potential of a component as defined in Equation2.12.

dG = −SdT + V dp+∑i

µidni = 0 (2.8)

In a closed reactive system, equality of temperature and pressure of pha-ses in contact are necessary for thermal and hydrostatic equilibrium,respectively. Hence Equation 2.8 reduces to a change in mole amount,dni, which may arise through transfer of a substance between phases bydiffusion, as well as by chemical reaction.

2.4.1 Phase equilibrium

Liquid - liquid system

Chemical equilibrium is reached in all phases regardless of which thereaction takes place in. Equation 2.8 is therefore valid for all phases incontact.

dGsystem = dG1 + dG2 = 0 (2.9)


Where 1 and 2 stand for different phases.

From mass conservation follows

dn1i = −dn2

i (2.10)

Finally, uniformity of chemical potentials in each of the individual phasesbetween which substances can freely diffuse is derived for equilibriumcondition

µ1i = µ2

i (2.11)

where the chemical potential of a component µi is related to its standardstate and to its thermodynamic activity ai according to Equation 2.12.The procedures followed for the estimation of activity coefficients arepresented in Sections 4.2.1 and 6.2.1.

µi = µ◦i +RT ln(ai) (2.12)

Solvent - polymer catalyst system

Specifically related to the second step of the targeted process, the inte-raction between resin catalysts and solvents and the impact on the courseof a reaction is presented in this section as a particular case of phase equi-libria.64,65 The hydrolysis of methyl formate is studied in Chapter 6 overa macroreticular-type cation-exchange resin, which consists of an inso-luble and elastic copolymer backbone of styrene-divinylbenzene functi-onalysed by sulfonic groups (−SO3H). When in contact with a solvent,the resin expands as the former is incorporated in the elastic polymermatrix until counteracting forces equilibrate. An equilibrium state is re-ached as the osmotic and eletrostatic forces forcing the solvent into thematrix are balanced out by the resistance to expansion, conferred bythe cross-link density in the gelular matrix. The magnitude of osmoticand electrostatic forces depends on the polarity and dieletric constant


of the solvent in contact with the ionic sites of the polymeric backbone.The degree of swelling relies thus largely on the properties and nature ofthe reactive mixture. The difference in affinities dictates the equilibriumdistribution of reactive components between the bulk liquid phase andthe one trapped in the resin pores. Consequently, they influence reactionperformance by affecting not only the level of catalyst-reactant contact,but also the accessibility in and out of the matrix proportionally to thelatter’s macropore dimensions.

The nature of the catalytic species is determined by the intensity of theinteraction between the solvent and the acid groups of the polymer back-bone.65 Type A reactions are characterised by aqueous or polar feeds.Although the packed bed comprises discrete particles, water hydratesthe protons of the sulfonic groups, which become mobile and catalysethe reaction within the particle pores that act as confined reaction ves-sels. This class is further divided into A1-type catalysis by fully swollenresins in aqueous systems, where total dissociation of the polymer-bound−SO3H groups is achieved, and A2-type catalysis by mixed organo-watersolvents with variable extent of proton hydration. Type B reactions aredefined by anhydrous feeds, where the sulfonic groups remain undissoci-ated and act as catalytic species in absence of solvating medium. Theyare not discussed here as they do not pertain to ester hydrolysis.

2.4.2 Reaction equilibrium

In a reactive system the variation in molar amount of each component dniin Equation 2.8 is derived from the reaction stoichiometry and the extentof reaction ζ (single reaction), where νi stands for the stoichiometriccoefficient of component i.

dG|T,p =∑i

µidni =∑i

νiµidζ = 0 (2.13)

In other terms, Equation 2.13 requires the effluent’s net chemical poten-


tial to be equal to the chemical potential of the feed at equilibrium.∑i

νiµi = 0 (2.14)

For a reversible reaction, the equilibrium constant Keq (Equation 2.15)states the relative concentrations at equilibrium.

Keq =k+

k−=∏i

aνii,eq =∏i

xνii,eq

∏i

δνii,eq = KxKγ (2.15)

where k+ and k− are the rate constants for the forward and reversereactions as defined in Equation 2.6, xi are the reactant molar fractionsand γi their activity coefficients. The equilibrium constant is also relatedto the standard Gibbs free energy as

ln Keq = −∆G◦r

RT(2.16)

Combining Equations 2.7 and 2.16 leads to the differential form of thevan’t Hoff equation, describing the dependence of the equilibrium con-stant on reaction temperature.

dln Keq

dT=

∆H◦r

RT 2(2.17)

Upon integration if ∆H◦r is assumed constant for small temperature

intervals, Keq yields

ln (Keq) = ln(K◦

eq

)− ∆H◦

r

R

(1

T− 1

T ◦

)(2.18)

The rate at which the chemical equilibrium is reached then depends onthe kinetics of the reaction, which itself varies in function of the catalystefficiency.

25

Chapter 3

Micro view-cell for in situRaman microspectroscopy

This chapter 1 is dedicated to the high-pressure microreaction plant usedto house the CO2 hydrogenation reactions and the features implementedto it to enable visual and spectroscopic inspection of the reaction zone.The focus is set on the strategy followed to design a Raman spectrometerand a view-cell compatible with one another, while allowing maximumflexibility in usage for a direct access into the stainless steel microre-actors. A versatile Raman spectrometer was designed to circumvent asfar as possible contingencies related to the variety of catalysts to besynthesised, such as fluorescence or thermo-lability. The view-cell wasdeveloped in a second step based on (i) the optical constraints dictatedby the Raman spectrometer itself, and (ii) the mechanical constraintsimposed by the type of reactions studied.

1. Reprinted with permission from: Reymond H., Rudolf von Rohr P.; (2017).Micro view-cell for phase behaviour and in situ Raman analysis of heterogeneouslycatalysed CO2 hydrogenation. Reviews of Scientific Instruments, 88(11):114103-1–6.

26 3. Micro view-cell for in situ Raman microspectroscopy

3.1 Introduction

Chemistries under harsh operating conditions are safely carried out inmicroreactors. What is more, the latter have been demonstrated to be avaluable tool for an efficient screening of reactions, online monitoring ofreaction and optimisation of operating conditions.45,48,66–70 Albeit typi-cal glass-silicon microreactors offer the optical access required for in situstudies, the Si-glass bonding does not withstand pressures and tempera-tures above 140 bar and 80 ◦C.71 The potential of view-cells for in situcollection of physical and chemical evidence of reactions has long beenrecognised,49,70,72–78 though an optical cell with high mechanical resis-tance and minimum internal volume has not been reported with adequateworking distance and sufficient angular aperture for microscope analysis.As a matter of fact, micro view-cells designed for various specific ana-lytical methods might offer a wider operating range than glass-siliconmicroreactors, but have little use for Raman microscopy of continuousflows due to characteristics such as inadequate window material, poorworking angle and optical path length, or batch operation.58,61,76,79–88

Optical cells for extreme pressures and temperatures are also availablecommercially; their large inner volumes in the millilitre range howevercause dead volumes, and mass and temperature gradients inappropriateto reproduce ideal plug flow conditions. Despite the variety of ingeni-ous designs, none of them can be used for the in situ Raman study ofcontinuous CO2 hydrogenation reactions in microreactors.

The chapter begins with a description of the existing hydrogenation plantand online GC analysis procedure, which also lays the foundation for theexperimental part of the follow-up chapters. Follows then an elaboratedescription of the view-cell built for the in situ collection of data atextreme conditions for reaction monitoring and for the study of phasebehaviour and catalyst surfaces. To prove the feasibility of the concept,the chapter closes with a series of diagnostic tests on simple non-reactivesystems which establish the performance of the view-cell as continuoustubular reactor and validate the subsequent operando-GC studies.

3.2 Continuous CO2 hydrogenation 27

3.2 Continuous CO2 hydrogenation

3.2.1 Microreaction setup

The hydrogenation of carbon dioxide to the different hydrocarbons re-ported in Chapters 4, 5, and 7 were carried out in the high-pressuremicroreaction plant schematically represented in Figure 3.1.32 The pre-fixes A,P and V used in the following description refer to the labelling inthe aforementioned figure. A pre-mixed 10 mol % He in H2 (Pangas) iscompressed intermittently up to a maximum of 1200 bar by a diaphragmpump (P1.1)(NovaSwiss, V09) in a coiled pipe (A1.1) acting as reservoir.Situated after the coiled pipe, a pressure relief valve (V1.6)(Tescom,ER3000) controls the volumetric flow rate of the He/H2 mixture bysetting a certain pressure drop across a nickel-clad fused silica capil-lary (A1.2)(�inner = 10 µm, L = 100 mm). Pressure transducers pla-ced at both extremities of the capillary enable the fine-tuning of thepressure drop from ∆p =10 to 150 bar to reach flows ranging 20 to300 µL. The CO2 flow rate (Pangas) is controlled independently from5 to 2.5× 104 µL min−1 by means of a jacketed high-pressure syringepump (P2.1)(Teledyne ISCO, 65D) maintained at 18 ◦C. Reactant stre-ams are contacted in a T-junction (VICI) before flowing through thetubular microreactor.

The microreactor (A1.3) consists in a 1/16” stainless steel tube (�inner =1 mm, L = 120 mm) fixed in between two 2-µm pore stainless steel filters(VICI). The reaction temperature is adjusted by fitting the reactor bet-ween two brass bodies heated by a PID controller (AllControl, AC302)connected to a K-type thermocouple and two resistive cartridges (Wi-sag, 200 W, 230 V). The system pressure is controlled with an automa-ted needle valve back pressure regulator (V3.1)(BPR, Tescom, ER3000).Pressures (Honeywell, Althen) as the locations indicated in Figure 3.1and the outlet flow rate (A1.4)(Bronkhorst) are monitored online via aLabView interface. The composition of the exiting stream is analysedonline by gas chromatography (Bruker, 450-GC). In order to prevent ex-tensive condensation, the BPR and transfer line to the GC (A1.5) areheated at 65 ◦C and 120 ◦C, respectively.


The system can be safely operated in continuous mode up to 1000 barfrom 25 ◦C to 400 ◦C. For more detailed information on the built, controland safety of the plant, the reader is referred to the work of Tidona etal..32,89

3.2.2 Online GC analysis

The GC is equipped with a thermal conductivity detector (TCD) anda flame ionisation detector (FID). Ar is used as carrier gas to ensure aclear TCD detection of all components of interest because of the highthermal conductivity differences. A 6-port valve switches the effluentsample from a 250µL loop to a CP-Sil 5CB column (60 m x 0.32 mm,dfilm = 8µm). This column separates the permanent gases (He, H2, CO,CH4) from other compounds like CO2, H2O, CH3OH, HCOOCH3, andCH3OCH3. The permanent gases are passed through a 4-port valve to aCP-Molsieve 5A column (25 m x 0.53 mm, dfilm = 50µm), while the othercompounds are passed to another CP-Sil 5CB column (15 m x 0.32 mm,dfilm = 5µm). A 4-port valve enables to switch the stream through amethaniser for the detection of CO2 by the FID. The total analysis timelasts 14 min during which the oven temperature is raised from 45 to110 ◦C by steps of 15 or 25 ◦C min−1. He serves as internal standard forthe calculation of reaction performance in terms of conversion, selectivityand yield. The calculation method is included in Appendix A.

3.3 Raman microscope

A fibre optic-launched confocal Raman microspectrometer (Renishaw,InVia) was used for the spectroscopic experiments. Merging of chemicalsensitivity and specificity of Raman scattering with the high spatial reso-lution of confocal microscopy improves the efficiency of data collection.The spectrometer consists in a stigmatic single pass spectrograph withthree independent beampaths with fully optimised optics for UV, visi-ble and near-IR excitations. The SynchroScan�scanning method and a

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set of motorised relocating spectrograph lenses grant optimised spectralresolution for the whole spectral range. Motorised neutral density fil-ters offer 16 different laser power levels from 5× 10−8 to 100 %. A deepdepletion UV-coating CCD array detector (1024× 256 pixels) with pel-letier cooling at −70 ◦C ensures optimal detection for wavelengths fromUV to near-IR. The Renishaw software version WiRe�4.1 was used forinstrument control, data acquisition and processing. The spectrometer isconnected with any of three excitation wavelengths λ =355 nm, 532 nmand 785 nm and, respectively, 2400 l/mm, 1800 l/mm and 1200 l/mm ho-lographic grating adjusted on a magnetic kinematic mount to ensureaccurate and repeatable positioning. Launched from the spectrometer,the light is connected to the remote probe through 5 m of robust fibreconduit.

The remote probes include plasma filters and a set of edge filters for spe-cific Raman cut-offs and are coupled to a white light camera for additi-onal visual monitoring. The light beam is focused to a few µm-diameterspot using a 20x magnification microscope objective for the visible andnear-IR lines and a 10x near-UV objective for the λ = 355 nm line. Ta-bles 3.1 and 3.2 tabulate specifications on the lasers, remote probes, andobjectives. The measurement configurations for the spectrum acquisitionare specified for each spectrum introduced in this thesis in terms of laserwavelength, exposure time, percentage of full power laser and number ofaccumulations.

Table 3.1 – Laser and remote probe specifications.

355 nm 532 nm 785 nmLight UV visible near-IRType DPSSL Diode laser Nd:YAG DPSSLFull power [mW] 4.3 60 75Grating [l/mm] 2400 1800 1200Cut-off [cm−1] 200 200 150Objective 10x 20x 20x

3.4 View-cell for phase behaviour and reaction analysis 31

Table 3.2 – Objective specifications.

Name Magnification WD NAOlympus SLW 20x 18 mm 0.35Thorlabs LMU-351 10x 15 mm 0.25

3.4 View-cell for phase behaviour andreaction analysis

In order to qualify as catalytic reactor and in situ cell for Raman mi-croscopy, the view-cell was constructed to meet the following require-ments: it had to (i) withstand the mechanical constraints imposed bythe operating conditions of the selected model reaction, the hydrogena-tion of CO2 into methanol, and (ii) fulfil the optical constraints dictatedby the specifications of the microscope objectives to reach optimum lightcollection efficiency with minimised optical path length and wide angularaperture. The latter are quantified in terms of working distance (WD)and numerical aperture (NA).

3.4.1 View-cell design

The view-cell is composed of a window unit and a cell body assembled asshown in Figure 3.2(a). Together, they form a channel-like chamber hol-ding the sample, which is easily adaptable via 1/16” stainless steel Valcofittings (VICI AG) at any position in the high-pressure microreactionsetup in Figure 3.1. The window unit was developed in collaborationwith SITEC-Sieber Engineering AG (Switzerland) on the basis of theirhigh-pressure vessels for optical measurements. Figure 3.2(b) details thewindow unit forming a hollow screw with a 5-mm thick sapphire win-dow at its extremity (� = 10 mm, flatness < λ/4). It is composed ofthree parts: (i) a M48x2 mm pressure screw (SS 1.4542), (ii) a windowplug (SS 1.4542) and (iii) a metal cap (SS 1.4435) enclosing the sapphirewindow. The sapphire window is pressed into the cap, which is itselfscrewed into the window plug. Preloaded in the pressure screw, the win-


dow plug presses against the sapphire to hold it fixed during utilisationunder pressure. The pressure screw serves to insert the entire windowunit into the stainless steel body (66× 64× 45 mm3, SS 1.4542) offeringa visual access from the top surface for Raman spectroscopy in back-scattering mode. The conical surfaces between the cell body and the thewindow plug with the inner groove (Figure 3.2(b)) achieve sealing bytwo means: the pressure screw presses against the seat of the cell body(τ = 60 N m) and the groove acts as a lip seal, further tightening thearea with increasing pressure.

A 1-mm diameter channel goes through through the cell body passingunder the window. A 1-mm thin slip of steel protrudes from the bodyunder the window to act as gasket delimiting a 1.5× 2.5 mm2 apertureand adapt the large sapphire surface to the narrow channel and pre-vent packed catalyst powder from spreading. This geometry reproducesthe microreactor dimensions as closely as possible and ensures an innervolume in the microliter range as well as plug flow hydrodynamics. Adetailed view of the junction between sapphire and steel is presented inFigure 3.2. Stainless steel provides high yield strength with good cor-rosion and abrasion resistance to ensure a chemically inert environmentresistant to hydrogen atmospheres. To complement, with high tempera-ture resistance, tensile (275-400 MPa at 20-1000 ◦C, 1 bar=105 Pa) andcompressive (2.0 GPa) strengths, the sapphire window grants robustnessfor extreme conditions and provides a plane surface with a transparency>80 % from near-UV to IR.90 As such, the optical cell withstands up to500 bar pressure and temperatures of at least 400 ◦C and features the op-tical characteristics highlighted in red in Figure 3.2(b). The microscopeobjective, mounted at the extremity of each Raman probe, is inserted in-side the �32-mm socket of the screw to perform in back-scattering mode.A focus point situated 1 mm below the window (i.e. at the bottom of thechannel) offers a WD of 14 mm and a NA 0.36, fulfilling the requirementsof both visible and UV objectives for 100 % light collection efficiency (a2 mm-focus point achieves a 15 mm WD and a NA 0.34).


6 6

45

6 4

5

Cell body

Window unit

Cutouts for assembly with heating body

Reactor channel

(a)

NA = 0.36 42.5°

WD

= 1

3+1

mm

(b)

Sapphire window Window cap

Window plug

Pressure screw

Groove

Figure 3.2 – (a) 3-D representation of the assembled view-cell composed of the cellbody, the window unit traced in blue, the inlet of the reactor channel on the frontside with two inserts for the heating element at the bottom. The sapphire windowis not visible in this perspective. (b) Cross-section of the window-unit formed bythe pressure screw, the window plug, the window cap and the sapphire window. TheNA and WD for the λ = 532 nm excitation line with a focus point 1 mm below thewindow are highlighted in red.


Flow

(a)

1.5

2 . 5

Flow

(b)

Figure 3.3 – Detailed view of the junction area between the window and the channel.(a) Sectional view of the cell along the reactor channel, (b) top view of the screwcavity.

3.4.2 Light collection efficiency

The optical characteristics result from a geometry optimisation proce-dure based on an adapted Monte Carlo method computing the fraction of10 Mio randomly scattered light beams effectively hitting the objectivelens as a function of varied window-unit dimensions. The required mini-mum width and maximum depth of the window socket were determinedfrom the cone of light required for the full usage of the 20x objectivelens for the excitation line λ = 532 nm. The characteristic half-angle θwas calculated according to Equation 3.1 to determine the basis of theconical diffraction pattern.

NA20x = n · sin θ = 0.35 (3.1)

where n is the refractive index of the media between the objective lensand the focus point: air (n = 1.000) and in sapphire (n = 1.772). Theθ-value of the 20x objective was used as boundary condition for thedimension of the view-cell.

θview−cell > 20◦ (3.2)

Snell’s law (Equation 3.3) accounted for the changes in refractive indicesas the light pencils refracted from air to sapphire and into the inner


Tubular reactor

Heating body

View-cell

Cartridge heaters

Remote probe

Focusing stages

Microscope objective

Figure 3.4 – Photo of the view-cell embedded in the heating body showing howthe remote probe is inserted inside the hollow window unit. During operation, anisolation body enclosed the heating body to minimise heat losses to the environment(not shown). A fan directed on the video-probe was used to keep the latter from overheating due to prolonged exposure and avoid damage.


channel, thus correcting the working distance accordingly.

sin θ1

sin θ2=λ1

λ2=n2

n1(3.3)

where the indices 1 and 2 refer to the immersion media air and sapphire,respectively.

Figure 3.5 illustrates the acceptance cone of light resulting from theoptimisation procedure. Pencils of light which can exit or enter the lensare traced as a function of the distance from a focus point situated inthe reactor channel, 1 mm below the window.

Objective

Bottleneck

Sapphire window

Focus point ∆x

θ1

θ2

Figure 3.5 – Acceptance cone of light for the 20x objective in function of the distanceto the focus point situated inside the channel, 1 mm below the sapphire window. Across-sectional view of the window unit is shown in background with the three mediashown in white for air, blue for sapphire and grey for inner channel. The continuousblack lines represent the maximum scattering angle allowed, while the dashed linedepicts the shorter working distance in air.

The three zones - air, sapphire, reactor channel - separating the frontlens from the sample are depicted with their corresponding half-angularapertures θ1 and θ2 causing the focus point to shift to a distance ∆x incomparison to a full air immersion (θ3 for reaction medium not shown).


For the sake of simplicity, air at atmospheric pressure was used as me-dium in the channel. Indeed, with a refractive index only slightly lowerthan reported for pure CO2, air exemplifies a more challenging workingdistance and avoids the use of a complex correlation between the re-fractive index of scCO2 and reaction conditions.91–93 The radii of thecone of light were calculated from trigonometric relations for randomscattering angles and compared to the actual radii of the window socketat its most critical positions. Pencils of light scattered at an angle wi-der than the cross-section of the socket are considered as invalid, hittingthe walls of the cell. Light loss by scattering in the reactor channel, byabsorption or by reflection are not accounted for in the model. Visiblein Figure 3.2(b), a 30° slanted surface is introduced at the most criticalpoint, labelled as bottleneck in Figure 3.5, to increase the half-angle atleast to the minimum required 20°. Consequently, all randomly scatteredrays land within the limits of the socket of the view-cell depicted in thebackground, thus yielding 100 % efficiency in light collection.

3.4.3 Sample focusing and heating

The view-cell is embedded in a brass body, which covers all sides of thecell with the exception of a locally open surface for the insertion of theobjective at the probe extremity into the window unit (Figure 3.4). Thebrass body is heated using two cartridge heaters (200 W, 230 V, Wisag,Switzerland) connected to a K-type thermocouple and a PID controller(AC302 Allcontrol, Switzerland). The two cutouts (Figure 3.2(a)) on thebottom surface of the view-cell allow optimal heating along the reactorchannel. A thermal insulation made of ceramic blocks and filled withceramic fibres ensures a close contact with the brass body on all sidesexcept for the opening for the window cavity (not shown in Figure 3.4for clarity). This arrangement combined with the small scale of the in-ner channel guarantees homogeneous heating and high heat transfer toeffectively dissipate the heat generated by the reaction or the laser beam.

Two mechanically stable movable mounts (Newport 405 Series, ThorlabsL490) are used to position the insulated cell precisely under the beam


allowing motion in xyz-directions. Additional focusing stages (Newport423 Series) allow the insertion and removal of the objective to/from theview-cell cavity to adjust the focus height precisely and avoid prolongedheat exposure, respectively. An air fan directed on the remote probeprotects it from excessive heat damage, with temperature not exceeding80 ◦C at the objective vs. 400 ◦C in situ.

3.5 View-cell applicability for in situmeasurements

The following sections present the results of diagnostic tests demonstra-ting the potential of the view-cell as practical tool for physical and che-mical in situ analysis. This potential shall allow to gain insight into basicaspects of reaction such as phase transitions, evolution of effluent com-position and detection of surface intermediate species.

3.5.1 Plug flow hydrodynamics

The area detailed in Figure 3.3 engenders a local irregularity in the cy-lindrical geometry of the channel. The potentially created dead zones,however, do not cause severe interference in the bed hydrodynamics (fluidbypassing the catalyst). This fact is confirmed by comparing the reactionoutcomes of a test reaction with the catalyst loaded in the view-cell usedas packed-bed reactor with a control experiment reacting the same cata-lyst amount packed in a standard stainless steel reactor (�inner = 1 mm).The synthesis of methanol over a commercial ternary Cu/ZnO/Al2O3 ca-talyst (Alfa Aesar, product number 45776) was selected as test reaction.Details on the experimental procedure are to be found in Chapter 4.

The almost identical reaction performances shown in Figure 3.6 validatethe operation of the view-cell as tubular reactor. The view-cell can thusbe used in static atmospheres with its outlets plugged, or under flowingmixtures in gas or liquid state for a trustworthy study of reactions.

3.5 View-cell applicability for in situ measurements 39

(a) (b)

CO

Methanol

Methyl formate

Figure 3.6 – Performance of view-cell as catalytic reactor for a molar feed ratioH2:CO2=3 at 200 bar. Dashed lines represent the catalyst packed in a 1-mm diameterstainless steel reactor, and the continuous lines stand for the catalyst packed in theview-cell implemented on a reactor. Reaction performance (a) CO2 conversion, (b)product selectivity

3.5.2 Detection of physical phase transition

The capability of the setup to detect phase changes and determine theirnature and relative composition was confirmed with the most representa-tive components of CO2 hydrogenation reactions. Being the most suscep-tible components in the reaction network introduced in Section 2.1 tocondense during reaction, water and methanol were studied in their gasand liquid states. CO2 phase transitions around its critical point werealso studied in reason of its properties as supercritical solvent.53,94 TheRaman spectra of the compounds used hereafter in their pure form aregrouped in Appendix A (Figures A.2-A.7).


Water and methanol structural changes

Figure 3.7 presents the pressure and temperature evolution of the OH-stretch in Raman spectra of pure water flowing at 100 µL min−1 at mo-derate pressures.

20°C, 3 bar

70°C, 3 bar

120°C, 3 bar

140°C, 5 bar

120°C, 3 bar

Figure 3.7 – Raman spectra of the OH-stretching vibration of water at 3 bar anddifferent temperatures. The spectrum at 140 ◦C at 5 bar is superimposed to highlightthe effect of pressure. The framed section is an enlarged version of the gas phasespectrum recorded at 200 ◦C at 3 bar. The spectra were recorded with 1 accumulationof 60 s exposure at a 50 % laser power, λ = 532 nm.

The spectrum of liquid water exhibits a large and intense feature cente-red around 3400 cm−1, representative of the extended hydrogen-bonding(HB) network. This feature can be decomposed into the symmetric andantisymmetric OH-stretch bands. With increasing temperature, the sig-nal intensity decreases and the distribution of intensity narrows down,while its maximum frequency steadily shifts to the blue. This blueshiftof the OH-stretch is known to be a direct consequence of the distanceand angular dependence of the intermolecular hydrogen-bonding inte-ractions, evidence of reduced ordering effects due to the lengthening and


weakening of these interactions. As the interactions weaken with tempe-rature, the intensity decreases until total breakdown of the network andthe maximum frequency attains its gas-phase value at 3653 cm−1 whenboiling (Tboil,3bar =133 ◦C, Tboil,5bar =151 ◦C), where the interactionbetween water monomers is effectively zero.95–98 Upon pressure incre-ase, the phase transitions back to liquid, resuming the evolution withtemperature.82

3 bar

5 bar

Gas

Liquid

(a) (b)

(c)

Figure 3.8 – Raman spectra of the CH3- and OH-stretching vibrations of an18 mol % methanol and water solution at 3 bar and 5 bar at 30 ◦C, 100 ◦C, 135 ◦Cand 200 ◦C. The spectra were recorded with 1 accumulation of 60 s exposure at a50 % laser power, λ = 532 nm. zMeOH stands for methanol molar fraction in the gasand liquid phase.

The same evolution is observed when studying methanol-water mixtures.In addition to the calibration line established for liquid mixture composi-tions, the relative Raman peak intensities of both compounds are used todetermine the nature of the phases. Figure 3.8 links the phase diagramof methanol-water binary mixture to Raman spectra recorded at cha-racteristic conditions at a total flow rate of 100 µL min−1. An 18 mol %methanol solution was heated at 3 bar above its dew point and cooled


down at 5 bar to observe the evolution in HB-structure described abovewith a simultaneous progressive decrease in intensity of the two metha-nol CH3-stretches at 2834 cm−1 and 2943 cm−1 (Figure 3.8(c)), and thesudden loss in intensity in the gas phase at 200 ◦C (Figure 3.8(b)).98–100

In the same way as observed with pure water, the two-phase zone marka transition where perturbed light-matter interaction through successiverefractive index changes between gas and liquid (not shown) impact theaspect of Raman spectrum due to low light collection efficiency.101 Theeffect of methanol on water structural transformation was not studiedbut has been reported in the literature.102 The increased intensity in thegas phase at 5 bar gives a hint of the benefits of high pressure for thestudy of gas-phase effluents discussed in the following phase behaviourstudy of CO2.

CO2 phase transitions

Figure 3.9 contrasts the pressure-dependence of the Raman shift andintensity of the two major bands of the CO2 Fermi dyad (1285 cm−1 and1387 cm−1) recorded in static atmosphere at 25 ◦C and 50 ◦C. Highlightedin Figure 3.9(a) are the paths along which the transitions were recorded.Figure 3.9(c) shows the evolution in density with pressure along thesame phase transitions at 25 ◦C and 50 ◦C. The density at the criticaltemperature at 31 ◦C is presented as reference.

Higher pressures cause the Raman signal intensity to increase due to thehigher density of molecules (Figure 3.9(c)), while shifting the bands to-wards lower frequency from shorter and stronger bonds (Figure 3.9(d)).The curves at 25 ◦C present an abrupt increase in intensity and simul-taneous drop in maximum shift between 60-80 bar corresponding to thesharp gas-liquid transition, whereas the smooth curves at 50 ◦C follow thecontinuous phase change from gas to supercritical state (pcritical = 74 bar,Tcritical = 31 ◦C).88,103 Figure 3.9(b) backs up this observation showinga stepwise increase in CO2 density at 25 ◦C as the medium transitionsfrom gas to liquid in contrast to the continuous increase expected at50 ◦C. The evolution passing through the critical temperature is shownas reference.


Liquid Supercritical fluid

Gas

Critical pointpc = 74 barTc = 31 °C

(a)

50°C

31°C

25°C(b)

(c) (d)

(e)

Figure 3.9 – (a) Phase diagram of CO2, (b)Evolution of CO2 density along phasetransition from gas to liquid at 25 ◦C, and from gas to supercritical fluid at 31 ◦C and50 ◦C. Effect of pressure on the Raman (c) intensity and (d-e) shift of the Fermi dyadbands of CO2 at 25 ◦C and 50 ◦C. The spectra were recorded with 1 accumulation of30 s exposure at a 100 % laser power, λ = 532 nm.

In a nutshell, the study of these simple systems shows the adequacy of theview-cell to correlate the signal intensity with the density of the probedmolecules and confirm Raman spectroscopy as a method of choice for thedetection of phase transitions even in water-containing systems.49 The


application can be extended to more complex multicomponent systemsand provide a means to understand their phase dynamics and study theimpact of the number and nature of phases on reaction performance interms of pressure drop, mass transfer, and product separation via in situcondensation.104

3.5.3 Monitoring of chemical composition

With the aim of monitoring the composition of reactions effluents, binarymixtures of H2 and CO2, and aqueous solutions of methanol were usedto evaluate the accuracy of the setup in quantitative chemical analysis.

The quantification strategy is based on the ratio of the most intenseRaman signals of components as introduced in Equations 3.4 and 3.5.Because the absolute intensity of a Raman signal is component-specificand directly related to the amount of molecules, considering the ratioof specific signals, RI , allows a qualitative analysis of the compositionirrespective of focus depth and refractive indices from different pressuresand temperatures.105,106

RI =IMeOH,2835 cm−1

IH2O,3450 cm−1

(3.4)

RI =IH2,584 cm−1

ICO2,1385 cm−1

(3.5)

Figure 3.10(a) presents the ratio of Raman signals as function of metha-nol molar fraction for the elaboration of a calibration curve of aqueoussolutions. For 12 different compositions, 4 Raman spectra were acquiredat various focus depths to show the correlation between Raman signalratio and methanol concentration. The relation is almost linear and canclearly distinguish differences as small as 3.9 mol % and 6 mol %. Thesame process for gas-phase methanol-water mixtures was however notsuccessful due to the poor peak definition achieved from small scattering


cross-sections at moderate pressures. Increasing pressure to sufficientlyimprove gas-phase signal for clear detection of methanol and water isnevertheless expected to generate high signal definition for gas-phase ca-libration as exemplified by H2-CO2 mixtures at 200 bar in Figure 3.10(b).Raman spectra were acquired as CO2 was started to flow concurrentlyto a H2 stream. Not only does the Raman ratio very accurately followthe ratio of molar fractions analysed by GC, but it also proves the poten-tial for a faster feedback. Indeed, approximatively 4 Raman spectra wereacquired during a single GC cycle. The system shows a level of accuracyhigh enough to distinguish between molar stream compositions as closeas RH2:CO2

= 2 and 3.

(a)RI = 3

RI = 2

(b)

Figure 3.10 – (a) Relation between the methanol-water Raman peak intensity ratioand the molar fraction of methanol in aqueous solutions, (b) Evolution of Ramanpeak intensity ratio of CO2 and H2 and the ratio of GC molar fractions for twodifferent initial feed compositions.

The accuracy of the setup to qualify chemical composition offers possi-bilities to monitor the real-time progress of CO2 reduction reactions. Tothis end, the view-cell was implemented downstream of the packed-bedreactor to analyse the flowing effluent before online GC analysis. It wasoperated at reaction pressure and kept at 280 ◦C to warrant a single gasphase and assess the real-time progress of the methanol synthesis. Usingthe intensity of the most intense band of each component (H2, CO2,


methanol and water respectively at 587 cm−1, 1387 cm−1, 2843 cm−1

and 3643 cm−1) as an expression of concentration, the Raman spectrasuccessfully trace the evolution of reactant and product at the reactoroutlet at 200 bar, translating the conversion and product selectivity mea-sured by GC. Unfortunately, the CO quantified by GC remains under thedetection limit of the Raman setup at 200 bar. In exchange, the detectionof water bears significant importance for mechanistic studies, giving areal appreciation of the quantity produced via the RWGS and hydro-genation reactions. From the starting reactant peak intensities at 25 ◦C,the initial molar ratio H2:CO2=3 can be calibrated with a correction fac-tor accounting for their different Raman activities. As reaction proceeds,the reactant bands decrease and the bands attributed to methanol andwater increase in the same stepwise manner as the temperature.

(a)

(b)

Figure 3.11 – Maximum Raman peak intensities of reactants (a) and products (b)showing the evolution of effluent composition during CO2 hydrogenation over Cu-based catalyst at 200 bar and 25 ◦C, 170 ◦C, 200 ◦C, 230 ◦C, 260 ◦C and 280 ◦C. Thespectra were recorded with 1 accumulation of 60 s exposure at a 100 % laser power,λ = 532 nm.


These results augur well to quantify compositions by means of proporti-onality factors obtained either by correlating the GC molar fractions orby extracting them from Raman intensity ratio from binary mixtures ofknown compositions. In addition to fast feedback for improved reactionmonitoring, the concept can be adapted for kinetic studies by positio-ning the view-cell after various amounts of catalyst, or using several ofthem in sequence, to allow the analysis of the effluent after increasingcontact times for a time-resolved composition profile along the reactor.Profiling the concentrations of key components dissolved in scCO2 likemethanol, water and carbon monoxide shall enable mechanistic insighton the reaction sequence in the intricate model network.105,107

3.5.4 Characterisation of catalyst surfaces

The view-cell was packed with commercial methanol synthesis catalystand operated as a reactor at 200 bar to prove its potential in catalystsurface characterisation under working conditions. Information concer-ning the nature of the commercial catalyst and the Raman spectra underoperating conditions are to be found in Chapter 4. Raman spectra of thefresh catalyst are presented in Figure 3.12(a) acquired ex situ using thethree excitation wavelengths λ =355 nm, 532 nm and 785 nm. Despitethe expected negligible Raman activity of the aluminum-oxide basedcatalyst,108 the different lasers yield similar features centered around300 cm−1, 600 cm−1 and 1100 cm−1 with very different intensity respon-ses. The features were attributed to oxides of copper and zinc on thebasis of spectra from the pure components. The highest signal intensityto acquisition time ratio is obtained with the visible excitation wave-length λ = 532 nm. Indeed, the latter exploits the full photovoltaic acti-vity of the CCD detector while benefiting from an intermediate energylight (∝ 1/λ4) to enhance signal detection. However, the steep baselineat larger Raman shifts is a clear indication of the potential limitation ofthis wavelength in case of fluorescent samples. In contrast, the UV andthe near-IR lines reveal a more constant baseline even with the longerrequired exposure times to reveal resolved peaks. It will consequentlynot be possible to study the structure of this catalyst, hence promising


the observation of intermediate surface species without interference fromthe bulk structure. Figure 3.12(b) compares the ex and in situ spectraof the commercial catalyst powder. When packed in the view-cell, thetotal catalyst signal intensity weakens and sharp sapphire peaks appearat 417, 447, 576 and 751cm−1 because of light absorption and refractionby the sapphire window. Nevertheless, the most intense features of thecatalyst remain visible and the rest of spectrum from 800 cm−1 onwardsis free from extraneous signals.

(a)

120 sec – 100% – 20 acc

10 sec – 5% – 40 acc

300 sec – 10% – 3 acc(b)

In situ

Ex situ

417

576

751

447

10 sec – 5% – 40 acc

60 sec – 100% – 3 acc

Figure 3.12 – (a)Raman spectrum of the commercial methanol synthesis catalystacquired with the three excitation wavelengths, (b) Raman spectrum of the samecatalyst with the λ = 532 nm laser in- and ex-situ.

Although the occurrence of fluorescence under the effect of tempera-ture cannot be predicted, these results prove that the three excitationswavelengths should provide sufficient freedom to find an optimum laserfor each catalyst candidate. In addition, the peaks from the reactantsand sapphire in the lower Raman shift region are clearly assigned andwill sharply contrast with the expected wider features originating fromintermediate surface species.

3.6 Conclusions 49

3.6 Conclusions

This chapter describes the mircoreaction plant and the implemented GCand Raman analytical methods used to carry out the CO2 hydrogenationreaction over a variety of heterogeneous catalysts to the hydrocarbonsintroduced in Chapters 4, 5, and 7. This experimental context led tothe description of an in-house designed high-pressure and -temperaturemicro view-cell for in situ visual and spectroscopic observation.

With an inner volume in the microliter range, the view-cell withstandsoperating pressures and temperatures up to 500 bar and 400 ◦C based onconical metallic seals between a flat sapphire window-unit and a stainlesssteel body. Easily loaded with solid catalyst particles, it was shown toperform ideally as plug flow reactor for reliable reaction analysis. In ad-dition to high resistance, the optical cell features a wide numerical aper-ture (NA = 0.36) and short working distance (WD = 14 mm) adequatefor non-intrusive in situ analysis by microscopy means with reduced re-fraction effects. The functionality of the view-cell with the remote Ramanspectrometer and its ability to collect physical and chemical data wasproven with a series of diagnostic tests using non-reactive binary mix-tures in flow and a commercial Cu/ZnO/Al2O3 catalyst. The setup hasshown good performance in characterising the nature and compositionof effluent streams as well as the surface state of the fresh catalyst.

On a wider scope, the view-cell complements the efficiency of microreac-tors in reaction screening and bridges the gaps for novel process windows.Indeed it allows for in situ analysis in microreactors to a variety of phe-nomena and chemical processes under high pressure and temperature.Filled with catalyst, it enables the spectroscopic observation of locallyresolved species in heterogeneous catalytic reactions under realistic con-ditions, while phase behaviour and chemical composition studies becomepossible when used as empty piping. At last, varying its location in thesetup, or using several in series, prospects spatially resolved studies alongthe reactor for mechanistic studies. The view-cell thus establishes itselfas a valuable tool to study, monitor and control heterogeneous catalyticreactions. More specifically to the targeted CO2 hydrogenation to liquid


chemical energy carriers, the view-cell will allow real-time spectroscopicfeedback under industrially relevant conditions. The following chapterswill dwell on operando-GC studies with the aim to unravel the influenceof phase behaviour on reaction performance and study fundamental as-pects of CO2 reduction mechanisms. The gained insight shall assist tomore rationally design an improved catalyst and modulate reaction con-ditions to keep it working at optimum performance.

51

Chapter 4

Methanol synthesis overCu/ZnO/Al2O3

This chapter 1 investigates the hydrogenation of CO2 into methanol overa Cu-based catalyst with the aim to shed light on the unsettled me-chanistic aspects introduced in Chapter 1. To this end, the view-celldescribed in Chapter 3 enabled the transition from an end-of-pipe toan operando Raman–GC analytical approach to characterise the natureand number of reacting phases, the main COx source and the natureof the catalytically surface species. The fundamental understanding thusgained confirmed the beneficial effects of high-pressure, temperature andprolonged residence time on CO2 conversion into multiple reduction pro-ducts through hydrogenation and the competing RWGS reaction. Con-densation phenomena at specific reaction conditions were observed andcorroborated by thermodynamic calculations to assess the intricate rela-tion between reaction and phase behaviour controlling methanol yields.Operating conditions should thus combine kinetic optimum with phaseoptimum to promote methanol synthesis beyond gas-phase equilibrium.

1. Reprinted with permission from: Reymond H., Amado-Blanco V., Lauper A.,Rudolf von Rohr P., (2017). Interplay between reaction and phase behaviour in carbondioxide hydrogenation to methanol. ChemSusChem, 10(6):1166–1174.

52 4. Methanol synthesis over Cu/ZnO/Al2O3

4.1 Introduction

A commercial Cu/ZnO/Al2O3 catalyst was chosen because of its esta-blished activity and stability in the industrial syngas (CO+H2) to met-hanol process. Although the former is a robust process which can bereadily adapted to CO2/H2 feeds,109 fundamental understanding of thereaction has the potential to expedite the development process for a ca-talyst tailored for the latter feeds by increasing the variety of informationalong with the quality and speed at which it is generated.110 In addition,the intrinsic physical and chemical characteristics of methanol synthesiscombined to the abundant literature, used as reference along this chap-ter, makes it a system of choice to further prove the performance of theconcept before the studies on catalysts selective for formic acid currentlyin development stage (Chapter 5).

Initially demonstrated by Hansen and Joensen,111 phase separation bycondensation proved to achieve conversions of syngas to methanol abovegas-phase thermodynamic expectations. Hence, the prospect of full one-pass conversion through in situ product condensation augurs well forhigh-pressure processing routes, obviating the need for costly reactantrecycle, making the most out of valuable H2 produced by water electro-lysis.112 Several studies investigated also theoretically phase separationin methanol synthesis from syngas.113–119 They used a reference model-ling case composed of H2:CO:CO2:CH4 = 74:15:8:3 vol.% and concludedon the formation of a liquid phase at 200 ◦C and 300 bar. Although fo-cused on syngas feedstocks, these studies pointed out the complex phasebehaviour of the multi-component reactive mixture. Van Bennekom et al.developed a model to calculate the simultaneously occurring phase andchemical equilibria of methanol synthesis.119 They expanded their inves-tigation to CO2-rich syngas and confirmed the likelihood of condensationat high pressures and low temperatures. In addition, they verified expe-rimentally a retrograde-like behaviour with increasing methanol contentunder high pressures using a view-cell operated in semi-batch mode.120

When CO2 was the main COx source, a single vapour phase was obser-ved at chemical equilibrium at 203 bar and temperatures above 234 ◦C.They claimed that phase separation is a function not only of operating

4.2 Materials and methods 53

conditions and feed composition, but also of extent of reaction. In thiscontext, Bros and Brilman exploited a two-temperature zone reactor toinduce condensation during CO2 hydrogenation to methanol at 50 barand drive the reaction close to complete conversion. They stated thatcooling is needed in the second reactor zone at moderate pressures, whendew point is lower than the reaction temperature in the first zone.104 Alt-hough primordial for catalyst and reactor design, triggering conditionsfor in situ condensation as well as the impact on sub-equilibrium reactionbehaviour remain unknown.

This chapter begins by confronting experimental reaction behaviour ofstoichiometric CO2 hydrogenation to methanol to thermodynamic equi-librium calculations. Following the Raman analysis used to detect phasetransitions, the dew points of characteristic reaction effluents are thenpresented to support dual COx reactive routes at high pressures overCu-based catalysts. A numeric estimation of the critical conditions com-pletes the full characterisation of the reaction mixture and testifies of themultiple phase transitions occurring during reaction. The chapter closeswith the identification of formate species on the working catalyst surfaceto provide a rationale for the development of CO2 reduction catalystsselective for formate species.

4.2 Materials and methods

4.2.1 Thermodynamic modelling

Chemical and phase equilibria

Reactions R.1-R.3 from Chapter 2 were considered for methanol synthe-sis. Thermodynamic equilibrium calculations were performed using theSoave-Redlich-Kwong (SRK) equation of state (EOS) implemented inthe Aspen Plus simulation tool for mildly polar mixtures of hydrocar-bons and light gases in the regions of high temperature and pressureincluding supercritical.121 In its simpler formulation, SRK-EOS fails to


represent highly non-ideal chemical systems. For this reason, the SRK-EOS binary interaction parameters for CO, CO2, H2, methanol and wa-ter were taken from the optimised values by van Bennekom et al. tocorrect for non-ideal effects.119 Calculations were performed by minimi-sation of the Gibbs free energy with respect to mole amounts at fixedreaction pressure and temperature (Equation 2.8). To represent the ex-perimental conditions, the molar composition of the reaction mixtureconsidered was CO2:H2:He=1:3:0.3.

Critical point of multicomponent mixtures

The criterion used to determine critical points followed the methodologyof Heidemann and Khalil, where the stability of a homogeneous phase isassessed in terms of the Helmholtz free energy A.122

For the detailed derivation of the methodology, the reader is referred tothe implementation of the conditions for the calculation of critical pointsusing the SRK-EOS explained by Stradi.123 The same equations wereused with the exception of α(T ) parameter in the attractive intermole-cular energy parameter ai of pure species. In this work, the Mathias124

modification of the original Soave121 α(T ) formulation was consideredto account for the vapour pressure of polar compounds. This formulationwas also used by van Bennekom in his thermodynamic calculations ona similar chemical system.119 A detailed listing of all equations used,including modifications, for the implementation of the model is given inAppendix A.

4.2.2 Catalyst

A commercial Cu-based ternary catalyst (Alfa Aesar, Cu/ZnO/Al2O3,Product number 45 776) was used for methanol synthesis in the high-pressure microreaction plant described in Chapter 3. The catalyst pelletswere crushed and sieved and separated into several size fractions of 28-45 µm and 45-63µm.


4.2.3 CO2 hydrogenation to methanol

The catalyst powder was loaded in the stainless steel microreactor sup-ported on a layer of smaller inert silicon beads and held in fix positionbetween two stainless steel filters. 100 mg (ca. 10 cm packed-bed) and130 mg (ca. 12 cm packed-bed) were loaded for the parametric studyand for the determination of dew point temperatures, respectively. Thecatalyst was reduced in a flow of 10 mol % He/H2 for 120 min at at-mospheric pressure at 330 ◦C. The reactor was allowed to cool down toroom temperature before pressurising the system with the He/H2 mix-ture. The desired flow of H2 was set before starting the correspondingCO2 flow for stoichiometric ratio of CO2:H2 = 1:3 at a total flow rate of29.2 mLN/min, corresponding to a gas hourly space velocity (GHSV) of22 300 h−1. Once a steady feed composition of 23 mol % CO2, 70 mol %H2 and 7 mol % He was reached, the reactor temperature was increasedat 10 ◦C min−1. The reaction temperature was maintained until reachinga steady-state composition and increasing the temperature to the nextsetpoint. Isothermal and -baric conditions were guaranteed respectivelyfrom the small reactor dimensions and from the negligible pressure dropacross the fixed-bed during operation. GHSV was defined as the ratio ofthe calculated total volumetric flow rate of the feed at normal conditions(0 ◦C, 1 bar) divided by the reactor volume occupied by the fixed-bed.

A series of control experiments verified the stability and necessity ofthe catalyst under harsh operating conditions respectively by achievingequal performance with a re-activated used catalysts, and by observingno activity during a blank test over a packed bed of inert glass beads at700 bar and 280 ◦C.

4.2.4 Dew point determination

Phase changes were detected through the view-cell implemented afterthe reactor for visual access into the product stream. The view-cell tem-perature was adjusted independently from the reaction temperature andset at the start at 330 ◦C to warrant gas-phase effluent. The tempera-


ture was then decreased stepwise allowing time to stabilise. Due to theair-cooling applied on the video probe for the sake of preservation, thetemperature in the view-cell was kept stable within ± 1.5 ◦C. This levelof accuracy was considered satisfactory as condensation was either verysharp, or progressive phase change at high pressure took place over awider temperature span. The remote probe was systematically removedfrom the view-cell cavity after each spectrum collected to protect it fromprolonged heat exposure and avoid damage.

The first signs of phase change were detected in a similar principle to afibre-optic reflectometer.125,126 As the dew line was crossed, the appari-tion of a second phase perturbed the incident and scattered light throughthe repetitive changes in curvature and refractive index in the inhomoge-neous medium. The consequent loss in light collection efficiency induceda sharp change in the aspect of the Raman spectrum, which coincidedwith the apparition of drops or waves on the lower sapphire surface visu-alised by video monitoring as represented in Figure 4.1 for 200 bar. Theprocess was reiterated by heating up again letting the system equilibratein gas-phase before cooling down gradually to confirm the transitiontemperature.

Figure 4.1 – (a) Raman spectrum of effluent stream in gas (black) and condensed(red) phase at 200 bar. (b) Snapshot of the inner window surface showing dropletsformed upon condensation at 200 bar.

4.3 Results and discussion 57

4.3 Results and discussion

4.3.1 Parametric study

Pressure and temperature effects on reaction performance

The effects of operating conditions on reaction performance were syste-matically studied and contrasted to the theoretical equilibrium expec-tations. Figure 4.2 and Figure 4.3 compile respectively theoretical equi-librium data and the experimental results obtained at pressures from200 bar to 700 bar between 170-280 ◦C at a GHSV of 22 300 h−1. A widetemperature range was investigated at total pressures of 200, 350, 500and 700 bar. Simulations at 50 bar are presented for the sake of compa-rison to industrial methanol synthesis from syngas.

Figure 4.2 contrasts calculations assuming a single vapour phase (con-tinuous lines) to ones accounting for vapour-liquid phase (dashed lines)to identify the reaction conditions which lead to condensation and high-light its effect on thermodynamic equilibrium. Methanol formation fromCO2 is an exothermic reaction with a reduction in molecule number. Inaccordance with Le Chatelier’s principle, improved conversion and met-hanol selectivity are predicted at elevated pressures and relatively lowtemperatures, whereas the selectivity to CO follows the opposite trendas a product of the endothermic RWGS. CO2 conversion is expected tofurther increase under methanol favourable operating conditions throughthe formation of a liquid phase. This prognostic is implied by the dis-tinct CO2 equilibrium conversion at all pressures except 50 bar (Figure4.3(a)). Phase separation would, however, not affect product selectivitiesat equilibrium state (Figure 4.2(b)), as chemical equilibrium is expectedregardless of which phase the reaction takes place in.

Experimentally, kinetic control was ascertained by testing the effect ofcatalyst particle size on reaction performance. Comparing the reactionperformance of runs with 45-63 µm and with 28-45µm catalyst particles,larger particles perform at least as well as the smaller ones, proving the


(a)

700 bar

500 bar

350 bar

200 bar

50 bar

(b)

MeOH

CO

Figure 4.2 – (a) Theoretical equilibrium CO2 conversion versus reaction tempera-ture for vapour only (continuous line) and vapour-liquid valid phases (dashed lines)at 50 bar (grey), 200 bar (black), 350 bar (blue), 500 bar (green) and 700 bar (red).(b) Respective equilibrium selectivities to methanol (dashed lines) and carbon mo-noxide (continuous lines) for vapour only as a function of reaction temperature andpressure (same colour scheme as in (a)). Vapour-liquid calculations yielded identicalselectivities.

absence of pore diffusion limitation at a GHSV of 22 300 h−1 (Appen-dix A, Table A.2). Moreover, the separate analysis of runs conductedat 200 bar (Figure 4.4) reveal the single occurrence of distinct outcomeswith particle size, showing up to 14 % CO2 conversion and 10 % met-hanol selectivity increase with larger particles. The fact that the latterachieve higher performance despite their longer internal diffusion pathand smaller specific surface area backs up the assumption of negligiblemass transfer hindrance. External mass transfer limitations are neglectedbecause of the high space velocities forcing intense mixing in the porousmicro packed-bed.127 Figure 4.3 thus presents the average conversionand selectivity values obtained with both catalyst particle sizes.

Figure 4.3(a) shows the advantageous effects of elevated pressures andtemperatures on CO2 conversion, where the strong temperature influence


(a) (b)

CO

MeOH

MF

Figure 4.3 – Experimental effect of pressure and temperature on (a) CO2 conversionand (b) product selectivity at GHSV =22 300 h−1 (same colour scheme as in Figure4.2). Continuous lines represent vapour-only theoretical calculations for comparison.Markers represent the average out of a minimum of two experiments (maximum five),and error bars the semi-dispersion. The lines connecting markers serve visual under-standing. Operating conditions and stream compositions correspond to experimentIDs A1-5, B1-5, C1-5 and D1-5 tabulated in Appendix A (Table A.2).

speaks in favour of a kinetic control of the process until the conversionapproaches theoretical equilibrium, causing the evolution to weaken at280 ◦C at 500 bar and 700 bar. Special attention is drawn to the averageconversion obtained at 200 bar and 280 ◦C (56 %), which exceeds theore-tical equilibrium (49 %) as a result of the distinctive outcomes presentedin Figure 4.4. An explanation for the conversion beyond equilibrium isincluded in a subsequent section discussing phase behaviour. In con-trast to conversion, carbon selectivities to methanol, carbon monoxideand methyl formate in Figure 4.3(b) present a more complex evolutiontranslating a gradual shift from kinetic to thermodynamic regime. As amatter of fact, an increase in reaction temperature should favour the en-dothermic RWGS reaction compared to the exothermic hydrogenationsof both COx to methanol, and thus lead to an increased CO selectivityas depicted in Figure 4.2(b). This trend is indeed observed in the expe-


(a) (b)

CO

MeOH

MF

Figure 4.4 – (a) Reaction performance and (b) product selectivity at 200 bar fordifferent catalyst particle size. The different behaviours were not related to particlesize effect, as no clear trend was observed upon change particle size change at otherpressures. The average of both runs corresponds to experiment ID A1-A5 labelled inAppendix A (Table A.2).

riments between 170-230 ◦C, where methanol selectivity at 170 ◦C startsoff at equilibrium at 90 % before decreasing to 24 % in favour of carbonmonoxide. However, the trend reverses in contradiction with the aboveconsideration as temperature is further raised: methanol selectivity re-increases, reaching 42 % at 280 ◦C and 700 bar - pressure at which theincrease is most pronounced. Although not clearly reversing at 230 ◦C,methanol selectivity at 200 bar and 350 bar does not follow the initialabruptly decreasing slope. Even at their extrema, the selectivities do notlevel-off and remain far from the equilibrium values predicted in Figure4.2(a). One can then expect that methanol selectivity would keep increa-sing until CO synthesis and hydrogenation balance out. Such behaviouris attributed to reactive pathways, detailed explanations of which areincluded in the subsequent sections.


Reaction pathway

The sequence of formation of methanol and carbon monoxide was stu-died to evaluate the interplay between Reactions R.1-R.3. To decouplemethanol synthesis from RWGS, the effect of GHSV on product dis-tribution at 350 bar from 170-280 ◦C was examined between 17 800 h−1

and 40 100 h−1. Conversions evolve monotonously with decreasing GHSVindicating that molecules react independently on the catalyst surface.Hence, the product selectivities observed are devoid of thermodynamicartefacts related to readsorption of products undergoing reverse reactionwith longer residence time. Even at 280 ◦C when fast supply of eductsis critical to sustain high turnover frequencies, substantial conversion isachieved, hinting at little diffusive hindrance as reported in literatureunder similar reaction conditions.128 The efficient utilisation of the ca-talyst surface is confirmed for all reaction conditions from the estimationof effectiveness factors close to unity yielding Weisz-Prater criteria smal-ler than 0.3 for the small particles, and slightly above the threshold forthe larger particles. Figure 4.5 presents the selectivities to the carbon-containing products against GHSV at the three characteristic tempera-tures: 200 ◦C, 230 ◦C and 280 ◦C. The results over the entire temperaturerange are presented in Appendix A (Figure A.1).

(a) (b) (c)

Figure 4.5 – Effect of GHSV on CO2 conversion (dashed lines) and product dis-tribution (continuous lines) at 350 bar and (a) 200 ◦C, (b) 230 ◦C, and (c) 280 ◦C.Operating conditions and stream compositions correspond to experiment IDs B1-B5,E1-F5 tabulated in Appendix A, Table A.2.


At 200 ◦C (Figure 4.5(a)), the selectivities to methyl formate and metha-nol increase as GHSV increases (i.e. contact time decreases), whereas COfollows the opposite trend. This evolution suggests that methyl formateand methanol are primary products of CO2 hydrogenation as revealedby their higher selectivities than CO at high GHSV, while CO is relega-ted to secondary reduction product through the weakly active RWGS.Although markedly higher selectivity to CO than methanol is observedat 230 ◦C, selectivities to all three products are almost independent ofGHSV as the control transitions from kinetic to thermodynamic (Figure4.5(b)), hinting at two parallel CO2 hydrogenation routes. The reducedconversions obtained at fast GHSV cause a shift in this transition regimetowards higher temperatures. At 280 ◦C (Figure 4.5(c)), the evolutivetrends in methanol and CO selectivities invert, whereby CO selectivityincreases with GHSV, while methanol is favoured by longer contact ti-mes and higher conversions. CO2 is efficiently converted to CO at 280 ◦Cand a GHSV of 40 100 h−1, but the subsequent hydrogenation of CO tomethanol does not take place due to too short a residence time. Theseresults adduce the deductions made by Urakawa and co-workers advan-cing CO as major COx source at high temperatures and pressures overa commercial methanol synthesis catalyst.128,129 The contrast between200 ◦C and 280 ◦C suggests an exchange in sequence of product formationas a consequence of a dual COx source in hydrogenation pathways.

These results are consistent with the theoretical study by Grabow et al.,according to which conclusions can be drawn only for specific conditionsrelating the COx source to the extent of reaction and the direction ofthe (R)WGS.130 CO2 hydrogenation has higher thermodynamic force atlow conversions and takes place when the WGS reaction is inactive. Asconversion increases, the direction of the WGS progressively reverses,thermodynamically promoting CO hydrogenation over the one of CO2.Moreover, the produced water via CO2 reduction and RWGS reactionwould favour CO reactive pathway which does not produce additionalwater. The results relate as well to the work of Ipatieff and Monroe sta-ting that CO hydrogenation prevails under high pressure conditions (ca.410 bar) over Cu-Al2O3 catalysts.131 They reported that CO is producedvia RWGS before its subsequent reduction to formaldehyde and furtherconversion to methanol via direct hydrogenation or Cannizzaro reaction.


4.3.2 Interplay between reaction and phasebehaviour

Concomitantly to the regime change discussed above, the reversing se-lectivities from 230 ◦C (Figure 4.3) may arise from in situ product se-paration via the formation of a liquid phase, composed of mainly waterand methanol, the less volatile components of the effluent.104,119 Equili-brium modelling in Figure 4.2 predict condensation for pure CO2 feedsfrom 200 bar upwards. Although equilibrated selectivities are calculatedto remain unaffected, kinetic selectivities can vary through the depletionof methanol and water in the gas phase. The induced change in localconcentrations promotes the synthesis of methanol over CO, which isexpected to remain mostly in the gas phase. To this aim, the reactionbulk was analysed by Raman microscopy and video camera through theoptical cell. The Raman spectra of the reaction feed and effluent are pre-sented in Figure 4.6. Features summarised in Table 4.1 were assigned tothe components of the reactive mixtures in reference to literature dataand on the basis of Raman spectra of the pure substances (Appendix A,Figures A.2-A.7).

Qualitatively, the gas-phase Raman spectrum of the reaction feed (Fi-gure 4.6(a)) shows sharp ground S0 rotational transitions of H2 (355, 587,812, 1033, 1246, and 1447 cm−1) as well as the 2ν2:ν1 fermi diad ([2l ] at1286 cm−1 and [2u] at 1387 cm−1) and weaker satellite hot bands ([3l ]at 1265 cm−1 and [3u] at 1408 cm−1) of CO2. As reaction proceeds, thefeature at 1033 cm−1 increases in intensity due to the combined effects ofS0(3) and of the CO-stretch of methanol while additional product peaksappear in the higher frequency range (Figure 4.6(b,c)). Although methylformate also presents two intense transitions at the same shifts,135 theRaman lines at 2843 cm−1 and 2950 cm−1 are attributed respectively tothe symmetric and asymmetric stretch of CH3 of methanol in reason ofits higher concentration in the effluent. The band at 3679 cm−1 is relatedto the OH-stretch of methanol monomer, while the band at 3643 cm−1

is attributed to the symmetric stretch of water monomer. The featurerecurrently appearing at 3566 cm−1 is tentatively interpreted as dimersof water-methanol. The extent of hydrogen-bond network is confirmed

1033

355H2

H2587

H2

H2

812

13871286CO2 CO2

1265CO2

1246H2

1447H2

1408CO2

CO2

1033

CO2140

CH3OH2843

CH3OH2950

H2O3643

CH3OH3679

H2O-CH3OH3566

(a)

(b)

(c)

Figure 4.6 – Gas-like phase Raman spectra of (a) reaction feed in stoichiometricmolar ratio CO2:H2=1:3 at 450 bar, and effluent of reaction carried out (b) at 240 ◦Cand (c) at 260 ◦C at the same pressure and various GHSV. The view-cell temperaturewas set to 330 ◦C to ensure gas-like conditions and preserve the remote probe fromexcessive heat exposure. Multiple line plots were shifted upwards in intensity of 0.2and of 0.4 to improve readability.


Table 4.1 – Experimental Raman band positions together with proposed assign-ments and literature references. Transitions are subject to shifts (up to a few cm−1)caused by different pressures and temperatures in the view-cell.

∆ν/cm−1 Assignment Ref.

355 H2 S0(0) 132,133

587 H2 S0(1) 132,133

812 H2 S0(2) 132,133

1033 CH3OH ν8 98,134,135

1033 H2 S0(3) 132,133

1246 H2 S0(4) 132,133

1265 CO2 ν(1)−

136–138

1286 CO2 ν− 135–138

1387 CO2 ν+ 135–138

1408 CO2 ν(1)+

136–138

1447 H2 S0(5) 132,133

2140 CO ν1 139

2843 CH3OH ν3 98,134,135,140

2950 CH3OH ν9 135,140

3566 H2O-CH3OHa 102

3643 H2O ν1 95,96,135–138,141

3679 CH3OH ν1a 98,134,135,135

a tentative assignment considering multiple factors affecting intensity and fre-quency in the stretching-mode region of H2O, such as operating conditions andchemical environment.

to strongly affect the Raman features of water, dictating its aggregationstate as pinpointed in [97,142]. A broadening of the peaks in the regionca. 3600-3700 cm−1 is observed at 200 bar when temperature decreases aspossible sign of the restoration of hydrogen-bond network in liquid state.The broadening becomes increasingly difficult to distinguish as pressureincreases. An example of a sharp aspect change in Raman spectrumafter condensation at 200 bar is presented in Figure 4.1, showing poorresolution and signal-noise ratio for the reasons explained in the experi-mental section. Quantitatively, peak intensities are expected to increaseproportionally to density with pressure, and were hence normalised inreference to the most intense Raman feature, the S0(1) transition of H2

at 587 cm−1.


The comparison of relative peak intensities at various reaction conditi-ons proves the gradual consumption of carbon dioxide and the varyingselectivities corroborating GC analysis: Raman features of methanol andwater intensify to above 0.6 and 0.2, respectively, as conversion increaseswith temperature and lower GHSV. In the opposite, the most intenseCO2 feature at 1387 cm−1 remains around 0.35 as a indication of theconstant stoichiometric H2:CO2 ratio. The evolution of carbon monox-ide transition intensity at 2140 cm−1 is not as distinct due to its smallerscattering cross-section than methanol. In the analysis of the inhomoge-neous effluent mixture, intensified methanol and water peaks were ob-served when precisely focusing on drops or waves, proving the latterproducts as major components of the condensed phase.

These results legitimate the following discussion on phase nature andcomposition from the relative band intensities of selected compounds.

4.3.3 In situ condensation

The dew temperature of reaction effluents of various compositions wasstudied experimentally at 200, 350 and 450 bar, at 240 ◦C and 260 ◦C.Higher pressures and temperatures were not tested as they yielded tran-sition temperatures exceeding the view-cell’s mechanical limits. The re-sults of condensation temperature versus CO2 conversion are presentedin Figure 4.7(a). Reactions were carried out at various temperatures,GHSV and levels of catalyst deactivation to span broader conversionand selectivity ranges. The corresponding selectivities to methanol andcarbon monoxide are tabulated in Figure 4.7(b).

Figure 4.7(a) shows that condensation occurs at higher temperature withincreasing pressure for similar effluent compositions (ca. 37 % conver-sion, 25 % methanol selectivity vs. 75 % CO selectivity). The incrementincreases from 0.1 ◦C bar−1 between 350-200 bar to 0.2 ◦C bar−1 between450 bar and 350 bar (points G6-7, H1, I2).

Condensation at 200 bar forms clearly visible droplets on the view-cellwindow as the density changes abruptly upon crossing the dew point in


I4I5

I6I1

I2I3 H6H7

H2H1H8

H3

G6G7G8

G2G3

G4

G5

G1

H5

H4


Table 4.2 – Selectivities to major reaction products, methanol and carbon monoxide,corresponding to xperiment IDs G1-8, H1-8 and I1-6.

Experiment Selectivity [%]ID MeOH COG1 35.7 67.2G2 24.2 74.9G3 23.0 76.1G4 23.7 75.3G5 21.0 79.0G6 24.8 75.2G7 23.8 75.3G8 21.5 77.6H1 28.4 70.1H2 25.4 73.4H3 23.2 75.8H4 19.7 79.4H5 23.0 75.0H6 25.8 72.8H7 26.4 72.4H8 17.7 81.4I1 25.6 73.2I2 23.7 74.0I3 20.7 77.2I4 37.1 61.8I5 23.0 75.9I6 21.1 77.8

change as pressure reaches such a level, that the gas density alreadyapproaches that of the dense liquid phase.

Although the critical line of the average mixture composition in Ta-ble 4.7(b) is unknown, the pressure approaches its critical region at450 bar, which causes a smoother phase and index change between gas-and liquid-like natures. Figure 4.7(a) shows as well continuously incre-asing condensation temperatures as reaction progresses, indicating thegradual enrichment of the effluent in the less volatile compounds met-hanol and water. The evolution at 200 bar is fairly linear over the CO2

conversions spanned. In contrast, results at 350 bar present a transitio-ning region at moderate conversions. A steep slope (H3-4-5-8), similar to

Figure 4.7 – (a) Condensation temperature of effluent stream in function of pressureand CO2 conversion. Empty markers represent reaction performed at 240 ◦C, andfilled markers reaction at 260 ◦C. The horizontal lines indicate reaction temperature at240 ◦C (dashed) and 260 ◦C (continuous). (b) Selectivities to major reaction products,methanol and carbon monoxide, corresponding to experiment IDs G1-8, H1-8 and I1-6. Complete reaction conditions and performance are tabulated in Appendix A, TableA.3.

a discontinuous phase change. This step-like function in isobaric densitychange with temperature is represented in the phase diagram of wa-ter in Figure 4.8, the least volatile compound, hence most susceptibleto trigger condensation. Similarly, the net difference in refractive indexbetween vapour and liquid phases causes considerable light collectionloss through reverberation, revealing a sharp drop in spectroscopic sig-nal intensity (Figure 4.1). Phase transition at 450 bar from gas-like toliquid-like phases resembles blurred waves passing through the cell, notaffecting the aspect of Raman spectra so dramatically. Condensation at350 bar resembles more the case of 450 bar than of 200 bar, albeit balan-ced between both. This dwindling impact reflects the smoother density


change as pressure reaches such a level, that the gas density alreadyapproaches that of the dense liquid phase.

100

bar

1bar

225 bar

500 bar

1000 bar

Figure 4.8 – Density of water in function of temperature at various pressures. Thegreen dashed line marks the liquid-vapour two-phase region. The red dot indicatesthe critical point of water. Data computed from the REFPROP database.143

Although the critical line of the average mixture composition in Ta-ble 4.7(b) is unknown, the pressure approaches its critical region at450 bar, which causes a smoother phase and index change between gas-and liquid-like natures. Figure 4.7(a) shows as well continuously incre-asing condensation temperatures as reaction progresses, indicating thegradual enrichment of the effluent in the less volatile compounds met-hanol and water. The evolution at 200 bar is fairly linear over the CO2

conversions spanned. In contrast, results at 350 bar present a transitio-ning region at moderate conversions. A steep slope (H3-4-5-8), similar tothe one obtained at 200 bar, proves a fast evolving condensation tempe-rature until roughly 30 %, before a milder slope is observed (H1-2-6-7-8)reflecting the smoother increase seen in experiments at 450 bar. Whilemost experiments show about 25 % methanol selectivity, experimentsG1 and I4 present larger methanol selectivities of 35 %; they neverthe-


less follow the afore-mentioned relations. The same observation is validfor experiments H4 and H8 which represent lower methanol selectivitiesof 17.7 % and 19.7 %, respectively.

Although relative product selectivities influence phase behaviour, CO2

conversion seems to bear more significantly on condensation of the re-active mixture. Multi-component phase diagrams can show strong depen-dence on minute fractions of a single compound as reported in the binarymixtures CH3OH-H2, CH3OH-CO2,144 and H2O-CO2.145,146 Even insmall concentrations at low CO2 conversions, water and methanol causean important shift in dew point towards higher temperatures. Whenconversion further increases, the dew point temperature presents a lesspronounced dependence as substantial amounts of methanol and waterare already produced.

Markers lying under the reaction temperature lines in Figure 4.7(a) re-present extents of reaction which do not experience in situ condensation.The results at 200 bar suggest a dew point lower than the actual reactiontemperature attesting of a single gas phase in the reactor. Points G3 andG6 represent borderline situations at 30 % and 37 % conversion whereboth, condensation and reaction, temperatures coincide at 240 ◦C and260 ◦C, respectively. Experiments at 350 bar maintain a single gas phaseuntil moderate conversion (points H3 and H8). However, as reactionextent proceeds above 30 %, markers lie well above the reaction tem-perature lines, thus implying condensation in the catalyst packed-bed.At 450 bar, all results suggest phase change at markedly higher tem-peratures than reaction temperature. In a similar way than at 350 bar,an extrapolation to lower conversions at 450 bar gives an estimate fromwhich extent of reaction condensation can be expected in the packed bedand shift reaction equilibrium.

Phase separation and/or higher mutual miscibilities can explain the im-proved performance observed in Figure 4.3(a) between 230 ◦C and 260 ◦Cat 500 bar and 700 bar as conversion exceeds a threshold value close to30 % conversion. Same conclusions can be drawn for 350 bar, however,the impact is more discrete most likely from condensation occurring to-wards the rear end of the packed-bed. The marked difference in reaction


performance observed at 200 bar advocates condensation as origin. Inthe less performant experiment, an equivocal situation arose at reactiontemperature of 260 ◦C where conversion reached 40 % coinciding withpoint G6-7 of Figure 4.7(a). As the latter points represent respectivemethanol selectivities of 23.8 % and 24.8 %, reaction most likely did notundergo in situ condensation with 21.5 % methanol selectivity (Figure4.4(b)). In view of the moderate conversion increase, reaction phase at280 ◦C was concluded verging on condensation. In comparison, the moreperformant run might have triggered condensation at 230 ◦C thanks tohigher conversions. Phase change subsequently accelerated reaction andallowed to react beyond equilibrium and improve methanol yields.

4.3.4 Critical temperature estimation

To evaluate the nature of the phase transition witnessed experimentally,the critical point of an average effluent composition was estimated withpredefined Matlab solvers on the basis of the methodology by Stradi.123

The performance of the implementation was tested beforehand on setsof reference mixtures. A satisfying fit was obtained for binary systems,while the deviation to the values reported by Stradi were found to incre-ase with the number of components in the system. The location of thecritical point is very sensitive to the model parameters used and onlythe binary interaction parameters were provided by Stradi. The compo-sition of the model effluent stream was simplified to its main componentsCO2 and H2 and the condensation products methanol and water. Hencea mixture composition of 78.46 mol % H2, 18.76 mol % CO2, 2.23 mol %H2O, 0.55 mol % methanol was scaled from the GC analysis of reactionG2 (Table 4.7(b)). The water content was calculated summing up themolar fractions of methanol and water following a 1:1 stoichiometric ratiofor each product according to Reactions R.1 and R.2. To keep computa-tional cost to a minimum, methyl formate, CO and He were not includedin reason of their low concentration, or lesser condensative effect.

A critical point for this mixture is calculated at −30.75 ◦C and 3290 bar.Despite the large uncertainty of this procedure stemming from the nume-


rous approximations, the critical temperature seems reasonable in that itlies between the critical temperatures of pure H2 at −240 ◦C and waterat 374 ◦C, weighted by their respective concentrations. The extremelyhigh critical pressure calculated is in fair agreement with trends repor-ting a critical pressure of 2520 bar for a 38 mol % H2 in water147 and upto 3600 bar for CO2–water systems.145

The results obtained from this calculation support then the experimen-tal observations concluding on a condensation from the gas phase to atwo-phase gas-liquid system in the pressure range investigated as high-lighted by the red arrow labelled as 1 in Figure 4.9. With a criticalpoint at −180 ◦C and 31 bar, the reactant feed of composition 7 mol %He/70 mol % H2/23 mol % CO2 is thus in supercritical phase before tran-sitioning to gas phase as reaction progresses to eventually separate into abiphasic regime upon crossing the dew point.143 Consequently, multiplephase transitions are expected during reaction accompanied by changesin physical properties of variable significance.

GAS

LIQUID

LIQUID + GAS

pmax

Tmax

Critical point 1

2

pc

Tc

Figure 4.9 – Phase diagram for a model binary mixture highlighting the possiblecondensation paths. Solid line, dew point; dashed line, bubble-point; filled circle, cri-tical point; diamond, maxcondenbar; square, maxcondentherm. Isobaric phase tran-sition: path 1 describes condensation from gas phase through a gas-liquid state, path2 follows condensation from a supercritical to liquid state. Adapted from [125]. Theenvelope pf the p, T -diagram varies depending on the composition of the mixture


4.3.5 Catalyst surface species and reactionmechanism

Using the view-cell as fixed-bed reactor, an operando Raman-GC studywas performed to study the relationship between reaction performance(Figure 3.6) and the surface species derived from the reaction at 200 barand 170-280 ◦C. As most aluminum oxides, the catalyst is very weaklyRaman active,108 thus its structure cannot be studied, allowing the ob-servation of surface species without interference. Figure 4.10 shows theλ = 532 nm Raman spectra of the catalyst at 200 bar at 230 ◦C, 260 ◦Cand 280 ◦C. Note that the baseline was corrected and that acquisitionwas limited to 2000 cm−1 due to high fluorescence.

1322

1574

1586

Figure 4.10 – Raman spectra of the Cu/ZnO/Al2O3 catalyst under H2/CO2 flowingat 200 bar at 230 ◦C, 260 ◦C and 280 ◦C. The C-O stretching region is shown acquiredwith 2 accumulations of 300 s exposure at 100 % laser power, λ = 532 nm.

The working catalyst presents two Raman features centered at 1322 and∼1580 cm−1. Not visible until 200 ◦C, the bands appear at 230 ◦C, gainin intensity at 260 ◦C and weaken at 280 ◦C. Contrary to the spectra at

4.4 Conclusions 73

230 ◦C and 260 ◦C with a band at 1574 cm−1 weaker than the one at1322 cm−1, the relative intensities at 280 ◦C are reversed with a domina-ting band blueshifted to 1586 cm−1. Although most probably broadenedfrom the distribution of bond lengths from different adsorption sites onthe ternary catalyst, the bands can be attributed to the symmetric andasymmetric νOCO stretches of formate species.30,148–152 Bidentate for-mates have been reported to dominate under reaction conditions,153,154

but they cannot be isolated from their monodentate counterparts on thebasis of these observations, neither can the presence of surface carbonatesand bicarbonates with similar features be ruled out.77,154–158 The blueshift at high temperatures might be related to a change of species rela-ted to the increasing CO concentration from the endorthermic RWGS asmeasured between 260 ◦C and 280 ◦C (Figure 3.6).

More mechanistic insight with other weaker Raman bands could notbe retrieved from this study in reason of the very low Raman activityof the catalyst and its fluorescence. The use of the UV excitation line(λ = 355 nm) improves the quality of the spectrum in terms of flu-orescence at high Raman shifts. Signals centered at 1350, 1510, and1620 cm−1 strengthen the formate pathway, but the power of the laser istoo low (5 mW at full transmission) to yield distinct formate bands νCH

reported around 2850 cm−1 and 2940 cm−1 (Appendix A, Figure A.17).These observations support numerous theoretical and experimental stu-dies on CO2 reduction mechanism.30,77,130,159 Control experiments anda combination of different in situ analysis techniques would be requi-red to accurately assign the peaks and conclude on the formate ratherthan carbonate nature of the intermediate species for methanol synthe-sis.157,160,161

4.4 Conclusions

A parametric study of the hydrogenation of carbon dioxide to methanolassessed the dependence of reaction performance on a broad range ofoperating conditions over a commercial Cu-based ternary catalyst. Highpressures, temperatures, and prolonged contact times showed beneficial


effect on CO2 conversion with little activity 200 ◦C and achieving equili-brium at temperatures approaching 280 ◦C (GHSV = 22 300 h−1). Wor-king at conversions below equilibrium enabled the study of the evolutionof selectivities to methanol and carbon monoxide with temperature andGHSV. Reverting selectivities at 230 ◦C at all pressures were attributedto a shift from a kinetically to a thermodynamically controlled reactionmarking a gradual transition in the COx hydrogenation pathway. Thisstudy supports other mechanistic studies advancing that operating con-ditions as well as conversion levels significantly influence the drivingforces for the competing CO and CO2 hydrogenations.130

In addition, the onset of condensation during methanol synthesis wasinvestigated below thermodynamic limitations in a separate set of expe-riments. This study showed that elevated pressures and moderate tempe-ratures favour in situ phase separation, however, that a certain extent ofreaction is necessary to initiate the change. Indeed, dew point tempera-tures were proven to increase with reaction pressure and with the amountof non-volatile products. Unless a threshold CO2 conversion is reached,and significant amounts of water and methanol are formed before exitingthe catalyst packed-bed causing the dew point temperature to rise abovethe actual reaction temperature, a single homogeneous reactive mixtureis maintained. Condensation at 200 bar was determined with the unequi-vocal apparition of droplets in the view-cell, whereas blurred waves at450 bar were considered to mark a softer density transition or/and incre-ased mutual miscibility. The same reasoning was applied for reactions at700 bar, although phase behaviour was not specifically tested.

All in all, the findings enlightened critical aspects in methanol synthesis,but also proved the adequacy of the view-cell for the study of reactivemixtures under catalytically relevant conditions. Consequently the cellshall be applicable to assist in controlled CO2 reduction to various speci-fic hydrocarbons over more Raman active oxide supports, such as ceriumoxide, particularly interesting for its unique structural reactivity.162,163

At last, the non-negligible amounts of methyl formate synthesised athigh-pressure, and the formate species observed at the reaction locusprovide a promising basis vouching for the feasibility of the two-stepprocess to formic acid.

75

Chapter 5

Methyl formate synthesis overheterogeneous catalysts

The objective of this chapter 1 is a direct conversion of CO2-derived met-hanol and formate intermediate to methyl formate. The former chaptershave demonstrated the efficient methanol synthesis from CO2/H2 feedsand the performance of the setup for in situ studies, both instrumental forthe optimisation of formate yields. Indeed, the minor quantities of methylformate by-produced in Chapter 4 hinted at the feasibility of the overallsynthetic perspective provided a viable heterogeneous CO2 hydrogena-tion catalyst selective for formic acid can be developed. Two strategieswere thus undertaken: (i) the heterogenisation of formate-active homo-geneous molecular complexes and (ii) the tuning of metal-supported he-terogeneous catalysts for methanol towards methyl formate production.The performance of an immobilised catalyst demonstrating potential inbatch operation to efficiently transform CO2 into formate products wasevaluated in continuous flow. The surface species identified as formateand methoxy by in situ Raman spectroscopy provided insight in sur-face chemistry of a variety of coinage metal supported on metal oxidesupports. Although both approaches still fall short of homogeneous cata-lysts, the synthesis of new carbonated products or the increased methylformate selectivities vouch for the catalysts’ efficiency.

1. The heterogeneous catalysts were synthesised by partners of the Sinergia pro-ject. Manuscripts in preparation H.K. Lo et al. and J.J. Corral et al.

76 5. Methyl formate synthesis over heterogeneous catalysts

5.1 Introduction

While heterogeneous catalysts, in particular Cu/ZnO/Al2O3, mainlyyield methanol, homogeneous processes dominate the selective hydro-genation of CO2 into formic acid and its derivatives (e.g. formate estersand formamides). Ever since the initial homogeneous CO2 hydrogenationcatalysts discovered in the 70’s,164,165 in the form of Ru complexes withphosphine ligands, numerous studies have been dedicated to its optimisa-tion by adapting ligands, transition metal, and external stabilising agentfor favourable thermodynamics. Selected milestones in catalyst develop-ment are referenced hereafter and tabulated in Table 5.1; detailed reviewsare provided in the literature.166,167 Noyori and co-workers disclosed intheir seminal work 20 years later the highly efficient Ru-phosphine cata-lyst precursor under scCO2 supercritical conditions.54,94,168,169 Furtherreports of optimised Ru catalysts include adapted surface-bound phos-phine ligands in solvent-free synthesis of formamide,170 or Ru-trimethylphosphine complexes in formic acid synthesis.171 Alternatively, othertransition metals have proven efficiency such as the Ir(III)-PNP pin-cer catalyst in aqueous KOH.172,173 To date, a Ru-PNP-Pincer com-plex sets the benchmark as record performance with an initial TOF of1 100 000 h−1 (TON of 20 000 after 1 h).174

Despite encouraging results with homogeneous molecular catalysts andthe need to develop continuous flow CO2 hydrogenation processes, com-bining the activity and selectivity of homogeneous molecular catalystswith the advantage of heterogeneous catalysts in a stable well-defined im-mobilised catalyst remains a challenge. Baiker et al. pioneered the immo-bilisation of active Ru complexes on silica gel matrices. While the mono-dentate phosphine ligands suffered from facile metal leaching,175,176 theimproved stability of the bidentate ligands did not reach the target of itshomogeneous counterpart.177 The preparation of these immobilised ca-talysts by direct sol-gel condensation of catalyst molecular precursors isbelieved to be the reason of the lower catalytic activity owing to diffusionresistances from poorly accessible active sites, probably entrapped in thesilica matrix. Alternative immobilisation strategies involving polymer-supported systems or molecular ligands grafted on silica surfaces gave

5.1 Introduction 77

catalytic performance weaker than the aforementioned system,178 andreports on supported pincer-type complexes have not been reported todate. Coperet et al. have shown that the use of mesostructured hybridorgano-silica materials prepared in the presence of a structure-directingagent is key to obtaining well-defined heterogeneous catalysts.179 Thisapproach allowed the introduction of functional groups into the material,and highly efficient heterogeneous catalysts were reported with catalyticperformance either close to, or in some cases greater than, their homo-geneous analogues for a range of reactions including CO2 hydrogenationby a Ru-NHC catalyst.180

Bulk metal and supported metal catalysts have shown some activity inthe conversion of CO2 to formate, but deactivation181 or their poor ca-talytic efficiency, with maximum TOF to date of 836 h−1 with 5 %Pd onactivated carbon,182 does not make them economically competitive.166

On Cu-based catalysts, a parametric study and IR spectroscopic ana-lysis independently showed the critical role of surface acidity in the re-lationship between surface formate intermediates and methyl formateyields.183,184 K promoters on alumina support enhanced the formationof surface formates, while suppressing methyl formate synthesis. Yu et al.supported the importance of the appropriate metal–metal oxide supportcombinations in defining the surface chemical properties of the catalystand intrinsic chemical activity in their work on liquid-phase mid-pressurehydrogenation of CO2 over unmodified and modified Cu/ZnO/Al2O3 ca-talysts.185 In excess methanol, metal promoters such as Ru, Ni, and Pdwere shown to have a positive impact on the catalytic performance, sho-wing the effect on the formation of activated surface formates. This studyalso demonstrated the thermodynamic limitations of the conversion ofCO2 to methyl formate, motivating a high-pressure approach to improvereactivity as reported for Au/ZrO2.186

Tab

le5.1

–S

um

mary

of

the

milesto

nes

inm

olecu

lar,

imm

ob

ilisedan

dsu

pp

orted

meta

lca

taly

std

evelo

pm

ent

reported

for

the

hyd

rogen

atio

nof

CO

2to

form

icacid

an

dderiv

ativ

es.

Year

Refe

rence

Cata

lyst

Base

,A

dd

itive

Solv

ent

p/b

ar

T/◦C

TO

F/h−1

Hom

ogeneou

sm

ole

cula

rcata

lysts

1994

Noyori

165

Ru

H2(P

Me3)4

NE

t3

scCO

2205

50

1400

1997

Baik

er170

Ru

Cl2

(dp

pe)

2M

e2N

H-

213

100

360

000

2002

Jesso

p171

Ru

Cl(O

Ac)(P

Me3)4

NE

t3,

C6H

F5O

HscC

O2

190

50

95

000

2009

Noza

ki172

IrH3 −

PN

P-P

incer

KO

HH

2O

50

200

150

000

2014

Pid

ko174

Ru−

PN

P-P

incer

DB

UD

MF

120

40

1100

000

Imm

obilise

dm

ole

cu

lar

cata

lysts

2000

Baik

er177

Ru

Cl2

[PM

e2(C

H2)2S

i(O−

Et)

3]3

Me2N

HscC

O2

215

100

970

2004

Zhen

g178

Si−

Et3 −

(NH

)Et3M

e−R

uN

Et3

EtO

H200

80

1384

Su

pp

orte

dm

eta

lcata

lysts

2007

Tsa

ng185

1w

t.%P

d/C

u/Z

nO

/A

l2O

3-

EtO

H60

150

38

2015

Han186

Au

/Z

rO2

-E

tOH

160

200

534

2015

Lin

182

Pd

/A

CN

H4H

CO

3H

2O

55

20

836


In the next sections, theoretical limits on equilibrium conversions areexamined and tested experimentally to maximise productivity and se-lectivity to methyl formate. The first part of this chapter introduces apincer-type Ru heterogeneous catalyst which has demonstrated promi-sing activity in batch operation in presence of a stabilising agent. Itsefficacy in high-pressure flow operation is then assessed over a range oftemperatures in absence of any additive. The second part of this chapterinvestigates the nature of surface species on Ag, Au and Cu supportedon CeO2 and SiO2 by in situ Raman spectroscopy to reinforce in situDRIFTS and computational studies. The data is then related to a pa-rametric study on catalytic performance leading to a discussion aboutpotential reaction pathways.


5.2.1 Catalysts

The methyl formate catalysts were synthesised by collaborators of theSinergia project. A N-heterocyclic carbene (NHC) pincer-type Ru he-terogeneous catalyst was prepared by H.K. Lo (Prof. Coperet, ETHZ).Details on the heterogenisation procedure and the catalytic efficiency inbatch operation are reported in [187].

A total of 9 supported metal catalysts were prepared by J.J. Corral(Prof. A. Urakawa, ICIQ, Spain), combining three coinage metals Ag,Au, and Cu with three metal oxide supports SiO2, CeO2, and Al2O3.Each catalyst contained 1 wt.% supported metal loading and was recei-ved pelletised, crushed and sieved to a 100-30µm size fraction.

The in situ microreaction plant (Chapter 3) was modified as describedbelow to evaluate the catalytic performance of the immobilised homoge-neous catalyst in flow and to characterise the surface of supported metalcatalysts.


XN

ON

N

N

N

Ru ClC

H ON

NN

OSiO Si

O

OO

O

The loading of Ru is about 0.6 wt%10.1 mg catalyst used in the batch reactor.5 bar CO2 and 35 bar H2.33 mmol DBU (base)Produce formate 10.8 mmolTon is about 18000.

Figure 5.1 – Immobilised Ru NHC-pincer complex synthesised by H.K. Lo.

5.2.2 CO2 hydrogenation over air-sensitive catalysts

To address the sensitivity of the immobilised heterogeneous catalyst toair, a reactor bypass was implemented in the setup with three needlevalves positioned as represented in Figure 5.2. Valves V1 and V2 at theextremities of the reactor enabled to handle the reactor avoiding contactwith air, while valve V3 was used to purge the piping.

PIH2

CO2

GC

V1 4

1

V3

V2

2

3

Figure 5.2 – Partial scheme of the modified high-pressure hydrogenation setup. (1)Reactor bypass for purging purposes operated via valves V1-V3, (2-3) Packed-bedreactor with heating, (4) Back pressure regulator.

30 mg of the NHC pincer-type Ru catalyst were packed (ca. 10 cm, pd ∼4 µm ) in the reactor under inert conditions in a Ar-filled glove box. The


packed-bed was supported on a dense plug made of quartz wool andheld between two stainless steel filters. The reactor, kept leak-tight withvalves V1 and V2, was implemented in the setup, which had been pur-ged beforehand with the He/H2 mixture via valve V3. The connectionat the bypass outlet was plugged before pressurizing the system to 200bar in He/H2. The reaction mixture (molar ratio CO2:H2 = 1:3) was fedthrough the reactor at a total flow rate of 14.5 mLN/min, correspondingto a GHSV of 11 100 h−1. After flow stabilisation, the reactor tempera-ture was increased at 10 ◦C min−1 to the desired operating temperatures.The temperature was maintained until a steady-state composition wasreached before increasing the temperature to the next setpoint.

5.2.3 In situ Raman analysis of heterogeneouscatalysts

The view-cell was filled with supported metal catalysts and implemen-ted after a standard reactor. The catalyst powders were loaded in theview-cell as received so as to completely fill its inner channel (equiva-lent to 5 cm packed bed) and continue upstream in the reactor (3 cmpacked-bed). The catalyst was reduced at atmospheric pressure in a flowof 10 % He/H2 at 10 ◦C min−1 to 260 ◦C for 1 h. The catalyst bed wasallowed to cool down to room temperature and the system was pressu-rised to reaction pressure with the He/H2 mixture. The flow of H2 wasset to 8.0 mLN/min before starting the CO2 flow rate at the same valuefor an equimolar feed ratio CO2:H2 of 1:1. The temperature was raisedstepwise upon reaching a steady composition from GC analysis, and thefeed composition and residence time at a fixed pressure were varied byadjusting independently the flow rates of both reactants.

Ex situ Raman spectra of fresh and spent catalysts were collected withall three excitation lines to determine the most adequate wavelengthfor in situ studies. In situ Raman spectra of 1 wt.% Ag/SiO2, 1 wt.%Ag/CeO2, 1 wt.% Au/CeO2, and 1 wt.% Cu/CeO2 were acquired withthe λ = 532 nm laser for reasons specified thereafter.



The addition of methyl formate in the thermodynamic equilibrium cal-culations carried out in Chapter 4 confirm the strong limitations to itsdirect synthesis (Reaction R.4, Chapter 2). Equilibrium calculations, per-formed in the same fashion as described in Chapter 4, predict extremelylow selectivities to methyl formate over the whole temperature and pres-sure range, not exceeding 5× 10−2 %. CO2 conversions as well as theselectivities to methanol and CO are left substantially identical to theones calculated in methanol synthesis. Hence catalysts are sorely neededto overcome the barrier to synthesise substantial amounts of formatesfrom CO2, H2 and in situ produced methanol (Reaction R.5 Chapter 2).

5.3.1 In flow performance of an immobilised RuNHC-pincer catalyst

Reaction performance

In batch operation, 10.1 mg of the Ru catalyst produced 37 % formatesfrom a 1:7 H2:CO2 atmosphere in presence of DBU (1M 1,8-diazabicyclo[5.4.0]undec-7-ene) to shift the reaction equilibrium and stabilise theproduct, corresponding to a TON of 18000 2 (33 mmol of DBU, 85 ◦C,40 bar, 24 h).187 Although the immobilised catalyst does not rival thehighest homogeneous TON to date of 3.5× 106,172 its promising activityis sufficient to evaluate its stability and performance in flow in absenceof external stabilising agent. 3

Following the procedure described in Section 5.2.2, the catalyst was tes-ted at 200 bar at temperatures ranging from 100 ◦C to 230 ◦C. GC ana-lysis of the reaction effluent shows decreased CO2 molar fractions as wellas weak signals from methyl formate and an unidentified compound. Fi-gure 5.3 traces the average CO2 molar fraction x with the evolution in

2. TON based on 0.6 wt.% Ru loading determined by elemental analysis.3. The batch experiments were performed by H.K. Lo (Prof. C. Coperet, ETHZ).


the average GC peak areas A of both products as a function of reactiontemperature.

18.7

3

18.9

3

8.74

1.102.93

0

18.3

2

14.9

0

9.84

18.0

8

22.1

5

47.4

4

19.5

0

34.9

3

651.

68

1631

.73

18.4

5 23.8

0

Figure 5.3 – CO2 molar fraction x in mol % and peak areas A in µV min−1 ofidentified and unidentified product in the effluent stream as a function of reactiontemperature.

The CO2 molar fractions, distinctly lower than in the starting feed com-position of ∼20.0 mol %, testify of a certain catalytic activity, with con-versions up to 10 %. The conversion do not evolve linearly with reactiontemperature as noticed from the CO2 molar fractions decreasing a littleuntil 180 ◦C, before re-increasing slightly at 210 ◦C and falling again at230 ◦C. In contrast to the seemingly constant CO2 conversion, the pro-duct peak areas, although small, steadily increase over the whole tem-perature range investigated. While the unknown product appears at theonset of the reaction at 100 ◦C, traces of methyl formate are formed onlyfrom 120 ◦C onwards, and appreciable amounts are not produced before210 ◦C. Methanol is not observed, nor is CO. Even at high temperatures,the quantity of methyl formate detected does not amount to the signi-ficant drop in CO2, resulting in a carbon loss of approximately 4-10 %.The open carbon balance and the negligible GC peak areas suggest met-hyl formate as by-product of the formation of a carbonated product withlow FID response factor such as formic acid. Because the signal of a FIDis proportional to the amount of oxidisable C−H bonds, the attempts to


qualitatively trace formic acid in diluted gas streams, let alone quantifyit, remained inconclusive. Although a similar GC signal was observed inthe analysis of more concentrated gas phase formic acid, the identity ofthe unknown product could not be unambiguously determined by otheranalytical means and stays as a strong hypothesis.

The little catalytic activity in flow is attributed to the short effectivecontact times with the catalyst (< 15 s) in comparison to batch operationwith residence time stretching up to 24 h. Nevertheless, the substantialTON measured in the first 10 min of operation suggest that the absenceof stabilising agent in flow caused the direct decomposition of formedformic acid back to reagents.

Directing the effluent to a downstream reactor loaded with an acidic ionexchange resin (Chapter 6) provided an alternative to promote formicacid synthesis by catalysing the hydrolysis of the methyl formate with theresidual air moisture trapped in the resin pores. Two consecutive reac-tors packed with single catalysts achieve similar levels of CO2 conversionunder the same conditions of pressure and temperature. The short-livedformic acid remain undetectable, however, the detection of traces of di-methyl ether (DME) in GC analysis confirms the hypothesis of hydrolysiswith co-production of methanol and consecutive dehydration followingReaction R.8. Methanol yields might further profit from shorter diffusionlengths with a closer interaction between methyl formate and the resincatalyst in a physical admixture of both catalysts.

Table 5.2 – Methanol dehydration to dimethyl ether.

∆H◦r(g)

∆G◦r(g)

Reaction kJ mol−1 kJ mol−1

(R.8) 2CH3OH CH3OCH3 + H2O −23.9 −16.6


5.3.2 Surface characterisation of metal supportedcatalysts

Reaction performance

The activity of metal supported catalysts were evaluated with excessmethanol to facilitate methyl formate formation. 4 Co-feeding methanolwith a molar feed composition of CO2:H2:MeOH = 4:4:1, a parametricstudy with the 9 catalysts highlighted Ag and CeO2 as most promisingsupported metal and oxide support, respectively: 1 wt.% Ag/CeO2 com-bined the highest methanol conversions up to 6 % and a selectivity tomethyl formate approaching 100 % at 240 ◦C and 260 ◦C and 300 bar.Alumina catalysts show similar conversions with markedly lower selecti-vity, whereas silica catalysts show increased selectivity at the cost oflittle conversion. On ceria, other coinage metal give rise to similar metha-nol conversions, but selectivity to methyl formate starts decreasing from240 ◦C in the order Ag > Cu > Au. As inferred in the introduction ofthis chapter, the Lewis acidity of a specific oxide support affects productselectivity through its interaction with the supported metal oxide.108,188

The parametric study was completed by switching to in situ formation ofmethanol by inclusion of a Cu/ZnO/Al2O3 catalyst to 1 wt.% Ag/CeO2

in a dual-bed configuration or as a physical admixture in a 1:2 wt. ratio.As expected from the severe thermodynamic limitation, pressure im-proves CO2 conversions, but selectivity to methyl formate remains sub-stantially unchanged regardless of reaction feed composition, tempera-ture and GHSV. The use of two sequential reactors operated at differenttemperatures maximises the exothermic methanol synthesis and subse-quently promotes the endothermic synthesis of methyl formate. Thus,with an equimolar feed ratio at 300 bar, methyl formate selectivity in-creases from 0.5 % to 2.9 % by loading Cu/ZnO/Al2O3 in the first reactorat 260 ◦C and packing a second reactor in series in a 1:1 wt. ratio with10 % Ag/Al2O3 operated at 140 ◦C.

4. The experiments with co-feeding and in situ formation of methanol were carriedout by J.J. Corral (Prof. A. Urakawa, ICIQ, Spain)


Catalyst surface species

Raman spectroscopy was applied to gain qualitative information on thesurface state of supported metal catalysts under controlled environmentsand feed back the information to the catalyst development to efficientlydesign more active and selective catalysts. The following paragraphshighlight the dynamic nature of these catalytic systems by reportingprominent changes on the surfaces dependent on the composition of thereaction mixture, pressure and temperature.

Ex situ spectra of the fresh catalysts listed in Section 5.2.1 confirmthe very weak Raman activity and fluorescent background of aluminasupported catalysts, which were consequently excluded from the in situstudy. 1 wt.% Cu/SiO2, Cu/CeO2, Au/CeO2, and Ag/CeO2 reveal exsitu well-defined features and a baseline free of fluorescence with theλ = 532 nm, promising for in situ characterisation.

1 wt.% Cu/SiO2 Although the catalytic activity of silica-supportedcatalysts is lower than the one of aluminum and cerium oxide catalysts,the inert lattice structure allows for the observation of surface speciesoriginating from the reaction on the supported metal, uncompromisedby metal-support interaction. Figure 5.4 presents the Raman spectraof the Cu-supported catalyst under reaction conditions revealing theformate nature of the surface species. Spectra were acquired at 200 barand 400 bar at 230 ◦C in a flow of CO2:H2 = 1:3.

Table 5.3 – Effect of pressure and temperature on surface species.

p T Band 1 Band 2bar ◦C Centre Area Centre Area1 230 1321 86 305 1590 262 221200 230 1325 181 645 1588 274 418400 230 1327 690 102 1583 316 189200 30 - - - -


Band 1 Band 2

Figure 5.4 – Effect of pressure on the Raman intensity of formate species on 1 wt.%Cu/SiO2. The spectrum at 1 bar was acquired directly after de-pressurising the systemafter reaction. The spectra were post-processed to correct the baseline. The sharppeaks labelled ◦ and O stand respectively for H2 and CO2. The spectra were acquiredwith 1 accumulation of 600 s exposure at 100 % power of the λ = 532 nm laser.

The absence of broad features amongst the sharp peaks from the reac-tants at room temperature and 200 bar (red line) stands for the freshly re-duced state of the surface before reaction took place. At reaction tempe-rature, 230 ◦C, the catalyst exhibits identical features whether at 200 bar(blue line) or at 400 bar (black line), suggesting an identical reactionmechanism. However, the larger area of the surface species at increasedpressure suggests a denser coverage of the catalyst surface with interme-diate species (Table 5.3). As a matter of fact, as previously mentioned,Raman intensities are directly proportional to the amount of moleculesprobed, which relates to surface coverage in case of such solid samplesin contrast to translucent samples where focus depth may be of influ-ence as exemplified by the intensified CO2 and H2 peaks. Although thearea of the peak centred at 1325 cm−1 shows stronger dependence onpressure than the peak centred around 1588 cm−1, both features can ori-ginate from the same species as they developed synchronously; or elsethey belong to different species, one of which may profit from a grea-ter stabilisation with pressure than the other. The pair remain visibleeven after de-pressurisation (green line) of the reaction system indicating


fairly stable intermediate species, uneasily desorbed.

The pair of peaks is consistent with symmetric and antisymmetric COstretches (νs(OCO) and νas(OCO), respectively) of formates adsorbedon copper. However, a clear assignment to bidentate formates cannot beconcluded on the basis of these Raman spectra alone in reason of thefluorescence overwhelming the distinctive CH peak in the region 2840-2950 cm−1. Complementary transient DRIFTS studies show transitionsat these wavelengths allowing, aided by first principle computationalstudies, the assignments of the intermediate species to uni- and bidentateformates alternately adsorbed on the support at the periphery of, as wellas on, the metal centre.189

1 wt.% Cu/CeO2 This paragraph focuses on the effect of operatingconditions on catalyst surface species; the changes in the lattice structurearising from high oxygen mobility do not pretend to elucidate the roleplayed by ceria in the catalytic mechanism. As a matter of fact, contraryto silica and alumina oxide supports interfering little with supportedoxides and with negligible Raman activity, ceria possesses intense Ramanbands below ca. 700 cm−1. They should, however, not interfere with thoseof the supported oxide expected in the range of 700-1100 cm−1.108

The ex situ analysis of 1 wt.% Cu/CeO2 (Appendix A, Figure A.11)shows distinctive peaks from the metal oxide lattice structure (460 and1043 cm−1) along with other peaks (831 cm−1, 1175 cm−1, 2846 cm−1,2937 cm−1 and 3656 cm−1) stemming from the adsorption of ambientCO2, O2 and moisture causing the solvation of the surface metal oxide.Unless explicitly specified differently in the figure caption, all in situspectra hereafter were acquired with 1 accumulation of 120 s exposureat 100 % power of the λ = 532 nm laser.

Catalyst activation Figure 5.5 traces the evolution in Raman bandintensity with temperature taking place during the activation of the ca-talyst.


Centre: 460 cm-1

Area: 2.0·106

Centre: 446 cm-1

Area: 1.5·105

23°C

50°C

70°C

80°C

90°C

110°C

230°C

Figure 5.5 – Bulk lattice and surface transformation during the activation of 1 wt.%Cu/CeO2.

At atmospheric pressure, in reducing atmosphere, temperature radicallyimpacts the intensity of all bands. The strong band centred ca. 450 cm−1,characteristic of the fluorite structure of CeO2, shows the reversible na-ture of lattice structural changes with a dependence inversely proporti-onal to temperature.190 The band quickly weakens and eventually va-nishes as temperature reaches 230 ◦C before slowly reappearing as thereactor cools down. The other bands of the support follow the same evo-lution, and the dehydrating conditions cause the desorption of residualmoisture. A close-up of the CH-stretch region shows the disappearanceof the transitions of special interest at 2845 cm−1 and 2937 cm−1, attri-buted to bidentate surface formates, revealing the reduced metal oxidesurface.

The conditions of H2-reduction (time and temperature) determine thedegree of reduction of cerium oxide, which was reported to impact the na-ture of the surface species adsorbed during a subsequent reaction.163,191,192

A minimum reduction temperature of 300 ◦C induces subsurface reorga-nisation with the reduction of bulk oxygen, while lower activation tem-peratures only superficially reduce cerium atoms and create oxygen va-cancies on the capping and surface layers. The generated surface active


sites Ce3+ or Ce4+ with oxygen vacancy (Ovac) were reported to play aprimary role in forming formates and inhibiting carbonates.

Effect of laser power In the process of determining best spectrosco-pic acquisition parameters, the effect of laser power on the Raman acti-vity of the working catalyst was investigated.

580

1090

1340

1590

450

Figure 5.6 – Effect of laser power on spectrum of 1 wt.% Cu/CeO2. The spectrawere acquired with 1 accumulation of 120 s exposure at 100 % (red) and 50 % (blue)power of the λ = 532 nm laser.

Increasing pressure to 200 bar with H2 does not alter the Raman spectrumof the reduced catalyst at room temperature (c.f. Figure 5.5) with theexception of the apparition of sharp H2 bands. However, formate speciesreadily adsorb when CO2 starts flowing as shown by the red curve in Fi-gure 5.6 by the features at 1340 and 1590 cm−1 at full laser power. Thesevibrational frequencies are similar but not identical to the ones reportedfor Cu/SiO2 and are hence assigned to the νs(OCO) and νas(OCO) offormate species on a different support. Because of the heterogeneous na-ture of the catalyst surface, different kinds of formates can adsorb eitheron the Cu-sites or on Ce3+ or Ce4+ with Ovac, or at the interface betweenmetal – support, giving rise to Raman bands at similar frequencies andcausing the cumulative features to convolute from 1100 to 1600 cm−1.192


Despite the absence of bands in the CH-stretch region due to obscuringbaseline, the majority of surface species is assigned to bidentate forma-tes on ceria, with a frequency difference separating the two local maximasmaller than the reported 250 cm−1. Monodentate formates on ceria typi-cally show a larger frequency difference of 350 cm−1.162,193 The intensityof the bands suggests a very high surface coverage, causing the formatesformed on Cu to migrate to the support. Experiments carried out onpure CeO2 (Appendix A, Figure A.17) present narrower features, lessintense, and redshifted to ca. 1310 cm−1 and 1586 cm−1 implying thatthe presence of a metal is not a requisite for the formation of formates.

With 50 % laser power, the spectrum changes radically as shown in bluein Figure 5.6. The formate features do not show, while bands of weakerintensity appear at 580 cm−1 and 1090 cm−1 together with the characte-ristic 460 cm−1 of cerium oxide. The presence of the latter peak indicatesa lower temperature at the sample suggesting that the heat generatedby the laser concentrated at the focus point induces structural reorga-nisation of the cerium oxide, stabilising different kinds of species at itssurface. While the peak at 580 cm−1 remains unattributed so far, theone at 1090 cm−1 is attributed to surface methoxy species. Pure CeO2

undergoes the same structural transformation at reduced laser powerto reveal the band at 1090 cm−1 as well as a weak band at 750 cm−1

from the deformation of OCO of formates (Appendix A, Figure A.17).On occasions, thanks to lower fluorescence possibly from the lower lo-cal temperature, weak bands at 2850 cm−1 and 2940 cm−1 of bidendateformates are observed.

The observation of this different Raman behaviour confirms the dyna-mic nature of cerium oxide lattice and its memory reported in the lite-rature.191,194 The white light camera in the remote probe enabled theconfirmation of the structural change as the spot after analysis at fulllaser power had turned to a darker colour. Even though both spectraare observed until 300 ◦C, the increase in temperature favours the for-mer spectrum, corroborated by the camera capturing the surface of thecatalyst gradually turning to darker shades. Consequently, the followingresults focus on the dominant Raman features acquired at 100 % laserpower.


Effect of operating conditions Reactions were carried out undervarious operating conditions to monitor the variation in surface species.Figure 5.7 with Tables 5.4-5.6 report the evolution in Raman spectra ofthe working catalyst when parameters were varied independently fromstandard conditions set at CO2:H2 = 1:3, p = 200 bar and T = 230 ◦C.

Table 5.4 – Effect of temperature on surface species (Figure 5.7(a)).

T Band 1 Band 2◦C Centre Width Area Centre Width Area120 1342 292 2.9× 106 1585 72 1.1× 106

260 1323 237 1.6× 106 1582 72 6.5× 105

Table 5.5 – Effect of pressure on surface species (Figure 5.7(b)).

p Band 1 Band 2bar Centre Width Area Centre Width Area200 1323 241 2.6× 106 1584 64 9.7× 105

400 1320 201 2.9× 106 1587 54 1.1× 106

Table 5.6 – Effect of feed composition on surface species (Figure 5.7(c)).

Ratio Band 1 Band 2CO2:H2 Centre Width Area Centre Width Area1:1 1321 265 2.0× 106 1586 67 6.4× 105

1:3 1321 283 3.7× 106 1584 67 1.0× 106


Band 1

Band 2

Band 1

Band 2

Band 1

Band 2

Figure 5.7 – Effect of reaction (a) temperature, (b) pressure, and (c) feed compo-sition on the surface formate bands on 1 wt.% Cu/CeO2.


The formate species intensify as temperature increases in a stepwisefashion from room temperature up to ca. 100 ◦C, before showing thegradual decrease until 260 ◦C presented in Figure 5.7(a) (Table 5.4). Con-comitantly, the maxima of Bands 1 and 2 undergo a significant redshiftfrom 1341 cm−1 to 1323 cm−1 and 1585 cm−1 to 1582 cm−1, respectively.The shifts are not necessarily attributed to a change in surface species,rather to a weakening of the interactions with temperature causing asmaller surface population as no obvious sharpening or broadening isobserved at high temperatures. The methoxy feature at 1091 cm−1 doesnot show particular evolution with temperature.

The effect of pressure on the intensity of formate bands in Figure 5.7(b)(Table 5.5) follows the same trend as reported for Cu/SiO2, where thelarger area at 400 bar than at 200 bar suggests a denser surface coverage.While the difference is relatively small between such high pressures, anstronger change in surface coverage is expected at lower pressures.

Figure 5.7(c) (Table 5.6) demonstrates the strong effect of feed compo-sition on surface formates. Proportionally to the H2 fraction in the feed,formate Bands 1 and 2 gradually intensify as the molar feed ratio is swit-ched from R = 1:1 to 1:3 and grow even larger upon completely stoppingCO2 supply. The intensity slowly decreases at 280 ◦C and retains sometraces after depressurisation.

Effect of re-activation The spectrum of the spent catalyst un-contaminated by ambient air in Figure 5.8 shows the reversibility overseveral hours of the thermal and reaction structural changes occurring incerium oxide, as well as the extreme stability of the generated bidentateformates. Consequently, the potential of re-activation was monitored toevaluate the possibility of cycling reactions.

Reducing the catalyst at 260 ◦C causes the partial decomposition offormates into carbonates as observed in Figure 5.9(a) by the shifts ofνs(OCO) and νas(OCO) at 1349 cm−1 and 1571 cm−1.195,196 The com-bination of residual formates with the freshly generated carbonate peaksresults in a large single feature spanning 1300-1575 cm−1.192,197 The


13731588

2850

457

Figure 5.8 – 1 wt.% Cu/CeO2 after reaction in view-cell, kept in reaction atmospherewithout contact to air. The spectrum was acquired with 1 accumulation of 120 sexposure at 50 % power of the λ = 532 nm laser

bands narrow down with reduction time marking a predominance of car-bonates at 260 ◦C, but perish when heated at 300 ◦C as shown in Figure5.9(b). Back under reaction conditions, the catalyst retrieves the samesurface formates as initially observed.

Effect of supported metal Highly similar Raman spectra wererecorded for the in situ study of Ag and Au supported on ceria, thecharacteristics of which are tabulated in Table 5.7. The Raman shiftsof methoxy and the νs(OCO) and νas(OCO) pair vary significantly upto 20 cm−1. This variation translates the adsorption of formates andmethoxy exposed to different environments, either directly on differentsupported metal sites or on structurally modified ceria by the presenceof the different metals. The band at 580 cm−1 reappears on all ceria-based catalyst and could be assigned to a structural change in the crystallattice.198

In an attempt to compare the ceria catalysts, the relative intensities wereconsidered as an indication of the surface coverage despite their differentspectroscopic acquisition parameters. The intensity of formate species is


1354

1555

1349 1571

1324

1584

Figure 5.9 – Reactivation and reaction 1 wt.% Cu/CeO2

highest on Ag closely followed by Cu and weakest on Au and pure ceria.Interestingly, the methoxy band intensifies on Ag compared to the twoothers.

Reaction mechanism The collection of in situ Raman spectra re-veal formates and methoxy as key species in the reduction of CO2 oncerium oxide. According to the literature on CO hydrogenation, formate


Table 5.7 – Raman shifts and relative intensitiesa of surface features on the 1 wt.%loaded Cu/SiO2, Cu-, Ag-, and Au-supported and pure CeO2 catalysts at standardreaction conditions.

Catalyst Acquisition Raman shift [cm−1]

Cu/SiO2 1 x 600 s-100 %1080 1325 1588w. w. w.

Cu/CeO2 1 x 120 s-50/100 %1091 1323 1585m. s. s.

Ag/CeO2 1 x 120 s-50/100 %1075 1335 1587

s. s. s.

Au/CeO2 1 x 120 s-/50 %1086 1332 1594w. s. s.

CeO2 1 x 120 s-50/100 %1090 1310 1586m. s. s.

aThe relative intensities are given as qualitative appreciation for comparison pur-poses. w. stands for weak, m. for medium, s. for strong.

species detected herein arise from the direct reaction of CO2 with surfacehydroxyls in the vicinity of active sites on the cerium oxide surface. As-suming extensive surface dehydroxylation after the catalyst activation,formate species might be generated at first from the reduced cerium ex-plaining the strategic factor of the surface chemical state. Metallic Cuwas reported to facilitate the creation of O-vacancies in ceria for sy-nergistic Cu-Ovac interactions.196 Later on, water vapour, generated asby-product in the RWGS and methanol/methyl formate synthesis, affectsthe extent of surface hydroxylation by dissociating on Ovac sites and pro-motes the formation of formates.188 However, the question of whethermethoxy species are formed from the hydrogenation of, or are decom-posed by oxidation into, formates and vice versa remains open. Basedon the theory of micro-reversibility, according to which the decomposi-tion of a compound should proceed via the same elementary steps as theformation of that compound, surface methoxy reveal the intermediateformate species needed to produce either methanol or methyl formate.An in situ study of the surface hydroxyls was hampered by the importantfluorescence at high Raman shifts, hence rendering unreliable an attemptto propose a specific structure-mechanism relationship merely from theresults of Raman spectra. Corresponding in situ analytical techniquesand computational chemistry could assist understanding.


Although very strong bands from formate species are detected after CO2

hydrogenation on all metal-supported cerium oxides, their intensities donot correlate the little catalytic activity measured by GC. Band intensi-ties decline with temperature, while the hydrogenation products increase,yet, surface formates are detected even on pure CeO2 with no catalyticactivity. What is more, the weakened formate features observed with anequimolar feed ratio do not reflect the higher CO2 conversions comparedto a stoichiometric ratio. Consequently, either the high stability of adsor-bed formates is detrimental to the catalytic efficiency, or other unobser-ved species contributing to the reaction impede the understanding of themechanism. It has been postulated that an amphoteric catalyst surfaceis required for a joint action of basic sites facilitating the adsorption andactivation of CO2 with acidic sites promoting the desorption of formedformates.186

5.4 Conclusions

In continuation of the promising progress with homogeneous catalystsand of the parallel efficiency of methanol synthesis, the development ofheterogeneous catalysts selective for formic acid and its derivative methylformate demonstrated the possibility of a one-step fixation of CO2 tomethyl formate using an immobilised Ru catalyst or metal supportedcatalysts under high pressure conditions.

The former catalyst, like its homogeneous counterpart, showed reasona-ble CO2 conversion promising high selectivity to formates in the absenceof stabilising base under flow conditions. The open carbon balance andthe traces of DME when combined with an acidic catalyst strongly poin-ted towards formic acid as main hydrogenation product in agreementwith its weak FID response factor. The severe lack of analytical meansfor the dilute gas-phase formic acid prevented a quantitative analysisof product yield and selectivity. Nevertheless, the immobilised molecularcatalyst proved high resistance under harsh flow conditions, without signof metal leaching or thermal degradation.

5.4 Conclusions 99

The second kind of catalyst showed higher conversions than the former inthe direct hydrogenation of CO2, but with little methyl formate selecti-vity due to the predominance of methanol and the by-production of COfrom the RWGS. Singled out in methanol co-feeding experiments for theirencouraging methyl formate yields at reaction conditions relevant to met-hanol synthesis (> 230 ◦C), SiO2 and CeO2 based catalysts were studiedin situ by Raman spectroscopy to establish some relationships betweensurface species and catalytic activity. Surface species derived from thereaction on the reduced catalysts included formates and methoxy, identi-fied on the basis of their characteristic Raman bands. The observation offormates on Cu/SiO2 confirmed the presence of formates on the Cu sites.The more Raman active ceria catalysts enabled the study of supportedmetal as well as operating conditions on surface species coverage. Con-trasting spectra obtained at different laser powers confirmed the greatstructural versatility of cerium oxide from its highly mobile lattice oxy-gen atoms, and suggested that thermally modified ceria sites adsorbeddifferent species. The instantaneous generation of formates upon contactwith CO2:H2 streams agreed with literature reporting the hyperactivityof cerium oxide with carbon oxides on partially reduced surfaces via resi-dual OH-groups. Extremely intense even at low temperatures, formatesstrongly adsorbed showing little dependence on temperature and decom-posing into carbonates above 300 ◦C, whereas an increase in pressure orH2 molar fraction resulted in increased formate bands. Operating condi-tions did not trigger changes in the methoxy bands, as much as the effectof supported metal, as witnessed by the stronger bands on Ag/CeO2.

Due to the lack of key information at high wavenumbers and the absenceof correlation between surface band intensity and product selectivity, thesequence of surface species generation remained unknown and the intri-cacy of the reaction network intact. On the one hand, the oxidation ofmethoxy by surface oxygen is reported to be responsible for the forma-tion of formates, but, on the other hand, the deprotonation and dehyd-roxylation of short-lived HCOOH can be accountable for the formationof formates as well. Although the Raman spectra could not distinguishbetween the two mechanistic scenarii, formate species were successfullyidentified as reaction intermediates in the reduction of CO2 with in situderived methanol.


Overall, the yields of formates achieved with both types of catalysts re-mained inconsequential either due to poor conversion or to poor selecti-vity. They nevertheless showed a net increase in comparison to the soleCu/ZnO/Al2O3 catalyst and selectivities higher than theoretically esti-mated. Hence, the study validated the potential of the two-step approachas a means to alleviate the thermodynamic constraints preventing for-mic acid synthesis and encourage further optimisation of catalyst designbased on the in situ characterisation of the working catalyst.

101

Chapter 6

Methyl formate hydrolysis byion-exchange resin

This chapter 1 dwells on the second step of the targeted continuous CO2

hydrogenation to formic acid process. The improved selectivity towardsformates testified by the supported metal oxide catalysts and by theimmobilised homogeneous catalyst in Chapter 5 provide a solid basisfor the subsequent hydrolysis of methyl formate to formic acid. On-going optimisation of these formate-selective catalysts involved in thefirst step prevented the determination of exact feed compositions for thesecond step. Consequently, a parametric study of the hydrolysis of met-hyl formate over an acidic resin catalyst was carried out to evaluate thesensitivity of the reaction performance on operating conditions and feedcomposition. The limited solubility of methyl formate in water forces thereactive system to partition into two liquid phases when the saturationconcentration is exceeded. The separate analysis of effluent liquid phaseswere thus used to establish a relationship between phase behaviour andkinetics, and unveiled a quasi-homogeneous catalytic process rooted inan aqueous layer at the resin surface, independent of the organic content.

1. Reprinted with permission from: Reymond H., Vitas S., Vernuccio S., Rudolfvon Rohr P., (2017). Reaction process of resin-catalyzed methyl formate hydrolysisin biphasic continuous flow. ChemSusChem, 56(6):1439–1449.

102 6. Methyl formate hydrolysis by ion-exchange resin

6.1 Introduction

The weakly endergonic nature of the equilibrium-limited methyl formatehydrolysis (∆G◦

r = 4.7 kJ mol−1) does not favour the spontaneous andfast synthesis of products. This feature, common to carboxylic ester hyd-rolysis, resulted in characterisation studies of similar liquid-phase rever-sible hydrolysis-esterification systems using various acidic catalytic me-dia.199–206 Process variations were reported to enhance reaction rate byrapid product complexation with an additive,207,208 or equilibrium con-version by furthering reaction through product separation in chroma-tographic reactors packed with ion-exchange resins.209–212 On a largerscale, patents described increased continuous formic acid synthesis usingester excess in a two-hydrolyser process,213 and shorter reaction timesby adding methanol to maintain a single reactive liquid phase to avoidmass transfer hindrance.214

While industrial scale hydrolysers have the drawback of prolonged resi-dence times, chromatographic reactors rely on a relative operational com-plexity and do not necessarily offer improved performance at high flow ra-tes for rapid synthesis.215 Moreover, the aforementioned studies on short-chain ester hydrolysis cover either a limited temperature or concentrationrange, ensuring a single homogeneous reactive phase. Although instru-mental for process intensification and on-demand production, where asustained source of reactant is primordial, reaction data in the two-phasedomain are sorely missing. To this end, this chapter reports continuousmethyl formate hydrolysis at concentrations beyond saturation. Impro-ved mass transfer between liquid phases was achieved exploiting the ad-vantageous surface-to-volume ratio of tubular millireactors. First, theresults of UNIFAC thermodynamic equilibrium calculations for methylformate hydrolysis are presented, modelled on the basis of the theoreti-cal background on phase and chemical equilibria introduced in Section2.4. A parametric study then investigates reaction performance over theion-exchange resin catalyst Amberlyst 15. Finally, the theoretical andexperimental knowledge gained on the reactive system are combined todevelop a kinetic model in continuous-flow operation.



6.2.1 Thermodynamic modelling

Thermodynamic equilibrium calculations based on Reaction R.6 (Chap-ter 2) were performed using the activity coefficient property methodUNIFAC216–218 implemented in the Aspen Plus simulation tool for itsability to simulate non-ideal mixtures at moderate pressures (< 10 bar)over a wide temperature range (17 ◦C to 147 ◦C). The calculations wereperformed by minimisation of the Gibbs free energy with respect to moleamounts at fixed reaction pressure and temperature (Equation 2.8). Theexistence of a vapour phase was discarded as experiments were carriedout under pressure and the gas volume caused by the evaporation of met-hyl formate was assumed negligible in comparison to the liquid volume.The activities of the components are defined in terms of the auxiliaryactivity coefficients γi related to operating conditions, where ζi standsfor any measure of concentration in the gas or liquid phase. We haveused component mole fractions xi.

ai = γiζi = γixi (6.1)

Activity coefficients in UNIFAC entail a combinatorial and a residualpart to account for the entropic contribution and the enthalpic interacti-ons.

lnγi = lnγCi + lnγR

i (6.2)

The group interaction parameters anm required for the estimation ofactivity coefficients were obtained from UNIFAC group specificationsdatabase (Appendix A, Tables A.6-A.7). The standard thermodynamicparameters for methyl formate hydrolysis used for the simulations werecalculated from the components’ formation enthalpies (Appendix A, Ta-bles A.1 and A.5).


6.2.2 Kinetic modelling

A general kinetic expression for the equilibrium-limited resin-catalysedhydrolysis was considered. Incidentally, the non-ideality of the liquidphase can be corrected for by replacing concentrations ci by the compo-nent activities ai.

203–205

r =1

νi

dnidt

= mcatk+

cAcB − cCcD/Keq

(1 +∑i kici)

n (6.3)

where mcat is the mass of catalyst, ki are the component adsorptionconstants and n is a parameter depending on the kinetic model. It definesthe catalytic mechanism following n = 0 for a quasi-homogeneous model(QH), n = 1 for a Eley-Rideal mechanism, and n = 2 for a Langmuir-Hinshelwood type mechanism.

The QH frame requires a complete swelling of the polymer matrix and anextensive dissociation of the sulfonic groups as in type A1 reactions. Forhydrolysis, such a model is safely assumed when the reaction is carriedout with an excess water to drive the reaction forward. In the case of ansignificant volumetric ester excess, the solvation capacity is greatly redu-ced and catalysis may occur from a combination of solvated protons anddirect interaction with the substrate as in type A2. As undissociated sul-fonic groups react differently, an adsorption-based heterogeneous (single-or dual-site mechanism, n = 1 or 2, respectively) may be considered intype A2.

A system of ordinary differential equations was constructed from Equa-tion 6.3 for each single species involved and numerically solved to calcu-late concentrations. The estimation of kinetic parameters was performedby minimising the sum of the squared residuals between experimentaland calculated concentrations through the objective function F (Equa-tion 6.4), while the prediction accuracy of the models was evaluated interms of the standard deviation σk (Equation 6.6).


F =

q∑k=1

n∑j=1

1

wj,k

m∑i=1

(yi,j,k − ci,j,k)2

(6.4)

The symbols yi,j.k and ci,j,k are respectively the experimental and calcu-lated concentrations, m is the number of experimental points recordedduring each run, n the number of detected species, and q the numberof experiments involved in the optimisation procedure. The weightingfactors wj,k are defined as

wj,k =1

m

m∑i=1

yi,j,k (6.5)

σk = 100×n∑j=1

1

wj,k

√∑mi=1 (yi,j,k − ci,j,k)

2

m

(6.6)

6.2.3 Catalyst and chemicals

Methyl formate (Sigma-Aldrich, 97%), formic acid (Sigma-Aldrich, 95%)and methanol (Sigma-Aldrich, 99.9%) were used as received from the ma-nufacturer without purification. The macroreticular resin Amberlyst 15in hydrogen form (Sigma-Aldrich) was used as catalyst. The physico-chemical properties of the reactive compounds are listed in Appendix A(Table A.8).

6.2.4 Swelling and sorption experiments

Swelling and phase equilibrium partitioning experiments were carriedout by contacting a known amount of dried resin Amberlyst 15 witheither single chemical species or non-reactive binary mixtures and left to


equilibrate in a thermostated incubator (Minitron INFORS HT) at 23 ◦C500 rpm. Precisely 1 mL of dried resin was placed in a 5±0.08 mL sealedgraduated cylinder with pure solvent (∼4 mL). The equilibrated volumeof the swollen resin was measured by eye after 24 h. Binary mixtures(∼40 mL) were prepared in 50-mL glass flasks containing a fixed mass ofdried catalyst (3± 0.01 g). Liquid samples were collected after 48 h forgas chromatography analysis.

6.2.5 Hydrolysis of methyl formate

The experimental setup for continuous high-pressure operation is shownin Figure 6.1. Two water-cooled (Huber CC-308) syringe pumps (Te-ledyne ISCO, 260D) allowed the individual adjustment of reactant flowrates for the delivery of distilled water and methyl formate. The reactantswere contacted in a T-junction (stainless steel, �inner = 1 mm) beforeflowing through a fixed-bed reactor (stainless steel, �inner = 1.52 mm.The system pressure was controlled by an automated needle valve back-pressure regulator (Jasco, BP-2080) while temperature was adjusted byfitting the reactor between two copper bodies heated by a PID controller(Eurotherm 2132) connected to a K-type thermocouple and two resistivecartridges (Wisag, 200 W, 230 V). Pressure at the packed-bed inlet andoutlet were monitored (Endress and Hauser) online via LabView soft-ware.219 The system can be operated in continuous mode up to 150 barfrom 25 ◦C to 350 ◦C. The catalyst beads were crushed and sieved, thenseparated into fractions of 71µm to 90µm, 90µm to 125µm and 450 µmto 500µm. A precise mass of fresh catalyst (75 mg of dried catalyst cor-responding to 5 cm packed-bed were used for base operating conditions)was loaded in the reactor without preliminary purification. The pressuredrop across the fixed-bed, depending on reaction temperature, did notexceed 10% for isobaric conditions. The system was allowed time to re-ach swelling equilibrium before heating to reaction temperature. At eachtemperature, the system achieved chemical and physical steady-statewithin 15 min on stream at a total liquid flow rate of 1 mL min−1 andsamples were collected for analysis. Reaction quenching was achieved byrapid cooling of the reactor outlet tubing with ice.


4

3

61

PI

Organic

Aqueous

PI

PIFIC

FIC

2

5

Methyl formate

Water

Figure 6.1 – Schematic representation of the hydrolysis setup. (1,2) Water-cooledhigh-pressure syringe pumps. (3) Packed-bed reactor. (4) Reactor bypass for blankmeasurements. (5) Back pressure regulator. (6) Sample vial.

6.2.6 Analytical method

At reaction steady-state, 5 mL effluent samples were collected in a gra-duated cylinder. The respective volumes of the organic and the aqueousphase in the sample were measured visually before separation by de-cantation. For biphasic effluents, the two phases were analysed off-lineseparately with a gas chromatograph (Bruker GC-450) equipped witha VF-WAXms separation column (25 m X 0.25 mm, coating thickness= 0.25 µm) and a flame ionisation detector (FID). The injection portand FID temperatures were 250 ◦C and 220 ◦C, respectively. The oventemperature was increased from 45 to 80 ◦C at a rate of 10 ◦C min−1

and further to 170 ◦C at 25 ◦C min−1 to remove trace of higher-boilingcompounds. A 2 vol.% solution of 1-hexanol (Sigma-Aldrich, ≥ 99.9 %)was used as internal calibration standard for the analysis of both phasesfrom which the respective amounts of methanol and methyl formate werecalculated according to their relative response factors. Formic acid wastraced qualitatively in the GC chromatograms. An analysis cycle lasted13 min in total which allowed, during the acquisition of the organic phase,the preparation of a second sample for the direct analysis of a freshlyacquired aqueous phase minimising artefacts from re-esterification. ThepH of both effluent phases were measured (Hamilton Polilyte Plus H S8120). The analysis of homogeneous effluents followed the same analyticalprocedures for the single phase.



6.3.1 Methyl formate hydrolysis phase equilibrium

Liquid-liquid equilibrium

Figure 6.2 presents the phase diagrams for the ternary system methanol-methyl formate-water at 10 bar and 20 ◦C, 60 ◦C and 110 ◦C. As expectedfor condensed phases, pressure does not affect the curved area delimitingthe immiscibility gap at low methanol fraction, while temperature greatlyimpacts the mutual miscibilities.

W A T ER0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95

0.05

0.1

.15

0.95

0.9

0.8

Water 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10

W A T ER0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95

0.05

0.1

.15

0.95

0.9

0.8

Water 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10

W A T ER0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95

0.05

0.1

.15

0.95

0.9

0.8

Water 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 W A T ER

MF

MEOH

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

0.95

0.9

0.85

0.8

0.75

0.7

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

Water 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10

(b) 20°C

(d) 110°C

(c) 60°C

(a) 10 bar, 20°C

Figure 6.2 – (a) Ternary map (mole basis) of the system methanol-methyl formate-water at 10 bar and 20 ◦C. Close up of immiscibility gap at 10 bar and (b) 20 ◦C,(c) 60 ◦C (d) 110 ◦C. Red markers represent the molar feed compositions used in theparametric study. The tie-lines connect the L-L compositions at equilibrium for eachfeed composition.

The additional diffusive resistance implied by the permeable liquid-liquid(L-L) interface was alleviated by the use of a milliscale reactor with highsurface-to-volume ratio, achieving improved mass transfer and advanta-geous performance compared to its macroscopic counterparts. In suchreactors where surface forces prevail, various flow patterns may be ob-served depending on the volume ratio of both phases, their superficial


velocities, and intrinsic physical properties.220 The partial solubility ofmethyl formate in water (Appendix A, Table A.8) induces a very lowinterfacial tension, thereby increasing the likelihood of parallel or an-nular flow. As such flow patterns expose only limited surface area forthe solubilisation of methyl formate, two modifications of the setup weretested in order to ensure that water reached saturation before enteringthe catalyst packed bed. Modifications improved diffusion via increa-sed recirculation (i) by breaking up the flow pattern by cofeeding aninert gas phase through a section of smaller diameter (Modification 1:�inner = 0.5 mm, Lcoiled = 50 mm + N2), and (ii) by thorough mix-ing over an inert silicon particle packed-bed (Modification 2: Si-beads70 µm to 110µm). According to Figure 6.2(b), a feed of molar composi-tion H2O:MF:MeOH = 77:22:1 splits into aqueous and organic phases ofmethyl formate content 8 mol% and 80 mol%, respectively. Although slig-htly lower than calculated as saturation reference, both phases containidentical methyl formate and methanol contents whether the standardsetup from Figure 6.1 (�inner=1 mm, L=20 mm) is used or any of theabove-mentioned modifications. The theoretical and experimental satu-ration concentrations are tabulated in Table 6.1.

Table 6.1 – Methyl formate saturation concentrations in [mol mL−1] in aqueous andorganic phases for a molar water:ester feed ratio of 3.6 at 23 ◦C and 1 bar.

Feed composition Aqueous Organic

Rn = 3.6 cMF cMeOH cMF cMeOH

Saturation 0.0050 miscible 0.0152 miscible

Standard setup 0.0038 0.0006 0.0148 0.0003

Modification 1 0.0042 0.0006 0.0145 0.0003

Modification 2 0.0038 0.0006 0.0140 0.0003

Solvent-polymer equilibrium

Non-reactive experiments were carried out contacting the resin with thereaction species to evaluate the volume increase by swelling, and thepartitioning ratio of each compound between bulk and polymer phase.The strong affinity of water toward this resin was evidenced for a si-


milar chemical system in the frame of liquid-phase esterification.209–211

Preferential sorption of water and methanol was reported compared tothe markedly lower resin affinity for the less polar carboxylic acids andtheir esters. Although different chemical species are involved herein, thefindings can be generalised to methyl formate hydrolysis and the follo-wing decreasing order of affinity concluded: water > methanol > formicacid > methyl formate. This hypothesis is consistent with the degree ofswelling measured in pure solvents reported in Table 6.2. Binary sorp-tion experiments remained inconclusive due to negligible concentrationdifferences.

Table 6.2 – Experimental swelling ratios in pure solvent at 23 ◦C.

Chemical species Swelling RatioWater 1.583Methanol 1.516Formic acid 1.455Methyl formate 1.417

The propensity of the components to sorb in the reaction locus controlsthe reaction yield and selectivity as the polymeric structure selectivelyremoves the components of weakest affinity into the bulk phase. Undercontinuous flow conditions, the strong selectivity of the resin with respectto the two educts implies a preferential water uptake at the reactionlocus susceptible of driving the chemical equilibrium more in favour ofthe products.

6.3.2 Parametric study

A parametric study was carried out for the hydrolysis of methyl formatecatalysed by the resin Amberlyst 15. The effects of operating conditionssuch as temperature, pressure, feed composition, and weight hourly spacevelocity (WHSV) on reaction performance are reported in the followingsections. The reliability of the results was ascertained by repeating aminimum of three runs of representative sets of experiments in order toensure the stability of the experimental method. Methanol content ser-


ved as basis for the calculation of methyl formate conversion. Materialloss by flash evaporation was found negligible from the constant liquidflow rates obtained under all conditions of pressure and temperature, andfrom the closed carbon balance. No side-reactions were observed in thetemperature range investigated, as secondary decomposition of formicacid to methanol and carbon monoxide, as well as catalyst degradation,are prone to happen only at temperatures above 120 ◦C. Although efflu-ent compositions were proved stable over several hours, re-esterificationartefacts were prevented by direct analysis after sampling. Reaction con-version and yield translate the overall reaction performance summing upconcentrations weighted by the volume fractions of each individual liquidphase.

Effects of weight hourly space velocity and catalyst particlesize

The different mass transfer resistances in the packed bed were proofedto ensure kinetic control of the process in the following reaction data.The negligible diffusion barrier at the L-L interface was previously shownfrom the fast solubilisation rate of methyl formate even in presence oflaminar parallel flows on the 20 mm distance separating the T-junctionfrom the packed bed entrance. The observation was confirmed by per-forming blank tests over a packed-bed of inert silicon beads (70µm to110 µm) at 110 ◦C. The increased turbulence and solubility cause littleevolution in the aqueous and organic exiting stream compositions. Thelow conversion (< 1 %) proves not only the fast diffusion rates, but alsothe necessity of an external catalyst.

Film diffusion limitation at the catalyst surface was minimised by highspace velocities forcing intensive mixing in the packed-bed.127 The influ-ence of residence time on reaction performance was studied by doublingthe catalyst loading and/or changing the total feed flow rate to achieveWHSV ranging from 380 h−1 to 760 h−1. Results in Figure 6.3(a) show in-creasing yields reaching faster equilibrium at longer contact times and/orlower space velocities due to the larger amount of acid sites available forreaction. However, a 2-fold increase in superficial velocity and catalyst


(a) (b)

Figure 6.3 – Effects of (a) WHSV and (b) catalyst particle size on reaction per-formance.

loading at a fixed WHSV of 760 h−1 reveals identical net methanol yieldsestablishing the absence of external mass transfer hindrance. WHSV of760 h−1 at 1 mL min−1 was hence used for the subsequent parameterstudies. Internal diffusion limitation was limited using catalyst particlesof 90 µm to 125 µm, in light of the lower conversions presented in Fi-gure 6.3(b) with markedly larger particles of 450 µm to 500 µm. Smallerparticles than the former however reach same conversions at the cost ofa higher pressure drop. The effect of catalyst swelling on intraparticleresistance was disregarded from the constant swelling extent observedirrespective of the reactant feed composition. Despite the 12 % lowerswelling ability in pure methyl formate than in water reducing accessibi-lity to the catalytically active sites (Table 6.2), the water fraction sufficesto improve swelling under reaction conditions and to dismiss pore diffu-sion hindrance in the case of reactions with methyl formate as continuouscarrier phase (i.e. in volumetric excess).


Effects of temperature and pressure

As pressure is changed from 10 bar to 20 bar, no difference is observed inreaction performance due to minimal liquid density change. A pressureof 10 bar was selected for all further experiments to avoid methyl formatevaporisation, while remaining within the validity frame of the UNIFACproperty method. The temperature study spanned 20 ◦C to 120 ◦C, wherethe maximum temperature was set to prevent the gradual desulfonisa-tion of the resin and investigate the impact of vapourisation on methylformate on reaction performance. Methyl formate conversions and met-hanol yields for the base conditions (10 bar, WHSV = 760 h−1, dp =90 µm to 125µm) are presented in Figure 6.4 together with the theoreti-cal equilibrium conversions for various feed compositions. Feed compo-sitions are described as the water:ester ratio in mole basis Rn = nW/nE

and corresponding volume basis Rv = vW/vE. Table 6.3 reports effluentcompositions, expressed as volume fraction of aqueous phase collected ateach reaction temperature for various feed compositions.

The vertical lines in Figure 6.4 mark the transition from a two liquid-phase system observed until 50 ◦C into a single homogeneous phase from60 ◦C to 120 ◦C. The increasing conversions with temperature result inthe gradual depletion of methyl formate, and increase in methanol andformic acid concentrations driving the system out of the immiscibilityzone. The onset temperature of the single-phase regime does not shiftsignificantly within the feed compositions studied as visible from Table6.3. This effect is attributed to the small variation in phase-equilibriumconcentrations dictated by the binodal curve and tie-lines of Figure 6.2,despite a stark difference in initial feed compositions. The strong ef-fect of temperature on the extent of reaction associated with the steadyevolution at the two- and single-phase regime transition infers an iden-tical reaction process and confirms again the absence of mass transferlimitation. The conversions at 50 ◦C and 60 ◦C, extrapolated to ternarymixtures water-methanol-methyl formate, lay on each side of the bi-nodal curve connecting the phases in equilibrium at room temperature(Figure 6.2(b)). A gradual shift from kinetic to thermodynamic regimetakes place as temperature approaches 100 ◦C for all feed compositions;

Tab

le6.3

–E

fflu

ent

liqu

id-liq

uid

volu

metric

com

positio

nat

basic

cond

ition

s.

FE

ED

EF

FL

UE

NT

Wate

rexcess

Aqu

eou

sp

hase

volu

me

fractio

n[%

]

Rn

Rv

23◦C

30◦C

40◦C

50◦C

60◦C

70◦C

80◦C

90◦C

100◦C

110◦C

120◦C

14.2

4.0

5.3

1.5

780

80

84

88

HO

MO

GE

NE

OU

S

3.6

1.0

68

68

74

92

AQ

UE

OU

S

2.7

0.7

558

58

61

74

1.8

0.5

246

46

46

46

0.9

0.2

520

20

12

HO

MO

GE

NE

OU

SO

RG

AN

IC


(a)Water molar excess

14.2

5.3

3.6

2.7

1.8

0.9

(b)

Figure 6.4 – Reaction performance as a function of temperature and molar feedcomposition Rn. (a) Equilibrated theoretical (solid lines) and experimental (dashedlines) methyl formate conversions, (b) experimental methanol yields. The vertical lineat 55 ◦C represents the transition from dual- to single phase effluent. Colours standfor different feed compositions quantified in terms of molar water excess reportedabove the corresponding equilibrium conversion line.

only the largest educt excess reaches equilibrium sooner, at 90 ◦C. pHanalysis of the organic and aqueous phases confirm equilibrium statusas their pH-values decrease monotonically until stabilising at the sametemperatures. Figure 6.5 traces the general pH evolution in the aqueousphase, although both pHs generally converge toward the same value ifnot coinciding. As expected from theoretical calculations (solid lines inFigure 6.4(a)), equilibrium compositions vary only slightly with tempe-rature due to the small equilibrium constant combined with the weaklyendothermic reaction enthalpy ∆H◦

r = 8.1 kJ mol−1 (Equation 2.17).The slight drop in activity at 120 ◦C is imputed to gaseous methyl for-mate from vapourisation at 111 ◦C causing slower solubilisation in water(Appendix A, Table A.8); such a decline is indeed not observed at 20 bar.


Figure 6.5 – pH of aqueous phase for the characteristic feed compositions Rn =1.8 (blue), 2.7 (green), and 3.6 (yellow). Same colour scheme as in Figure 6.4. Bothphases pH generally converged toward the same value if not identical.

Effect of initial feed composition

Running the hydrolysis reaction in a plug flow mode does not exploit theseparator properties of the resin to react beyond the equilibrium by se-gregating products and prevent re-esterification. Nonetheless, a reactantexcess displaces the equilibrium toward product formation as predictedby Le Chatelier’s principle. The effect of feed composition on reactionequilibrium was investigated by varying the Rn ratio from 0.9 to 14.2.The solubility limit of methyl formate in water at 20 ◦C would corre-spond to a molar ratio of 11; thus the largest ratio investigated in thiswork, 14.2, speaks for a single-aqueous phase reaction. The lowest valueof Rn = 0.9 represents a two-phase reaction regime where the orga-nic fluid is in molar excess in contrary to all other ratios. The initialmolar ratio compositions are represented by red dots in the ternary di-agrams of Figure 6.2. As depicted on Figure 6.4(a), except for the factthat a larger excess of one of the educts increases the reaction rate, allconversion curves present the same evolution trend, whether homogene-ous or biphasic, pointing toward an identical reaction process. Methylformate equilibrium conversions increase with the magnitude of water


excess from 26 % at Rn = 0.9 to 81 % at Rn = 14.2. The theoreticalequilibrium conversions show a satisfactory fit (< 5 % discrepancy) tothe experimental equilibrium values. Figure 6.4(b) presents the net met-hanol yields, i.e. methanol in the effluent from which the initial methanolcontent in the feed was subtracted. In accordance with theoretical prog-nostics, the optimal experimental compromise between reactant excessand limiting reactant conversion is obtained at molar ratios of Rn = 2.7-3.6 with a methanol yield peaking at 57 mmol min−1 g−1. In comparison,the homogeneous system with Rn = 14.2 reaches 36 mmol min−1 g−1 atits maximum at 100 ◦C.

6.3.3 Reaction locus

Figure 6.6 traces the biphasic effluent composition as a function of tem-perature and feed composition. The separate analyses of methyl formate(Figure 6.6(a)) and methanol (Figure 6.6(b)) concentrations in the or-ganic and aqueous phases serve to highlight the evolution of the simul-taneously occurring phase and chemical equilibria.

(a) (b)

Figure 6.6 – (a) Methyl formate and (b) methanol concentrations in aqueous (solidlines) and organic (dashed lines) phases. Same colour scheme as in Figure 6.4.


Concentrations at 20 ◦C match the simulated equilibrium compositionsprojected from the tie-lines (Figure 6.2); where formic acid concentra-tion is omitted because of the little conversions at low temperatures. Theweakly increasing initial aqueous methyl formate concentration with de-creasing water excess in feed confirms the binodal curve predicting si-milar equilibrium compositions for all feed ratios. Improved solubilitywith temperature causes the aqueous methyl formate concentration toinitially increase to a maximum: point at which the biphasic regimedisappears and the totality of methyl formate is solubilised. After theregime transition, the concentration decreases steadily until equilibrium,in consistency with the shift from kinetic to thermodynamic control.The homogeneous hydrolysis at Rn = 14.2 shows in contrast a monoto-nically decreasing methyl formate concentration. The inverted sequenceobtained at 60 ◦C compared to the one obtained at room temperaturereflects the different extents of reaction and therefore the maximum es-ter concentration in water is reached with Rn = 1.8. Methyl formateconcentration in the organic phase evolves in a complementary way asthe initial fraction drops and the other species’ fractions increase. Theconstant increase of methanol concentration translates the progressingreaction. As long as two phases coexist, methanol concentrations aresimilar in both phases which correlates the simultaneous pH drop mea-sured in the aqueous phase as well as in the organic phase, sign of anidentical behaviour for formic acid concentrations.

In principle hydrolysis can happen in any phase as long as reactantsare susceptible to contact the catalytic medium. The in situ-generatedformic acid acts as homogeneous autocatalyst in both phases after aninduction period. However, due to the resin’s stronger acidity, heteroge-neous catalysis certainly dominates the process. Furthermore, conside-ring its strong selectivity toward water, the latter is expected to forma stagnant film around the resin particles, modifying the distribution ofmethyl formate in favour of the bulk solution. Reaction might thus takeplace in both phases by homogeneous autocatalysis, but primarily in theaqueous phase via general liquid-solid reaction carried out on a porouscatalyst. The increasing product concentrations in the organic phases aremerely a consequence of the intense mixing and fast diffusion throughthe L-L interface equilibrating chemical potentials.


6.3.4 Kinetic modelling

Because of the high polarity of the reaction medium, complete swellingand protonation of the polymeric catalyst motivates a QH kinetic mo-del, as successfully applied to similar reactive systems with Amberlyst15.204–206 Kinetic data between 20 ◦C and 40 ◦C at Rn = 3.6 were accor-dingly correlated with two different ideal QH models: a pseudo-first-orderreaction model and an equilibrium-based model. Both models correspondto Equation 6.3 with n = 0, with the former model being a particularcase of the latter. The pseudo-first-order assumption considers the con-centration of water constant at the reaction locus and re-esterificationas negligible at limited conversions. The reagent ratio Rn = 3.6 was se-lected for its high yields with a substantial ester volume fraction. Figure6.7(a) highlights the calculated and experimental concentration profilesof formic acid at various temperatures in the two-liquid regime, and Fi-gure 6.7(b) groups all species at 40 ◦C assuming ideal pseudo-first-orderreaction. The adjustable parameters k+

0 and Ea resulting from the fittingprocedures are grouped in Table 6.4, along with their 95 % confidenceintervals, the average standard deviations σ, and the minimised value ofthe objective function F .

From their similarly small F -values, both models yield equal predictionaccuracy, confirming the easy access of reactants to the mobile activesites without strong interaction between components and the solid ca-talyst. The activation energy values obtained for both models are veryclose, Ea = 65 kJ mol−1, and in the same magnitude range as reportedin the literature for similar Amberlyst 15-catalysed ester hydrolysis.203

The high activation energy confirms the intrinsic kinetic control of theprocess even in the biphasic regime. Also, little deviation is observedin the Arrhenius plots for all feed compositions confirming an identi-cal reaction process irrespective of the nature of the continuous phase.Although a shift from A1 to A2 type could have been envisaged at Rn= 0.9 as defined previously, QH catalytic species remain dominant viaaccumulated water forming a hydration shell around the polymer phasethrough which the organic carrier diffuses at steady-state under flow con-ditions. The fact that even the simple pseudo-first order assumption for


(a) 40°C

30°C

21°C

(b)Water

Methanol

Methyl formate

Formic acid

Figure 6.7 – Experimental (markers) and pseudo-first order calculated (lines) con-centrations of (a) formic acid at 21 ◦C (circles), 30 ◦C (squares) and 40 ◦C (diamonds)and (b) all reactive species at 40 ◦C.

a plug flow reactor reveals good agreement with the experimental data(mean value of σ <5 %) supports this hypothesis as well as the absenceof side reactions and product inhibition. Although theoretically equal atlimited conversions, the equilibrium-based model is consistent at conver-sions approaching equilibrium as it considers the backward reaction asdescribed in Equation 6.3. The use of activities to account for the liquid-phase non-ideality does not reveal substantially higher accuracy than itssimpler ideal pseudo-first-order counterpart. As opposed to conventionalheterogeneous catalysis, unaffected by swelling and solvent selectivity,most sophisticated modelling would be based on polymer-phase activi-ties rather than bulk effluent concentrations. However, such an approachwas judged irrelevant in this kinetic study because of the imperceptibleconcentration changes of the involved species revealed in binary sorptionexperiments, obliterating the need for complex partitioning constants.The fast solubilisation of methyl formate is concluded more critical inthe continuous flow process than the negligible extent to which equili-brium distribution might displace the organic reactant in favour of thebulk phase, detrimentally affecting the water-enriched polymer phase.

6.4 Conclusions 121

Table 6.4 – Kinetic parameters for the two ideal quasi-homogeneous models.

Kinetic model

Pseudo 1st order QH equilibrium

Ea (65.0± 3.2) kJ/mol (65.8± 3.3) kJ/mol

k+0 (1.16± 0.06)× 1010/(g s) (5.50± 0.28)× 1011 mL/(mol g s)

F < 1.5× 10−5 1.7× 10−5

σ 4.6 % 4.9 %

6.4 Conclusions

The continuous hydrolysis of methyl formate in the heterogeneous liquid-liquid regime was successfully achieved using the cation-exchange resinAmberlyst 15 as catalyst. A wide range of reaction conditions was sy-stematically investigated with the aim to understand the relationshipgoverning the simultaneously occurring physical and chemical equilibriaand their impact on the reaction process. The milli-scale reactor ensu-red high mass transfer in the two-liquid phase regime which was provedto dominate over an extended temperature range at the 10 bar pressureused to overcome the high-volatility of methyl formate.

The fast solubilisation of methyl formate in water unlocked feed compo-sitions in the L-L regime for high product yields. With 57 mmol/(min g)yield, optimum water:ester molar ratios between 2.7 and 3.6 led to a 1.6-fold increase in methanol yields compared to the homogeneous systemwith Rn = 14.2. The calculated thermodynamic equilibrium values wereconsistent with the experimental phase and reaction data, validatingUNIFAC as a competent model for the description of such multiphasereactive systems.

The resin Amberlyst 15 was ideally suited for this modelling study, ca-pable of catalysing the reaction in the organic phase as well as in theaqueous phase. Nevertheless, water was found to be strongly adsorbed inthe resin, ensuring complete swelling of the polymer matrix even underlarge methyl formate excess. The presence of a stagnant water film un-der flow conditions prompted a simple quasi-homogeneous kinetic model.


Expressed in terms of concentrations, the latter model was able to accu-rately describe the experimental data, thereby confirming the nature ofthe reaction locus and obviating the need for more complex adsorption-based models. The high value of the activation energy confirmed thatthe reaction in the catalyst pores was rate-controlling independent ofthe number of liquid phases and feed molar composition.

In the frame of the continuous two-step hydrogenation of CO2 to for-mic acid, substantial conversion without mass transfer impediment isexpected also in the gas phase irrespective of the Rn value resultingfrom the first step of the process. The minor fraction of methanol in theinlet feed was shown in this chapter not to impact the reaction outcome,however, the effect it may have on the reaction equilibrium in case ofa larger content will have to be considered. Such a situation is likelyto occur in case of imbalanced activities between methanol and methylformate synthesis.

123

Chapter 7

Formic acid synthesis viamethyl formate intermediary

This chapter embodies a feasibility study of the continuous synthesis offormic acid from CO2/H2 feeds via the intermediate synthesis of met-hyl formate. The previous chapters laid the foundations for the two-stepprocess by studying separately each reaction involved. The first stepof the process exploits a two-reactor configuration where the optimalconditions for methanol synthesis from Cu/ZnO/Al2O3 (Chapter 4) arecombined to the knowledge gained in the methyl formate synthesis overAg-based catalysts (Chapter 5) to maximise methyl formate yields. Thesecond step of the process uses the information collected on methyl for-mate hydrolysis (Chapter 6) over the acidic resin Amberlyst 15 with theaim to generate formic acid. To combine the reactions into a coherentwhole, preliminary trials investigated a set of configurational possibilitiesvarying the operating conditions of the acidic catalyst. A three-reactorsystem secluding the catalysts from each other achieved record methylformate yields, while an admixture of methyl formate synthesis and hyd-rolysis catalysts showed firm evidence of greater formic acid formation.Despite the difficulty in analysing the product, the study highlighted theimportance of acting on the sensitive equilibrium nature of the intricatereaction network to tune the process efficiency through ripple effect.

124 7. Formic acid synthesis via methyl formate intermediary

7.1 Introduction

Exemplified in the reaction network in Chapter 2, the success of the mul-tistep reactive route investigated is rooted in the control of the numerousequilibria composing it. Although at the heart of the problematic, the pr-opensity of formic acid towards decomposition was not represented in thenetwork for simplicity. For a trustworthy representation of the full pro-cess, formic acid dehydrogenation/decarboxylation (Reaction R.9) anddehydration/decarbonylation (Reaction R.10) reactions thus need to beaccounted for.

Table 7.1 – Formic acid decomposition

∆H◦r(g)

∆G◦r(g)

Reaction kJ/mol kJ/mol

(R.9) HCOOH CO2 + H2 −15.5 −43.4

(R.10) HCOOH CO + H2O +25.7 −14.8

For reversible hydrogen storage, it is crucial to manage formic acid synt-hesis and decomposition in a consistent and timely manner. Decomposi-tion follows either pathway in Table 7.1 depending on temperature, for-mic acid concentration and catalytic surface: dehydration is thermallydriven, whereas the desired dehydrogenation is an actively researchedcatalytic process.221–224 The reverse process, that is the synthesis of for-mic acid from CO2/H2, is generally controlled using a stabilising agentin batch operation (Section 5.1).

In absence of a stabilising agent in flow operation, the inclusion of anot-her catalyst to transform formic acid into a transient stable compoundgives the possibility to favourably disturb the overall chemical equili-brium, by reacting it in a subsequent step back into formic acid along adistinct reactive route. The extent to which the balance between chemi-cal equilibria can be shifted towards formic acid synthesis then directlyensues from the catalysts selected and the operating conditions. Since thereaction equilibrium of the second step depends on the stream composi-tion exiting the first one, the predominance of one or another compound


engenders downstream a cascade of counteracting responses responsiblefor the whole process efficiency. In the present chapter, methyl formateassumes the pivotal role as intermediate promoting either hydrolysis or ofre-esterification in function of its concentration relative to those of waterand methanol. A molar excess of methyl formate or water would hamperesterification, shifting the equilibrium in favour of formic acid synthesis,whereas an excess of methanol would induce the opposite effect as soonas a little formic acid is formed.

The remainder of this chapter evaluates the potential of the two-stepprocess to circumvent the thermodynamic constraints in a series of con-figurational variations of a three consecutive reactor system. As currentmost active catalyst for methyl formate synthesis, 1 wt.% Ag/Al2O3 wasused in combination to the commercial Cu/ZnO/Al2O3 and cation ex-change resin catalysts. In total, seven configurations are presented va-rying the vicinity of the methyl formate synthesis to hydrolysis catalystand the hydrolysis conditions. Explanations for the diverging efficienciesare advanced before closing the chapter with a brief statement on thepurification of the effluent and on the economic feasibility.


7.2.1 Catalysts

The commercial catalysts Cu/ZnO/Al2O3 (dp = 45-63 µm) and Amber-lyst 15 (dp = 90-125 µm) were used in combination with 1 wt.% Ag/Al2O3

(dp = 100-300µm).

7.2.2 Continuous multistage reduction of CO2

The high-pressure CO2 hydrogenation plant in Chapter 3 was adapted tohouse three consecutive reactors as schematically represented in Figure7.1.


PIH2

CO2

GC

R1 R2 R3

Step 1 Step 2

Figure 7.1 – Partial scheme of the multistage setup with the three reactors R1,R2, and R3 connected in series with independent heating systems. The back pressureregulator was implemented either between reactors R2 and R3, or after R3 before theonline GC (configuration shown here).

The temperature of each reactor was controlled independently by meansof three resistive heatings, while the pressure was controlled by meansof the back pressure regulator placed after either reactor R2 or reactorR3 depending on the operating pressure of the third reactor.

The Cu/ZnO/Al2O3 and the 1 wt.% Ag/Al2O3 catalysts were activatedfollowing the procedures described in chapters 4 and 5, respectively. Thepressurisation of the system and the setting of the reagent flows followedthe steps listed in the same two chapters. The total flow rate was fixedat 17.5 mLN/min, and details on the catalyst loadings, GHSVs, and re-action pressure and temperature are tabulated for each configuration inthe following feasibility study.


7.3.1 Synthetic feasibility

The first approach to the two-step process consisted in maximising themethyl formate yields at the outlet of the first process step, entering thesecond step. Chapter 5 attested the need for two consecutive reactors toachieve higher methyl formate productivity from CO2-derived methanol


excess. Thus the synthetic path introduced in Chapter 1 changed wherethe first step of the continuous process henceforth refers to the combina-tion of two sequential reactors respectively packed with catalysts 1 and2 as represented in Figure 7.2.

CO2 + H2 HCOO(H) CH3OH* HCOOCH3*

Catalyst 1 Catalyst 2 Catalyst 3

REACTION STEP 1

INTERMEDIATE

HCOOH

CH3OH

REACTION STEP 2


Figure 7.2 – Adapted scheme of the two-step synthetic approach for the continuousconversion of CO2 and H2 into formic acid and methanol via formate intermediate.

Table 7.2 presents the setup configurations tested and the operatingconditions of individual reactors together with the overall performanceestimated on the basis of the components detected by GC. The outcomeof the various configurations are described and compared to a reference2-reactor system with the methanol and formate synthesis catalysts ope-rated at standard conditions.

The standard operating conditions represented in the reference case usethe methanol synthesis catalyst packed in reactor R1, followed by the1 wt.% Ag/Al2O3 in reactor R2 and eventually reactor R3 loaded withthe acidic resin Amberlyst 15. Section 4.3.1 implied that the opera-tion of reactor R1 with a stoichiometric molar feed ratio at 300 barand 260 ◦C guarantees fair methanol yields from reasonable CO2 con-version (> 55 %) and methanol selectivity (∼ 29 %)(Section 4.3.1). Theco-production of CO is noticed not to distinctly impact the operation ofthe downstream reactors as not directly involved with the intermediatemethyl formate, and acting against the thermally driven dehydrationof formic acid. Subsequently, applying the findings of the parametricstudy in Section 5.3.1, the excess methanol at the outlet of R1 promo-tes the synthesis of methyl formate through transesterification betweenadsorbed methoxy and formates on acid sites of the Ag-based catalyst


in reactor R2 operated at 140 ◦C. The synthesis of methyl formate frommethanol dehydrogenation is judged unlikely due to the abundant H2 inthe stream. A greater catalyst loading prolongs contact time with thecatalyst to increase as much as possible methyl formate yields as in-termediary. In turn, operating reactor R3 under pressure at its highestallowed temperature of 120 ◦C offers maximum conversion regardless ofthe upstream composition.

C1 The acid catalyst reactor R3 at 300 bar leads to high CO2 conversion and

selectivities to methanol and methyl formate. Traces of DME appear from the

dehydration of methanol catalysed by the acid catalyst according to Reaction

R.8 (Chapter 5).

C2 An increase in temperature to 160 ◦C in R2 results in lower CO2 conversion

and favoured methanol synthesis over methyl formate, although the selecti-

vity of the latter remains above the reference value. This outcome can be

the result of esterification and consecutive formic acid dehydrogenation cau-

sing the co-production of methanol and simultaneous consumption of methyl

formate.

C3 Operating reactor R3 at atmospheric pressure achieves the lowest net CO2

conversion by promoting the dehydrogenation of in situ generated formic acid,

and the simultaneous consumption of methanol via dehydration to DME.

C4 Shortening the contact time in R3 limits potential re-esterification from the

lower methyl formate selectivity and CO2 conversion arising from formic acid

decomposition.

C5 Equimolar feed ratio promotes methyl formate selectivity to an unpreceden-

ted 4.22 % alongside methanol, although at the cost of lower CO2 conversion.

C6 Physically mixing 1 wt.% Ag/Al2O3 catalyst to the resin results in similar

CO2 conversions at 120 ◦C, but increases CO and DME selectivities, whereas

methyl formate selectivity reaches its lowest value. The proximity of the acid

sites on the resin might then either enable shorter diffusion paths for the fast

hydrolysis of methyl formate and dehydration of methanol, or else almost

completely suppress methyl formate synthesis similarly to the effect of acid

promoted supported catalysts.183,184

C7 The stable activity upon increasing the admixture temperature proves the

resistance of Amberlyst 15 at 140 ◦C, and shows favourable methyl formate

and DME synthesis over CO. The lower CO2 conversion despite the higher

temperature than in C6 hints at a decomposition of formic acid.

Tab

le7.2

–R

eact

ion

per

form

an

ceof

thre

ere

act

or

con

figu

rati

on

ste

sted

for

the

conti

nu

ou

ssy

nth

esis

of

form

icaci

d.

The

two

react

or

syst

emop

tim

ised

for

met

hyl

form

ate

synth

esis

(Chap

ter

5)

isp

rese

nte

das

refe

ren

cefo

rco

mp

ari

son

.O

per

ati

ng

con

dit

ion

sare

spec

ified

for

each

con

figu

rati

on

inte

rms

of

vari

ati

on

sfr

om

stan

dard

cond

itio

ns

intr

od

uce

din

con

figura

tion

C1.

Confi

gurati

on

an

dop

erati

ng

con

dit

ions

Conv.%

Sele

cti

vit

ites

%Y

ield

%

Fee

dra

tio,

cata

lyst

s,G

HS

V,p,T

XCO

2SCO

SM

eOH

SM

FSDM

EYM

F

C1

R =

3 :

1 C

u-ca

t A

g-ca

t

R =

1 :

1

13’3

72 h

-1

300

bar

260

°C

11’1

44 h

-1

300

bar

140

°C

13’3

72 h

-1

300

bar

120

°C 1

bar

160

°C

27’4

54 h

-1

27’4

54 h

-1

Adm

ix.

120

°C

120

°C

140

°C

140

°C

Res

in

59.4

769.3

626.9

83.2

00.4

61.9

1

C2

R =

3 :

1 C

u-ca

t A

g-ca

t

R =

1 :

1

13’3

72 h

-1

300

bar

260

°C

11’1

44 h

-1

300

bar

140

°C

13’3

72 h

-1

300

bar

120

°C 1

bar

160

°C

27’4

54 h

-1

27’4

54 h

-1

Adm

ix.

120

°C

120

°C

140

°C

140

°C

Res

in

54.3

266.5

630.0

72.9

20.4

51.5

9

C3

R =

3 :

1 C

u-ca

t A

g-ca

t

R =

1 :

1

13’3

72 h

-1

300

bar

260

°C

11’1

44 h

-1

300

bar

140

°C

13’3

72 h

-1

300

bar

120

°C 1

bar

160

°C

27’4

54 h

-1

27’4

54 h

-1

Adm

ix.

120

°C

120

°C

140

°C

140

°C

Res

in

51.8

071.2

223.7

52.7

32.3

11.4

1

C4

R =

3 :

1 C

u-ca

t A

g-ca

t

R =

1 :

1

13’3

72 h

-1

300

bar

260

°C

11’1

44 h

-1

300

bar

140

°C

13’3

72 h

-1

300

bar

120

°C 1

bar

160

°C

27’4

54 h

-1

27’4

54 h

-1

Adm

ix.

120

°C

120

°C

140

°C

140

°C

Res

in

52.2

572.6

624.8

32.2

70.2

51.1

8

C5

R =

3 :

1 C

u-ca

t A

g-ca

t

R =

1 :

1

13’3

72 h

-1

300

bar

260

°C

11’1

44 h

-1

300

bar

140

°C

13’3

72 h

-1

300

bar

120

°C 1

bar

160

°C

27’4

54 h

-1

27’4

54 h

-1

Adm

ix.

120

°C

120

°C

140

°C

140

°C

Res

in

27.0

166.8

128.7

84.2

20.1

91.1

4

C6

R =

3 :

1 C

u-ca

t A

g-ca

t

R =

1 :

1

13’3

72 h

-1

300

bar

260

°C

11’1

44 h

-1

300

bar

140

°C

13’3

72 h

-1

300

bar

120

°C 1

bar

160

°C

27’4

54 h

-1

27’4

54 h

-1

Adm

ix.

120

°C

120

°C

140

°C

140

°C

Res

in

57.8

770.9

725.3

20.7

32.9

80.4

2

C7

R =

3 :

1 C

u-ca

t A

g-ca

t

R =

1 :

1

13’3

72 h

-1

300

bar

260

°C

11’1

44 h

-1

300

bar

140

°C

13’3

72 h

-1

300

bar

120

°C 1

bar

160

°C

27’4

54 h

-1

27’4

54 h

-1

Adm

ix.

120

°C

120

°C

140

°C

140

°C

Res

in

54.1

965.8

522.5

41.0

810.5

30.5

9

Ref

.

R =

3 :

1 C

u-ca

t A

g-ca

t

R =

1 :

1

13’3

72 h

-1

300

bar

260

°C

11’1

44 h

-1

300

bar

140

°C

13’3

72 h

-1

300

bar

120

°C 1

bar

160

°C

27’4

54 h

-1

27’4

54 h

-1

Adm

ix.

120

°C

120

°C

140

°C

140

°C

Res

in

56.3

871.6

225.6

62.7

2-

1.5

3


It is evident that the absence of tangible estimation of formic acid pro-duction prevents any firm statement about effects of operating conditi-ons on performance. Nevertheless, the yields, calculated as the productof CO2 conversion and selectivity, give an appreciation of the potential ofthe multi-stage approach. A high methyl formate yield as in configurati-ons C1, C2, C4, and C5 is a promising indication of an efficient formatecatalyst tuning, exceeding by far theoretical thermodynamic equilibriumvalues as well as experimental record yields of Chapter 4, where a sin-gle reactor as R1 reached a maximum 1.50 % at 700 bar, and yields at350 bar did not exceed 0.38 % at 260 ◦C.

Alternatively, lower formate yields translate a decrease in CO2 conver-sion accompanied by a decrease in methyl formate selectivity, which canalso be interpreted as efficient conversion into formic acid. As a mat-ter or fact, the absence of the latter at the outlet of R2, and the fastdehydration of methanol to DME over Amberlyst 15, suggest the favou-rable hydrolysis of the little methyl formate with water. The unclosedcarbon balance in all configurations, especially in C6 (XC = 71.34 %)and C7 (XC = 75.75 %), suggests the production of an undetected car-bonated product. As a matter of fact, the carbon deficit thus engenderedsurpasses up to 12 % the ones usually caused by methanol solubilisingin condensed water as in the reference configuration (XC = 83.10 %)(Chapter 4). In addition, the sporadic condensation of the reaction efflu-ent caused by high conversion in R1 revealed the acidity of the mediumby qualitative analysis on pH paper in all 7 configurations. The amountof liquid generated is however too little to use a pH probe for a grada-tion of performances. Signs of formic acid in NMR analysis confirmed theidentity of the unknown carbonated acid product in the reaction effluentof configuration C7 bubbled in a 1M KOH aqueous solution.

These preliminary results keeping the methanol synthesis catalyst underconstant conditions confirm the pivotal position of methyl formate andestablish the needs for a tuning of methanol concentration: an excessof methanol improves methyl formate synthesis, however such an excessis detrimental to the subsequent hydrolysis. Consequently, the fast con-sumption of methanol into DME is a potential solution to continuouslyremove the former from the balance and promote formic acid synthesis


beyond equilibrium while avoiding re-esterification. Hence, an admixtureof methyl formate synthesis and hydrolysis catalysts concretises the mostpromising configuration, reshaping the two-step process as in Figure 7.3.

CO2 + H2 HCOO(H)

HCOOH

CH3OH

H+-cat.

REACTION STEP 1

300 bar, 230°C

REACTION STEP 2

300 bar, 140°C

INTERMEDIATE

CH3OH* Cu-cat. Ag-cat.

HCOOCH3*

H+-cat.

CH3OCH3*


Figure 7.3 – Scheme of the optimal two-step synthetic approach for the continuousconversion of CO2 and H2 into formic acid and DME via formate intermediate.

The ratios of water:methyl formate and methanol:methyl formate in bothsteps need to be optimised by adjusting the residence time and varyingthe catalyst ratios. Varying the amount of methanol synthesis catalystwith respect to that of methyl formate catalyst leads to a tunable met-hanol excess. Additionally, two moles of water are produced against onemole of methyl formate through Reactions R.1-R.3 and R.5 (Chapter2) so that a molar excess of water is guaranteed. Finally, a admixedcatalyst load in the second step adapted to the upstream catalytic per-formance should efficiently consume methanol and water to respectivelylimit backward reaction and ease downstream product purification.

7.3.2 Effluent purification

A practical aspect of importance for the feasibility of the multi-stageprocess remains in the downstream purification of the product stream.Methyl formate, and methanol to a lesser extent, profit from very diffe-rent volatilities at atmospheric pressure rendering their separation pos-sible as head product of distillation. The bottom product would howe-ver require further processing for the separation of the azeotropic mix-ture formed by formic acid and water (45 % formic acid, 55 % water,


Tboil,1 bar = 106.8 ◦C). The maximum-boiling azeotrope can be separa-ted via different routes such as pressure-swing or extractive distillation.Else, liquid-liquid extraction with alcoholic solvents225 or organic saltslike ionic liquids containing NEt3 as base226,227 are used to facilitate thedistillation for formic acid recovery. Such solutions are sensible for mostapplications requiring free HCOOH, however they would defeat the pur-pose of the continuous process investigated herein. For H2 storage pur-poses, recent heterogeneous catalysts proved high activity with diluteformic acid solutions.221,224 Consequently, an alternative to the energy-intensive distillation –and therefore CO2-emitting– using the conversionof methanol into gaseous DME, would not only promise increased formicacid yields, but also simplify purification from the liquid effluent. To thisaim, the versatility of the acidic zeolite H-ZSM-5 with potential full met-hanol conversion to DME228,229 and simultaneous hydrolysis of methylformate, would allow the omission of costly dehydration steps. Indeed,not only did H-ZSM-5 (Alfa Aesar, NH4-ZSM-5, Product number 45 881)reach selectivities to DME exceeding 95 % in dual bed configuration withCu/ZnO/Al2O3 during CO2 hydrogenation at 350 bar, it was also ableto efficiently catalyse the liquid phase hydrolysis of methyl formate.

7.3.3 Energetic and economic feasibility

This section expands the theoretical and experimental underpinnings ofthe targeted reactive route to energetic and economic aspects. The pro-cess efficiency is currently too low to extract a realistic cost and energybalance, nonetheless general findings from life cycle assessments (LCA)on CO2 hydrogenation to hydrocarbon fuels provide a sound startingpoint for a feasibility analysis. The LCAs examined the environmentalimpact of different technologies for the generation of green H2 and cap-ture of CO2 from point sources or from the atmosphere.167,230–232

From an environmental perspective, assuming CO2 capture from con-centrated sources such as flue gases of coal-fired power plants and H2

production from water electrolysis powered by wind would not allow acarbon-neutral process, but an alternative to sequestration and as one-

7.4 Conclusions 133

time recycle of CO2, slowing its net rate of release to the atmosphere.From an energy perspective, the cost of high-pressure operation wasshown to be pressure independent, where the the compression work upto 400 bar was compensated by advantageous reaction thermodynamics,before slightly lagging behind at higher pressures.109 The surveys agreeon the fact that the cost associated to the electrolysis of water weighssignificantly more in the total power requirement (60 %) than reactantcompression (25 %). To this respect the equimolar feed ratio shown inSection 7.3.1 to increase formate yields brightens up perspectives for thepresented reactive route.

Evidently the technology cannot currently compete for the production offormic acid at an acceptable rate and cost, while protecting environmentwith a negative carbon footprint, yet the future amount of anthropoge-nic atmospheric CO2 motivates the development of formic acid as energycarrier. With the joint progress on carbon capture, water electrolysis andformic acid dehydrogenation, the better understanding of heterogeneouscatalysts drives the design of new generation catalysts, which could in-crease the technology readiness level (TRL) to competitive reach withinan acceptable time frame.

7.4 Conclusions

The findings of the previous chapters were strategically combined into acontinuous multi-reactor system to tune methanol and water contents tosuch an extent that the second reaction step inclined towards hydrolysis,and to counteract decomposition.

Making use of Cu/ZnO/Al2O3, 1 wt.% Ag/Al2O3, and the resin Am-berlyst 15, different operating parameters evidenced, by GC analysis, arange of outcomes from substantially enhanced methyl formate yieldsto significant carbon loss attributed to the production of an undetectedcarbonated product. When operating methanol synthesis at 260 ◦C togrant methanol excess for an efficient methyl formate synthesis in sub-sequent esterification at 140 ◦C, the position of the resin catalyst played


a decisive role in the composition of the effluent stream.

Amongst the tested configurations, secluding the resin in the third reac-tor at high pressure and/or using an equimolar feed ratio reached highmethyl formate selectivity and yields. In contrast, operating the resin atatmospheric pressure, or physically mixing it with the formate catalystat high pressure, promoted the dehydration of methanol into DME. Thesignificant loss in carbon balance in all but one configuration, reachingas low as 71.34 %, hinted at substantial production of formic acid. Alt-hough formic acid was not detected per se, the identity was grounded insolid evidence of the production of an acid compound from pH analysiscombined to a one-time NMR analysis. The assumption was supportedby the lower CO2 conversion caused by dehydrogenation of formic acidfavoured at low pressure and high temperature.

The mere existence of formic acid qualitatively proved the potential ofthe two-step reactive pathway to alleviate the severe thermodynamicconstraints of the direct hydrogenation to formic acid over heterogeneouscatalysts. The preliminary experiments also clearly put forward the needfor a deeper parametric study based on a reliable estimation of the acidformed. The knowledge of the impact of conditions on the individualand overall reaction tendencies shall lead to maximised formic acid spacetime yields. Eventually, the deeper understanding shall assist the designof new generation of robust catalysts, which would render the multi-stepprocess technically, energetically and economically more viable.

135

Chapter 8

Conclusion and perspectives

The need to accelerate the transition to a low-carbon economy promotesthe discovery of advanced catalytic materials and process designs for CO2

hydrogenation as opportunity for innovations in cleaner hydrocarbon fu-els. Indeed, liquid chemical energy carriers synthesised from environmen-tally harmful waste are an ideal vector for the distribution and storage ofenergy from intermittent and renewable energy sources. What is more,reports predict plunging costs with the apparition of improved techno-logies in wind and solar power rendering their economics increasinglycompelling.233,234 To date, the designs of catalysts and processes for acontrolled CO2 hydrogenation to specific hydrocarbons remain defeatedby unfavourable thermodynamics and intricate reactivities. An efficientprocess hence calls for a rational exploration of the reaction space.

This thesis is part of a multidisciplinary workforce targeting the simul-taneous synthesis of formic acid and methanol via methyl formate. Itcontributes to innovate heterogeneous CO2 catalysis by developing aspectroscopic tool for operando Raman microscopy at catalytically rele-vant conditions, and by engineering a continuous process to maximise theselectivity and reactivity of CO2 to formic acid and derivatives. Herein,a summary highlights the most relevant results achieved in this work. Ageneral conclusion is then drawn on the feasibility of the two-step pro-cess before reflecting on its future implementation. Finally, the chapterends with an outlook providing directions to tackle unsettled challengesto improve the process from a chemical engineering point of view.

136 8. Conclusion and perspectives

8.1 Summary

Mechanistic reaction understanding is key to process efficiency in thesame way that in situ study of process dynamics is fundamental for asound understanding of CO2 reduction pathways over solid catalysts. Aview-cell is presented in Chapter 3 fulfilling the combined optical, me-chanical and hydrodynamic constraints for the analysis by microscopy ofchemical flow processes at extreme operating conditions. Tailored to thecharacteristics of a standard fibre-coupled Raman microscope, the opti-cal access into the microreaction setup allows the selective identificationof active species on catalytic surfaces, the measurement of chemical com-position of a flowing medium, as well as the detection of phase transitionsduring operation up to 400 ◦C and 500 bar.

The view-cell strategically complements online analytical methods byproviding physical and chemical insights into the realms of the flow re-actor. In Chapter 4, a parametric study of CO2 hydrogenation over a Cucatalyst reports the benefits of high-pressure on methanol yields, surpas-sing in cases equilibrium limitations. The view-cell enables the detectionof condensation of the reactive mixture and provides a rationale behindthe transgressing reaction behaviour. The resulting in situ phase separa-tion is shown to enhance process efficiency by segregating the productsfrom the reactants locally impacting the reaction equilibrium. The fairagreement between the dew point temperatures experimentally determi-ned and the calculated critical conditions of a representative reactionmixture confirms a transition to a dense gas-liquid biphasic system de-pending on pressure, temperature and extent of reaction.

In Chapter 5, the view-cell is used to gain chemical insight on catalyti-cally active surfaces to rationalise CO2 hydrogenation to methyl formateon supported metal catalysts. The study of SiO2- and CeO2-based cata-lysts evidences the involvement of surface formate and methoxy speciesand their evolution with reaction conditions. In combination with ot-her spectroscopic techniques and atomistic simulations, the operandoRaman-GC analysis successfully assisted the elucidation of the origin ofthe catalytic activity in methyl formate synthesis through reactive sur-

8.1 Summary 137

face formate formation. An alternative synthesis route to formic acidconsists in the immobilisation of active homogeneous Ru molecular ca-talysts. Although purposely designed for continuous processing, such acatalyst was operated for the first time in flow showing promising ca-talytic activity in spite of the absence of stabilising medium. The littleCO2 conversions and the complications stemming from the difficulty inanalysing formic acid derivatives in dilute gas steams justify the needsfor multiple reactor systems to circumvent thermodynamic barriers andstabilise formic acid.

In Chapter 6, the evaluation of the effect of operating conditions on thehydrolysis of methyl formate to formic acid and methanol demonstratedfast kinetics even in presence of additional diffusion resistances impliedby the biphasic liquid feed. The high reaction rates regardless of the feedcomposition warrant a successful implementation of the hydrolysis stepin the targeted two-step process, irrespective of the composition of thereactive stream exiting the first operation step, and of the TRL of theoverall process.

Chapter 7 combines the knowledge gained from the parametric analysisof the single reactions in Chapters 4-6 and leads to the design of a threeconsecutive reactor system to optimise methyl formate yields at the in-termediary. The overall efficiency of the two-step approach relies on twocritical aspects: (i) a substantial synthesis of methanol over Cu catalystand (ii) a close contact between the Ag/Al2O3 and the resin catalysts.The presence of methanol synthesised in the first reactor is shown toengender esterification in the subsequent reactors with surface derivati-ves of formic acid to form methyl formate as intermediate, while shortdiffusion paths to the resin ensure the fast hydrolysis of methyl formate.Although no optimal reactor configuration could be singled out due tothe lack of appropriate analysis technique to quantify efficiency, the two-step synthesis of formic acid and methanol in one continuous process hasbeen preliminary but successfully demonstrated. The semi-analytical so-lution recognised the effect of reactor configuration on the equilibriumnature governing the final outcome and broadens possibilities for thedesign of a well-conditioned multi-step process.


8.2 Conclusion and perspectives

The continuous multistep approach is the first work of its kind aimingat rationalising the discovery and integration of advanced catalytic ma-terials for an efficient CO2 valorisation into formic acid. It is evidentlytoo far from being a mature technology to discuss implementation andsubstantially contribute to the targets set by industry and governmentsin terms of CO2 emission reductions. Yet, the fundamental and appliedknowledge gained in this project is expected to have vast impacts on rela-ted CO2 hydrogenation reaction, rational development of solid catalystsselective for formates, and in situ/operando Raman spectroscopy. Thecurrent results are insufficient to lead to a qualitatively sound evaluation,nevertheless the ample room for continued improvement in catalyst andreactor designs encourages a holistic evaluation using LCA.

The next steps to be taken towards improvement of the process are sugge-sted in the following sections. A brief description is given and preliminaryresults are included where applicable.

Determination of carbon oxide source in methanol

In Chapter 4, the variation in residence time revealed an evolution inthe sequence of products of reaction. In combination to the possibility todetermine by Raman spectroscopy the chemical composition of a flowingstream as shown in Chapter 3, the composition of the CO2 hydrogenationeffluent could be analysed over a larger range of contact times for kineticunderstanding and determine the origin of COx in methanol.

Using a transparent tubular reactor made of sapphire and discrete cata-lyst beds, preliminary results 1 demonstrate the potential to decouple thehydrogenation reactions and elucidate their sequence in function of ope-rating conditions. Unfortunately, the absence of signal for CO at 200 bar,due to its small scattering cross-section and the intensity loss from the

1. H. Reymond, R. Gaikwad, A. Urakawa and P. Rudolf von Rohr, unplublishedresults.

8.2 Conclusion and perspectives 139

curved optical surface, restrict the analysis to the reagents (and metha-nol). A higher light collection efficiency or pressure would enable its de-tection and assert the analysis with product selectivity. To this purpose,the developed view-cell would enable easier pressure and temperatureramp up, improved signal detection from its flat surface, and increasedRaman activity at higher pressures. The same study principle could thenbe achieved by either connecting multiple view-cells and stainless steelreactors alternately, or using a single view-cell varying the amount of ca-talyst packed upstream. The preliminary results are presented hereafteras starting point to a more refined study.

Materials and methods Preliminary trials were carried out using thehigh-pressure microreaction plant (Chapter 3). The reactor consisted in asapphire tube (�inner= 1 mm, L = 100 mm) packed with the commercialCu/ZnO/Al2O3 catalyst. Leak-tight connections with the stainless steelfittings were achieved by increasing adhesion to the sapphire surface bycoating the ends of the tube with a thin layer of polyimide and usinggraphite reinforced polyimide ferrule. The catalyst pellets were crushedand sieved to 63-80 µm particle size. 15 mg of catalyst were loaded in thesapphire tube arranged in three separate packed beds (each ca. 5 mg and5 mm) segregated by void sections (100-120 mm) as depicted in Figure8.1. The alternating sections of void and catalyst were segregated bymeans of dense plugs made of quartz wool. To prevent the quartz woolplugs from sliding and combining under pressure, a stainless steel rod(�outer= 0.6 mm) was inserted in each void section to support the plugsat its extremities.

The composition of the reactor outlet stream was analysed online byGC, while the four void sections enabled the analysis of the intermediatereactive stream at discrete positions by Raman microscopy applying theconcept presented in Equation 3.5 in Section 3.5.3 (Raman band areasconsidered in place of maximum intensity). Mounted on a motorisedlinear actuator (PI, M-404), the Raman probe was moved along thereactor axis to focus on either one of the positions marked as P1-P4.Position P1 allowed the analysis of the unreacted feed, and positionsP2, P3, and P4 analysed the effluent stream after packed-bed sections,


Inlet Outlet

SS rod10 mm

Catalyst bed5 mg - 5 mm

Glass wool2 mm

Raman probe position

P1 P2 P3 P4

…….…….…….…….…….…….…….…….

…….…….…….…….…….…….…….…….

…….…….…….…….…….…….…….…….

Figure 8.1 – Schematic representation of the sapphire reactor with multiple packed-beds and alternating void sections. Positions P1-P4 denote the focusing positions forthe remote Raman probe.

monitoring the evolution in composition at increasing contact times.

The activation of the catalyst and pressurisation of the system followedthe procedure of Chapter 4. Reactions were carried out at a pressure of200 bar and four temperatures 180 ◦C, 230 ◦C, 260 ◦C and 290 ◦C. Thetotal flow rate of 16 mLN/min corresponded to a GHSV of 80 000 h−1.A molar feed composition of H2:CO2 = 2.5:1 was used in order to in-crease the density of the reacting stream and improve the quality ofthe Raman spectra. Indeed the higher density resulting from the largerscCO2 fraction improved the signal-to-baseline ratio in comparison to astoichiometric feed ratio of 3:1. Disturbance in light-matter interactionarose from intense refraction and reflection caused by the circular cross-section of the cylindrical sapphire tube, thus decreasing the signal inten-sity collected by the remote probe in backscattering mode. However, thelight-collection efficiency loss caused from light transmitting through thetransparent reactor was lessened by a higher stream density.

Preliminary results and discussion Figure 8.2 presents the flowcomposition measured by GC as representation of the effluent at positionP4, while Figures 8.3(a) and (b) trace the evolution of the Raman arearatio along the reactor axis at different reaction temperatures.


SMeOH

SCO

XH2

Figure 8.2 – Hydrogen conversion (XH2) and selectivities to methanol (SMeOH) and

CO (SCO) in function of reaction temperature.

In Figure 8.3(a), the initial area ratio at P1 is scaled to 2.5 to representthe molar ratio of unreacted feed confirmed by GC measurement. Thereaction at 180 ◦C shows a slight decrease in area ratio from position P1to P2 before re-increasing at P3 beyond its initial value and no majorchange is observed moving from P3 to P4. The results at 290 ◦C pre-sent an opposite behaviour with an initially increasing ratio at P2 whichthen decreases at P3 before stabilising at P4. In the case of direct met-hanol synthesis from CO2 as in Reaction R.1 (Chapter 2), 3 moles ofH2 would be consumed per mole of CO2 for the synthesis of methanol,making H2 the limiting reactant. In the opposite, if CO2 were consumedin the synthesis of CO via Reaction R.2 (Chapter 2), CO2 would be-come limiting reactant. A decrease in the area ratio H2/CO2 would thussignify an excessive H2 consumption as in the former case of direct met-hanol synthesis, whereas an increasing ratio would be a sign of a gradualCO2 shortage. In case methanol is a secondary product obtained fromthe subsequent hydrogenation of CO (Reaction R.3, Chapter 2), the ra-tio is expected to decrease, as an equivalent amount of H2 is requiredwhichever the COx source to methanol. Consequently, at low reactiontemperature 180 ◦C, the exothermic methanol synthesis is favoured atP2, before the RWGS produces CO in reason of prolonged contact times


(a)(a) (b)

Figure 8.3 – H2/CO2 area ratio at the four positions P1-P4 along the reactor (a)at 180 ◦C and 290 ◦C and (b) at 230 ◦C and 260 ◦C.

at P3-4 (Section 4.3.1). At 290 ◦C, the endothermic RWGS is promotedcausing the area ratio to initially increase at P2, before decreasing at P3as a sign of subsequent methanol synthesis from CO. The re-increasingratio at P4 hints at a CO synthesis more active than methanol, which isa result of most favoured high temperature RWGS. The presence of COwas identified by Raman analysis at P4 at 290 ◦C due to the high COcontent as confirmed by GC analysis.

Following the same deductions for Figure 8.3(b), the increasing ratio atshort contact times speaks in favour of CO as primary CO2 hydrogena-tion product. Methanol seems to be relegated to secondary product atlonger contact times. The sharp drop in area ratio at 260 ◦C at P4 coinci-ded with the observation of condensation at the rear end of the packed-bed. Condensation was observed by means of a conventional portablecamera as well as with a white-light camera implemented in the remoteRaman probe (Figure 8.4). The void section where the effluent was ob-served to have condensed was heated by means of heat-blower at 300 ◦Cto keep the stream under gas phase and prevent any light disturbance inthe Raman analysis. The condensation is believed to enhance methanol


synthesis via CO or CO2, by in situ separation of the less volatile com-ponents water and methanol. Indeed, when focusing on the dense phase,the methanol peak shows increased intensity and the area ratio H2/CO2

drops to even lower values, proving the higher miscibility of scCO2 thanH2. Interestingly, despite higher conversion, no condensation was obser-ved even at P4 at 290 ◦C. This observation is directly correlated to thelower methanol selectivity calculated from GC. As a matter of fact, theGC analysis reveals a larger CO content than methanol only at 290 ◦C.The lower fraction of methanol and concomitant higher fraction of COcause the dew point of the reaction effluent to drop significantly belowthe actual reaction temperature, hence containing the reactive stream ina homogeneous gas phase (Section 4.2.4).

Glass woolSS rod P4

Condensed phase

Figure 8.4 – In situ condensation at position P4, 200 bar and 260 ◦C.

Improvement of intermediate methyl formate yields

Concerning the synthesis of methyl formate, first-generation catalystswere proposed, synthesised and identified, and their activity were confir-med. The insight acquired from the synergetic workforce in the Sinergiaproject provides solid basis for the development of second-generationcatalysts with improved activity, selectivity and stability and start anot-her round of testing and feedback. As far as in situ Raman spectroscopygoes, modulation spectroscopy would complement the steady-state cha-


racterisation of the catalyst surface by monitoring transient behaviourupon changes in feed composition and hence identify Raman bands ori-ginating from kinetically separable species. Diagnostic adsorption testswith isotope labelling would also assist in the distinction of catalyticallyactive from spectator species.

In situ Raman spectroscopy could also be applied for the characterisa-tion of the immobilised molecular catalysts for mechanistic studies athigh pressure. In addition, if a candidate can be suggested as reference,in situ Raman spectroscopy of the reaction effluent could assist in theidentification of the unknown carbonated product postulated in Section5.3.1. In term, transforming formic acid into another compound detec-table by GC bears the potential to shun the problem of its analysis.To this end, provided stable separate flows can be obtained, contactingthe product stream of a reactor packed with the immobilised Ru cata-lyst with the effluent of a parallel reactor containing Cu/ZnO/Al2O3,and then flowing the joint outlets in a third reactor containing the resinAmberlyst 15, could offer an alternative to the search of reliable formicacid analysis. Indeed, a parallel configuration should form the productsnecessary to the esterification of formic acid with methanol into waterand quantifiable methyl formate, while ensuring no poisonous contactbetween methanol and the Ru-catalyst as well as avoiding the decompo-sition of formic acid on the Cu catalyst.

Rigorous optimisation of continuous two-step process

The main challenge hampering any cogent argument on process efficiencyand hence any conclusion on practical viability remains in a reliable ana-lysis of formic acid in dilute gas streams. At least qualitative appreciationof the amounts of formic acid produced is a prerequisite to compare theoutcomes of various reactor systems and operation. Finally, the multi-ple reactor system generates a vast set of variables to be tuned for anoptimal efficiency. However, an exhaustive parametric study of all pro-cess configurations, studying the effect for each single reactor of pressure,temperature, contact time, catalyst loading, reactor dimensions and con-figuration, is unrealistic, leading to either loss of time from a combinato-


rial explosion or loss of information by simplification. The automation ofreaction control and monitoring techniques would open strategies to op-timise the two-in-one process. In combination with artificial intelligence,an automated characterisation in flow offers smarter, rigorous, yet ef-ficient screening of the multivariate space by capturing and evaluatinginformation in real time. This autonomous feedback could potentiallyconstitute a platform towards finding optimal operating conditions. Suchan approach was already successfully implemented in material researchin the study on carbon nanotube growth.235 Similarly, machine-assistedautonomous approaches have been recently actively researched in conti-nuous flow chemistry for an ”accelerated reaction development”.236,237

147

A Appendix

The appendix groups the information used in the theoretical equilibriumcalculations as well as the analysis of the experimental output data. TheRaman spectra of a selection of compounds, reactants and catalysts, arealso contained herein, used as reference in the assignment of the in situspectra presented in the results and discussion sections of Chapters 3, 4and 5.

148 A. Appendix

Thermodynamic values of formation

Table A.1 – Standard enthalpies ∆H◦f and Gibbs free energies ∆G◦

f of forma-

tion.35,36

Component ∆H◦f [kJ/mol] ∆G◦

f [kJ/mol]

CO2 −393.5 −394.4

CO −110.5 −137.2

CH3OH (g) −201.0 −162.3

CH3OH (lq) −239.1 −166.6

H2O (g) −241.8 −228.6

H2O (lq) −285.8 −237.1

HCOOCH3 (g) −357.4 −297.2

HCOOCH3 (lq) −386.1 −295.6201

HCOOH (g) −378.0 −351.0

HCOOH (lq) −424.7 −361.4

CH3OCH3 −184.1 −112.6

149

CO2 hydrogenation efficiency

Helium factor

A helium factor f′

He is introduced based on the inert internal standardHe premixed to the H2 gas, in order to correct for the absence of waterin the GC analysis.

nHe,0 = nHe

Dividing by the total amount of moles in the feed before reaction,

nHe,0

ntot,0=

nHe

ntot,0· n′

tot

n′tot

where n′

tot accounts for the reduction in total mole number stemmingfrom the loss of water in analysis.

n′

tot = ntot − nH2O

xHe,0 = x′

He ·n′

tot

ntot,0

The helium factor f′

He is obtained by reformulating,

f′

He =n′

tot

ntot,0=xHe,0

xHe

150 A. Appendix

Conversion

The conversion Xi of a component i defines the fraction of the convertedamount of component i in relation to the total amount of component ifed to the reaction at time t = 0. The conversion is calculated for thereactants of CO2 and H2 in terms of molar fractions xi obtained fromGC chromatograms.

Xi =xi,0 − x

′

i · f′

He

xi,0

where xi,0 is the mole fraction in the feed at room temperature, xi thesteady-state mole fraction at reaction temperature.

Selectivity

In case of multiple products formed in the reactive process, the selectivityS defines the amount of a specific product species p formed relative tothe total amount of reactant i consumed.

Sp,i =xp − xp,0xi,0 − xi

|νi||νp|

Because the water formed during hydrogenation is not quantified by GC,only the selectivities of the carbon-containing products are evaluated onthe basis of the amount of carbon converted XC .

XC = f′

He(N∑j

x′

p,j · rp,j)

where rp stands for the number of carbon atoms contained in a moleculeof product j.

151

The carbon-selectivity reads then

Sp,C =x′

p · rp · f′

He

XC

Space time yield

The space time yield Y is used as measure to quantify and comparethe amount of a product p produced normalised per amount of catalystand per unit of time. It is calculated on the basis of the CO2 conversionand selectivities, and considers the mass of catalyst used as well as theresidence time in the reactor.

Yp =nCO2,0

·XC · Sp,Cmcat

[mmol

g · h

]

Methanol synthesis: supplementary data

(a) (b)

CO

MeOH

MF

Figure A.1 – (a) Effect of GHSV on reaction performance and (b) product se-lectivity at 350 bar. Operating conditions and stream compositions of correspondingexperiment IDs B1-B5,E1-F5 are tabulated in Table A.2.

Tab

leA

.2–

Exp

erimen

tID

s,C

O2

conversio

nan

dp

rod

uct

selectivities

for

diff

erent

cata

lyst

particle

sizes.T

he

data

referto

reactio

nco

nd

ition

su

sedfo

rth

eeff

ectof

op

eratin

gco

nd

ition

son

cata

lytic

perfo

rman

ce.

Particle

size

Particle

size

45-6

3µm

28-4

5µm

Run

pT

GH

SV

XCO

2SM

eO

HSM

FSCO

XCO

2SM

eO

HSM

FSCO

IDbar

◦C

/h

%%

A1

200

bar

170

22’3

00

18.3

92.2

7.8

-6.4

87.9

12.1

-

A2

200

20.4

95.9

4.1

-13.9

95.5

4.5

-

A3

230

34.8

23.4

1.0

75.6

28.7

25.8

1.1

73.1

A4

260

50.4

24.2

0.0

75.8

40.2

21.5

0.6

77.9

A5

280

62.7

32.5

0.0

67.5

49.6

22.5

0.0

77.5

B1

350

bar

170

22’3

00

4.8

93.0

7.0

-5.0

89.9

10.1

-

B2

200

14.0

51.2

5.2

43.6

15.7

51.0

4.1

44.9

B3

230

31.4

26.6

1.3

72.1

33.5

23.8

1.5

74.7

B4

260

47.4

23.1

0.8

76.1

49.8

34.2

1.1

64.7

B5

280

62.0

35.3

0.8

63.9

60.5

31.6

1.2

67.2

C1

500

bar

170

22’3

00

7.2

91.3

8.7

-6.7

88.6

11.4

-

C2

200

18.4

38.8

5.2

56.0

15.0

41.6

4.6

53.8

C3

230

38.4

27.7

4.2

68.1

33.3

23.1

2.8

74.0

C4

260

63.3

28.8

1.4

69.8

69.3

30.4

1.1

68.5

C5

280

73.3

42.2

1.5

56.3

72.6

36.3

1.3

62.4

D1

700

bar

170

22’3

00

9.7

79.6

20.4

-5.3

80.3

19.7

-

D2

200

17.7

42.2

7.5

50.3

17.6

39.1

8.3

52.6

D3

230

37.0

24.4

4.3

71.2

39.7

20.4

3.5

76.1

D4

260

74.5

36.2

1.9

61.9

77.7

32.5

1.7

65.8

D5

280

78.0

42.6

1.4

56.0

77.0

38.9

1.9

59.2

E1

350

bar

170

17’8

00

9.5

88.4

11.6

-

E2

200

23.7

41.0

3.3

55.7

E3

230

39.4

26.5

1.1

72.5

E4

260

57.9

29.2

1.2

69.6

E5

280

67.4

31.0

0.6

68.3

F1

350

bar

170

40’1

00

7.1

89.5

10.5

-

F2

200

13.8

90.1

9.9

-

F3

230

29.8

26.2

2.0

71.7

F4

260

42.0

18.5

0.8

80.8

F5

280

50.7

21.6

0.8

77.6

Table A.3 – Experiment IDs, CO2 conversion and product selectivities for differentcatalyst particle sizes. The data refer to reaction conditions used for the determinationof dew point temperatures. Particle size 28-45 µm.

Run p T GHSV XCO2SMeOH SMF SCO Tdew

ID bar ◦C /h % ◦C

G1 200 bar 240 9’300 16.0 35.7 1.6 62.7 183

G2 17’800 32.4 24.2 0.9 74.9 245

G3 17’800 29.5 23.0 0.9 76.1 240

G4 22’300 27.8 23.7 0.9 75.3 232

G5 260 9’300 24.7 21.0 0.0 79.0 220

G6 17’800 37.2 24.8 0.0 75.2 260

G7 17’800 36.9 23.8 0.9 75.3 255

G8 22’300 33.2 21.5 0.8 77.6 253

H1 350 bar 240 10’000 37.2 28.4 1.5 70.1 272

H2 17’800 40.1 25.4 1.1 73.4 275

H3 22’300 21.9 23.2 1.1 75.8 242

H4 29’700 27.1 19.7 0.9 79.4 252

H5 39’600 20.7 23.0 2.0 75.0 235

H6 260 10’000 57.4 25.8 1.3 72.8 285

H7 17’800 48.4 26.4 1.2 72.4 282

H8 22’300 27.8 17.7 0.9 81.4 265

I1 450 bar 240 11’100 46.0 26.6 1.2 73.2 300

I2 17’800 37.3 23.7 2.3 74.0 290

I3 22’300 34.3 20.7 2.1 77.2 287

I4 260 11’100 66.4 37.1 1.1 61.8 327

I5 17’800 55.5 23.0 1.1 75.9 320

I6 22’300 49.0 21.1 1.0 77.8 307

153

154 A. Appendix

Critical point of methanol effluent

The equations derived for the determination of the critical points followsthe procedure of Heidemann and Khalil122 and Stradi123. The modifica-tions implemented are also indicated. The signification of the variablesare summarised in table A.4. Starting with the conditions for the criticalpoint,

Q∆n = 0

andC∑k=1

C∑j=1

C∑i=1

(∂3A

∂nk∂nj∂ni)∂ni∂nj∂nk = 0

Q is a CxC matrix, their elements qij are defined by:

qij = (∂2A

∂nj∂ni)

These conditions are reformulated to:

Q∆n =

C∑j=1

Aij∆nj =RT

n(∆niyi

+ F1(βiN + β + βiF21 β))

+a

bn(βiβF3 −

F5

a

C∑j=1

aij∆nij + F6(βiβ − αiβ − αβi))

(i = 1...C)

155

∑k

∑j

∑i

(∂3A

∂nk∂nj∂ni)∂ni∂nj∂nk =

RT

n2(−

C∑i=1

∆n3i

y2i

+ 3N(βF1)2 + 2(βF1)3)

+a

n2b(3β

2(2α− β)(F3 + F6)− 2β

3F4 − 3βaF6)

Additionally the variation in the component mole number is normalisedto ensure a non-zero variation.

∆nT∆n− 1 = 0

The first condition results in C equations, the second condition and thenon-zero variation of the mole number add two equations to the system,yielding a total system of C+2 equations.

The parameters a, b and aij are determined by the SRK-EOS. The follo-wing equations describe the equations needed for the calculation of thesethree parameters. The calculation of ai was adapted from the equationproposed by Stradi, to includ the polar interaction parameter pi accoun-ting for the presence of polar molecules.

a =

C∑i=1

C∑j=1

(ninjn2

)aij , aij = (aiaj)0.5(1− kij)

a0.5i = 0.42748

(RTci)2

Pci(1 + ci(1−

T

Tci)0.5 − pi(1−

T

Tci)(0.7− T

Tci))

ci = 0.48 + 1.574ωi − 0.176ω2i

156 A. Appendix

b =

C∑i=1

(nin

)bi, bi = 0.08664RTcipci

Additionally to the parameters for the EOS additional parameters areused for the computation of the critical points, without physical meaning:

N =

C∑i=1

∆ni

αk =

∑Ci=1 yiaika

, α =

C∑i=1

∆niαi, a =1

a

C∑i=1

C∑j=1

∆ni∆njaij

βi =bib, β =

C∑i=1

∆niβi

F1−6 are auxiliary functions and defined as:

F1 =1

K1 − 1, F2 =

2

K + 1, F3 =

1

(K + 1)2

F4 =1

(K + 1)3, F5 = 2 ln(

K + 1

K), F6 = 2

[1

K + 1− ln(

K + 1

K)

]

The parameter K stands for the dimensionless volume defined as:

K =ν

b

157

Once the total C+2 unknowns (∆n (C unknowns), T , ν) are determined,the critical pressure is calculated with the SRK-EOS as follows.

p =RT

ν − b− a

ν(ν + b)

Table A.4 – Nomenclature of parameters used in the derivation. The abbreviationsnot listed here are auxiliary parameters of no physical meaning.

Variable Signification

A Helmholtz free energy

C total number of components

ni mole number of species i

n total number of moles

∆ni change in mole number of species i

R ideal gas constant

T absolute

yi mole fraction of component i

a average energy parameter in the van der Waals mixing rules

aij energy of interaction parameter between species i and j

ai energy parameter of species i

pi polar interaction parameter of species i

ωi acentric factor of species i

Tci critical temperature of species i

pci critical pressure of species i

kij binary interaction parameter between species i and j

b average van der Waals molar volume

bi van der Waals molar volume of species

ν specific volume

p absolute

158 A. Appendix

Raman spectra of selected compounds

Gases: CO2, H2 and CO

1281

1385

12611407

Figure A.2 – Raman spectrum of CO2 at 140 bar and room temperature acquiredwith 15 s/100 %/4acc., λ = 532 nm.

357

589

8161037

1248 1451

Figure A.3 – Raman spectrum of H2 at 350 bar and room temperature acquiredwith 10 s/100 %/1acc., λ = 532 nm.

The Raman spectrum of CO (not shown due to its very weak Ramanactivity) exhibits a single band at 2140/cm at high pressures (> 200 bar).

159

Liquids: methanol, methyl formate, formic acid and water

130

1034

1462

2834

2943

Figure A.4 – Raman spectrum of liquid methanol acquired with 20 s/100 %/5acc.,λ = 532 nm.

34273236

Figure A.5 – Raman spectrum of liquid water acquired with 60 s/60 %/1acc., λ =532 nm.

160 A. Appendix

120331

768637

912

13811449

1720

2740

2841

2958

3040

31081030,1160,1212

Figure A.6 – Raman spectrum of liquid methyl formate acquired with20 s/100 %/5acc., λ = 532 nm.

125

200

677

1059

1202

1398

1667

2778

2957

Figure A.7 – Raman spectrum of liquid formic acid acquired with 20 s/100 %/5acc.,λ = 532 nm.

161

In situ Raman spectra of Cu/ZnO/Al2O3

Excitation wavelength: λ = 532 nm

417750

1638 1816

Figure A.8 – Raman spectrum of Cu/ZnO/Al2O3 showing the important fluores-cence after activation when acquired with 240 s/100 %/4acc., λ = 532 nm.


1350 1510 1620

Figure A.9 – Raman spectrum of the Cu/ZnO/Al2O3 showing weak surface specieswhen acquired with 180 s/100 %/6acc., λ = 355 nm.

162 A. Appendix

Ex situ Raman spectra of metal supported catalysts

1 wt.% Cu/SiO2

3737

605, 794, 905, 974, 1054

131, 291, 443

Figure A.10 – Ex situ Raman spectra of Cu/SiO2 fresh (120 s/100 %/3acc.) andspent (120 s/100 %/1acc.), λ = 532 nm.

1 wt.% Cu/CeO2

3656

2846 2937

831,1043,1175

127, 271

607

Figure A.11 – Ex situ Raman spectra of Cu/CeO2 fresh (240 s/5 %/1acc.) andspent (10 s/5 %/1acc.), λ = 532 nm.

163

1 wt.% Ag/CeO2


129

460

597

832 1076

Figure A.12 – Ex situ Raman spectra of Ag/CeO2 fresh (120 s/5 %/1acc.) and spent(60 s/10 %/2acc.), λ = 532 nm.


1241, 1356, 1399, 1525, 1652182, 248

460

Figure A.13 – Ex situ Raman spectra of Ag/CeO2 fresh (60 s/100 %/1acc.) andspent (60 s/100 %/1acc.), λ = 785 nm.

164 A. Appendix

1 wt.% Au/CeO2


134, 258578

832

460

Figure A.14 – Ex situ Raman spectra of Au/CeO2 fresh (120 s/5 %/1acc.) andspent (120 s/5 %/1acc.), λ = 532 nm.


1249, 1359, 1411, 1532, 1646

171, 248

460

Figure A.15 – Ex situ Raman spectra of Au/CeO2 fresh (60 s/100 %/1acc.) andspent (60 s/100 %/1acc.), λ = 785 nm.

165

Pure CeO2

Ex situ

129, 254

460

598

1067,1175, 1322, 15503521

3701

Figure A.16 – Ex situ Raman spectra of pure CeO2 (30 s/10 %/1acc.), λ = 532 nm.

In situ

1311

1581

2848

3661

1086741

453

Intensity x2

Figure A.17 – In situ Raman spectra of pure CeO2 at 200 bar at 230 ◦C in reactionflow (120 s/50-100 %/1acc.), λ = 532 nm.

166 A. Appendix

Methyl formate hydrolysis: supplementarydata

Table A.5 – Liquid-sate standard enthalpies and Gibbs free energies of reaction35,36.

∆H◦r=+8.13 kJ/mol

Kx(20 ◦CC) = 0.14b Kγ(20 ◦CC) = 0.27b∆G◦r=+4.74 kJ/mol

K◦eq = 0.06a

a evaluated from the Gibb’s free energy, b evaluated by UNIFAC property method.

Table A.6 – UNIFAC group specifications and sample group assignments238

Group numbers Group Volume Surface Area

Main Secondary Name R Q

1 1 CH3 0.9011 0.848

6 15 CH3OH 1.4311 1.432

7 16 H2O 0.9200 1.400

12 23 HCOO 1.2420 1.188

20 43 HCOOH 1.5280 1.532

Table A.7 – UNIFAC Group-group interaction parameters, amn, in Kelvins238

Main Group n=1 6 7 12 20

m=1 0.0 697.2 1318 507.0 663.5

6 16.51 0.0 -181.0 179.7 -202.2

7 300.0 289.6 0.0 233.87239 -14.09

12 329.3 227.8 124.63239 0.0 -268.1

20 315.3 339.8 -66.17 193.9 0.0

Table A.8 – Reactants physico-chemical characteristics.143,238

Component Property Value

Methyl formate Tboil(1 bar) 31 ◦C

Tboil(10 bar) 111 ◦C

ρ(20 ◦C) 974.2 kg/m3

µ(20 ◦C) 0.000 347 Pa s

σ(20 ◦C) 24.6 mN/m

σ(100 ◦C) 12.9 mN/m

solubility in water(20 ◦C) 30 %

Water Tboil(1 bar) 100 ◦C

Tboil(10 bar) 180 ◦C

ρ(20 ◦C) 998.2 kg/m3

µ(20 ◦C) 0.001 01 Pa s

σ(20 ◦C) 72.7 mN/m

σ(100 ◦C) 58.9 mN/m

Amberlyst 15 skeleton styrene divinylbenzene

type strong acid

structure macroreticular

functional groups sulfonic (SO3H)

ionic form hydrogen

particle size < 300µm

mass loss (105 ◦C) 0.8 %

Cacid sites 4.7 meq/g of dry resin

Tmax 120 ◦C (desulfonisation)

167

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List of publications 197

List of publications

Journal publications

1. H. Reymond and P. Rudolf von Rohr. Micro view-cell for phasebehaviour and in situ Raman analysis of heterogeneously cataly-sed CO2 hydrogenation. Rev. Sci. Instrum., under revision.

2. H. Reymond, V. Amado-Blanco, A. Lauper and P. Rudolf vonRohr. Interplay between reaction and phase behaviour in car-bon dioxide hydrogenation to methanol. ChemSusChem, 10:1166–1174, 2017.

3. H. Reymond, S. Vitas, S. Vernuccio and P. Rudolf von Rohr.Reaction process of resin-catalyzed methyl formate hydrolysis inbiphasic continuous flow. Ind. Eng. Chem. Res., 56:1439–1449,2017.

Conference contributions

1. H. Reymond, P. Rudolf von Rohr. Operando Raman spectroscopyof high-pressure CO2 hydrogenation. 14th International Confe-rence on Microreaction Technology, Beijing, 2016.

2. H. Reymond, P. Rudolf von Rohr. View-cell for in situ analysisin microreactors under harsh operating conditions. 14th Interna-tional Conference on Microreaction Technology, Beijing, 2016.

3. H. Reymond, P. Rudolf von Rohr. Continuous formic acid synthe-sis in a milli-reactor. 4th Zing Continuous Flow Chemistry Con-ference, Algarve, 2016.

4. H. Reymond, A. Lauper and P. Rudolf von Rohr. High pressureCO2 reduction: operando heterogeneous characterisation setup.Faraday Discussions, Carbon dioxide utilisation, Sheffield, 2015.

5. B. Tidona, H. Reymond, A. Bansode, A. Urakawa, P. Rudolf vonRohr. CO2 hydrogenation to methanol at supercritical reactionconditions. 9th World Congress of Chemical Engineering, Seoul,2013.

Documents

Gaining Light into High-Pressure Carbon Dioxide Hydrogenation to Chemical Energy Carriers