Journal Name

Naphtha reforming process development methodol-ogy based on the identification of catalytic reactivitydescriptors

Olivier Said-Aizpurua,b, Florent Allaina, Fabrice Diehla, David Farrussengb, Jean-François Jolya, Aurélie Dandeua∗

Several major refining catalytic processes show a strong dependence of overall performancesupon catalyst formulation and structure. This observation is particularly true in naphtha catalyticreforming for which product distribution is sensitive to slight changes in active phase formulation.This review presents different issues encountered in naphtha reforming that are related to activephase formulation change.Two research and development approaches are usually proposed. (i) Rationalise the developmentof new catalytic formulations in order to increase valuable products selectivity (ii) Design simula-tors that would guide operation in order to maintain process performances. Current limitations toa faster process development come from the disjunction between kinetic modelling and catalystoptimisation.This critical review is not an extensive analysis of existing naphtha reforming kinetic modellingmethodologies or advance in catalytic behaviour elucidation. Rather, it proposes an original andpragmatic process development approach aimed at merging catalyst development with kineticmodelling through the identification of “effective” and “measurable” catalytic descriptors. Thespecificities of the naphtha reforming catalysts structure/property relationship are reviewed andtaken into account in order to list potential catalytic descriptors for this process. A focus is madeon a current bottleneck faced in the description of the active phase which lies in understanding therole of the proximity and the interaction between acid and metallic sites over the balance betweendifferent bi-functional pathways. A variety of experimental approaches that can be used to mea-sure these naphtha reforming active phase catalytic descriptors are presented and compared.

1 IntroductionCatalysts play a key role in the refining industry and new commer-cial formulations are frequently introduced in order to increasethe performances of existing processes. Given the scale of refin-ing industrial units, significant overall yields and selectivity gainsmight arise from tiny catalytic structural adjustments. Depend-ing on the severity of the operating conditions, stability issuesare responsible for a degradation of initial performances over thetime on stream and catalytic structure is also engineered in orderto address deactivation. As a consequence, operating a catalyticunit leads to two questions: Which catalyst is required to fulfilthe performance objectives? How to adjust process conditions inorder to maintain the performances over time?

a IFP Énergies nouvelles, Rond-Point de l’échangeur de Solaize - BP 3, 69360 Solaize,France. E-mail: aurelie.dandeu@ifpen.frb Univ Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, F-69626, Villeur-banne, France.

Simulation tools are required to guide operation decisions andthe influence of the catalyst over the chemical transformation isdescribed through the kinetic model. Hence, there is a need for animprovement of simulators robustness towards catalytic formula-tion change and for a better structural description of the catalystwithin the kinetic model.

The intricate link between the physico-chemical properties ofthe catalyst and the overall process performances is particularlystriking in naphtha reforming catalysis. Since the developmentof first units in the 1940’s, engineers struggled for the slightestrise in selectivity and productivity. Naphtha reforming is oper-ated in a narrow range of operating conditions, close to the op-timum.1–3 Temperature and partial pressures are carefully set inorder to satisfy several requirements: thermodynamically favourdehydrogenation reactions, compensate endothermicity and han-dle deactivation by coke deposition. However, the selection of thefresh catalyst properties as well as the catalyst regeneration con-ditions offer some flexibility in process operation. The demand

Journal Name, [year], [vol.],1–19 | 1

for higher reformate yields and for a better control of process se-lectivity is met by a frequent renewal of the commercial catalysts.This results in the introduction of more than tens of new com-mercial catalytic formulations every five years.1,4–6 The catalyststructural properties can also be tuned during process operationthanks to the adjustment of chlorination and hydration regener-ation conditions. Given the specificities of naphtha reforming, abetter description of catalyst structure within process simulatorspaves the way for an optimised and more flexible operation ofreformers. In this study, a focus is made on the description ofthe active phase of a reforming catalyst. Based on a literaturereview, we show that the effect of an active phase formulationchange on reforming selectivity is not fully understood and thatthe structural investigation of the active phase remains challeng-ing. This review does not provide an extensive description ofthe state-of-the-art advancement in structural investigation of thecatalyst nor a complete report of intrinsic naphtha reforming ki-netic modelling methodologies. Here, we propose a pragmaticapproach driven by industrial considerations in order to unravelthe key physico-chemical properties of the catalyst that controlthe product distribution at the process scale. These properties arereferred to as reactivity descriptors. Then, a method is suggestedin order to implement catalytic descriptors within reforming sim-ulators. This review is divided in three parts. Part one explainsthe need to unravel selectivity descriptors in naphtha reforming.Potential descriptors as well as their measurement methods arethen listed in part two. Part three proposes a way to integratethose descriptors into predictive tools and discusses the advan-tages of such an approach in process development.

2 The problem of selectivity control andprediction in naphtha catalytic reforming

Naphtha reforming process operation as well as catalyst develop-ment is mainly about controlling the selectivity of a broad reac-tion network. As shown in Figure 1 and Figure 2, naphtha re-forming consists in converting a hydrocarbon cut with a boilingpoint inferior to 180–200◦C (a naphtha) into a hydrocarbon cutwith roughly the same distribution in carbon atoms but enrichedwith high Research Octane Number (RON) compounds such asaromatics and isoparaffins (the reformate). Nowadays, almost allrefineries in the world are equipped with at least one catalyticnaphtha reforming unit as this process contributes to three cru-cial functions in these plants: (i) producing bases for gasolineproduction (ii) producing aromatic compounds and (iii) gener-ating dihydrogen that can be sent towards hydrogen consumingunits in the refinery.2,7,8

Naphtha reforming chemical transformations can be sorted outin two main categories:- Reactions that are targeted as they permit the increase inoctane number such as naphthenes dehydrogenation and alkanesdehydrocyclisation (strongly endothermic, equilibrated andgenerating H2) or isomerisation of paraffins and naphthenes(equilibrated and slightly exothermic).- Parasite reactions, usually exothermic and thermodynamicallyenhanced. This group includes reactions such as hydrogenolysis,

hydrocracking, toluene dismutation or dealkylation of branchedaromatics.

Figure 2 illustrates the variety of reactions expected in n-heptane reforming. Reactions leading to aromatics are stronglyendothermic and equilibrated, and they are associated to a risein octane number. Isomerisation reactions are slightly exother-mic and also afford the rise in octane number. On the contrary,cracking reactions are exothermic. This differences of thermalbehaviour have a strong impact on process operation.

2.1 Catalyst performance requirements

The development of radial bed technologies in the 1970’s allowedfor the reduction of process pressure. Until then, the increase inthe production of aromatics by reformers was inhibited by ther-modynamic limitations. Now that reformers can be operated inoptimal dehydrogenation thermodynamic conditions, the biggesthurdle to higher valuable product yields lies in the design of theactive phase itself. The role of the catalyst is paramount in con-trolling the balance between these two sets of reactions.9 Reform-ing reactions require a bi-functional catalysis through hydrogenand proton transfer reactions. Naphtha reforming catalysts aretherefore usually constituted of a metallic phase (platinum asso-ciated or not with other metals) affording hydrogen transfer re-actions and dispersed at the surface of a mild acidic solid (usuallychlorinated alumina).10,11 A fine tuning of the reaction networkis needed to adjust product distribution to the needs of the re-finery. Some units are optimised to reach the maximal produc-tivity of gasoline bases (C+

5 products) whereas other units areaimed at producing aromatics (which allows lower C+

5 yields andhigher aromatic selectivity). Depending on the targeted selectiv-ity, different commercial active phase formulations are available.Around tens of new industrial reforming catalysts are developedevery five years4, which leads to a broad variety of commerciallyavailable active phase formulations. Catalyst development en-abled a gradual increase of reformate RON and C+

5 yields. De-spite 70 years of continuous optimisation, there is still a tiny butsignificant theoretical performance increase margin. As an exam-ple, today, a reformer can be operated at RON values between90-105, C+

5 yield up to 90%wt and H2 yields of 3-4%wt . The re-maining 6%wt is composed of C1-C2 and C3-C4 undesired prod-ucts. Therefore, the objective is to decrease this 6%wt amount ofby-products. These values typically depend on feedstock compo-sition. This theoretical potential improvement is very significantin naphtha reforming and would arise from the optimisation ofthe catalyst itself. In spite of the achievements in the structuralcharacterisation of the catalyst active phase12, the identificationof selectivity descriptors is still lacking to a rational design of newcatalytic systems. Up to now, conventional screening methodsand trial and error approaches were conducted in naphtha re-forming catalyst optimisation. Those approaches might not bebest suited to meet the current theoretical performance increasemargin.

2 | 1–19Journal Name, [year], [vol.],

AromaticsNaphthenesIsoparaffinsParaffinsNaphthaReformate

Number of carbon atoms

Pro

port

ions

(%

wt)

Fig. 1 Composition of a naphtha cut before and after catalytic reforming.

3 4 5 6 7 8 9 10 110

20

40

60

80

100

120

140

160

Number of carbon atoms

Res

earc

h O

ctan

e N

umbe

r

Paraffins IsoparaffinsNaphthenes Aromatics

Dehydrocyclisation

Dehydrogenation

Cracking

Isomerisation

Fig. 2 RON of different hydrocarbons present in a naphtha cut.

2.2 Kinetic modelling based predictions

Controlling naphtha reforming product distribution goes well be-yond the choice of the fresh catalyst formulation and the optimalthermodynamic conditions. In addition to the transformation ofreagents on active sites, the transport of species within the cat-alyst pores as well as deactivation phenomena have a deep in-fluence on the process. Kinetic models, by giving an insight intothese diverse but entangled phenomena play a key role in theelaboration of better simulation tools that would guide operation.

2.2.1 Diffusion in catalyst pores

When it comes to the description of the influence of textural prop-erties on catalytic performance, the challenges lie on the one handin characterising the actual porous structure of the catalyst andon the other hand in designing a diffusion model that matchesthe level of detail in the description of the catalyst. Once the dif-fusion model established, it is of prime importance to couple itwith the kinetic model in order to predict the influence of textu-ral properties on the overall catalytic performances. The reader isreferred to comprehensive book chapters and reviews regardingtopics such as porous diffusion in naphtha reforming catalysis, itscoupling with full reforming kinetic models as well as naphthareforming catalytic pore network optimisation.13–16

2.2.2 Catalyst deactivation

Reforming catalysts are sensitive to deactivation phenomena suchas support specific surface loss, metallic particles sintering, poi-soning of active phase by impurities (heavy metals, Fe, Si. . . ),acidity loss by dechlorination as well as carbonaceous deposit atthe surface of the catalyst.1 Coke deactivation is the main deacti-vation cause. The kinetics of coke formation and its influence onthe kinetics of other reactions should also be integrated into pro-cess simulators. Dating back from the pioneering work of Myersin this field17, kinetic deactivation models based on coke forma-tion are built on a mechanistic description of site poisoning by

coke deposit. One is referred to the work made by the team ofRumschitzki18–20 or to dedicated book chapters1 for examplesof naphtha reforming models taking into account deactivation bycarbonaceous deposition.

2.2.3 Complex feedstock description

The development of naphtha reforming kinetic models has beendriven by a need for more simulation details as well as more ac-curate reformer performance prediction. Naphtha cuts consist ofseveral thousands of compounds and present a huge composi-tion diversity depending on its origin. Most of proposed kineticschemes rely on the very first metal/acid bi-functional reformingmechanism proposed by Mills in 1953.21

Lumped kinetic models are the most widespread in naphthareforming. The level of sophistication of these models increasedfrom four lumps22 to more than twenty pseudo-compounds23.Such models are compatible with the description of surface ad-sorption and Langmuir-Hinshelwood kinetics24, the inclusion ofdeactivation kinetics25 or the integration of transport modelwithin the pores13. Detailed lumped strategies were found tobe successful approaches for the design of industrial reactors26,27

as well as the description feedstock composition change effects28.

The single event methodology is an example of molecular de-tailed kinetic modelling used in naphtha reforming. Single eventmodels are built on the description of reaction intermediate (car-bocation) reactivity at the surface of the catalyst and compute therate constants of the whole reaction network from the kinetics ofintermediates transformations.29–32 Mixed approaches conciliat-ing the flexibility of a lump model with the level of accuracy ofa single event model were also introduced.33–36 State of the artmethodologies are therefore satisfactory when it comes to predictreformate properties and unit performances with different naph-tha feed compositions.

Journal Name, [year], [vol.],1–19 | 3

-Industrial catalyst n-Simulator suitable for catalyst n

-Industrial catalyst n+1-Simulator suitable for n+1

Catalyst n+1

Pilot plant experimentation

Kinetic parameters fittingKinetic scheme adjustment

Process Development

Fig. 3 Traditional catalytic process development scheme.

2.3 Current limits and challenges in reforming performanceprediction

Naphtha reforming is lacking reliable methodologies to model theissues related to active phase formulation modification. Yet, theseissues are significant for both process operation and reformingdevelopment.

2.3.1 Operation issues linked to performance prediction

The technical and economic requirements of the refinery shouldbe taken into account by the operators when it comes to choosethe nature of the fresh catalyst and to set the operating condi-tions. As long as a recommended fresh commercial catalyst ischarged, corresponding kinetic models are perfectly able to iden-tify a set of operating conditions matching the targeted reformateproduct distribution. However, in order to face deactivation phe-nomena or feed property modifications during the run, operationalso consists in adjusting process and regeneration conditions tomaintain catalyst performance over the time.6,37 Different regen-eration solutions remediate the deactivation issues. Regenerationusually consists in burning the coke deposits, re-dispersing the ac-tive phase, compensating chlorine loss and reducing the catalyst.1

This actually corresponds to a real time active phase formulationadjustment of the loaded catalyst. The amount of added chlo-rine, the introduction of water or the regeneration conditions aretherefore example of manifolds used by the operator to controloverall performance. Similarly, adjusting chlorination or hydra-tion of a reforming catalyst to the nature of a feedstock is a com-mon practise. It should be pointed out that nowadays, practicaland empirical know-how is more valuable than modelling abili-ties when it comes to such issues.

2.3.2 Constraints faced in process development

It has been previously pointed out that the identification of cat-alytic selectivity descriptors is required for the development ofnew catalysts as well as more reliable predicting tools (especiallytools that regeneration operation would benefit from). Anotheraspect is related to the necessity to shorten the time to marketdesign of both catalysts and kinetic models. Reforming catalyticprocess development follows the frame represented in Figure 3and usually consists in two distinct steps. Optimisation of the cat-alyst is first afforded thanks to dedicated methodologies such ascatalyst screening combined with scale up studies. Afterwards,once the final structure of the new catalyst is obtained, specificexperiments are conducted to provide the data required for theelaboration of a suitable kinetic model. Lumped kinetic modelsimplemented in naphtha reforming simulators handle overall ki-

netic parameters such as apparent orders, activation energies andrate constants. These parameters are not often robust to catalystchange and therefore, the fitting procedure is to be repeated eachtime a new catalytic formulation is developed. This makes ki-netic model development a cumbersome step and a bottleneck inthe case of processes known for a high frequency of commercialcatalyst formulation renewal.

3 Research and measurement of naphthareforming catalytic descriptors

Classical reforming catalysts consist of a reduced metallic phase(Pt sub-nanometric particles usually associated with other met-als) that is supported on a weak acidic support (chlorinated γ-Al2O3).1,8 Given the very high dispersion of the metallic phase,its characterisation requires advanced techniques.38,39 Chlorina-tion has deep consequences on active phase properties. First, thisleads to an overall increase of alumina surface acidity. Chlorinealso interacts with the metallic phase. It contributes to the stabil-isation of Pt particles40 and might affect the intrinsic activity ofreduced metal particles in hydro-dehydrogenation.40,41 As a re-sult of reforming catalyst peculiarities, formulation changes mightaffect the properties of both acid and metallic functionalities aswell as their interaction. This section reviews the reactivity asso-ciated at each phase and reviews potential reforming selectivitydescriptors.

3.1 Naphtha reforming principal reaction families

One can distinguish three groups of reforming reactions: thosecatalysed by metal sites, the one occurring on acid sites and thereactions that require an interaction between the two kinds ofsites. The main reforming reactions are listed as follows andsorted out by family as shown in Table 1. The transformationof linear paraffins into aromatics through the formation of naph-thenes plays a key-role in process operation insofar as it leadsto the formation of high value compounds and generates a highquantity of hydrogen. Dehydrocyclisation, that is to say the trans-formation of linear paraffins into naphthenes, is considered asthe first step of aromatisation. N-heptane reforming is a systemthat can be proposed to illustrate paraffin aromatisation transfor-mations. N-heptane dehydrocyclisation mechanisms haven’t beenfully unravelled yet. Three potential mechanisms are advanced,out of which two are believed to require acid and metallic sitecooperation. These pathways are presented hereafter:Bi-functional path with cyclisation on an acid site:One can imagine the formation of an olefin on a metallic site fol-lowed by an isomerisation on a Brønsted acid site and a last stepof dehydrogenation on a metallic site.Bi-functional path with cyclisation on metallic sites:A second bi-functional mechanism was suggested for paraf-fin dehydrocyclisation. It assumes the direct transformationof a paraffin into a five atoms ring naphthene on a singlemetallic site. At that point, a ring expansion catalysed bya Brønsted site would lead to a 6 atoms ring naphthene.Mono-functional dehydrocyclisation:Studies carried on non-acid catalysts suggested that dehydrocy-

4 | 1–19Journal Name, [year], [vol.],

clisation could happen on metallic sites only. For example, thegroup of Davis reported n-heptane dehydrocyclisation activity us-ing Pt/Al2O3-K whereas the team of Arcoya obtained similar re-sults on Pt/BaKL zeolite.42,43 Still, Davis states that dehydrocy-clisation through the mono-functional pathway is hundred timesslower than bi-functional dehydrocyclisation.44

3.2 Acid functionality descriptorsOver the past twenty years, an increasing number of studiesdedicated to the identification of solid acid reactivity descriptorshas been reported. Significant advance in unravelling key struc-ture/activity relationships was achieved for systems of industrialinterest. So far, this advance was limited to systems exhibitingwell defined acids sites on which mechanistic studies can be con-ducted. For example, the combination between kinetic studiesand the computation of site adsorption energies brought conse-quential insight for catalysts such as polyoxometallates51,52 orzeolites53,54. Gamma alumina based supports are also used as in-dustrial acid catalysts but do not range among well-defined acidsites systems.55,56 The alumina surface exhibits a variety of un-coordinated AlIV atoms and hydroxyl groups that are respectivelypotential Lewis and Brønsted acid sites. However, the point ofzero charge of this solid is equal to 8. It means that the acidityof gamma alumina is very low. The fixation of chlorine atomson the γ-Al2O3 surface is a way to enhance the acidity of sur-face HO groups. Chlorine fixation is generally assumed to pro-ceed through the exchange of a Cl atom with a surface hydroxylgroup.40,55 This overall increase in acidity is either explained byelectron withdrawing effect57 or by the disturbance of the H-bondnetwork at the alumina surface58. In naphtha reforming cataly-sis, the amount of Cl is set around 1%wt to provide the desiredmoderate acidity to these systems. Cutting edge characterisationand computation techniques give an atomic insight into chlori-nated gamma alumina structure. What is still poorly understoodis the structural features underlying the moderate acidic reactivityrequired for naphtha reforming reactions.

3.2.1 Chlorinated γ-Al2O3 surface characterisation tech-niques.

The combination between various spectroscopy techniques andmolecular modelling permits an extensive characterisation ofgamma-alumina. These techniques are able to identify and quan-tify each type of hydroxyl surface moieties and uncoordinatedaluminium at the surface of a model reforming catalyst. Infraredspectroscopy is a classical and historical tool used for the charac-terisation of γ-Al2O3 surface hydroxyls. The measurement of theH-O bond elongation frequency for a hydroxyl group is directlycorrelated to the H-O bond strength and therefore to the hydroxylacidic properties. The H-O elongation frequency depends both onthe geometric environment of the hydroxyl group as well as thenature of the Al atoms bearing it and different historical mod-els were built for the assignation of the associated IR peaks.59,60

Ab initio. modelling was later used for the attribution of H-Opeaks obtained in IR spectroscopy of chlorinated gamma aluminasurfaces58. More recently, Batista et al. extended the investiga-tion to crystallite edge sites by proposing a new attribution of 1H

Bulk atoms

AlIV: octaheral uncoordinated site AlIII: tetrahedral uncoordinated site

(100) Surface (110) Surface

Fig. 4 Comparison between (100) and (110) γ-alumina dehydrated struc-tures. 72

NMR hydroxyl surface groups on chlorinated alumina based ona structural Density Functional Theory (DFT).61 Other analyti-cal techniques also proved to be successful in the characterisationof chlorinated-gamma alumina surfaces. 1H NMR, 27Al NMR aswell as bi-dimensional NMR were used to assess the acidity ofprotons borne by HO groups.62–64 The adsorption of some probemolecules can also be suggested to probe the impact of chlori-nation on acidity (e.g. trimethylamine adsorption followed by IRspectroscopy65). Adsorption of trimethylphosphine followed by31P NMR (which requires toxic hazard management) was foundto be very well suited to the study of the chlorination impact forreforming catalysts.66,67

3.2.2 Computation of descriptors by molecular modelling.

As discussed above, accurate structural characterisations of chlo-rinated gamma-alumina relies on DFT based models.68 Exampleof bulk and surface representations shown in Figure 4 and Figure5 are derived from ab initio. calculations according to Krokidismodel.69 Nevertheless, some conclusions based on DFT studies ofγ-alumina surfaces dynamics are debated issues. DFT calculationswere carried out by different groups in order to study the relativestability of gamma alumina crystallite facets. Results obtainedby Raybaud and Sautet indicate a preferential stability of (100)facets compared to (110) and (111) facets.70 On the contrary,the team of Pinto lead similar studies and found that (100) and(111) surfaces show similar energy and are more stable than the(110) facet.71 Molecular modelling of chlorinated gamma alu-mina surfaces is not only a powerful tool to provide their detailedstructural representation, but it is also a way to study the dy-namics of these surfaces with regards to variations in operatingconditions. Molecular modelling is a way to study in details thesubstitution of hydroxyl groups by chlorine atoms on γ-Al2O3 sur-faces and its consequences on the acidity of HO groups. Thesecalculations give access to the free enthalpy associated to theseexchange reactions (endothermic on the whole).58,71 Carrying onthese calculations at different temperatures and HCl partial pres-sures conditions helps determining the most stable structure indifferent conditions. This leads to the drawing of surface energystate diagrams. Figure 6 is an example of chlorinated γ-Al2O3

surface diagram obtained thanks to DFT studies led by the teamof Raybaud and Sautet. These results are consistent with a well-known observation in naphtha catalytic reforming which is thestrong interaction between HCl and H2O partial pressures andthe acido-basic properties of the surface.

Journal Name, [year], [vol.],1–19 | 5

Table 1 Diversity of the catalytic naphtha reforming reaction scheme

Single site reactions Comments Studies

Catalysis by metals

Dehydrogenations Fast and strongly endothermic

Unsaturated intermediates likely to be transformed on othersites

45–47

Hydrogenolysis Leads to C1/C2

Direct cyclisation Rare and slow 42,44

Position isomerisation Perhaps anecdotic 48

Acid catalysis

Olefin isomerisation Likely to occur on both Lewis and Brønsted sites 49

Olefin cyclisation(g)(g)

Carried on mild Brønsted sites

Naphthene expansion

(g) (g)

Carried on mild Brønsted sites 50

Cracking Unsaturated intermediates likely to be transformed on othersites

Multiple site reactions

Hydroisomerisation Dehydrogenation of the linear paraffin on a metal site fol-lowed by isomerisation on an acid site. Branched olefin ishydrogenated on a second metal site.

Hydrocracking(g)

+ Leads to C3/C4

Aromatisation Several possible pathways. Requires at least one metallic site.

Coking(g)

(s)

n. + H2(g)

The formation of coke can also be described as a catalyticreaction requiring catalysis by acid and metal sites

6 | 1–19Journal Name, [year], [vol.],

800

1200

1000

600

400

Tem

pera

ture

(K

)

log(PHCl/P°)-3 -2 -1 0 1 2 3

HCl and H2O desorption

Cl-OH substitutionHCl desorption

Cl-OH substitutionH2O adsorption

OH-Cl substitutionHCl adsorption

H2O adsorptionCl-OH substitution

OH-Cl substitutionH2O desorption

HCl and H2O adsorption

Initial conditions: PHCl=0.016bar, T=800K, PH2O=0.001bar

HCl adsorptionOH-Cl substitution

iso-coverage

Fig. 5 Crystallographic γ-alumina bulk structure obtained by ab initio.calculations according to Krokidis et al. 69 The repartition between AlVIand AlIV atoms differs from what is expected for a pure spinel structure.

(100)(110)

(111)

x

y

z

fcc node positionsTetrahedral siteOctahedral site

Oxygen

Aluminium

Al IV (~25%)

Al VI (~75%)

O

Krokidis bulk model for

gamma alumina

Spinel structure cristallographic plans

Miller indexation

Fig. 6 Surface phase diagram obtained by ab initio. calculations. (100) γ-alumina-Cl surface evolution mechanisms in function of the temperatureand the HCl partial pressure. Water partial pressure was fixed at 0.001bar. 73

3.2.3 Probing acidity by model reactions.

The interpretation of data acquired by a catalytic test involvingacid catalysed reactions is also a way to study the properties ofacid sites (provided the reaction mechanism is well known andthe data are significant). One advantage presented by these meth-ods is that the investigation of the catalyst is made in conditionsthat are close to the industrial ones. These kinds of chemical reac-tions are usually referred to as “model reactions”. An ideal modelreaction satisfies the following criteria: (i) being unique (ii) hav-ing an unravelled mechanism (iii) involving chemical species thatcan be identified by routine analytic techniques.

In acid catalysis, the kinetics of the transformation depend onsite nature, strength, density and reaction temperature. Apply-ing these principles, Bourdillon and Gueguen managed to iden-tify the distribution in strength and in density of the acid sitesof a HY zeolite.49 Seven model reactions were used, each oneprobing a specific range of acid strength. These reactions canbe sorted out by increasing required acid site strength as fol-lows: 3,3-dimethylbutene isomerisation, isomerisation (and dis-mutation) of 1,2,4-trimethylbenzene, isomerisation (and dismu-tation) of orthoxylene, 2,2,4-trimethylpentane cracking, isomeri-sation (and cracking) of 2,4-dimethylpentane, isomerisation (andcracking) of 2-methylpentane as well as isomerisation and crack-ing of n-hexane. In order to quantify the strength of the acidBrønsted sites that are probed, authors use minimal pyridine des-orption temperature for which a significant activity is detected(noted TD). This value is calculated in each case by the follow-ing procedure: the whole catalytic surface site is first poisoned bypyridine at low temperature. At that point, the middle is heatedup little by little, which leads to a progressive desorption of pyri-dine from the weakest to the strongest Brønsted sites. Finally, ateach temperature, one calculates the ratio between the measuredactivity and the activity that would be measured in the same con-ditions but in the absence of pyridine. Figure 7 represents theresults obtained by Bourdillon and Gueguen and shows the reac-tions considered. For each reaction, a kinetic law is fitted and isused to extrapolate an activity at the same temperature (350◦C).The authors are then able to associate a rate constant obtained atthe same temperature for each reaction and use it as descriptorof the number of active sites for a given range of site strength.

The work of Bourdillon and Gueguen shows that a set of modelreactions can be used to characterise a variety of acid sites on agiven catalyst. Reciprocally, one can use a single reaction, target-ing a given range of acid sites, in order to compare different cata-lysts. This second approach was tested by Morra et al. to comparethe density in mild acid sites for a set of different metallic phasessupported either on δ or αθ alumina.73,74 3,3-dimethylbutene(3,3-DMB) isomerisation is taken as the model acid reaction. Thereagent structure is not suitable for a π-allylic rearrangement.Thus, the reaction entails a proton exchange. Pines suggested twoisomerisation mechanisms (shown in Figure 8) for this reactionaccording to the Lewis or Brønsted nature of the active site.75 Yet,it is generally admitted that this reaction is carried on with mildBrønsted sites only. The authors used 3,3-dimethylbutene (3,3-DMB) as a single model reaction, which is known to target acid

Journal Name, [year], [vol.],1–19 | 7

3,3-DMB

1,2,4-tmb

2,2,4-tmC5

o-Xylene

2,4-dmC5

2mC5

n-C6AR

T(°C)

1

0.5

300 400 500TD°

Fig. 7 Use of model reactions to probe and characterise the range ofacidity of the different surface sites of a zeolite HY. AR : reaction rate inpresence of pyridine divided by its value without pyridine. 49

slow step

3,3-DMB

2,3-DMB1

2,3-DMB2

Fig. 8 3,3-DMB isomerisation mechanism according to Pines. 75

sites (TD between 200 and 400◦C). This reaction is performed ona set of catalysts and a ranking is based on the fitted first orderrate constant. Figure 9 shows the results obtained by the authors.In this case, the first order isomerisation rate constant is directlytaken as a reactivity descriptor insofar as it enables to rank anddistinguish the activity of different catalysts. On can note that3,3-DMB isomerisation involves a limited number of species thatcan easily be distinguished by gas chromatography, which makethis reaction very convenient to handle.

Particular attention should be paid to dichlorinated aluminiumatoms at the γ-Al2O3 surface. As shown in Figure 10, these groupsact as strong Lewis acid site that can be turned into superacidBrønsted site under high HCl partial pressures.76–78 The group ofPrimet associates these strong Lewis sites to a pyridine tempera-

APdMo1.06APt

ANiMn0.44S

A

BPdMo1.06

BPtB

BPtSn0.8S

BRhMn0.36S

0.01 0.1 1 10 100

Ainitial(mmol.h-1.gcata

-1)

Fig. 9 Acidity scale relying on the initial activity in 3,3-Dimethylbuteneisomerisation measured at 260◦C for two different solids. 74 Solid A: δ -alumina. Solid B: αθ -alumina.

AlOAl

Cl

Cl

+ HCl = AlOAl

Cl

Cl

Cl

-

+ H+

Lewis strong site Brønsted strong site

Fig. 10 Strong Lewis and Brønsted sites associated to dichlorinated Alsurface species. 42,76

ture desorption higher than 623K. The formation of dichlorinatedaluminium surface atoms was found to be strongly dependent onthe method of chlorination. As an example, the use of perchlo-rates (CCl4) is known to favour the fixation of two Cl on a singlesurface aluminium through the formation of an AlCl3s intermedi-ate. N-butane isomerisation is a model reaction used to probe thecatalytic activity arising from dichlorinated aluminium atoms.

Depending on the study, Lewis or Brønsted strong activities areconsidered to be at stake in n-butane isomerisation (contrary toother acid sites at the surface of a chlorinated γ-Al2O3). Primetet al. used this reaction to investigate the presence of such siteson chlorinated alumina as well as naphtha reforming-like catalyst(Pt/γ-Al2O3-Cl). Though chlorination of industrial naphtha re-forming catalysts is usually afforded by oxy-chlorination (whichis less likely to form dichlorinated species than the method usingCCl4), over-hydrocracking activity is sometimes suspected to arisefrom a tiny amount of dichlorinated atoms at the catalyst surface.Therefore, the injection of small and controlled quantities of wa-ter to remove these strong acid sites takes part to the operatingprocedures of a naphtha reforming unit.

3.3 Metallic functionality descriptors

Naphtha reforming catalysts exhibit sub-nanometric platinumparticles (usually associated to other metals such as Sn or Re) atthe surface of chlorinated γ-Al2O3. Under reductant conditions,this metallic phase is able to perform hydro/dehydrogenation andhydrogenolysis reactions. Platinum being expansive, high disper-sion levels guarantee a high active metallic surface at low Pt con-centrations (around 0.3%wt for industrial catalyst). These plat-inum particles present a very small diameter, usually lower thanone nanometre, which makes the direct observation and study ofthese particles difficult owing to the limits of resolution of mostanalytical devices. The identification of descriptors should alsotake into account the role of chlorine on platinum nanoparticles.The study of the metallic phase is even more complex in the caseof multi-metallic systems such as PtSn/γ-Al2O3-Cl that are usedwith Continuous Catalytic Recycling (CCR) reforming technolo-gies.

3.3.1 Pt only catalysts.

Even though the share of Pt only catalysts on the current reform-ing market is shrinking, these systems remain favoured for ad-vance structural studies.12 Routine electronic microscopy is notsuitable with an easy observation of sub-nanometric structures.Thus, more sophisticated microscopy techniques are required for

8 | 1–19Journal Name, [year], [vol.],

Fig. 11 Two Transmission Electronic Microscopy observation modes ofthe same photograph representing the surface of Pt(3%wt )/γ-Al2O3 cat-alyst. 39 Photograph A was provided by Z contrast techniques. Whitezones are associated by authors with platinum particles. Photograph Bwas taken by “Bright Field” methods and gives an atomic resolution ofthe sample. According to the authors, circled structures correspond tosub-nanometric particles of 1 to 4 Pt atoms.

a direct observation of metallic nanoparticles. High ResolutionTransmission Electronic Microscopy (HR-TEM) as well as HighAngle Annular Dark Field Scanning Transmission Electron Mi-croscopy (HAADF-STEM) are examples of techniques that can beused for an accurate imaging of metallic particles. The direct ob-servation of these systems is complicated by the high intensity ofthe electron beam that can cause particles sintering during TEMimaging.

Another issue that can be faced is the strong interaction be-tween the particles and the support that can complicate the in-terpretation of microscopy pictures. The group of Frenkel con-sidered a model of platinum particles constituted by 15±9 Ptatoms to interpret naphtha reforming catalyst MET pictures show-ing particles with a diameter ranging between 0.7 and 1.1 nm.39HAADF- STEM allows for an observation of even smaller struc-tures. The team of Nellist was able to identify Pt structuresformed by 1 to 3 atoms as shown in Figure 11.38 ab initio. mod-elling of supported Pt particles and the role of chlorine atomson the surface properties of reforming like catalysts is also car-ried on. A systematic study led by Mager-Maury et al. consistedin predicting the most stable configuration of Pt13/γ-Al2O3 sys-tems at different temperatures and H2 partial pressures. Figure12 corresponds to the surface thermodynamic diagram providedby this study.79 The same team also simulated the influence ofsurface chlorination on the given system. One major conclu-sion was that in reforming conditions at chlorine rates close tothe value of the industrial catalysts, the migration of chlorineand hydrogen atoms from the support to the metallic particlesis likely to occur, thus leading to a stronger interaction betweenPt particles and gamma alumina. Hence, chlorination of the cat-alysts participates to the stabilisation of sub-nanometric particlesat the surface of naphtha reforming catalysts.40 X-ray AbsorptionSpectroscopy (XAS), when coupled with ab initio. calculationsis also advantageous for the characterisation of Pt/γ-Al2O3 metalphase.80 XAS analyses can be carried on under a hydrogen at-mosphere. They are then suited for in situ. reductant conditions

experiments. An XAS experiments provides both XANES (X-rayAdsorption Near Edge Structure) and EXAFS (Extended X-ray Ad-sorption Fine Structure) spectra. The nature and the oxidationnumber of elements present on the sample can be inferred fromXANES data whereas the EXAFS spectrum gives indications aboutthe interaction between a metal and its neighbour atoms. EXAFSstudy give access to the number of metal atoms within a metallicparticle or average inter-atomic distances.81–84 To sum up, thecombination between high resolution microscopy, DFT calcula-tions and XAS spectroscopy agrees with the following observa-tions. Reforming platinum particles appear as systems comprisingbetween 10 to 13 atoms with preferential flat or bi-planar mor-phology. Size and shape rearrangement are to be expected fromH2 surface partial pressure shifts. Chlorine contributes to the sta-bilisation of both particles and single-atoms. Therefore, particlesurface density, particle size and shape, Pt-Pt distances as well asparticles/single-atom ratios are potential descriptors that can beprovided by state-of-the-art techniques.

3.3.2 PtSn only catalysts.

The introduction of tin into a naphtha reforming catalyst compli-cates its structure and the structural study of this kind of cata-lyst is less documented. Owing to the electronegativity differencebetween Pt and Sn, Pt catalytic activity might be influenced bycharge transfer. Understanding the oxidation state of Sn atomsas well as their interaction with Pt is required for the identifica-tion of CCR catalyst descriptors. This can be achieved throughdifferent experimental techniques.

Temperature Programmed Reduction data usually agree witha strong interaction between Sn and the γ-Al2O3 surface. Ac-cording to Carvalho, around 80% of Sn atoms are linked to sup-port oxygen atoms and an irreversible reduction of these atoms inSn(II) occurs during a TPR analysis.85 Burch et al. realised TPRexperiment on γ-alumina successively impregnated with Pt, Snand Re.86,87 The authors confirm that most of the tin atoms arepresent as Sn(II). According to them, a fraction of the tin atomsis irreversibly linked to the support while some Sn atoms couldform a solid solution with Pt particles Mö ssbauer spectroscopy isa useful tool to determine the oxidation number but is restrictedto certain elements such as Fe or Sn. This technique was appliedby Olivier-Fourcade et al. to the study of PtSn/γ-Al2O3 systemsconsidering different methods of tin introduction.88–90 Whateverthe preparation method, authors state that Pt3Sn and PtSn alloysare present before reduction. After reduction by H2, oxometallicPt-Sn bonds are detected in small quantities and indicate that Snis partially linked to O atoms from the gamma alumina surface.

XANES experiments were applied by Gorczyca et al. to thestudy of the metallic phase of PtSn CCR reforming catalysts.91,92

Ab initio. calculations were used for the interpretation of XANESresults. It was found that the Pt10Sn3 stoichiometry was associ-ated to the best description of XANES spectra and was thereforethe most likely. According to these results H2 would be preferen-tially adsorbed on Pt instead of Sn. The introduction of Sn withinPt particles was found to be favoured far from the γ-Al2O3 sur-face and would result in an electron enrichment of the Pt atomsinside the particle. Nevertheless, the authors were not able to de-

Journal Name, [year], [vol.],1–19 | 9

H2/HC=100 H/cluster~10-26

H2/HC=1-10 H/cluster~6-18

H2/HC=0.01 H/cluster~4

log(

PH

2/P

°)

Temperature (K)

Fig. 12 Thermodynamic state diagram computed by DFT for Pt13 particles supported on a non-chlorinated (100) γ-Al2O3 surface. The number ofhydrogen atoms adsorbed at the surface of the particle is given in function of the temperature and the PH2/PC2H6 ratio. 79

Pt

H+

Pt Pt

Pt

Pt

+

A7 C3/C4

iP7N7C7-

H++

P7

Fig. 13 Bi-functional n-heptane reforming pathways showing competitionbetween different site transformations.

termine the relative distribution between SnO and SnO2 oxideson the basis of this XANES experiment.

3.3.3 Model reactions.

As for acid catalysis, model reactions can be used to generatecatalytic descriptors of the metallic phase. Naphthenes dehydro-genation reactions satisfy several requirements to be consideredas model reactions. First, these reactions are important in naph-tha reforming. They greatly contribute to the increase in octanenumber as well as to hydrogen production and they are respon-sible for the overall endothermic nature of the process. Second,their selectivity is high under reforming conditions. Third, cy-clohexane and methylcyclohexane experimental dehydrogenationinto aromatics is well documented.93–95 However, naphthene de-hydrogenations are strongly endothermic and fast compared toother naphtha reforming reactions, which makes the experimen-tal study of their intrinsic kinetics difficult. In spite of beingwell studied, the precise mechanism of this reaction on the sur-face of an industrial reforming catalyst has not been totally un-ravelled.96 In the case of methylcyclohexane aromatisation totoluene, different mechanistic pathways could be suggested. Re-verse Horiuti-Polanyi, Langmuir-Hinshelwood-Hougen-Watson or

Eley-Rideal are all possible mechanisms relying on different hy-pothesis regarding reactant adsorption. Then, the expression ofthe corresponding micro-kinetic model depends on the assumedmechanism.32,46 In case naphthene dehydrogenation kinetic pa-rameters are selected as metallic activity descriptors, one shouldpay a very close attention to the expression of the kinetic law thatis used.

3.4 Bi-functional descriptors

The association of a hydro/dehydrogenation function with a pro-ton transfer activity is required for various refining processessuch as paraffin hydroisomerisation or heavy feedstock hydroc-racking. These catalysts associate a metallic phase (affording hy-dro/dehydrogenations) and acid sites located on the support (of-ten zeolites or SiO2/Al2O3). These catalysts are often designed,compared and optimised on the basis of criteria such as site bal-ance,97,98 site intimacy99,100 or support porous structure101.

Direct extrapolation of these concepts to reforming catalysts isnot straightforward. First, γ-Al2O3 support surface is less organ-ised than what is observed with zeolites.13 Second, as previouslymentioned, naphtha reforming transformations involve a weakacidity and are therefore predominantly afforded on the metallicphase.102,103 According to Weitkamp classification103,104, naph-tha reforming catalysts can be described as (M+,A-), whichmeans that the activity of the metallic phase is high compared tothe one of the acid sites. As a consequence, cracked products areexcepted to result from hydrogenolysis reactions at the surface ofthe catalyst. In this same formalism, hydrocracking and hydroi-somerisation catalyst are rather noted (M+,A+) or (M-,A+).

3.4.1 Describing reaction competition.

In spite of the significant variety of metal/acid bi-functional re-fining catalysts, there is a limited amount of possible transforma-tions on each kind of sites (see sections above). The succession of

10 | 1–19Journal Name, [year], [vol.],

A

AS1 BS1

B

BS2 CS2

C

Catalyst surface

Site diffusion from

S1 to S2

Fig. 14 Bi-functional transformation mechanism at the surface of a catalystaccording to the Weisz formalism. 99

CA Bk1

k-1

k2

C

A B

D

Site1

Site2

Site1

Fig. 15 Two examples of bi-functional transformations. The second caseshows the possible interception of a bi-functional reaction by a doublemono-functional transformation.

active phase transformations as well as the competition betweenthe adsorption of different species is responsible for the varietyof catalytic applications. Tuning reaction competition is thereforethe key to selectivity control. Figure 13 indicates that it is a typicalissue encountered in naphtha reforming catalysis, for which theintermediate of the aromatisation pathway could be interceptedby other reactions (cracking, coking, and isomerisation). Inter-mediate species distribution between active sites is often consid-ered as the main phenomenon ruling bi-functional transforma-tions. The traditional and general framework that is employedto describe this issue was developed by Weisz and published in1962. Weisz’s formalism relies on multi-step transformations atthe surface of a heterogeneous catalyst.99 An illustration is givenin Figure 14. The generic transformation of a species A to C isrepresented through successive reactions on distinct surface sites,S1 and S2. This example introduces B as a species generated on asurface site and transformed on another. B can be desorbed andre-adsorbed on the surface.

One purpose of the Weisz study was to determine conditionsfor which the diffusion of the intermediate species from onesite to another can be neglected compared to the intrinsic kinet-ics of the site transformations. The mathematical derivation ofthese conditions leads to the so-called “Weisz intimacy criterion”.Consequently, according to Weisz, the diffusion of B can be ne-glected when the Thiele modulus of the reaction B→C is lowerthan 1, that is to say when the following condition is fulfilled:d[C]dt . 1

[Beq]L2

D < 1. In this expression,[Beq]

refers to the concentra-tion of B in case the equilibrium A = B is reached. L representsthe characteristic length associated to the diffusion of B whereasD corresponds to the diffusion coefficient of B.

The Weisz intimacy criterion is often used in catalyst optimisa-tion. Catalysts whose acid and metallic sites are close enough toguarantee the satisfaction of the Weisz criterion are often targetedin order to optimise the activity of the catalyst. Nevertheless, theWeisz formalism relies on strong assumptions. It supposes a ho-mogeneous distribution of sites, which is not necessarily guaran-teed with systems such as naphtha reforming catalysts. Moreover,it is not always easy to determine an average distance betweensites as well as a characteristic length of diffusion. In absenceof detailed information about the reaction mechanism, it can also

be difficult to determine if the intermediary actually diffuses fromone site to another (as in the Weisz model), or if all the transfor-mations occur in adsorbed phase.

Tuning of intermediary species diffusion from one site to an-other does not only matters in order to optimise catalytic activity,but has also consequences on selectivity. Let it consider that theintermediary B also reacts on site A. In this situation, one has toface the issue presented in Figure 15. Two competitive reactionscan occur provided B is re-adsorbed on S1 or on S2. One can con-clude, within the Weisz framework, that the ratio between S1-S1and S1-S2 distances controls the probability of B to be re-adsorbedon S1 or S2 and as a consequence, to control the selectivity of theoverall transformation.

3.4.2 Experimental research of catalytic descriptors on bi-functional catalysts.

Within the Weisz formalism, inter-site distance is expected to bea relevant activity descriptor. Depending on the catalytic systemwhich is studied, different authors were able to confirm or not theimportance of site intimacy at the catalytic surface on the activityand selectivity of the reaction.100,105,106 Studying the validity ofWeisz intimacy criterion with these systems is challenging inso-far as it means controlling the nanoscale distance between sites.Fatty acid hydrocracking on Pt supported on an acidic supportswas investigated be Zecevic et al..100 In this case, the authorsmanaged to tune the nanoscale repartition between sites by con-trolling the grafting of Pt particles on a hybrid support (nanoscalemixture between acid HY zeolite and inert alumina). This studyconfirms the strong link between inter-site distance and productselectivity, even below the distance defined by the Weisz inti-macy criterion. Instead of quantifying inter-site distance usingadvanced analytic techniques to estimate Weisz intimacy crite-rion, Mendes et al. have suggested the use of model reactions toprobe the inter-site distance.105

3.4.3 Probing active site interaction in naphtha reforming.

The separate investigation of acid and metallic phase reactivity(thanks to the methods suggested on the previous section) is aprerequisite to the study of site interaction. A couple of issueslinked to mono-functional catalysis are to be addressed first. Forinstance, bi-functional hydrocracking might be ruled out depend-

Journal Name, [year], [vol.],1–19 | 11

ing on the ability of the acid of phase to catalyse or not branchedparaffin cracking. As pointed out, direct dehydrocyclisation ofparaffins into aromatics on the metallic phase only has been re-ported in the literature. The rates of mono and bi-functional de-hydrocyclisation paths should therefore be compared. The frac-tion of the acid sites that is actually involved in bi-functionaltransformations is still unclear. For a given metallic site, coopera-tion with acid sites might be limited to neighbouring OH groups.Hence the interest of probing the properties of the acid phase onlyin presence or not of the metallic phase.

To our knowledge, experimental investigation of Weisz crite-rion relevance in naphtha reforming chemistry hasn’t been re-ported in the literature yet. Though technically challenging, tun-ing the location of active sites at the crystallite scale thanks todedicated preparation techniques is possible. Then, carrying onreactions that target both acid and metallic sites can be suggestedto test Weisz’s approach in naphtha reforming catalysis. If not,inter-site distance might not be a relevant bi-functional descrip-tor contrary to the overall balance between acid and metallic sitesor to the intrinsic properties of individual sites.

Examples of naphtha reforming model reactions are comparedin Table 2. N-heptane reforming is a common simplified systemwhich is used to assess the reactivity of a naphtha reforming cata-lyst. First, n-heptane is one of the main components of a naphthaand represents between 5 and 15%wt of the feedstock (C7 cut pro-portion being around 35-40%wt). Moreover, n-heptane is likely toundergo most of reforming transformations (see Figures 2 and13), including its aromatisation into toluene which is associatedto a high increase in RON. Most of model reactions mentioned inTable 2 are actually encountered in n-heptane reforming. One canalso point out that n-heptane reforming presents a good compro-mise between the diversity of its reaction network and the num-ber of parameters required in order to build a detailed kineticmodel.

4 Integration of active phase descriptorswithin performance predictive tools

The following discussion proposes an approach to integrate in-trinsic reactivity descriptors into the overall naphtha reformingprocess development scheme. The aim of this methodology is toanswer the need for a better prediction of active phase formula-tion change in current reformer simulators stressed out in Sectionone.

4.1 Identifying active phase descriptors inlets

One of the main challenges encountered in building descriptorbased predicting tools lies in the correlation between numerousand diverse phenomena. First, transport limitation and cata-lyst deactivation interfere with the active phase transformations.However, as summed up in Table 3, the corresponding descrip-tors are distinct. Second, the intrinsic reactivity of the catalystitself reflects the contribution of various active sites transforma-tions. This problem is precisely faced with conventional naphthareforming catalyst screening techniques that give few clues aboutthe intermingling between metal/acid site interaction and coke

deposition.

This hurdle can be lowered through the choice of an adequatetesting methodology. The aim is to acquire experimental reac-tion data that are sensitive to active phase descriptor variations.Dedicated experimental methodologies address this point. Thechoice of the reactor system can be adapted to lower the impactof heat and mass transfer. Testing powders in a miniature isother-mal reactor is an example of conventional methodology suitedfor intrinsic kinetics studies. Using a set of model reactions (see2), preferably not prone to coke formation, is also recommendedto probe the reactivity of each kind of sites separately and in in-teraction. Therefore, the kinetic parameters of model reactionsobtained in these specific testing conditions are considered as thefirst gate into which reactivity descriptors can be integrated (seeFigure ??).

4.2 Model reaction descriptor based models

A quantitative expression of model reaction intrinsic kinetic pa-rameters in function of descriptors is sought in the proposedmethod. Dedicated statistical correlation tools exist and couldbe applied to the identification of the descriptors that are actuallycorrelated to the targeted parameters. The Quantitative StructureActivity Relationship (QSAR) formalism has already been appliedto the design of heterogeneous catalytic descriptor libraries115

and to the elaboration of statistical predictive models in bothacid74 and bi-functional catalysis116. Figure 17 illustrates a fourstep approach aimed at building descriptor based models. A set ofcatalysts presenting a diversity of formulations is first gathered.It is then tested on a target reaction. A set of kinetic parame-ters corresponding to the same lumped model is then fitted foreach catalyst. Given the variety of active phase descriptors andpossible model reactions, this approach goes along with a signif-icant amount of experiments. Acquisition of kinetic informationwould benefit from parallelisation and miniaturisation of experi-ments provided by high throughput experimentation techniques.This reactor configuration also allows for the operation of reactorin isothermal mode and closer to ideal hydrodynamics, which isa valuable asset for kinetic studies. The high amount of formula-tions that are to be tested cannot be reasonably fully characterisedby the set of sophisticated techniques (XANES, HR-TEM. . . ) re-quired for a thorough structural investigation. Industrial consid-erations would insist on descriptors that are easily measurableand controllable thanks to a preparation method (H2/O2 titration,XRF. . . ). This does not mean that the state-of-the art structuralinvestigation of reforming catalyst is useless. On the contrary,recent advance provide atomic scale crystallite models that canbe used from the interpretation of routine analysis. For exam-ple, thermodynamic state of the art DFT models can be suggestedto deduce chlorine or hydroxyl crystallite repartition from overallchlorine content (measurable by XRF) and crystallite shape (de-duced from TEM observation on the alumina precursor).

12 | 1–19Journal Name, [year], [vol.],

Table 2 Example of reactions that could be used to probe surface properties of a naphtha reforming catalyst

Reaction Competitive reactions Comments Studies

Acid catalysis

+(g)(g) (g)

None (under mild conditions) Targets mild Brønsted sites 49

+(g)(g) (g)

None (under mild conditions) Isomers ratio is a function of acid strength and targets mildBrønsted sites

107–109

(g) (g)

Cracking Targets strong acid sites (dichlorinated Al species) 77,78

(g)

+

+ +

(g) (g)

(g) (g)

Cracking (and dehydrogenation on metal sites) Poor selectivity in presence of metal sites 50

Catalysis by metals

Cracking (severe conditions) and contraction (on acid sites) Well studied, kinetic studies available but mechanism still de-bated

45–47

+3H2(g) (g) (g)

Ring expansion and cracking (under severe conditions) Reactions sensitive to deactivation 110

(g)(g)

Isomerisation and cracking Poor selectivity in presence of acid sites 111

Bi-functional catalysis

Complex reactive scheme Unknown mechanism 20,44,112

+3H2(g) (g) (g)

Complex reactive scheme Well studied 44,113,114

(g)

+ +

(g) (g) (g)

Cracking under severe conditions Does not represent the overall diversity of reforming reac-tions

115,116

Journal Name, [year], [vol.],1–19 | 13

Issues

Catalyst characterisation

Simulator

Suitable for a reference catalyst and a set of feedstocks

Reactor model

Pore diffusion model= f(textural descriptors)

Pore plugging model=f(textural descriptors,deactivation descriptors)

Intrinsic kinetics scheme

Deactivation model=f(deactivation descriptors)

Coking

Dechlorination

mono-functional reaction = f(active phase descriptor)

bi-functional descriptor = f(active phase descriptors)

Kinetic model

Next catalyst generation property adjustment

Ex-regenerator catalyst performance prediction

Unconventional catalyst performance prediction

Active phase descriptors

Textural descriptors

Deactivation descriptors

Fig. 16 Different descriptors inlets into complete reforming simulators.

Table 3 Examples of parameters suggested to describe the performances of naphtha reforming catalysts

Issues Possible descriptors Quantitative integration

Textural properties Modelling intra-particular diffusion

Specific surface (SBET)Porous volume (N2 adsorption isotherm)Crystallite size (TEM)Porosity (ε ,Hg porosimetry) Tortuosity (τ)Fractal dimension (N2 adsorption isotherm)

Enables mass balance in a grainDe f f = f (ε,τ)(effective diffusion coefficient)φ = L

√( k

De f f) (Thiele modulus)

Acid functionalityDetermining the nature and the strength of an acid siteAssigning a type of reactions to a range of acid sitesQuantifying the acid sites

%wtCl (FX analysis)ν-OH (FTIR)covering rates given by DFT calculation(ΘOH ,ΘCl )

TOF = f (nbsites) (Turn Over Frequency)(k,Ea,n) (kinetic parameters of acid model reactions)

Metallic functionalityIdentifying the metallic sites and their structureQuantifying the metallic sitesAssessing hydrogen transfer efficiency

%wtPt (FX)Metal dispersion (H2 chemisorption)Singlet atom/Particle ratioOxidation number (XANES)Shape of nanometric particles (HAADF-TEM/DFT)Size of sub-nanometric particles (HAADF-TEM)ΘH (covering rate from DFT calculations)

TOF = f (nbsites) (Turn Over Frequency)(k,Ea,n) (kinetic parameters of acid model reactions)

Bi-functional catalysis

Determining the interaction andthe proximity between acid and metallic sitesIdentifying the fraction of acid siteactually involved in bi-functional transformations

Pt/H+ proximity (inferred from 1H 2DNMR+EXAFS+HAADF-TEM+DFT calculations)Number of OH at the Pt neighbouringAcid/Metal overall site ratio

(k,Ea,n) (bi-functional reaction)

Coking

Estimating catalytic deactivation over the timeEstimating the amount of coke producedin function of the operating conditionsAssessing the ability to regenerate the catalystin function of thenature of the carbonaceous deposit

%wtCl (Overall amount ofcoke from elemental analysis or TPO)Tc (temperature of combustionobtained from TPO)Deposit crystallographic structure (XRD, TEM)H/C coke ratio

Deactivation function parameters

14 | 1–19Journal Name, [year], [vol.],

Statistical correlations

Sets of kinetic

parameters(k,Ea,n)

1

2

Set of catalystformulations

Catalyticdescriptors

Descriptor based model: (k,Ea,n) = f(descriptors)

3

4

Characterisations,model reactions,

etc

Catalytic tests

Kinetic modelling

Fig. 17 Method suggested to build descriptor based kinetic modelsthanks to the identification of statistical correlations between kinetic pa-rameters and descriptors.

4.3 Extrapolation of naphtha reforming kinetic parametersfrom model reaction testing

The nature of the kinetic models fitted on model reaction datashould be discussed. The use of model reactions allows for the useof detailed kinetic models based on a mechanistic description ofthe transformation. One can also suggest simpler rate expressions(for instance power laws) that will be comparable to the actualexpressions handled in most of naphtha reforming lumped kineticmodels. Integrating catalytic descriptors into simple power lawsseems to bring less information on mechanisms but presents theadvantage of handling a limited number of parameters. As illus-trated in Figure 18, this also permits an easier extrapolation ofparameters fitted on a model reaction towards parameters associ-ated to similar reactions in a full lumped reforming model.

When it comes to adjust a kinetic model to a new catalyst for-mulation, several issues encountered in the parameter estimationprocedure can be distinguished. What are the reactions that arethe most affected by a given formulation change? Which initialvalues might be reasonably suggested for the initialisation of theoptimisation procedure? Both question are hard to solve in theconventional process development scheme (see Figure 3). Di-rect extrapolation of values provided by model reaction descriptorbased models could therefore lead to a valuable fastening of fit-ting procedures. For instance, if a formulation change is found toaffect the reactivity of metallic phase mono-functional reactionsonly, this information can be directly applied to simplify the fittingprocedure. The guess of initial parameters is equally a valuableasset in order to choose between several sets of parameters ob-tained by optimisation procedures and to deal with the problemof local extrema.

Despite the tremendous amount of species in a naphtha cut,reactions can be gathered in a limited number of families. Thisobservation paves the way for a dramatic reduction of the lib-

New catalysts

Descriptors Kinetic parameters for a model system

Acquisition of descriptors

Descriptor based model

Complete kinetic model

Initial guess of parameters

kinetic studies

parameter optimisation

Final set of parameters

Extrapolation to a family of reactions by LFER

Fig. 18 Use of descriptor based model for a faster kinetic model fittingprocedure.

erty degrees in kinetic modelling. One can legitimately assume acommon dependence of kinetic parameters for a given family ofreactions. For instance, the impact of a formulation change onisomerisation kinetics should be extrapolated between differentkinds of paraffins. This principle has been successfully applied bythe group of Klein on the basis of Linear Free Energy Relationships(LFER).117,118 This methodology enables the author to extrapo-late the kinetics of complex refining process schemes from a lim-ited number a reactions. This method was successfully applied tothe identification of catalytic structural descriptors.33 From thisperspective, a similar approach is suitable to naphtha reformingissues.

4.4 Prospects and benefits of descriptor integration intomodels

The establishment of descriptor based models is particularlypromising with regards to catalyst process development. The-oretically, it could circumvent the difficulties mentioned aboveby shifting from a two phases process development scheme illus-trated in Figure 3 to a continuous and simultaneous developmentof both catalysts and models as shown in Figure 19. This alter-native process improvement methodology consists in coupling thedevelopment of new catalysts with the elaboration of their kineticmodel and presents several advantages. First, the catalytic testingstrategy is more efficient. The usual scheme consists in carryingon two distinct families of tests (screening and kinetic studies).Here, we propose the inclusion of model reaction testing thatbenefit both to the determination of catalyst performance and theacquisition of some kind of kinetic information. De-correlating in-trinsic kinetic studies from deactivation and heat and mass trans-fer thanks to the experimental strategy eases the elaboration ofmodels.

Provided the descriptor based models are predictive, descrip-tor based models can also be used backwards in order to opti-mise catalyst formulation thanks to in silico. screening. Withinthis framework, descriptor based kinetic parameters fitting is acontinuous process and integrates the reaction data obtained allalong the screening steps. This results in a significant speed upof the full process development scheme. The integrated catalyticprocess development methodology which is proposed is thereforeattractive but one can insist on the need in truly reliable and easy-access effective descriptors that can be easily tailored through thecatalyst preparation steps. Otherwise, the strategy presented inFigure 19 might be difficult to execute. Very similar approacheshave already been successfully implemented by several authors

Journal Name, [year], [vol.],1–19 | 15

Industrial catalyst Unknown solids

Obsolete set of kinetic parameters

"Pre-descriptors"

First fit of kinetic parameters

New set of kinetic parameters

Suitable kinetic model

New possible catalysts (hits)

Rough ranking of hits performances

Promising catalysts (leads)

New possible catalysts (hits)

1) Characterisations

2) Catalytic testing

3) In silico screening

4) Catalytic testing

5) Scale up

Scientific breakthrough, new technical constrainsts...

Catalyst development

Fig. 19 Descriptor based approach coupling the development of new catalysts with the elaboration of their associated kinetic models.

when it comes to the modelling of transport phenomena withinheterogeneous porous catalysts and their backward applicationto optimise the textural properties of this solids. For instance,the teams of Szczygieł and Coppens have already achieved meso-porous γ-Al2O3 textural optimisation on the basis of a transportmodel.14,15,119,120 Another example, in the case of zeolites, isprovided by the group of Marin.121 Recently, a comprehensivedescriptor based heterogeneous catalyst development methodol-ogy was proposed by Pirro et al.122 This methodology focuseson kinetic model based catalyst design and reviews the differentstatistic tools that can be used for the identification of catalyticdescriptors before being applied on methane oxidative coupling.Therefore, the descriptor based process development shown inFigure 19 can fully benefit from the discussion made by Pirro andco-workers. The mutual benefit of catalyst and model develop-ment is not afforded in the traditional approach depicted in Fig-ure 3. It is shown here that the interest in handling descriptor

based kinetic models at the process scale can go beyond the op-timisation of new solids. Such an approach also accelerates theadjustment of kinetic models to new catalysts and paves the wayfor a description of the structural evolution of the catalyst duringoperation.

5 ConclusionA couple of naphtha reforming industrial issues are linked to ac-tive phase formulation changes. The choice of the fresh catalystformulation as well as the choice of regeneration parameters areimportant operating issues that call for a better understanding ofthe structure/activity relationship of the catalyst. The elaborationof predicting tools for the next generation of catalysts hampersthe reduction of simulators time to market. This problem alsoasks for the implementation of reactivity descriptors in the designof kinetic models.

This review presents possible descriptors and highlights the

16 | 1–19Journal Name, [year], [vol.],

gaps for which descriptors are currently missing. Recent studiesprovided a deep insight on chlorinated gamma alumina surfacestructure and reforming catalyst nanoparticle properties. How-ever, the physico-chemical parameters controlling the selectivityof reforming bi-functional transformations are still unclear andthe intricate relation between chlorination, metallic phase disper-sion and surface acidity remains to be investigated.

A pragmatic process development methodology involving theuse of descriptor based kinetic models is suggested. This methodrelies on the identification of measurable and quantifiable over-all catalytic descriptors. The addition of model reaction kinetictesting to the process development scheme is advised in order toprobe active phase properties and to get access to the valuable ki-netic information. Descriptor based kinetic modelling approachesrequire a heavy and well settled experimental strategy. However,as discussed here with the example of naphtha reforming, thesemethods appear as a powerful tool to unlock significant issuesencountered with processes for which operating conditions aredictated by catalyst handling.

Conflicts of interestThere are no conflicts to declare.

AcknowledgementsThe authors would like to thank colleagues from IFPEN Catalysis,Biocatalysis and Separation division for fruitful discussions andadvice regarding the application of a descriptor based approachin naphtha reforming.

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