Methane to Methanol What’s Known and Questions/Challenges

Tobin Marks NAS Workshop March 7-8, 2016

1. Properties of Methane 2. Naïve Generalizations 3. Conventional Approaches to Methanol 4. Homogeneous Catalytic Approaches 5. Immobilized Homogeneous Catalytic Approaches 6. Enzymatic Approaches 7. Research Challenges

Properties of Methane Selective methane activation challenging because of “noble

gas”-like electronic configuration

Large Bond Dissociation Enthalpies

D(CH3–H) = 104 kcal/mol D(CH2–H) = 106 kcal/mol D(CH–H) = 106 kcal/mol D(C–H) = 81 kcal/mol

O2 Oxidation Processes

∆G° kJ/mol

800 (1050)ºC (1) 2 CH4 + O2 →

C2H4 + 2 H2O -307 (-313)

(2) 2 CH4 + ½ O2 → C2H6 + H2O

-117 (-104)

(3) CH4 + ½ O2 → CH3OH

-75.3 (-62.3)

(4) CH4 + 2 O2 → CO2 + 2 H2O

-792 (-797)

(5) C2H4 + 3 O2 → 2 CO2 + 2 H2O

-1294 (-1286)

Thermodynamics of Oxidative Processes

Selectivity Challenges? Thermal Management Challenges?

Gas phase: I.P. = 12.6 eV, P.A. = 5.5 eV

Some Naïve Generalizations Creative Catalytic Chemistry Important but Alone is Insufficient Excellent Engineering is Essential for a Successful Process Issues:

• Thermodynamics • Heat and Mass Transfer Management • Management of Toxic Intermediates and Byproducts • Catalytic Selectivity • Product Separation, Purification • Catalyst Cost & Supply Security • Catalyst Lifetime, Regeneration • Others?

Emerging Tools for Catalyst Discovery, Optimization, Downselection • Operando and Ex-situ Spectroscopy to Probe Catalyst Structure & Dynamics • New Chemical/Analytical Techniques to Probe Mechanism • High Throughput Experimentation for Optimization, Discovery • Materials Science of Catalyst Supports, Plant Construction Materials • Ligand Supply, Design • High-Powered Computation for Both Understanding and Prediction • Others?

Methane → Methanol. Thermodynamic Considerations

Practiced on a huge scale ICI Process

Dream

Current Indirect US MeOH Price ≈ $0.75/Gallon Uses Earth-Abundant Catalysts

Direct Methane to Methanol Conversion. Heterogeneous Catalytic

Casey, P.S. et al Ind. Eng. Chem. 1994, 33, 1120-1130. Hutchings, G.J.; Scurrell, M.S.; Woodhouse, J.R. Chem. Soc. Rev. 1989, 18, 251-283.

Gesser, H. D.; Hunter, N. R.; Prakash, C. B. Chem. Rev. 1985, 85, 235 Lunsford, J. H. Catal. Today 2000, 63, 165 Alvarez-Galvan, M. C.; Mota, N.; Ojeda, M.; Rojas, S.; Navarro, R. M.; Fierro, J. L. G. Catal. Today 2011, 171, 15 Tabata, K.; Teng, Y.; Takemoto, T.; Suzuki, E.; Bañares, M. A.; Peña, M. A.; Fierro, J. L. G. Catal. Rev. 2002, 44, 1 Holmen, A. Catal. Today 2009, 142, 2 Brown, M. J.; Parkyns, N. D. Catal. Today 1991, 8, 305 Zhang, Q.; He, D.; Zhu, Q. J. Nat. Gas Chem. 2003, 12, 81

Many attempts using huge variety of conditions and catalysts Very high dilution Excess methane Short contact times Low temperatures Any selectivity achieved at expense of conversion, yields typically 1-3%

Methanol Synthesis on Cu/ZnO/Al2O3 with H2 + CO + CO2 Both CO and CO2 hydrogenation pathways, depending on rxn conditions Cu active site; rxn temperature = 230 – 280ᵒC; 40-100 atm pressure 99% yield (when recirculating); ~ 25% conversion per pass Simultaneous WGS; Possible ZnO synergistic effects

Potential energy surface for methanol synthesis reactions after fitting of DFT & microkinetic model to experimental data

Agreement between experimental & computed TOF data

Grabow & Mavrikakis ACS Catal., 2011, 1, 365–384 http://bioweb.sungrant.org/Technical/Bioproducts/Bioproducts+from+Syngas/Methanol/Default.htm

This Technology Has Been Heavily Engineered, Refined

Olah, G. A.; Goeppert, A.; Czaun, M.; Mathew, T.; May, R.B.; Prakash, G. K. S. J. Am. Chem. Soc. 2015, 135, 8720–8729. Olah, G. A.; Goeppert, A.; Czaun, M.; Prakash, G. K. S. J. Am. Chem. Soc. 2013, 135, 10030–10031

Single Step Bi-reforming & Oxidative Bi-Reforming of Methane (Natural Gas) with Steam & Carbon Dioxide to

Metgas (CO-2H2) for Methanol Synthesis

NiO/MgO and CoO/MgO catalysts in tubular flow reactor up to 42 atm & 830–910°C. Catalysts for metgas production stable for 100s of hours. No obvious demonstration of MeOH formation in peer-reviewed publication.

Concept:

ARPA-E

Shilov. Homogeneous Catalytic Oxidation of Methane Original. Pt(IV) as oxidant

Shilov discovered that the combination of [PtCl4]2− and [PtCl6]2−, under conditions similar to those of CH4 H/D exchange, oxidizes alkanes to a mixture of products, primarily alcohols and alkyl chlorides

Labinger and Bercaw JOMC, 2015, 793, 47-53; Nature 2002, 417, 507-514.

Bercaw and Labinger:

Shilov Chemistry. Homogeneous Catalytic Oxidation of Methane

Labinger & Bercaw JOMC, 2015,793,47-53

Bercaw & Labinger refinement with Cu(II) oxidant

Original Cycle. Pt(IV) as oxidant

TON ≈ 100 for p-TOSH

Mechanistic Dissection

Electrophilic Methane Functionalization → Methyl Derivatives

Periana, R. A. et al. Science 1998, 280, 560–564; Accts. Chem. Res. 2012, 45, 885-898.

Electrophilic Main Group Metals: Hg (II) & Tl(III) in Strong Acids

• HgSO4 system catalytic in conc. H2SO4

• 85% Selectivity to MeSO4H in 50% yield • 15% selectivity to CO2 • Air regenerates catalyst • Catalyst inhibited by reaction product • MeOH formation requires product • hydrolysis, separation from water

Catalytica system, uses (bipyrimidine)Pt(II) complex as catalyst and fuming sulfuric acid as solvent/oxidant; capable of efficiently functionalizing methane at temperatures ~ 200 °C. The ‘protected’ product is not of direct use and must be separately converted to a more useful compound, such as methanol, using a scheme such as that shown in Fig. 7b. At least at present, such an integrated multistep process seems not economically competitive with the currently used technology, the indirect conversion of methane to methanol via synthesis gas.

Periana, R. A. et al. Science 1998, 280, 560–564; Accts. Chem. Res. 2012, 45, 885-898. Labinger and Bercaw JOMC, 2015, 793, 47-53; Nature 2002, 417, 507-514.

Homogeneous Methane Oxidation by H2SO4/SO3 Mediated by Pt bipyrimidine Complexes (Periana)

Selectivity: 81% at >90% conversion TOF < 10/hr Reactor corrosion issues?

Methane Oxidation by Zeolite-Supported Catalysts M-ZSM-5, M = Fe, Cu; H2O2 Oxidant

Hutchings G.J. et al Angew. Chem. Int. Ed. 2010, 51, 5129-5133; ACS Catal. 2013, 3, 1835-1844.

Aqueous medium, H2O2 oxidant Low temperature (50ᵒC) helps selectivity Both Fe2 and Cu required for max selectivity Closed catalytic cycle Cu suppresses selectivity-lowering ∙OH TOF = 2,200 – 14,000 h-1

Selectivity > 90% ( + CO2)

Computation: Need proximate Fe centers Transient, closely controlled ∙CH3 formation, transfer

HCO2H

MeCOOH MeOH CO2

ZSM-5; No Fe, Cu

Catalyst performance very sensitive to metal loading & location in zeolite

Immobilized Periana Catalysts for Methane Oxidation

Palkovits, R.; Antonietti, M.; Kuhn, P.; Thomas, A.; Schüth, F. Angew. Chem. Int. Ed. 2009, 48, 6909-6912. Palkovits, R.; Schüth, F. et al, Chem. Commun. 2013, 49, 240-242.

Under the same conditions (fuming H2SO4), TON ≈ Periana catalyst MeOH selectivity ≈ 75%; rest is CO2 Catalyst can be multiply recycled

SEM

Pt EDX map

B. N-Doped Carbon Support

MeSO4H TON ≈ 6 - 9x of the system A above; 92 – 95% methyl selectivity, Other product, CO2

Lobster shell

A.

Enzymatic Methane to Methanol Conversion Example: Fe2 Dioxygenases: Methane Monooxygenase Enzyme Structure CH4 Activating Q Site. O-O Cleavage Homolytic?

Banergee, R.; Proshlyakov, Y.; Lipscomb, J.D.; Proshlyakov, D.A. Nature 2015, 518, 431–434 Wang, W.; Liang, A.D.; Lippard, S.J. Acc. Chem. Res., 2015, 48 , 2632–2639.

Overall reaction requires separate pathways to channel 3 reagents

1. Electrons and protons via a three-amino-acid pore adjacent to Fe2 center 2. O2 migrates via hydrophobic cavities 3. Methane reaches active site via hydrophobic channel or linked cavities 4. Above rates closely coupled to avoid unproductive destruction of reductant by oxidant

Multi –Copper Oxidases also Known

ARPA-E

Thoughts. Need Your Input! Amazing how much we have learned about key catalytic mechanisms New tools emerging to screen catalysts, catalytic mechanisms as never before Materials science of heterogeneous catalysts advancing rapidly Computation has made impressive advances in understanding, predicting

Are alternative oxidants such as H2O2, N2O, SO3 realistic? Are alternative products such as CH3SO3H realistic? Are there “softer” oxidants which can replace or “tame” O2 or the above oxidants for greater selectivity? Advances have been made through biomimicry. Can we extend further, remembering that Nature is frequently trying to solve a different problem than we are? Are enzymatic processes for methanol realistic? What about product separation from aqueous solutions? Is the use of noble metal catalysts realistic? Are homogeneous catalysts realistic? What about “single-site” heterogeneous catalysts? Are we using computation in the most effective way? What are we missing? Scientifically? Programmatically?