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Poster presentation at the 7th Molecular Quantum Mechanics Symposium, Lugano, Switzgerland, 2-7 June, 2013
Mo/ZSM-5 is a promising catalyst for non-oxidative methane dehydroaromatization with benzene as the main product. Initial Mo oxide species in ZSM-5 are known to convert to carbide or oxycarbide nanoparticles but the structure of these nanoparticles has not been systematically studied. The current study systematically evaluates the structure and activity of Mo carbide nanoparticles as a function of their size and composition
Structure and Catalytic Activity of Zeolite-Supported Molybdenum Carbide Nanoparticles for Methane Conversion
Structure and Catalytic Activity of Zeolite-Supported MolybdenumCarbide Nanoparticles for Methane ConversionGeorge Fitzgerald1, Jie Gao2, Simon Podkolzin21Accelrys, Inc.2Dept. of Chemical Engineering and Materials Science, Stevens Institute of Technology, USAMo/ZSM-5 is a promising catalyst for non-oxidative methane dehydroaromatization with benzene as the main product. Initial Mo oxidespecies in ZSM-5 are known to convert to carbide or oxycarbide nanoparticles but the structure of these nanoparticles has not beensystematically studied. The current study systematically evaluates the structure and activity of Mo carbide nanoparticles as a functionof their size and composition.• Methane conversion over catalytic Mo/ZSM-5, offers apromising approach for converting methane into liquidaromatics6 CH4 → C6H6 + 9 H2• No other reactants needed: ideal for processingstranded natural gas.• Challenges:• Limited conversion and rapid catalyst deactivation• Characterization shows presence of MoCx and/orMoOxCy nanoparticles, but composition & structurenot known• This work investigates the structure of MoCxnanoparticles, develops the reaction mechanism forcatalytic methane activation over these nanoparticles,and establishes relationships between the catalyststructure and its activity.AbstractIntroduction ResultsAnchoring sites for Mo in ZSM-5• H+ prefers C-atom near the anchored Mo• CH3 prefers Mo atom while H prefers neighboring Catom, i.e.,• CH3-H activation takes place across Mo-C bond• Earlier work on MoOxidentified anchoring sites forMo in the ZSM-5 frameworkas Al Lewis acid sites.• Calculated normal modes forMo(=O)2 bonded as shownmatch experimental Ramanbands.MoC Structures• MoCx, MoOy, and MoCxOy are expected to bepresent in the zeolite• We start with MonCx and investigate the effect ofcluster size and stoichiometry• Nanoparticles modelled to date:Mo2C2 Mo2C3 Mo2C4 Mo2C6Mo4C2 Mo4C4 Mo4C6• Determine minimum energy structures of MonCx usingGA search• Full geometry optimization of MonCx in ZSM-5 model• Find minimum energy site of H+ (required forneutrality)• Find minimum energy sites of CH3 and H on MonCx• Determine energy barrier to CH4 → CH3 + H on MonCx• Computational details:• DMol3 DFT calculations in Accelrys Materials Studio• RPBE functional• DNP basis set• Semi-core relativistic pseudopotentials on Mo• QM/MM using MS QMERA (Chemshell)• GULP Universal FF for MM region• Subtractive mechanical embedding schemeComputational ProcedureCatalyst ∆E (kcal/mol)E (barrier)(kcal/mol)Mo2C2 -27 17Mo2C3 -8 34Mo2C4 -10 29Mo2C6 -3 44Mo4C2 -22 18Mo4C4 -36 17Mo4C6 -23 16Summary & Conclusions• A mechanism of methane activation over catalytic MoCnanoparticles was developed for the first time.• In this mechanism, methane is activated by a particularcatalytic site:• Neighboring Mo-C pair of atoms is required.• Active site splits gas-phase methane by stabilizingCH3 on a Mo atom and H on neighboring C atom.• DFT energies indicate this mechanism is viable• Optimized structures of isolated and zeolite-supportedMo2 and Mo4 carbide nanoparticles determined for thefirst time• Higher catalytic activity (lower activation barriers) ispredicted for larger Mo4 compared to Mo2 nanoparticles• Catalytic activity of Mo4 carbides is predicted to bepractically independent of the Mo:C ratio.• Subsequent calculations will investigate effects of largerQM regions in QM/MM models.ResultsEnergies of reaction and energy barriers for CH4 activation overcatalytic MonCx nanoparticles in a 10T zeolite cluster modelQM zeolitemodel size ∆E (kcal/mol)E (barrier)(kcal/mol)10T -5 4322T -25 2638T -25 22Energies of reaction and energy barriers for CH4 activation overcatalytic Mo2C4 nanoparticles in QM/MM zeolite models with variablenumber of QM atoms• Results for Mo2 nanoparticles vary significantly withcomposition, increasing with increasing C content.• Environment of Mo significantly affected by thenumber of bonding C atoms• Results for Mo4 nanoparticles are insensitive tocomposition changes: barrier ~17 kcal/mol• Using QM/MM models yields significantly higherbarrier: ~30 kcal/mol• Preliminary results have not yet converged withrespect to cluster size• Natural gas (mostly methane)is an abundant resource,but• 30-60% of natural gasreserves are classified asstranded• 150 billion m3 of natural gashave been flared or ventedworldwide annually• Use single framework Alsite in 10T cluster• Terminated with H atoms• Frozen O and H atoms• Subsequently use QM/MMperiodic models with QMregions of 10, 22, 38 T-sites2D slice of HOMO throughCH3-H and Mo-C bonds.Bonding character of theorbital changes significantlywith number of C atoms