Poster presentation at the 7th Molecular Quantum Mechanics Symposium, Lugano, Switzgerland, 2-7 June, 2013 Abstract: 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

1. Structure and Catalytic Activity of Zeolite-Supported Molybdenum
Carbide Nanoparticles for Methane Conversion
George Fitzgerald1, Jie Gao2, Simon Podkolzin2
1Accelrys, Inc.
2Dept. of Chemical Engineering and Materials Science, Stevens Institute of Technology, USA
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.
• Methane conversion over catalytic Mo/ZSM-5, offers a
promising approach for converting methane into liquid
aromatics
6 CH4 → C6H6 + 9 H2
• No other reactants needed: ideal for processing
stranded natural gas.
• Challenges:
• Limited conversion and rapid catalyst deactivation
• Characterization shows presence of MoCx and/or
MoOxCy nanoparticles, but composition & structure
not known
• This work investigates the structure of MoCx
nanoparticles, develops the reaction mechanism for
catalytic methane activation over these nanoparticles,
and establishes relationships between the catalyst
structure and its activity.
Abstract
Introduction Results
Anchoring sites for Mo in ZSM-5
• H+ prefers C-atom near the anchored Mo
• CH3 prefers Mo atom while H prefers neighboring C
atom, i.e.,
• CH3-H activation takes place across Mo-C bond
• Earlier work on MoOx
identified anchoring sites for
Mo in the ZSM-5 framework
as Al Lewis acid sites.
• Calculated normal modes for
Mo(=O)2 bonded as shown
match experimental Raman
bands.
MoC Structures
• MoCx, MoOy, and MoCxOy are expected to be
present in the zeolite
• We start with MonCx and investigate the effect of
cluster size and stoichiometry
• Nanoparticles modelled to date:
Mo2C2 Mo2C3 Mo2C4 Mo2C6
Mo4C2 Mo4C4 Mo4C6
• Determine minimum energy structures of MonCx using
GA search
• Full geometry optimization of MonCx in ZSM-5 model
• Find minimum energy site of H+ (required for
neutrality)
• 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 scheme
Computational Procedure
Catalyst ∆E (kcal/mol)
E (barrier)
(kcal/mol)
Mo2C2 -27 17
Mo2C3 -8 34
Mo2C4 -10 29
Mo2C6 -3 44
Mo4C2 -22 18
Mo4C4 -36 17
Mo4C6 -23 16
Summary & Conclusions
• A mechanism of methane activation over catalytic MoC
nanoparticles was developed for the first time.
• In this mechanism, methane is activated by a particular
catalytic site:
• Neighboring Mo-C pair of atoms is required.
• Active site splits gas-phase methane by stabilizing
CH3 on a Mo atom and H on neighboring C atom.
• DFT energies indicate this mechanism is viable
• Optimized structures of isolated and zeolite-supported
Mo2 and Mo4 carbide nanoparticles determined for the
first time
• Higher catalytic activity (lower activation barriers) is
predicted for larger Mo4 compared to Mo2 nanoparticles
• Catalytic activity of Mo4 carbides is predicted to be
practically independent of the Mo:C ratio.
• Subsequent calculations will investigate effects of larger
QM regions in QM/MM models.
Results
Energies of reaction and energy barriers for CH4 activation over
catalytic MonCx nanoparticles in a 10T zeolite cluster model
QM zeolite
model size ∆E (kcal/mol)
E (barrier)
(kcal/mol)
10T -5 43
22T -25 26
38T -25 22
Energies of reaction and energy barriers for CH4 activation over
catalytic Mo2C4 nanoparticles in QM/MM zeolite models with variable
number of QM atoms
• Results for Mo2 nanoparticles vary significantly with
composition, increasing with increasing C content.
• Environment of Mo significantly affected by the
number of bonding C atoms
• Results for Mo4 nanoparticles are insensitive to
composition changes: barrier ~17 kcal/mol
• Using QM/MM models yields significantly higher
barrier: ~30 kcal/mol
• Preliminary results have not yet converged with
respect to cluster size
• Natural gas (mostly methane)
is an abundant resource,
but
• 30-60% of natural gas
reserves are classified as
stranded
• 150 billion m3 of natural gas
have been flared or vented
worldwide annually
• Use single framework Al
site in 10T cluster
• Terminated with H atoms
• Frozen O and H atoms
• Subsequently use QM/MM
periodic models with QM
regions of 10, 22, 38 T-sites
2D slice of HOMO through
CH3-H and Mo-C bonds.
Bonding character of the
orbital changes significantly
with number of C atoms