Structures, Mechanisms, and Kinetics of Ammoxidation and Selective Oxidation of Propane Over the M2 Phase of MoVNbTeO Catalysts

Abstract

We report here first-principles-based predictions of the structures, mechanisms, and activation barriers for propane activation by the M2 phase of the MoVNbTeO multi-metal oxide catalysts capable of the direct conversion of propane to acrylonitrile. Our approach is to combine extensive quantum mechanical (QM) calculations to establish the mechanisms for idealized representations of the surfaces for these catalytic systems and then to modify the parameters in the ReaxFF reactive force field for molecular dynamics (MD) calculations to describe accurately the activation barriers and reaction mechanisms of the chemical reactions over complex mixed metal oxides. The parameters for ReaxFF are derived entirely from QM without the use of empirical data so that it can be applied to novel systems on which there is little or no data. To understand the catalysis in these systems it is essential to determine the surface structures that control the surface chemistry. High quality three-dimensional (3D) Rietveld structures are now available for the M1 and M2 phases of the MoVNbTeO catalysts. However the details of the chemical mechanisms controlling selectivity and activity have remained elusive because the catalytically important sites in these Rietveld structures are occupied by mixtures of Mo and V atoms, obscuring the actual distributions of the metals and oxides at the active sites. To solve this problem we use a supercell of the Rietveld structure sufficiently large that all atoms can be whole, then we use Monte Carlo techniques based on ReaxFF to resolve these partial occupations into the optimum configuration of whole atoms still consistent with the X-ray data. We will report the ReaxFF resolved 3D structures for the M2 phase of the MoVNbTeO system. Using the resolved 3D structures we consider the distribution of sites on the important surfaces and carry out ReaxFF Reactive Dynamics (RD) calculations to follow the initial steps of the reactions. Such studies provide insights into the chemical reaction steps on MMO catalysts that should be useful in designing more selective and more active systems. We find that this suggests the critical role of the Te IV oxo chains for activating propene but not propane in the M2 phase. This suggests a new mechanism for this phase.Keywords Mechanism Á ReaxFF Á Ammoxidation Á Acrylonitrile Á Theory Á M2 phase Á M1 phase Á Mixed metal oxide Á Propane

In Silico Design of Highly Selective Mo-V-Te-Nb-O Mixed Metal Oxide Catalysts for Ammoxidation and Oxidative Dehydrogenation of Propane and Ethane

“…13 However it is in sharp contrast to previous suggestions for vanadium 22 and molybdenum oxides, 23 that oxygen bound directly with V or Mo are responsible for initial alkane C-H activation.…”

Propane Ammoxidation over Mo–V–Te–Nb–O M1 Phase Investigated by DFT: Elementary Steps of Ammonia Adsorption, Activation and NH Insertion into π-Allyl Intermediate

“…So far, however, only a handful of theoretical studies have been performed to better understand the catalytic behavior of the Mo-V-Te-Nb-O M1 and M2 phases [10][11][12][13][14]. Previously, we have performed DFT calculations using cluster models of the proposed active surface center of the M1 phase to study the hydrogen abstraction steps from propane to p-allyl intermediate on the various metal sites [15][16][17].…”

The Mechanism of Alkane Selective Oxidation by the M1 Phase of Mo–V–Nb–Te Mixed Metal Oxides: Suggestions for Improved Catalysts

“…[25] We discussed in some detail the M2 catalyst at Irsee IV, where we used the ReaxFF reactive force field to resolve the partial occupations in the Rietveld crystal strucures to form a 2×3×4 supercell that resolved the structure into whole atoms. [26] We then built models of the stable surfaces and examined the activation of propene. We found that the surface V=O and Mo=O species could not activate propene, whereas most surface Te=O sites could.…”

“…Indeed we did this for the M2 phase using the ReaxFF reactive force field to find the optimum configuration, which required a 2×3×4 supercell. [26] A similar supercell for M1 would require 3840 atoms, far too large for QM. Thus previous computational studies have focused on developing algorithms to determine the exact locations of molybdenum and vanadium atoms rather than studying the reaction mechanism.…”