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Mechanism of Selective Ammoxidation of Propene to Acrylonitrile on Bismuth Molybdates from Quantum Mechanical Calculations

Pudar, Sanja and Oxgaard, Jonas and Goddard, William A. III (2010) Mechanism of Selective Ammoxidation of Propene to Acrylonitrile on Bismuth Molybdates from Quantum Mechanical Calculations. Journal of Physical Chemistry C, 114 (37). 15678-15694 . ISSN 1932-7447. https://resolver.caltech.edu/CaltechAUTHORS:20101004-095501676

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Abstract

In order to understand the mechanism for selective ammoxidation of propene to acrylonitrile by bismuth molybdates, we report quantum mechanical studies (using the B3LYP flavor of density functional theory) for the various steps involved in converting the allyl-activated intermediate to acrylonitrile over molybdenum oxide (using a Mo_3O_9 cluster model) under conditions adjusted to describe both high and low partial pressures of NH_3 in the feed. We find that the rate-determining step in converting of allyl to acrylonitrile at all feed partial pressures is the second hydrogen abstraction from the nitrogen-bound allyl intermediate (Mo−NH−CH_2−CH═CH_2) to form Mo−NH═CH−CH═CH_2). We find that imido groups (Mo═NH) have two roles: (1) a direct effect on H abstraction barriers, H abstraction by an imido moiety is (~8 kcal/mol) more favorable than abstraction by an oxo moiety (Mo═O), and (2) an indirect effect, the presence of spectator imido groups decreases the H abstraction barriers by an additional ~15 kcal/mol. Therefore, at higher NH_3 pressures (which increases the number of Mo═NH groups), the second H abstraction barrier decreases significantly, in agreement with experimental observations that propene conversion is higher at higher partial pressures of NH_3. At high NH_3 pressures we find that the final hydrogen abstraction has a high barrier [ΔH‡_(fourth-ab) = 31.6 kcal/mol compared to ΔH‡_(second-ab) = 16.4 kcal/mol] due to formation of low Mo oxidation states in the final state. However, we find that reoxidizing the surface prior to the last hydrogen abstraction leads to a significant reduction of this barrier to ΔH‡_(fourth-ab) = 15.9 kcal/mol, so that this step is no longer rate determining. Therefore, we conclude that reoxidation during the reaction is necessary for facile conversion of allyl to acrylonitrile.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/jp103054x DOIUNSPECIFIED
http://pubs.acs.org/doi/full/10.1021/jp103054xPublisherUNSPECIFIED
ORCID:
AuthorORCID
Goddard, William A. III0000-0003-0097-5716
Additional Information:© 2010 American Chemical Society. Received: April 5, 2010; Revised Manuscript Received: June 28, 2010. Publication Date (Web): August 25, 2010. We thank Dr. Kimberly Chenoweth and Dr. Adri van Duin for many helpful discussions. This material is based upon work supported as part of the Center for Catalytic Hydrocarbon Functionalization, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001298. The facilities were supported by ARO-DURIP and ONR-DURIP funds. We also thank Bob Grasselli and Jim Burrington for early discussions on the mechanistic issues. Supporting Information: Computational details including Cartesian coordinates, energies, and vibration frequencies for all structures reported. This material is available free of charge via the Internet at http://pubs.acs.org.
Funders:
Funding AgencyGrant Number
Center for Catalytic Hydrocarbon FunctionalizationUNSPECIFIED
U.S. Department of Energy, Office of Science, Office of Basic Energy SciencesDE-SC0001298
Army Research Office - Defense University Research Instrumentation Program (ARO-DURIP)UNSPECIFIED
Office of Naval Research - Defense University Research Instrumentation Program (ONR-DURIP)UNSPECIFIED
Issue or Number:37
Record Number:CaltechAUTHORS:20101004-095501676
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20101004-095501676
Official Citation:Mechanism of Selective Ammoxidation of Propene to Acrylonitrile on Bismuth Molybdates from Quantum Mechanical Calculations Sanja Pudar, Jonas Oxgaard, William A. Goddard III The Journal of Physical Chemistry C 2010 114 (37), 15678-15694
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:20270
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:16 Nov 2010 19:57
Last Modified:26 Nov 2019 11:15

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