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Published September 23, 2010 | Supplemental Material
Journal Article Open

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

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.

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.

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