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 Mo3O9 cluster model) under conditions adjusted to describe both high and low partial pressures of NH3 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−CH2−CHCH2) to form Mo−NHCH−CHCH2). 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 NH3 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 NH3. At high NH3 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.

The mechanism and kinetics of propene ammoxidation over α-bismuth molybdate

“…The instability to hydrolytic decomposition in air at room temperature of Mo 6+ =NR groups in organometallic complexes has also been reported [35,36,37]. We note further that theoretical calculations of the transition state enthalpy change for the reaction of an allyl species with a Mo 3 O 9 cluster to produce bound acrolein is 10.9 kcal/mol [38], and the transition state enthalpy change for the reaction of allyl species with a Mo 3 O 8 NH cluster to produce bound acrylonitrile is very similar at 11.4 kcal/mol [39]. Together, this evidence suggests that the surface concentration of Mo=NH groups relative to Mo=O groups on the surface of Bi 2 Mo 3 O 12 is likely to be very low, and therefore the former groups are unlikely to be responsible for the observed high selectivity to acrylonitrile during propene ammoxidation.…”

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

“…We find that step 4 to form 5 is much more favorable when species 4 or 7 is first oxidized. All reported values are for the doublet state (M s = ) [25].…”

Quantum mechanics based mechanisms for selective activation of hydrocarbons by mixed metal oxide heterogeneous catalysts – A tribute to Robert Grasselli

“…Later, Sanya Pudar continued the QM investigations of the mechanism [7,8,9], Leading to much more detail on the reaction barriers and intermediates as shown in figures 4 and 5. Figure 3.…”