Mechanism of Selective Oxidation and Ammoxidation of Propene on Bismuth Molybdates from DFT Calculations on Model Clusters Yun Hee Jang ² and William A. Goddard III* Materials and Process Simulation Center (139-74), California Institute of Technology, Pasadena, California 91125 ReceiVed: March 26, 2002 In this paper, we use first principles quantum mechanical methods (B3LYP flavor of Density Functional Theory) to examine the mechanism of selective oxidation and ammoxidation of propene by BiMoO x catalysts. To do this, we use finite clusters chosen to mimic likely sites on the heterogeneous surfaces of the catalysts. We conclude that activation of the propene requires a Bi(V) site, whereas all subsequent reactions involve di-oxo Mo(VI) sites adjacent to the Bi. We find that two such Mo sites are required for the most favorable reactions. These results are compatible with current experimental data. For ammoxidation, we conclude that ammonia activation would be easier on Mo(IV) rather than on Mo(VI). Ammonia would be activated more easily for more reducing condition. Because ammonia and propene are reducing agents, higher partial pressures of them could accelerate the ammonia activation. This is consistent with the kinetic model of ammoxidation proposed by Grasselli and co-workers that imido sites (ModNH) are more abundant in higher partial pressures of feed. Our calculations also indicate that allyl groups produced as a result of the hydrogen abstraction from propenes would be adsorbed more easily on imido groups (ModNH) than on oxo groups (ModO) and that the spectator oxo effect is larger than spectator imido effect. Thus, we propose that the best site for ammoxidation (at least for allyl adsorption) is the imido group of the “oxo-imido” species. 1. Introduction Despite the keen industrial interest in selective oxidation and ammoxidation of small alkanes (CH 4 ,C 2 H 6 ,C 3 H 8 , and C 4 H 10 ) by mixed metal oxides, there is little in the way of definitive mechanism known for the most effective catalysts. To learn more about the reaction mechanism of these important catalysts, we are using a variety of first principles theoretical approaches to calculate the fundamental steps. As the first step, we examined the selective oxidation and ammoxidation of propene 1,2 for which there are some experi- mental data relating to mechanism One of the most active and selective catalysts for these reactions is based on bismuth molybdate. 2-4 The mechanism has been proposed as follows: 5,6 (1) Allylic H abstraction at a bismuth site resulting in an allyl intermediate adsorbing on a molybdenum site (rate-determining step), (2) O insertion into the allyl intermediate at the molybdenum site and abstraction of a second H, (3) Elimination of the H 2 O to remove the H from the bismuth oxide, and (4) Reoxidation of the Bi and Mo sites. The “dual-site” concept is widely accepted, where the bismuth site is responsible for the C-H activation and the molybdenum site is responsible for allyl adsorption and oxygen insertion. Ammoxidation proceeds essentially in the same way as oxida- tion, except that (1) ammonia is first activated on molybdenum site to create imido groups (dNH) from oxo groups (dO) and (2) NH rather than O is inserted into the allyl group. In this study, we investigated this reaction path on the model clusters of the component oxides (Bi 2 O 3 and MoO 3 ) using ab initio quantum-mechanical (QM) methods (DFT-B3LYP). The same reactions had been investigated theoretically in 1980s. 7 Calculation methods and computers have been dramatically improved since then, and our work is reexamining the reaction with more advanced methods and fewer simplications. This work could also be a theoretical background to a current catalyst development with a greater opportunitysammoxidation catalysts operating on propane rather than propene. 2. Calculation Details 2.1. Model Clusters. A schematic representation of the crystal structures of the component oxides of bismuth molybdate, R-MoO 3 and R-Bi 2 O 3 , are given in Figure 1. In R-MoO 3 , 8 each molybdenum site consists of a tetrahedron with two terminal oxo oxygens [r(ModO) ≈ 1.7 Å] and two bridging oxygens [r(Mo-O) ≈ 1.9 Å] connected to a molybdenum atom in its nearest-neighbor coordination shell. In R-Bi 2 O 3 , 9 each bismuth has three nearest-neighbor oxygens connected to it and one lone pair. Similar bonding configuration can be found in one of the bismuth molybdate, R-Bi 2 Mo 3 O 12 , 10,11 as shown in Figure 1c. * To whom correspondence should be addressed. E-mail: wag@ wag.caltech.edu. ² Current address: School of Chemistry, Seoul National University, Seoul 151-747, Korea. CH 3 CHCH 2 (propene) + O 2 9 8 300-450 °C CH 2 CHCHO (acrolein) + H 2 O (1) CH 3 CHCH 2 (propene) + NH 3 + 3 2 O 2 9 8 400-460 °C CH 2 CHCN (acrylonitrile) + 3H 2 O (2) 5997 J. Phys. Chem. B 2002, 106, 5997-6013 10.1021/jp0208081 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/18/2002