Selective (Amm)Oxidation of Propene on Bismuth Molybdates:

ab initio Calculations

Yun Hee Jang,# Terumasa Yamasaki,§ and William A. Goddard III#

#Materials and Process Simulation Center (139-74),

California Institute of Technology, Pasadena, California 91125, USA

§Asahi Chemical Ind. Co., Ltd., Fuji, Shizuoka 416-8501, Japan

Despite the keen industrial interest in selective oxidation and ammoxidation of small alkanes (CH4, C2H6, C3H8, and C4H10) by mixed metal oxides, there is little in the way of definitive mechanisms known for the most effective catalysts. In order 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 a first step, we are examining the selective oxidation of propene to acrolein for which there is some experimental data relating to mechanism. One of the most active and selective catalysts is based on bismuth molybdate. The mechanism of this reaction has been proposed as:

  1. allylic H abstraction at a bismuth site resulting in the 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 H2O to remove the H from the bismuth oxide, and

  4. Reoxidation of the Bi and Mo sites

We are investigating this reaction path using ab initio quantum-mechanical (QM) methods (DFT-B3LYP). We started with propene adsorbed on small and moderate sized clusters of