(718d) Tuning Reducibility for Improved Catalytic Performance in Bismuth Molybdovanadate Catalysts

In 1955, Standard Oil of Ohio was awarded a patent for the production of acrolein and acrylic acid from propene using a bismuth molybdate-based catalyst. Since that time, manufacture of these chemicals has become a million ton per year industry. In the intervening years, the addition of various modifiers, promoters, and stabilizers has substantially improved the performance of the bismuth molybdate catalyst. However, the exact means by which modifying elements influence the elementary reaction steps is still not well understood.

In order to better understand the effects of modifiers on reaction chemistry, we have synthesized and characterized a series of catalysts of composition Bi_1-x/3Mo_xV_1-xO₄, where x=Mo/(Mo+V) ranges from 0 to 1. In all cases, a single phase system is produced. Substitution of Mo by V produces a rate enhancement, with the maximum rate observed for x ~ 0.45. Diffuse reflectance UV-Vis spectroscopy has shown that  formulations showing greater steady-state reaction rates also show higher initial rates of reduction when exposed to propene. X-ray near edge absorption measurements reveal that during reduction, Mo⁺⁶ and V⁺⁵ are reduced to Mo⁺⁴ and V⁺⁴ respectively, while bismuth remains in the Bi⁺³ state throughout. Calculations using density functional theory support the conclusion that the combined vanadium-molybdenum catalyst is more reducible than either bismuth molybdate or bismuth vanadate. This greater reducibility in turn increases catalytic activity by facilitating the rate-determining step in propene oxidation. A better understanding of the physical principles underlying the enhanced reducibility seen in this particular mixed vanadate-molybdate system can in turn be used to rationally formulate mixed metal oxides with improved activity for a variety of selective oxidation reactions.