Relationship between solid state structure and catalytic activity of rare earth and bismuth-containing molybdate ammoxidation catalysts

Abstract

The formation of solid solutions in rare earth and bismuth-containing molybdate catalysts plays a key role in the selective ammoxidation of propylene to acrylonitrile. Solid state structural studies of the Bi 2 − x Ce x (MoO 4) 3 system reveal that bismuth dissolves readily in the Ce 2(MoO 4) 3 lattice which crystallizes in the low-temperature La 2(MoO 4) 3 structure, yielding a single-phase material when x ≥ 1. The solubility of cerium in the Bi 2(MoO 4) 3 structure is less extensive, with maximum solubility occurring at x − 0.2. Structural substitution of bismuth or cerium by rare earth cations such as La, Pr, Nd, and Y results in lattice parameter changes which indicate that bismuth and cerium cations randomly occupy equivalent positions in BiCe-molybdate solid solutions. Catalytic activity maxima correlate well with phase compositions and occur in the two single phase regions, where there is maximum solubility of bismuth in the cerium molybdate phase and maximum solubility of cerium in the bismuth molybdate phase. A third catalytic maximum is observed in the binary phase region of the two saturated solid solutions and coincides with equal solubility of Ce in the Bi-molybdate phase and Bi in the Ce-molybdate phase. At these three optimum catalyst compositions, maximum interactions exist between the three key catalytic components: Bi (α-H-abstracting element), Ce (oxygen and electron transfer element), and Mo (olefin chemisorption and nitrogen insertion element).