KINETICS AND CHARACTERIZATION OF BISMUTH MOLYBDATE CATALYSTS .2. REACTION STUDIES OVER VARIOUS BIMOLYBDATES

The model reaction, 1-butene to 1,3-butadiene, was studied over various compositions of unsupported bismuth molybdates to rank the catalysts' reactivities. Following Matsuura et al. we confirmed that γ-Bi2MoO6 doped with bismuth to yield a surface Bi Mo > 2.0 is unselective; selective catalyst species need a few mole% excess MoO3. All catalysts with a surface Bi Mo ≤ 1.5 show selectivities to butadiene greater than 95%. As the amount of MoO3 increases, reducing the surface Bi/Mo ratio, the activity increases down to Bi Mo = 1.5, then remains constant down to Bi Mo = 1, and then decreases steadily down to Bi Mo = 2 3 Characteristics of the partial oxidation catalysis change drastically going from γ-Bi2MoO6 to α-Bi2MoO12 catalysts. For the model reaction, Bi2MoO6 + 4% MoO3 exhibits an Ea = 40 kJ/mol (9.5 kcal/mol) at temperatures above 673 K. This catalyst is completely poisoned by its product butadiene below 673 K, while the inhibition is lifted above this temperature. We suggest that the pores of this catalyst, estimated to be 24 nm in diameter, become filled with "polybutadiene"; the heat of butadiene adsorption was estimated to be 375 kJ/mol. In contrast, α-Bi2Mo3O12 is not inhibited by butadiene, and has a lower activity (even calculated per unit surface) with an activation energy of 93 kJ/mol over the entire temperature range 660-713 K, studied. We propose models which describe two distinct ensemble effects operative in determining the kinetics over selective γ-Bi2MoO6 and α-Bi2Mo3O12 catalysts. © 1990.