Aspects of Selective Oxidation and Ammoxidation Mechanisms over Bismuth Molybdate Catalysts. 3. Substituent Effects in the Selective Oxidation of Allylbenzenes

Article abstract of DOI:10.1021/jo00322a026

The relative rates of catalytic oxidation of p-XC6H4CH2CH=CH2 (X=H, CH3, Cl, OCH3) at 320 deg C over MoO3, Bi2MoO6, Bi2Mo3O12, and a multicomponent system (M_a²⁺M_b³⁺Bi_xMo_yO_z) have been determined at low (<2.5percent) conversion.The relative rates for the bismuth-containing catalysts are 1.00 (H), 1.55 (CH3), 1.98 (Cl), and 3.03 (OCH3), while for MoO3 the values are 1.00 (H), 1.78 (CH3), 1.38 (Cl), and 3.42 (OCH3).The main products of the reaction under these conditions are cinnamaldehyde and indene, while increasing ammounts of CO and CO2 are also produced at higher conversions.Relative cinnamaldehyde selectivity decreases with increasing electron-donating ability of X (Cl > H > CH3 > OCH3) and decreasing Bi content in the catalyst (Bi2MoO6 > Bi2Mo3O12 > MoO3).The results are interpreted in terms of the formation of a radical-like phenylallyl-molybdenum surface ? complex in the rate-determining step and, subsequently, the corresponding ?-O molybdate complex which is the intermediate for both cinnamaldehyde and indene formation.These results strongly suggest that in the selective oxidation of propylene over bismuth molybdate catalysts the ?-allylic surface complex formed in the rate-determining step is radical-like in character.