Influence of the active phase structure Bi-Mo-Ti-O in the selective oxidation of propene

Bismuth molybdenum titanium oxides were prepared in a highly dispersed state by the sol–gel method and in a largely crystallized state by coprecipitation and impregnation. The catalysts contained 14 or 25 wt.% of bismuth molybdate. Their catalytic performance in propene oxidation to acrolein was studied. The increase in activity of Bi-Mo-Ti oxides in propene oxidation as compared to that of bulk bismuth molybdate can be tentatively related, in the case of the sol–gel samples, to the stabilization of small aggregates of Bi- and Mo-containing phases due to the beneficial presence of the titania matrix. The catalytic performance of the samples prepared by coprecipitation or impregnation is enhanced once a crystal size of bismuth molybdate close to 23 nm is reached, i.e., the crystal size should not be too small (samples prepared by the sol–gel method) nor too large (samples prepared by impregnation and coprecipitation), but an optimum size should exist to enhance the catalytic performance.