Study on the effects of boron doping Co-Cr and Bi-Mo composite oxides on styrene epoxidation: Why is it so different?

Cobalt-chromium (CoCr₂O_x) and bismuth-molybdenum composite oxides (BMO-3) before and after boron-doping are prepared by co-precipitation and their catalytic performance on styrene epoxidation were studied, respectively. Interestingly, their changes in catalytic performance are completely different after boron doping, styrene conversion and epoxidation selectivity on boron-doped CoCr₂O_x (B-CoCr₂O_x) catalyst was enhanced (from 31.5 % and 64.6 % to 45.8 % and 71.6 %) while that of on boron-doped BMO-3 (B-BMO-3) catalyst was weakened (from 28.8 % and 66.0 % to 7.5 % and 66.5 %). For B-CoCr₂O_x catalyst, the presence of boron induced some electrons to be transferred from Cr to Co species, resulting in enhanced electron cloud density around Co active species, and then the content of Co²⁺ species was improved, as a result, the increased oxygen vacancies near Co active species enhanced the ability to activate molecular oxygen, and then the catalytic performance of B-CoCr₂O_x oxide was improved. For B-BMO-3 catalyst, the presence of boron pushes some electrons from Bi to Mo species, resulting in a lower electron cloud density and fewer oxygen vacancies around the Bi active species, finally, the reduction of oxygen vacancies near the Bi active species reduces the production of reactive oxygen species and then the catalytic performance is weakened.