Theoretical study of propylene epoxidation heterogeneous‐homogeneous mechanism over MoO_x/SiO₂ catalyst

Abstract

The green direct propylene (C3H6) epoxidation on MoOx is well studied by experimental methods, but detailed molecular reaction mechanism studies using in‐silico experiments method are few. Here, the different oxidation heterogeneous‐homogeneous pathways for MoOx/SiO2 catalyst are calculated, mainly involving Mo=O on di‐oxo tetracoordinate MoOx, allyl peroxy (C3H5OO•), and allyloxy (C3H5O•) radicals. The results show that, for surface reaction mechanism with Mo=O, the barriers of propylene oxide (PO) and acetone generation are too high; in comparison, the byproduct acrolein is more beneficial product with a lower barrier. In heterogeneous‐homogeneous pathways, the desorbed allyl (C3H5•) from the surface can easily combine with O2 to synthesize C3H5OO• radical, and in the partial oxidation of propylene with C3H5OO• as an oxidant, PO is more beneficial with a low barrier compared to byproducts such as propanal, acetone, acetaldehyde, etc. These indicate that (a) gas‐phase free radical reactions have important effects on PO generation, in which C3H5OO• is the main active species; (b) on MoOx surface, Mo=O is difficult to be used as the active O species for PO production. Further research is needed on other active sites such as Mo‐O‐Mo or defective sites.