Pd-Sb Bimetallic Catalysis for Propylene Acetoxylation: Identification of Active Sites and Catalytic Mechanism

Allyl acetate, an essential building block in petrochemical sector, can be produced through catalytic acetoxylation of propylene. However, an efficient catalyst may not be developed without a clear understanding of the property-performance relationship and reaction mechanism. In this work, we found antimony could be a quite effective promoter for propylene acetoxylation resulting in a selectivity nearly 100.0% (by propylene) and a space-time yield of 2.0 h-1 on the Pd catalyst by forming the Pd7Sb1 bimetallic active sites. With the systematical characterizations (HRTEM, XPS, in-situ DRIFTS, etc) and DFT calculations, we elucidated the geometric and electronic effects of Sb on the Pd catalysts. Electron transfer from Sb to Pd occurs in the bimetallic Pd-Sb alloy, which could efficiently promote the reduction of Pd species and maintain the stability of Pd0. The theoretical simulations also show that the addition of Sb can lower the d-band center of Pd and weaken the strong adsorption of propylene and acetic acid, which inhibits the over-oxidation. Furthermore, the coupling reaction between the dissociatively adsorbed C3H5* and OAc* species was determined to be the rate-determining step at low and high coverages of acetate, and it can be efficiently promoted at the Pd7Sb1 bimetallic sites.