Differences in catalytic sites for selective oxidation of propane and direct propene epoxidation using H2 and O2 on Au/TS-1 catalysts

This work addresses the differences in catalytic sites for selective oxidation of propane to acetone and direct propene epoxidation to propene oxide (PO) using H2 and O2 (simplified as the HOPO process) on TS-1 supported nanogold catalysts. Au nanoparticles (NPs) and clusters immobilized on the external surface of uncalcined TS-1 (i.e., TS-1-B) show much higher acetone formation rate than that of the Au/TS-1 catalyst with Au clusters located in micropores of TS-1, though the latter exhibits higher conversion of hydrogen. However, the Au/TS-1 catalyst exhibits a promising PO formation rate in the HOPO reaction but shows lower hydrogen conversion compared to the selective oxidation of propane with H2 and O2. These results demonstrate that, unlike in the HOPO reaction, the Au NPs and clusters located on the external surface of TS-1 are more active for the acetone formation. Moreover, the Au/TS-1-B catalyst shows superior acetone formation rate compared to the tandem catalyst synthesized by physically mixing uncalcined silicalite-1-supported Au NPs and TS-1-B sample, where the Ti sites are remote from the Au sites. This suggests that Au-Ti proximity sites are more active for acetone formation, which differs from the case where remote Ti sites have been shown to contribute more significantly to PO formation. Furthermore, the Au/TS-1-B catalyst exhibits significantly enhanced acetone formation rate when reduced at higher temperatures. This is because the higher reduction temperature favors the decomposition of more TPA+ templates that covered on the Ti active sites, thereby exposing more accessible Au-Ti proximity sites.