Propylene oxide inhibits propylene epoxidation over Au/TS-1

The kinetics of propylene epoxidation over Au supported on titanium silicalite-1 (Au/TS-1) catalysts were measured in a continuous stirred tank reactor (CSTR) free from temperature and concentration gradients. Apparent reaction orders measured at 473 K for H₂ (0.7 order), O₂ (0.2), and propylene (0.2) for a series of Au/TS-1 with varied Au (0.02–0.09 wt%) and Ti (Si/Ti: 75–143) contents were consistent with those reported previously. Co-feeding propylene oxide enabled measurement of the apparent reaction order in propylene oxide (−0.4 to −0.8 order) and the determination that relevant pressures of propylene oxide reversibly inhibit propylene epoxidation over Au/TS-1, while co-feeding carbon dioxide and water had no effect on the propylene epoxidation rate. Analysis of previously proposed two-site reaction mechanisms in light of reaction orders for O₂ (0.4), H₂ (1), and C₃H₆ (0.4), corrected to account for propylene oxide inhibition, provides further evidence that propylene epoxidation over Au/TS-1 occurs via a simultaneous mechanism requiring two distinct, but adjacent, types of sites, and not a sequential mechanism that invokes migration of H₂O₂ formed on Au sites to PO forming Ti sites. H₂ oxidation rates are not inhibited by propylene oxide, implying that the sites required for hydrogen oxidation are distinct from those required for propylene epoxidation.