Gas-phase epoxidation of propylene in the presence of H_2 and O_2 over small gold ensembles in uncalcined TS-1

摘要

A novel catalyst consisting of gold nanoparticles supported on an uncalcined (with template in) titanium silicalite-1 (Au/U-TS-1) and having a very long (~10-20 h) and unique activation period for gas-phase propylene epoxidation in the presence of H_2 and O_2 is reported. Propylene oxide (PO) rate per gram of catalyst, H_2 selectivity, and BET apparent surface area of the catalyst were all found to increase during the course of activation, indicating that the number of the active sites in the Au/U-TS-1 for the PO reaction increased together with the formation of nanopores near the surface of the U-TS-1. The activated Au/U-TS-1 catalysts showed high stability and slightly higher H_2 selectivity (~40% vs. ~25%) relative to their TS-1 counterparts at the same gold loading ~0.04 wt%. Comparison of transient kinetic responses of the PO rate for the Au/U-TS-1 samples pretreated in different environments suggests that in situ hydrogen peroxide generated from O_2 and H_2 over the Au sites is the cause of template removal. The average gold particle sizes of the Au/U-TS-1 samples tested at different periods of time-on-stream (1, 7, 58 h) at ~200 ℃ were all found to be ~5-6 nm. Since (1) larger gold particles (>2 nm) have been found to be less active for the PO reaction in the Au/TS-1 system, (2) the average gold particle size does not correlate with the increasing PO rate during the course of the catalyst activation, and (3) there was an unexpectedly high surface Au/Si content (determined by XPS) for a spent Au/U-TS-1 sample with a low density of gold nanoparticles and large gold particles (~5-6 nm), we propose that the migration of gold species into the in situ formed nanopores generates the active sites (Au-Ti) for the PO reaction in the sublayer of the U-TS-1 surface, resulting in the long induction time observed in Au/U-TS-1 catalysts. The mechanistic implication of this unique phenomenon is that gold clusters inside the nanopores in the TS-1 can serve as the active sites for the PO reaction, which is also supported by the similar apparent activation energies (28-36 kJ mole~(-1)) observed among the Au/U-TS-1, Au/TS-1 and Au supported on S-1 coated TS-1 catalysts.