Gas-phase epoxidation of propylene in the presence of H-2 and O-2 over small gold ensembles in uncalcined TS-1
Date of this Version5-2014
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 (similar to 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, H2 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 (similar to 40% vs. similar to 25%) relative to their TS-1 counterparts at the same gold loading similar to 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 similar to 200 degrees C were all found to be similar to 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 (similar to 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. (C) 2014 Elsevier Inc. All rights reserved.
Nanoscience and Nanotechnology