Superior Performance of Gold Supported on Titanium-Containing Hexagonal Mesoporous Molecular Sieves for Gas-Phase Epoxidation of Propylene with Use of H₂ and O₂

Titanium-containing hexagonal mesoporous silicas (Ti-HMS) with wormhole structure and Si/Ti molar ratios ranging from 10 to 40 have been prepared by using long-chain alkyl primary amines as template agents. The Ti-HMS supported Au catalyst (Au/Ti-HMS) was obtained by a deposition−precipitation method for direct gas-phase epoxidation of propylene with use of O2 and H2. The structures of Ti-HMS and Au/Ti-HMS samples were characterized by X-ray diffraction, N2-physisorption, scanning electron microscopy, transmission electron microscopy, UV−vis diffuse reflectance spectroscopy, UV Raman spectroscopy, ammonia-temperature-programmed desorption, and atomic adsorption spectroscopy. The results showed that the Au/Ti-HMS catalyst exhibited superior performance in terms of propylene conversion, propylene oxide (PO) selectivity, and H2 efficiency in comparison with the Au catalysts supported on the conventional Ti-containing mesoporous materials. Besides the Si/Ti molar ratio, the chain length of alkylamine for the Ti-HMS preparation was crucial for the enhancement of catalytic performance. Specifically, 9.0% of propylene conversion, 97.3% of PO selectivity, and 30.4% of H2 efficiency can be obtained at 373 K in the initial 30 min of time-on-stream on the Au/Ti-HMS catalyst, where the Ti-HMS having a Si/Ti molar ratio at 20 was prepared by using tetradecylamine as the template agent. Regeneration of the spent catalyst by calcination in air gave almost no change in the PO selectivity but about 25% loss in the propylene conversion. The enhanced catalytic performance of Au/Ti-HMS catalyst may be essentially attributed to the homogeneous dispersion and uniformity of titanium species in combination with accessible pore structure.