Effect of surface chemical properties and texture of mesoporous titanosilicates on direct vapor-phase epoxidation of propylene over Au catalysts at high reaction temperature

Abstract

Two kinds of organically modified mesoporous titanosilicate supports, Ti-MCM-48- and Ti-MCM-41-type, with different surface chemical properties and texture were prepared by two-step and one-step synthesis, respectively. The direct vapor-phase epoxidation of propylene with O 2 and H 2 was performed over Au nanoparticles deposited on the above supports. Both the supports and the Au catalysts were characterized by XRD, UV-Vis, N 2 adsorption–desorption isotherms, FT-IR, ICP, TEM, and ADF-EELS. Over Au-deposited on trimethylsilylated Ti-MCM-48 supports, an optimum reaction temperature for the highest yield of propylene oxide (PO) while maintaining selectivity above 80% was 523 K. This temperature was 100 K higher than over the non-silylated Au catalyst. Catalyst deactivation with time-on-stream was appreciably depressed for the silylated ones, which could be explained by the change of surface property from hydrophilic to hydrophobic. Over Au-deposited on Ti-MCM-41-type amorphous supports, but not on pure Ti-MCM-41, excellent catalyst stability with time-on-stream was observed from the very early stage of reaction at 473 K. In terms of PO formation rate per unit weight of Au, Au/Ti-MCM-41-type amorphous support with a very low Au loading of 0.015 wt.% and Ti/Si molar ratio of 1/100 was found to be more active by one order of magnitude than Au/Ti-MCM-48 with a high Au loading of 0.57 wt.% and Ti/Si molar ratio of 2/100; the former also presented higher H 2 efficiency. Organic modification did not change this feature. FT-IR spectra showed strong adsorption of water even on the Au catalyst composed of trimethyl silylated Ti-MCM-48 and the partial regeneration of hydroxylated surfaces during reaction at 523 K. This may partly explain why the silylated catalysts still deactivate in the reaction mixture.