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

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 An, 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 An loading of 0.57 wt.% and Ti/Si molar ratio of 2/100; the former also presented higher H 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. (C) 2003 Elsevier B.V. All rights reserved.