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

摘要

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,and 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 hydrop0hilic 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 o fmagnitude 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.