Photocatalytic hydroxylation of benzene to phenol over titanium oxide entrapped into hydrophobically modified siliceous foam

Abstract

The selective photocatalytic conversion of benzene to phenol was achieved on titania incorporated in hydrophobically modified mesocellular siliceous foam (MCF). Titanium oxide nanoparticles entrapped into mesocellular siliceous foam (TiO2@MCF) were prepared by “co-condensation” method using block copolymer as a template. TiO2@MCF was further modified by surface organo-grafting with silylation agent to make it hydrophobic (TiO2@MCF/CH3) and the grafted organic functional groups on the surface of TiO2 were selectively removed by post UV-irradiation to obtain the catalyst (TiO2@MCF/CH3/UV) that has a higher selective activity for benzene oxidation. By modifying the hydrophobicity of the mesoporous siliceous cage environment, both the adsorption of reactants (benzene) and the desorption of the desired products (phenol) on photocatalytic sites can be facilitated with an increase in selectivity and yield of phenol production. The titania nanoparticles incorporated into MCF cage were characterized by X-ray diffraction, nitrogen adsorption–desorption, high-resolution transmission electron microscopy, energy-dispersive X-ray analysis, and UV–visible diffuse reflectance spectroscopy. The hydrophobic modification of the catalysts was confirmed by Fourier transform infrared spectroscopy, water contact angle measurement, and thermo-gravimetric analysis. The adsorption isotherms of benzene and phenol on various catalysts suspended in aqueous solution were measured to investigate the effect of hydrophobic MCF cage. The hydrophobically modified TiO2@MCF/CH3/UV sample showed the highest selectivity and yield for the photocatalytic hydroxylation of benzene to phenol in aqueous solution. This could serve as an efficient and green photocatalyst for the selective oxidative conversion of a hydrophobic reactant to a hydrophilic product.