The Coupling Between Selective Oxidation And Photocatalytic Reactions On Ti-Modified Mesoporous Sieves

Titanium-containing mesoporous molecular sieves have attracted considerable attention because of their remarkable catalytic properties in selective oxidation reactions using H2O2 in aqueous solution as the oxidant. The high cost and low efficiency of H2O2 however, limits its applications on an industrial scale. The hydroxylation of benzene in the liquid phase results in the formation of non-framework Ti centers in [Si, Ti]-MCM-41 and the environment of these Ti centers is similar to that in bulk TiO2. This suggests that these non-framework Ti centers can be used as photocatalytic sites on which molecular oxygen is transformed to the active oxidant species, either for the hydroxylation of benzene on framework Ti centers or to interact directly with benzene to produce phenol.In this paper, Ti-modified mesoporous molecular sieves containing both selective oxidation and photocatalytic centers were prepared by the following three methods:a) directly synthesis in one step reaction;b) post-synthesis modificationc) via a precursor.The catalysts were characterized by powder XRD, low temperature N2 adsorption-desorption measurements, and FT-IR, Raman, UV-visible diffuse reflectance and XPS spectroscopy. The materials were shown to contain both framework Ti centers (or Ti species with the same environment as framework Ti) and non-framework Ti centers (or Ti species with the same environment as non-framework Ti). Apart from the catalyst prepared by post-synthesis grafting with TiCl4 the materials all possess well ordered long-range and pore structures with high specific surface areas and a narrow distribution of pore diameters.The photocatalytic performance was followed by determining the production of H2O2 and ?OH. The selective oxidation performance was investigated in the hydroxylation of benzene using H2O2 in aqueous solution as the oxidant. It was found that the framework and extra-framework Ti centers exhibited selective oxidation catalytic activity and photocatalytic activity respectively.The extent of the coupling between selective oxidation and photocatalysis reactions in these catalysts was studied using the photocatalytic hydroxylation of benzene as the probe reaction. All of the catalysts exhibited coupling. From the relationship between the coupling activity and the structure of the active centers, it can be deduced that selective oxidation and photocatalytic centers must both be present.The extent of coupling was enhanced by varying the composition of the selective oxidation and photocatalytic centers, or by adding of spin trap or an additional oxidant. The effect of varying the structure of the selective oxidation active centers, the number of photocatalytic active centers and the mode of synthesis were also investigated.