Abstract and subjects
The influence of the synthesis conditions of SBA-15 silica support in the development of Ti-SBA-15 epoxidation catalysts has been studied in detail. In this study, efficient and stable Ti-SBA-15 epoxidation catalysts were prepared using a recently developed postgrafting method based on the insertion of a chelated titanium alkoxide precursor inside the SBA-15 silica mesophase. First, the nature of the SBA-15 supports was analyzed as a function of different synthesis parameters, such as hydrothermal aging temperature and calcination temperature, by solid-state NMR spectroscopy, thermal analysis, and nitrogen physisorption at -196 degrees C. Subsequently, titanium-substituted SBA-15 materials were characterized by DR UV-vis spectroscopy, elemental analysis, and nitrogen physisorption. As a catalytic test reaction, the activity, selectivity, and catalyst regenerability were studied in the epoxidation of cyclohexene. Our findings show that the density of silanol groups on the support greatly influences the retention and coordination number of the grafted titanium species. This characteristic of the mesoporous silica supports also has an influence on the catalytic activity of the resulting titanosilicate materials. The conversion of cyclohexene obtained with Ti-SBA-15 catalysts synthesized using noncalcined mesoporous silica was found to increase with the hydrothermal aging temperature of the support. Furthermore, the results showed that higher conversions of cyclohexene were obtained with catalysts prepared using SBA-15 calcined at 550 degrees C prior to the Ti grafting step, although lower dispersion of the titanium species was usually observed for these materials. A higher accessibility of the substrates to the active sites on the surface of these materials can explain these results. However, a dramatic deactivation of the catalysts was observed upon recycling of these active materials. In contrast, the grafted catalysts prepared using uncalcined mesoporous silica supports showed a substantially lower catalytic deactivation upon different reaction cycles as compared to those grafted after calcination.