The effect of acid treatment on the active sites and reaction intermediates of the low-cost TS-1 in propylene epoxidation

This study investigated the roles of the different titanium species of low-cost TS-1 in propylene epoxidation and the influence of acid treatments. The low-cost TS-1 zeolites with different titanium species were synthesized using the hydrothermal method and characterized by ultraviolet (UV)-Raman, X-Ray Diffraction (XRD), ultraviolet visible diffuse reflectance spectroscopy (UV-Vis), N-2 physical absorption and NH3 temperature programmed desorption (NH3-TPD) techniques. The roles of the different titanium species in the low-cost TS-1 samples were investigated by gas chromatography-Raman spectrometry (GC-Raman) during the propylene epoxidation process. The framework titanium species was found to be active for propylene epoxidation, while the extra-framework Ti species were found to be harmful for propylene epoxidation due to their acidity. The extra-framework Ti species can be removed by acid treatments. It was found that the acid treatment of the as-synthesized TS-1 is more effective since the amorphous TiO2 was not transformed into anatase TiO2 upon calcination. Strong acids, such as HCl, HNO3 and H2SO4, are more efficient for the elimination of the extra-framework TiO2, but do not significantly affect the framework titanium species. After treatment with an acid of a suitable concentration, the amount of the active intermediate species Ti-OOH(eta(2)) can be increased, and a higher conversion of propylene and yield of PO can be achieved.