Synthesis Of Mesoporous Titanosilicate Materials In Supercritical Fluids

As excellent catalytic materials for selective oxidation of hydrocarbons, titanium-containing molecular sieves have attracted more and more attention since the discovery of microporous titanosilicate (TS-1). Especially, the mesoporous titanosilicates with bigger pore size have better reactivity, and show better catalytic perfromance on fine chemistry, medical chemistry and photocatalysis, compared with traditional microporous materials. Therefore, the researches on the synthesis of mesosturctured titanosilicates with large pore diameter are always in the progress. Recently, supercritical fluids (SCFs) have been widely used in preparation of composites as alternatives for conventional organic solvents, because of their unique properties such as high diffusivity, low viscosity and strong solvent power. Among the commonly used SCFs, supercritical (SC) CO₂-based solutions are environmentally friendly. In this work, two kinds of mesoporous titanosilicate materials, Ti-SBA-15 and TiO₂/SBA-15, which are different in the titanium coordination environment, were prepared in SC CO₂-ethanol solutions via a post-synthetic approach according to different preparation procedures. Furthermore, the catalytic activity of Ti-SBA-15 and TiO₂/SBA-15 were tested by cyclohexene epoxidation reaction and photodegradation for methylene blue, respectively.Firstly, a series of Ti-substituted mesoporous SBA-15 materials with different Ti contents (Ti-SBA-15) were successfully prepared via a post-treatment route in supercritical CO₂-ethanol solution, followed by calcination. The resultant Ti-SBA-15 materials were characterized by different techniques including X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, N₂ sorption isotherms and ²⁹Si MAS NMR. It was demonstrated that the resultant materials remained the similar structure to the parent SBA-15, and the high amount of Ti (Si/Ti=10) was incorporated into the framework of SBA-15 successfully without forming any extra framework TiO₂. Ti species was in tetrahedral environment. At high Ti content (i.e, Si/Ti=5), TiO₂ phase formed and was coated onto the inner surface of SBA-15 besides the incorporation of Ti in the framework. Cyclohexene epoxidation reaction with dry tert-butyl hydroperoxide (TBHP) as an oxidant was studied to evaluate the catalytic properties of Ti substituted mesoporous catalysts. It was demonstrated that Ti-SBA-15 (Si/Ti=10) exhibited the highest performance for the oxidation of cyclohexene with a conversion of 100% and selectivity to cyclohexene oxide of 92% within 24h.Secondly, TiO₂/SBA-15 composites with various TiO₂ loading content were prepared via Ti-alkoxide hydrolysis in SCF solution, followed by calcination. The products were characterized by different techniques. It was demonstrated that the materials with titanium dioxide (anatase) was formed and Ti species were in octahedral coordination environment. The mesoporous SBA-15 as a substrate inhibited the anatase to rutile phase transformation as well as the growth of TiO₂ crystal grain. The phase of the nanocrystals did not change even calcined at 650℃. The TiO₂/SBA-15 composite with TiO₂ content of 25wt%, prepared in SC CO₂-ethanol solution and calcined at 650℃, showed higher photodegradation ability for methylene blue (MB) than the commercial TiO₂ nanoparticles P-25.