Characterization Of Titanium Species In Ti-MWW Zeolite And Construction Of High External Surface Structure

Due to excellent catalytic performance in selective oxidation,Ti-MWW zeolite has attracted much attention nowadays.However,there are still many scientific issues unsolved,such as,what's the role of acid treatment,how to simply and efficiently expose 12-membered ring semi-supercages for better diffusion.In this dissertation,the characterization of titanium species during post-treatment of Ti-MWW zeolite and the construction of high external surface structure have been carried out,and the main results are summarized as follows:Titanium species in Ti-MWW zeolite has been thoroughly characterized by UV resonance Raman spectroscopy with the excitation line at 244 nm and the role of acid treatment has been revealed.It was found out that non-framework TiO6 species was the absolutely dominant titanium species in as-synthesized Ti-MWW zeolite.Acid treatment of as-synthesized Ti-MWW not only removed about half of the absolutely dominant non-framework TiO6 species but also made the other part converted into framework Ti04 species,while acid treatment of directly calcined Ti-MWW made most of the dominant non-framework TiO6 species transformed into anatase TiO2.These understandings have never been reported before.Only the samples obtained from acid treatment of as-synthesized Ti-MWW and subsequent calcination showed catalytic activity in 1-hexene epoxidation,fully demonstrating that the definite role of acid treatment is transforming inactive non-framework TiO6 species into active framework Ti04 species.With organosilane-based in situ delamination strategy,we have obtained Ti-MWW zeolite with external specific surface area of about 210 m2/g,almost two times of that of conventional Ti-MWW zeolite.Due to higher external specific surface area and better diffusion performance,the Ti-MWW samples prepared by organosilane-based synthetic strategy showed superior catalytic performance in epoxidation of bulky olefins than conventional Ti-MWW samples.Spectroscopic characterizations indicated that there were more framework TiO4 species in the Ti-MWW samples prepared by organosilane-based synthetic strategy.However,because of the framework TiO4 species was more flexible(poor),the Ti-MWW samples prepared by organosilane-based synthetic strategy showed inferior catalytic performance than conventional Ti-MWW in 1-hexene epoxidation.These means that organosilane-based synthetic strategy is not an ideal way to improve the diffusion performance of Ti-MWW zeolite.Both the diffusion and the state of the catalytic activity site are the crucial factors influencing the catalytic performance of zeolite catalysts,which are needed to be fully considered during the development of zeolite catalytic materials.