Synthesis And Post-Modification Of Titanosilicates And The Study On The Catalytic Performance In Epoxidation Reaction

Since the discovery of the titanosilicate TS-1,titanosilicates have achieved great success in the field of green catalytic oxidation with H₂O₂ as oxidant and have been extended to the synthesis of many oxygen-containing chemicals.Subsequently,MWW,MOR,MSE,*BEA and other titanosilicates with different topological structures were developed,which were used in a variety of catalytic oxidation reactions.However,some titanosilicates are limited by synthesis and poor activity.Therefore,it is necessary to explore new synthestic methods or post-treatment modification to achieve the best catalytic performance.In this paper,we mainly focused on the development of novel synthetic method of Ti-MSE and the exploration of postsynthesis modification of Ti-Beta,and evaluate the catalytic performance of the constructed catalysts in the epoxidation reaction.In the first part,using the commercially available dimethyldipropylammonium hydroxide（DMDPAOH）as the organic structure-directing agent and ultra-stable FAU zeolite（USY）as the silica and alumina sources,the pure-phase UZM-35 zeolite was rapidly synthesized without adding MSE zeolite seeds,while the Si/Al ratio in the synthetic gel was adjusted with additional silicon source.Ti-UZM-35 was then obtained through subsequent dealumination and Ti Cl₄ vapor treatment.The prepared Ti-UZM-35 showed excellent catalytic performance in the liquid-phase epoxidation of n-hexene,and could be reused after calcination and regeneration.In the second part,nanocrystalline Ti-Beta zeolite with high hydrophilicity and Ti-rich exterior was successfully prepared via a dissolution-recrystallization method.With the post treatment of tetraethylammonium hydroxide（TEAOH）solution at high temperature,the Si and Ti species within the Ti-Beta matrix were partially dissolved and recrystallized on the outer surface of crystals,resulting in Ti-rich exterior and the generation of silanol groups,which improved the accessibility of the active Ti sites,framework hydrophilicity and thus the enrichment of H₂O₂.Simultaneously,some of the close Ti（OSi）₄ species were transformed to more active open Ti（OSi）₃OH or Ti（OSi）₂（H₂O）₂（OH）₂ species.The modified Ti-Beta zeolite exhibited greatly enhanced catalytic performance in the epoxidation of cyclohexene in comparison to the parent Ti-Beta.