Acid-modulated Synthesis Of Ti-MWW Zeolites And Their Catalytic Performance In 1-hexene Epoxidation

At present,titansilicate has been widely used in chemical industry.Because of its unique solvent effect and pore-structure system,Ti-MWW catalysts have received widespread attention and show a superior catalytic performance in the selective oxidation with hydrogen peroxide.Therefore,it is of great significance to study the synthesis and catalytic performance of Ti-MWW.However,in the synthesis of Ti-MWW by prevailing methods,some extra-framework octahedral Ti species are usually formed,which can promote the ring opening of the epoxide in the catalytic oxidation reaction,and has an adverse effect on the catalytic performance.Usually,acid treatment is required to remove non-framework octahedral Ti species.Therefore,it is necessary to develop a simple synthesis method to introduce Ti species into the framework of zeolite without non-framework octahedral titanium species.In the present study,an acid-modulated synthesis method was developed to prepare Ti-MWW without the formation of extra-framework octahedral Ti species,and the prepared catalyst is applied to the epoxidation reaction of 1-hexene.In addition,the structural characteristics and formation mechanism of framework titanium species on Ti-MWW were investigated in detail,and the main results are summarized as follows:（1）With deboronated MWW zeolite as precursor,the Ti-MWW zeolites were successfully prepared by the acid modulated synthesis strategy to implant Ti species into the framework of the zeolite in the acid aqueous solution containing Ti sources.In addition,the Ti-MWW zeolite were synthesized by the prevailing method,and a comparison,in terms of physicochemical properties and catalytic performance of1-hexene epoxidation between the Ti-MWW zeolite prepared by the two different methods,was conducted to demonstrate the advantage of the currently developed acid modulated synthesis strategy.The lamellar structure of the MWW zeolite can effectively be restored by the reversible structure conversion,which plays a decisive role in introducing Ti species into the zeolite framework.Moreover,the extraframework octahedral Ti species can be effectively inhibited in a strong acid aqueous solution with a relatively high concentration.Unusually,Ti-MWW zeolites,with the open sites of Ti（OSi）₃OH as the primary Ti species,prepared by the acid-modulated synthesis method,possess a stronger Lewis acidity and better catalytic properties in 1-hexene epoxidation than those prepared by the prevailing method.Additionally,the developed catalysts are reusable.（2）With MWW-type boronsilicate zeolite（ERB-1）as the precursor,Ti-MWW with the open sites of Ti（OSi）₃OH as the primary Ti species was synthesized by acid-assisted isomorphous substitution strategy.Moreover,the non-framework octahedral titanium species and anatase were effectively inhibited.In addition,the effects of titanium source,acid concentration,acid treatment temperature and time on the synthesis of Ti-MWW zeolite were systematically investigated,and the optimal preparation conditions of Ti-MWW zeolite were clarified.The prepared Ti-MWW catalyst exhibit a good catalytic performance in the epoxidation of 1-hexene.