A Study On The Regulation Of Microenvironment Of Active-Site Of Ti-MWW And Its Catalytic Mechanism In Alkenes Epoxidation

Epoxides are widely used in the production of fine chemicals,food additives,pharmaceutical intermediates and even agricultural chemicals.Epoxy groups have high reactivity,because such compounds are easy to produce a variety of chemicals through selective ring opening or functional group conversion,such as diol（ring opening）,lactones,carbonates,etc.,making epoxy compounds ubiquitous in organic synthesis and playing an important role in the national economy.Therefore,the research and development of olefin epoxidation is of great practical significance.Based on the concept of"green chemistry",the design and construction of high activity catalytic system has been the pursuit of the goal in the field of catalysis.TS/H₂O₂ catalytic system has the advantages of mild reaction conditions,high atomic utilization rate,and only water by-product.However,there are still many problems to be solved in this catalytic system,such as the confirmation of the structure of reactive intermediates,the nature of solvent effect,the confirmation of reactive active sites,etc.Therefore,to further improve the catalytic oxidation activity must be considered and started to solve the above problems.The theory of interfacial states（PBIS）with lower overall energy level induced by the overlapping of p orbitals of adjacent heteroatoms（O,N,P,S,B,etc.）,which has high energy delocalized electrons andπ→π^*orbitals,emits intense fluorescence and provides additional pathways for electron and proton transfer in catalytic reactions.Therefore,this theory has great advantages in regulating the catalytic reaction,and can provide a new idea and understanding for studying the movement of electrons and protons in the reaction.In this paper,under the guidance of PBIS theory and with the help of spectral characterization,the nature of olefin epoxidation reaction and the method of regulating the reaction activity are systematically studied,mainly including the following three aspects:（1）The epoxidation activity of Ti-MWW/H₂O₂ system for 1-hexene can be greatly improved by adding base.It was found that the addition of alkali had no effect on the Ti-MWW system through the characterization of a series of catalysts.Then,it was found that the interface state existed in the Ti-MWW system through the characterization of the interface state by fluorescence spectrum.Here,OH^-can promote the proton dissociation of H₂O₂,which makes H₂O₂ easier to coordinate with Ti to form Ti-OOH,and then constructs the interface state activation O-O bond with neighboring Si-OH to promote the transfer of reactive oxygen species.A series of confirmatory experiments have proved that solvent plays the role of proton transfer in the system,and PBIS theory has also made a reasonable explanation for the solvent effect.（2）Based on PBIS theory,Ti-MWW catalyst was hydrothermal treated by PI and Zn²⁺respectively,and the interfacial OH^-was introduced to promote the formation of PBIS,thus greatly improving the yield of the product.Through characterization,we found that Ti had the same state and microenvironment in the catalysts treated with PI and Zn²⁺,indicating that the six coordination active species formed by the coordination of PI and Ti reported in previous literatures may not be the reason for improving the reaction activity.Therefore,combined with PBIS theory and spectral characterization,a model of enhanced catalytic activity at the interface of Ti-MWW molecular sieve was constructed,and the mechanism of catalytic reaction was proposed.（3）PBIS can significantly regulate the electron and proton transfer process in catalytic reaction,and plays a decisive role in the reaction.In order to further prove the role of PBIS,we added Na OH and small organic amines to the free molecular sieve system to construct the interface state,so as to activate H₂O₂ to complete the transfer of reactive oxygen species.The 20%product yield proves that PBIS is the active site for the epoxidation of olefin.This is the first time that such a high yield is obtained by using H₂O₂ as oxidant in free molecular sieves,which breaks through the traditional catalytic mechanism with Ti as the active center and proposes a new epoxidation mechanism of olefin.