Solid State NMR Study On Reaction Mechanism Of Methanol To Olefin Over Zeolite

As basic chemical raw materials,light olefins such as ethylene and propylene can be widely used in industrial production.At present,it mainly from the petroleum cracking reaction.However,the fluctuation of oil price will increase the production cost of light olefins.In view of the gradual depletion of petroleum resources and the increasing demand for light olefin,it is particularly important to find an alternative way to prepare it.Methanol to olefins on zeolite has attracted worldwide attention because of its wide source of raw materials and low cost.Since the development of MTO technology,a lot of research has been done on its reaction process and many reaction mechanism have been proposed.For example,the dealuminization of ZSM-5can promote the formation of surface methoxy species bond to an extra-framework Al（SMS-EFAL）,which can react with formaldehyde to form the initial C-C bond species acetaldehyde and eventually to form ethylene.However,the mechanism of the formation of surface methoxyl species is not well understood,especially the molecular sieve framework structure and the active sites how to affect the formation of methoxyl species.Methoxyl species are generally believed to be formed at acidic sites of molecular sieve,but there no direct experimental evidence for such complex structure.Then,the shape selectivity of zeolite affect the interaction between species in the channal,and researching these interactions is also of great importance to understanding MTO reaction process.This paper mainly carries out the following work:1.The formation and reactivity of surface methoxy species in methanol reaction over SSZ-13 zeolite were investigated by using solid-state NMR technique.The surface methoxy species bounded on Br?nsted acid sites is for the first time identified by two-dimensional ¹³C-²⁷Al（Heteronuclear Multiple-Quantum Correlation,HMQC）NMR experiment.The formation of another surface methoxy species bounded on Lewis acid sites is also observed.The combination of ¹³C NMR and GC-MS shows that the two kinds of surface methoxy species possess high reactivity,contributing to the formation of hydrocarbon pool species and hydrocarbons via the hydrocarbon pool mechanism.2.Carbocations are important hydrocarbon pool species in Methanol-to-Hydrocarbon（MTH）reaction,largely affecting the product selectivity as well as the deactivation process.However,the reaction mechanism involved of carbocation is still unclear due to the complex reaction networks in MTH reaction.Based on the two-dimensional ¹³C-¹³C correlation spectrum and DFT calculation,we studied the interaction between carbocation and benzene molecules in molecular sieve channels and the effect of different molecular sieve channel threshold effect on the intermolecular interaction.The cationic-πinteraction between benzene and cyclopentene carbocation with positive charge was found.The strength and interaction model of this interaction were affected by the channel size of the molecular sieve,and the chemical reaction ability between the two molecules was finally affected.