The Mechanism Of Dimethyl Ether Formation From Methanol Catalyzed By HSAPO-34 Zeolites

Ethylene,propylene and other low carbon olefins are important organic chemical raw materials.The traditional method of producing ethylene and propylene is light oil cracking process.With the depletion of petroleum resources,the process of producing low carbon olefins from coal or natural gas by methanol provides an important alternative route.In order to control the generation activity and selectivity of ethylene,propylene and other products better,as-well as to realize the design of zeolite catalyst,it is particularly important to understand the catalytic mechanism of MTO process.In this paper,the methanol adsorption and the formation mechanism of dimethyl ether in MTO reaction were studied by the first-principles density functional theory(DFT).Several possible adsorption conformations were obtained,as well as the spatial orientation and the free energy.The reaction heat and reaction energy barrier of two different mechanisms were calculated respectively.The calculated results indicated that methanol could be adsorbed on the Bronsted acid site in either the hydrogen bonding or in the protonation.The adsorption configurations coexisted,slightly varying with the channel.When two CH3OH molecules were considered,they were likely located at both ends of the twelve-ring window via the strong and the weak hydrogen bonding.The reaction energy barrier for the formation of surface methoxide was very low and it could occur before the formation of dimethyl ether.The dimethyl ether could be generated either directly from two methanol molecules or indirectly from a surface methoxyl group intermediate.The calculations indicated that the indirect mechanism was the main mechanism for the formation of dimethyl ether from methanol.