Mechanistic Investigation Of Olefins Formation In The Process Of MTH Catalyzed By ZSM-5 Zeolite:A DFT Study

Methanol to hydrocarbons(MTH)is a reaction in which coal and other non-oil carbon-containing resources can be transformed into liquid fuels or light olefins.The reactant,namely methanol,can be obtained from almost any gasifiable carbonaceous resource.And the MTH products can range from light olefins to gasoline.This technology can turn China's energy supply disadvantage into an advantage,which in large part can ease the oil crisis.Therefore,MTH has attracted more and more attention.The ZSM-5 molecular sieve is considered to be an effective catalyst for MTH,but also has serious carbon deposition problem,which restricts the efficient utilization of ZSM-5.The mechanisms of coke inactivation of the catalyst is mainly divided into two types: the active sites covered and the pores blocked.Through analysis of the current experimental work,we found that the basically accepted option of carbon deposition is due to the large volume of the by-product.And the volume of polycyclic aromatic hydrocarbons(PAHs)is too large and plug the catalyst channel.The generation of PAHs begins with the first C-C bond.Therefore,with the method of DFT-GGA,we have studied the generation of first C-C bond and the initial olefin using the 34 T cluster and the Al / Ga-ZSM-5 periodic model.The following conclusions can be drawn by calculations:1.There are two reaction mechanisms for the formation of dimethyl etheron the ZSM-5.The first is the synergistic mechanism,that is,two methanol molecules react with each other catalyzed by Br?nsted acid to form dimethyl ether directly;the second is the step-by-step reaction mechanism,that is,the formation of dimethyl ether is completed by the methoxy intermediates.The theoretical results show that the formation of dimethyl ether is mainly through the synergistic mechanism.2.In the process of MTH on ZSM-5 catalyst,the calculation results show that the first C-C bond specis is 1,2-dimethoxyethane.3.For the formation of the first C-C bond,the activation barrier obtained on the 34 T cluster model is significantly higher than the activation barrier obtained on the periodic model,indicating that the periodic model can more fully reflect the full cage properties of ZSM-5 molecular sieve.4.Through demonstration on the periodic model,it is concluded that route5(including reactions:ZOCH3 + CH3CH2OCH3 ? ZO-+ CH3O+(CH3)CH2CH3;ZO-+CH3O+(CH3)CH2CH3?ZOCH2CH3+DME;ZOCH2CH3?ZOH+ CH2=CH2)is the best reaction path for the formation of the initial olefin,and the initial olefin is ethylene.