Effect Of Br(?)nsted Acid And Lewis Acid On ZSM-5 Catalyzed The Conversion Mechanism Of Cyclic Hydrocarbon In MTA Reaction

Methanol to aromatics（MTA）is an important technical path for the preparation of aromatics from non-petroleum routes.ZSM-5 has the advantages of high temperature resistance,high catalytic activity,easy adjustment of acidity,and good shape-selective effect on aromatics,and is widely used in MTA process.In the MTA process,cyclic hydrocarbons such as cycloalkanes and cycloalkenes are the main intermediates for the formation of aromatics.It is of great significance to study the micro-reaction mechanism of the conversion of cyclic hydrocarbons to aromatics is of great significance for promoting aromatics production and rational design of catalysts.The current research on MTA is mainly focused on adjusting the Br（?）nsted acid site of ZSM-5 and introducing metals as Lewis acid to improve the selectivity of aromatics.Acidity adjustment is a key factor affecting the aromatics selectivity.It plays an important role to study the effects of Br（?）nsted acid and Lewis acid on the micro-reaction mechanism of cyclic hydrocarbons to aromatics in understanding the active centers of catalysts.Further,it can clarify the microstructure characteristics of catalysts from a new perspective and provide theoretical clues for the rational design of catalysts.In this work,methylcyclopentane to benzene is used as the reaction model and the ONIOM calculation method is adopted to study the effect of Br（?）nsted acid and Lewis acid on methylcyclopentane to benzene from two aspects:the different kinds of Al distributions on the ZSM-5 catalyst and the introduction of Ga and Zn.And the above reactions are compared with the cracking process of methylcyclopentane on the same catalysts.The main research contents and conclusions of this paper are as follows:The microscopic mechanism of the formation of methylcyclopentane to benzene on ZSM-5 with three kinds of different Al distributions was studied.When ZSM-5 zeolites with different kinds of Al distributions maintains the same Br（?）nsted acid active centers,the dehydrogenation is the rate determining step in the process of methylcyclopentane to benzene in all H-type catalysts.H-Z with single Al,H-Z₂ with nearest double Al,H-Z₄ and H-Z₆ with three nearest Al exhibit good catalytic effects on methylcyclopentane to cyclohexene and cyclohexene to benzene.The reason is that H-Z,H-Z₂,H-Z₄ and H-Z₆ have stronger Br（?）nsted acid acidity.Strong Br（?）nsted acid acidity is good for promoting the dehydrogenation reaction,and it is also good for ring expansion and hydrogen transfer reaction.Comparing the reactions between methylcyclopentane to cyclohexene and the cracking of methylcyclopentane on H-Z,H-Z₂,H-Z₄ and H-Z₆,it can be seen that the free energy barrier required for the cracking is lower than methylcyclopentane to cyclohexene,although the dehydrogenation process is easier than those on other catalysts,the Br（?）nsted acid protons still tend to crack for methylcyclopentane,instead of dehydrogenation to convert it to benzene.The process of methylcyclopentane to benzene after introducing Ga and Zn into ZSM-5 was studied.The introduction of Ga on the basis of the H-Z₁ and H-Z₂ models significantly reduces the energy barrier required for the dehydrogenation.The conversion of methylcyclopentane to benzene on Ga-ZSM-5 is the most advantageous,even without cracking which inhibits the formation of by-products.After introducing Zn on the basis of H-Z₃,H-Z₄,H-Z₅and H-Z₆ models,the energy barrier for dehydrogenation is also significantly reduced,however,the hydrogen transfer reaction of tertiary carbonium ions prevents the processof methylcyclopentane to benzene.Meanwhile,and the energy barrier of B acid attacking the C atomsand making C-C bond of methylcyclopentane to crack is lower than that of benzene generation on Zn-ZSM-5,which causes that benzene is not easy to produce from methylcyclopentane on Zn-ZSM-5.In addition,whether it is a dehydrogenation process,a ring expansion process or a hydrogen transfer process,the free energy barrier on Ga-ZSM-5 is lower than that on Zn-ZSM-5,and the reason of the catalytic performance of Ga-ZSM-5 being better than Zn-ZSM-5 is that Ga-ZSM-5 has stronger Br（?）nsted acid acidity and Lewis acid acidity as well as two closer H atoms to participate in the dehydrogenation reaction.In ZSM-5,although the strong Br（?）nsted acid acidity is beneficial to promote the dehydrogenation,it is more conducive to the cracking of methylcyclopentane,which ultimately makes the conversion to benzene difficult.ZSM-5 modified by Ga and Zn can significantly reduce the free energy barrier of dehydrogenation on H-type ZSM-5.For Zn-ZSM-5,due to a higher barrier for hydrogen transfer reaction,it difficult for methylcyclopentane to benzene,while for Ga-ZSM-5,due to the low proton transfer energy of Br（?）nsted acid,stronger Br（?）nsted acid acidity and Lewis acid acidity as well as closer H atoms to participate in the dehydrogenation,which can not only inhibit the cracking of methylcyclopentane,but also promote the production of benzene.The information of these microscopic reaction mechanisms provides important theoretical clues for the rational design of ZSM-5 to promote the formation of aromatics.