Sources Of Aromatic Hydrocarbons Co-catalysts In MTO Reaction Over ZSM-5 Zeolite And The Effect Of Acid Sites On Mechanism

ZSM-5 zeolite is one of the most used catalysts for methanol-to-olefins because of its good stability and proper pore structure.The selectivity of the low-carbon olefin is low in methanol to olefins catalized by ZSM-5 zeolite due to many by-products.In addition,the source of aromatic hydrocarbons in the aromatics cycle of methanol-to-olefins is unclear.In this work,density functional theory was used to study the source of aromatic hydrocarbons co-catalyst of MTO reaction,and the effect of single and double acid sites on the MTO mechanism over the ZSM-5 zeolite.For the source of aromatic hydrocarbons co-catalyst of MTO reaction,the reaction mechanism of ethene and propene coupling to form benzene was studied,which provided pathways for aromatic hydrocarbons sources.For the effects of the single and double acid sites of the ZSM-5 zeolite over the MTO reaction were investigated through the follwing work.Firstly,the reaction mechanism of MTO catalyzed by T12 sites of B and Ga modified ZSM-5 zeolite was studied,which the effect of single acid site and acid strength on MTO reaction was determined.Then the reaction mechanism of MTO catalyzed by T3-T12 sites of Al-Al and B-Al substituted ZSM-5 zeolite was studied,through which the influence of double acid sites and acid strength on the MTO reaction was clarified.The main conclusions of this work are as follows:1.Ethene or propene self-coupling to form benzene undergoes reaction pathways through 1-methyl-2-cyclopentane carbocation M17 intermediate and 1-methyl-1-cyclopentane carbocation M19 intermediate,respectively.In the process of ethene or propene self-coupling to form benzene,the rate-limiting step of the reaction route through the 1-methyl-2-cyclopentane carbocation M17 intermediate is the one from methylcyclopentane M8-1 to 1-methyl-3-cyclopentyl alkoxide and methane molecule M9-1.In the process of ethene self-coupling to form benzene,the rate-limiting step of the reaction route through the 1-methyl-1-cyclopentane carbocation M19 intermediate is the one from ethoxide with ethene M3 to form butoxide M4.In the process of propene self-coupling to form benzene,the rate-limiting step of the reaction route through the 1-methyl-1-cyclopentane carbocation M19 intermediate is cyclohexene M22-1 to form cyclohexene carbocation and methane molecule M23-1.The activation energies of the rate-determining steps in the above reaction pathways are approximately,therefore both pathways are possible in the process of ethene or propene self-coupling to benzene.In addition,the activation energies of the rate-limiting steps for ethene or propene self-coupling to form benzene are similar to that in the MTO reaction,indicating that the co-catalyst aromatics of the hydrocarbon pool mechanism may be derived from the self-coupling of ethene or propene.This study confirms that the co-catalyst aromatic hydrocarbon for the MTO reaction can be derived from the products ethene and propene,providing evidence for our understanding of the origin of aromatic hydrocarbon for the MTO reaction.2.After the B or Ga modified the T12 site of the ZSM-5 zeolite(Al12ZSM-5)with single acid site,the Br?nsted acid strength is weakened relative to the Al12ZSM-5 zeolite.Compared with the Al12ZSM-5 zeolite,the Ga modified ZSM-5 zeolite has little effect on the activity and selectivity of ethene and propene in the aromatic side-chain mechanism of MTO reaction.The activation energy of MTO reaction over the B-modified ZSM-5 zeolite is increases compared to the Al12ZSM-5 zeolite,indicating that the reaction rate is reduced.However,the selectivity of ethene formation is increased compared to the selectivity of propene formation.3.The Br?nsted acid strength was enhanced of the HAl3-HAl12ZSM-5 and HB3-HAl12ZSM-5 zeolite with double acid sites compared to the Al12ZSM-5 zeolite with single acid site.Compared with Al12ZSM-5 zeolite,the activity of methanol to ethene over HAl3-HAl12ZSM-5 zeolite is improved,while the activity of methanol to propene is lowered.At the same time,the selectivity of ethene formation is higher than the selectivity of propene formation.Over the HB3-HAl12ZSM-5 zeolite,the activity of methanol to ethane and propene is reduced than that over the Al12ZSM-5 zeolite.The selectivity to ethene formation is higher than that of the propene formation.Compared with HAl3-HAl12ZSM-5 zeolite,the activity and selectivity of the MTO reaction over the HB3-HAl12ZSM-5 zeolite is decreased.