Theoretical Study On The Reaction Mechanism Of Methanol-to-olefin Catalyzed By H-RUB-50 And Zn-modified H-SAPO-34 Zeolite

Light olefins such as ethene and propene are important basic chemical raw materials.With the development of social economy,the market demand for light olefins has increased steadily year by year.The methanol-to-olefins（MTO）process,which uses coal and natural gas as feedstock,provides an important non-petrochemical route for the production of light olefins and has attracted widespread attention from academia and industry in the past decades.The energy structure of China is characterized by"rich in coal,poor in oil,and low in gas".Therefore,MTO technology has a broad application prospect in China and is of great significance for reducing the dependence of China’s economic development on oil resources.Currently,zeolites are widely used to catalyze MTO reactions.Numerous studies have found that the topology structure（i.e.,confinement effect）and acidity（including acid sites and acid strength）of zeolites are the key factors affecting their MTO catalytic performance,which determines the selectivity of products.H-SAPO-34 zeolite with an 8-membered ring window and CHA cage structure is a commonly used zeolite catalyst in MTO process due to its moderate acidity,excellent hydrothermal stability,and high methanol conversion.H-SAPO-34 exhibits high catalytic activity and high selectivity toward light olefins（>80%）in MTO reactions,and the ratio of ethene to propene is close to 1:1.Given that ethene has higher economic value than propene,how to further optimize the MTO reaction catalysts to improve the selectivity of zeolite for the target products（e.g.,ethene）is one of the focal issues of current MTO research.Recent experimental studies have shown that the 8-membered ring small pore H-RUB-50 zeolite with LEV topology and H-SAPO-34 zeolite modified with extra-framework Zn exhibit good MTO reaction performance with higher selectivity of ethene than propene in the products.However,due to the complexity of the MTO reaction network and the lack of effective characterization of the zeolite structures（such as reactive sites and spatial microenvironment）and intermediate species in the reaction,it is a great challenge to deeply understand the microscopic mechanism of ethene selectivity improvement caused by catalyst structural changes.In this thesis,using density functional theory（DFT）,the mechanism of olefin-based cycle at four Br（?）nsted acid（B acid）sites in H-RUB-50 zeolite in the steady-state stage of MTO reaction and the mechanism of olefin aromatization and diene formation in the transition stage of MTO reaction catalyzed by Zn-modified H-SAPO-34 were systematically investigated.Theoretical calculations were performed to investigate the effects of different Al sites,B-acid active centers and the local confinement environment of zeolite on the selectivity of ethene/propene products in MTO reaction,and to reveal the synergistic effect of B acid and Lewis acid（L acid）on the formation of aromatic hydrocarbon pool species and diene precursors in the transition period of MTO after the introduction of metal ions.This thesis work helps to deepen the understanding of the formation mechanisms of products and key hydrocarbon pool species in the transition and steady-state periods of MTO reactions,and provides some theoretical references and basis for the experimental preparation and modification of zeolite.The main results achieved in this thesis are as follows:1.Effects of acid strength and local confinement environment on the methanol to olefins reaction catalyzec by in H-RUB-50 zeoliteFour different B acid sites can be generated in the 8-membered ring window by Al sitting at T1 and T2 sites of H-RUB-50 zeolite.In order to evaluate the effects of different local confined spaces,active acid sites,and acid strengths caused by different Al sites on the catalytic performance of MTO reaction,this work systematically investigated the mechanism of olefin-based cycle using2,3-dimethyl-2-butene as the initial hydrocarbon-pool species.DFT results show that the olefin-based cycle is conducive to the formation of ethene at the four B acid sites in H-RUB-50 zeolite,and the B acid site with the strongest acid strength is the most active site for MTO reaction.The catalytic activity and ethene selectivity of H-RUB-50 at T1 site is much better than that at T2 site.Microkinetic calculations further indicate that the acid strength is closely related to the formation of ethene,but for the formation of propene,not only the acid strength but also the local confined space of the active center plays a more important role.The present work provides some theoretical guidance and reference for the experimental synthesis and adjustment of Al sites to improve the catalytic performance of MTO.2.Synergistic effect of Br（?）nsted/Lewis acid in olefin aromatization during MTO over Zn²⁺-modified H-SAPO-34 zeoliteIn the early stage of the MTO reaction,the introduction of the extra-framework Zn²⁺ions promotes the formation of the low methylbenzene hydrocarbon pool species in H-SAPO-34 and increases the proportion of the aromatic-based cycle in the dual-cycle,thus improving the selectivity of ethene in light olefins.However,the specific promotion mechanism is still unclear.Therefore,in the present work,the reaction mechanism of the aromatization of 3-methyl-1,3-pentadiene to m-xylene was studied by using density functional theory.The results show that the spatial proximity of B acid sites and L acid sites in Zn@H-SAPO-34 zeolite enhances the acid strength of B acid sites,thereby improving the methylation activity.The dehydrogenation reaction,occurring mainly at the L acid sites provided by Zn²⁺,proceeds via the"carbenium"mechanism,which was further confirmed by the metadynamics simulations.The synergistic effect of Br（?）nsted acid and Lewis acid enables Zn²⁺-modified H-SAPO-34 to have high olefin aromatization activity.This work shows that improving the olefin aromatization activity in MTO reaction can achieve the regulation of ethene selectivity and provide insights for the optimal design of MTO catalysts.3.Theoretical study of the influence of H-SAPO-34 modified with Zn²⁺on the formation of butadiene in MTO reactionIn the above work,we investigated the promotion mechanism of introduced Zn²⁺on the aromatization of olefin in H-SAPO-34.Dienes are the precursors for the formation of aromatic species in the MTO reaction.Given that the effect of the introduction of Zn²⁺on the mechanism of diene formation in the transition period is not clear,the present work investigated the reaction mechanism of butadiene formation from propene in H-SAPO-34 with or without Zn²⁺doping using density functional theory.It was found that the modification of Zn²⁺enhanced the acidic strength of adjacent B-acid sites,which was conductive to the formation of butadiene via the B-acid catalyzed alkene-mediated pathway.Meanwhile,the introduction of Zn²⁺provided L-acid reaction sites for the transfer dehydrogenation reaction of propene with methanol and promoted the formation of formaldehyde and the formaldehyde-mediated reaction pathway.Microkinetic calculations further confirmed that the formaldehyde-mediated pathway was the optimal reaction pathway for butadiene formation in Zn@H-SAPO-34 zeolite,revealing the synergistic promotion effect of B acid sites and L acid sites on butadiene formation.This work contributes to a better understanding of the microscopic mechanism of metal ion modification at the transition stage of the MTO reaction and provides guidance for product regulation in zeolite.