Research On Embedded Metal Zeolite Catalyst

Ethane is the second-largest component of shale gas.Due to the upgrading of shale gas mining technology,the production capacity of ethane has been greatly increased.With the in-depth development of China's industrialization,the demand for ethylene and aromatics has grown rapidly,and the supply-demand gap has further expanded.The new technology to produce ethylene and aromatics has broad prospects.Non-oxidative dehydrogenation of ethane and ethane non-oxidative aromatization are important ways to effectively utilize ethane.With the continuous reduction of traditional non-renewable resources such as oil and coal,the conversion of ethane to ethylene and aromatics will be an effective supplement to traditional oil and coal-based production routes,and it also meets with China's major energy and resource needs.The preparation of metal zeolite catalysts for non-oxidative conversion of ethane by loading metals and regulating the synergistic effect of acid sites and metal active sites in catalysts is one of the research hotspots.However,the traditional metal-supported zeolite catalysts quickly deactivate due to coke deposits and metal sintering and agglomeration,which hinders its further industrial development.Platinum-based catalyst is the most active for direct dehydrogenation of ethane to ethylene(EDH).The H-type embedded Pt@HZSM-5 catalyst was successfully and directly prepared by using Pt/Si O₂ as an only silicon source and metal source through the hydrothermal synthesis method.The Pt@HZSM-5 catalyst realizes excellent stability during 50 h EDH reaction without any regeneration,while the ethylene selectivity and ethane conversion rate are kept at 88.40%and 15.2%,respectively,under the serve reaction conditions(90%ethane in the feed gas,GHSV=2000 m L·g^-1_cat·h^-1,P=0.1 MPa,T=550?).However,the conventional Pt/HZSM-5 catalyst exhibits dynamic transformation of Pt chemical state,particle size,and its surrounding environment,resulting in rapid deactivation induced by much more carbon deposition and aromatization of the formed ethylene.Based on various characterization results,including HAADF-STEM,H₂-TPR,in-situ XPS,in-situ CO-DRIFTS,Py-FTIR,and MAS NMR,we have in-depth analyzed the embedded structure of the catalyst,metal active species,acidic sites distribution of zeolite,and coke deposits.Meanwhile,in-situ characterization techniques,such as in-situ/operando synchrotron XAS and in-situ DRIFTS,are used to explore the interaction between metal species of embedded catalyst and its surrounding environment.It is concluded that on the one hand,the Pt⁰ species embedded in the zeolite framework of Pt@HZSM-5catalyst is highly dispersed,which not only inhibits the agglomeration of Pt particles but also transfers electrons from the zeolite framework to the Pt⁰species to increase the electron cloud density of the Pt⁰ species.Those Pt⁰species with high electron density weaken the Pt-(C=C)bond strength to promote the desorption of olefins and delay the deep dehydrogenation of ethane on the Pt⁰ species,which would avoid the deep dehydrogenation of ethane to form coke deposits.On the other hand,because the embedded Pt@HZSM-5catalyst is prepared by the hydrothermal synthesis method,the zeolite framework is generated in situ around the metal to embed the metal.The surrounding environment of metal species is less disturbed by metal active species so that there are no extra-framework Al sites in the embedded catalyst,and there are more Al atoms located in the straight and sinusoidal channels of the zeolite,which is more beneficial for olefin circulation,and avoids the subsequent cyclization reaction of ethylene intermediate to form aromatics.In addition,for zinc-based catalysts,the rapid deactivation caused by coke deposits is the main obstacle to the industrial application of ethane non-oxidative aromatization(EDA).The H-type embedded Zn@HZSM-5 catalyst was successfully directly prepared by the two-step crystallization method,using Zn/Si O₂ as an only silicon source and metal source.Under the serve reaction conditions(90%ethane in the feed gas,GHSV=2000 m L·g^-1_cat·h^-1,P=0.1 MPa,T=550?),the conversion rate was maintained at 14.3%,and the aromatics selectivity reached 43.6%after 10 h reaction for Zn@HZSM-5 catalyst.Meanwhile,the final selectivity of light aromatics(C_6-8)in aromatics exceeds79.3%.However,conventional Zn/HZSM-5 catalyst was rapidly deactivated due to severe coke deposits under the same reaction conditions.Furthermore,we also carried out a 35 h long-time stability test for Zn@HZSM-5 catalyst.The catalyst has excellent stability while maintaining the selectivity of the desired product(ethylene+aromatics)at 86.6%,and the aromatics account for more than 41.1%of the desired product.Multi-characterizations,including HAADF-STEM,H₂-TPR,in-situ XPS,UV-vis DRS,Py-FTIR,MAS NMR,and in-situ DRIFTS were utilized to in-depth analyze the embedded structure of the catalyst,metal active species,acidic sites distribution of zeolite,coke deposits,and the interaction between metal species of embedded catalyst and its surrounding environment.It is concluded that on the one hand,the embedded structure not only inhibits the agglomeration of Zn O species but also makes the Br(?)nsted acid sites of zeolite combine with metallic zinc,forming a dominant stable Zn(OH)⁺species in Zn@HZSM-5 catalyst.These Zn species not only reduce the formation of undesired coke and CH₄ from deep dehydrogenation of ethane but also have the ability for both H₂ dissociation and re-combinative desorption to promote the aromatization reaction.Meanwhile,due to its embedded structure,the dynamic transformation of Zn species,which is different from conventional Zn/HZSM-5 catalyst,improves the selectivity of aromatics during the reaction.On the other hand,due to the zeolite framework around the metal particles of the embedded catalyst is generated in situ during the crystallization process,the surrounding environment of metal species is less disturbed by metal active species so that there are no extra-framework Al sites in the embedded catalyst,and more Al atoms are located in the straight and sinusoidal channels of the zeolite,which is beneficial to promote the olefin cycle and help suppress the excessive aromatization of the product.In addition,due to the confinement effect of zeolite,the generated light aromatics will not be further converted into heavy aromatics,thereby greatly reducing the coke deposits formed by heavy aromatics.