Study On Performance Of Catalysts For Hydrogenation Of CO₂ To Produce Low Carbon Hydrocarbons

With the rapid development of the economy and the progress of industrial technology,CO₂emissions are increasing yearly,China has proposed 2030"Carbon Peak",which gradually reduces CO₂emissions after 2035 and achieves the"Carbon Neutralization"development goals by 2060.High value-added carbon-containing resources by CO₂hydrogenation is an effective way of CO₂reuse,which increase CO₂recycling efficiency.Simultaneously,low carbon hydrocarbons can be used as raw materials for many downstream high value-added products in the chemical field,and attracted much attention on"how to convert CO₂directly into low carbon hydrocarbons".Although the traditional Fischer-Tropsch synthesis（FTS）route can effectively hydrogenate CO₂but the carbon chain is long and complex,and product distribution is limited by Anderson-Schlulz-Flory（ASF）distribution.It is difficult to achieve high selectivity for a single target product.Herein,we adopted a bifunctional catalyst.Firstly,CO₂hydrogenation converts methanol as intermediate on metal hydrogenation catalyst.Secondly,it realizes the conversion process of methanol to C₂-C₄low carbon hydrocarbons through zeolite catalyst.Coupled multi-step reaction to achieve efficient CO₂hydrogenation to synthesize low carbon hydrocarbons.XRD,SEM-EDS,BET,NH₃-TPD and H₂-TPR characterization techniques were employed to analyze the physical properties of the catalyst,and the activity of the catalyst was evaluated by continuous reaction in a fixed-bed reactor.Based on a physically mixed bifunctional catalyst which was designed,in order to synthesize low carbon hydrocarbons via the route of CO₂hydrogenation by methanol intermediates.The Cu-based catalyst（marked CZZ below）was prepared by co-precipitation method,then CO₂hydrogenation converted to methanol.Then a modified zeolite catalysts（Me SAPO-34,Me represented Zn,Mn and Zr,respectively）were prepared by incipient wetness impregnation method,then methanol was converted to low carbon hydrocarbons,and the series reaction can be realized by physical mixing of two catalysts in a certain proportion.Cu-based catalyst can efficiently transform CO₂into intermediate product methanol,then through coupling Zr modified SAPO-34 zeolite,methanol can be further converted to low carbon hydrocarbons.The rapid conversion of methanol was significant to enhance methanol formation reaction and promote CO₂hydrogenation.After modification of zeolite with different alkaline metals,the properties of CZZ/Me SAPO-34 series catalysts were studied.The experimental results demonstrated that the Zr modified SAPO-34zeolite exhibited good catalytic activity and the Cu-based metal catalyst maintained the strong hydrogenation ability.After physically mixing two catalysts,they can cooperate efficiently and realize the rapid conversion of methanol.Among them,CZZ/5%Zr SAPO-34 catalyst showed good catalytic performance,the selectivity of C₂-C₄low carbon hydrocarbons reached 90.3%,in which the selectivity of low alkanes（C₃⁰-C₄⁰,LPG）was as high as 86.1%and the selectivity of light olefins and C₅⁺were not detected,by the method of independent calculation CO in products.It breaks through the product distribution limitation of the FTS route.Simultaneously,the product distribution was concentrated and by-products were few,providing the basis for further regulation of catalyst structure.Since the overall hydrogenation capacity of above CZZ/Me SAPO-34 catalysts was efficient,and Cu may act as the Inverse Water Gas Shift Reaction center,resulting in high CO selectivity.According to the synthesis mechanism of the above reaction route,the structure and composition of catalyst were further analyzed,to investigate the physical mixing of non-Cu-based catalysts（Zn Zr O₂,marked as ZZ）with modified zeolite Me SAPO-34（Me represented Zn,Mn and Zr,respectively）as bifunctional catalysts to investigate distribution of low carbon hydrocarbons products via the reaction route of CO₂hydrogenation by methanol intermediates.The ZZ catalyst was prepared by precipitation method,modified zeolite catalyst Me SAPO-34was prepared by hydrothermal synthesis method.Compared with the CZZ/Me SAPO-34 catalyst,the selectivity of low alkanes C₂⁰-C₄⁰was significantly reduced and the selectivity of light olefins C₂⁼-C₄⁼was greatly increased with lower CO selectivity in ZZ/Me SAPO-34 catalyst.The selectivity of C₂-C₄low carbon hydrocarbons reached 95.3%in 13%ZZ/Mn_0.1SAPO-34 catalyst,consists 33.6%low alkanes C₂⁰-C₄⁰and 61.7%light olefins C₂⁼-C₄⁼.The above physically mixed metal catalysts with zeolite to realize series reaction on bifunctional catalyst.The core-shell structure bifunctional catalyst was designed which based on the problem of non-uniform mixing of components in physical mixed catalyst.The CZZ,ZZ and Mn SAPO-34 were prepared using the methods in the above sections.Subsequently,a layer of Silicalite-2（marked as Si-2）was introduced into different positions of the core-shell structure catalysts by hydrothermal synthesis to make the core-shell catalysts.Compared with the performance of the core-shell catalysts,which Mn SAPO-34 was directly coated on the surface of the core（CZZ or ZZ）,and the influence of different core-shell structure on the catalysts performance was investigated.Compared with the physical mixed catalysts,the results indicated that core-shell catalysts（CZZ@Mn SAPO-34 and ZZ@Mn SAPO-34）undoubtedly decreased the selectivity of CO,the selectivity of low olefins C₂⁼-C₄⁼increased and the selectivity of low alkanes C₂⁰-C₄⁰decreased slightly.Furthermore,the ZZ/Mn SAPO-34@Si-2 and CZZ/Mn SAPO-34@Si-2 catalysts were prepared,in which showed significant low alkanes C₂⁰-C₄⁰selectivity（82.1%）,and light olefins C₂⁼-C₄⁼selectivity（70.0%）,respectively.