Design And Preparation Of Cu-Zn-M?Zr?Ti?/SAPO-34 Bifunctional Catalyst And Its Catalytic Performance For CO₂ Hydrogenation To Light Olefins

Chemical conversion of CO2 into light olefins by hydrogenation has become an important research topic.It is known that CO2 hydrogenation to light olefins proceeds via a methanol synthesis or oxide intermediates which can achieve the tandem reactions including the activation of CO2 to methanol and methanol conversion to olefins.The process can break the product distribution of Anderson-Schulz-Flory(ASF)in Fischer-Tropsch synthesis,and achieves high selectivity to light olefins.On traditional Cu-based catalyst for methanol synthesis,the concentration of the intermediate products is low at high reaction temperature.The strong hydrogenation ability of Cu based catalysts leads to higher selectivity of alkanes.Furthermore,in methanol dehydration(MTO)process,the amount and acidity of zeolite has great influence on the product distribution,and the occurrence of side reactions also affects the selectivity of light olefins.Herein,we report the design and preparation of Cu-Zn-M(Zr?Ti)/SAPO-34 composite catalysts to enhance the selectivity of light olefins on Cu-based catalysts.Factors such as composite methods,mass ratio of CZM and SAPO-34 and the acidity of SAPO-34 are investigated to improve the product selectivity.The structure and surface properties of the catalysts were characterized by XRD?CO2-TPD?NH3-TPD?SEM?EDS?N2 adsorption-desorption methods,respectively.The effects of composite methods of Cu-based catalyst and SAPO-34,the amount of SAPO-34 and its acid density on the product selectivity was discussed.The main research and results are as follows.1.The composite methods,mass ratio and the acidity of SAPO-34 had great influence on product selectivity.The results showed that:(1)there existed competition between the formation of methane and the tandem reaction with regulating the content of two active components.The increase of acid content of SAPO-34 could inhibit the formation of methane and promote the tandem reaction.With the increase of SAPO-34 content,the mass ratio of the composite catalyst was 1:2,and the selectivity of CH4 was reduced from 56%to 39%.The hydrogenation of intermediate methanol on metal surface sites was directly inhibited during the reaction process.With the reaction time goes on,the product distribution became stable,and the selectivity of C2-C4 olefins was 14%;(2)The zeolite SAPO-34 acidity had a significant effect on the selectivity for light olefins.After reducing acidity,the secondary hydrogenation of primary olefins was inhibited by Zn-SAPO-34 zeolite.The selectivity of C2-C4 olefins increased about 60%,and the O/P value increased from 0.3 to 3.5;(3)The composite method of the active component has great influence on the product distribution.That is,the contact distance between the two active sites can be adjusted by changing the composite method.The secondary reaction of olefins can be adjusted and the hydrogenation capacity of the reaction can be reduced by increasing the contact distance properly.The selectivity of C2-C4 olefin was increased from 17%to 43%,and the CH4 selectivity decreased from 59%to 14%.Also,with the increasing of contact distance,the O/P value was increased.2.CuZnZrO2@(Zn-)SAPO-34 catalyst with core-shell structure was prepared by physical encapsulation.The synergistic reaction of two steps was controlled,CH4 was reduced and the selectivity of C2-C4 olefins was improved.The study demonstrates that high selectivity of light olefins can be obtained on core-shell structural CuZnZrO2@(Zn-)SAPO-34.The occurring probability of side reactions such as hydrogenation and hydrogen transfer of the primary olefin product decreased due to reduced contact interface and low concentration of intermediate methanol.Also,the reduced acidity of SAPO-34 by Zn modification inhibited secondary hydrogenation reactions.In addition,at low concentration of intermediate methanol environment,C3 and C4 hydrocarbons were promoted formed owing to the hydrogenation and hydrogen transfer reactions of primary products.In CO2 hydrogenation,the CuZnZrO2@SAPO-34(4:1)catalyst showed a significant decrease in CH4 selectivity(<10%).After reducing the acidity of SAPO-34,the selectivity of C2-C4 olefins reached 67%over CuZnZrO2@Zn-SAPO-34(4:1).Moreover,the selectivity of C2-C4 olefins could reach 72%with increase in the space velocity,and the O/P value was 8.6.In addition,the catalyst was highly stable within 110 h of time on stream.