Preparation Of ZnO-ZrO₂@Al₂O₃@SAPO-34 Bifunctional Core-shell Catalysts And Hydrogenation Of CO₂ To Olefins

In the tandem reaction process of CO₂ hydrogenation via methanol to lower olefins,the existing catalytic system has the problem of excessive CO content of by-products and low yield of lower olefins.In order to further promote the catalytic performance of ZnO-ZrO₂/SAPO-34 in the hydrogenation reaction of CO₂ to hydrocarbons,in view of the influence of composite phase interface characteristics on the catalytic performance of ZnO-ZrO₂/SAPO-34 bifunctional catalysts,the paper proposes the preparation,characterization and performance evaluation of ZnO-ZrO₂oxide solid solution and acidic SAPO-34 molecular sieve from the perspective of physical and chemical compounding.The effect of various Zn-Zr mass ratio,aging gel time,calcination time,precipitation pH value and calcination temperature on the catalytic performance of ZnO-ZrO₂/SAPO-34 mixed catalysts were investigated by physical blending.The results shown that the suitable preparation conditions of ZnO-ZrO₂ were 30 wt%Zn,precipitation pH=7,aging for 3 h and calcination at 500℃for 3 h.The mesoporous ZnO-ZrO₂ solid solution prepared under suitable conditions exhibits structural characteristics of weak base,specific surface area of 32 m²·g^-1,pore volume of 0.24cm³·g^-11 and average pore diameter of 12.16 nm.The effects of two-phase mass ratio,H₂ reduction temperature on the structural properties and hydrogenation of CO₂ performance of the composite catalyst were investigated systematically based on physical blending strategy,and the catalyst evaluation conditions were optimized.The physicochemical properties of ZnO-ZrO₂/SAPO-34 mixed catalysts prepared by various H₂ reduction temperatures were characterized by XRD,XPS,CO₂-TPD and NH₃-TPD.The results shown that the suitable mass ratio of SAPO-34 to ZnO-ZrO₂ was 1:3,the reaction temperature,pressure and volume space velocity were 380℃,3.0 MPa and 3500 h^-1,respectively.The catalyst obtained by hydrogen reduction at 500℃for 3 h exhibited higher catalytic performance with 73%selectivity for all hydrocarbon products（the selectivity of lower olefins was 70%）,and CO by-product was 27%.Compared with the catalyst without hydrogen reduction,the conversion of CO₂ was increased by 3%,and the selectivity of CO by-product was reduced by 14%.From the chemical point of view,the ZnO-ZrO₂@SAPO-34 composite catalyst was prepared by hydrothermal coating method,and the ZnO-ZrO₂/SAPO-34 mixed catalyst was used as a reference.the effects of liquid precipitation coating method and hydrothermal coating on the physicochemical properties and catalytic performance of the composite catalysts were investigated.These obtained products were extensively characterized by XRD,XRF,XPS,BET,SEM,HRTEM-EDS,CO₂-TPD and NH₃-TPD techniques to investigate their crystalline phase,morphology,element distribution and binding energy,pore structure,surface acidity and alkalinity.The results shown that the composite preparation method had a great influence on the structural properties of the catalyst.A two-phase of composite structure did not existe in the SZ-LPC composite sample fabricated via liquid phase precipitation coating process.The ZnO-ZrO₂@SAPO-34 composite catalyst fabricated via hydrothermal coating method had formed a core-shell structure,but the ZnO-ZrO₂ component was deactivated,resulting in a significant decrease in catalytic performance.The CO₂conversion rate was 13%,the selectivity of lower olefins was 21%among all hydrocarbon products,and the selectivity of CO by-product was 55%.From the chemical point of view,ZnO-ZrO₂@Al₂O₃@SAPO-34 bifunctional composite catalyst was fabricated via hydrothermal coating method.The effects of Al₂O₃ content,Al₂O₃ calcination temperature,two-phase mass ratio and other hydrothermal coating conditions on the structural properties and hydrogenation of CO₂ performance of the composite catalyst were investigated systematically.These obtained products were extensively characterized by XRD,XPS,BET,SEM-EDS,HRTEM,CO₂-TPD and NH₃-TPD techniques to investigate their crystalline phase,morphology,element binding energy,pore structure,surface acidity and alkalinity.Using hydrothermal coating method,under the conditions that the mass ratio of Al₂O₃:ZnO-ZrO₂ was 1:2,the calcination temperature was 600℃,and the mass ratio of ZnO-ZrO₂@Al₂O₃:SAPO-34 was 1:1,ZnO-ZrO₂@Al₂O₃@SAPO-34 was prepared to existed a uniform core-shell composite phase,meso-microporous structure(the total specific surface area was 258 m²·g^-1,the specific surface area of micropores was176 m²·g^-1,the specific surface area of mesopores was 82 m²·g^-1,and the total pore volume was 0.2 cm³·g^-1),and acid-base characteristics.Under the conditions of V（H₂）/V（CO₂）=3,reaction temperature of 380℃,reaction pressure of 3.0 MPa,reaction volumetric space velocity of 3500 h^-1,CO₂ conversion and lower olefins among all hydrocarbon products were 21%and 75%,respectively.Compared with the ZnO-ZrO₂/SAPO-34 composite catalyst,the conversion of CO₂ was increased by 4%,and the selectivity of lower olefins were increased by 5%among all hydrocarbon products.