Catalytic Hydrogenation Of CO₂ To Methanol And Low-carbon Olefins Over In Modified Cu-ZnO-Al₂O₃ Hydrotalcite Catalyst

Methanol is an important organic raw material and clean energy,and catalytic hydrogenation of CO₂ to methanol could effectively easy up energy crisis and greenhouse effect,which is an important and significant research work.Cu-Zn O-Al₂O₃ catalyst has emerged as a remarkable catalyst for the hydrogenation of CO₂ to methanol.However,the yield of methanol product is unsatisfactory due to the low dispersion of Cu,poor activation capacity of CO₂,and competitive reaction of reverse water gas reaction（RWGS）.In this work,a series of In-doped Cu-Zn O-Al₂O₃catalysts were synthesized from the Cu-based hydrotalcite precursors and employed in the hydrogenation of CO₂ to methanol.The uniformly dispersed metal ions,synergistic effects and adjustable surface acidity and alkalinity were in favor of improving the dispersion of Cu,the activation capacity of CO₂ and the selectivity of methanol.（1）A series of In-doped Cu-Zn O-Al₂O₃ hydrotalcite precursors(HT-Inx,x=n(In³⁺)/(n(In³⁺)+n(Al³⁺)),x is 0,0.1,0.2,0.3,0.4 and 0.5,respectively)were synthesized via a hydrothermal method,and two-dimensional layered catalysts CHT-Inx and r-CHT-Inx,with different In contents were obtained after calcination and reduction.The prepared catalysts were extensively characterized using various techniques such as XRD,TG,CO₂-TPD,NH₃-TPD,H₂-TPR and XPS,and the catalysts were evaluated in the hydrogenation of CO₂ to methanol.（2）The two-dimensional layered structure of the catalyst precursor was maintained in the 5 samples with different In contents.Only Cu O crystalline phase structure was detected in the calcined catalysts（CHT-Inx）,and other metals were in the amorphous phase.The oxygen vacancies in the reduced catalyst r-CHT-Inx increased with the increase of In content.The dispersion of Cu and specific surface area of Cu showed a volcanic trend with the increasing of In content,where the dispersion and specific surface area of Cu reached its maximum value in r-CHT-In0.3.The adsorption capacity of the catalyst for CO₂ increased first and then decreased with the increasing of In content,and the r-CHT-In0.4 catalyst showed the strongest activation capacity of CO₂ adsorption.（3）The conversion of CO₂ and the selectivity of methanol showed a volcanic trend with the increasing of In content at 513 K,3 MPa with a space velocity of 8000h^-1,and the highest conversion of CO₂ and selectitity towards methanol were achieved its maximum value over r-CHT-In0.3,that was 19.1%and 79.6%,respectively.（4）The reaction pathway of CO₂ hydrogenation to methanol was investigated by in situ infrared diffuse reflectance（DRIFTS）,and HCOO*and CH₃O*were found to be the main intermediates in the reaction,which were further hydrogenated to form methanol.The two intermediate were proved to the key species to improve the selectivity for methanol.The r-CHT-In0.3 catalyst prepared in the above study was mixed with SAPO-34and further employed in the CO₂ hydrogenation to C₂-C₄ olefins.The effect of catalyst mixing way on the selectivity for olefins was investigated in detail.The results showed that increasing the catalytic contact area could effectively improve the selectivity for C₂-C₄ olefins,and the total selectivity of C₂-C₄ olefins reached 43.2%with the reaction conditions of 653 K,2 MPa,and 8000 h^-1.Furthermore,the amount of surface acid of SAPO-34 reduced with the incorporation of Zn and the selectivity towards C₂-C₄ olefins olefins in the product could reached to 51.5%over a mixture of r-CHT-In0.3/SAPO-34.