Construction And Optimization Of Zn_xCe_yZr_zO/HZSM-5 Catalyst For Carbon Dioxide Hydrogenation And Benzene Alkylation

Alkylation of benzene using CO₂and H₂to toluene and xylene（TX）can not only give value-added chemical products but also alleviate the greenhouse effect.This paper focuses on the reaction process of benzene using CO₂and H₂to aromatics,and designed a high-efficient bifunctional catalyst composed of metal oxide and molecular sieve（Zn_xCe_yZr_zO/HZSM-5）,which usesa reaction path,coupling CO₂hydrogenation to methoxy and methoxy and benzene to aromatics,to achieve direct conversion of benzene using CO₂and H₂to aromatics in one step.In thebifunctional catalyst,Zn_xCe_yZr_zO efficiently hydrogenates CO₂to methoxy groups,while themodification of HZSM-5 realizes that benzene and methoxy groups have suitable active sitesand product shape selectivity.Studies have shown that the precise coupling of metal oxides andmolecular sieve active sites has a significant effect on the alkylation performance of benzenewith CO₂and H₂.The main research contents and conclusions are as follows:（1）Influence of bifunctional catalyst Zn_xCe_yZr_zO/Z5 on the process of alkylation of benzene with CO₂and H_2.The key to the alkylation of benzene with CO₂and H_2.is the development of efficient catalysts.Herein,a class of bifunctional catalysts containing a series of Zn_xCe_yZr_zO mixed oxide and HZSM-5 was prepared and studied for the catalytic properties.Higher CO₂conversion and alkylation utilization were achieved using the Zn_xCe_yO coupling with HZSM-5 compared to its counterpart Zn O and Ce O₂,revealing synergy between the Zn O and Ce O₂.More strikingly,the Zn_xCe_yZr_zO,synthesized by insertion of Zirconia（Zr）atoms into Ce O₂,can not only improve the concentration of oxygen vacancy but also facilitate CO₂adsorption.After coupling with HZSM-5,the alkylation utilization of CO₂was significantly improved and the unwanted CO selectivity was obviously reduced by hindering the RWGS reaction.The optimal bifunctional catalyst Zn₃Ce₁Zr₃O/Z5 showed alkylation utilization of 24%,unwanted CO selectivity of only 64%and the TX selectivity as high as 82%.In situ DRIFTS results confirmed that the methanol generated on the Zn_xCe_yZr_zO mixed oxide via the formate-methoxy intermediates mechanism was rapidly alkylated on HZSM-5 with benzene to produce toluene and xylene.（2）Study on the catalytic performance of zinc and silicon modified HZSM-5 for the alkylation of benzene with CO₂and H_2.W%Zn/Z5 catalysts with different zinc impregnation amount were prepared by impregnation method.W%Zn/z5sn was obtained by modifying HZSM-5 by liquid phase deposition（CLD）on the basis of the optimal amount of zinc impregnation.The crystallinity,specific surface area,pore size and surface acidity of the modified catalyst were analyzed by XRD,bet,FT-IR and NH₃-TPD,and its effect on the alkylation of benzene with CO₂ and H₂was further investigated.The experimental results show that Zn modification can change the acid distribution of HZSM-5 and transform some strong acid sites into weak acid sites.Through characterization,it was found that some strong acid sites of the modified molecular sieve were transformed into weak acid sites,and the selectivity of p-xylene was increased to 33%.CLD modification can modify the orifice and surface acidity of HZSM-5.After three times of modification,the selectivity of p-xylene in xylene is increased to 50%.