Sutdy On Preparation Of ZnO-Mn₂O₃ Catalyst And Catalytic Hydrogenation Of CO₂ To Methanol

CO₂ hydrogenation to methanol can not only solve the problem of CO₂ emissions effectively,but also realize the utilization of CO₂ resources.The supported catalysts for CO₂hydrogenation to methanol have the problems of low CO₂ conversion rate,severe reverse water gas reaction at high temperature,resulting in excessive CO content in the by-products and low methanol selectivity of the product,oxide catalysts have poor high temperature stability.According to the mechanism of high-temperature CO₂ activation of oxygen vacancies in bimetallic oxide catalysts,in view of the unique redox and structural properties of manganese oxide?Mn_xO_y?,Mn-O structures are prone to form a large number of rich oxygen vacancies,while ZnO has a unique electronic structure on its surface In the field of heterogeneous catalysis,it exhibits excellent heterocrack hydrogen overflow ability.Based on the design of solid solution structure catalyst based on dual active sites,this paper proposes to construct a ZnO-Mn₂O₃ composite oxide to catalyze the reaction of CO₂ hydrogenation to methanol,with a view to achieve the catalyst has good high-temperature stability and excellent catalytic performance.The research work was carried out on the analysis and evaluation of the structural properties and catalytic performance of MnO_x catalyst,the preparation and characterization of ZnO-Mn₂O₃ composite oxide,the performance evaluation of catalytic hydrogenation of CO₂ to methanol,and the reaction mechanism.The catalytic performance of MnO_x and ZnO were prepared and investigated respectively.The results show that MnO_x changes from MnO₂ to Mn₂O₃ and Mn₃O₄ in sequence with increasing temperature;among the three manganese oxides,the catalytic activity of Mn₂O₃ for CO₂ hydrogenation to methanol is the best,with CO₂ conversion to 7.4%,methanol selectivity and STY are 55.5%and 0.20 g_MeOH·h^-1·g^-1_cat,respectively;CO₂ conversion of ZnO is 6.3%,methanol selectivity and STY are 74.2%and 0.23 g_MeOH·h^-1·g^-1_cat,respectively.It can be seen that although Mn₂O₃ has a high CO₂ conversion rate,the methanol selectivity is insufficient.Conversely,although Zn O has a high methanol selectivity,the CO₂ conversion rate is very low.The focus is on the preparation and catalytic performance of ZnO-Mn₂O₃ composite oxide.Firstly,the effects of three different preparation methods:co-precipitation method,impregnation method and material mixing method on the structural properties and catalytic performance of ZnO-Mn₂O₃ composite oxide are compared,the results show that the ZnO-Mn₂O₃ composite oxide prepared by co-precipitation method has the best catalytic performance.performance.On the basis of investigating the preparation method of ZnO-Mn₂O₃ composite oxide and its influence on structural properties and catalytic performance,the structure-activity relationship of ZnO-Mn₂O₃ composite oxide prepared by co-precipitation method in the reaction of CO₂ hydrogenation to methanol was studied.?1?With the help of XRD,XPS,XRF,CO₂-TPD,H₂-TPR,O₂-TPO,SEM,TEM and other characterization methods,the preparation conditions of ZnO-Mn₂O₃ composite oxide were determined,such as manganese-zinc mass ratio,pH study precipitation end point value,aging Temperature and calcination temperature.The effects of the structural performance and catalytic performance of the bimetallic oxide catalyst on the catalyst structure-activity relationship were studied.The results show that the ZnO-Mn₂O₃ composite oxide prepared under the conditions of ZnO content of 30 wt%,precipitation pH=7,aging temperature of70°C,and air atmosphere calcination at 500°C is a ZnMn₂O₄ spinel catalyst with the best ZnMn₂O₄ chemical composition,with good dispersion and high crystallinity.There are many basic sites?6.76 cm³/g?and oxygen vacancies?relative oxygen vacancy concentration is 66%?,and the catalytic performance is good,under the conditions of reaction temperature 380?,reaction pressure 3.0 MPa,reaction volume space velocity 14400m L/g·h,methanol selectivity is 88.9%,CO₂ conversion rate is 17.8%,STY is 0.78 g_MeOH·h^-1·g^-1_cat,by-products are mainly CO and dimethyl ether,a small amount of CH₄ appears,CO selectivity is 5.9%,dimethyl ether selectivity is 5.9%,CH₄ selectivity is 0.5%.?2?By means of XRD,TG,FT-IR and O₂-TPO-MS combined analysis and characterization of the ZnMn₂O₄ catalyst after the reaction,it was found that alcohols,benzene rings and alkanes existed in the carbon deposits after the reaction,These high-temperature macromolecules will generate a small amount of CO and a large amount of CO₂ after oxidation and carbonation.The reaction mechanism of ZnMn₂O₄ catalyst for CO₂ hydrogenation to methanol was analyzed.In-situ infrared?DRIFTS?was used to investigate the catalytic reaction process of ZnMn₂O₄ bimetallic oxide catalysts under different reaction time,temperature,pressure and space velocity.The results show that the reaction mechanism of ZnMn₂O₄ catalyzing the conversion of CO₂ to methanol should follow the formate pathway.CO₂ is adsorbed on the oxygen vacancies of-Mn-O-and-Zn-O-structures for activation,and then reacts with H*Form formic acid HCOO*,then hydrogenate HCOO*to dioxymethylene?HCOOH*?,and then further hydrogenate to H₂COOH*,cleave the carbon-oxygen bond to formaldehyde?H₂CO*?and hydroxyl?OH*?,and adsorb next to the oxygen vacancy The H₂CO*on the metal is further hydrogenated to methoxy?H₃CO*?,and finally hydrogenated to obtain the product methanol.The optimal reaction temperature of ZnMn₂O₄ catalyst is 380?,the reaction activity increases with the increase of pressure,and the reaction activity increases with the increase of space velocity within a certain space velocity range.