Study On Preparation Of CuO-ZnO-ZrO₂ By Citric Acid Assissted Route And Its Catalytic Performance For Methanol Synthesis From CO₂ Hydrogenation

The recovery and transformation of CO₂ is of the main aspect in realizing the comprehensive utilization of global carbon resources.Among various conversion routes,catalytic conversion of CO₂ to methanol has good application prospects.Recently,a new kind of ternary catalyst,which is composed of Cu/Zn/Zr,is applied in this conversion process.and good catalytic performances have been achieved for its special physicochemical properties.However,the interaction between different components and how these affect the catalytic reaction are still controversial,which is important both for understanding the catalytic reaction process and further developing high activity/high stability catalysts.Therefore,in this work.Cu/Zn/Zr ternary catalyst is prepared by citrate method as the research object,and the preparation paralmeters are systematically studied.Meanwhile,the interaction between different components is also discussed to reveal how these affect the catalytic process.The main results are as follows:1.Preparation of CuO-ZnO-ZrO₂ catalysts by citrate methodCatalyst CuO-ZnO-ZrO₂ was prepared by citrate method.Effects of preparation and pretreatment conditions on catalyst structure and catalytic performances were systematically investigated.The results show that the amount of citric acid and the dehydration approach of gel are the main factors which can greatly influence the catalytic performances.When the amount of citric acid is equal to the stoichiometric ratio,the performance is optimal,and the yield of methanol reaches to 105.6 g·kg-1·h-1（2.6 Mpa,240 0C,3600 h-1）.The effects of calcination temperature and pre-reduction conditions on the structure and catalytic properties were investigated.The results show that pre-reduction conditions can significantly affect the interaction between metal and oxides,and moderate reduction is beneficial to form the copper species with suitable ductility,which plays a positive effect for the conversion reaction.The best pre-treatment conditions were calcined under 450 ℃,4 h and reducted in 10 vol%H2 in N2,250 0C,2 h.The effects of reaction conditions were also investigated.With increasing the temperature,the conversion rate is increased with increasing the temperature and the maximum yield of methanol is achieved at 240 ℃ which is 144.6 g·kg-1·h-1.High reaction pressure,high space velocity and high H2/CO₂（volume ratio）are all favorable for yielding of methanol.2.Analysis of the interaction between different componentsThe synthesis of methanol is positively related to the number of medium strength alkaline sites existed on catalyst surface.When CuO/ZnO is 2/7,the CuO-ZnO-ZrO₂ catalyst（Cu0.2Zn0.7Zr0.1）owns highest content of medium strength basic position,and the time and space yield of methanol is also the highest 193.9 g·kg-1·h-1.Based of Cu0.2Zn0.9Zr0.1 catalyst,di-component and duo-component catalysts were fabricated.The synergistic effect in different components was investigated by CO₂/H2-TPD and XRD methods.The results show that single Cu component has no conversion activity at low pressure（0.8 MPa）,unless it was composited with other metal oxides,which indicates that there be some interaction between Cu species and those oxides.ZnO and ZrO₂ can provide adsorption sites and help motivating those adsorbed molecules.Therefore,abundant active reactants could be supplied to the conversion reaction,and further improve the catalytic ability of this catalyst,leading to achieve high yield of methanol.3.Investigation of long term operation stabilityThe stability of catalyst CuO-ZnO-ZrO₂ was investigated.Catalyst can maintain the conversion rate of C02 over 17%for 500 hours during the reaction process（2.6 MPa,240 ℃,3600 h-1）,and the methanol selectivity is over 40%.Results show that active Cu species are slowly oxidized when the catalyst is contact with CO₂,leading to a decrease of methanol yield for the loss of active sites.30 hours later,this oxidation process is balanced with the reduction process of hydrogen to Cu oxides,the catalytic performance goes to a stable stage.