Preparation And Performance Study Of Cuo-ZnO/TiO₂Catalyst For Methanol Synthesis From Greenhouse Gas Carbon Dioxide Hydrogenation

A large amount of carbon dioxide emission led to global greenhouse effect andenvironmental pollution. Research of carbon dioxide emission reduction andutilization is of focus and urgent issues in the world. Methanol synthesis by carbondioxide hydrogenation is one of approaches of carbon dioxide utilization. Methanolis considered an important raw material of fundamental organic chemical industryand a new-type environmental protection clean fuels. Carbon dioxide hydrogenationto methanol is a new pathway for strategic adjustment of energy structure, and it isimportant significance to solve the increasingly environmental pollution and energyquestions.In the process of carbon dioxide hydrogenation to methanol, the existingproblems of the general preparation methods of catalysts are lower to carbon dioxideconversion and methanol selectivity. In regard to these problems, CuO-ZnO/TiO₂catalysts were prepared by a novel parallel-slurry-mixing-precipitation methodcombined with addition of cetyltrimethylammonium bromide （CTAB） surfactant.Structure and catalytic performance of catalysts were improved effectively.CuO-ZnO/TiO₂catalysts were prepared by parallel-slurry-mixing-precipitationmethod combined with addition of surfactant, two-step precipitation methodcombined with addition of surfactant, parallel-slurry-mixing-precipitation andimpregnation method, respectively. Characterizations of physical and chemicalproperties of the catalysts were investigated by BET, XRD, SEM, EDS, H₂-TPR,H₂-TPD and CO₂-TPD. The effects of different preparation methods on catalyticperformances of CuO-ZnO/TiO₂catalysts were studied. The catalytic performancesof CuO-ZnO/TiO₂catalysts for methanol synthesis from CO₂hydrogenation wereevaluated by fixed bed reactor.The research results show that CuO-ZnO/TiO₂catalysts of different preparationmethods are mesoporous materials. BET specific surface area and pore volume forthe CuO-ZnO/TiO₂catalysts of precipitation methods are greater than that ofimpregnation method, and dispersion of CuO and ZnO is better. TiO₂preventedgrowth of CuO crystal particles and promoted the dispersion of CuO species. In theprecipitation methods, CuO reduction temperature of CuO-ZnO/TiO₂catalyst ofparallel-slurry-precipitation method combined with addition of surfactant is lowest,and its reduction is easiest. CuO reduction temperature of that of impregnationmethod is highest, and its catalytic activity is lowest. H₂-TPD showed that hydrogendesorption peak area was large at high temperature and strong adsorption state, andactive hydrogen molecules on catalyst surface were more and more, and methanol synthesis from CO₂hydrogenation was promoted. CO₂-TPD revealed that CO₂adsorption center of moderate intensity was closely related to methanol synthesisfrom CO₂hydrogenation. From high to low, the activity and methanol selectivity ofCuO-ZnO/TiO₂catalysts of different preparation methods wasparallel-slurry-mixing-precipitation method combined with addition of surfactant,two-step precipitation method combined with addition of surfactant,parallel-slurry-mixing-precipitation and impregnation method. Addition ofsurfactant improved effectively catalytic performances of CuO-ZnO/TiO₂catalysts.CuO-ZnO/TiO₂catalyst of parallel-slurry-precipitation method combined withaddition of surfactant for methanol synthesis from CO₂hydrogenation was best.Under the conditions of reaction pressure of2.5MPa, space velocity of2100h-1,reaction temperature of230℃, H₂/CO₂molar ratio of3:1and reaction time4h,carbon dioxide conversion and methanol selectivity and maximum of methanol yieldwere13.26%,23.95%and3.18%, respectively.Additives of catalysts used in methanol synthesis from carbon dioxidehydrogenation are important effect on structure and performance of catalyst.CuO-ZnO/TiO₂catalyst added ZrO₂, Al2O3and MnOχ, respectively.CuO-ZnO-ZrO₂/TiO₂, CuO-ZnO-Al2O3/TiO₂and CuO-ZnO-MnOχ/TiO₂catalystswere prepared by parallel-slurry-mixing-precipitation method combined withaddition of CTAB surfactant. The structure characterization and chemical propertiesof the catalysts were investigated by BET, XRD, SEM, EDS, H₂-TPR, H₂-TPD,CO₂-TPD. The effects of different promoters and ZrO₂content on catalyticperformance of CuO-ZnO/TiO₂catalyst were studied. In the presence ofCuO-ZnO-ZrO₂/TiO₂catalyst, reaction mechanism of carbon dioxide hydrogenationto methanol was discussed by XRD, XPS, H₂-TPD, CO₂-TPD and DRIFT.The research results show that addition of different promoters increasedspecific surface area and pore volume for the CuO-ZnO/TiO₂catalysts. The additionof MnOχon CuO-ZnO/TiO₂catalyst increased the reduction temperature of CuO,and decreased adsorptive hydrogen concentration and catalytic activity. AddingZrO₂or Al2O3promoter decreased the reduction temperature of CuO, enhancedcapacity of H₂and CO₂adsorption, and promoted methanol synthesis. In theCuO-ZnO/TiO₂catalysts containing different promoter, CuO-ZnO-ZrO₂/TiO₂catalyst exhibited an optimal catalytic performance. Under the conditions ofreaction pressure of2.5MPa, space velocity of2100h-1, reaction temperature of230℃, H₂/CO₂molar ratio of3:1and reaction time4h, carbon dioxide conversionand methanol selectivity and maximum of methanol yield were18.24%,31.52%and5.75%, respectively.ZrO₂content is important effect on the structure and performance ofCuO-ZnO/TiO₂catalysts. Along with the increase of ZrO₂content, specific surface areas and pore volume of the catalysts increase, and pore size distribution isconcentrated. ZrO₂promoted the dispersion of CuO and ZnO species and decreasedthe reduction temperature of CuO, and enhanced catalyst activity. H₂-TPD showedthat increasing ZrO₂content of catalysts promoted strong adsorption of hydrogenand adsorptive hydrogen of higher concentration on catalyst surface, and possessedfunction of hydrogen storage and enhanced reaction rate of CO₂hydrogenation.CO₂-TPD revealed that ZrO₂improved CO₂adsorption and activation. IncreasingZrO₂content of catalysts enhanced catalytic performance.On the CuO-ZnO-ZrO₂/TiO₂catalyst, CuO being reduced to Cu species are theactive sites of H₂adsorption and dissociation. ZnO and ZrO₂species are the activesites of CO₂adsorption. The dual-site reaction mechanism of adsorption andactivation for H₂and CO₂is reasonable for the directly synthesis methanol by CO₂hydrogenation on the CuO-ZnO-ZrO₂/TiO₂catalyst.