Study On Catalyst For Direct Synthesis Of Dimethyl Ether By Hydrogenation Of Carbon Dioxide

The technological development for direct synthesis of dimethyl ether (DME) via CO₂ hydrogenation is of great importance to utilization of CO₂ resources, to dispel the greenhouse effect of CO₂, to improve the current structure of power source, and to promote the utilization of reproducible energy.The synthesis of DME by CO₂ hydrogenation is a complicated process; it consisted of the CH3OH synthesis step by CO₂ hydrogenation and the dehydration step of formed CH3OH to DME. In this thesis, the Cu-based CO₂ hydrogenation component and the HZSM-5 zeolite dehydration component were selected as main research object to constitute the hybrid catalyst for direct DME synthesis, the effects of preparation conditions of catalyst, including compositions, precipitating conditions, calcinations conditions, promoters, and combination modes of two different functional components, on the performances of catalyst were investigated systematically. Using the techniques of BET, TEM, XRD, IR, XPS, TPR, NH3-TPD, H2-TPD and CO₂-TPD, some physico-chemical properties of the hybrid catalyst samples, such as macroscopic physical properties, phase structure, adsorption behavior to reactants, disperse properties, element composition and structure on surface etc., were characterized more roundly. The catalytic performances of hybrid catalyst samples were evaluated on the micro fixed bed reactor-gas chromatographic apparatus combined equipment. The related possible reaction mechanism, as well as the technical and economical prospective about this synthesis route were also discussed.The H2-TPR studies showed that the only single peak was observed in all TPR profiles of concerned catalyst samples, and the peak temperature was lower than that of pure CuO, which indicated the possible formation of some solid solution of CuO and ZnO, thus the interaction between both oxides improve the reduction behavior of Cu2+ species. In all XRD patterns, the characteristic diffraction peaks for HZSM-5 were observed, which indicated the phase structure of HZSM-5 was perfectly maintained after a series of treatment procedure,whereas the half width of characteristic diffraction peaks for ZnO and CuO and the specific surface area of different catalyst samples showed apparent difference. These facts indicated that different preparation methods and treatment procedures of catalyst could result in great difference of their surface properties and bulk structure. In the IR absorption spectra of hybrid catalyst samples, an absorption peak at the wave number of1630cm-1 appeared, which was not characteristic for ZnO and CuO. It could be attributed to the characteristic vibration absorption of Cu-O-Zn bands, which was only formed in the catalyst sample with finely mixed components and after reduction treatment.The HZSM-5 zeolite component provided the surface acidic sites for hybrid catalyst samples. But NH3-TPD experimental results indicated that the acidity of hybrid catalyst samples, either acidic amount or acidic intensity, was rather different from those of HZSM-5 zeolite. These differences might come from the metallic ion exchange with those in zeolite skeleton and the coverage of metal oxide on HZSM-5. The adsorption studies of reactants H2 and CO₂ on catalyst surface showed that there existed two types of adsorption sites for H2 and CO₂ both, the weak ones and strong ones. The adsorption properties for reactant could be correlated with the catalytic activity of catalyst samples. The stronger adsorption sites for H2 and CO₂ were corresponded to higher de-sorption peak temperature. From the IR spectra of catalyst samples adsorbed CO₂, it was concluded that the inorganic carbon could be reduced to the HCOO- radicals, CH3O- radicals and CH3OH species, and the activated CO₂ species could be converted to formate species ( HCOO- radicals).During the preparation process of catalyst by the co-precipitation method, the kinds of precipitant and precipitation modes can influence the properties of catalyst obviously. When Na2CO3 aqueous solution was used as precipitant, the obtained catalyst samples showed higher specific surface area and better dispersion degree, and the optimal calcinations temperature was 350℃. The calcination at excessive high temperature or under nitrogen atmosphere would cause the sintering of catalyst, thus serious decreasing in surface area and catalytic activity. The XPS characterization data suggested that calcinations treatment under nitrogen protection would make the Cu atoms on catalyst surface migrate into bulk phase of catalyst, whereas the Zn atoms enrich onto surface of catalyst to some extent.Among other preparation methods of catalyst, the catalyst sample prepared by the precipitation-deposition method showed less dehydration ability, because the deposed oxides particulates resulted in the blockage of pore mouth, thus inhibited the efficient diffusion of methanol molecules onto the acidic sites on surface seriously, so the dehydration process proceeded rather difficultly. In the catalyst samples prepared by the impregnation method, the metal oxide particulates aggregated and attached around HZSM-5 zeolite, both combined rather tightly. During H2-TPR and CO₂-TPD experiments, it was found that there existed two kinds of CuO species, i.e. the Cu cations into HZSM-5 skeletal structure, and the CuO deposed on zeolite surface.There existed no necessary relationship between the tightness degree of combination of hydrogenation sites with dehydration sites on catalyst samples and their catalytic activity for DME synthesis.When adding the promoters into hydrogenation component, it was found that Al2O3, MnO and Cr2O3 were good structural promoters, which enlarge surface area of catalyst samples, improve the disperse degree of active components, also the reactivity for DME synthesis. MnO and Cr2O3 showed also some electronic promoting effects improved the adsorption ability to reactants of catalyst samples. After addition of NiO and CoO, the hydrogenation of CO₂ was strongly repressed, because they decreased the adsorption ability to CO₂ of catalyst samples.The activity evaluation test indicated that the main products of CO₂ hydrogenation over Cu-based/HZSM-5 catalyst included DME, CH3OH and CO. The effects of reaction conditions on product distribution were investigated systematically, and the optimal reaction conditions were found. With the increase of reaction temperature, the CO₂ conversion enhanced, and the maximal DME/CH3OH selectivity appeared at 240℃. With increase of reaction system pressure, the CO₂ conversion and total DME/CH3OH selectivity both increased, higher pressure benefited the formation of DME and CH3OH. With the increase of spatial velocity, the CO₂ conversion decreased, but higher special velocity could improve the total selectivity for object product DME and CH3OH.The study on adsorption properties to reactants CO₂ and H2 indicated that the obvious differences in the adsorption ability among the different catalyst samples appeared. The disperse property, the intensity of characteristic IR absorption peak positioned at 1630cm-1, the adsorption ability to reactants, and the activity of catalyst samples showed the mutual relationship to some extent. The hybrid catalyst samples with better Cu disperse degree, stronger IR absorption peak at 1630cm-1 showed also stronger adsorption ability to CO₂ and It was concluded that the IR absorption at 1630cm-1 could be attributed to Cu-O-Zn structure, and the Cu/ZnO species were active centers for CO₂ hydrogenation. When the reduced hybrid catalyst samples were utilized to CO₂ adsorbed IR test, a new absorption peak positioned at 962 cm-1 on IR spectra appeared, which could be attributed to the activated CO₂ species. From the experimental data on CO₂ adsorption IR, CO₂-TPD and H2-TPD, it was concluded that the formation of adsorbed and activated CO₂ species on catalyst surface was the prerequisite for CO₂ farther hydrogenation to oxygenated organic products.