Research On Heterogeneous Catalytic Systems For The Indirect Transformation Of Carbon Dioxide To Synthesize Organic Alcohols And Esters

Clean synthesis of organic alcohols and esters derived from carbon dioxide is a hot research topic of chemical utilization of carbon dioxide. However, direct transformation of CO2 to organic alcohols and esters usually suffers from the problems of low reaction efficiency, harsh reaction conditions and low product yields due to the thermodynamic stability and dynamics inertia of CO2 molecular. It is worth noting that CO2 can react with epoxide to highly efficiently synthesize ethylene carbonate. CO2 can also react with ammonia and ethanol to easily produce ethyl carbamate. In addition, a large volume of fatty acid triglyceride can be achieved via biological carbon sequestration of CO2. Therefore, ethylene carbonate, ethyl carbamate and fatty acid triglycerides can be used as CO2 carrier to further produce high-valued organic alcohols and esters. With the aim of the transformation of the ester group of the three above-mentioned CO2 carrier, this dissertation mainly focused on developing heterogeneous catalytic systems for the hydrogenation of ethylene carbonate to co-produce methanol and ethylene glycol, ethanolysis of ethyl carbamate to synthesize diethyl carbonate, and transesterification of fatty acid triglycerides with methanol to synthesize fatty acid methyl esters. Systematic and in-depth investigations such as the thermodynamics calculation, characterization of catalysts, relationship between catalyst structure and activity, catalyst reusability, catalytic mechanism, and reaction conditions were carried out. The main research contents and conclusions are as follows:(1) Thermodynamics calculation of the hydrogenation of ethylene carbonate to co-produce methanol and ethylene glycol was firstly carried out, which revealed that this reaction was a thermodynamic favorable exothermic reaction. Then, three copper-based catalysts Cu/KIT-6, Cu/MCM-41 and Cu/SBA-15 were successfully prepared by ammonia evaporation method using three ordered mesoporous silicas KIT-6, MCM-41 and SBA-15 as supports. The species of Cu0 and Cu+ with varied ratio were found to co-exist on the catalyst surfaces after reduction, which were verified to originate from CuO and copper phyllosilicate, respectively. The catalytic performances of the three as-prepared copper-based catalysts revealed that among the three catalysts, Cu/SBA-15 exhibited a better catalytic activity. It was found that high copper dispersion and suitable Cu0/Cu+ ratio were responsible for the excellent activity of Cu/SBA-15. Under the optimized condition, the conversion of ethylene carbonate reached 100%, and the yields of methanol and ethylene glycol reached 62.3% and 94.7%, respectively. The reusability of Cu/SBA-15 showed that, the gradually decreasing catalytic activity of Cu/SBA-15 during recycling experiments was probably ascribed to the agglomeration of Cu and Cu2O particles.(2) A series of 2%-30%Cu/SiO2-AE catalysts with different copper loadings were controllably prepared using silica sol as the silica source by ammonia evaporation method. The as-prepared 2%-30%Cu/SiO2-AE catalysts were also used in the hydrogenation of ethylene carbonate to co-produce methanol and ethylene glycol. The effects of copper loading on the catalyst properties and catalytic activities of Cu/SiO2-AE catalysts were systematically investigated. The systematic catalyst characterization results demonstrated that different proportions of Cu0 and Cu+species co-existed in the Cu/SiO2-AE catalysts, which derived respectively from the reduction of CuO and copper phyllosilicate. Catalytic performance evaluation results showed that the 10%Cu/SiO2-AE catalyst with moderate copper loading exhibited better catalytic activity, mainly attributing to the synergetic effect of Cu0 and Cu+ species and the suitable proportion of Cu+/(Cu0+Cu+). Under the optimized condition, the conversion of ethylene carbonate reached 100%, and methanol yield and ethylene glycol yield were further increased to 70.8% and 98.0%, respectively. A plausible catalytic mechanism that Cu0 promoted the dissociation of H2 and Cu+ adsorbed on the carbonyl group of ethylene carbonate was proposed. The catalyst reusability result showed that the catalytic activity of 10%Cu/SiO2-AE catalyst gradually declined along with recycling experiments. The agglomeration of Cu and Cu2O particles was responsible for the decreasing catalytic activity of 10%Cu/SiO2-AE catalyst during recycling experiments.(3) An efficient catalyst MgO-SC-450 was prepared using sodium carbonate as the precipitant and applied in the ethanolysis of ethyl carbamate (EC) to synthesize diethyl carbonate (DEC). Catalyst characterization results revealed that MgO-SC-450 possessed a high specific surface area of 245 m2 g"1, nanosheet morphology and a larger amount of appropriate medium basic sites. MgO-SC-450 exhibited higher activity and selectivity with a TOF of 3522 mgDEC gcat-1 h-1 obtained at 200℃. Under the optimized reaction condition, an excellent DEC yield of 58.0% with a high DEC selectivity of 92.1% and EC conversion of 62.9% were achieved over MgO-SC-450 catalyst. The structure and surface basicity of the different MgO catalysts showed that more abundant medium basic sites were favorable for MgO-SC-450 to obtain much superior catalytic activity. Moreover, the catalytic activity and structure properties of MgO-SC-450 could be essentially retained during recycling experiments. Quasi in situ FT-IR experiments were carried out to elucidate the adsorption behaviors of reactants. It was found that EC could be effectively activated and ethanol could be dissociated to a strong nucleophilic ethoxy group by MgO. In addition, the theoretical calculation results proved that EC and ethanol co-adsorbed on the MgO surface.(4) A highly efficient waste carbide slag based catalyst CS-650 was developed using widely available and low-cost waste carbide slag as the solid calcium-based catalyst materials by facile thermal activation. The as-prepared CS-650 was employed in the transesterification of fatty acid triglyceride with methanol to synthesize fatty acid methyl esters (FAME). It was found that the catalytic activity of CS-650 was much higher than commercial CaO. The relationship between the surface basicity and activity indicated that stronger basicity of catalyst was responsible for the higher activity of CS-650. Under the optimized condition,91.3% FAME yield with TOF of 182.6 gFAME gcat-1 h-1 were achieved over CS-650. In addition, the reusability and structure changes of CS-650 showed that the formation of calcium glyceroxide resulted in the decreasing activity of CS-650 to 80.1% along with recycling experiments.