Study Of Copper Coordition Compound Catalyst For Synthesis Of Dimethyl Carbonate By Carbonylation

Dimethyl carbonate (DMC) is a green chemical attracted high attention. The carbonylation of methanol to DMC is highly promising for industrial application due to the inexpensive substrates, simple process, and low pollution. The catalyst CuCl is widely used for the liquid phase carbonylation of methanol to DMC; however, the application of CuCl is hindered by the shortcomings of low stability, strong corrosiveness, and difficult separation of reaction mixture. As a result, the improvement of catalytic performance becomes a hot research topic. We choose CuBr2 instead of CuCl as research direction because of the better stability and solubility as well as lower corrosiveness of CuBr2 than CuCl, while the catalytic activity of Cu(II) is relatively low. Taking account of the special crystalline structure and coordination property of metal complexes, we employed transition metal complexes as catalysts and systematically investigated their catalytic performance, which includes the preparation and characterization of complexes, thermodynamics, mechanism, and process simulation.Firstly, we choose quaternary ammonium salts and phosphonium salts as ligands to form complexes with CuBr2. Interaction between R4N+ or (R4P+) in ligand and Br" have been changed to enhance the activation degree of Br- in CuBr2, which are favorable for the generation of reaction intermediates and the insertion of CO, O2 and methanol, and subsequently enhance the reaction activity. The effect of ligand quaternary ammonium salts is obvious than phosphonium salts, which is consistent with the shield extent of nitrogen and phosphor atoms. Among them, the ligand (C3H7)4NBr in quaternary ammonium salts showed the best promoting effect. N-methyl-imidazole and 2,2-bipyridine can react with CuBr2 to form stable copper complexes, which is not beneficial to the addition of reactant molecules. Hence, the promotion effects are worse than those of (C3H7)4NBr and (C6H5)4PBr.Secondly, on the basis of the results of FTIR, XRD, EA, ICP-AES, and XPS, the structure of copper complexes is [Cu2Br6]2-rather than the common structure [CuX4]2-. The chemical formula of copper complexes is [(C3H7)4N]2Cu2Br6. The optimal preparation conditions were the molar ratio of CuBr2 to (C3H7)4NBr 1:2, ethanol as solvent, the temperature of 353 K, the reaction time of 1～1.5 h, and dried in low-temperature vacuum system. The complexes catalyst can be obtained with the yield of 59.2%. The catalytic performance of the complexes is higher than that of CuBr2, and is almost equivalent to that of CuCl catalyst. Methanol conversion and DMC selectivity are 27.4% and 98.3%, respectively. Thirdly, thermodynamic analysis to the process of liquid phase oxidative carbonylation of methanol to DMC was obtained under certain reaction conditions, which reveals that the tendency to spontaneous reaction is evident. The reaction mechanism was also proposed according to the fact that the constant valence state of Cu(II) in the reaction. DMC was produced from the reaction of methanol, CO, and O2 activated by Cu2+through the variation of ligand in [Cu2Br6], rather than via the conventional oxidation-reduction pathway. The effects of acidic or basic environment, dehydration agents, and metal additives were also investigated. The optimal composition of composite catalysts was obtained by orthogonal experiments and consisted of K2CO3, ZnBr2, and copper complexes. Under the optimal conditions, the methanol conversion can reach 44.9% with DMC selectivity of 95.1%.Finally, on the catalysis of the complex catalysts, we evaluated the effects of process conditions, recycling of catalysts, and gas continuous process on catalytic activity. Under the conditions of reaction temperature 363～378 K, reaction pressure 2.8-3.5 MPa, reaction time 4-6 h, gas velocity 15～18 L·h-1, total catalysts concentration 0.31 g·mL-1, and solvent dimethylacetamide volume 0.30 mL·mL-1, the methanol conversion and DMC selectivity can reach 58.1% and 93.5%, respectively. When the concentration of raw materials O2 is controlled below 20%, CO selectivity could reach 95%. We investigated the feasibility and reliability of the whole carbonylation process by ASPEN process simulation. The results may provide important technical supports for potential industrial applications.