| Carbon dioxide fixation has received worldwide attention in decades, since CO2 is not only one of the greenhouse gases, but also an abundant, economical, and biorenewable resource. One of the most promising reactions in this area is the reaction of CO2 and epoxides to prepare polymeric or cyclic carbonates. Since salen-metal complexes [(salen)MX] are stable to air and water, easy to prepare, and have higher catalytic activity for the copolymerization of CO2 and epoxides, among the numerous metal-based catalyst systems, the salen-metal complex system is of special interest. Using binary catalyst system of (salen)MX as catalyst in conjunction quaternary ammonium/phosphonium salt cocatalyst could markedly enhance the catalytic activity and selectivity. However, it has been only about ten years since salen-metal complex was firstly applied to catalyze the reaction of CO2 and epoxides. Therefore, while some achievements have been made in this area, there are still many areas we need to explore, such as the modification and development of (salen)MX or investigating the reaction mechanism in more detail, In this paper, we focus on the (salen)MX- quaternary ammonium/phosphonium salt catalytic system. Several new salen metal complexes are synthesized and applied to catalyze the reaction of the CO2 and eopxides. In addition, the catalytic activity of the quaternary ammonium/phosphonium salts on such reaction is studied in more detail.Up to now, diamines, as the framework for H2salen ligands, focus only on some ortho-diamines, such as 1,2-cyclohexenediamine,1,2-phenylenediamine, and 1,2-ethylenediamine. So far as we know, the copolymerization catalyzed by salen-metal complex from non-ortho-diamine was absent. It prompted us to verify whether the salen-metal complex synthesized from non-ortho-diamine could effectively catalyze the copolymerization of CO2 and epoxides. We prepared a natural lysine-based (salen)Crâ…¢Cl, and investigated the copolymerization of CO2 and cyclohexene oxide(CHO) catalyzed by this catalyst. The results showed that the (lys-salen)Crâ…¢Cl, with PPNC1 (PPN+=bis(triphenylphosphoranylidene)ammonium) as cocatalyst, could effectively catalyze the alternating copolymerization. In contrast to the traditional salen-metal catalysts, the (lys-salen)Crâ…¢Cl catalyst was prepared from lysine, which is commercially available and optically pure. Although the activity of (lys-salen)Crâ…¢Cl catalyst is not better than traditional salen-metal catalysts, it shows some advantages such as high carbonate linkage, narrow molecular weight distribution, and good selectivity. In addition, the ESI-MS measurement of (lys-salen)Crâ…¢Cl indicated that one molecule of water coordinated to the central metal ion, responding to the bimodal GPC curve of copolymers.The catalytic activity of (lys-salen)Crâ…¢Cl-PPNCl on the reaction of CO2 and propylene oxide(PO) was also investigated, and it exhibited that this catalytic system could catalyze the coupling reaction of CO2 and PO in low catalytic concentration, producing cyclic carbonate. Since the copolymerization of CO2 and PO is usually catalyzed by salen cobalt complexes, the cobalt complex of lys-salen ligand [(lys-salen)CoOAc] was also prepared and applied to catalyze the reaction of CO2 and PO. It is a pity that it is also a coupling reaction between CO2 and PO.Many studies show that the ionic-liquid-supported catalysts are of particular advantage in many reactions. Can we synthesize a quaternary ammonium/phosphonium-supported salen metal complex? Is such a bifunctional salen metal catalyst of particular advantage in the reaction of CO2 and epoxides? We tried to modify the phenyl of salen ligand, in order to prepare quaternary ammonium-supported salen metal complex, and finally several ammonium-supported salen metal complexes were synthesized. However, most of these complexes have almost no catalytic activity. We believe that low solubility in PO and the short distance between the supported ammonium and metal center are main factors for the deactivation of catalysts.In most cases, a better catalyst is selected by comparing the reaction results with different catalysts after practical experiment. From the standpoint of resource conservation, how to select catalysts simply and cost-effectively is also an important topic. The relationship of activity and electrical conductivity of catalyst, which contains quaternary ammonium/phosphonium and alkali halides, in water-assisted coupling reaction of CO2 and PO was studied. It exhibited that the PO conversion was closely correlated with the electrical conductivity, when different catalysts, different amounts of water, and different hydroxy solvents were added in the reaction. This phenomenon not only proves that the catalytic activity was closely related to the ionic mobility of catalysts, but also indicates the simple operation of conductivity measurement might be helpful in deciding which catalyst should be chosen, which solvent should be used, how much solvent should be added in these reactions, and even in some other halide salts-aided reaction.To sum up, several new salen metal complexes were synthesised, and combined with quaternary ammonium/phosphonium salts, which were applied to catalyze the reaction of CO2 and epoxide. In addition, the activity of quaternary ammonium/phosphonium salt was studied. We except it can be used for the futher development of the (salen)MX-quaternary ammonium/phosphonium salt catalytic system for the chemical fixation of CO2.
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