Catalytic Synthesis Of Cyclic Carbonates Under Supercritical Conditions

The present thesis originates from catalytic activation of CO2, and studies the cycloaddition of CO2 with epoxides to produce corresponding cyclic carbonates under supercritical conditions. Some advantages for the utilization of supercritical CO2 as both a solvent and a reactant will be discussed in catalytic synthesizes of some organic compounds.For understanding the phase behavior during homogeneous catalytic reaction of CO2 and ethylene oxide to form ethylene carbonate under supercritical conditions, vapour-liquid equilibria in the system CO2/ethylene oxide at various temperatures and in the system CO2/ethylene oxide/ethylene carbonate at 110'Chave been studied in detail. It can be confirmed that the binary system of CO2 and ethylene oxide has a continuous critical locus between both critical points of the pure components. The results also indicate that phase change must occur during the cycloaddition reaction of CO2 and ethylene oxide under supercritical conditions.Homogeneous catalytic formation of ethylene carbonate from supercritical CO2/ethylene oxide mixture was realized by using tetradentate Schiff-base aluminum complexes as catalysts. The effect of phase change during the reaction on the rate was also explored.Various tetradentate Schiff-base aluminum complexes, which can be dissolved in supercritical CO2/ethylene oxide mixture, are effective catalysts for the cycloadditions of CO2 and epoxides. Axial group, substitution on the aromatic rings of SalenAlX, and co-catalyst all affect catalytic properties of these aluminum complexes. Among them, the binary catalyst consisted of SalenAl-(OCH2CH2)3Cl and n-Bu4NBr was found to be most effective and exhibited a catalytic activity of upto 2340 turnovers/h at 110"C, which was one of the highest for the cycloaddition of CO2 to epoxides amongst the reported catalysts. With the same binary catalyst, the formation rate of ethylene carbonate under supercritical condition was about 2 times of that under 4.0MPa CO2 pressure at same temperature.The interactions between ethylene oxide and tetradentate Schiff-base aluminum complexes (SalenAlX) were confirmed by means of NMR method. It was proposed that ethylene oxide was ring-opened according to base-catalyzed cleavage, and then inserted the Al-X bond of SalenAlX. Activation of CO2 and its insertion towards Al-O bond of aluminum complexes to form intermediate條inear carbonates, which formed cyclic carbonate by intermolecular substitution, were studied by using FTIR methods. The plausible mechanisms for cycloaddition of CO2 and ethylene oxide are proposed with aluminum complex alone, or combined with a Lewis base as co-catalyst. The synergistic effect of SalenAlX/n-Bu4NBr bifunctional catalyst for the cycloaddition reaction was discussed in detail.Soluble aluminum phthalocyanine and Schiff-base aluminum, cobalt complexes were covalently bonded to the silica surface of ordered mesoporous MCM-41 molecular sieve. The anchoring resulted complexes were characterized by means of FTIR, XRD, UV-VIS and TGA methods, and all exhibited high catalytic activities towards the cycloaddition of CO2 and ethylene oxide, when a quaternary ammonium salt was used as co-catalyst.In the presence of n-Bu4NBr, the cobalt complex anchored onto MCM-41 was used as catalyst for the cycloaddition of CO2 and ethylene oxide to continuous produce ethylene carbonate in a fixed bed reactor. The conversion of ethylene oxide is up to 85.6%. This is first successful example that supercritical CO2 was used as both a solvent and a reactant for continuous catalytic syntheses of organic compound.IVThe removal of up to 92% of the template from mesopores has been successfully achieved by treating as-synthesized pure siliceous MCM-41 with supercritical CO2 modified with CH2Cl2/MeOH mixture. The extracted template retained its structure and property, and thus could be reused for preparation of MCM-41 material. Compared to high temperature calcination, the proposed method can avoid structure shrinkage, and retain highly uniform...