Effects of reaction conditions on the conversion of epoxides to cyclic carbonates through CO2 inclusion

Abstract

This thesis targets to the synthesis of cyclic carbonates as they are compounds of high importance in a number of different applications. The one-pot synthesis of cyclic carbonates consists of two sequential reactions of epoxidation of the olefin and the subsequent cycloaddition reaction of CO2 with the epoxide. To obtain more information on the roles of catalyst components, the epoxidation of 1-decene (first step) and the cycloaddition of CO2 with epoxide (second step) were conducted individually. The supported cobalt catalysts, prepared by a wet-impregnation method, were active in the epoxidation of 1-decene in the presence of oxygen from air as the primary oxidant and a very small amount of the radical initiator at 80°C. Using TiO2 as a support for cobalt resulted in a significant reduction in the quantity of the leached cobalt catalyst compared with the use of MgO as a support. 1-Decene epoxidation was also performed over supported gold catalysts in the presence of a very small amount of the radical initiator using oxygen as the oxidant. Supported gold catalysts prepared by the sol-immobilisation method displayed the highest activity. Gold nanoparticles supported on TiO2 and SiO2 showed the highest activity. The cycloaddition of CO2 with different epoxides were studied using different catalysts. Tetrabutylammonium bromide (Bu4NBr) was the most active quaternary ammonium salts. Other heterogeneous catalysts such as polydiallyldimethylammonium bromide and imidazole supported onto silica were found to be effective catalysts for this reaction. The compatibility between these two catalysts for the two different steps before coupling them in a one-pot reaction for the direct synthesis of cyclic carbonate was also investigated. The epoxide selectivity was significantly reduced in the presence of Bu4NBr or polydiallyldimethylammonium bromide (40% PDDABr/SiO2) or imidazole supported on silica (Imid/SiO2). No effect of supported gold catalysts was observed on the cycloaddition of CO2 with 1,2-epoxydecane. A simple and highly efficient preparation of cyclic carbonates from 1-decene was achieved by the use of 1%Au/support-Bu4NBr/ZnBr2 catalysts. The oxidative carboxylation process for a range of different cycloalkenes is challenging. For the epoxidation step, it was shown previously in our group that smaller ring size, such as cyclopentene, became less selective to the epoxidation. However, regarding the cycloaddition step, the opposite trend was found. Cyclopentene oxide and cyclohexene oxide gave high selectivity for cyclic carbonate,whereas the insertion of CO2 in carbonylation of cyclooctene oxide and cyclododecane oxide to form cyclic carbonate was a challenging step and the main product was ketone