Cycloaddition of CO2 to Epoxides over Solid Base Catalysts

Abstract

The cycloaddition of carbon dioxide to various epoxides (ethylene oxide, epoxybutene, and epoxypropylbenzene) over solid base cata-lysts (KX zeolite, Cs-loaded KX zeolite, Cs-doped alumina, and MgO) was performed in a batch autoclave reactor at 423 K and with excess CO2. Catalysts were characterized by elemental analysis, N2 physisorption, and CO2 adsorption microcalorimetry. The combination of results from elemental analysis and microcalorimetry showed that the occluded base sites on the Cs/KX were actually composed of occluded Cs and K species that could be removed by washing with water. The activity of the zeolite catalysts for ethylene oxide conversion to ethylene carbonate depended on the basicity of the sample, with the sample containing occluded alkali metal oxides being the most active. In addition, the site-time yields of ethylene carbonate formation, based on CO2 adsorption capacity, over Cs/KX, Cs/Al2O3, and MgO were similar to each other and were within a factor of 4 of the site-time yield seen with the homogeneous catalyst [N(C2H5)4]Br. The rates of epoxypropylbenzene conversion over the solid base catalysts were much lower than the rates of ethylene oxide conversion, presumably due to steric hindrance of the bulky side group on the former. Rate measurements with a mixture of ethylene oxide and epoxypropylbenzene over Cs/KX and MgO indicated that most of the basic sites on Cs/KX are located in the zeolite micropores. A high catalytic activity of Cs/Al2O3 for all of the epoxides studied suggests that proximity of surface Lewis acid sites to surface base sites is needed for the cycloaddition reaction. In the case of a zeolite catalyst, addition of a small amount of water enhanced the rate ethylene oxide reaction without significant formation of glycol side products.