Molecular design and experimental study on synergistic catalysts for synthesis of cyclocarbonate from styreneoxide and CO2
Taking the reaction between styrene oxide and CO2 to yield cyclocarbonate as the target, the activities of synergistic catalysts, which are composed by Br- and alcohol compounds served as hydrogen bond donors (HBDs), were predicted by DFT calculations and confirmed by subsequent experiments. Intramolecular reactions of active intermediate as well as intermolecular reactions between active intermediate and styrene oxide were possible side reactions. DFT calculation results show that the most likely by-product is phenylacetone which yields from the intramolecular reaction of active intermediate. According to the calculation result, the catalytic activity of a synergistic catalyst was related to the pKa of the HBD, and the optimal calculated pKa was about 3~4 higher than the pKa of 2-bromo-1-phenylethanol which is the protonated active intermediate of the target reaction. The combination of water and tetraethylammonium bromide (TEAB) was found to be a promising synergistic catalyst, and it can give a conversion of 70% and yield of 64% at the condition of catalyst load 1% (mol%), solvent free, 100 ℃, 1.0 MPa and 1 h. Additionally, the TEAB can be reused. The observed catalytic activity of water is higher than expected but similar to that of styrene glycol which is the hydration product of styrene oxide. Water is therefore an indirect catalyst and catalytic activity is mainly contributed by the styrene glycol.