(698h) Carboxylation of Propylene Oxide to Propylene Carbonate

Polycarbonates, a class of thermoplastic polymer, have wide ranging applications in the transportation, construction, and packaging industries. Non-phosgene route for making polycarbonates involves transesterification of bis-Phenol-A and diphenyl carbonate, made from cyclic carbonates. Cyclic carbonates are produced from the cycloaddition of CO₂and epoxides employing either homogenous or heterogeneous catalysts. The main challenges of these systems are low catalyst activity and selectivity, and high CO₂pressures. In this study, we systematically investigate both homogeneous (TBAB) and heterogeneous catalysts (Ion exchange resins) (which show promise among the catalysts evaluated) aimed at enhancing catalyst performance with respect to activity and selectivity. We measured the liquid-phase solubility of CO₂in various solvents to better delineate solvent effect on carboxylation of propylene oxide to propylene carbonate (PC). The experiments were performed in a 100 mL Parr reactor with provision for temperature & pressure monitoring and in-line sampling of liquids. At the end of each experiment, the liquid products are analyzed for concentrations of PO, PC and solvent to assess the material balance. Various commercial solvents were investigated, and the order of activity is: Methanol > Ethanol > Isopropyl alcohol > Dimethyl formamide > N-methyl-2-pyrrolidone > Propylene carbonate > Dimethyl carbonate. The polar protic solvents (such aslinear alcohols) are strong nucleophiles, strong hydrogen-bond donors and acceptors and interact strongly with the electron-deficient electrophiles, epoxides and CO₂. In contrast, polar aprotic solvents are weak nucleophiles compared to the polar protic solvents resulting in low activity. These conclusions, based on CO2 solubility measurements in methanol and PC solvents along with kinetic studies of PO carboxylation, will be presented and discussed.