Cesium-phosphorous-silicon mixed oxide (Cs-P-Si oxide), a typical acid–base bifunctional catalyst, efficiently catalyzes propylene carbonate synthesis from CO2 and propylene oxide under supercritical conditions (8-10MPa). This catalyst contains no halogens and requires no additional solvents.A portion of the catalyst was eluted into the product solution during catalysis. It was also found that Cs3PO4, one of the species that may be eluted from the Cs-P-Si oxide, is an effective non-halogen homogeneous catalyst.

Polyfluoroalkyl phosphonium iodides, Rf3RPI (Rf=C4F9C2H4, C6F13C2H4, C8F17C2H4; R=Me, Rf), catalyzed propylene carbonate synthesis from propylene oxide and carbon dioxide under supercritical CO2 conditions, where propylene carbonate was spontaneously separated out of the supercritical CO2 phase. The Rf3RPI catalyst could be recycled with maintaining a high CO2 pressure and temperature by separating the propylene carbonate from the bottom of the reactor followed by supplying propylene oxide and CO2 to the upper supercritical CO2 phase in which the Rf3RPI remained.

Tetraalkylammonium salts of transition-metal-substituted polyoxometalates, such as [(n-C7H15)4N]6[α-SiW11O39Co] and [(n-C7H15)4N]6[α-SiW11O39Mn], efficiently catalyze cyclic carbonate synthesis from carbon dioxide and epoxide. The catalytic activity is significantly influenced by the type of transition metal and the countercation (Co2+≈Mn2+>Ni2+>Fe3+»Cu2+; (n-C7H15) 4N+>(n-C4H9)4N+»K+). Co- or Mn-substituted catalysts required neither additional organic solvents nor additives. Thus, polyoxometalates are promising as nonhalogen anionic components of catalysts for cyclic carbonate synthesis.

Insoluble ion exchange resins, one type of polystyryl supported catalysts containing an ammonium salt or amino group, and the polar macroporous adsorption resin, are efficient and reusable heterogeneous basic catalysts for the synthesis of propylene carbonate from propylene oxide and CO2 under supercritical CO2 conditions (373K, 8MPa), which requires no additional organic solvents either for the reaction or for the separation of product. Various parameters affecting the reaction were examined. A quantitative yield (>99%) together with excellent selectivity (>99%) was obtained. The purity of product separated directly by filtration from the reaction mixture, reached more than 99.3% without further purification processes. The catalyst can be easily recovered and reused without significant loss of its catalytic activity. The process represents a simple, ecologically safer, cost-effective route to cyclic carbonates with high product quality, as well as easy product recovery and catalyst recycling.

Dibutyltin oxide or dibutyltin dimethoxide was first used as a remarkable selective catalyst for the synthesis of propylene carbonate from propylene glycol and carbon dioxide. The effects of the reaction parameters, such as reaction time, temperature and CO2 pressure on the amount of propylene carbonate were also experimentally studied. Under the optimized conditions, the amount of propylene carbonate was nearly proportional to PG concentration. The use of N, N-dimethylformamide as a co-solvent in this study significantly enhanced the catalytic activity, and the ketals as dehydrating agents greatly improved the yield of PC, which can be limited by the equilibrium. A postulated mechanism for the dibutyltin oxide-catalyzed carboxylation of propylene glycol was also discussed.