Metal catalyst for CO2 capture and conversion into cyclic carbonate: Progress and challenges

Fossil fuel has empowered a remarkable era of prosperity and development of the welfare of human society. However, the resulting large anthropogenic CO2 emissions had an adverse impact on global temperature. Furthermore, the scarcity of limited fossil fuel resources will eventually force them to look for alternative carbon sources to sustain a sustainable economy. Chemical fixation of CO2 into fuels and valuable chemicals via renewable energy sources has been attracting human society for not only alleviating CO2 emissions but also reducing reliance on non-renewable energy sources and minimizing the impact on the environment from displaced fossil fuel fractions. Cyclic carbonate is a valuable CO2 product that can be used as aprotic solvent, the electrolytic solvent in lithium batteries, degreasing solvents, and intermediates for the synthesis of polycarbonates, drugs, and cosmetics. It can be synthesized via cycloaddition of CO2 with epoxide under the catalytic condition due to the low reactivity of CO2 (thermodynamic as well as kinetic inert). An ideal catalyst for this conversion is composed of a Lewis acid to activate the epoxide ring and a Lewis base to open the epoxide ring. Efforts have been done to synthesize various catalytic systems for cyclic carbonate formation. The review is focused on metal-catalyzed cyclic carbonate formation. It begins with carbon capturing, storage, and utilization (CCSU) along with the importance of cyclic carbonate. The mechanism for cyclic carbonate formation was classified into two categories including binary and bifunctional systems based on the presence of nucleophilic moiety either as a separate entity or attached to a catalyst. Various metal catalysts such as metal salen, metal porphyrin, metal salts, metal-organic framework, and zeolitic imidazolate framework are discussed with recent progress in the development. It was believed that homogeneous catalysts showed high catalytic activity but difficult product separation whereas heterogeneous catalysts can be easily separated by simple filtration. Finally, the conclusion and the future outlook in the development of catalysts for cyclic carbonate formation are mentioned.