Cyclic carbonates: Treasure of fine chemicals obtained from waste stream CO2 over carbon-based heterogeneous catalysts

Cyclic carbonates are used extensively in pharmaceutical and fine chemical sectors as valuable building blocks. The coupling of CO2 with epoxides is a promising CO2 fixation strategy for fulfilling the goals of green and sustainable chemistry. Efforts have been made to design a variety of active and selective catalytic systems to produce valuable products from CO2 under benign conditions and carbon-based heterogeneous catalysts provide a good opportunity in this regard. Different catalysts (metal supported and metal free) prepared from graphene oxide, graphene, graphitic carbon nitrides, carbon nanotubes and activated carbon have been used in this regard. The key factor for designing catalyst is to provide an activating site that can polarise either epoxide or the carbon dioxide molecule through acidic or basic sites respectively. The active sites in the catalyst are typically generated through surface modification in presence of nitrogen (N), oxygen (O) and sulfur (S) rich compounds, or by depositing metals using an appropriate metal precursor. The acidic sites promote adsorption and ring opening of epoxide resulting in the formation of reaction intermediates, whose stability (on catalyst surface) is important for the success of reaction. Whereas, basic sites mainly activate the CO2 molecule making it proficient for further reaction. This reaction is useful for generating a pool of fine chemicals including compounds such as 1,3-dioxolan-2-one, 4-methyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one and 4-phenyl-1,3-dioxolan-2-one. Various functional groups, nature of substrate, co-catalyst, reaction conditions play a significant role and a detailed account of these factors with most current development is provided in this review.