Study of Superbase-Based Deep Eutectic Solvents as the Catalyst in the Chemical Fixation of CO2 into Cyclic Carbonates under Mild Conditions

Superbases have shown high performance as catalysts in the chemical fixation of CO₂ to epoxides. The proposed reaction mechanism typically assumes the formation of a superbase, the CO₂ adduct as the intermediate, most likely because of the well-known affinity between superbases and CO₂, i.e., superbases have actually proven quite effective for CO₂ absorption. In this latter use, concerns about the chemical stability upon successive absorption-desorption cycles also merits attention when using superbases as catalysts. In this work, ¹H NMR spectroscopy was used to get further insights about (1) whether a superbase, the CO₂ adduct, is formed as an intermediate and (2) the chemical stability of the catalyst after reaction. For this purpose, we proposed as a model system the chemical fixation of CO₂ to epichlorohydrin (EP) using a deep eutectic solvent (DES) composed of a superbase, e.g., 2,3,4,6,7,8-hexahydro-1H-pyrimido[1,2-a]pyrimidine (TBD) or 2,3,4,6,7,8,9,10-octahydropyrimido[1,2-a]azepine (DBU), as a hydrogen acceptor and an alcohol as a hydrogen bond donor, e.g., benzyl alcohol (BA), ethylene glycol (EG), and methyldiethanolamine (MDEA), as the catalyst. The resulting carbonate was obtained with yields above 90% and selectivities approaching 100% after only two hours of reaction in pseudo-mild reaction conditions, e.g., 1.2 bars and 100 °C, and after 20 h if the reaction conditions of choice were even milder, e.g., 1.2 bars and 50 °C. These results were in agreement with previous works using bifunctional catalytic systems composed of a superbase and a hydrogen bond donor (HBD) also reporting good yields and selectivities, thus confirming the suitability of our choice to perform this study.