Optimization of CO₂ Cycloaddition to Epoxides Using Vinylimidazole-Based Ionic Compounds: Influence of Reaction Parameters on Catalytic Performance

In this study, a series of catalysts for the cyclic carbonate (CC) formation process via the cycloaddition of CO2 to epoxides─namely, 1-vinylimidazole ionic compounds with different functional moieties such as –OH, –COOH, and –NH2─were obtained. These compounds were comprehensively characterized using spectral techniques, including NMR, FTIR, and elemental analysis, and tested as catalysts under various conditions to find out the most suitable circumstances and functional moieties of the catalyst molecule for different application modes. The results indicate that most catalysts provide a main product yield exceeding 70% with selectivity above 90% and turnover frequency ranging from 19 to 25 h–1 under model conditions: 2 mol % catalyst loading, temperature of 90 °C, and CO2 pressure of 1 MPa within 2 h. The yield of the main product grows with the catalyst amount; however, obtained compounds exhibit high catalytic activity even at such low concentrations as 0.5% mol. With temperature variation between 50 and 110 °C, proceeding with the reaction at 90 °C leads to a significant increase in yield of the desired product without a decrease in selectivity, while higher temperatures favor the formation of side products. Increasing CO2 pressure from 0.1 to 1 MPa moderately improved product yield; however, further pressure increases up to 2 MPa either resulted in a slight additional increase or a decline in conversion, depending on the catalyst composition. The reaction time of 2 h was sufficient to achieve high epichlorohydrin conversion at 1 MPa CO2 and 90 °C. Finally, two optimal conditions were identified for CC synthesis using the developed ionic catalysts: (a) energy-efficient conditions, with 90 °C, 1 MPa initial CO2 pressure, 2 mol % catalyst loading, 1 h; (b) high-efficiency conditions, with 110 °C, 2 MPa initial CO2 pressure, 2 mol % catalyst loading, 1 h. Among the tested catalysts, those containing Br– and I– counterions exhibited the highest efficiency, achieving product yields above 94% with selectivities exceeding 93%.