The Role of Water Revisited and Enhanced:A Sustainable Catalytic System for the Conversion of CO2 into Cyclic Carbonates under Mild Conditions

The role of water as highly effective hydrogen bond donor (HBD) for promoting the coupling reaction of CO2 with a variety of epoxides was demonstrated under very mild conditions (25 °C ≤ T ≤ 60 °C, 2 bar ≤ p(CO2) ≤ 10 bar). Water led to a dramatic increase in the cyclic carbonate yield when employed in combination with Bu4NI, while it had a detrimental effect with the corresponding bromide and chloride salts. The efficiency of water in promoting the activity of the organic halide was compared with state-of-the-art HBDs. Although water requires higher molar loadings to achieve similar degree of conversion of CO2 and styrene oxide into the corresponding cyclic carbonate under the same, mild reaction conditions, its environmental friendliness and much lower cost make it a very attractive alternative as HBD. The effect of different parameters such as the amount of water, CO2 pressure, reaction temperature, and nature of the organic halide used as catalyst was investigated using a high-throughput reactor unit. The highest catalytic activity was achieved with either Bu4NI or PPNI: with both systems, the cyclic carbonate yield at 45 °C with different epoxide substrates could be increased by a factor two or more by adding water as a promoter, while retaining high selectivity. Water is an effective hydrogen bond donor even at room temperature, allowing to reach 85% conversion of propylene oxide with full selectivity towards propylene carbonate in combination with Bu4NI (3 mol%).