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 (25–60 °C, 2–10 bar CO2). Water led to a dramatic increase in the cyclic carbonate yield when employed in combination with tetrabutylammonium iodide (Bu4NI) whereas 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 three state‐of‐the‐art hydrogen bond donors, that is, phenol, gallic acid and ascorbic acid. Although water required higher molar loadings compared to these organic hydrogen‐bond donors to achieve a 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 hydrogen‐bond donor. 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 by using a high‐throughput reactor unit. The highest catalytic activity was achieved with either Bu4NI or bis(triphenylphosphine)iminium iodide (PPNI): with both systems, the cyclic carbonate yield at 45 °C with different epoxide substrates could be increased by a factor of two or more by adding water as a promoter, retaining high selectivity. Water was 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 %). For the conversion of epoxides in which PPNI is poorly soluble, the addition of a cyclic carbonate as solvent allowed the formation of a homogeneous solution, leading to enhanced product yield. Just add water: Water acts as a hydrogen bond donor under very mild reaction conditions [25–45 °C, p(CO2)=10 bar], boosting the performance of organic iodides in catalyzing the fixation of CO2 into cyclic carbonates. The efficiency of water in promoting the activity of the organic halide is compared with three state‐of‐the‐art hydrogen bond donors (i.e., phenol, gallic acid and ascorbic acid).