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 (Bu₄ NI) 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 Bu₄ NI 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 Bu₄ NI (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.