Quantification of Ylide Formation in Phosphonium-Based Ionic Liquids Reacted with CO₂

Phosphonium-based ionic liquids (ILs) paired with aprotic heterocyclic anions (AHAs) are found to react with CO₂ to form both a carbamate product and a carboxyl product. The carboxyl product is formed primarily at elevated temperatures through the formation of a phosphonium ylide intermediate. The formation of the carboxyl product leads to the formation of the neutral azole, which can lead to an irreversible process if the neutral azole is sufficiently volatile. To understand how the ILs would behave in a CO₂ capture process operated at elevated temperatures, it was necessary to quantify the two reaction products. CO₂ was reacted with seven different AHA ILs to determine the equilibrium amounts of carbamate and carboxyl, the equilibrium constants for both reactions, and the rate of CO₂ absorption by each reaction path. The reactions were tracked and quantified in situ by using ATR-FTIR spectroscopy, while NMR spectroscopy was used after equilibrium was reached to determine the extent of each reaction at multiple temperatures and pressures. It was found that both the basicity and molecular size of the anion play key roles in the formation of the phosphonium ylide. In the extreme case of [P₆₆₆₁₄][4-Triaz] only half of the reacted product was the desired carbamate at 60 °C. Although there is a significant amount of the carboxyl product formed, the carbamate is kinetically favored.