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 [P-66614] [ 4-Triaz] only half of the reacted product was the desired carbamate at 60 degrees C. Although there is a significant amount of the carboxyl product formed, the carbamate is kinetically favored.

Automated summary

Phosphonium-based ionic liquids (ILs) paired with aprotic heterocyclic anions (AHAs) react with CO2 to form carbamate and carboxyl products, with the latter primarily formed at elevated temperatures through the formation of a phosphonium ylide intermediate. The basicity and molecular size of the anion play key roles in the formation of the phosphonium ylide, while the carbamate product is kinetically favored despite the significant amount of carboxyl product formed under certain conditions.