Ammonium-, phosphonium- and sulfonium-based 2-cyanopyrrolidine ionic liquids for carbon dioxide fixation

The development of carbon dioxide (CO2) scavengers is an acute problem nowadays because of the global warming problem. Many groups around the globe intensively develop new greenhouse gas scavengers. Room-temperature ionic liquids (RTILs) are seen as a proper starting point to synthesize more environmentally friendly and high-performance sorbents. Aprotic heterocyclic anions (AHA) represent excellent agents for carbon capture and storage technologies. In the present work, we investigate RTILs in which both the weakly coordinating cation and AHA bind CO2. The ammonium-, phosphonium-, and sulfonium-based 2-cyanopyrrolidines were investigated using the state-of-the-art method to describe the thermochemistry of the CO2 fixation reactions. The infrared spectra and electronic and structural properties were simulated at the hybrid density functional level of theory to characterize the reactants and products of the chemisorption reactions. We conclude that the proposed CO2 capturing mechanism is thermodynamically allowed and discuss the difference between different families of RTILs. Quite unusually, the intramolecular electrostatic attraction plays an essential role in stabilizing the zwitterionic products of the CO2 chemisorption. The difference in chemisorption performance between the families of RTILs is linked to sterical hindrances and nucleophilicities of the alpha- and beta-carbon atoms of the aprotic cations. Our results rationalize previous experimental CO2 sorption measurements (Brennecke et al., 2021).

Automated summary

The development of carbon dioxide (CO2) scavengers is an urgent problem due to global warming. Room-temperature ionic liquids (RTILs) are considered as a promising starting point for synthesizing environmentally friendly and high-performance sorbents. This study investigates the thermodynamics and properties of CO2 fixation reactions in RTILs containing weakly coordinating cations and aprotic heterocyclic anions (AHA). The results provide insights into the CO2 capturing mechanism and the differences between different families of RTILs.