Formation of choline salts and dipolar ions for CO2 reactive eutectic solvents

Abstract

Choline-based sorbents derived from imidazole (ImH), phenol (PhOH), pyrrole-2-carbonitrile (CNpyrH), and 1,2,4-triazole (TrzH) are developed for CO₂ capture to enable alternative regeneration approaches over aqueous amines. During synthesis, the equilibrium between [Ch]⁺[OH]⁻ and Ch^± dipolar in water shifts to support the formation of Ch^±ImH and Ch^±PhOH in the presence of ImH and PhOH upon drying. In contrast, salts of [Ch]⁺[CNpyr]⁻ and [Ch]⁺[Trz]⁻ were obtained with CNpyrH and TrzH, as confirmed by NMR and FTIR spectroscopy. Density functional theory (DFT) calculations support a spontaneous proton transfer from CNpyrH and TrzH to Ch^±, while they show an energy barrier in the case of ImH. These sorbents formed eutectic solvents upon mixing with ethylene glycol (EG) where deprotonation of EG and subsequent binding of CO₂ contributed to capacities up to 3.56 mol CO₂ kg⁻¹ at 25 °C and 1 bar of CO₂. The regenerability of the eutectic solvents was demonstrated by dielectric heating via microwaves (MWs) in support of renewable energy utilization. This study shows the impact of proton sharing on the CO₂ capacity and regenerability of eutectic sorbents as molecular design guidance.