Tetrabutylphosphonium acetate and its eutectic mixtures with common-cation halides as solvents for carbon dioxide capture

Abstract

The absorption and desorption isotherms of CO2 in the ionic liquid tetrabutylphosphonium acetate ([P4 4 4 4][OAc]) were experimentally determined at 343.2 K in the pressure range 0–15 bar. Analysed in a mass basis, the CO2 absorption capacity of [P4 4 4 4][OAc] is particularly competitive among that of phosphonium ionic liquids. A contribution of chemical absorption was evidenced from the shape of the isotherms, and a reaction scheme based on the abstraction of an acidic proton in the cation by the basic acetate anion, with a 1:2 (CO2 to ionic liquid) overall stoichiometry, was proposed. The investigation of the solid–liquid equilibria of the mixtures of [P4 4 4 4][OAc] with its homologous halides, namely [P4 4 4 4]Cl and [P4 4 4 4]Br, led to the identification of eutectic behaviours, with the eutectic compositions having melting temperatures below 308.2 K, remarkably lower than the melting temperature of pure [P4 4 4 4][OAc]. The CO2 absorption capacities of these eutectic compositions were then explored at 308.2 K, and also 343.2 K for comparative purposes. While advantageously allowing their utilisation as CO2-capturing solvents at lower temperatures, in comparison to pure [P4 4 4 4][OAc] the eutectic mixtures did not lead to a major loss in CO2 absorption capacity, with greater physisorption compensating for the reduction in chemisorption.