High pressure CO2 solubility and physical properties of tetrabutylphosphonium l-prolinate

Abstract

This paper reports the CO2 solubility, density, viscosity, and electrical conductivity for a kind of amino acid ionic liquid (AAIL), tetrabutylphosphonium l-prolinate ([P4444][Pro]). The physical properties of [P4444][Pro] showed the ordinary temperature dependency, i.e. the density and viscosity decreased and the electrical conductivity increased with a temperature rising. One mole of [P4444][Pro] absorbed a 0.970 mol of CO2 at atmospheric pressure of CO2 and 333.2 K, and desorbed only ∼17% of the absorbed CO2 at 393.2 K. NMR spectra revealed that a CO2 molecule formed a complex with [Pro]−, and [P4444]+ had a little effect on the CO2 absorption. The CO2 solubility increased steeply at lower pressures of CO2 < 0.1 MPa and very gradually in the higher pressure range from 0.1 to 6 MPa at 333.15 K. Other tetrabutylphosphonium AAILs with (S)-2-methylprolinate ([P4444][MePro]) and glycinate ([P4444][Gly]) showed the similar pressure dependencies. The contributions of physical and chemical absorptions to the total CO2 solubility were analyzed to discuss the effect of anion on the high-pressure CO2 solubility. [P4444][Pro] and [P4444][Gly] showed the higher physical and chemical CO2 absorptions (in molarity scale) in the present AAILs, respectively.