Molecular mechanism of CO2 absorption in phosphonium amino acid ionic liquid

Abstract

In this investigation, we examine the molecular mechanism of high pressure CO₂ absorption in tetra-butylphosphonium lysinate Ionic Liquid (IL) using molecular dynamics simulations. The calculations show that on CO₂ absorption, a structured bilayer of charges (in opposite phases) at the CO₂–IL interface is formed. The interface formation starts within a short span of tens of picoseconds, and attains saturation around ten nanoseconds, where CO₂ molecules remain absorbed in the bulk IL layers. The simulations predict a 0.9 molar absorption of CO₂ at T = 298 K and p_CO2 = 20 bar, with a further increase of 17% at a reduced temperature. The structural properties show that CO₂ molecules strongly and preferentially interact with the two terminal amine sites of the anions, where the presence of carboxylate group further enhances CO₂ absorption. The interaction and absorption of CO₂ molecules leads to enhanced mobility of the cations and anions of the IL. The mobility of anions is slightly higher than cations due to the preferential interaction of CO₂ molecules with anions. The molecular mechanism examined in this work can be used as a predictive tool to develop efficient amino acid functionalized ILs for CO₂ absorption.