CO2 capture and viscosity of metal chelate-based ionic liquids: Influence of the structure and substitution of the azole-based anion

Abstract

In this work, the effect of the structure and substitution of azole-based anion on the CO2 capture and viscosity (η) of metal chelate-based dual functional ionic liquids (DFILs) with [K(DGA)2]+ cation was investigated. The CO2 absorption capacity of DFILs at 0.1 MPa and 333.2 K shows that methyl and nitro groups on the azole-based anion, such as pyrazolide ([Pyr]−) and imidazolide ([Im]−), attenuate their reactivity with CO2 through the steric hindrance and electron-withdrawing effects, respectively. Furthermore, the CO2 absorption capacity of [K(DGA)2][Pyr] is larger than that of [K(DGA)2][Im], which is due to the interaction of the two adjacent N atoms in the [Pyr]− anion with CO2, as confirmed by DFT calculations. The CO2 absorption mechanism shows that both [Pyr]− anion and [K(DGA)2]+ cation of [K(DGA)2][Pyr] can chemically interact with CO2, making its saturated uptake at 333.2 K as high as 1.47 mol CO2 per mole IL. Moreover, CO2 interacts preferentially with [Pyr]−, and the [K(DGA)2]+–CO2 interaction is enhanced when the CO2 uptake is greater than 0.5 mol CO2 per mole IL. η of pure [K(DGA)2][Pyr] is lower than that of pure [K(DGA)2][Im]. η of [K(DGA)2][Pyr] increases with the increase of CO2 absorption, and the rapid increase in η after CO2 uptake greater than 0.5 is attributed to the formation of [K(DGA)2]+–CO2 interactions.