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.