Does the active hydrogen atom in the hydantoin anion affect the physical properties, CO2 capture and conversion of ionic liquids?

Abstract

Compared to the effect of the active hydrogen atom in the cation in protic ionic liquids (ILs) on their properties and applications, there are very few reports on the role of the active hydrogen atom in the anion. In order to better understand the role of the active hydrogen atom in the anion, the physical properties, CO₂ capture and conversion of three hydantoin-based anion-functionalized ILs ([P₄₄₄₂][Hy], [P₄₄₄₂]₂[Hy], and [HDBU][Hy]) have been investigated via experiments, spectroscopy, and DFT calculations in this work. The results show that the active hydrogen atom in the anion can form anionic hydrogen bonding networks, which significantly increase the melting point and viscosity and decrease the basicity of the IL, thereby weakening its ability to capture and convert CO₂. Interestingly, [P₄₄₄₂][Hy] undergoes a solid/liquid two-phase transition during CO₂ absorption/desorption due to the formation of quasi-intramolecular hydrogen bonding between the active hydrogen atom and the O⁻ atom of the absorbed CO₂, suggesting that the presence of the active hydrogen atom gives [P₄₄₄₂][Hy] the potential to be an excellent molecular switch. As there is no active hydrogen atom in the anion of [P₄₄₄₂]₂[Hy], it shows excellent CO₂ capture and conversion performance through the double-site interaction. [HDBU][Hy] shows the weakest catalytic CO₂ conversion due to the presence of active hydrogen atoms on both its anion and cation. Therefore, the active hydrogen atom in the anion may play a more important role in the properties and potential applications of ILs than the active hydrogen atom in the cation.