FTIR investigation of the interfacial properties and mechanisms of CO₂ sorption in porous ionic liquids

Porous liquids, which are liquids with permanent porosity, have received significant attention as a new class of materials with the potential for far-reaching impacts in a variety of applications including gas separation. In this work, in situ Fourier transform infrared spectroscopy measurements were conducted to investigate the mechanism of carbon dioxide absorption in a porous ionic liquid consisting of ZIF-8 combined with 8,8′-(3,6-dioxaoctane-1,8-diyl)bis(1,8-diazabicyclo[5.4.0]undec-7-en-8-ium) bis(trifluoromethanesulfonyl)imide ([DBU-PEG][(Tf₂N)₂]). While the vibrational modes of the pure ionic liquid remain relatively unchanged, the incorporation of carbon dioxide leads to slight structural fluctuations in the ZIF-8 framework whether it is pure solid or as integrated into the porous ionic liquid. The analysis of the vibrational modes of the porous ionic liquid suggests that the interaction of the CO₂ occurs more strongly with the ring structure of the ZIF-8 framework. The splitting of the asymmetric stretch of the CO₂ into multiple peaks upon sorption indicate the presence of multiple environments, which could be a combination of free and physisorbed CO₂ or simply multiple binding sites within the porous ionic liquid. A better understanding of gas sorption mechanisms in this unique material could lead to new porous ionic liquids with enhanced separations properties.