Exploring the Effect of Ionic Liquid Conformation on the Selective CO2 Capture of Supported Ionic Liquid-Phase Adsorbents Based on ZIFs

Abstract

The CO2 adsorption capacity and the CO2/N2 selectivity of a series of Supported Ionic Liquid-Phase adsorbents (SILPs), including the novel inversely structured SILP “Inverse SILPs”, are thoroughly investigated. ZIF-8, ZIF-69 and ZIF-70 were involved as the solid matrix, while ILs, having tricyanomethanide (TCM) as an anion and alkyl-methylimidazolium of different alkyl chain lengths (C2, C6, C8) as a cation, were used as the liquid constituents of the SILPs. The ultimate target of the work was to ratify a few recently reported cases of enhanced CO2 absorptivity in ILs due to their incorporation in ZIFs and to corroborate phenomena of CO2/N2 selectivity improvements in ZIFs, due to the presence of ILs. This ambiguity originates from the vague assumption that the pores of the ZIF are filled with the IL phase, and the free pore volume of a SILP is almost zero. Yet, through the integration of theoretical predictions with N2 porosimetry analysis of an actual sample, it is suggested that a thin layer of IL covered the exterior surface of a ZIF crystal. This layer could act as an impermeable barrier for N2, inhibiting the gas molecules from reaching the empty cavities laying underneath the liquid film during porosimetry analysis. This consideration is based on the fact that the solubility of N2 in the IL is very low, and the diffusivity at 77 K is negligible. In this context, the observed result reflects an averaged adsorptivity of both the IL phase and the empty pores of the ZIF. Therefore, it is incorrect to attribute the adsorption capacity of the SILP solely to the mass of the IL that ‘hypothetically’ nests inside the pore cavities. In fact, the CO2 adsorption capacity of SILPs is always less than the average adsorptivity of an ideal ZIF/IL mixture, where the two phases do not interact. This reduction occurs because some ZIF pores may become inaccessible, particularly when the IL forms a layer on the pore walls, leaving only a small empty core accessible to CO2 molecules. Additionally, the IL layer masks the active sites on the ZIF’s pore walls. It should also be noted that the CO2/N2 selectivity increases only when the ZIF’s pores are completely filled with the IL phase. This is because ILs have a higher CO2/N2 selectivity compared to the bare ZIF.