Effect of cation's alkyl chain length of imidazolium-driven PVDF-based supported ionic liquid membranes on the CO2 separation performance from biogas and flue gas

This study investigates the impact of the alkyl chain length in imidazolium-based ILs on SILMs using poly (vinylidene fluoride) (PVDF) as a polymer substrate on the membrane's physicochemical properties and CO2 separation performance from CO2/CH4 and CO2/N2 gas mixtures. Particularly, 1-Ethyl, 1-Butyl and 1-Hexyl 3- methylimidazolium bis(trifluoromethylsulfonyl)imide, i.e. [EMIM][NTf2], [BMIM][NTf2], and [HMIM][NTf2]- based SILMs, were assessed. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) confirmed and quantified the presence of ILs within the polymeric matrix, while differential scanning calorimetry (DSC) suggested larger electrostatic interactions for shorter alkyl chains on imidazolium cations. CO2, CH4 and N2 permeability of each SILM were assessed by single-gas permeability tests and their CO2/N2 and CO2/CH4 selectivity were achieved. ILs with shorter alkyl chain presented stronger interactions with CO2 molecules. [EMIM][NTf₂] SILM exhibited the highest single-gas CO₂ permeability (357.0 Barrer), CO2/CH4 selectivity (αCO2/CH4 =23.5) and CO2/N2 selectivity (αCO2/N2 =16.4), which were competitive when benchmarked in an upper bound plot. Afterwards, this SILM was tested in extended experiments under dry and humid conditions (≥ 90 h each), in which CO2 feed composition was modified between CO2/CH4 50/50 %v/ v and 25/75 %v/v. CH4 permeability increased as its proportion in the feed was reduced (from 15.4 Barrer using 100 %v/v CH4 to 20.5 and 19.9 Barrer when CH4 was fed at 50 %v/v or higher, under dry and humid conditions, respectively), affecting CO2/CH4 selectivity (from ~24 for single gas to ~17 and ~19 in dry and humid ex periments, respectively), remaining competitive regarding the technological benchmarking