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

• [EMIM][NTf 2 ] had larger electrostatic interactions, which increased CO 2 affinity. • CO 2 /CH 4 selectivity was higher for shorter IL alkyl chains. • [EMIM][NTf 2 ] SILM CO 2 permeability remained during the dry extended test. • Humidity slightly increased [EMIM][NTf 2 ] SILM CO 2 permeability. • Feed composition increased CH 4 permeability slightly reducing CO 2 /CH 4 selectivity. 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 CO 2 separation performance from CO 2 /CH 4 and CO 2 /N 2 gas mixtures. Particularly, 1-Ethyl, 1-Butyl and 1-Hexyl 3-methylimidazolium bis(trifluoromethylsulfonyl)imide, i.e. [EMIM][NTf 2 ], [BMIM][NTf 2 ], and [HMIM][NTf 2 ]-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. CO 2 , CH 4 and N 2 permeability of each SILM were assessed by single-gas permeability tests and their CO 2 /N 2 and CO 2 /CH 4 selectivity were achieved. ILs with shorter alkyl chain presented stronger interactions with CO 2 molecules. [EMIM][NTf₂] SILM exhibited the highest single-gas CO₂ permeability (357.0 Barrer), CO 2 /CH 4 selectivity (α CO2/CH4 =23.5) and CO 2 /N 2 selectivity (α CO2/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 CO 2 feed composition was modified between CO 2 /CH 4 50/50 %v/v and 25/75 %v/v. CH 4 permeability increased as its proportion in the feed was reduced (from 15.4 Barrer using 100 %v/v CH 4 to 20.5 and 19.9 Barrer when CH 4 was fed at 50 %v/v or higher, under dry and humid conditions, respectively), affecting CO 2 /CH 4 selectivity (from ∼24 for single gas to ∼17 and ∼19 in dry and humid experiments, respectively), remaining competitive regarding the technological benchmarking.