Impact of MOF‑5 on Pyrrolidinium-Based Poly(ionic liquid)/Ionic Liquid Membranes for Biogas Upgrading

Bearing in mind that metal organic frameworks (MOFs) have remarkable CO2 adsorption selectivity and mixed matrix membranes (MMMs) have been identified as potential solutions for advancing the current state of the art of membrane separation technology, this work investigates the effect of combining a MOF, with high adsorption properties toward CO2 when compared to CH4 (MOF-5), with a blend of poly­(ionic liquid)/ionic liquid (PIL/IL) for biogas upgrading. The blend system consisted of a pyrrolidinium-based PIL, poly­([Pyr11]­[Tf2N]), and a free imidazolium-based IL, [C2 mim]­[BETI]. The MOF-5 was incorporated at different loadings (10, 20, 30 wt %), and MMMs were prepared by solvent evaporation and characterized by diverse techniques (FTIR, SEM, TGA, puncture tests, and single gas transport). The results showed that the free IL is miscible with the PIL, while MOF-5 particles were uniformly dispersed into the PIL/IL matrix. The formed PIL/IL/MOF-5 membranes revealed suitable thermal stability (T onset up to 656 K) for biogas upgrading processes, but a loss of mechanical stability was found after the incorporation of MOF-5. Thus, more rigid and fragile membranes were obtained. At 30 wt % of MOF-5 loading the CO2 permeability increased 133% when compared to that of the pristine PIL/IL membrane, mainly due to the adsorption capacity of the MOF, as well as its porous structure. The presence of a porous structure may also be the reason why the ideal selectivity decreases by 88% for the MMM with the highest loading. It was possible to demonstrate the relevance of studying different components within the polymeric matrix in order to assess not only thermal, mechanical, and chemical properties but also gas transport response.