Cr-based MOF/IL composites as fillers in mixed matrix membranes for CO₂ separation

New composite materials made of bromide-based ionic liquids (ILs) and metal–organic framework (MOF) MIL-101(Cr) were produced using two different ILs. The powdered composites [PMIM][Br]@MIL-101(Cr) and [BMIM][Br]@MIL-101(Cr) were rigorously characterized, and it was confirmed that both ILs were incorporated into the MOF structure. Single-component CO₂ and N₂ adsorption–desorption isotherms for the pristine MIL-101(Cr) and [BMIM][Br]@MIL-101(Cr), at 303 K and up to 10 bar, showed that the composite has lower gas adsorption capacity and selectivity when compared with the pristine MOF due to the IL incorporation. Mixed Matrix membranes (MMMs) were prepared by the solvent evaporation method, using Matrimid®5218 as the polymeric matrix, and MIL-101(Cr) and IL@MIL-101(Cr) composites as fillers with different loadings (10, 20 and 30 wt%). All prepared membranes were dense, except for the Matrimid®5218/[PMIM][Br]@MIL-101(Cr) ones, and their mechanical properties were improved by the presence of the IL in the composite fillers. Single-gas permeation experiments with CO₂ and N₂ were performed at 303 K for the Matrimid®5218/MIL-101(Cr) and Matrimid®5218/[BMIM][Br]@MIL-101(Cr) membranes, as the Matrimid®5218/[PMIM][Br]@MIL-101(Cr) showed voids. Independently of the filler percentage, the Matrimid®5218/MIL-101(Cr) membranes showed superior CO₂ permeability than the Matrimid®5218/[BMIM][Br]@MIL-101(Cr) ones. In every case, the best CO₂/N₂ selectivity was achieved with a 20 wt% of filler loading, which indicates the existence of an optimum loading that yields the best membrane separation performance.