Mixed matrix membranes based on MIL-101 metal–organic frameworks in polymer of intrinsic microporosity PIM-1

This work presents a study on mixed matrix membranes (MMMs) of the polymer of intrinsic microporosity PIM-1, embedding the crystalline Cr-terephthalate metal-organic framework (MOF), known as MIL-101. Different kinds of MIL-101 were used: MIL-101 with an average particle size of ca. 0.2 µm, NanoMIL-101 (ca. 50 nm), ED-MIL-101 (MIL-101 functionalized with ethylene diamine) and NH2-MIL-101 (MIL-101 synthesized using 2-aminoterephthalic acid instead of terephthalic acid). Permeability, diffusion and solubility coefficients and their corresponding ideal selectivities were determined for the gases He, H2, O2, N2, CH4 and CO2 on the "as-cast" samples and after alcohol treatment. The performance of the MMMs was evaluated in relation to the Maxwell model. The addition of NH2-MIL-101 and ED-MIL-101 does not increase the membrane performance for the CO2/N2 and CO2/CH4 separation because of an initial decrease in selectivity at low MOF content, whereas the O2 and N2 permeability both increase for NH2-MIL-101. In contrast, MIL-101 and NanoMIL-101 cause a strong shift to higher permeability in the Robeson diagrams for all gas pairs, especially for CO2, without significant change in selectivity. Unprecedented CO2 permeabilities up to 35,600 Barrer were achieved, which are among the highest values reached with PIM-1 based mixed matrix membranes. For various gas pairs, the permeability and selectivity were far above the Robeson upper bound after alcohol treatment. Short to medium time aging shows that alcohol treated samples with MIL-101 maintain a systematically higher permeability in time. Mixed gas permeation experiments on an aged as-cast sample with 47 vol% MIL-101 reveal that the MMM sample maintains an excellent combination of permeability and selectivity, far above the Robeson upper bound (CO2 = 3500–3800 Barrer, CO2/N2 = 25–27; CO2/CH4 = 21–24). This suggests good perspectives for these materials in thin film composite membranes for real applications.