Fabrication of MOFs/PDMS mixed matrix membranes and defect-free composite membranes for efficient air separation

Membrane is a potential candidate to solve the challenge of air separation to meet the huge industrial demand of N₂ or O₂. In this contribution, four types of prominent metal organic frameworks (MOFs) with different pore sizes and series, including ZIF-8 (3.4 Å), ZIF-71 (4.2 Å), UiO-66 (6 Å) and MIL-53(Al) (16.3 Å) were synthesized, and used as a nanoporous filler combined with polydimethylsiloxane (PDMS) substrate to prepare MOFs/PDMS mixed matrix membranes (MMMs) for air separation, which were compared to derive the optimal MOFs with high O₂/N₂ selectivity (α_O2/N2) and O₂ permeability (P_O2). Gas permeation test was done to analyze the influences of MOFs content, feed gas pressure and gas composition on the O₂/N₂ separation performance. 10 wt% ZIF-71/PDMS MMMs exhibits the optimal separation performance (α_O2/N2 = 3.14, P_O2 = 630 Barrer), while pressure has small effect on O₂/N₂ separation performance. P_O2 is higher under pure gas test conditions, while α_O2/N2 is better under mixed gas conditions, which is caused by the competitive adsorption-diffusion of O₂ and N₂ molecules. Further, ZIF-71/PDMS/PVDF composite membrane that could produce 30 vol% oxygen-enriched permeate stream was prepared, and the P_O2 (1047 Barrer) is improved by 66% compared with MMMs.