Ideal Molecular Sieving with a Dense MOF for Helium Upgrading with Highly Diffusion Selective Mixed Matrix Membranes

Helium is one of the most critical resources of our planet, as it is a finite resource, cannot be produced from radioactive decay in sufficient amounts and escapes our atmosphere, while being extraordinarily important for high tech applications in research and medicine. We demonstrate a concept of using the “dense” metal-organic framework (MOF) MIL-116(Ga) as a molecular sieve specifically allowing diffusion of He. Incorporating up to 20 wt.% MIL-116(Ga) into polysulfone, a chemically stable, mechanically robust, and commercially available polymer, high performance mixed matrix membranes were fabricated and tested in gas permeation. The membranes reach He permeabilities up to 37.4 Barrer and He/CH4 selectivity of 1190, mimicking process conditions with He concentration of 4 % in CH4. With increasing filler content, permeability of He increases, while CH4 permeability decreases. Microstructural analysis of the MIL-116(Ga) reveals that the crystals grew into druse-like hollow crystals, highly beneficial for fast He permeability. CH4, N2 and CO2 cannot enter the crystal, as proven by sorption experiments, providing high diffusional selectivity. Furthermore, polymer filler interactions are investigated by scanning electron microscopy and energy dispersive x-ray spectroscopy. We benchmarked the performance to existing composites and polymers, where MIL-116(Ga)-formate stands out with extraordinary membrane performance.