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 this planet, as it is a finite resource, cannot be produced from radioactive decay and escapes the atmosphere, while being extraordinarily important for high-tech applications in research and medicine. A concept of using the “dense” metal–organic framework (MOF) MIL-116(Ga) as a molecular sieve specifically allowing diffusion of He is demonstrated. Incorporating up to 20 wt.% MIL-116(Ga) into polysulfone, a chemically stable, mechanically robust, and commercially available polymer, high-performance mixed matrix membranes are fabricated and tested in gas permeation. The membranes reach He permeabilities up to 37.4. Barrer and He/CH4 selectivity of 1190, mimicking He concentration of 4% of a natural gas reservoir. With increasing filler content, the 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 demonstrating ideal compatibility. The performance is benchmarked to existing composite materials and polymers, where MIL-116(Ga)-formate stands out with extraordinary membrane performance.