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, N-2, 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.