Synthesis and helium separation performance of polycrystalline membranes of the high precision molecular sieve MIL-116(Ga)

Helium is a finite but essential resource with important applications in medicine, research, and aerospace. Conventional He-upgrading from natural methane gas (CH 4 ) is done by cryogenic distillation. This study presents energy-efficient He-upgrading by membrane separation using the “dense” metal-organic framework (MOF) MIL-116(Ga), which enables high precision molecular sieving of non-adsorptive gases. MIL-116(Ga) membranes were synthesized on alumina disks after developing a method to homogeneously intergrow the MOF into a polycrystalline film. Single gas permeation experiments reveal exceptional ideal selectivity of α ideal (H 2 /CH 4 ) = 122 and α ideal (He/CH 4 ) = 87. In mixed gas permeation, the separation selectivity surpasses α ideal (He/CH 4 ) = 207 GPU. Mixed-gas tests at realistic 4:96 He/CH 4 feed gas composition reached a selectivity of α(He/CH 4 ) = 31.9 and permeance for He of P(He) = 227 GPU despite the low chemical potential. Electron microscopy uncovered a complex, grain-boundary microstructure, which limits perfect molecular sieving but still enables superior separation performance. This work demonstrates that dense MOFs, at the example of MIL-116(Ga) enables high performance He-upgrading, setting a new benchmark amongst reported MOF-based membranes. We highlight the potential of dense MOFs for the separations of small, non-adsorptive gases and the need to address grain boundary diffusion in polycrystalline MOF membranes.