Synthesis and helium separation performance of polycrystalline membranesofthehigh 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 (CH4) is done by cryogenic distillation. This study presents energy-efficient He-upgrading by membrane separation using the “dense”metal-organicframework(MOF)MIL-116(Ga),whichenableshighprecisionmolecularsieving 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 αideal(H2/CH4) = 122 and αideal(He/ CH4)=87. In mixed gas permeation, the separation selectivity surpasses αideal(He/CH4)=207 GPU. Mixed-gas tests at realistic 4:96 He/CH4 feed gas composition reached a selectivity of α(He/ CH4)=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.