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 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 ) = 116 and α _ideal (He/CH ₄ ) = 78. In mixed gas permeation, the separation selectivity surpasses α _ideal (He/CH ₄ ) = 200 in a 1:1 mixture. Mixed-gas tests at realistic 4:96 He/CH ₄ feed gas composition reached a selectivity of α(He/CH ₄ ) = 31.2 and permeance for He of P(He) = 1800 GPU. 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. The paper highlights 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.