Tuning the Phase Composition of Metal-Organic Framework Membranes for Helium Separation through Incorporation of Fullerenes

Metal-organic framework (MOF) membranes have attractedsignificantresearch interest in gas separation, but efficient helium (He) separationremains a challenge due to the weak polarizability of He and the intrinsicpore size flexibility of MOFs. Herein, incorporated fullerenes (C-60 and C-70) were used to tune the crystallographicphase composition of ZIF-8 membranes, thus creating small and fixedapertures for selective He permeation. The fullerene-modified ZIF-8(C-60@ZIF-8 and C-70@ZIF-8) membranes containabout 20% of the rigid-lattice ZIF-8_I-43m phase and have been preparedas 200-350 nm thick supported layers through electrochemicalsynthesis. They show a significantly enhanced molecular sieving forHe/N-2,CH4 together with a satisfactory He permeanceof >200 GPU. Specifically, the He/N-2 selectivity oftheC(70)@ZIF-8 membrane is up to 30.4, which is much higherthan that of the fullerene-free ZIF-8 membrane (5.1) and nearly anorder of magnitude higher than those of other reported He-selectiveMOF membranes. A continuous long-term gas permeation test over 780h under dry and humid conditions proved the excellent stability ofthe fullerene-modified ZIF-8 membranes. The general validity and versatilityof the proposed strategy for MOF membrane preparation are also demonstratedby the enhancement of the separation performance of a fullerene-modifiedZIF-76 membrane.

Automated summary

Utilizing fullerenes to tune the crystallographic phase composition of ZIF-8 membranes creates small and fixed apertures for selective helium permeation, significantly enhancing the separation performance.