Highly porous metal-organic framework glass design and application for gas separation membranes.

Abstract

Crystalline metal-organic frameworks (MOFs) exhibit enormous potential application in gas separation, thanks to their highly porous structures and precise pore size distributions. Nevertheless, the inherent limitations in mechanical stability of crystalline MOFs cause challenges in processing MOF powders into bulky structures, particularly for membrane filtrations. Melt-quenched MOF glasses boast excellent processability due to liquid-like properties. However, the melting process diminishes the inherent porosity, leading to reduced gas adsorption capacities and lower gas diffusion coefficients. In this work, we demonstrated that enhancing the porosity of MOF glasses is achievable through topological engineering on the crystalline precursors. Crystalline zeolitic imidazolate frameworks (ZIFs) with large 12-membered rings pores, including AFI and CAN topology, were synthesized by using both structure-directing agents and mixed organic ligands. The large pores are partially preserved in the melt-quenched glass as evidenced by high-pressure CO₂ absorption at 3000 kPa. The a_gAFI-[Zn(Im)_1.68(bIm)_0.32] glass was then fabricated into self-supported membranes, which shows high gas separation performance, for example, CO₂ permeance of 3.7 × 10⁴ GPU with a CO₂/N₂ selectivity of 14.8.