In situ growth of defect-minimized ZIF-8 membranes suitable for practical propylene/propane separation processes: Elucidation of mechanistic insights into ZIF-8 crystal growth led to an optimal synthetic route

초록

Despite energy-efficient separations, feasible defect control and scale-up in metal-organic framework membrane manufacturing largely remain limited. In this study, we experimentally and theoretically elucidated the liquidphase crystallization of zeolitic imidazolate framework-8 (ZIF-8), whose evolution depends on solvent polarity, metal precursor, and ion solvation. Such insights into direct in situ growth allowed for rigorous time-resolved analysis on ZIF-8 membrane formation. In particular, electrostatically assisted heterogeneous nucleation on hydroxylated alpha-Al2O3 has been verified under mildly alkaline conditions. Guided by these findings, coordinated tuning of composition and temperature in optimal solvent (here, water) yielded defect-minimized ZIF-8 membranes on alpha-Al2O3 disc supports, showing excellent C3H6/C3H8 separation performance (its separation factor up to 128) and remarkable pressure resistance and thermal stability. Furthermore, the in situ strategy was successfully extended to alpha-Al2O3 tube supports: The resulting tubular membranes showed C3H6/C3H8 separation factors of 79.3 +/- 5.4. Desirably, a four-channel membrane module, now having an effective area of 150 cm2, exhibited good long-term C3H6/C3H8 separation stability. Finally, the in situ method was effective in synthesizing large-area ZIF-8 membranes on flat polyacrylonitrile (PAN) supports (300 cm2), with satisfactory mechanical strength and C3H6/C3H8 separation performance comparable to those of the alpha-Al2O3 disc- or tube-supported ones. In this work, we proposed a fundamental solution (rather than a trial-and-error approach) for in situ ZIF-8 membrane growth independent of supports: Mechanism-driven control with manufacturable processing provided universal and practical design rules for high-performance ZIF-8 membranes.

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