Ammonia-Induced Metal-Organic Framework Membrane Pore Tuning for Hydrogen Separation

Ammonia has attracted great attention as a key material for clean hydrogen production, such as the ammonia cracking process. Remaining trace ammonia in the purification step is considered a critical impurity due to its toxic and corrosive nature. Herein, we introduce ammonia as a tuning agent for MOF membranes to control their gas transport behavior. When exposed to ammonia, the ZIF-8 membrane undergoes partial surface amorphization due to strong interactions between ammonia and the framework, while the adjacent ZIF-8 region becomes rigid. As a result, the ammonia-treated ZIF-8 membrane exhibits the intrinsic gas-selective property rather than the flexible one, yielding excellent H₂/N₂ separation performance. The ammonia-treated ZIF-8 membrane achieves an H₂ permeance of 1.26 × 10^-7 mol/m²·Pa·s and an ideal H₂/N₂ selectivity of 122.39. Additionally, molecular dynamics simulations further revealed the interaction between ammonia and the ZIF-8 framework and how the local structural modification influences molecular diffusion. This study presents a new approach to utilizing reactive small molecules for precise control of structural flexibility and separation properties in MOF-based membranes. In addition, while most gas separations have been conducted in ideal conditions without impurities, it can be critical in practical applications to maintain the high separation performance of MOF-based membranes.