(642a) Thermodynamic Gate-Opening Behavior Study on Network Nanostructured ZIF-8 to Control Gas Transport Selectivity

Inorganic materials such as metal–organic frameworks (MOFs) are of significant interest in achieving high-performance gas separation membranes with good stability. Unfortunately, MOF crystals are difficult to process into continuous films and expensive to make as modules. Instead, MOFs are often incorporated into polymers to form mixed-matrix membranes (MMMs), where percolation network formation between MOF fillers could play a significant role in fully taking advantage of high-performance MOFs. However, particles tend to aggregate, preventing the MOF network in the polymer matrix from forming and even resulting in non-selective defects. Thus, even at high MOF loadings, particle percolation, and hence, MOF-like transport is rarely accessible.

From our previous work, a facile method to synthesize branched ZIF-8 (BZ) nanoparticles at ambient conditions has been developed for efficient gas separations that can achieve this percolation network in the continuous polymeric matrix, resulting in significantly faster gas transport compared to its bulk counterpart traditional ZIF-8 composite. In addition, due to the small particle dimensions of BZ and the high surface-area-to-volume ratios, ZIF-8 ligand motion is suppressed when combining into the polymer–ZIF composites. This gate opening suppression results in exceptionally high selectivities for small gases (e.g., H2-based) compared to the typical propylene/propane size cut-off observed for traditional ZIF-8 control (CZ) particles. Additionally, mechanical property analysis and NMR relaxation studies indicate that the BZ morphology effectively stabilizes polymer chains, preventing plasticization. Here, this confinement effect has been further studied thermodynamically by varying temperatures to quantify the activation enthalpy for diffusion of penetrants. This systematic study allows us to finely tune the selectivity of different gas pairs whilst keeping the fast transport via percolated network.