How Crystallite Size and Topology Control Intrusion-Extrusion Hysteresis in Metal-Organic Frameworks

Heterogeneous lyophobic systems (HLSs) are of interest for diverse applications, including energy materials and column chromatography. Within these systems, tuneability is a key property to broaden the scope of application. In this work, we explore how the exogeneous property of crystallite size can be used to tune in the intrusion-extrusion characteristics of various ZIF MOFs of differing chemistries and topologies. We employ a combination of experimental and in silico techniques to investigate and analyze the behavior of distinct ZIFs during high-pressure water intrusion-extrusion. We observed that ZIF-71, despite differing both chemically and topologically from the previously studied ZIF-8, demonstrated a reduction in intrusion/extrusion pressures and intruded volume with a reduction in crystallite size down to the nanoscale. This was corroborated by a stochastic model of the intrusion of the microporous framework, where control of network size could qualitatively match the experimentally observed behavior. For ZIF-67, the broader distribution of crystallite size led to a staggered intrusion step, which was replicated by the model by averaging the PV-isotherms simulated for various network sizes. These results pave the way for the diversification of HLSs to include the known ZIF family capable of water intrusion-extrusion. Furthermore, controlled crystallite size ranges can be used to mimic the performance of pore size distribution in silica without the inherent associated randomness.