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

Abstract

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.