Crystal Size Dependent Flexibility in ZIF-7: From Macro- to Nanoscale

Abstract

Flexible metal–organic frameworks (MOFs) are highly desirable materials for gas separation, but most of them become rigid when the particle size is reduced toward nanoscale. We aim to comprehend the effect of textural properties such as crystal size, its distribution, and morphology on the gate-opening behavior stimulated by the adsorption of guest molecules in ZIF-7. The synthesis conditions are varied to obtain ZIF-7 batches with crystal sizes ranging between 0.05 and 15 μm with various size distributions. We report for the first time a CO2-filled open pore phase of ZIF-7 at 195 K (OP2) derived from in situ powder X-ray diffraction (PXRD) data measured in parallel to CO2 physisorption. The adsorption of CO2 on ZIF-7 indicates persisting flexibility for all particle size regimes; with the crystal size, its distribution, and morphology having a significant impact on both gate-opening and gate-closing pressures and slope of CO2 adsorption isotherms. In situ PXRD measurement indicated further expansion of the ZIF-7 framework in the presence of methanol as guest species. The capability of ZIF-7 to accommodate molecules larger than its 0.3 nm window diameter signifies the importance of intermolecular interactions to overcome the energy barrier for linker movement/gating of the framework.

Controlling the steepness of gate-opening behavior on elastic layer-structured metal–organic framework-11 via solvent-mediated phase transformation

“…There is a hypothesis that the steepness of the S-shaped curve represents the dispersity of particle sizes (specifically, the activation energy) in a sample used for the adsorption measurement. 16,34–36 That is, the gate-opening pressure varies depending on the particle size, resulting in a smeared S-shape as a collection of step shapes, each with slightly different threshold pressures. However, this hypothesis may not apply to this study for two main reasons.…”

“…One hypothesis for this phenomenon is that it reflects a significantly broader activation energy distribution in small grains. 16,34–36 Elucidating key factors that determine the magnitude of the slope of the adsorption isotherm is crucial, as most potential applications of flexible MOFs rely on their distinctive responsiveness.…”

Plastic Avalanches in Metal–Organic Framework Crystals Due to the Dynamic Phase Mixing

“…Although the phase transition due to compression was also observed in our recent MD simulations of bulk HKUST-1, it was found to occur cooperatively throughout the whole structure without any phase coexistence process. We attribute this difference to the different stress distributions in the different HKUST-1 structures considered, as the flexibility of MOFs has been proven to be dependent on their textural properties . In bulk HKUST-1 under compression, the stress uniformly distributes, causing the simultaneous collapse of all nanopores .…”

Thermomechanical properties of metal–organic framework HKUST-1 crystals

“…, extrusion and pellet formation) expose MOF crystals to significant mechanical stimuli. 5,6 In the past few decades, the intrinsic mechanical properties of various MOFs have been widely studied by numerous experimental techniques 7–16 and simulation methods. 16–25 Specifically, nanoindentation experiments have been widely used to measure the Young's modulus and hardness of various MOF crystals, 8–10,14,15 while some atomistic simulation methods such as density functional theory calculations and molecular dynamics (MD) simulations have also been employed to reveal the impact of some structural factors on the mechanical properties of crystalline MOFs.…”