We have found that the 3D zeolitic imidazolate framework ZIF-7 exhibits far more complex behavior in response to the adsorption of guest molecules and changes in temperature than previously thought. We believe that this arises from the existence of different polymorphs and different types of adsorption sites. We report that ZIF-7 undergoes a displacive, nondestructive phase change upon heating to above ∼700 °C in vacuum, or to ∼500 °C in CO2 or N2. This is the first example of a temperature-driven phase change in 3D ZIF frameworks. We predicted the occurrence of the high-temperature transition on the basis of thermodynamic arguments and analyses of the solid free-energy differences obtained from CO2 and n-butane adsorption isotherms. In addition, we found that ZIF-7 exhibits complex behavior in response to the adsorption of CO2 manifesting in double transitions on adsorption isotherms and a doubling of the adsorption capacity. We report adsorption microcalorimetry, molecular simulations, and detailed XRD investigations of the changes in the crystal structure of ZIF-7. Our results highlight mechanistic details of the phase transitions in ZIF-7 that are driven by adsorption of guest molecules at low temperature and by entropic effects at high temperature. We derived a phase diagram of CO2 in ZIF-7, which exhibits surprisingly complex re-entrant behavior and agrees with our CO2 adsorption measurements over a wide range of temperatures and pressures. We predicted phase diagrams of CH4, C3H6, and C4H10. Finally, we modeled the temperature-induced transition in ZIF-7 using molecular dynamics simulations in the isobaric-isothermal ensemble, confirming our thermodynamic arguments.

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The kinetics of the irreversible phase conversion of covalent organic frameworks-1 (COF-1) has been investigated using time-resolved, in situ environmental X-ray diffraction (EXRD) and modeled with the Avrami–Erofe’ev model. Tightly fitting mesitylene solvent is found to be present in both the AB staggered and AA eclipsed polymorphs, which plays a key role in the phase change. Solid-state NMR (SSNMR) showed the presence of discrete dipolar coupling between residual mesitylene solvent and the framework in both polymorphs, indicative of a host–guest adsorptive interaction. Binding energy calculations indicate two different adsorbed mesitylene configurations in the AB and AA phases, both with short distances to the framework pore walls to generate the observed dipolar coupling. The mechanism of phase change has been illustrated using molecular dynamics simulations and was found to be a displacive transition from AB staggered to AA eclipsed COF-1 structures, which was made possible due to low in-plane shear modulus of 2D COFs. Our findings highlight the polymorphic nature of COF-1 material mediated by the interactions with guest molecules and the irreversibility for polymorph formation and conversion.

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