John Zengel

New High- and Low-Temperature Phase Changes of ZIF-7: Elucidation and Prediction of the Thermodynamics of Transitions.

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.

Loading‐Dependent Transport Properties of Zeolitic Imidazolate Frameworks Probed by In‐Situ PFG NMR

Zeolitic imidazolate frameworks (ZIFs), a unique subclass of metal‐organic frameworks (MOFs) recently reported by Yaghi group and others, are attracting worldwide attention. Our systematic work on the adsorption and molecular transport in ZIFs has revealed some unique properties of these new materials that could not have been anticipated from standard structural characterization results. More specifically, the rearrangement of the imidazolate linkers (and in some cases, the framework structure) driven by adsorbate‐adsorbent interaction causes the window sizes and accessible pore space of ZIFs to deviate (in some cases, drastically) from the values determined by low‐temperature single‐crystal X‐ray crystallography and typical physisorption experiments carried out at liquid nitrogen temperature (77 K) with H2, N2 and Ar. Here we show the high degree of structural flexibility of ZIFs at near ambient temperatures and report the first time their unique adsorption and transport characteristics.