We provide a general thermodynamic framework for the understanding of guest-induced structural transitions in hybrid organic-inorganic materials. The method is based on the analysis of experimental adsorption isotherms. It allows the determination of the free energy differences between host structures involved in guest-induced transitions, especially hard to obtain experimentally. We discuss the general case of adsorption in flexible materials and show how a few key quantities, such as pore volumes and adsorption affinities, entirely determine the phenomenology of adsorption, including the occurrence of structural transitions. On the basis of adsorption thermodynamics, we then propose a taxonomy of guest-induced structural phase transitions and the corresponding isotherms. In particular, we derive generic conditions for observing a double structural transition upon adsorption, often resulting in a two-step isotherm. Finally, we show the wide applicability and the robustness of the model through three case studies of topical hybrid organic-inorganic frameworks: the hysteretic hydrogen adsorption in Co(1,4-benzenedipyrazolate), the guest-dependent gate-opening in Cu(4,4'-bipyridine)(2,5-dihydroxybenzoate)2 and the CO2-induced "breathing" of hybrid material MIL-53.

Thermodynamics of Guest-Induced Structural Transitions in Hybrid Organic-Inorganic Frameworks

Elucidation of flexible metal-organic frameworks: Research progresses and recent developments

Porous glasses from metal-organic frameworks (MOFs) represent a new class of functional inorganic-organic materials, which have been proposed for applications ranging from solid electrolytes to radioactive waste storage. So far, just a few zeolitic imidazolate frameworks (ZIFs), a subset of MOFs, have been reported to melt and the structural and compositional requirements for MOF melting and glass formation are poorly understood. Here, we show how the melting point of the prototypical ZIF-4/ZIF-62(M) frameworks (composition M(im)2-x(bim)x; M2+ = Co2+, Zn2+; im- = imidazolate; bim- = benzimidazolate) can be controlled systematically by adjusting the molar ratio of the two imidazolate-type linkers im- and bim-. By covering the entire range from x = 0 to 0.35, we unveil a delicate transition from ZIF materials showing sequential amorphization/recrystallization to derivatives exhibiting coherent melting and a liquid phase that is stable over a large temperature window. The melting point of this ZIF system is a direct function of x and can be lowered from ca. 430 °C to only 370 °C, by far the lowest melting point reported for a three-dimensional porous MOF. On the basis of our results, we postulate compositional requirements for ZIF melting and glass formation, which may guide the search for other meltable ZIFs. Moreover, gas physisorption experiments establish that the ZIF glasses adsorb technologically relevant C3 and C4 hydrocarbons. Importantly, the adsorption kinetics are much faster for propylene compared to propane and are also dependent on the im-:bim- ratio, thus demonstrating the potential of these ZIF glasses for applications in gas separation.

Meltable Mixed-Linker Zeolitic Imidazolate Frameworks and Their Microporous Glasses: From Melting Point Engineering to Selective Hydrocarbon Sorption

Studying inter-dimensional phase transitions of zeolitic-imidazolate frameworks (ZIFs) is essential for developing strategies in controlling morphology and properties. Herein, the inter-dimensional topotactic phase transformations of 3D ZIF-67 to 2D ZIF-L are investigated in detail by employing a simple and efficient solvent-induced growth method. In addition to ZIF-67 and ZIF-L, a series of novel core-shell composites of ZIF-67@ZIF-L, with unprecedented morphologies, are also obtained and well-defined. The different behaviors of the amine hydrogen of 2-MIM in the solvents play a pivotal role for inter-dimensional phase transformations, and in combination with the concentration of 2-MIM, the 2D to 3D phase transformations are also revealed. The findings are very beneficial for morphological design of the ZIFs, along with exploration of the corresponding properties. Impressively, Co-ZIFs exhibit interesting tunable CO2 adsorption behaviors with the phase evolution, which might bring broader understanding for designing CO2 detection and adsorption devices.

The Solvent Induced Inter-Dimensional Phase Transformations of Cobalt Zeolitic-Imidazolate Frameworks.

Zeolitic imidazolate framework ZIF-8 displays flexibility of its structure by rotation of its imidazolate linker. This "swing effect" has been widely described in the literature, both experimentally and theoretically, as a bistable system where the linker oscillates between two structures: "open window" and "closed window". By using quantum chemistry calculations and first-principles molecular dynamics simulations, it is shown that the deformation upon adsorption is in fact continuous upon pore loading, with thermodynamics of packing effects being the reason behind stepped adsorption isotherms experimentally observed. Finally, we study a variant of ZIF-8 with a different linker, highlighting the influence of the linker and the balance of microscopic interactions on the framework's flexibility.

Molecular Mechanism of Swing Effect in Zeolitic Imidazolate Framework ZIF‐8: Continuous Deformation upon Adsorption

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 ...

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

The flexibility of zeolitic imidazolate frameworks (ZIFs) is often interpreted as a result from their bulky linkers, the free rotations of which sometimes generate enough momentum to fluctuate the pore opening. In some extreme cases, all metal centers’ atomic positions are changed correspondingly, and in a three-dimensional network, it is intriguing how the displacement happens coordinately. Here, we bridged the learnings from zeolite sodalite (SOD) topology and applied it in studying ZIF-7’s structural transformation. We discovered a new extra-large pore ZIF-7 following the same rhombohedral symmetry. We have examined side-by-side the structural changes from X-ray diffraction (XRD) and solid-state NMR and successfully identified the precise adsorption sites. We applied ZIF-7’s flexibility in alcohol separation and used a series of alcohol molecules to study the adsorption mechanism.

Insights into the Flexibility of ZIF-7 and Its Structural Impact in Alcohol Adsorption

Methods of preparing organosilica materials, which is a polymer comprising independent siloxane units of Formula [Z 3 Z 4 SiCH 2 ] 3 (I), wherein each Z 3 represents a hydroxyl group, a C 1 -C 4 alkoxy group or an oxygen atom bonded to a silicon atom of another siloxane unit and each Z 4 represents a hydroxyl group, a C 1 -C 4 alkoxy group, a C 1 -C 4 alkyl group, or an oxygen atom bonded to a silicon atom of another siloxane, in the absence of a structure directing agent and/or porogen are provided herein. Processes of using the organosilica materials, e.g., for gas separation, etc., are also provided herein.

Methods of producing organosilica materials and uses thereof

A heat transfer fluid can be used as part of a multi-phase adsorption environment to allow for improved separations of gas components using a solid adsorbent. The heat transfer fluid can reduce or minimize the temperature increase of the solid adsorbent that occurs during an adsorption cycle. Reducing or minimizing such a temperature increase can enhance the working capacity for an adsorbent and/or enable the use of adsorbents that are not practical for commercial scale adsorption using conventional adsorption methods. The multi-phase adsorption environment can correspond to a trickle bed environment, a slurry environment, or another convenient environment where at least a partial liquid phase of a heat transfer fluid is present during gas adsorption by a solid adsorbent.

Meghan Nines

Papers

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

Insights into the Flexibility of ZIF-7 and Its Structural Impact in Alcohol Adsorption

Methods of producing organosilica materials and uses thereof

Cyclic thermal swing adsorption with direct heat transfer

Cyclic thermal swing adsorption with direct heat transfer using a heat transfer fluid