We report the first systematic study experimentally investigating the effect of changes to the divalent metal node on the thermodynamic stability of three-dimensional (3D) and two-dimensional (2D) zeolitic imidazolate frameworks (ZIFs) based on 2-methylimidazolate linkers. In particular, the comparison of enthalpies of formation for materials based on cobalt, copper, and zinc suggests that the use of nodes with larger ionic radius metals leads to the stabilization of the porous sodalite topology with respect to the corresponding higher-density diamondoid (dia)-topology polymorphs. The stabilizing effect of metals is dependent on the framework topology and dimensionality. With previous works pointing to solvent-mediated transformation of 2D ZIF-L structures to their 3D analogues in the sodalite topology, thermodynamic measurements show that contrary to popular belief, the 2D frameworks are energetically stable, thus shedding light on the energetic landscape of these materials. Additionally, the calorimetric data confirm that a change in the dimensionality (3D → 2D) and the presence of structural water within the framework can stabilize structures by as much as 40 kJ·mol-1, making the formation of zinc-based ZIF-L material under such conditions thermodynamically preferred to the formation of both ZIF-8 and its dense, dia-topology polymorph.

This study experimentally explores the energetics for the formation of boron-imidazolate frameworks (BIFs), which are synthesized by mechanochemistry. The topologically similar frameworks employ the same tetratopic linker based on tetrakis(imidazolyl)boric acid but differ in the monovalent cation metal nodes. This permits assessment of the stabilizing effect of metal nodes in frameworks with sodalite (SOD) and diamondoid (dia) topologies. The enthalpy of formation from endmembers (metal oxide and linker), which define thermodynamic stability of the structures, has been determined by use of acid solution calorimetry. The results show that heavier metal atoms in the node promote greater energetic stabilization of denser structures. Overall, in BIFs the relation between cation descriptors (ionic radius and electronegativity) and thermodynamic stability depends on framework topology. Thermodynamic stability increases with the metallic character of the cation employed as the metal node, independent of the framework topology. The results suggest unifying aspects for thermodynamic stabilization across MOF systems.

We report the first experimental and theoretical evaluation of the thermodynamic driving force for the reaction of metal-organic framework (MOF) materials with carbon dioxide, leading to a metal-organic carbonate phase. Carbonation upon exposure of MOFs to CO2 is a significant concern for the design and deployment of such materials in carbon storage technologies, and this work shows that the formation of a carbonate material from the popular SOD-topology framework material ZIF-8, as well as its dense-packed dia-topology polymorph, is significantly exothermic. With knowledge of the crystal structure of the starting and final phases in the carbonation reaction, we have also identified periodic density functional theory approaches that most closely reproduce the measured reaction enthalpies. This development now permits the use of advanced theoretical calculations to calculate the driving forces behind the carbonation of zeolitic imidazolate frameworks with reasonable accuracy.

Rare metal-organic framework (MOF) minerals stepanovite and zhemchuzhnikovite can exhibit properties comparable to known oxalate MOF proton conductors, including high proton conductivity over a range of relative humidities at 25 °C, and retention of the framework structure upon thermal dehydration. They also have high thermodynamic stability, with a pronounced stabilizing effect of substituting aluminium for iron, illustrating a simple design to access stable, highly proton-conductive MOFs without using complex organic ligands.

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