Experimental and Theoretical Evaluation of the Stability of True MOF Polymorphs Explains Their Mechanochemical Interconversions

Abstract

We provide the first combined experimental and theoretical evaluation of how differences in ligand structure and framework topology affect the relative stabilities of isocompositional (i.e., true polymorph) metal-organic frameworks (MOFs). We used solution calorimetry and periodic DFT calculations to analyze the thermodynamics of two families of topologically distinct polymorphs of zinc zeolitic imidazolate frameworks (ZIFs) based on 2-methyl- and 2-ethylimidazolate linkers, demonstrating a correlation between measured thermodynamic stability and density, and a pronounced effect of the ligand substituent on their stability. The results show that mechanochemical syntheses and transformations of ZIFs are consistent with Ostwald's rule of stages and proceed toward thermodynamically increasingly stable, more dense phases.

Magnetically responsive horseradish peroxidase@ZIF-8 for biocatalysis

“…Conversely, for pure ZIF-8, diffraction peaks of sod are observed after B20 seconds. Although other crystalline phases are indicated as more thermodynamically stable, 36,37 our syntheses yielded sod for all the MOF composites prepared.…”

Phase Dependent Encapsulation and Release Profile of ZIF-Based Biocomposites

“…diamondoid (dia), katsenite (kat), ZIF-L). [29][30][31][32][33] Similarly, for ZIF-based biocomposites, a variety of topologies can be accessed by modulating the reaction conditions. 19,34 In these studies, a fixed amount of biomacromolecule was employed while the concentrations of the ZIF components were varied.…”

Modulation of Metal-Azolate Frameworks for the Tunable Release of Encapsulated Glycosaminoglycans

“…These data indicate that the GAGs enhance the rate of formation of the ZIF yielding the less thermodynamically stable sod topology. [46] The PXRD pattern of ZIF-90, prepared in the absence of GAGs, is consistent with the dia polymorph. However, in presence of the biomolecule the resultant GAG@ZIF-90 biocomposites show a mixture of kinetic sod-ZIF-90 and the thermodynamic polymorph dia-ZIF-90 ( Figure 1b).…”