Reliably Modeling the Mechanical Stability of Rigid and Flexible Metal–Organic Frameworks

Abstract

ConspectusOver the past two decades, metal–organic frameworks (MOFs) have matured from interesting academic peculiarities toward a continuously expanding class of hybrid, nanoporous materials tuned for targeted technological applications such as gas storage and heterogeneous catalysis. These oft-times crystalline materials, composed of inorganic moieties interconnected by organic ligands, can be endowed with desired structural and chemical features by judiciously functionalizing or substituting these building blocks. As a result of this reticular synthesis, MOF research is situated at the intriguing intersection between chemistry and physics, and the building block approach could pave the way toward the construction of an almost infinite number of possible crystalline structures, provided that they exhibit stability under the desired operational conditions. However, this enormous potential is largely untapped to date, as MOFs have not yet found a major breakthrough in technological applications. One of the remaining challenges for this scale-up is the densification of MOF powders, which is generally achieved by subjecting the material to a pressurization step. However, application of an external pressure may substantially alter the chemical and physical properties of the material. A reliable theoretical guidance that can presynthetically identify the most stable materials could help overcome this technological challenge.In this Account, we describe the recent research the progress on computational characterization of the mechanical stability of MOFs. So far, three complementary approaches have been proposed, focusing on different aspects of mechanical stability: (i) the Born stability criteria, (ii) the anisotropy in mechanical moduli such as the Young and shear moduli, and (iii) the pressure-versus-volume equations of state. As these three methods are grounded in distinct computational approaches, it is expected that their accuracy and efficiency will vary. To date, however, it is unclear which set of properties are suited and reliable for a given application, as a comprehensive comparison for a broad variety of MOFs is absent, impeding the widespread use of these theoretical frameworks.Herein, we fill this gap by critically assessing the performance of the three computational models on a broad set of MOFs that are representative for current applications. These materials encompass the mechanically rigid UiO-66(Zr) and MOF-5(Zn) as well as the flexible MIL-47(V) and MIL-53(Al), which undergo pressure-induced phase transitions. It is observed that the Born stability criteria and pressure-versus-volume equations of state give complementary insight into the macroscopic and microscopic origins of instability, respectively. However, interpretation of the Born stability criteria becomes increasingly difficult when less symmetric materials are considered. Moreover, pressure fluctuations during the simulations hamper their accuracy for flexible materials. In contrast, the pressure-versus-volume equations of state are det...

Solvent‐Free Encapsulation at High Pressure with Carboxylate‐Based MOFs

“…Conspicuously characteristic peaks below 10° disappeared while most of the others tended to merge into broad peaks. These findings are in agreement with previous works on computational characterization of the mechanical stability of flexible MIL‐53(Al) and rigid UiO‐66 , . These works predicted the loss of crystallinity of rigid UiO‐66 at 1.83 GPa.…”

“…In agreement with previous publications dealing with the liquid phase encapsulation of drugs in MOFs,, , , , , , , XRD, N 2 adsorption, NMR and TGA techniques were employed to evidence the encapsulation carried out here by the new solvent‐free methodology proposed. XRD is qualitative proof of encapsulation, since the guest molecule intensity in its contact with the MOF decreases due the adsorption in the MOF structure.…”

“…These findings are in agreement with previous works on computational characterization of the mechanical stability of flexible MIL-53(Al) and rigid UiO-66. [60,61] These works predicted the loss of crystallinity of rigid UiO-66 at 1.83 GPa. In turn, this coincides with the experimental work carried out on UiO-66 that concluded that this MOF was not amorphized below 2 GPa, even though a gradual loss of crystallinity was detected at moderated pressures (0.3-1.7 GPa) in terms of broadening of the XRD peaks.…”

Strontium‐Carboxylate‐Based Coordination Polymers: Synthesis, Structure and Dielectric Properties

“…It is recognized that order‐disorder of ions and solvent molecules in the lattice could influence the dielectric constant of CPs/MOFs . Rigid ligands such as benzene di‐, tri‐ and tetra‐carboxylic acids provide better thermal and mechanical stability; in addition, they tend to form solvent‐free networks . On the other hand, flexible ligands favour more open frameworks facilitating incorporation of guest molecules .…”

“…[32][33][34] Rigid ligands such as benzene di-, tri-and tetra-carboxylic acids provide better thermal and mechanical stability; in addition, they tend to form solvent-free networks. [35][36][37][38] On the other hand, flexible ligands favour more open frameworks facilitating incorporation of guest molecules. [39][40][41][42] The rigidity, flexibility and inclusion of the lattice solvents greatly influences its dielectric behaviour.…”