Toward an Accurate Modeling of the Water−Zeolite Interaction: Calibrating the DFT Approach

The performances of nine selected exchange-correlation functionals for the description of the interaction of water with clusters modeling the silicalite-1 zeolite have been assessed. The chosen structural models cover different classes of interactions between water and a zeolite backbone, ranging from electrostatics to van der Waals types. Almost all of the considered functionals give qualitatively correct results for the considered systems with respect to the reference MP2 data. However, among all of the models, only two (M05-2X and B97-D) provide a quantitative agreement for all of the clusters taken into account. These functionals can thus be envisaged as affordable methods to study weakly interacting systems as well as to provide a database for the development of accurate and transferable intermolecular potentials for classical simulations

Stress-Based Model for the Breathing of Metal−Organic Frameworks

Gas adsorption in pores of flexible metal−organic frameworks (MOF) induces elastic deformation and structural transitions associated with stepwise expansion and contraction of the material, known as breathing transitions between large pore (<b>lp</b>) and narrow pore (<b>np</b>) phases. We present here a simple yet instructive model for the physical mechanism of this enigmatic phenomenon considering the adsorption-induced stress exerted on the material as a stimulus that triggers breathing transitions. The proposed model implies that the structural transitions in MOFs occur when the stress reaches a certain critical threshold. We showcase this model by drawing on the example of Xe adsorption in MIL-53 (Al) at 220 K, which exhibits two consecutive hysteretic breathing transitions between <b>lp</b> and <b>np</b> phases. We also propose an explanation for the experimentally observed coexistence of <b>np</b> and <b>lp</b> phases in MIL-53 materials

Investigating the Pressure-Induced Amorphization of Zeolitic Imidazolate Framework ZIF-8: Mechanical Instability Due to Shear Mode Softening

We provide the first molecular dynamics study of the mechanical instability that is the cause of pressure-induced amorphization of zeolitic imidazolate framework ZIF-8. By measuring the elastic constants of ZIF-8 up to the amorphization pressure, we show that the crystal-to-amorphous transition is triggered by the mechanical instability of ZIF-8 under compression, due to shear mode softening of the material. No similar softening was observed under temperature increase, explaining the absence of temperature-induced amorphization in ZIF-8. We also demonstrate the large impact of the presence of adsorbate in the pores on the mechanical stability and compressibility of the framework, increasing its shear stability. This first molecular dynamics study of ZIF mechanical properties under variations of pressure, temperature, and pore filling opens the way to a more comprehensive understanding of their mechanical stability, structural transitions, and amorphization

Molecular Simulation of a Zn–Triazamacrocyle Metal–Organic Frameworks Family with Extraframework Anions

We report an investigation by means of adsorption experiments and molecular simulation of the behavior of a recently synthesized cationic metal–organic framework. We used a combination of quantum chemistry calculations and classical forcefield-based Grand Canonical Monte Carlo simulations to shed light onto the localization of extra-framework halogenide anions in the material. We also studied the adsorption of small gas molecules into the pores of the material using molecular simulation and investigated the coadsorption of binary gas mixtures

Softening upon Adsorption in Microporous Materials: A Counterintuitive Mechanical Response

We demonstrate here that microporous materials can exhibit softening upon adsorption of guest molecules, at low to intermediate pore loading, in parallel to the pore shrinking that is well-known in this regime. This novel and counterintuitive mechanical response was observed through molecular simulations of both model pore systems (such as slit pore) and real metal–organic frameworks. It is contrary to common belief that adsorption of guest molecules necessarily leads to stiffening due to increased density, a fact that we show is the high-loading limit of a more complex behavior: a nonmonotonic softening-then-stiffening

Molecular Simulation of a Zn–Triazamacrocyle Metal–Organic Frameworks Family with Extraframework Anions

We report an investigation by means of adsorption experiments and molecular simulation of the behavior of a recently synthesized cationic metal–organic framework. We used a combination of quantum chemistry calculations and classical forcefield-based Grand Canonical Monte Carlo simulations to shed light onto the localization of extra-framework halogenide anions in the material. We also studied the adsorption of small gas molecules into the pores of the material using molecular simulation and investigated the coadsorption of binary gas mixtures