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

Abstract

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.

Thermal and mechanical stability of zeolitic imidazolate frameworks polymorphs

“…It also shows that this stabilitization is dominated by a strong and generic pore filling effect, even in the absence of specific host-guest interactions (as is the case here with methane), in line with earlier work on the topic. 20,37 The same is true of frameworks of group (ii), especially ATN which was not stable without guest molecules at 150 K. For frameworks of group (i), however, there was no major effect: only the standard adsorption-induced variation of volume (of the order of 0.2%, and thus not depicted on Fig. 2) was observed.…”

Theoretical prediction of the mechanical properties of zeolitic imidazolate frameworks (ZIFs)

“…16 The weak resistance against shear deformation was later veried by the observation that ZIF-8 might undergo a rapid and catastrophic porosity collapse during ball-milling.…”

“…Therefore, the theoretical computation of the mechanical modulus of ZIFs has become an important alternative, 12,15,16 although it remains challenging to obtain accurate values through computational methods. 24 Yet a comparison of the mechanical modulus values obtained for different ZIFs might yield important insights.…”

Fast and efficient optimization of Molecular Dynamics force fields for microporous materials: Bonded interactions via force matching

“…The last two terms, instead, refer to the nonbonded ones: Lennard-Jones (12-6 van der Waals interaction, where R min is the location of the minimum and is the well depth), and Coulomb (electrostatic interaction between partial charges). We choose this functional form because it is suitable to adequately model the systems here under investigation [26,16,15] and because it is implemented in most modern MD simulation packages, exploiting the full power of massively parallel architectures and even GPUs [27][28][29][30][31][32].…”

“…1, which is currently receiving great attention from the scientific community [59,[72][73][74][75]26,76]. This structure has a cubic unit cell of side 1.69856 nm, made of metal ions (Zn) and organic linkers (2-methylimidazole, MeIM), arranged as silicon and oxygen atoms in zeolites.…”