Despite forming the critical basis for technologies in separation, catalysis, nuclear waste containment and biomedical applications, the relationship between the structural topology of the microporous framework materials and energetics of their formation and stability remain poorly understood. ZIF-8 (Zn(MeIm)2, MeIm = 2-methylimidazolate) is one of the most representative ZIF materials with a zeolite sodalite topology. Katsenis et al. (2015 Nat. Comm.) have shown that ZIF-8 transforms to a series of denser polymorphs by ball milling ultimately forming a dense, non-porous polymorph of ZIF-8 with a diamondoid topology (dia-Zn(MeIm)2). In this study, ZIF-8 is investigated using density functional theory (DFT) to reveal the structural changes of porous structure at high pressure and to compare with the result from mechanochemistry. Phonon calculations are also performed to predict the phonon density of states from which the thermodynamic properties of each polymorph are derived. Ab initio molecular dynamic simulations using CP2K package under NPT (constant pressure and temperature) ensemble were performed at pressures ranging from 0 to 4 GPa for ZIF-8. In order to detect the pressure effect, the changes in the volume of the simulation cell are recorded with increasing pressure. In addition, the reversibility effect is explored by de-pressurizing compressed structures. Molecular dynamics snapshots at different stages in compression and decompression are used to identify possible pressure-induced phase transitions, including pressure-induced amorphization, and their decompression to the crystalline starting point. Phonon calculations were performed for both ZIF-8 and dia-Zn(MeIm)2 to extract the phonon density of states and are compared with results from inelastic neutron scattering. Thermodynamic properties such as heat capacities are calculated and compared with calorimetric data focusing on the low-temperature regime where the phonon density of states of ZIF8 and dia-Zn(MeIm)2 vary the most.

DFT study of ZIF8 and its polymorph dia-Zn(MeIm)2

2019 Spring National Meeting

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