Melting of Zeolitic Imidazolate Frameworks with Different Topologies: Insight from First-Principles Molecular Dynamics

Abstract

Metal-organic frameworks are chemically versatile materials, and excellent candidates for many applications from carbon capture to drug delivery, through hydrogen storage. While most studies so far focus on the crystalline MOFs, there has been a recent shift to the study of their disordered states, such as defective structures, glasses, gels, and very recently liquid MOFs. Following the publication of the melting mechanism of zeolitic imidazolate framework ZIF-4, we use here molecular simulation in order to investigate the similarities and differences with two other zeolitic imidazolate frameworks, ZIF-8 and ZIF-zni. We perform first principles molecular dynamics simulations to study the melting phenomena and the nature of the liquids obtained, focusing on structural characterization at the molecular scale, dynamics of the species, and thermodynamics of the solid-liquid transition. We show how the retention of chemical configuration, the changes in the coordination network, and the variation of the porous volume in the liquid phase are influenced by the parent crystalline framework.

Investigating the Melting Behaviour of Polymorphic Zeolitic Imidazolate Frameworks

“…63 As discussed earlier, Gaillac et al concluded that ZIF-8 does not melt on its own because of the high energy barrier to linker diffusion in its open network structure. 22,32,33 These observations can rationalise both the results in this paper and those previously reported. 37 ZIF-76-mbIm, like ZIF-8, is a relatively porous, cubic framework.…”

“…37,56,57 Hence, the dispersion forces in TIF-4 are stronger and more energy is required for the methyl groups to move past one another and for the glass to transition to a more liquid-like, mobile state. 32,37 The use of halogenated linkers therefore may provide a promising strategy for lowering the melting and glass transition temperatures of MOFs.…”

“…23,37 Interestingly, another open, cubic framework, ZIF-8 [Zn(mIm)2] (mIm -2-methylimidazolate -C4H5N2 -), decomposes prior to melting. 22,32,33 Theoretical calculations performed by Gaillac et al demonstrated ZIF-8 melts significantly above its experimental decomposition temperature. 32,41 This is attributed to the relatively high barrier to linker diffusion between zinc tetrahedra due to the open network structure.…”

“…[29][30][31] Firstprinciples molecular dynamics simulations have demonstrated that melting of ZIFs occurs on a picosecond timescale: rapid decoordination of an imidazolate linker from a metal ion is followed by dissociation and re-coordination of a new linker in its place. 22,32 The temperature at which the transition to the liquid state is complete is referred to as the melting temperature (Tm). The liquids formed upon heating can then be quenched upon cooling, yielding melt-quenched glasses.…”

Investigating the melting behaviour of polymorphic zeolitic imidazolate frameworks

“…[29][30][31] First-principles molecular dynamics simulations have demonstrated that melting of ZIFs occurs on a picosecond timescale: rapid de-coordination of an imidazolate linker from a metal ion is followed by dissociation and re-coordination of a new linker in its place. 22,32 The temperature at which the transition to the liquid state is complete is referred to as the melting temperature (T m ). The liquids formed upon heating can then be quenched upon cooling, yielding melt-quenched glasses.…”