Flexibility of zeolitic imidazolate framework structures studied by neutron total scattering and the reverse Monte Carlo method

Abstract

The zeolitic imidazolate framework ZIF-4 undergoes an amorphization transition at about 600 K, and then transforms at about 700 K to ZIF-zni, the densest of the crystalline ZIFs. This series of long-range structural rearrangements must give a corresponding series of changes in the local structure, but these have not previously been directly investigated. Through analysis of neutron total diffraction data by reverse Monte Carlo modelling, we assess the changes in flexibility across this series, identifying the key modes of flexibility within ZIF-4 and the amorphous phase. We show that the ZnN4 tetrahedra remain relatively rigid, albeit less so than SiO4 tetrahedra in silicates. However, the extra degrees of freedom afforded by the imidazolate ligand, compared to silicate networks, vary substantially between phases, with a twisting motion out of the plane of the ligand being particularly important in the amorphous phase. Our results further demonstrate the feasibility of reverse Monte Carlo simulations for studying intermolecular interactions in solids, even in cases, such as the ZIFs, where the pair distribution function is dominated by intramolecular peaks.

Recent advances in the application of total scattering methods to functional materials

“…Total scattering approach was recently applied also to a new subfamily of metal organic frameworks which show non-crystalline structures and for this reason are termed as amorphous MOFs (aMOFs) [44][45][46][47] . The absence of Bragg diffraction peaks in these materials precludes the determination of the atomic positions and of the long range structure.…”

New Insights into Complex Materials Using Reverse Monte Carlo Modeling

“…Drawing further parallels to the SiO 2 system, a model based on the structure of amorphous SiO 2 , appropriately enlarged to accommodate the imidazolate ligands, was used to fit the amorphous ZIF data, and an excellent agreement was obtained (Figure 7). The flexibility of the framework in these ZIF structures was investigated by using total-scattering data collected at variable temperature (55). Analysis of bond angles in the RMC configurations revealed that the [ZnN 4 ] tetrahedra are fairly rigid and that the flexibility arises from twisting of the imidazole ligands or from movement of the Zn atoms out of the plane of the rings.…”

Extreme Flexibility in a Zeolitic Imidazolate Framework: Porous to Dense Phase Transition in Desolvated ZIF‐4

“…Both sets of reflections co‐exist from 150 K to 120 K; the increasing intensities of the new reflections upon cooling come at the expense of the ZIF‐4(Zn)‐DS_HT reflections. As‐prepared ZIF‐4(Zn), with DMF guest molecules in the pores, has been reported to show flexibility on application of mechanical pressure (converting to a monoclinic structure before amorphizing21) and on heating (where it amorphizes before recrystallizing as ZIF‐zni22). Furthermore, its desolvated form has one of the lowest bulk moduli of any known MOF (2.56 GPa), recently confirmed by DFT calculations 21.…”