Tracking thermal-induced amorphization of a zeolitic imidazolate framework via synchrotron in situ far-infrared spectroscopy

Abstract

We present the first use of in situ far-infrared spectroscopy to analyze the thermal amorphization of a zeolitic imidazolate framework material. We explain the nature of vibrational motion changes during the amorphization process and reveal new insights into the effect that temperature has on the Zn-N tetrahedra.Vibrational spectroscopy has been shown to be an excellent method of analyzing framework materials and gaining a better understanding of their molecular structure. The mid-infrared (MIR) region of the vibrational spectra is related to the local characteristic vibrations (e.g. localized bond stretching and bending) and is therefore of limited interest for providing information regarding lattice dynamics of a specific framework. However, there has recently been a strong focus on the framework specific vibrational motions located in the far-infrared (FIR) region (< 700 cm -1 ), which have been shown to reveal the nature of quasi-localized and collective modes.One group of framework materials exhibiting a particularly rich variety of collective vibrational motions are metal-organic frameworks (MOFs), 1, 2 which are a topical class of hybrid (inorganic-organic) crystalline materials. Their nanoscale pore structures and long-range order have attracted significant scientific and industrial interest, which originates from their wide range of potential applications including, carbon sequestration, photonics and microelectronics, and drug encapsulation and delivery. 3,4 The diverse structural behavior of MOFs 5, 6 results in the FIR region of the vibrational spectra providing a gateway to understanding the physical behavior and underlying flexibility of these highly promising next-generation functional materials.We have shown that inelastic neutron scattering (INS) and synchrotron-based FIR spectroscopy, in conjunction with ab initio density functional theory (DFT) can be used to explain these framework specific motions. 7 Specifically, we related quasi-local vibrations to the deformation of the organic linkers and Zn-based coordination polyhedra (ZnN4) in zeolitic imidazolate frameworks (ZIFs), 8 and linked them to various physical phenomena, including "gate-opening", "breathing" and shear destabilization modes. 7 We also reported molecular rotational dynamics, trampoline-like vibrational motions, and demonstrated how collective vibrations can be connected to the elasticity and mechanical stability of the framework structure, revealing the origin of auxeticity (negative Poisson's ratio), in the copper paddle-wheel MOF, HKUST-1. 9 Unlike in situ MIR spectroscopy, 10, 11 which can be performed via benchtop equipment, detailed characterization of the FIR region in situ requires a more sophisticated experimental setup, explaining the relative absence of reports to date.The structural dynamics of MOFs, at the molecular level, is of increasing importance. For example, phase transitions in ZIF-7 upon exchange of guest molecules and ZIF-8 upon gas adsorption have been ascribed to "breathing" and "gate-opening" mechanisms...

Quasi‐Harmonic Lattice Dynamics of a Prototypical Metal–Organic Framework

“…By distorting the framework and reducing its symmetry according to the normal mode, we could find a symmetry‐reduced lattice configuration (space group F) and resolve the structural instability on compression. This is shown in Figure where the middle panel is a graphical representation of the normal mode, that can be described as a simultaneous deformation of the ZnO polyhedra (similar to that previously reported to be linked to the mechanical and thermal instability of other Zn‐based MOF materials) and rotation of the organic linkers. This mode is also clearly linked to the mechanism underlying the NTE of the framework .…”

Improving the Mechanical Stability of Metal–Organic Frameworks Using Chemical Caryatids

“… 6 These and related studies concluded that for these materials the mechanical properties can be described with the low density–stiffness correlation. 6 , 29 − 32 As these experiments require sufficiently large single crystals, the number of studied structures is relatively small compared to the total number of possible ZIFs. In this work, we expand the studied materials to, in addition to the known ZIF structures, a large set of in silico constructed materials using 50 different zeolite topologies 33 with four types of ligands.…”

Structural evolution in a melt-quenched zeolitic imidazolate framework glass during heat-treatment

“…The peaks located at 1681-1475 cm À1 are characteristic of CQC, CQN and imidazolate ring stretching. 20,21 Both the decreased absorption of the peak near 1680 cm À1 and the increased absorption of the associated weak peak near 1600 cm À1 indicate possible distortion of the imidazolate rings in the ZIF-4 glass upon melt-quenching and heat-treatment. This is further confirmed by the noticeable change of the C-H bending peak near 1387 cm À1 .…”