Structure and Properties of an Amorphous Metal-Organic Framework

Thomas D. Bennett, Andrew L. Goodwin, Martin T. Dove, David A. Keen, Matthew G. Tucker, Emma R. Barney, Alan K. Soper, Erica G. Bithell, Jin-Chong Tan, and Anthony K. Cheetham

Phys. Rev. Lett. 104, 115503 – Published 19 March 2010

Abstract

ZIF-4, a metal-organic framework (MOF) with a zeolitic structure, undergoes a crystal–amorphous transition on heating to $300 ° C$ . The amorphous form, which we term $a$ -ZIF, is recoverable to ambient conditions or may be converted to a dense crystalline phase of the same composition by heating to $400 ° C$ . Neutron and x-ray total scattering data collected during the amorphization process are used as a basis for reverse Monte Carlo refinement of an atomistic model of the structure of $a$ -ZIF. The structure is best understood in terms of a continuous random network analogous to that of $a − {SiO}_{2}$ . Optical microscopy, electron diffraction and nanoindentation measurements reveal $a$ -ZIF to be an isotropic glasslike phase capable of plastic flow on its formation. Our results suggest an avenue for designing broad new families of amorphous and glasslike materials that exploit the chemical and structural diversity of MOFs.

Received 8 January 2010

DOI:https://doi.org/10.1103/PhysRevLett.104.115503

Authors & Affiliations

Thomas D. Bennett¹, Andrew L. Goodwin^2,3, Martin T. Dove², David A. Keen^4,5, Matthew G. Tucker⁴, Emma R. Barney⁴, Alan K. Soper⁴, Erica G. Bithell¹, Jin-Chong Tan¹, and Anthony K. Cheetham^1,*

¹Department of Materials Science, University of Cambridge, Cambridge CB2 3QZ, United Kingdom
²Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, United Kingdom
³Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom
⁴ISIS Facility, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0QX, United Kingdom
⁵Department of Physics, Oxford University, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom

^*akc30@cam.ac.uk

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Vol. 104, Iss. 11 — 19 March 2010

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Images

Figure 1
Representations of the Zn-im-Zn and Si-O-Si linkages in tetrahedral ZIF and silicate networks, respectively.Reuse & Permissions
Figure 2
(a) Low- $Q$ ( $Q < 5.5 Å^{- 1}$ ) region of the neutron total scattering patterns of (top) the desolvated ZIF-4 framework at $300 ° C$ , (middle) the in situ amorphized $a$ -ZIF product at $320 ° C$ , and (bottom) subsequently recrystallized ZIF-zni at $400 ° C$ . (b) Neutron differential correlation functions $D (r)$ [21]; the inset illustrates the practically identical local structure in all phases. (c) $D (r)$ functions compared for $a$ -ZIF and $a − {SiO}_{2}$ [28], where the latter data have been stretched in $r$ by a factor of two to reflect approximately the difference in $Zn \dots Zn$ and $Si \dots Si$ distances in the two materials.Reuse & Permissions
Figure 3
(a) A region of the final RMC model of $a$ -ZIF (center) shown relative to comparable regions of the ZIF-4 (top) and ZIF-zni (bottom) framework structures; the corresponding unit cells are shown in outline. Deuterium atoms have been omitted for clarity. (b) RMC fits (thin red lines) to experimental total scattering data (thicker black lines), calculated using the single atomistic configuration illustrated in the center panel of (a): (top) Neutron differential correlation function $D (r)$ , (center) neutron total scattering structure factor $F_{N} (Q)$ , and (bottom) x-ray total scattering structure function $F_{X} (Q)$ [21]. Although the level of agreement with the x-ray data is slightly lower than for the neutron data, we did not adjust weightings to force an improved agreement because we judge the neutron data and corresponding corrections to be of higher quality.Reuse & Permissions
Figure 4
Typical optical micrographs of: (a) a desolvated ZIF-4 single crystal, where the desolvation process has induced visible surface cracking; (b) a recovered $a$ -ZIF monolith, showing curved external surfaces and curved internal cavities; (c) a ZIF-zni single crystal prepared independently (see SI for further details); (d), (e) a partially recrystallized monolith consisting of $a$ -ZIF (“dark”) and ZIF-zni (“bright”) phases—panel (e) is that portion of the monolith in (d) enclosed within the white square and shows the indentations used to measure mechanical behavior of the two phases [30]. Electron diffraction patterns from: (f) an $a$ -ZIF monolith together with the radially averaged intensity distribution for qualitative comparison with the x-ray scattering function $F_{X} (Q)$ ; (g) a crystallite of ZIF-4 prior to heating; (h) a polycrystalline monolith after transformation to ZIF-zni. (i) Young’s moduli ( $E$ ) and hardnesses ( $H$ ) determined for the different phases in panels (a)–(e) along known orientations, over surface penetration depths of 150 to 1000 nm (“T,” “S,” “F” refer to the transverse, sagittal, and frontal sections). These measurements were used to assign the dark and bright phases in panel (c) as $a$ -ZIF and ZIF-zni, respectively, indicating that ZIF-zni nucleates at the monolith surfaces during recrystallization.Reuse & Permissions

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