Flux Melting of Metal-Organic Frameworks

Abstract

<div>Recent demonstrations of melting in the metal-organic framework (MOF) family have created</div><div>interest in the interfacial domain between inorganic glasses and amorphous organic polymers. The</div><div>chemical and physical behaviour of porous hybrid liquids and glasses is of particular interest,</div><div>though opportunities are limited by the inaccessible melting temperatures of many MOFs. Here,</div><div>we show that the synthetic processing technique of flux melting, ‘borrowed’ from the inorganic</div><div>domain, may be applied to MOFs in order to melt materials which do not possess an accessible</div><div>liquid state in their pure form. We employ the high-temperature liquid state of one MOF as a solvent</div><div>for a secondary, non-melting MOF component. Differential scanning calorimetry, small- and wideangle</div><div>X-ray scattering, electron microscopy and X-ray total scattering techniques are used to show</div><div>the flux melting of the crystalline component within the liquid. Gas adsorption and positron</div><div>annihilation lifetime spectroscopy measurements show that this results in enhanced, accessible</div><div>porosity to a range of guest molecules, in the resultant flux melted MOF glass.</div>

Guest Size Limitation in Metal-Organic Framework Crystal-Glass Composites

“…S19), in agreement with the reported measurement of 0.18 mmol/g and representing a decrease from the reported value of 1.21 mmol/g in the crystalline ZIF-62. 29 Hysteresis is observed in the agZIF-62 isotherms as a result of the diffusion limitations through the amorphous structure, increasing in magnitude with the kinetic diameter of the adsorbent, also consistent with prior work. 30,31 Remarkably, in addition to the reported selectivity towards propane (C3H8) and propene (C3H6) by agZIF-62 by Frentzel- 30 some selectivity towards ethene (C2H4) over ethane (C2H6) is observed here with uptakes of 0.46 and 0.29 mmol/g respectively (Fig.…”

Metal-Organic Framework Crystal-Glass Composites

“…The two key requirements for the crystalline component in such a composite, are that the temperature of decomposition (T d ) should exceed the glass-forming matrix T m , and that the chemical (in)compatibility is such that no flux melting occurs 24 . The two frameworks we chose, MIL-53(Al) [Al(OH)(O 2 C-C 6 H 4 -CO 2 )] and UiO-66 [Zr 6 O 4 (OH) 4 (O 2 C-C 6 H 4 -CO 2 ) 6 ] both fulfil these criteria 25,26 .…”

Investigating the Melting Behaviour of Polymorphic Zeolitic Imidazolate Frameworks

“…Longley et al have reported the use of one high temperature MOF liquid, as a solvent for a secondary MOF component, which itself does not melt in the pure state under ambient pressure. 63 In the glass domain, the high temperature MOF liquid would be referred to as a flux. 63 They used ZIF-62 as a flux to induce melting in ZIF-8.…”

“…63 In the glass domain, the high temperature MOF liquid would be referred to as a flux. 63 They used ZIF-62 as a flux to induce melting in ZIF-8. 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.…”

“…63 They used ZIF-62 as a flux to induce melting in ZIF-8. 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.…”