Hybrid glasses connect the emerging field of metal-organic frameworks (MOFs) with the glass formation, amorphization and melting processes of these chemically versatile systems. Though inorganic zeolites collapse around the glass transition and melt at higher temperatures, the relationship between amorphization and melting has so far not been investigated. Here we show how heating MOFs of zeolitic topology first results in a low density ‘perfect' glass, similar to those formed in ice, silicon and disaccharides. This order–order transition leads to a super-strong liquid of low fragility that dynamically controls collapse, before a subsequent order–disorder transition, which creates a more fragile high-density liquid. After crystallization to a dense phase, which can be remelted, subsequent quenching results in a bulk glass, virtually identical to the high-density phase. We provide evidence that the wide-ranging melting temperatures of zeolitic MOFs are related to their network topologies and opens up the possibility of ‘melt-casting' MOF glasses.

We report the encapsulation of the hydrophilic model molecule calcein in the Zr-based MOF UiO-66, followed by amorphization of the framework by ball-milling. We show controlled release of calcein over more than 30 days, compared with the 2 day release period from crystalline UiO-66.

We have used a family of Zr-based metal–organic frameworks (MOFs) with different functionalized (bromo, nitro and amino) and extended linkers for drug delivery.

The mechanism and products of the structural collapse of the metal–organic frameworks (MOFs) UiO-66, MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal–ligand bonding in each case. The amorphous products contain inorganic–organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4 clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140 frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked.

X-ray diffraction has been used to investigate the kinetics of amorphization through ball-milling at 20 Hz, for five zeolitic imidazolate frameworks (ZIFs) - ZIF-8, ZIF-4, ZIF-zni, BIF-1-Li and CdIF-1. We find that the rates of amorphization for the zinc-containing ZIFs increase with increasing solvent accessible volume (SAV) in the sequence ZIF-8 > ZIF-4 > ZIF-zni. The Li-B analogue of the dense ZIF-zni amorphizes more slowly than the corresponding zinc phase, with the behaviour showing a correlation with their relative bulk moduli and SAVs. The cadmium analogue of ZIF-8 (CdIF-1) amorphizes more rapidly than the zinc counterpart, which we ascribe primarily to its relatively weak M-N bonds as well as the higher SAV. The results for the ZIFs are compared to three classical zeolites - Na-X, Na-Y and ZSM-5 - with these taking up to four times longer to amorphize. The presence of adsorbed solvent in the pores is found to render both ZIF and zeolite frameworks more resistant to amorphization. X-ray total scattering measurements show that amorphous ZIF-zni is structurally indistinguishable from amorphous ZIF-4 with both structures retaining the same short-range order that is present in their crystalline precursors. By contrast, both X-ray total scattering measurements and (113)Cd NMR measurements point to changes in the local environment of amorphous CdIF-1 compared with its crystalline CdIF-1 precursor.