Continuous structure modification of metal-organic framework glasses via halide salts

Melting and glass formation in metal-organic frameworks (MOFs) provide promising avenues for processing them into bulk materials. However, two major challenges persist: only a small fraction of MOF crystals undergo melting and glass-formation, and there are no well-established strategies exist for continuously tuning MOF glass structures and properties in non-stoichiometric compositions. This study addresses both of these challenges through co-melting of zeolitic imidazole frameworks (ZIFs), a subset of MOFs, with heterocycle-based halide salts. The salts act as chemical “modifiers”, akin to the role of alkali modifiers in traditional silicate glasses, lowering the melting temperature of MOFs crystals. Specifically, we employ heterocycle-based halide (Cl-, Br-, I-) salts, such as benzimidazolium chloride, and co-melt them with ZIF crystals (ZIF-62 [Zn(Im)1.75(bIm)0.25], ZIF-4 [Zn(Im)2], and ZIF-8 [Zn(mIm)2], where Im: imidazolate, bIm: benzimidazolate, 2-mIm: 2-methylimidazole). This approach enables melting below 300 °C, very low glass transitions temperatures (<150 °C), and the formation of bubble- and defect-free bulk-sized samples (>1 cm). Using high-field solid-state NMR spectroscopy, in situ X-ray total scattering, and molecular dynamics simulations, we elucidate the structure of the modified glasses and the mechanisms underlying their formation. Specifically, our findings reveal that the salts depolymerize the ZIFs to create bridging and non-bridging imidazolate linkers, i.e., the imidazolate salts act as modifiers by partially replacing the imidazolium-based organic linkers with the halide anions. The discovered approach enables continuous tuning of the glass transition temperature and mechanical properties and allows for melting of ZIF-8, which otherwise undergoes decomposition prior to melting.