Tailoring the Melting and Glass Transition Behavior of Zeolitic Imidazolate Frameworks via Ammonium Halide Salts

Why It Matters

By reducing the thermal barrier for ZIFs, the technique makes large‑scale MOF glass manufacturing feasible, expanding applications in catalysis, gas separation, and advanced optics. It also introduces a versatile, inorganic route to tailor network connectivity without relying on organic modifiers.

Key Takeaways

• Ammonium halide salts lower ZIF melting points by up to 200 °C
• Zn–N bonds break, Zn–Cl/Br/I bonds form during co‑melting
• Enabled centimeter‑scale ZIF glass sheets and melt‑processed ZIF‑8
• In situ X‑ray imaging visualizes salt infiltration and crystal collapse
• Modifier approach broadens MOF glass design beyond hybrid organic additives

Pulse Analysis

Metal‑organic framework (MOF) glasses have attracted attention for their hybrid organic‑inorganic chemistry, yet most crystalline MOFs decompose before they can be melted into a glass. Early strategies relied on organic or polymeric modifiers to depress the melting point, but these additives often compromise the intrinsic porosity or introduce unwanted organic residues. The recent shift toward inorganic modifiers, specifically ammonium halide salts, offers a cleaner, more controllable method to adjust thermal behavior while preserving the framework’s functional sites.

In the study, researchers co‑melted ZIF crystals with ammonium chloride, bromide, or iodide. Real‑time X‑ray imaging captured the molten salts penetrating the crystalline domains, effectively breaking Zn‑N bonds and establishing new Zn‑X (X = Cl, Br, I) linkages. Pair distribution function analysis confirmed this bond reconfiguration, which reduces network connectivity and lowers both the melting temperature and the glass‑transition temperature. The result is a homogeneous melt that can be quenched into a glass, even for ZIF‑8, a material previously deemed un‑meltable. The ability to produce centimeter‑scale glass sheets demonstrates the scalability of the process.

The implications extend beyond academic curiosity. Lowering the processing temperature enables cost‑effective, continuous‑flow manufacturing of MOF glasses, facilitating their integration into membranes, optical components, and catalytic monoliths. Moreover, the inorganic nature of the modifiers minimizes contamination, preserving the high thermal stability and chemical resistance prized in industrial settings. Future work will likely explore other halide salts, mixed‑anion systems, and the impact on mechanical and transport properties, positioning modified MOF glasses as a versatile platform for next‑generation functional materials.