# High Pressure-Based Synthesis of Nanoporous Metal–Organic Framework ZIF-93 Giving Rise to a Phase for Proton Conduction

## TL;DR

A new nanoporous material, ZIF-93_HP, was synthesized under high pressure and shows proton conductivity, offering potential for new material applications.

## Contribution

A solvent-free, high-pressure synthesis method produced a new ZIF phase with proton conductivity, not achievable via traditional methods.

## Key findings

- ZIF-93_HP was synthesized using high pressure and temperature without solvents.
- ZIF-93_HP exhibits proton conductivity at room temperature and moisture conditions.
- The material's surface area increased significantly after water washing.

## Abstract

This study aims to develop a green, solvent-free synthesis
of ZIF-93
(ZIF stands for zeolitic imidazolate framework) and to explore the
formation of different phases. We report the solvent-free synthesis
of a previously unreported nanoporous ZIF phase, ZIF-93_HP (HP referring
to “high-pressure”), from zinc oxide using a dual high-pressure
(150 MPa) and thermal (110 °C) method. The influence of key synthesis
parameters, such as the amount of NH4NO3 promotor
and reaction steps, was systematically investigated to maximize the
conversion of ZnO into the intermediate ZIF-93_HP, while, in parallel,
preventing its further conversion into nanoporous ZIF-93 phase. The
material was extensively characterized by X-ray diffraction, thermogravimetry,
electron microscopy and N2 and CO2 adsorption,
which revealed insights into the structure, morphology and nanoporosity
of ZIF-93_HP. ZIF-93_HP, with empirical formula of Zn­(C5N2OH5)2·1.2­(NH4NO3)·(H2O), is related to the previously reported
ZIF-93 (Zn­(C5N2OH5)2).
Water washing of this phase led to the transformation into ZIF-93
and a significant increase in the BET specific surface area (from
4 to 181 m2/g). In addition, the presence of NH4
+ and NO3
– ions into its
structure makes ZIF-93_HP proton conductor at room temperature and
moisture conditions (3.76 × 10–3 S/cm), a property
that decreases with increasing temperature due to dehydration. The
discovery of ZIF-93_HP highlights the potential of the high-pressure,
solvent-free synthesis as a powerful tool for the exploration of different
ZIFs and reticular materials that are inaccessible through traditional
solvothermal methods. As crystallization under solvent-free conditions
is often influenced by nonthermodynamic equilibrium, this approach
holds a great potential for expanding the material landscape by enabling
the discovery of different phases and structures with unique properties,
such as the promising proton conductivity demonstrated here.