Zeolitic imidazolate framework-22: high hydrophilicity, water resistance, and proton conduction

Among metal-organic frameworks (<b>MOF</b>s), zeolitic imidazolate frameworks (<b>ZIF</b>s) enable precise design of pore structures, and most of them exhibit high water resistance. However, no hydrophilic ZIF that maintains water resistance and adsorbs water vapor in low-pressure range has been achieved. In the current work, as a <b>MOF</b> with both high structural stability and hydrophilicity, we focused on ZIF-22 that contains one polar uncoordinated <i>N</i>-heteroatom in its organic linker. ZIF-22 exhibited high water resistance due to presence of an appropriate number (one) of uncoordinated <i>N</i>-heteroatoms. The added polarity from <i>N</i>-heteroatoms allowed ZIF-22 to exhibit the highest hydrophilicity among <b>ZIF</b>s. Furthermore, ZIF-22 exhibited the highest proton conductivity (1.77 × 10^-3 S cm^-1 at 363 K and 95% RH) among <b>ZIF</b>s without acidic groups or guest proton carriers. These findings provide a design strategy of <b>MOF</b>s that achieve hydrophilization while maintaining water resistance and broaden their application range in aqueous environments.