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

Among metal-organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs) 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 MOF with both high structural stability and hydrophilicity, we focused on ZIF-22 that contains one polar uncoordinated N-heteroatom in its organic linker. ZIF-22 exhibited high water resistance due to presence of an appropriate number (one) of uncoordinated N-heteroatoms. The added polarity from N-heteroatoms allowed ZIF-22 to exhibit the highest hydrophilicity among ZIFs. Furthermore, ZIF-22 exhibited the highest proton conductivity (1.77 × 10^-3 S cm^-1 at 363 K and 95% RH) among ZIFs without acidic groups or guest proton carriers. These findings provide a design strategy of MOFs that achieve hydrophilization while maintaining water resistance and broaden their application range in aqueous environments.