Effect of Elevated Air Humidity on the Structure and Proton Conductivity of Porphyrin-Based Zr(IV)-MOFs

Metal-organic frameworks (MOFs) based on Zr₆(μ₃-O)₈ oxometallic clusters are attracting attention as potential proton conductors due to their high surface area, ease of further substitution, and exceptional chemical stability. We hereby present an examination of two Zr(IV)-MOFs with a tetrakis(4-carboxyphenyl)porphyrin (TCPP^4-) linker, PCN-222 and PCN-224, as proton conductors. It was found that, in spite of their excellent stability in aqueous suspensions, in the environment of elevated air humidity, serious changes in their bonding system occur, mainly involving breakage of the carboxylate coordination bonds and hydration of the Zr₆(μ₃-O)₈ clusters, which leads to gradual amorphization and loss of porous character. The stability of the structures can be improved by postsynthetic modification with diphenylphosphinic acid (DPPA) to some extent. Inclusion of host imidazole molecules facilitates proton mobility in the pore system of the MOFs, further accelerating the structural degradation. Even though the original structures of the MOFs collapse under the conditions of proton conductivity measurement, the resulting amorphous solids still reveal a proton conductivity up to 6.7 × 10^-6 S·cm^-1 at ambient temperature and a 92% relative humidity, which is comparable to that of other Zr(IV)-MOFs with well-preserved structures. The presented study demonstrates an important phenomenon that has to be considered with any investigation using MOFs as proton conductors.