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 (TCPP4-) 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.

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Metal-organic frameworks (MOFs) are attracting increasing attention as adsorbents of contaminants of emerging concern that are difficult to remove by conventional processes. This paper examines how functional groups covering the pore walls of phosphinate-based MOFs affect the adsorption of specific pharmaceutical pollutants (diclofenac, cephalexin, and sulfamethoxazole) and their hydrolytic stability. New structures, isoreticular to the phosphinate MOF ICR-7, are presented. The phenyl ring facing the pore wall of the presented MOFs is modified with dimethylamino groups (ICR-8) and ethyl carboxylate groups (ICR-14). These functionalized MOFs were obtained from two newly synthesized phosphinate linkers containing the respective functional groups. The presence of additional functional groups resulted in higher affinity toward the tested pollutants compared to ICR-7 or activated carbon. However, this modification also comes with a reduced adsorption capacity. Importantly, the introduction of the functional groups enhanced the hydrolytic stability of the MOFs.

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