Zirconium-based metal-organic frameworks: the relation between linker connectivity, structure stability, and catalytic activity towards organophosphates

Metal-organic frameworks (MOFs) are studied for many applications, however, there are only a few examples of commercialization. One of the reasons behind this is that the stability of MOFs is still unknown. Much attention has been devoted to the rational synthesis of novel MOFs, yet the predictability of MOF stability is so far limited. The present study compares the stability in a water environment with pH ranging from 3.0 to 11.0 of four zirconium-based MOFs constructed from ditopic, tritopic, and tetratopic linkers, namely UiO-66 (benzene-1,4-dicarboxylic acid), MOF-808 (benzene-1,3,5-tricarboxylic acid), MIP-200 (5,5 '-methylenediisophthalic acid), and PCN-222 (5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin). Finally, to delineate the nature of the defects induced by the linker release, we tested the MOFs treated at a certain pH for the degradation of model organic pollutant methyl paraoxon. It is clear that both MOFs based on tetratopic linkers are much more stable than UiO-66 and MOF-808 composed of di- and tritopic linkers, respectively. It should be noted that the kinetics of the linker release were also significantly slower for tetratopic linkers. At the same time, the connectivity of the Zr6 cluster did not play such an important role. MIP-200 proved to be the most stable MOF from the series in an aqueous environment; however, the loss of a small amount of monocarboxylic acid from the structure allowed thermal recrystallization of MIP-200 to an unknown phase so far. The stability of MOFs in aqueous environments is influenced by the linker topicity rather than by the connectivity. Only MIP-200 survives the alkaline environment.