MOF-based Catalysis of Hydrolytic Destruction of Nerve Agent Simulants: The Complex Role of Lewis-acid Strength

In practical applications of metal-organic frameworks (MOFs) as catalysts for hydrolytic destruction of chemical warfare agents (CWA), such as those entailing integration of the MOF with fabric for protective equipment, co-incorporation of a basic buffer is essential for sustained catalytic turnover – in part, because the salient nucleophile is the hydroxide ion. These buffers will have different pH values, and it has become clear that pH differences can translate to dramatic effects on MOF-catalyzed rates. Predicting the relative magnitude and even the direction of rate variations, however, is hindered by an incomplete understanding of the rate laws involved in CWA (or agent simulant) hydrolysis and their relationship to the nature of the catalyst active-site. Here, we have experimentally examined, with a simulant, a series of Lewis acidic UiO-66-series MOFs (UiO-66-NO2, UiO-66(-H), and UiO-66-NH2) with varying active-site Lewis acidities as determined by their functional groups, and correlate their catalytic performance at different pH values with their measured relative Lewis acidity and with catalyst characteristics governed by Lewis acidity. This comparison is extended to Lewis basic MOFs Zn- and Cu-MFU-4L, with the conclusion that the Lewis acidity of the active site plays a complex, but decisive role in determining the pH that maximizes the rate of hydrolysis. Furthermore, an explanation for this behavior based on the rate-determining steps for hydrolysis is presented, with the rates for hydrolytic attack and for node aqua ligand displacement by an agent simulant being the source of the correlation between optimal pH values and active-site Lewis acidity. Notably, the observation of a peaked maximum in plots of hydrolysis rate versus pH points to a pH-correlated change in rate-determining step. Finally, these results suggest that the selection of certain MOFs as the “best” for nerve agent hydrolysis is largely based on an arbitrary selection of buffer and pH value.