Abstract
Metal-organic frameworks have been utilized as heterogeneous catalysts for the degradation of chemical warfare agents, typically organophosphorous nerve agents. Vibrational spectroscopy techniques coupled with nuclear magnetic resonance (NMR) were utilized to study the adsorption and degradation of dimethyl phosphite (DMP), a simulant molecule of the organophosphorus nerve agent Soman (GD), by Zr- and Hf-UiO-66 as a function of particle size, defect type, and defect density. Defective Zr- and Hf-UiO-66 have been synthesized via a modulated synthesis protocol to engineer missing linker and missing cluster defects into the crystal structure. The adsorption of DMP to UiO-66 was observed to be surface-limited, suggesting maximal DMP adsorption occurs with maximized external surface area. In addition, Hf-UiO-66 samples engineered with large quantities of missing cluster defects are observed to more efficiently hydrolyze DMP into phosphonic acid when compared to less-defective samples. The increase in reactivity is attributed to the greater accessibility of the internal particle volume and, thus, access to a higher number of Lewis-acidic open metal sites, facilitated by missing cluster defects. Taken together, these two observations indicate that to create maximally efficient MOF catalysts for chemical warfare agent degradation one must obtain frameworks with large surface area to maximize adsorption of the simulant as well as a large accessible free volume obtained through the introduction of missing cluster defects to maximize the degradation of the simulant at the Lewis acid sites found in the interior of the framework.