Hydrolysis of Dimethyl Phosphite by Zr- and Hf-UiO-66

Metal–organic\nframeworks have been utilized as heterogeneous\ncatalysts for the degradation of chemical warfare agents, typically\norganophosphorous nerve agents. Vibrational spectroscopy techniques\ncoupled with nuclear magnetic resonance (NMR) were utilized to study\nthe adsorption and degradation of dimethyl phosphite (DMP), a simulant\nmolecule of the organophosphorus nerve agent Soman (GD), by Zr- and\nHf-UiO-66 as a function of particle size, defect type, and defect\ndensity. Defective Zr- and Hf-UiO-66 have been synthesized via a modulated\nsynthesis protocol to engineer missing linker and missing cluster\ndefects into the crystal structure. The adsorption of DMP to UiO-66\nwas observed to be surface-limited, suggesting maximal DMP adsorption\noccurs with maximized external surface area. In addition, Hf-UiO-66\nsamples engineered with large quantities of missing cluster defects\nare observed to more efficiently hydrolyze DMP into phosphonic acid\nwhen compared to less-defective samples. The increase in reactivity\nis attributed to the greater accessibility of the internal particle\nvolume and, thus, access to a higher number of Lewis-acidic open metal\nsites, facilitated by missing cluster defects. Taken together, these\ntwo observations indicate that to create maximally efficient MOF catalysts\nfor chemical warfare agent degradation one must obtain frameworks\nwith large surface area to maximize adsorption of the simulant as\nwell as a large accessible free volume obtained through the introduction\nof missing cluster defects to maximize the degradation of the simulant\nat the Lewis acid sites found in the interior of the framework.