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Abstract
Ambient pressure in situ synchrotron-based spectroscopic techniques have been correlated to illuminate atomic-level details of bond breaking and formation during the hydrolysis of a chemical warfare nerve agent simulant over a polyoxometalate catalyst. Specifically, a Cs8[Nb6O19] polyoxoniobate catalyst has been shown to react readily with dimethyl methylphosphonate (DMMP). The atomic-level transformations of all reactant moieties, the [Nb6O19]8− polyanion, its Cs+ counterions, and the DMMP substrate, were tracked under ambient conditions by a combination of X-ray absorption fine structure spectroscopy, Raman spectroscopy, and X-ray diffraction. Results reveal that the reaction mechanism follows general base (in contrast to specific base) hydrolysis. Together with computational results, the work demonstrates that the ultimate fate of DMMP hydrolysis at the Cs8[Nb6O19] catalyst is strong binding of the (methyl) methylphosphonic acid ((M)MPA) product to the polyanions, which ultimately inhibits catalytic turnover.
Highlights
Organophosphorus (OP) compounds are typically found in battlefield or agricultural settings and present a health hazard to most living organisms
Towards the goal of in situ and computational studies at the OP-POM interface, we studied the reaction of vaporized dimethyl methylphosphonate (DMMP) with solid Cs8[Nb6O19]·xH2O
The rate-limiting step of the reaction involves: i) dissociation of a water molecule on the CsPONb, which leads to protonation of the polyniobate oxygens and the generation of hydroxide, and ii) nucleophilic addition of the nascent hydroxide to the phosphorus atom of DMMP to generate a pentacoordinated phosphorus intermediate (P5)
Summary
Organophosphorus (OP) compounds are typically found in battlefield or agricultural settings and present a health hazard to most living organisms. Materials and methods aimed at the sequestration and decomposition of OP compounds have been developed in the past, including oxidation with bleach and related reagents, detoxification by (catalytic) hydrolysis, and degradation involving biochemical approaches[3,4,5]. Significant efforts have sought to develop effective filtration materials, coatings/fabrics or skin protectants that maintain high efficacy for deactivation of gaseous toxic compounds[6, 7] Motivated by their noteworthy reactivity, many metal oxide/hydroxide-based formulations[8,9,10,11], metal organic frameworks (MOFs)[12,13,14,15,16,17,18], polyoxometalates (POMs)[19,20,21], and relevant composites[22], have attracted significant recent attention as potential catalysts of nerve-agent decomposition. In contrast to other [Nb6O19] alkali salts, the Cs-based analogue exhibits a higher nerve- agent- simulant decontamination rate for reaction on the POM surface than for the analogous reaction in solution[21], which makes it a promising material for decomposition of OP compounds at the solid-gas interface and greatly broadens the applicability of these materials
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