Reaction Mechanism of Nerve-Agent Decomposition with Zr-Based Metal Organic Frameworks

We present a detailed study of the decomposition of Sarin on the Zr-based UiO-66 and MOF-808 metal organic frameworks (MOFs) using electronic structure calculations. The central step of the mechanism involves nucleophilic addition of OH to the nerve agent coordinated to a Zr atom of the MOF. This addition process generates a phosphorus pentacoordinated intermediate from which phosphonic acid products are formed through an elimination step, which also produces HF or isopropanol. Two major mechanisms have been probed. In the lowest-energy mechanism, a hydroxide ligand coordinated to a MOF Zr atom acts as the nucleophile in the addition step. In the second mechanism, which exhibits a slightly larger barrier, this Zr-OH group acts as a base to deprotonate a water molecule and generate a hydroxide moiety that concertedly adds to the nerve agent. In both mechanisms, the phosphonic acid products of the nerve-agent decomposition are strongly bound to the MOFs, suggesting that regeneration of the catalyst at the gas-surface interface might necessitate thermal treatment. The atomistic details of the reaction mechanism revealed by this work augment a growing body of experimental efforts that have recently demonstrated efficient catalytic decomposition of nerve agents by Zr-based MOFs in solution, but have not yet probed the reaction at the gas surface interface.