Bioinspired Catalytic Reduction of Aqueous Perchlorate by One Single-Metal Site with High Stability against Oxidative Deactivation

Abstract

Reduction of perchlorate (ClO4–) with an active and stable catalyst is of great importance for environmental, energy, and space technologies. However, after the rate-limiting oxygen atom transfer (OAT) from inert ClO4–, the much more reactive ClOx– (x ≤ 3) intermediates can cause catalyst deactivation. The previous Re–Pd/C catalyst contained a [ReV(O)(hoz)2]+ site (Hhoz = 2-(2′-hydroxyphenyl)-2-oxazoline) and readily reduced ClO4–, but ClOx– intermediates led to rapid formation and hydrolysis of [ReVII(O)2(hoz)2]+. While microbes use delicate enzymatic machinery to survive the oxidative stress during ClO4– reduction, a synthetic catalyst needs a straightforward self-protective design. In this work, we introduced a methyl group on the ligand oxazoline moiety and achieved a substantial enhancement of catalyst stability without sacrificing the performance of ClO4– reduction. A suite of kinetics measurement, X-ray photoelectron spectroscopy characterization, reaction modeling, stopped-flow photospectrometry, and 1H NMR monitoring revealed the underlying mechanism. The most critical and unexpected effect of the methyl group is the deceleration (for 2 orders of magnitude) of OAT from ClO3– to [ReV(O)(Mehoz)2]+. However, the rate of OAT with ClO4– was not affected. The methyl group also slowed down the hydrolysis of [ReVII(O)2(Mehoz)2]+ and allowed the introduction of methoxy onto the phenolate moiety to further accelerate ClO4– reduction. With 1 atm H2 at 20 °C, the Re–Pd/C catalyst used [ReV(O)(MehozOMe)2]+ as the only reaction site to reduce multiple spikes of 10 mM ClO4– into Cl– without decomposition. This work showcases the significant effect of simple ligand modification in improving catalyst stability for high-performance ClO4– reduction.

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