Catalytic Perchlorate Reduction Using Iron: Mechanistic Insights and Improved Catalyst Turnover

Abstract

Perchlorate is a non-coordinating and kinetically inert anion commonly used in solid rocket fuels and explosives. Upon introduction to the environment, it is a pervasive groundwater pollutant that poses remediation challenges. There remain few homogeneous systems capable of deoxygenating perchlorate to chloride, and the reported catalysts suffer from deactivation as the concentration of chloride increases during catalysis. This report improves the turnover number of perchlorate reduction by an iron catalyst containing a secondary coordination sphere by an order of magnitude. The use of a zinc reagent mitigates chloride inhibition and promotes catalyst turnover. Additionally, the initial rate of perchlorate reduction was examined using time-resolved UV–visible spectroscopy, furnishing insights into the mechanism of perchlorate reduction.

A Bioinspired Molybdenum Catalyst for Aqueous Perchlorate Reduction

“…For example, ClO4 − reduction by a Fe complex relied on hydrogen bonds in the secondary coordination sphere (Fig. 1D) and thus required the use of an anhydrous medium (12,13). Furthermore, a singlefunction metal complex or isolated reductase needs special electron donors (e.g., methyl viologen, hydrazine, ferrocene, and phosphine) to sustain the redox cycle of the OAT metal (12,14,15).…”

“…The complexity of biological systems challenges the design of an artificial ClO 4 – reduction system, especially in the aqueous phase under ambient conditions. For example, a Fe complex relies on hydrogen bonds in the secondary coordination sphere to reduce ClO 4 – (Figure d) and thus requires an anhydrous environment. , Moreover, a single-function metal complex or isolated reductase requires special electron donors (e.g., methyl viologen, hydrazine, ferrocene, and phosphine) to sustain the redox cycle of OAT metals. ,, Hence, a robust catalyst that can reduce aqueous ClO 4 – with H 2 is highly desirable. , Although the immobilization of Re complexes (Figure e) , onto Pd/C has achieved this function, , Re is a rare metal, and the presynthesized Re complexes are subject to irreversible decomposition …”

“…For example, a Fe complex relies on hydrogen bonds in the secondary coordination sphere to reduce ClO 4 − (Figure 1d) and thus requires an anhydrous environment. 17,18 Moreover, a single-function metal complex or isolated reductase requires special electron donors (e.g., methyl viologen, hydrazine, ferrocene, and phosphine) to sustain the redox cycle of OAT metals. 17,19,20 Hence, a robust catalyst that can reduce aqueous ClO 4…”

Inspired by Nature—Functional Analogues of Molybdenum and Tungsten-Dependent Oxidoreductases

“…In both substrate cases, the first effective homogenous catalyst was a rhenium species: [ReOCl(hoz) 2 ] (hoz = 2-(2 -hydroxyphenyl)-2-oxazoline, Figure 11c) for perchlorate reduction [130] and a methyl rhenium oxide [Re(CH 3 )O 2 ] for nitrate and perchlorate reduction [131]. Another example for an active homogenous perchlorate and nitrate reducing catalyst is an iron complex reported by Fout et al [132,133].…”

Recent advances in metal-mediated nitrogen oxyanion reduction using reductively borylated and silylated N-heterocycles

“…There is precedence for perchlorate reduction using a bioinspired iron complex that also performs nitrate reduction. 32,75 An inherent challenge with perchlorate and phosphate reduction is coordinating these weak nucleophiles to transition metal centers. While many transition metal perchlorate and carbonate complexes are crystallographically characterized, phosphate complexes are much rarer.…”