Copper-Catalyzed Oxidation of Cyanide by Peroxide in Alkaline Aqueous Solution

Abstract

The oxidation of cyanide by peroxide in alkaline aqueous solution is catalysed by copper complexes. In the presence of excess cyanide, copper(II) is reduced to form the tricyanocuprate (I) complex. The cyanogen oxidation product is hydrolysed with disproportionation to cyanate and cyanide:

2Cu^II+2CN^-→ 2Cu^I+(CN)₂

(CN)₂+2OH^- → OCN^-+CN^-+H₂O

Cu^I+3CN^- ↔ Cu(CN)₃^2-

The stoichiometry and kinetics of the catalysed oxidation have been investigated. Hydrogen peroxide oxidizes coordinated cyanide with a rate that is first order in peroxide and first order in copper but independent of cyanide concentration in the presence of excess cyanide.

Cu(CN)₃^2-+H₂O₂→ Cu(CN)₂^-+OCN^-+H₂O

Cu(CN)₂^-+CN^-↔ Cu(CN)₃^2-

When the excess cyanide is consumed and Cu(CN)₂^- becomes the dominant species, the reaction becomes more complex and less efficient. Under certain conditions the stoichiometry revealed a peroxide-to-Cu(CN)₂^- ratio of about 6 : 1, instead of the minimum of 2.5:1 required for the oxidation of the coordinated cyanide to cyanate and the Cu^I to Cu(OH)₂. This suggests that peroxide is consumed by a copper- catalysed disproportionation, in competition with oxidation of the coordinated cyanide. An intermediate yellow complex forms while peroxide is present, before Cu(OH)₂ finally precipitates.

The consequence of this mechanism is that the most efficient process for the destructive oxidation of cyanide has a high cyanide-to-copper ratio, to minimize the final concentration of Cu(CN)₂^- which consumes peroxide inefficiently. The rate of the reaction depends on the concentration of copper, however, which must be large enough for a satisfactory turnover.