Reaction kinetics and mechanism of formation of [MnMo9O32]6– by hypochlorous acid oxidation of Mn2+(aq) in the presence of molybdate

Abstract

Oxidation of Mn²⁺(aq) by HOCl in weakly acidic solution, in the presence of molybdate, results in the formation of the soluble heteropolymolybdate [MnMo₉O₃₂]^6–. The speciation of Mn²⁺ in solution prior to oxidation has been examined using ESR spectroscopy and it has been established that the principal species is [Mn(OH₂)₆]²⁺, there being evidence for weak interactions only with molybdate and isopolymolybdate anions. In keeping with this conclusion, the secondary manganese(II) polymolybdate species Na₄[MnMo₈O₂₇]·20H₂O was crystallized from an acidic (pH 4.5) solution containing Mn²⁺(aq) and isopolymolybdate ions by slow vapour diffusion of ethanol into the aqueous phase. The compound crystallizes in the triclinic space group P, a= 9.568(2), b= 9.868(2), c= 11.703(3)Å, α= 103.60(1), β= 100.86(1) and γ= 96.05(1)°, and the structure was determined by X-ray diffraction methods to an R of 0.026 (R′= 0.040) for 3182 independent observed reflections. It consists of polymerized octamolybdate units linked through the sharing of common oxygen atoms and hydrated Mn²⁺ groups resulting in a sheet structure, with hydrated Na⁺ ions also attached to the octamolybdate units. Uncomplexed water is also present. The kinetics of oxidation of Mn^II by HOCl was studied at 5–20 °C over the range pH 4.0–5.4 and found to exhibit solution autocatalytic behaviour. The oxidation kinetics followed the expanded rate expression +d[MnMo₉O₃₂^6–]/dt=k_AC[Mn²⁺][MnMo₉O₃₂^6–][HOCl][MoO₄^2–][HMoO₄^–] based on an examination of [HOCl], pH and total [MoO₄^2–] dependences, and arguments based on molybdate speciation under the reaction conditions. An approximate value of k_AC at 25 °C, 1.36(10)× 10⁸ dm¹² mol^–4 s^–1, was estimated from an extrapolated value of the second-order autocatalytic rate constant using an Arrhenius plot and the reported formation constants describing the speciation of molybdate, isopolymolybdate and their protonated analogues in solution. A mechanism involving sequential one-electron oxidation steps of Mn²⁺ through to Mn⁴⁺via Mn³⁺, together with the equilibrium Mn²⁺+ Mn⁴⁺⇌ 2Mn³⁺, which favours Mn²⁺ and Mn⁴⁺, is able to account for the observed autocatalysis.