Kinetics and mechanism of the oxidation of Mn(II)_(aq) by bromate and peroxomonosulfate in the presence of molybdate to form [Mn~(IV)Mo_9O_(32)]~(6-)

摘要

The oxidation of Mn(II) by both BrO_3~- and HSO_5~- in the presence of MoO_4~(2-), in weakly acidic solution over the pH ranges 3.9-5.5 and 4.4-5.5,respectively, results in the formation of the heteropolyoxomolybdate [Mn~(IV)Mo_9O_(32)]~(6-) in each case. The kinetics of oxidation were studied at 40.0 deg C for BrO_3~- and 30.0 deg C for HSO_5~-, along with temperature dependence studies, and for each oxidant were found to exhibit solution autocatalytic behaviour. For BrO_3~- the oxidation kinetics followed the expanded rate expression +d[MnMo_9O_(32)~(6-)]/dt=kappa_(AC(1))[Mn~(2+)][MnMo_9O_(32)~(6-)][HMoO_4~-]~2[BrO_3~-] based on an examination of the individual [BrO_3~-], pH and actual [MoO_4~(2-)] dependences, with a value for kappa_(AC(1)) of 9.09(34) X 10~6 dm~(12) mol~(-4) s~(-1). For HSO_5~- the oxidation kinetics followed the expanded two-tern rate expression +d[MnMo_9O_(32)~(6-)]/dt=kappa_(AC(2a))[Mn~(2+)][MnMo_9O_(32)~(6-)][HMoO_4~-]~(-5)[SO_5~-] + kappa_(AC(2b))[Mn~(2+)][MnMo_9O_(32)~(6-)][HMoO_4~-]~(-5)[HSO_5~-] based on [HSO_5~-], pH and [MoO_4~(2-)] dependences. The values of kappa_(AC(2a)) and kappa_(AC(2b)) are 4.6(4) X 10~(11) dm~(-9) mol~3 s~(-1) and 2.6(4) X10~(-15) dm~9-9) mol~3 s~(-1). For BrO_3~- oxidation, from the composition of the transition state, it is proposed that the product [MnMo_9O_(32)]~(6-) species combines with Mn(II) and two HMoO_r~- ions to generate a Mn(II)-substituted lacunary [Mn~(II)Mn~(IV)Mo_(11)O_(39)]~(6-) anion based on a Keggin structure,with the Mn(IV) located at the centre of the polyoxomolybdate framework. Extended-Huckel molecular orbital calculations have been used to investigate the stability of the proposed Mn(II)-substituted lacunary species, based on an alpha-Keggin structure, relative to the unassembled components.For HSO_5~- oxidation, the two parallel pathways indicate oxidation by both HSO_5~- and its deprotonated from SO_5~-. The two mechanisms reflect the differences in how BrO_3~- and HSO_5~- operate oxidatively and have been highlighted by the facile nature of polyoxomolybdate polymerization. In each case following oxidation of Mn(II) to Mn(IV) fast separation of the two Mn(IV) centres must subsequently occur, along with rapid assembly of the polyoxomolybdate frameworks around each centre to yield the product species.