Equilibria and dynamics of some aqueous peroxomolybdate catalysts: a 17O NMR spectroscopic study

Abstract

The equilibrium speciation at 278 K and the dynamics in the systems pH+ + qMoO42− + rH2O2 + sL ⇌ (H+)p(MoO42−)q (H2O2)r Ls (L = SO42− or Cl−) in 0.3 M Na2(SO4) and 0.6 M Na(Cl) medium has been determined from 17O NMR integral and shift data in the range 0.7 ≤ pH ≤ 4.1 and at excess of peroxide ([H2O2]tot/[Mo]tot = 3). In 0.3 M Na2(SO4) medium species with the following compositions were found: MoX2− (1,1,2,0), MoX2 (2,1,2,0), MoX2S2− (2,1,2,1), MoX2S− (3,1,2,1), Mo2X42− (2,2,4,0), Mo2X4− (3,2,4,0) and Mo2X62− (2,2,6,0), where (Mo) corresponds to MoO42−, (X) to O22− or HOO− and (S) to SO42−. In 0.6 M Na(Cl) medium the diperoxomolybdosulfate complex MoX2S− is replaced with a novel diperoxomolybdochloride complex MoX2Cl− (2,1,2,1), where (Cl) corresponds to Cl−. The numbers in parentheses refer to the values of p,q,r and s in the formula above. The stoichiometries of each complex are given by the abbreviated formula MoqXrSs(2q + 2s − p)– and MoqXrCls(2q + s − p)–. The following formation constants with 3σ were obtained with values for 0.6 M Na(Cl) in parenthesis: log β1,1,2,0 = 11.61 (11.61); log β2,1,2,0 = 13.70 ± 0.10, pKa = 2.09 (13.86 ± 0.10, pKa = 2.25); log β2,1,2,1 = 14.57 ± 0.07 (13.87 ± 0.15 ); log β3,1,2,1 = 15.27 ± 0.17, pKa = 0.70, log β2,2,4,0 = 24.06 ± 0.05 (24.08 ± 0.04); log β3,2,4,0 = 25.99 ± 0.18, pKa = 1.93 (26.23 ± 0.17, pKa = 2.15); log β2,2,6,0 = 24.02 ± 0.15 (23.9 ± 0.3). The pKa value for HSO4− was determined to be 1.06 ± 0.11. At 278 K the MoX2 and Mo2X4 complexes are in chemical exchange and the rate of exchange increases upon protonation. At 312 K there is also measurable exchange between MoX2S and the MoX2 and Mo2X4 complexes. The dynamics in 0.6 M Na(Cl) medium seem to be very similar to those in the sulfate medium.