Characterisation of aqueous peroxomolybdate catalysts applicable to pulp bleaching

Abstract

The equilibrium speciation in the system pH⁺ + qMoO₄²⁻ + rH₂O₂ + sSO₄²⁻ ⇌ (H⁺)_p(MoO₄²⁻)_q(H₂O₂)_r(SO₄²⁻)_s in 0.300 M Na₂(SO₄) medium at 25 °C has been determined from potentiometric titration data in the ranges 2.0 ≤ pH ≤ 5.5, 5.00 ≤ [Mo]_tot/mM ≤ 80.00, 0 ≤ [H₂O₂]_tot/mM ≤ 245 and 273.07 ≤ [SO₄²⁻]_tot/mM ≤ 320.20. Species with the following compositions were found: MoX⁻ (1,1,1,0), MoX (2,1,1,0), MoX₂⁻ (1,1,2,0), MoX₂ (2,1,2,0), MoX₂S²⁻ (2,1,2,1), Mo₂X₄²⁻ (2,2,4,0), Mo₇X⁶⁻ (8,7,1,0), Mo₇X⁵⁻ (9,7,1,0), Mo₇X⁴⁻ (10,7,1,0) and Mo₇X³⁻ (11,7,1,0). The numbers in parentheses refer to the values of p, q, r and s in the formula above. The numbers and charges of molybdenum (Mo), peroxide (X) and sulfate (S) in each species are given in the abbreviated formula Mo_qX_rS_s^{(2q + 2s − p)−}. The following formation constants with 3σ were obtained; log β_1,1,1,0 = 8.53 ± 0.03, log β_2,1,1,0 = 11.22 ± 0.04 (pK_a = 2.69), log β_1,1,2,0 = 11.61 ± 0.03, log β_2,1,2,0 = 13.77 (± 0.06) (pK_a = 2.16), log β_2,1,2,1 = 14.50 ± 0.06, log β_2,2,4,0 = 23.77 ± 0.11, log β_8,7,1,0 = 56.71 ± 0.11, log β_9,7,1,0 = 62.00 ± 0.05 (pK_a = 5.29), log β_10,7,1,0 = 65.74 ± 0.06 (pK_a = 3.74), log β_11,7,1,0 = 68.23 ± 0.08 (pK_a = 2.49). Most complexes have been verified by ⁹⁵Mo NMR, ¹⁷O NMR, and the novel MoX₂S²⁻ complex also by FTIR spectroscopy.