Kinetics and mechanism of the heterogeneous catalyzed oxidative degradation of indigo carmine

The kinetics of the oxidative degradation of the indigo carmine (IQ dye (disodium salt of 3,3-dioxobi-indolin-2,2-ylidine-5,5-disulfonate) with hydrogen peroxide catalyzed with the supported metal complexes have been investigated. The complexes used are [Cu(amm)(4)](2+), [Co(amm)(6)](2+), [Ni(amm)(6)](2+), [Cu(en)(2)](2+), and [Cu(ma)(4)](2+) (amm = ammonia, en = ethylenediamine, and ma = methylamine). Silica, alumina, silica-alumina (25% Al2O3), and cation-exchange resins (Dowex-50W, 2 and 8% DVB) are used as supports. The reaction is first order with respect to [IC] while the order with respect to [H2O2] was dependent on the initial concentration and the type of the catalyst used. At lower [H2O2](0) the order was first, which then decreases with increasing [H2O2)](0), finally reaching zero. This aspect is consistent with the formation of a colored peroxo-complex on the catalysts surface. The reactivity of catalysts is dependent on the redox potential of the metal ions, the amount of complex loaded per gram of dry catalyst, the type of ligand, and the support. Moreover, the reaction rate was strongly dependent on the pH of the medium, the cationic and anionic surfactants, and the irradiation with UV-light. The reaction is enthalpy controlled as confirmed from the isokinetic relationship. A reaction mechanism was proposed with the formation of free radicals as reactive intermediates. (C) 2002 Elsevier Science B.V. All rights reserved.