Ion exchangers as catalysts. II. Catalytic decomposition of hydrogen peroxide with resin-ethylenediamine-copper(II) complex ions

Summary The slow decomposition of H₂O₂ in the presence of Dowex-50 W resin in the form of an ethylenediaminecopper(II) complex ion in water is accompanied by an induction period. The reaction is first order with respect to [H₂O₂] and the rate constant (perg of dry resin) was deduced. Autocatalytic behaviour was found for the H₂O₂ decomposition with 2% crosslinked divinylbenzene. The induction period disappeared and the reaction rate increased when the decomposition was carried out with a resin in the form of a peroxo-copper complex, which proves that the formation of an intermediate (active species) retards the reaction rate. The precursor of the active species, formed during the induction period, was not the amine-copper(II) complex ion but a product of the latter with H₂O₂. It proved impossible to carry out the decomposition in acid or buffer solutions, in which the resin is regenerated.     

Adsorption characteristics and the kinetics of the cation exchange of rhodamine-6G with Na+-montmorillonite

The adsorption and the kinetics of the cation exchange of rhodamine-6G (Rh-6G) with Na(+)-montmorillonite (Na(+)-MMT) have been studied. The binding parameters of Rh-6G have been determined by applying Freundlich and D-R isotherms. The enthalpy and the entropy of adsorption have been determined. The isosteric heat of adsorption has also been determined and decreases with increasing the concentration of Rh-6G. Increasing the concentration of Rh-6G led to a decrease in the adsorption capacity, which attributed to the formation of Rh-6G aggregates. Kinetic measurements of the cation exchange were followed up using a stopped-flow electrical conductivity detection unit. The cation-exchange process exhibited first-order kinetics with respect to the dye concentration and inversely proportional to the clay concentration. The measurements were accomplished at different temperatures and the activation parameters were determined. Increasing the Na(+)-MMT concentration led to a decrease in the rate constant. The latter is also affected by changing the exchangeable cation.     

Immobilization of transition metal ion complexes and their catalytic activity towards the activation of hydrogen peroxide

Transition metal ion-imidazole complexes have been immobilized on silica, silica–alumina (25%Al₂O₃), and alumina supports by adsorption and functionalization methods. The catalytic activity of these supported complexes in the decomposition of H₂O₂ has been studied. The reaction exhibits first-order kinetics with respect to [H₂O₂] and the quantity of catalyst. The rate of reaction decreases as [H₂O₂]₀ increases. The order of catalytic reactivity is strongly dependent on the type of metal ion, support, and the immobilization method. The complex anchored via adsorption exhibited a higher activity compared to the corresponding complex anchored via functionalization of the surface. The reaction proceed via formation of the peroxo-intermediate, which has an inhibiting effect on the reaction rate. The reaction is enthalpy-controlled as is concluded from the isokinetic relationship. A mechanism is proposed involving the generation of HO₂ radicals from the peroxo-intermediate in the rate-determining step.     

Ion Exchangers as catalysts I. Catalytic decomposition of hydrogen peroxide in presence of Dowex 50 W resin in the form of ethylamine-copper(II) complex ion in aqueous medium

Summary Dowex 50 W resin in the form of an ethylamine-Cu¹¹ complex ion was used as potentially active catalyst for the decomposition of H₂O₂ in aqueous medium. The stoichiometry of the amine-Cu¹¹ complex on the resin, determined experimentally, was found to have the total [Cu²⁺]: [ethylamine]=14 concentration ratio. The kinetics of the decomposition was studied and the calculated rate constant (per g of dry resin) was found to decrease with increase the degree of resin crosslinking. The active species, formed as an intermediate at the beginning of the reaction, had an inhibiting effect on the reaction rate. The brown peroxo-copper complex formed as a result of H₂O₂ decomposition, was found to contain the catalytic active species. The order of the reaction increased with decreasing initial H₂O₂ concentration, a sign of a step-wise mechanism. A quantitative treatment of the decomposition of H₂O₂ was provided in terms of activation parameters.     

Kinetics of the Oxidation of Tartrazine with Peroxydisulfate in the Presence and Absence of Catalysts

 The food dye tartrazine is oxidized with peroxydisulfate in the absence and in the presence of Ag(I) and Fe(III) catalysts. In the absence of these metal ions, the reaction shows second-order kinetics, first-order in each of the reacting species. With the Ag(I) ion in the medium the reaction proceeds considerably faster, but still follows second-order kinetics. The reaction rate depends on the concentration of Ag(I) and S₂O₈ ²⁻, but is independent of the concentration of the substrate. When Fe(III) acts as the catalyst, a marked enhancement in the reaction rate is observed, and the reaction proceeds through two parallel pathways, the catalyzed and the noncatalyzed. The catalyzed path follows third order kinetics, being first-order in substrate, oxidant, and catalyst concentration. Mechanisms of the noncatalyzed as well as the Ag(I) and Fe(III) catalyzed reaction systems are proposed.     

Oxidation kinetics and fluorescence quenching of coumarin-1 laser dye with peroxodisulfate

Summary The fluorescence of coumarin-1 laser dye (C1; 7-diethylamino-4-methyl-coumarin) proved to be sensitive to the presence of peroxodisulfate (S₂O ₈ ^2– ). The emission intensity decreases with increasing in peroxodisulfate concentration. The decrease is assigned to a quenching process connected with a ground state peroxodisulfate oxidation of the dye. The kinetics of the reaction have been investigated spectrophotometrically at 382 nm. The reaction follows second order kinetics, being first order for each reactant. The reaction rate ispH dependent; higher rates are observed in alkaline media. Addition of sodium dodecyl sulfate (SDS) retards the oxidation process remarkably.

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Kinetik der Oxidation und Fluoreszenzlöschung des Laserpigments Coumarin-1 mit Peroxodisulfat

Zusammenfassung Die Fluoreszenz des Laserpigments Coumarin-1 (C1; 7-Diethylamino-4-methyl-coumarin) reagiert auf die Anwesenheit von Peroxodisulfat (S₂O ₈ ^2– ). Die Emissionsintensität nimmt mit steigender Peroxodisulfatkonzentration ab. Die Abnahme wird einem Löschvorgang zugeschrieben, der mit einer Oxidation des Grundzustands durch Peroxodisulfat verbunden ist. Die Kinetik der Reaktion wurde bei 382 nm spektrophotometrisch untersucht. Sie verläuft nach zweiter Ordnung (nach erster Ordnung bezüglich jedes Reaktanden). Die Reaktionsgeschwindigkeit istpH-abhängig; in alkalischen Medien werden höhere Geschwindigkeitskonstanten gefunden. Zusatz von Natriumdodecylsulfat (SDS) hemmt die Oxidation beträchtlich.

     

Kinetic Studies of the Oxidationof Coumarin-540 Laser Dye

Summary.  3-(2-Benzothiazolyl)-7-diethylaminocoumarin (C-540) has been irradiated at 254 nm in halomethane solvents, and the first order rate constant of the photo-oxidation reaction was determined. The hydrogen bond donation of the solvents plays a substantial role in controlling the rate of formation of the photoproduct. The linearity of the isokinetic relationship supports the similarity of the mechanism in all solvents and emphasizes that the activation process is controlled by diffusion of dye into the solvent cage. Moreover, the chemical oxidation of C-540 with peroxodisulfate anion catalyzed by Ag⁺ ions has been studied. The reaction follows second-order kinetics, first order for each individual reactant. Received October 4, 1999. Accepted (revised) February 14, 2000     

Catalytic effect of supported metal ion complexes on the induced oxidative degradation of pyrocatechol violet by hydrogen peroxide

Kinetics of the oxidative degradation of pyrocatechol violet dye (PCV) [2-[(3,4-dihydroxyphenyl)(3-hydroxy-4-oxocyclohexa-2,5-dien-1-ylidene) methyl]-benzenesulfonic acid] by H(2)O(2) catalyzed by supported transition metal complexes have been studied. The reaction was followed by conventional UV-vis spectrophotometer at lambda(max)=440 nm in a buffer solution at pH 5.1. The supports used were silica gel and cation exchange resins (Dowex-50W, 2 and 8% DVB), while the complexes were [Cu(amm)(4)](2+), [Cu(en)(2)](2+), [Cu(ma)(4)](2+), [Co(amm)(6)](2+), and [Ni(amm)(6)](2+) (amm=ammonia, en=ethylenediamine, and ma=methylamine). The reaction exhibited first-order kinetics with respect to [PCV] and [H(2)O(2)]. The reactivity of the catalysts is correlated with the redox potential of the metal ions, the type of support, and the amount of supported complexes. The rate of the reaction increases with increasing pH and the addition of NaCl. Addition of SDS and CTAB showed inhibiting effects. The reaction is enthalpy-controlled as confirmed from the isokinetic relationship. A reaction mechanism involved the generation of free radicals as an oxidant has been proposed.