Ahmed B. Zaki

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

The kinetics of the oxidative degradation of the indigo carmine (IC) 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.

Kinetics and mechanisms of decomposition reaction of hydrogen peroxide in presence of metal complexes

Hydrogen peroxide was discovered in 1818 and has been used in bleaching for over a century [1]. H2O2 on its own is a relatively weak oxidant under mild conditions: It can achieve some oxidations unaided, but for the majority of applications it requires activation in one way or another. Some activation methods, e.g., Fentons reagent, are almost as old [2]. However, by far the bulk of useful chemistry has been discovered in the last 50 years, and many catalytic methods are much more recent. Although the decomposition of hydrogen peroxide is often employed as a standard reaction to determine the catalytic activity of metal complexes and metal oxides [3,4], it has recently been extensively used in intrinsically clean processes and in end-of-pipe treatment of effluent of chemical industries [5,6]. Furthermore, the adoption of H2O2 as an alternative of current industrial oxidation processes offer environmental advantages, some of which are (1) replacement of stoichiometric metal oxidants, (2) replacement of halogens, (3) replacement or reduction of solvent usage, and (4) avoidance of salt by-products. On the other hand, wasteful decomposition of hydrogen peroxide due to trace transition metals in wash water in the fabric bleach industry, was also recognized [7]. The low intrinsic reactivity of H2O2 is actually an advantage, in that a method can be chosen which selectively activates it to perform a given oxidation. There are three main active oxidants derived from hydrogen peroxide, depending on the nature of the activator; they are (1) inorganic oxidant systems, (2) active oxygen species, and (3) per oxygen intermediates. Two general types of mechanisms have been postulated for the decomposition of hydrogen peroxide in the presence of transition metal complexes. The first is the radical mechanism (outer sphere), which was proposed by Haber and Weiss for the Fe(III)-H2O2 system [8]. The key features of this mechanism were the discrete formation of hydroxyl and hydroperoxy radicals, which can form a redox cycle with the Fe(II)/Fe(III) couple. The second is the peroxide complex mechanism, which was proposed by Kremer and Stein [9]. The significant difference in the peroxide complex mechanism is the two-electron oxidation of Fe(III) to Fe(V) with the resulting breaking of the peroxide oxygen-oxygen bond. It is our intention in this article to briefly summarize the kinetics as well as the mechanisms of the decomposition of hydrogen peroxide, homogeneously and heterogeneously, in the presence of transition metal complexes.

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.

Characteristic mechanisms of the homogeneous and heterogeneous oxidation of aromatic amines with transition metaloxalate complexes

Abstract The oxidation–reduction reactions of aromatic amines {o-aminophenol (o-AP), o-phenylenediamine (o-PDA) and p-phenylenediamine (p-PDA)} with Co(III), Mn(III), and Cu(II)oxalate complexes have been investigated in homogeneous and heterogeneous systems. The kinetics of the redox reaction of both systems have been studied spectrophotometrically. The redox products were identified and characterised by diffuse reflectance spectroscopy (DRS), and mass spectrometry. The redox reactions follow first-order kinetics with respect to each of the reactants and first order in [amines] in homogeneous and heterogeneous (supported complexes on Amberlite IRA 904 anion-exchange resin) phases, respectively. However, the specific oxidation rate of the amines is found to follow the order, p-PDA>o-AP>o-PDA. The effect of the oxalate anion on the reaction rate of both systems was examined. There is a significant difference in the behaviour of the oxidation of o-AP and o-, p-PDA with all the metal complexes in both systems. Moreover, the oxidation of o-AP and o-, p-PDA appears to follow the outer- and inner-sphere mechanistic classification, respectively.

Kinetics of degradation of allura red, ponceau 4R and carmosine dyes with potassium ferrioxalate complex in the presence of H2O2

The degradation of title dyes with ferrioxalate and H(2)O(2) couple has been investigated spectrophotometrically. In the absence of either one of the oxidizing agents no degradation occurred. The reaction rate was proportional to moderate concentrations of the dye and H(2)O(2). At high concentration of the dye and H(2)O(2) the reaction rate decreased. With regard to the concentration of the ferrioxalate complex the rate of reaction increased even over a wide range of complex concentration. Degradation of dyes does not occur in acidic medium. It is slow in neutral but thoroughly fast in alkaline medium. The reaction rate reaches a maximum value at pH 11.5. This behavior was again observed if HCl or NaOH were added. With HCl the reaction rate decreases with increasing acid concentration but greatly increases with NaOH concentration. Isopropanol showed inhibiting effect due to scavenging the in situ generated hydroxyl radical (()OH). Oxalate ion enhanced the rate, confirming an outer sphere mechanism. The activation parameters of the reaction are estimated and a possible mechanism is proposed. The mechanism is well confirmed with data simulation procedure.

Catalytic decomposition of hydrogen peroxide in the presence ofN, N′-bis(salicylidene)-o-phenylenediamineiron(III) sorbed on to Dowex-50W resin

SummaryThe tetradentate Schiff-base ligandN, N′-bis(salicylidene)-o-phenylenediamine (salph) is very strongly sorbed by cation exchange materials with transition metal counter ions, forming stable complexes. The kinetics of catalytic decomposition of H2O2 in the presence of (salph)-FeIII sorbed on Dowex-50W resin has been studied in aqueous medium. The reaction is first order with respect to [H2O2]. The rate constant, k (per g of dry resin) decreased with increasing degree of resin cross-linkage due to a salting out effect. The activation parameters were calculated and a reaction mechanism is proposed.

The role of resin-amine transition metal complexes in hydrogen peroxide decomposition

Abstract Dowex-50W resin in the form of ethylamine- and dimethylamine-transition metal ion (Co II , Ni II , Mn II ) complexes have been used as potentially active catalysts for H 2 O 2 decomposition in an aqueous medium. The rate constant (per g of dry resin) was evaluated with a resin containing 8% divinylbenzene crosslinkage over the temperature range 25 – 40 °C. With both ligands the reaction rate was directly proportional to [H 2 O 2 ] for complexes with Co II and Ni II and to [H 2 O 2 ] 2 for Mn II complexes. Probable mechanisms for the reactions have been proposed. The activation energy with both ligands was found to increase in the following sequence: Ni II II II . The activation energies of the complexes with the Me 2 NH ligand were smaller than those with the EtNH 2 ligand. The change in the entropy of activation, the rate constant (per g of dry resin) and the probability of activated complex formation with the secondary amine (Me 2 NH)-transition metal complexes were smaller than those with the primary amine (EtNH 2 )-transition metal complexes. This is due to the steric effect of the methyl groups in the Me 2 NH ligand.

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

SummaryDowex 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]=1∶4 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 and mechanism of o-aminophenol oxidation by the supported mesoporous silica (HISiO2) in the binary system with Amberlite resin

HISiO2 (hexagonal mesoporous silica) was synthesized with a high concentration of a non-ionic template. The synthesized HISiO2 materials have a well defined porous architecture with surface area (760 m2 g?1), and pore SIZE=35 A. Cu(II)–aquo complex was anchored onto silica (S–CuII) through the coordination with amino-functionalized HISiO2 (S–NH2) without impregnation on the surface. The oxidation of o-aminophenol (o-AP) with (S–CuII), Cu(II)-oxalate complex supported on Amberlite resin (R–CuII), (R–CuII)/S–NH2 (0.05 g), and a mixture (1:1) of (R–CuII)/(S–CuII), has been studied at different temperatures (25–40 °C) ±0.1. The oxidation product has been monitored kinetically and spectrophotometrically. All redox reactions were characterized by first-order kinetics. The rate constant of the oxidation reaction of o-AP decreases in the following order; (S–CuII)>(R–CuII)/(S–CuII)>(R–CuII)/(S–NH2)>(R–CuII). This sequence reflects the role of the effective surface area of HISiO2 on the redox reaction. The activation parameters for the amine oxidation have been estimated. Besides, a mechanism of the oxidation process of o-AP has been proposed.

Role of resin-copper(II) complexes containing ethanolamines in hydrogen peroxide decomposition

SummaryDowex-50W resins in the form of mono (mea)-, di (dea)-and tri-ethanolamine (tea)-CuII complexes have been used as potentially active catalysts for H2O2 decomposition in an aqueous medium. The rate constant (per g of dry resin) was evaluated with resins containing 2,8 and 12% divinylbenzene (DVB) crosslinkage, over the temperature range 25–40°C. The reaction was first order with respect to [H2O2] with mea for 8 and 12% DVB (50–100 mesh), second order with mea for 2% DVB (50–100 and 200–400 mesh) and third order with dea and tea for 8% DVB (50–100 mesh). The value of the rate constant (per g of dry resin) of the mea-CuII/CoII binary system was compared with that of the mea-CuII/NiII binary system. With a given degree of resin crosslinkage the activation energy increased in the sequence mea < dea < tea, which is the inverse sequence of the basic strength of the free amines. The activation parameters were calculated. Probable mechanisms were proposed for the reaction with the three ethanolamines.