Ali H. Gemeay

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.

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.

Kinetics and mechanism of the reduction of some azo-dyes by inorganic oxysulfur compounds

Abstract The kinetics of the reduction of Sunset Yellow (E-110, C.I. 15985), SY, and Ponceau-4R (E-124, C.I. 16255), PR, dyes by dithinoite and bisulfite was studied using stopped-flow and conventional UV/vis spectrophotometer under pseudo- first order conditions. The reaction followed a total one and half-order kinetics, first order with respect to dye and half-order with respect to dithionite, while with bisulfite overall second-order kinetics has been revealed. The reduction using dithionite has higher rate constant than bisulfite. With dithionite, PR has higher rate constant compared with SY, while the reverse has been observed with bisulfite. The direct involvement of SO2·− radical in the reduction mechanism has been established. The formation of carbon-4 sulfonate adducts was concluded in the case of using bisulfite. Addition of β-cyclodextrin showed an enhancement in the reduction rate of SY, while no detectable effect was noticed with PR. This increase in the rate constant is attributed to the inclusion of p-sulfophenylazo side of SY into β-cyclodextrine cavity. Addition of cationic surfactant, CTAB, retarded the rate for both dyes due to the squeezed of dye molecules in the micellar core. The reaction was found to be an enthalpy controlled. Finally the reduction mechanism was proposed.

Activity of silica-alumina surface modified with some transition metal ions

Abstract Kinetics of H 2 O 2 decomposition have been investigated in the presence of a silica-alumina surface (25% Al 2 O 3 ) modified with transition metal ions including Cu(II), Co(II), and Fe(III). The decomposition reaction proceeded with first-order kinetics with respect to the peroxide concentration. The rate of reaction depends upon the type of ions. The reaction rate and the energy of activation decrease in the order Cu(II) > Co(II) > Fe(III). The initial concentration of H 2 O 2 has a great influence on the reaction rate and the coloration of the surface. The decomposition reaction involved the generation of free radical species as confirmed by using the reference chromogen 2,2′-azino-bis(3-ethylbenzthiazoline)-6-sulphonate diammonium salt as a probe. An isokinetic relationship indicates that the reaction is entropy-controlled. A probable mechanism has been suggested for the decomposition process which agrees with the results obtained.

Kinetics of heterogeneous decomposition of hydrogen peroxide with some transition metal complexes supported on silica-alumina in aqueous medium

Abstract Silica-alumina in the form of diethylamine (deam)-, dimethylamine (dmam)-, ethylamine (eam)-, ammonia (amm)- and aniline (an) -copper(II) complexes as well as deam-cobalt (II) complex have been used as potentially active catalysts for H2O2 decomposition in an aqueous medium. The reaction was first order with respect to the H2O2 concentration and the rate constants (per g of dry silica-alumina) were determined. A coloured compound, a peroxo-metal complex, which formed at the beginning of the reaction in each case, was found to contain the catalytically active species. The activation energy and the change in the entropy of activation increased with the basicity of the ligands in the following order: an

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.

Catalytic activity of silica gel surface modified by transition metal-aminosilane complexes in the decomposition of hydrogen peroxide

Abstract The decomposition of H 2 O 2 catalyzed by Cu(II), Co(II), and Mn(II)aminosilane complexes immobilized on silica gel has been investigated. The reaction exhibited first-order kinetics in the presence of both the Co(II) and the Mn(II) complexes, whereas with the Cu(II) complex second-order kinetics were observed. As the amount of catalyst increased, the rate of reaction increased and first-order kinetics with respect to the amount of catalyst are confirmed. The silica gel surface modified by adsorbed metal ion complexes exhibited higher activity compared to the corresponding silica gel surface modified by bonded complexes. The reaction was entropy-controlled, as confirmed from an isokinetic relationship. A probable mechanism for the decomposition of H 2 O 2 consistent with the kinetic data 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.

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

SummaryThe 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.