Mohamed Y. El-Sheikh

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.

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.

The catalytic effect of transition metal-ion ammine complexes on the decomposition of hydrogen peroxide in the presence of Dower-50W resin in aqueous medium

SummaryThe kinetics of the catalytic decomposition of hydrogen peroxide was studied in the presence of Dowex-50W resin in the form of some transition metal-ion ammine complexes in an aqueous medium. The transition metal-ions, Co2+, Ag+, Cd2+ and Zn2+ were chosen in this study and the rate constants (per gram of dry resin) were evaluated at various resin weights in the 25–40°C range. A coloured compound (peroxo-metal complex), which formed at the beginning of the reaction in each case, was found to contain the catalytic active species. Probable mechanisms for the reactions are proposed. The activation energy and the change in the entropy of activation increased in the following sequence: [Ag(NH3]2]+< [Cd(NH3)6]2+<[Co(NH3)6]2+<[Zn(NH3)6]2+, which is also the probability sequence for the formation of the activated complex.

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.

The role of resin—transition metal complexes containing hydroxy amine ligand in hydrogen peroxide decomposition

Abstract The catalytic decomposition of hydrogen peroxide was studied in aqueous medium in the presence of cobalt(II)—ethanolamine complexes sorbed on Dowex-50W resin. Mono-, di- and triethanolamines were used as ligands. The reaction was first order with respect to [H2O2] in all cases. The rate constant decreased with increasing degree of resin crosslinkage owing to a salting-out effect. Also, the rate constant decreased in the following order: mono->di->triethanolamine, which is the same as the order of base strength of the three amines. The activation energy decreased with increasing degree of resin crosslinkage. The other activation parameters were calculated, and the probable reaction mechanism is proposed.

Kinetics and mechanism of the decomposition of hydrogen peroxide catalyzed by Mn(II)-bis-salicylaldimine complexes

SummaryThe tetradentateSchiff bases N,N′-bis(salicylidene) ethylenediamine (salen), N,N-bis-(salicylidene) hexylenediamine (salhex), and N,N′-bis(salicylidene)-o-phenylenediamine (sal-o-phen) are very strongly adsorbed by cation exchange resins (Dowex-50W) with manganese(II) as a counter ion, forming stable complexes. The kinetics of the catalytic decomposition of H2O2 in presence of these complexes has been studied in aqueous medium. The decomposition reaction is first order with respect to H2O2 in the case ofsalen andsal-o-phen and third order in the case ofsalhex. The greater the ligand methylene chain length or the greater the steric effect of the ligand, the greater will be the rate of reaction. The reaction is governed by the entropy of activation. A reaction mechanism is proposed.ZusammenfassungDie teradentatenSchiffschen Basen N,N′-bis-Salicyliden-ethylendiamin (salen), N,N′-bis-Salicyliden-Hexylendiamin (salhex) und N,N′-bis-Salicyliden-o-phenylendiamin (sal-o-phen) werden von Kationenaustauschen (Dowex-50W) mit Mangan(II) als Gegenion unter der Bildung stabiler Komplexe adsorbiert. Die Kinetik der katalytischen Zersetzung von H2O2 in Gegenwart dieser Komplexe wurde in wäßrigem Medium untersucht. Die Zersetzungsreaktion ist erster Ordnung bezüglich H2O2 in den Fällensalen undsal-o-phen und dritter Ordnung im Fall vonsalhex. Die Reaktionsgeschwindigkeit steigt mit der Länge der Methylenkette des Liganden und mit dessen Raumbedarf und wird von der Aktivierungsentropie bestimmt. Ein Reaktionsmechanismus wird vorgeschlagen.