A kinetic study of the oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-methionine by water soluble colloidal MnO<Subscript>2</Subscript>

Aqueous solution of water soluble colloidal MnO₂ was prepared by Perez-Benito method. Kinetics of l-methionine oxidation by colloidal MnO₂ in perchloric acid (0.93 × 10⁻⁴ to 3.72 × 10⁻⁴ mol dm⁻³) has been studied spectrophotometrically. The reaction follows first-order kinetics with respect to [H⁺]. The first-order kinetics with respect to l-methionine at low concentration shifts to zero order at higher concentration. The effects of [Mn(II)] and [F⁻] on the reaction rate were also determined. Manganese (II) has sigmoidal effect on the rate reaction and act as auto catalyst. The exact dependence on [Mn(II)] cannot be explained due to its oxidation by colloidal MnO₂. Methionine sulfoxide was formed as the oxidation product of l-methionine. Ammonia and carbon dioxide have not been identified as the reaction products. The mechanism with the observed kinetics has been proposed and discussed.     

Kinetics of the reduction of water soluble colloidal MnO<Subscript>2</Subscript> by mandelic acid in the absence and presence of non-ionic surfactant triton X-100

Kinetics of the title reaction has been studied spectrophotometrically in presence of perchloric acid at 30°C both in the absence and presence of Triton X-100 (TX-100). The reaction-time curves suggest the involvement of non-autocatalytic and autocatalytic reaction paths. The reaction follows first-order kinetics with respect to colloidal MnO₂ and mandelic acid. The reaction has acid-dependent and acid-independent paths and, in the former case, the order is fractional in [H⁺]. Addition of nonionic surfactant (TX-100) catalysed the reaction which is explained on the basis of hydrogen bonding between the oxygen of polyoxyethylene chains of TX-100 and hydroxy groups of mandelic acid/colloidal MnO₂. The kinetic data are rationalized in terms of model proposed by Tuncay et al. On the basis of the observed results, a possible mechanism has been proposed and discussed.     

Reduction of soluble colloidal MnO<Subscript>2</Subscript> by <Emphasis Type="SmallCaps">DL</Emphasis>-malic acid in the absence and presence of nonionic TritonX-100

Kinetics of oxidation of DL-malic acid by water soluble colloidal MnO₂ (prepared from potassium permanganate and sodium thiosulfate solutions) have been studied spectrophotometrically in the absence and presence of nonionic Triton X-100 surfactant. The reaction is autocatalytic and manganese(II) (reduction product of the colloidal MnO₂) may be the autocatalyst. The order of the reaction is first in colloidal [MnO₂] as well as in [malic acid] both in the absence and presence of the surfactant. The reaction has acid-dependent and acid-independent paths and, in the former case, the order is fractional in [H⁺]. The effect of externally added manganese(II) is complex. The results show that the rate constant increases as the manganese(II) concentration is increased. It is not possible to predict the exact dependence of the rate constants on manganese(II) concentration, which has a series of reactions with other reactants. In the presence of TX-100, the observed effect on k is catalytic up to a certain [TX-100]; thereafter, an inhibitory effect follows. The catalytic effect is explained in terms of the mathematical model proposed by Tuncay et al. (in Colloids Surf A Physicochem Eng Aspects 149:279 3). Activation parameters associated with the observed rate constants (k_obs/k) have also been evaluated and discussed.     

Kinetics of colloidal MnO<Subscript>2</Subscript> reduction by L-arginine in absence and presence of surfactants

The kinetics of the oxidation of L-arginine by water-soluble form of colloidal manganese dioxide has been studied using visible spectrophotometry in aqueous as well as micellar media. To obtain the rate constants as functions of [L-arginine], [MnO₂] and [HClO₄], pseudo-first-order conditions are maintained in each kinetic run. The first-order-rate is observed with respect to [MnO₂], whereas fractional-order-rates are determined in both [L-arginine] and [HClO₄]. Addition of sodium pyrophosphate and sodium fluoride enhanced the rate of the reaction. The effect of externally added manganese(II) sulphate is complex. It is not possible to predict the exact dependence of the rate constant on manganese(II) concentration, which has a series of reactions with other reactants. The anionic surfactant SDS neither catalyzed nor inhibited the oxidation reaction, while in presence of cationic surfactant CTAB the reaction is not possible due to flocculation of reaction mixture. The reaction is catalyzed by the nonionic surfactant TX-100 which is explained in terms of the mathematical model proposed by Tuncay et al. Activation parameters have been evaluated using Arrhenius and Eyring equations. On the basis of observed kinetic results, a probable mechanism for the reaction has been proposed which corresponds to fast adsorption of the reductant and hydrogen ion on the surface of colloidal MnO₂.     

Reactivity of some sulphur- and non-sulphur-containing amino acids towards water soluble colloidal MnO<Subscript>2</Subscript>. A kinetic study

Kinetic data for the oxidation of glutathione (reduced, GSH), cysteine, glycine and glutamic acid by colloidal manganese dioxide, (MnO₂) _n are reported. Colloidal MnO₂, oxidized glutathione to disulphide (glutathione, oxidized), was reduced to manganese (II). Glycine and glutamic acid (structural units of glutathione) are not oxidized by colloidal MnO₂, but the other structural unit, cysteine, is also oxidized by the same oxidant under similar experimental conditions. This is interpreted in terms of the rate-determining colloidal MnO₂-S bonded intermediate. The reactivity of GSH towards colloidal MnO₂ is very much higher than cysteine. Kinetics of oxidation of GSH and cysteine by colloidal MnO₂ were performed spectrophotometrically as a function of [GSH], [cysteine], colloidal [(MnO₂) _n ], [HClO₄], temperature and trapping agents sodium fluoride and manganese (II) (reduction product of colloidal MnO₂). The purpose of this work was to study the role of –NH₂, –COOH, –SH groups present in the carbon chain of the above amino acids. It was found that the reactivity of –SH group is higher than –NH₂ and –COOH groups. The mechanisms, involving a colloidal MnO₂ complex with GSH and cysteine, are proposed. The complexes decompose in a rate-determining step, leading to the formation of free radical and manganese (III), which is also an intermediate. The dimerization of radicals takes place in a subsequent fast step to yield the products.     

The Kinetics of Oxidation of L-Tryptophan by Water-Soluble Colloidal Manganese Dioxide

The kinetics of the oxidation of L-tryptophan by water-soluble colloidal MnO₂ (prepared from potassium permanganate and sodium thiosulfate solutions) has been carried out in aqueous perchloric acid medium at different temperatures. Monitoring the disappearance of the MnO₂ spectrophotometrically at 390 nm was used to follow the kinetics. The first-order kinetics with respect to [L-tryptophan] at low concentrations shifted to zero-order at higher concentrations. The reaction followed first-order with respect to [MnO₂] but fractional-order with respect to [HClO₄]. Adding trapping agents enhanced the rate of the reaction. The Arrhenius and Eyring equations were found valid for the reaction between 35°C and 55°C and different activation parameters (E_a, ΔH^#, ΔS^#) have been evaluated. On the basis of various observations and product characterization a plausible mechanism has been envisaged for the reaction taking place at the colloid surface. The results suggest formation of an adsorption complex between L-tryptophan and MnO₂. The complex decomposes in a rate-determining step, leading to the formation of free radical, which again reacts with the colloidal MnO₂ in a subsequent fast step to yield products. Freundlich isotherm is used to explain the adsorption of L-tryptophan on the colloidal MnO₂.     

Reduction of water-soluble colloidal manganese dioxide by thiourea: a kinetic and mechanistic study

Kinetic data for the colloidal MnO₂–thiourea redox system are reported for the first time. The reduction of water-soluble colloidal MnO₂ by thiourea (sulfur containing reductant) in aqueous perchloric acid medium has shown that it proceeds in two stages, i.e., a fast stage followed by a relatively slow second stage. The log (absorbance) versus time plot deviates from linearity. The kinetics of both the stages was investigated spectrophotometrically. The first-order kinetics with respect to [thiourea] at low concentration shifts to zero-order at higher concentration. The reaction rate increases with [HClO₄] and the kinetics reveals complex order dependence in [HClO₄]. Addition of P₂O ₇ ⁴⁻ and F⁻ in the form of Na₄P₂O₇ and NaF, respectively, has inhibitory effect on the reaction rate. The reaction proceeds through the fast adsorption of thiourea on the surface of the colloidal MnO₂. A mechanism involving the protonated thiourea as the reactive reductant species is proposed. The observed results are discussed in terms of Michaelis–Menten/Langmuir–Hinshelwood model. From the observed kinetic data, colloidal MnO₂–thiourea adsorption constant (K _ad1) and rate constant (k ₁) were calculated to be 1.25×10¹⁰ mol⁻¹ dm³ and 3.1×10⁻⁴ s⁻¹, respectively. The variation of temperature does not have any effect on the reaction rate.     

Water-soluble colloidal manganese dioxide as an oxidant for l-tyrosine in the absence and presence of non-ionic surfactant TX-100

Water-soluble colloidal manganese dioxide has been used to oxidize l-tyrosine in aqueous-acidic medium. The kinetics of the reaction was studied in the absence and presence of non-ionic surfactant (TX-100) using a spectrophotometric technique. As the reaction was fast under pseudo-first-order conditions ([l-tyrosine]  [MnO₂]), the rate constants as a function of [l-tyrosine], [MnO₂], [HClO₄] and temperature were obtained under second-order conditions. The rate of the reaction increased and decreased with the increase in [l-tyrosine] and [MnO₂], respectively. Perchloric acid, sodium pyrophosphate and sodium fluoride showed catalytic effect. The effect of externally added manganese(II) sulphate is complex. It is not possible to predict the exact dependence of the rate constants on manganese(II) concentration, which has a series of reactions with other reactants. The reaction is inhibited by the non-ionic surfactant TX-100. Activation parameters have been evaluated using Arrhenius and Eyring equations. Based on observed kinetic results, a probable mechanism for the reaction has been proposed which corresponds to fast adsorption of the reductant and hydrogen ion on the surface of colloidal MnO₂ followed by one-step two-electron transfer rate limiting process.     

Influence of MnO<Subscript>2</Subscript> on the photocatalytic activity of P-25 TiO<Subscript>2</Subscript> in the degradation of methyl orange

Influences of α-MnO₂, β-MnO₂, and δ-MnO₂ on the photocatalytic activity of Degussa P-25 TiO₂ have been investigated through the photocatalytic degradation of methyl orange. The TiO₂ photocatalyst, before and after being contaminated by MnO₂, was characterized by UV-visible diffuse reflectance spectroscopy (UV-vis DRS), photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). The results showed that photocatalytic activity of TiO₂ could be inhibited significantly or completely deactivated due to the presence of even a small amount of MnO₂ particles. It was found that the poisoning effect varied with the crystal phases of MnO₂ and the effect was in the order δ-MnO₂ >α-MnO₂ >β-MnO₂. The poisoning effect was attributed to the formation of heterojunctions between MnO₂ and TiO₂ particles. The heterojunctions changed the chemical state of Ti⁴⁺ and O²⁻ sites in the crystalline phase of TiO₂. MnO₂ in contact with TiO₂ particles also broadens the band-gap of TiO₂, which decreases UV absorption of TiO₂. It can also create some deep impurity energy levels serving as photoelectron-photohole recombination center, which accelerates the electron-hole recombination. Supported by the National Natural Science Foundation of China (Grant No. 20477009) and the Natural Science Foundation of Hebei Province (Grant No. E2005000183)     

Formation,characterization and stabilization of water-soluble colloidal MnO<Subscript>2</Subscript> in the oxidation of methionine,thiourea and thioacetamide by permanganate

Stabilization and characterisation of water soluble colloidal MnO₂ during the oxidation of sulphur-containing organic reductants “thiourea, thioactamide and methionine” by permanganate in aqueous neutral media are reported for the first time. Upon addition of permanganate to a solution of methionine, a transient species appears within the time of mixing, which is stable for several weeks. On the other hand, the transient species is unstable in the presence of thiourea and thioacetamide, respectively. The nature of manganese (IV) species present in the solution was characterized by spectrophotometric and coagulation measurements. On addition of HClO₄, there is a decrease in the absorbance of the reaction mixture. Under pseudo first-order conditions ([reductants] > []), the reduction rate was very fast up to the formation of water soluble colloidal MnO₂. The effect of various parameters, such as hydrogen ion concentration, amount of and concentration of reductants were investigated. Mechanisms consistent with the observed results have been proposed and discussed.     

Mechanism of the oxidation of d-glucose onto colloidal MnO2 surface in the absence and presence of TX-100 micelles

Aqueous colloidal manganese dioxide (MnO₂) was prepared via titration by using potassium permanganate and sodium thiosulphate in aqueous neutral medium. The kinetics of oxidation of d-glucose onto the surface of colloidal MnO₂ have been studied spectrophotometrically. The results show that the rate of initial stage (nonautocatalytic path) increases with increasing the [d-glucose], [H⁺], and temperature and also upon addition of nonionic surfactant Triton X-100 (TX-100), which indicates that the surfactant enhances the concentration of d-glucose at the surface of the colloidal MnO₂. Hydrogen bonding interaction seemingly arises between –OH groups of d-glucose and oxygen of the ether linkages of polyoxyethylene chain of TX-100. A possible mechanism of the oxidative degradation of d-glucose is discussed in terms of d-glucose/TX-100 and colloidal MnO₂ interaction.     

Unusual stabilization of water-soluble colloidal MnO2 during the oxidation of paracetamol by

The kinetics of the formation and decomposition of water-soluble colloidal MnO₂ in the paracetamol– redox system have been investigated spectrophotometrically in aqueous-neutral media at 30 °C. Upon mixing aqueous solutions of permanganate and paracetamol, a readily distinguishable brown color appears and then disappears slowly. Experiments have been done to confirm the nature of intermediate (Mn(IV)) formed during the reduction of permanganate by paracetamol. The stoichiometry was found to be 1:1. Formation and decomposition of water-soluble colloidal MnO₂ depend upon the experimental conditions, i.e., [paracetamol] and [H⁺]. The effect of total [paracetamol], and [H⁺] on the rate of the reaction was determined. On the basis of various observations, two mechanisms are proposed: one for MnO₂ formation and the other for decomposition.     

Kinetics and mechanism of oxidation of malonic acid by permanganate and manganese dioxide in acid perchlorate medium

Summary The kinetics of oxidation of malonic acid by both [MnO₄]^– and MnO₂ have been studied in an HClO₄ medium. The oxidation product of the organic acid was found to be glyoxylic acid. A reaction mechanism assuming complexation between MnO₂ and malonic acid is suggested. The rate is independent of [H⁺].     

Liquid-phase MnO<Subscript>2</Subscript>-modification of clinoptilolite

Formation mechanism of the MnO₂ phase in the reaction of heterogeneous synthesis between Mn²⁺ and MnO ₄ ^- ions on a solid aluminosilicate surface in aqueous solutions was studied. It was shown that, for lowsilica forms, the Mn²⁺ ion is oxidized by the MnO ₄ ^- ion uniformly across the grain depth to give the MnO₂ phase and manganese manganites. For high-silica materials, the MnO₂ phase is formed on the outer surface of grains, with the decomposition of the MnO ₄ ^- ion and formation of the MnO₂ phase and molecular oxygen. It was found that, for the clinoptilolite rock used as a solid support, the yield of the MnO₂ phase and its distribution over the particle volume depend on the penetration capacity of the MnO ₄ ^- ion into the porous structure of this rock, determined by its composition. It is shown that the amount of the MnO₂ phase grows with increasing concentration of the MnO ₄ ^- ion and treatment duration, with the phase thickness being 15–20 and 350–1050 μm for, respectively, high- and low-silica samples.     

Decay of H<Subscript>2</Subscript>O<Subscript>2</Subscript> under the action of the colloidal catalyst based on iron(III) oxides in a neutral medium

The catalytic activity of the colloidal catalyst based on iron(III) hydroxide was studied in the decomposition of H₂O₂ in a neutral medium (pH 6.7). A colloidal micellar solution of iron(III) hydroxide after preparation was kept at 19–20 °С for 2 or 20 h without additives or with C₂H₅OH additives. The decomposition of H₂O₂ under the action of the colloidal catalyst (20 h) proceeds via the first-order reaction with the decay rate constant k_d = 1.26?10^–4 s^–1, whereas the decay rate of the first-order reaction is k_d = 0.77?10^–4 s^–1 for the colloidal catalyst (2 h) prepared in the presence of C₂H₅OH.     

Nitration of Aromatic Compounds Catalyzed by Divanadium-Substituted Molybdophosphoric Acid,H<Subscript>5</Subscript>[PMo<Subscript>10</Subscript>V<Subscript>2</Subscript>O<Subscript>40</Subscript>]

Summary. The nitration of aromatic compounds was carried out in the presence of divanadium-substituted molybdophosphoric acid, H₅PMo₁₀V₂O₄₀, as catalyst and a mixture of nitric acid and acetic anhydride as nitrating agent. In the presence of this heteropolyacid the ortho- and para-nitro compounds were obtained in good to excellent yields under mild reaction conditions.     

Evidence of protonation during the oxidation of some aryl alcohols by permanganate in perchloric acid medium and mechanism of the oxidation processes

The oxidation of benzyl alcohol and methoxy-, chloro-, and nitro- substituted benzyl alcohols by permanganate has been studied in aqueous and acetic acid medium in presence of perchloric acid. The reaction is first-order in [MnO₄^?] and [XC₆H₄CH₂OH], but the order is complex with respect to [H⁺]. Different thermodynamic parameters have been evaluated. The reaction occurs through the protonation of alcohol in a fast preequilibrium followed by a slow rate-determining oxidation step. A two-electron transfer oxidation step has been suggested for benzyl alcohol and chloro- and nitro- substituted alcohols, while the oxidation of methoxy compounds involves a one-electron transfer via a free-radical mechanism. © 1995 John Wiley & Sons, Inc.     

Synthesis of hydroxy derivatives from adamantanecarboxylic acids in the system MnO<Subscript>2</Subscript>–H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

A convenient procedure has been developed for the synthesis of mono- and dihydric cage alcohols from adamantanecarboxylic acids and their esters using the MnO₂–H₂SO₄ system. The reaction at elevated temperature involved both decarboxylation and decarbonylation of the initial acid or ester.     

Study of nitrosation of hexaammineruthenium(II): Crystal structure of <Emphasis Type="Italic">trans</Emphasis>-[RuNO(NH<Subscript>3</Subscript>)<Subscript>4</Subscript>Cl]Cl<Subscript>2</Subscript>

The nitrosation of [Ru(NH₃)₆]²⁺ in hydrochloric acid and alkaline ammonia media has been studied; the patterns of interconversion of ruthenium complexes in reaction solutions have been proposed. In both cases, nitrogen(II) oxide acts as the nitrosation agent. The procedure for the synthesis of [Ru(NO)(NH₃)₅]Cl₃ · H₂O (yield 75–80%), the main nitrosation product of [Ru(NH₃)₆]²⁺, has been optimized. Thermolysis of [Ru(NO)(NH₃)₅]Cl₃ · H₂O in a helium atmosphere has been studied; the intermediates have been identified. One of these products is polyamidodichloronitrosoruthenium(II) whose subsequent decomposition gives an equimolar mixture of ruthenium metal and dioxide. The structure of trans-[RuNO(NH₃)₄Cl]Cl₂, formed in the second stage of thermolysis and as a by-product in the nitrosation of [Ru(NH₃)₆]Cl₂, has been determined by X-ray diffraction.