New coordination polymers. III: Oxidation of poly(vinyl alcohol) by permanganate ion in alkaline solutions. Kinetics and mechanism of formation of intermediate complex with a spectrophotometric detection of manganate(vi) transient species

The kinetics of formation of the [PVA. Mn^VIO] intermediate complex have been measured by the conventional spectrophotometric technique. The rate of formation of the complex was found to be given by the expression d[Complex]/dt = k₁[MnO-₄][PVA^?]. The results showed that the rate of formation is dependent on the base concentration. A first-order reaction in permanganate and fractional order with respect to the poly(vinyl alcohol) (PVA) concentration were observed. The activation parameters have been evaluated and a tentative reaction mechanism has been suggested.     

Decay of the HO2 and O−2 Radicals Catalyzed by Superoxide Dismutase. A Pulse Radiolytic Investigation

The pulse radiolysis technique has been employed in the investigation of the dismutation of superoxide radicals, O^?₂ and HO₂, in the presence of superoxide dismutase in aqueous solutions. The decay of superoxide radicals in the presence of the enzyme was found to be first order in both enzyme and superoxide concentrations. An apparent second order reaction rate constant was found to be about 2 × 10⁹ M^?1 sec^?1, decreasing slightly as the pH is increased from 5 to 9.5. A mechanism which accounts for all our observations is proposed. It includes two steps: (1) formation of a product (EO^?₂ or E^?) from one enzyme (E) molecule and one O^?₂ radical ion; (2) regeneration of E by a reaction of this product with an additional O^?₂ ion radical. The reaction rate constants k = (1.4 ± 0.2) × 10⁹ and k = (1.9 ± 0.6) × 10⁹ M^?1 sec^?1 were measured at pH = 7 in an oxygenated 0.16 M sodium formate solution.     

In situ synchrotron powder diffraction study of the setting reaction kinetics of magnesium-potassium phosphate cements

This work reports a kinetic study of the formation of magnesium-potassium phosphate cements accomplished using in-situ synchrotron powder diffraction. The reaction: MgO + KH₂PO₄ + 5H₂O → MgKPO₄ · 6H₂O was followed in situ in the attempt of contributing to explain the overall mechanism and assess the influence of periclase (MgO) grain size and calcination temperature (1400-1600 °C) on the reaction kinetics. Numerical kinetic parameters for the setting reaction have been provided for the first time. The best fit to the kinetic data was obtained using a weighted nonlinear model fitting method with two kinetic equations, representing two consecutive, partially overlapping processes. MgO decomposition could be described by a first order (F1) model followed by a Jander diffusion (D3) controlled model. Crystallization of the product of reaction was modelled using an Avrami model (A_n) followed by a first order (F1) chemical reaction. A reaction mechanism accounting for such results has been proposed.     

Kinetics of carbon dioxide reaction with diethylaminoethanol in aqueous solutions

Differential rates of CO₂ adsorption into 0.90, 0.47 and 0.24 M aqueous solutions of 2-(diethylamino)ethanol (DEAE) were measured at 323 K over a wide range of carbonation ratios. A rigorous thermodynamic model was used to define species activities which were coupled with Danckwerts' gas-liquid reaction model to deduce the kinetics. The reaction of CO₂ with this highly basic tertiary amine occurs by two pathways: (1) a minor path via the CO₂ reaction with hydroxide ion and (2) a predominant reaction pathway that can be characterized by its first order dependency on the free amine concentration. The second reaction was proposed to involve an internal salt-like intermediate,.     

Radical terpolymerization of zincacrylate,acrylonitrile, and styrene initiated by As2S3-styrene complex

Terpolymerization was investigated by dilatometry for zincacrylate (ZnA₂), acrylonitrile (AN), and styrene (St), radically initiated by a As₂S₃-styrene complex (I) in dimethyl sulphoxide (DMSO), at 90 ± 0.1°C for 1 h under inert atmosphere. The system follows non-ideal kinetics, due to primary radical termination as well as a degradative chain transfer reaction. The kinetic expression for the system is R_pα(I)^0.27 (St)^0.31 (AN) ^0.22[ZnA₂]^0.12. The value for the activation energy and k/k_t are 55 kJ mol^?1 and 1.87 × 10^?7 1 mol^?1 s^?1 respectively. The terpolymer has been characterized by IR and NMR spectroscopy. The thermal stability and solubility of the terpolymer have also been studied.     

Investigation of the TiCl4 reaction order in quasiliving styrene polymerization with 1-phenylethyl chloride/TiCl4/Bu2O

Summary The reaction order with respect to TiCl₄ was investigated in the quasiliving polymerization of styrene using a 1-phenylethyl chloride/TiCl₄/Bu₂O initiating system in mixture of 1,2-dichloroethane and n-hexane (55:45 v/v) at -15 °C. It was found out that the reactions order with respect to TiCl₄ was closer to 1. A mechanistic scheme to explain the kinetic behavior of quasiliving styrene polymerization in 1-phenylethyl chloride/TiCl₄/Bu₂O is proposed.     

Variable thermochemical stability of RE2Si2O7 (RE = Sc,Nd, Er,Yb, or Lu) in high-temperature high-velocity steam

Five rare-earth (RE) disilicates (RE₂Si₂O₇, RE = Sc, Nd, Er, Yb, or Lu) were synthesized and exposed to high-velocity steam (up to 235 m/s) for 125 hours at 1400°C. Water vapor reaction products, mass loss, average reaction depths, and product phase microstructural evolution were analyzed for each material after exposure. Similar to steam testing results in the literature, RE₂Si₂O₇ (RE = Er, Yb, Lu) underwent silica depletion producing gaseous silicon hydroxide species, RE₂SiO₅, and RE₂O₃ product phases. Sc₂Si₂O₇ reacted with high-velocity steam to produce only a Sc₂O₃ product layer with no stable Sc₂SiO₅ phase detected by X-ray diffraction or microscopy techniques. Further, Nd₂Si₂O₇ rapidly reacted with steam to produce with no Nd₂SiO₅ or Nd₂O₃ reaction products. All RE₂Si₂O₇ that produced a silicate reaction product (RE = Nd, Er, Yb, Lu) showed densification of the product phase at steam velocities above 150 m/s that resulted in enhanced resistance. The results presented in this work demonstrate that rare-earth silicates show diverse steam reaction products, reaction product microstructures, and total reaction depths after high-temperature high-velocity steam exposure. Of the materials in this study, RE₂Si₂O₇ (RE = Yb, Lu) were most stable in high-temperature high-velocity steam, making them most desirable as environmental barrier coating candidates.     

Reaction rate and MWD changes in crosslinking of PVC,with dithioltriazine

The reaction rate of crosslinking of PVC with dithioltriazine has been studied by following gel formation and changes in the molecular weight distribution (MWD). Compounding was performed on a roll mill at 145°C and crosslinking by heat treatment at 180 or 90°C. In this system crosslinking is executed by the thiolate anion, formed in situ by reaction with MgO. We have studied the catalyzing effect of several polyols in order to achieve a more efficient reaction. Most likely, these catalysts work by chelating the Mg²⁺ ions, thus increasing the nucleophilic character of the thiolate. With the most efficient ones, ditrimethylolpropane and HO(CH₂CH₂)_6-7H, complete crosslinking can be obtained in 3 min at 180°C, i.e., at processing temperatures. We also followed the changes in the MWD before gelation at a considerably lower temperature, 145°C, and found an extensive molecular enlargement even after 5-10 min. Most surprisingly, μM_n increased up to 100% without formation of insoluble material. By ¹H-NMR measurements on low molecular weight extracts, we have shown this to be due to a fast and selective reaction with allylic chlorine in the unsaturated end groups, ～ CH₂? CH?CH? CH₂Cl, formed in the mechanism of chain transfer to monomer. Due to this reaction, formulations with too high reactivity may crosslink during processing, which calls for a careful balancing of the reactivity for each processing case.     

Turnover rate and kinetic mechanism for the reaction of hydrodechlorination of 1,1-dichlorotetrafluoroethane (CF3-CFCl2) over a polycrystalline Pd foil

The kinetics (turnover rate, activation energy, reaction order) of catalytic hydrodechlorination of 1,1-dichlorotetrafluoroethane or CFC 114a (CF₃-CFCl₂) were determined on a ~ 0.5 cm² palladium polycrystalline foil as a catalyst. The reaction rates were measured in the temperature range of 80-200°C at a total pressure of 770 Torr. Using a reaction cell that was connected to an ultrahigh vacuum surface characterization chamber the composition of the metal surface was monitored before and after reaction. The products formed in a parallel reaction network and the turnover rates at 150°C, 50 Torr CFC 114a, 100 Torr H₂, and 0.1 Torr HCl were 2.1 s^-1 for CF₃-CFH₂, 3.0× 10^-1 s^-1 for CF₃-CFClH, and 6.4×10^-2 s^-1 for CF₃-CH₃. For the two most abundant products, the reaction order is 1 in CF₃-CFCl₂, 0.5 in H₂ and -1 in the reaction product HCl. These results suggest that the rate determining step for the reaction is the associative chemisorption of CF₃-CFCl₂ on the palladium surface. The palladium surface was free of adsorbates after the reaction with the exception of sulfur impurity that may accumulate during reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

From single crystals to supported nanoparticles in experimental and theoretical studies of H2 oxidation over platinum metals (Pt, Pd): Intermediates, surface waves and spillover

The present paper reviews our investigations concerning the mechanism of H₂ + O₂ reaction on the metal surfaces (Pt, Pd) at different structures: single crystals (Pt(1 1 1), Pt(1 0 0), Pd(1 1 0)); microcrystals (Pt tips); and nanoparticles (Pd–Ti³⁺/TiO₂). Field electron microscopy (FEM), field ion microscopy (FIM), high-resolution electron energy loss spectroscopy (HREELS), XPS, UPS, work function (WF), TDS and temperature-programmed reaction (TPR) methods have been applied to study the kinetics of H₂ oxidation on a nanolevel. The adsorption of both O₂ and H₂ and several dissociative products (H_ads, O_ads, OH_ads) was studied by HREELS. Using the DFT technique the equilibrium states and stretching vibrations of H, O, OH, H₂O, adsorbed on the Pt(1 1 1) surface, have been calculated depending on the surrounding of the metal atoms. Sharp tips of Pt, several hundreds angstroms in radius, were used to perform in situ investigations of the dynamic surface processes. The FEM and FIM studies on the Pt-tip surface demonstrate that the self-oscillations and waves propagations are connected with periodic changes in the surface structure of nanoplane (1 0 0)-(hex) ↔ (1 × 1), varying the catalytic property of metal. The role of defects (Ti³⁺-□_O) in the adsorption centers formation, their stabilization by the palladium nanoparticles, and then the defects participation in H₂ + O₂ steady-state reaction over Pd–Ti³⁺/TiO₂ surface have been studied by XPS, UPS and photodesorption techniques (PhDS). This reaction seems to involve the “protonate” hydrogen atoms (H⁺/TiO_x) as a result of spillover effect: diffusion of H_ads atoms from Pd particles on a TiO_x surface. The comprehensive study of H₂, O₂ adsorption and H₂ + O₂ reaction in a row: single crystals → tips → nanoparticles has shown the same nature of active centers over these metal surfaces.     

The transformation: Brushite → calcium monofluorophosphate under aqueous conditions

The reaction between CaHPO₄. 2H₂O and 10^?1 M solution of Na₂PO₃F have been studied in the pH range 7 to 5 at 25 °C. The formation of CaPO₃F · 2H₂O, rather than CaF₂ or Ca₅F (PO₄)₃ by initial hydrolysis and subsequent reaction with the F^? formed, appeared to occur. The value of ΔG for CaPO₃F · 2H₂O has been calculated as – 530.9 kcal mol^?1. A comparison is drawn to the exchange reactions reported here, and to the similar reactions reported to occur in calcium deficient apatite.     

Density‐Dependent Formation of the Pure trans‐Isomer of the Unsaturated Alcohol by Selective Hydrogenation of Citral in Supercritical Carbon Dioxide

Selective hydrogenation of citral to unsaturated alcohol [geraniol (trans) + nerol (cis)] was carried out in supercritical carbon dioxide (scCO₂) using an MCM‐41 supported plantinum catalyst (∼1 wt% Pt). A remarkable rate of isomerization of the unsaturated alcohol [nerol (cis) to geraniol (trans)] during the hydrogenation of citral was achieved simply by tuning the density of CO₂. Optimum reaction conditions were developed to obtain only geraniol (trans) with a selectivity of 98.8% and citral conversion of 99.8%. A significant change in the cis:trans ratio of the product (1:82.3) from the substrate (1:1.3) was observed depending on the various reaction parameters like carbon dioxide and hydrogen pressure, reactant concentration, reaction time and, particularly, the total selectivity for unsaturated alcohol [geraniol (trans) +nerol (cis)]. It has been observed that the presence of hydrogen is necessary for isomerization. Our results were explained in terms of a density‐dependent, two‐step model. The kinetic behaviour shows that the rate of isomerization was higher in scCO₂ compared to other organic solvents and the pure form of geraniol (trans) was obtained exclusively. A probable reaction pathway was proposed in order to explain the isomerization during hydrogenation of citral in scCO₂ medium.     

Radiation Chemistry of Ethylene Glycol,Meso-Erythritol, 2-Deoxy-D-Ribose and Alkyl Phosphates as DNA Model Compounds

The γ-radiolysis of ethylene glycol, meso-erythritol, 2-deoxy-ribose, and alkyl phosphates has been studied in diluted (10^?2 M), N₂ O-saturated, deoxygenated aqueous solutions. Products and their G values have been determined. Reaction schemes were derived from complete material balances. In the polyols the primary attack of the radiolytically formed OH radicals and H atoms leads to α, β-dihydroxyalkyl radicals (A) which eliminate water to give substituted α-carbonyl-methyl radicals: (B) Disproportionation reactions of the radicals A and B yield products having the structural units ? CO? CHOH? and ? CO? CH₂ —. In ethylene glycol a chain reaction is induced, radical B abstracts a hydrogen from ethylene glycol to give acetaldehyde and radical A. In 2-deoxy-ribose the major attack is at C-1. Products from this radical are 2-deoxy-ribonic acid and 2,5-dideoxy-ribonic acid. In the formation of the latter a rearrangement is involved. With trimethyl phosphate, the reaction of the solvated electron is only small (2 × 10^s 1/mole sec), as shown by pulse radiolysis. Dimethyl and methyl phosphates do not react with the solvated electron to a measurable extent. The attack of the OH radical apparently leads to a cleavage of the alkyl phosphate linkage.     

Osmium(VIII) catalyzed oxidation of a sulfur containing amino acid—a kinetics and mechanistic approach

The kinetics of osmium (VIII) catalyzed oxidation of DL-methionine by hexacyanoferrate(III) (HCF) in aqueous alkaline medium at a constant ionic strength of 0.50 mol dm^?3 was studied spectrophoto-metrically. The reaction between hexacyanoferrate(III) and DL-methionine in alkaline medium exhibits 2:1 stoichiometry (2HCF:DL-methionine). The reaction is of first order each in [HCF] and [Os(VIII)], less than unit order in [alkali] and zero order for [DL-methionine]. The decrease in dielectric constant of the medium increases the rate of the reaction. The added products have no effect on the rate of reaction. The main products were identified by spot test. A free radical mechanism has been proposed. In a prior equilibrium step Os(VIII) binds to OH^? species to form a hydroxide species and reacts with [Fe(CN)₆]^3? in slow step to form an intermediate species(C₁). This reacts with a molecule of DL-methionine in a fast step to give the sulfur radical cation of methionine and yields the sulfoxide product by reacting with another molecule of [Fe(CN)₆]^3?. The rate constant of the slow step of the mechanism is calculated. The activation parameters with respect to slow step of the mechanism are evaluated and discussed.     

A Novel Process for Ultrasound‐Induced Radical Polymerization in CO2‐Expanded Fluids

Summary: A strong viscosity increase upon polymerization hinders cavitation and subsequent radical formation during an ultrasound‐induced bulk polymerization. In this work, ultrasound‐induced radical polymerizations of methyl methacrylate (MMA) have been performed in CO₂‐expanded MMA in order to reduce the viscosity of the reaction mixture. For this purpose, the phase behavior of CO₂/MMA systems has been determined. With temperature oscillation calorimetry, the influence of CO₂ on the viscosity and on the reaction kinetics of ultrasound‐induced polymerizations of MMA has been studied. In contrast to polymerizations in bulk, this technique shows that a low viscosity is maintained during polymerization reactions in CO₂‐expanded MMA. As a consequence, a constant or even increasing polymerization rate is observed when pressurized CO₂ is applied. Moreover, the ultrasound‐induced polymer scission in CO₂‐expanded MMA is demonstrated, which appears to be a highly controlled process. Finally, a preliminary sustainable process design is presented for the production of 10 kg/h pure PMMA (specialty product) in CO₂‐expanded MMA by ultrasound‐induced initiation.

Process flow diagram of the ultrasound‐induced polymerization of MMA in CO₂‐expanded MMA.     

Kinetics of absorption of CO2 into buffer solutions containing carbonic anhydrase

The rate of absorption of pure CO₂ into equimolar phosphate (pH = 6.6 and 1 1) and carbonate (pH ≈ 9.6) buffers in the presence of crude carbonic anhydrase has been measured in a stirred cell at a temperature range of 5-35°C and at atmospheric pressure. Some experiments with carbonate buffer were also repeated in a wetted wall column.Experimental results were analysed using classical gas absorption with chemical reaction theory in order to extract information about the kinetics of the enzymic hydration of CO₂ and the catalytic power of carbonic anhydrase. At high pH-values (9.6-11.1) this reaction is first order in CO₂ the reaction rate constant being proportional to enzyme concentration with a rate constant of about 0.90 1/mg sec at 25°C and with an activation energy of 9.0 kcal/g mole.At low pH-values (pH ≈ 6.5-6.7) the catalytic power of the enzyme is considerably reduced and the results are not compatible with a simple first order reaction mechanism.     

Synthesis of β-SiAlON: A combined method using sol–gel and SHS processes

β-SiAlON has been prepared by a SHS reaction performed under high nitrogen pressure on a compacted mixture of silicon and mullite. In order to optimise this reaction, the solid mixture was prepared from a liquid solution in which the mullite (2SiO₂-3Al₂O₃) was formed by a sol–gel process. The SHS samples were characterized by using Scanning Electron Microscopy (SEM) and X-ray diffraction. A Rietveld refinement performed on the recorded diffractograms revealed a very high formation rate of β-SiAlON (more than 80 wt%) when the reaction was carried out under low nitrogen pressure. From the whole of our results, a mechanism of β-SiAlON formation has been suggested.     

EXTRACTION OF PALLADIUM(II) AND PLATINUM(IV) AS CHLOROCOMPLEX ACIDS INTO BASIC ORGANIC SOLVENTS

Solvent extraction of platinum(IV) and palladium(II) with tri-n-octylamine(TOA) in o-xylene from 4.0M (Na,H)(Cl, HSO₄) has been investigated at 298K in order to compare with similar extraction by trioctylphosphine oxide(TOPO). The dependence of the distribution ratio of platinum(IV) and pal 1adium(II) on TOA, hydrogen ion and chloride ion concentrations suggests the dominant extracted species of these metals would be 2:1 for proto-nated amine : chlorocomplex anion and the extraction should be essentially anion exchange reaction. The extraction constants (Kex = D[H⁺(T0A)C1 ] ²[C1 ]²), were evaluated as 10^6-3 and 10^5.8 for platinum(IV) and pal ladium(II), respectively. The anion exchange with protonated TOPO salt represented by H(T0P0)₂ ⁺C1  is less than that with protonated TOA salt by a factor of one order.     

Zur Polarographie von Peroxiden im Null-Volt-Bereich II: Oszillopolarogramme von Hydroperoxiden,Quecksilbersalzen und Hg-Ionen in Gegenwart von Diacylperoxiden

Polarography of Peroxides in Zero-Volt Range II: Oscillo-Polarogramms of Hydroperoxides, Mercury Salts and Hg-Ions in the Presence of Diacyl Peroxides Further experiments carried out in order to explain the occurrence of each of the two polarographic peak currents and current step-ups, A₁ and A₂ respectively, of peroxides in zero-volt range are reported. Occurrence of both the peak currents is probably due to the reduction of monovalent mercury ions. A part of the Hgions which were formed by the oxidation of the Hg-electrode due to the peroxides, is reduced at A₁, and the other part is chemically oxidized by the peroxides further to Hg²⁺ions. The latter can, in a similar fashion, give rise to Hgions by reaction with the Hg electrode. The Hgions are probably reduced at A₂, the reduction being influenced by the nature of binding of the Hg-ions and the catalytic action of chlorides.