The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. IV. Fixation of CCA to wood

The kinetic behavior of copper–chromium–arsenic (CCA) wood preservatives in their reaction with wood, lignin, cellulose, and their respective simple model compounds, guaiacol and D(+)-glucose, has been elucidated. Copper chromates^VI/lignin guaiacyl units and chromium^III arsenates/lignin guaiacyl units, which are stable and insoluble complexes formed during the reaction, have been characterized. No copper arsenates were formed during the reactions (which were carried out using a CCA of type C). The sequence, nature, and combined effect of the reactions of CCA with wood, lignin, and cellulose are similar to that already described for the “CrO₃ alone,”¹ Cu/Cr², and Cr/As³ systems. Rate constants, energies of activation, and Arrhenius equations have been obtained and reported. The distribution of Cr^VI, Cr^III, As^v, and Cu²⁺ in CCA-C-treated wood has been calculated. The results obtained have allowed meaningful comparisons of several commercial CCA formulations.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. I. Fixation of chromium on wood

The kinetic behavior of CrO₃ in its reaction with wood has been elucidated. Various reactions take place between CrO₃ and the lignin and cellulose in wood. CrO₃ reacts with cellulose in a two-step reaction: the first step is an adsorption of Cr^VI onto the cellulose to form Cr^VI/cellulose activated complexes. The second step is a Cr^VI → Cr^III reduction taking place on the cellulose surface. The Cr^III formed is only physically adsorbed to the cellulose or very weakly bound as small amounts of Cr^III can be released into the reaction medium. The Cr^VI adsorbed by cellulose appears mainly to be reduced to Cr^III. The reaction of Cr^VI with lignin has been shown to be the composition of the three successive reaction of Cr₂O₇^2?, HCrO₄^?, and CrO₄^2? with the guaiacyl units of lignin. Insoluble and stable Cr^VI/lignin complexes in which chromium maintains its hexavalent oxidation state are formed. Rate constants and energies of activation for all the reactions have been determined. The fixation of CrO₃-derived compounds on wood has been explained as the combination of the various reactions investigated. The results indicate that 60% of Cr is fixed irreversibly to the lignin of wood as Cr^VI and 40% is weakly bound, probably just precipitated, on the cellulose surface as Cr^III of which small amounts can be released in a water medium. The complex Cr^VI and Cr^III species forming complexes with the guaiacyl units have been identified.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. III. Fixation of a Cr/As system on wood

The kinetic behavior of a Cr/As system in its reaction with wood is described, as are its reactions with lignin and carbohydrates of wood for Cr^VI alone. Chromium^III arsenate complexes with the guaiacyl units of lignin, as well as the usual chromic acid/guaiacyl unit complexes already characterized, was formed. The interference of As concentration on the various zones of fixation of chromium are identified. Complexes between guaiacol and chromium arsenates indicate that in this Cr/As system Cr^III can also be linked to the guaiacyl units of lignin when in the form of CrAsO₄·6H₂O contrary to what obtained with systems not containing As. An initial increase in the rate of reduction Cr^VI → Cr^III by the carbohydrates fraction in wood is ascribed to the catalytic effect of the arsenic. The percentages of chromium fixed onto lignin and cellulose of wood compare well with experimental values.     

Ca10(CrVO4)6(CrVIO4), a disordered mixed‐valence chromium compound exhibiting inversion twinning

The structure analysis of so‐called 9CaO·4CrO₃·Cr₂O₃ proved it to be the title compound, decacalcium hexakis[chromate(V)] chromate(VI), with the simultaneous presence of unusual chromium oxidation states. The structure determination was carried out on a crystal that had inversion twinning. The Cr^VIO₄ tetrahedron is situated on a threefold axis and is disordered over two possible orientations that share three O atoms, while the Cr^VO₄ tetrahedra are in general positions and are ordered. The charge is balanced by Ca²⁺ cations, one of which is located on a threefold axis. The Ca²⁺ ions are coordinated by six, seven or eight O atoms. The compound is a significant phase in the CaO–CrO_x system and its formation reduces the refractoriness of calcium‐rich compositions in an oxidizing atmosphere.     

CrOx/SiO2催化剂上丙烷在CO2气氛中脱氢反应的研究

采用XRD、UV-vis DRS、ESR和微分吸附量热等技术,考察了铬担载量分别为2.5、5和10wt%的CrO_x/SiO₂催化剂的结构、表面性质和氧化还原性能。结果表明,催化剂表面上存在多种Cr的氧化态和聚集形式。随着Cr担载量从2.5wt%到10wt%的逐渐增大,催化剂表面占主导地位的Cr物种由CrO₃单体转为多聚CrO₃和Cr₂O₃晶相。在CO₂气氛中催化剂对丙烷转化率和丙烯选择性的大小顺序为2.5wt%CrO_x/SiO₂>5wt%CrO_x/SiO₂>10wt%CrO_x/SiO₂,反应过程中的原位ESR和UV-visDRS测定结果表明,催化剂表面的反应活性中心为Cr⁵⁺,Cr⁵⁺可由催化剂预处理过程中Cr³⁺的氧化及丙烷反应过程中CrO₃单体的还原产生,在反应中CO₂可使Cr³⁺重新氧化为Cr⁵⁺.     

Investigation of the chemical modifications of beech wood lignin during heat treatment

Holocellulose, Klason lignin and milled wood lignin (MWL) of beech wood were extracted before and after heat treatment and analysed using CP MAS ¹³C NMR, ¹³C NMR, ³¹P NMR and size exclusion chromatography (SEC). Experimental results showed that the thermal treatment degrades hemicelluloses and affects lignin polymer through depolymerisation due mainly to cleavage of β-aryl-ether linkages and recondensation reactions. The spectroscopic analysis of MWL demonstrated that these recondensation reactions involved mainly guaiacyl units through formation of 5,5′-biphenolic and diarylmethane structures.Analysis of molecular weight distribution of MWL by SEC indicated that average molecular weights of heat treated milled wood lignin were lower than those of native milled wood lignin.     

Étude spectrophotométrique UV. de solutions diluées de chromate dans l'acide sulfurique concentré

Dilute solutions ( < 1· 10^?4M ) of CrO₄K₂ in concentrated sulfuric acid, were investigated spectrophotometrically. Spectral, kinetic and equilibrium studies confirm the instability of Cr^VI species and their conversion to Cr^V complexes, as proposed by MISHRA & SYMONS . Reaction mechanisms are proposed.     

Trace metal-ion catalysis: kinetics and mechanism of oxidation of l-ascorbic acid by chromium(VI) in phosphate buffers

Summary The kinetics of oxidation of l-ascorbic acid (H₂A) by Cr^VI with and without added Cu^II conforms to the stoichiometry represented by the equation: 2Cr^VI + 3H₂A 2Cr^III + 3A + 6H⁺ where A is dehydroascorbic acid. The mode of the electron transfer from H₂A to Cr^VI is suggested to involve oxidative decomposition of an intermediate complex. Catalysis by Cu^II is indicated via complexation of the catalyst and substrate. The inhibitory effects of Cl^–, NO _f3 ^p– and SO _f4 ^p2– ions indirectly support complexation between Cr^VI and H₂A.     

Reactions anodiques et cathodiques des composes de Cr(VI) en milieu de nitrates alcalins fondus

The electrochemical behavior of chromate and dichromate, ions in molten NaNO₃-KNO₃ at 250°C has been studied by single-sweep voltammetry and cyclic voltammetry. The anodic dimerization of CrO₄^2- ions has been shown to proceed in three steps, the reaction: CrO₄^2-→CrO_4ad^?+e^? being rate-determining. The cathodic reduction of Cr₂O₇^2- ions includes the coupled acid-base reactions; it forms solid chromium oxide Cr₂O₃ and the soluble chromate and chromite ions.     

Removal of chromate from aqueous solution by reduction with nanoscale Fe–Al layered double hydroxide

Nanoscale layered double hydroxides of Fe^II and Al^III (Fe–Al LDH) have been applied for removal of chromate (Cr^VI) from aqueous solution. Given the reaction stoichiometry, Cr^VI was completely reduced to Cr^III and coprecipitated with Fe^III and Al^III oxyhydroxides. The extent of Cr^VI removal decreased with increasing initial pH and decreasing molar ratio of Cr^VI/structural Fe^II in the LDH. The chromate reduction rate at different initial concentrations of Cr^VI was well described by the pseudo-second-order model with reaction rate constant ranging from 197.4 to 13.53 (mmol min)^?1. Initial pH and substitution of various amounts of Fe^III in the LDH structure had little effect on the reaction rate. Backtransformation of Cr^III to Cr^VI by birnessite Mn oxide (δ-MnO₂) after 40 days of reaction was less than 1% of the initial Cr (as Cr^III solid), indicating high stability of the final reaction products and high efficiency of nanoscale Fe–Al LDHs for removal of chromate from aqueous solution.     

Formation of Cr(II) Species in the H2/CrO3 System. Parameter control

The thermal behaviour of CrO₃ on heating up to 600°C in dynamic atmospheres of air, N₂ and H₂ was examined by thermogravimetry (TG), differential thermal analysis (DTA), IR spectroscopy and diffuse reflectance spectroscopy (DRS). The results revealed three major thermal events, depending to different extents on the surrounding atmosphere: (i) melting of CrO₃ near 215°C (independent of the atmosphere), (ii) decomposition into Cr₂(CrO₄)₃ at 340–360°C (insignificantly dependent), and (iii) decomposition of the chromate into Cr₂O₃ at 415–490°C (significantly dependent). The decomposition CrO₃ → Cr₂(CrO₄)₃ is largely thermal and involves exothermic deoxygenation and polymerization reactions, whereas the decomposition Cr₂(CrO₄)₃ → Cr₂O₃ involves endothermic reductive deoxygenation reactions in air (or N₂) which are greatly accelerated and rendered exothermic in the presence of H₂. TG measurements as a function of heating rate (2–50°C min⁻¹) demonstrated the acceleratory role of H₂, which extended to the formation of Cr(II) species. This could sustain a mechanism whereby H₂ molecules are considered to chemisorb dissociatively, and then spillover to induce the reduction. DTA measurements as a function of the heating rate (2–50°C min⁻¹) helped in the derivation of non-isothermal kinetic parameters strongly supportive of the mechanism envisaged. This revised version was published online in August 2006 with corrections to the Cover Date.     

The detection of preservatives in wood with pyrocatechol violet

The detection ot As⁺⁵, BO₃^-3. Cu⁺², Cr⁺³, Cr⁺⁶, Zn⁺², and F^- in treated wood with Pyrocatechol Violet is described.Es wird eine Method vorgeschlagen zum Nachweis von As⁺⁵, BO₃^-3, Cu⁺², Cr⁺³, Cr⁺³, Zn⁺² and F^- in imprägniertem Holz unter Verwendung von Pyrocatechol Violet.     

The influence of addition of silver and copper on the reducibility of CrAl<Subscript>3</Subscript>O<Subscript>6</Subscript> system

The influence of addition of silver and copper on the reduction behavior of binary oxide CrAl₃O₆ was investigated. The formation of copper chromite CuCr₂O₄ and silver chromate Ag₂CrO₄ during calcinations process was observed. The intermediate phase CrO was detected when copper and silver-copper systems were reduced at temperatures above 500°C. This intermediate is formed from Cr₂O₃, which is yielded by the initial reduction of Cr₂O₄²⁻ species.     

Complexes of a tridentate ONS Schiff base. Synthesis and biological properties

Several new complexes of a tridentate ONS Schiff base derived from the condensation of S-benzyldithiocarbazate with salicylaldehyde have been characterised by elemental analyses, molar conductivity measurements and by i.r. and electronic spectra. The Schiff base (HONSH) behaves as a dinegatively charged ligand coordinating through the thiolo sulphur, the azomethine nitrogen and the hydroxyl oxygen. It forms mono-ligand complexes: [M(ONS)X], [M=Ni^II, Cu^II, Cr^III, Sb^III, Zn^II, Zr^IV or U^VI with X = H₂O, Cl]. The ligand produced a bis-chelated complex of composition [Th(ONS)₂] with Th^IV. Square-planar structures are proposed for the Ni^II and Cu^II complexes. Antimicrobial tests indicate that the Schiff base and five of the metal complexes of Cu^II, Ni^II, U^VI, Zn^II and Sb^III are strongly active against bacteria. Ni^II and Sb^III complexes were the most effective against Pseudomonas aeruginosa (gram negative), while the Cu^II complex proved to be best against Bacillus cereus (gram positive bacteria). Antifungal activities were also noted with the Schiff base and the U^VI complex. These compounds showed positive results against Candida albicans fungi, however, none of them were effective against Aspergillus ochraceous fungi. The Schiff base and its zinc and antimony complexes are strongly active against leukemic cells (CD₅₀ = 2.3–4.3 μg cm⁻³) while the copper, uranium and thorium complexes are moderately active (CD₅₀ = 6.9–9.5 μg cm⁻³). The nickel, zirconium and chromium complexes were found to be inactive. This revised version was published online in June 2006 with corrections to the Cover Date.     

MgSO4·11H2O and MgCrO4·11H2O based on time‐of‐flight neutron single‐crystal Laue data

Hexaaquamagnesium(II) sulfate pentahydrate, [Mg(H₂O)₆]SO₄·5H₂O, and hexaaquamagnesium(II) chromate(II) pentahydrate, [Mg(H₂O)₆][CrO₄]·5H₂O, are isomorphous, being composed of hexaaquamagnesium(II) octahedra, [Mg(H₂O)₆]²⁺, and sulfate (chromate) tetrahedral oxyanions, SO₄²⁻ (CrO₄²⁻), linked by hydrogen bonds. There are two symmetry‐inequivalent centrosymmetric octahedra: M1 at (0, 0, 0) donates hydrogen bonds directly to the tetrahedral oxyanion, T1, at (0.405, 0.320, 0.201), whereas the M2 octahedron at (0, 0, ) is linked to the oxyanion via five interstitial water molecules. Substitution of Cr^VI for S^VI leads to a substantial expansion of T1, since the Cr—O bond is approximately 12% longer than the S—O bond. This expansion is propagated through the hydrogen‐bonded framework to produce a 3.3% increase in unit‐cell volume; the greatest part of this chemically induced strain is manifested along the b* direction. The hydrogen bonds in the chromate compound mitigate ∼20% of the expected strain due to the larger oxyanion, becoming shorter (i.e. stronger) and more linear than in the sulfate analogue. The bifurcated hydrogen bond donated by one of the interstitial water molecules is significantly more symmetrical in the chromate analogue.     

Zum Chemischen Transport von Cr2O3 mit Cl2 und mit HgCl2 ? Experimente und Modellrechungen

On the Chemical Transport of Cr₂O₃ with Cl₂ and with HgCl₂ — Experiments and Model Calculations The migration of Cr₂O₃ in a temperature gradient (1 000°C → 900°C) in the presence of low concentrations of chlorine and water (from the wall of silica ampoules) is a result from the endothermic reactions (1) Cr₂O_3,s + H₂O_g + 3 Cl_2,g = 2 CrO₂Cl_2,g + 2 HCl_g (2) Cr₂O_3,s + 1/2 O_2,g + 2 Cl_2,g = 2 CrO₂Cl_2,g With higher concentrations of chlorine, the transport reaction is (3) Cr₂O_3,s + 5/2 Cl_2,g = 3/2 CrO₂Cl_2,g + 1/2 CrCl_4,g The gas phase of the transport system Cr₂O₃/Cl₂ can be reduced step by step by adding small amounts of chromium, so that CrCl₃ and finally also CrCl₂ become more important. Further, at a lower ratio n°(Cl)/n°(Cr) three transport reactions have to be taken into consideration; with the participation of CrOCl_2,g (5). (4) Cr₂O_3,s + 9/2 CrCl_4,g = 3/2 CrO₂Cl_2,g + 5 CrCl_3,g (5) Cr₂O_3,s + 3 CrCl_4,g = 3 CrOCl_2,g + 2 CrCl_3,g (6) Cr₂O_3,s + H_2,g + 4 HCl_g = 2 CrCl_2,g + 3 H₂O_g The reactions (1), (2) and (6) become possible through the cooperation of two transport agents at a time. The migration of Cr₂O₃ with HgCl₂ can also be described with reactions (1) – (3). The decomposition of HgCl₂ Produces the small chlorine pressure for the transport reaction. The oxidation potential of the transport agent HgCl₂ is too low for the oxidation of Cr^III to Cr^VI.     

Kinetics and mechanism of the chromic oxidation of 3-O-methyl-d-glucopyranose

The oxidation of 3-O-methyl-d-glucopyranose (Glc3Me) by Cr^VI in acid medium yields Cr^III, formic acid and 2-O-methyl-d-arabinose as final products when a 50-times or higher excess of Glc3Me over Cr^VI is used. The redox reaction takes place through the combination of Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III pathways. Intermediacy of free radicals and Cr^II in the reaction was demonstrated by the observation of induced polymerization of acrylamide and detection of CrO₂²⁺ formed by reaction of Cr^II with O₂. Intermediate oxo-Cr^V–Glc3Me species were detected by EPR spectroscopy. In 0.3–0.5 mol/L HClO₄, intermediate Cr^V rapidly decompose to the reaction products, while, at pH 5.5–7.5, where the redox processes are very slow, five-coordinate Cr^V bis-chelates of the pyranose and furanose forms of Glc3Me remain more than 15 h in solution. The C1–C2 bond cleavage of Glc3Me upon reaction with Cr^VI distinguishes this derivative from glucose, which is oxidized to gluconic acid.     

Magnetic behavior of tetrahedral Cr4+ compounds: Sr2CrO4, Ba2CrO4, and Ba3CrO5

The preparation of the compounds Sr₂CrO₄, Ba₂CrO₄, and Ba₃CrO₅ is described. The characterization of these three Cr⁴⁺ compounds by X-ray and magnetic susceptibility experiments has been conducted. The magnetic moments for Sr₂CrO₄, Ba₂CrO₄, and Ba₃CrO₅ were determined to be in good agreement with the calculated value expected for a tetrahedral Cr⁴⁺ ion. Weak antiferromagnetic ordering for all three compounds is indicated from the small paramagnetic Weiss constants determined from the susceptibility data in the temperature region 80–300 K. Distortions of the tetrahedra from ideality, as determined from the structural features, further cause a reduction in the magnetic moments from the theoretical values.     

Synthesis,Properties, and Structures of Chromium(VI) and Chromium(V) Complexes with Heterocyclic Nitrogen Ligands

The reaction of CrO₂Cl₂ with 2, 2′‐bipyridyl or 1, 10‐phenanthroline (diimine) in CCl₄ or anhydrous CH₃CO₂H solution, produces orange‐brown diamagnetic [CrO₂Cl₂(diimine)]. The X‐ray structure of [CrO₂Cl₂(2, 2′‐bipy)] shows a six‐coordinate central chromium(VI) atom with cis‐dioxo groups trans to the diimine. In contrast, the diimines react with CrO₃ in CH₃CO₂H / conc. aqueous HCl to form bright red paramagnetic Cr^V complexes, [CrOCl₃(diimine)]. The X‐ray structure of [CrOCl₃(2, 2′‐bipy)] shows a six‐coordinate central chromium atom with mer‐chlorines and the diimine trans to O/Cl. The addition of [2, 2‐bipyH₂]Cl₂ to a solution of CrO₃ in CH₃CO₂H saturated with HCl gas, produces the Cr^V anion [2, 2′‐bipyH₂][CrOCl₄]Cl, which loses HCl on heating in vacuo to form [CrOCl₃(2, 2′‐bipy)]. IR, UV/Vis, and ¹H NMR spectra (Cr^VI only) are reported for the new complexes. Attempts to extend these routes to oxygen donor ligands, including ethers and phosphine oxides, were unsuccessful. The diimine complexes are the first structurally autheticated adducts of chromium(VI) and (V) oxide‐chlorides with neutral ligands.     

Oligo and polyuronic acids interactions with hypervalent chromium

Selective oxidation of galacturonic residues of oligo and polyuronic acids by Cr^VI affords CO₂/HCO₂H, oxidized uronic acid, and Cr^III as final redox products. Kinetic studies show that the redox reaction proceeds through a mechanism combining Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III pathways. The mechanism is supported by the observation of free radicals, CrO₂²⁺ and Cr^V as reaction intermediates. The EPR spectra show that five- and six coordinated oxo-Cr^V intermediates are formed. Penta-coordinated oxo-Cr^V species are present at any [H⁺], whereas hexa-coordinated ones are only observed at pH <1. At low pH Cr^V predominating species are coordinated by carboxylate groups and O^ring (g_iso = 1.9783/5). At pH 7.5, the predominating ones are those coordinated by alcoholate groups of the ligand (g_iso = 1.9800). Polygal can reduce Cr^VI and efficiently trap Cr^III. This behaviour represents an interesting model for the study of biomaterials, which possess a high proportion of polygal, in order to remove chromium from polluted waters.