Model study of pyrite demineralization by hydrogen peroxide oxidation at 30 °C in the presence of metal ions (Ni2+, Co2+ and Sn2+)

Dissolution of pyrite involving oxidation by hydrogen peroxide (H₂O₂) in the presence of metal ions (Ni²⁺, Co²⁺ and Sn²⁺) has been investigated. Before oxidation, pure and well crystalline structure of the acid washed pyrite sample, used in the present investigation, was confirmed by X-ray diffraction and chemical analysis. Oxidation of pyrite was examined by the determination of soluble sulfur. The rate of oxidation of pyrite with H₂O₂ is best represented by determining the rates of total soluble sulfur production. Each experiment was carried out for short (1–4 h) and extended (24 h) time periods. Pyrite is oxidized by H₂O₂ (1:1) up to the extent of 31.3% at short time period, which however remained the same even at extended time period. Increased amount of soluble sulfur has been observed when pyrite was oxidized by H₂O₂ (1:1) in the presence of Ni²⁺ or Co²⁺ or Sn²⁺ ion at short time period. The effectiveness of these metal ions in relation to pyrite oxidation at short time period decreases in the order Co²⁺>Sn²⁺>Ni²⁺, while at extended time period the order is Co²⁺>Ni²⁺>Sn²⁺. With Co²⁺ ion, the highest pyrite oxidation is obtained both at short (34.0%) and extended (35.0%) time period, while it is the lowest 31.3% with Ni²⁺ ion at short time and 25.3% with Sn²⁺ ion at extended time period. The effect of chloride ion on the rate of oxidation of pyrite is not pronounced in the metal ion containing systems. Substantial depletion in the concentration of externally added metal ions is in good agreement with the level of oxidation and infers certain adsorption or precipitation of metal ions on pyrite surface. The results of this study throw a new light of the influence of metal ions in the dissolution of pyrite in oxidation systems and has considerable applications in fields of demineralization, desulfurization and environmental science.     

Catalytic effects of NaOH and ZrO2 for partial oxidative gasification of n-hexadecane and lignin in supercritical water☆

Partial oxidative gasification of n-hexadecane (n-C₁₆) and organosolv-lignin (lignin) was studied by use of a batch type reactor in supercritical water: 673 K, 0.52 cm⁻³ of water density (40 MPa of water pressure at 673 K), and 0.3 of O/C ratio for the n-C₁₆ experiments; 673 K, 0.35 cm⁻³ of water density (30 MPa of water pressure at 673 K), and 1.0 of O/C ratio for the lignin experiments. The experiments without O₂ were also conducted for lignin (lignin decomposition). For all the cases (n-C₁₆ partial oxidation, lignin decomposition, lignin partial oxidation), NaOH or zirconia (ZrO₂) was added in the system as catalysts. Through n-C₁₆ studies, the catalytic effect of NaOH and ZrO₂ on partial oxidation in supercritical water were examined. In the case of lignin partial oxidation, we studied the possibility of partial oxidation in supercritical water for gasification technique of wastes. The yield of H₂ from n-C₁₆ and lignin with zirconia was twice as same as that without catalyst at the same condition. The H₂ yield with NaOH was 4 times higher than that without catalyst. Thus, a base catalyst has a positive effect on partial oxidation of n-C₁₆ and lignin to produce H₂. The catalytic effect of NaOH and ZrO₂ was found to be enhancement of decomposition of intermediate (aldehyde and ketone) into CO, through n-C₁₆ studies. In the case of lignin studies, the enhancement of decomposition of the carbonyl compounds by catalytic effect of NaOH and ZrO₂ inhibit char formation and promotes CO and thus H₂ formation.     

THE INFLUENCE OF THE BLEACHING MEDIUM ON CHLORINE DIOXIDE DELIGNIFICATION

Chlorine dioxide delignification (D₀) stages performed in 90% ethanol resulted in post-D₀ kappa numbers that were 2 to 2.5 units lower than those performed in water. In spite of the reduction in kappa number, various gravimetric and UV spectroscopic techniques for measuring residual lignin indicated that the ethanol medium did not significantly enhance D₀ stage lignin removal. Differences in post-D₀ kappa numbers between the ethanol-based and aqueous systems were ascribed to how the bleaching medium affects ClO₂ oxidation of residual lignin. Ethanol-based D₀ stages resulted in an oxidized lignin that contains fewer muconic acid structures and more quinone structures than the aqueous-based systems. Indirect evidence from sodium hydrosulfite reduction implied that quinone moieties consume less KMnO₄ than aromatic structures in lignin during the kappa number test.     

Phenol Formaldehyde Resin Modification with Lignin

Abstract

Lignin (poly‐phenylpropane units) was precipitated from waste black liquor produced by paper manufacture from rice straw. The use of this lignin as partial substitution of phenol in phenol formaldehyde resin is investigated. The effect of different variables [e.g., lignin concentration, ratio of formaldehyde to phenol lignin (phenol + lignin), temperature, and polymerization time on the properties of the produced resin is studied]. The effect of the chemical modification of lignin by oxidation, reduction, and hydrolysis on the properties of the produced phenol lignin formaldehyde resin is also studied. Hydrolysis of lignin by hydrochloric acid increases the reactivity between lignin, formaldehyde, and phenol in producing resin with good properties more than untreated lignin. Also, the percent of replaced phenol with lignin in the resin can be increased by hydrolysis of lignin with hydrochloric acid. So, the reactivity of lignin toward resin formation has the following sequence: hydrolyzed lignin > oxidized lignin with dichromate > treated lignin with sodium hydrosulfite.     

The electroactive species in the catalysis of cyclohexadiene to benzene by Rh2(TM4) 4 +2 (TM4=2,5-diisocyano-2,5-dimethylhexane)

Upon further investigation of the recently reported electrocatalytic oxidation of 1,4-cyclohexadiene to benzene by Rh₂(TM4) ₄ ⁺² (TM4=2,5-diisocyano-2,5-dimethylhexane), we have obtained data which strongly implicates the 2e^– oxidized d⁷-d⁷ complex as the electroactive species. This contrasts with the original report which suggested that the le^– oxidized d⁷-d⁸ radical acted as the key species via hydrogen atom abstraction from 1,4-cyclohexadiene. A possible mechanism for the catalysis is proposed.     

Primary reaction steps and active surface sites in the rhodium-catalyzed partial oxidation of methane to CO and H2

The nature of surface sites responsible for methane activation and CO_x formation on Rh catalysts for the partial oxidation of methane to syngas was investigated. The interaction of H₄ with Rh-black after oxidative and reductive pretreatments was studied applying (a) pulse experiments at reduced total pressure (10^–4 Pa) and 1013 K in the temporal-analysis-of-product (TAP) reactor and (b) in situ DRIFTS at 973 K. The saturation of the metal surface sites with oxygen was found to inhibit methane dissociation. Direct methane oxidation to CO₂ on the oxidized surface sites proposed earlier was excluded. Methane is first dissociated on reduced surface sites; the carbon species formed, then, react with surface oxygen to CO₂. Rh sites responsible for methane activation are neither related to the formation of the Rh₂O₃ nor Rh⁰. Probably the partially oxidized species (Rh⁺) or highly dispersed Rh³⁺ entities act as active surface centers for the dissociation of methane. For supported catalyst, such sites are stabilized by the support, which on the other side acts as a source of active oxygen involved in the oxidation of surface carbon and hydrogen.     

Catalytic organosolv fractionation of willow wood and wheat straw as pretreatment for enzymatic cellulose hydrolysis

BACKGROUND: Ethanol‐based organosolv fractionation of lignocellulosic biomass is an effective pretreatment technology for enzymatic cellulose hydrolysis to produce sugars and lignin within a biorefinery. This study focuses on the catalytic effect of H₂SO₄, HCl, and MgCl₂ on organosolv pretreatment of willow wood and wheat straw. RESULTS: The use of catalysts improved fractionation of both feedstocks. The maximum enzymatic cellulose digestibility obtained was 87% for willow wood (using 0.01 mol L^?1 H₂SO₄ as catalyst) and 99% for wheat straw (0.02 mol L^?1 HCl). Non‐catalytic organosolv fractionation at identical conditions resulted in 74% (willow wood) and 44% (wheat straw) glucose yield by enzymatic hydrolysis. Application of catalysts in organosolv pretreatment was particularly effective for wheat straw. The influence of the acid catalysts was found to be primarily due to their effect on the pH of the organosolv liquor. Acid catalysts particularly promoted xylan hydrolysis. MgCl₂ was less effective than the acid catalysts, but it seemed to more selectively improve delignification of willow wood. CONCLUSION: Application of catalysts in organosolv pretreatment of willow wood and wheat straw was found to substantially improve fractionation and enzymatic digestibility. The use of catalysts can contribute to achieving maximum utilization of lignocellulosic biomass in organosolv‐based biorefineries. Copyright © 2011 Society of Chemical Industry     

Structural characterization of residual lignins isolated with cyanamide-activated hydrogen peroxide from various organosolvs pretreated wheat straw

The posttreatment of various organosolvs pretreated wheat straw with cyanamide-activated hydrogen peroxide was studied. About 44–80% of the total residual lignin and 38–85% of the total residual hemicelluloses were released or degraded during the posttreatment with 1.8% H₂O₂–0.18% cyanamide at 50°C under pH 10.0 for 4 h from different aqueous organic acids or alcohols pretreated straw. The seven degraded residual lignin preparations were subjected to a comprehensive physicochemical and structural characterization by UV, FTIR, and ¹H and ¹³NMR spectroscopy, and GPC. The nitrobenzene oxidation method was also applied to the in situ lignins. It was found that the seven residual lignin preparations contained large amounts of noncondensed syringyl and guaiacyl units, together with fewer noncondensed p-hydroxyphenyl units, esterified p-coumaric acid, and mainly etherified ferulic acid. All of the lignin fractions are free of associated polysaccharides and had molecular-average weights ranging between 2980 and 3820 g mol⁻¹. Analysis of these low molecular weight degradation products revealed an oxidation of residual lignin had occurred. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Formation mechanism of Cu2O particles using layered CaSi2 as a reduction/oxidation mediator

We demonstrate that Cu₂O particles can be produced along with siloxene formation by simply dispersing layered CaSi₂ into an aqueous solution of CuCl₂ and HCl at room temperature. The Cl⁻ ions induce oxidative extraction of Ca from CaSi₂ to form siloxene and trigger the reductive deposition of Cu particles. All particles are then gradually oxidized to form Cu₂O particles under optimized conditions as follows. A trace amount of residual CaSi₂ is dissolved in the solution, which provides OH⁻ ions, and a portion of the formed Cu particles are dissolved as [Cu(OH)₄]²⁻ ions. Accordingly, Cu₂O particles would be formed through the comproportionation reaction between Cu and [Cu(OH)₄]²⁻ ions in the solution. However, under conditions with an excess amount of Cl⁻ ions results in further oxidation of Cu to also form Cu₂Cl(OH)₃. Thus, CaSi₂ acts as an effective reduction and/or oxidation mediator to tune the number of Cl⁻ and OH⁻ ions and control the oxidation state of Cu.     

Hydrothermal synthesis and characterization of surface-modified δ-MnO2 with high Fenton-like catalytic activity

Surface-modified δ-MnO₂ with a hierarchical structure was synthesized via a hydrothermal redox reaction between toluene and KMnO₄. Toluene is oxidized to produce benzoic acid which rapidly adsorbs onto the surface of δ-MnO₂. The δ-MnO₂ sample prepared with toluene exhibited a much higher catalytic activity for the Fenton-like oxidation of methylene blue in aqueous solution than that prepared without toluene. The enhanced catalytic activity might be attributed to the surface-bound persistent free radicals produced from the electron transfer from the adsorbed C₆H₅–COO⁻ to Mn(IV).     

Electrochemical synthesis of nanocrystalline CoFe2O4 thin films and their characterization

The electrochemical oxidation (anodization) of electrodeposited CoFe₂ alloy films in alkaline solution (1 N KOH) is depicted as a novel and powerful route for preparing spinel nanocrystalline CoFe₂O₄ thin films. CoFe₂ alloy films were electrodeposited from simple aqueous sulphate bath. Several key processing parameters were optimized to obtain good quality CoFe₂ alloy films with maximum thickness. The films resulting from electrochemical oxidation for 30 min at 5 mA/cm² current density possess high quality surface and well crystallized spinel cubic CoFe₂O₄ structure. The effects of air annealing of electrochemically oxidized alloy films on various properties were studied.     

Mechanism of GAPDH Redox Signaling by H2O2 Activation of a Two−Cysteine Switch

Oxidation of glyceraldehyde−3−phosphate dehydrogenase (GAPDH) by reactive oxygen species such as H₂O₂ activate pleiotropic signaling pathways is associated with pathophysiological cell fate decisions. Oxidized GAPDH binds chaperone proteins with translocation of the complex to the nucleus and mitochondria initiating autophagy and cellular apoptosis. In this study, we establish the mechanism by which H₂O₂−oxidized GAPDH subunits undergo a subunit conformational rearrangement. H₂O₂ oxidizes both the catalytic cysteine and a vicinal cysteine (four residues downstream) to their respective sulfenic acids. A ‘two−cysteine switch’ is activated, whereby the sulfenic acids irreversibly condense to an intrachain thiosulfinic ester resulting in a major metastable subunit conformational rearrangement. All four subunits of the homotetramer are uniformly and independently oxidized by H₂O₂, and the oxidized homotetramer is stabilized at low temperatures. Over time, subunits unfold forming disulfide−linked aggregates with the catalytic cysteine oxidized to a sulfinic acid, resulting from thiosulfinic ester hydrolysis via the highly reactive thiosulfonic ester intermediate. Molecular Dynamic Simulations provide additional mechanistic insights linking GAPDH subunit oxidation with generating a putative signaling conformer. The low−temperature stability of the H₂O₂−oxidized subunit conformer provides an operable framework to study mechanisms associated with gain−of−function activities of oxidized GAPDH to identify novel targets for the treatment of neurodegenerative diseases.     

Comparative study of lignins from ultrasonic irradiated sugar‐cane bagasse

After ultrasonic irradiation of sugar‐cane bagasse (SCB) in distilled water at 55 °C for 40 min and continuing stirring for 80 min, the irradiated SCB was sequentially treated with 0.5 M NaOH, 0.5 %, 1.0 %, 1.5 %, 2.0 % and 3.0 % H₂O₂ at pH 11.5, and 2 M NaOH at 55 °C for 2 h, which solubilized 3.3, 59.7, 15.5, 4.4, 5.5, 1.7, 0.6 and 0.6 % of the original lignin, and 13.4, 38.2, 11.9, 4.5, 9.0, 2.4, 1.2, and 11.3 % of the original hemicelluloses, respectively. In total, the sequential extractions resulted in a release of 91.2 % of the original lignin and 91.9 % of the original hemicelluloses, which were higher than in the case of corresponding extractions without ultrasound pre‐treatment. The eight lignin fractions were comparatively characterized by chemical composition, structural features, molecular weights, and thermal stability using alkaline nitrobenzene oxidation, acid hydrolysis, UV, FT‐IR, ¹H and ¹³CNMR spectroscopy, GPC, and thermal analysis. Compared to the lignins obtained without ultrasound, the lignin fractions prepared with the assistance of ultrasound had the same primary structure, composition, and similar or slightly higher purity. Copyright © 2004 Society of Chemical Industry     

Oxidized porous diamond/pyrocarbon nanocomposites as improved field electron emitters

Porous diamond/sp²-bonded carbon nanocomposites were studied for field electron emission properties and electroconductivity depending on composition and post-growth oxidation. It is found that the oxidation in nitric acid can sufficiently improve the field emission from the diamond samples. The oxidized composites exhibit lower emission threshold fields E_th=1–2 V/μm compared to E_th=4–6 V/μm for non-oxidized samples, and the emission uniformity (the number of emission sites per unit area) is also improved. More porous composites typically show better results. A possible reason of the emission improvement by oxidation in this two-phase carbon system is the lowering of the tunneling barrier due to formation of the dipole layers on the oxidized carbon surfaces.     

Oxidation of unsaturated and hydroxy fatty acids by ruthenium tetroxide and ruthenium oxyanions

The reactions of ruthenium VIII tetroxide (RuO₄) and the ruthenium VII and VI oxyanions, perruthenate (RuO₄ ⁻) and ruthenate (RuO₄ ⁼) with hydroxy substituted and unsaturated fatty acids have been studied. At a 1:1 molar ratio, ruthenium tetroxide (RuO₄) and both oxyanions (RuO₄ ⁻ and RuO₄ ⁼) oxidized 12-hydroxystearic acid to 12-ketostearic acid. With 9, 10-dihydroxystearic acid, the type of oxidation products obtained depended on the amount of ruthenium oxidant used. At high ratios of oxidant to substrate, cleavage to pelargonic and azelaic acids occurred whereas at lower ratios, partial oxidation to diketo and acyloin derivatives predominated. The oxidation of oleic acid with excess ruthenium tetroxide (RuO₄) or perruthenate anion (RuO₄ ⁻) gave the cleavage products pelargonic and azelaic acid through the intermediate formation of dihydroxy and diketo intermediates. Ruthenate anion (RuO₄ ⁼) did not react with the double bond of oleic acid. Agricultural Research Service, U.S. Department of Agriculture.     

Stimulus Responsive Poly(ferrocenylsilanes): Redox Chemistry of Iron in the Main Chain

Redox chemistry of organometallic poly(ferrocenylsilane) polymers (soluble in organic or aqueous environment) is discussed. Poly(ferrocenyldimethylsilane) (PFDMS) was oxidized in CH₂Cl₂ solution with different oxidants. Oxidation was accomplished with stepwise increasing amounts of ferric chloride (FeCl₃), iodine (I₂) and tris(4-bromophenyl)ammoniumyl hexachloroantimonate ( ). The process was followed by UV–visible spectroscopy. Mixed-valence salts with different Fe(III)/Fe(II) ratios were obtained. The oxidation products were characterized by Mössbauer spectroscopy, which proved to be a very valuable tool to analyze mixed valence ferrocenyl compounds. Mössbauer analysis demonstrated that 100% oxidation could be achieved with TBPA^{· +}. Decamethylferrocene (DMFC) was found to be a suitable reducing agent for the oxidized polymer. The reduction process was monitored by UV–visible spectroscopy. The polymer was characterized by gel permeation chromatography (GPC) following the oxidation/reduction cycle and evidence of some fragmentation of the chains was observed. A water-soluble PFS polycation was successfully oxidized with FeCl₃ for pH values between 4 and 5. Ascorbic acid (vitamin C) was found to completely reduce the oxidized water-soluble polymer.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

Redox reaction of tri(2-deoxy-2-L-ascorbyl)amine. Formation of a persisting free radical species in water

Electrochemical studies showed that tri(2-deoxy-2-L-ascorbyl)amine ($\text{1}$), a new compound isolated as one of the products of reaction of dehydro-L-ascorbic acid with phenylalanine (with added L-ascorbic acid) in ethanol, is oxidized in aqueous solution in two reversible one-electron transfer steps, on mercury or platinum electrode. The first step occurs through the dianion, and its product is an unusually stable blue anion radical giving a characteristic ESR signal. The product of the second step of oxidation is labile and is slowly converted into the oxidized form of di(2-deoxy-2-L-ascorbyl)amine, presumably by hydrolysis with splitting of L-ascorbic acid.     

N2O Decomposition Catalysed by Transition Metal Ions

The ignition processes for the catalytic partial oxidation of methane (POM) to synthesis gas over oxidic nickel catalyst (NiO/Al₂O₃), reduced nickel catalyst (Ni⁰/Al₂O₃), and Pt-promoted oxidic nickel catalyst (Pt–NiO/Al₂O₃) were studied by the temperature-programmed surface reaction (TPSR) technique. The complete oxidation of methane usually took place on the NiO catalyst during the CH₄/O₂ reaction, even with a pre-reduced nickel catalyst, and Ni⁰ is inevitably first oxidized to NiO if the temperature is below the ignition temperature. It is above a certain temperature that Ni⁰ is formed again, which leads to the start of the POM. The POM can be initiated at a much lower temperature on a Pt–NiO catalyst because of Pt promotion of the reduction of NiO. The POM in a fluidized bed can be easily initiated due to the addition of Pt.     

Oxidative Degradation of Thiaproline Derivatives in Aqueous Solutions Induced by •OH Radicals

The participation of neighboring amino and carboxyl groups in thiazolidyne-4-carboxylic acid (thiaproline) and its two derivatives, thiazolidyne-2-carboxylic acid and thiazolidyne-2,4-dicarboxylic acid, is demonstrated during ^.OH-radical-induced oxidation in aqueous solutions. The reaction of ^.OH radicals with these compounds does not lead to the expected one-electron oxidized sulfur-centered radical cations and subsequently to the intermolecularly (S∴S)-bonded dimeric radical cations (even at very high concentration of thiaprolines (>50 mM )), but rather to the elimination of CO₂ with the parallel formation of αN radicals. The latter radicals formed in thiaproline and thiazolidyne-2,4-dicarboxylic acid subsequently undergo β-fragmentation of the C S bond, leading to the thiyl radicals (RS^.). The αN and RS^. radicals were probed and quantified by one-electron reduction of p-nitroacetophenone (PNAP) and by one-electron oxidation of the monoanion of ascorbic acid (AH⁻), respectively. The calculated stabilization enthalpies of the αN radicals show that the loss of CO₂ is thermodynamically a favorable process. The Gibbs free energies of β-fragmentation processes in thiaproline and thiazolidyne-2,4-dicarboxylic acid are equal to −3.7 and −10.2 kJ mol⁻¹, respectively, showing that at room temperature both are exergonic. A general ^.OH-radical-induced oxidation mechanism of thiaproline derivatives in aqueous solutions is proposed.