Studies in chemically modified celluloses. VIII. Hypochlorite oxidation of cellulose in the presence of cuprous hydroxide

Cotton cellulose has been oxidized with aqueous sodium hypochlorite solution in the presence of cuprous hydroxide for a short time (10 min). The copper hydroxide content, the pH of the oxidizing medium, and the concentration of the oxidant were varied and their effect on the chemical and physical properties of cellulose has been studied. Cuprous hydroxide has been shown to possess intense catalytic activity in this oxidation. The oxidized products are further modified by chlorous acid and sodium borohydride treatments and their effects on the properties of the oxidized products studied.     

Graft copolymerization of vinyl monomers onto modified cotton. IX. Hydrogen peroxide–thiourea dioxide redox system induced grafting of 2-methyl-5-vinylpyridine onto oxidized celluloses

Cotton cellulose was independently oxidized with potassium periodate, potassium dichromate-sulfuric acid, and potassium dichromate–oxalic acid, and the resulting oxidized celluloses were further modified by treatment with chlorous acid or sodium borohydride. The various modified celluloses so obtained were grafted with 2-methyl-5-vinylpyridine using a thiourea–dioxide–H₂O₂ redox system. It was found that the initiation characteristics of the cellulose samples vary widely with the oxidizing agent used. Further modification of the oxidized celluloses by treating them with chlorous acid enhances considerably their susceptibility toward grafting. The opposite holds true when these oxidized celluloses were modified by sodium borohydride treatment. Excluding thiourea dioxide from the polymerization system offsets grafting onto cotton cellulose while considerable grafting takes place on the various oxidized celluloses and their further modified samples. The work was also extended to study the factors which affect the graft uptake, homopolymer formation, and total conversion. In addition, the reactions involved in initiation of grafting were elucidated.     

Studies in chemically modified celluloses. I. Oxidative susceptibility of chemically,modified cellulose

The oxidative susceptibility to different oxidizing agents such as dichromate–sulfuric acid, dichromate–oxalic acid and sodium hypochlorite in presence of leuco Cibanone Orange R of cellulose oxidized with sodium metaperiodate and its corresponding borohydride-reduced product was studied and compared with that of cellulose. From the changes in the chemical properties of the oxidixed products, an attempt was made to study the mechanism of oxidation of periodate oxycellulose and sodium borohydride-reduced periodate oxycellulose with the above mentioned oxidizing agents.     

Studies in chemically modified celluloses. II. Some aspects of physical properties of chemically modified celluloses

The changes in physical properties such as moisture regain, tensile strength, elongation at break, bursting strength, and resistance to abrasion of cellulose modified with different oxidizing agents such as dichromate–sulfuric acid, sodium hypochlorite in presence of leuco vat dyes, and potassium metaperiodate and of the corresponding products obtained as a result of reduction with sodium borohydride and those by oxidation with chlorous acid have been studied. Results are critically discussed in the light of existing knowledge on the changes that occur in the fine structure as well as in the chemical structure of cellulose during the mentioned modifications. It is pointed out that complex changes in physical properties of cotton cellulose brought about by chemical modification could hardly be evaluated and interpreted in terms of one or two physical or chemical parameters.     

缓冲溶液对TEMPO/NaClO/NaBr选择性氧化纤维素的影响

TEMPO/NaClO/NaBr氧化体系被广泛应用于选择性氧化糖类C6位伯醇羟基,使其成为羧酸盐物质,从而获得更好的水溶性或其他特定功能。但在氧化反应过程中,pH随反应的进行下降,并且存在pH值持续控制繁琐等问题。实验比较分析TEMPO/NaClO/NaBr氧化纤维素在碳酸钠―碳酸氢钠缓冲溶液、硼砂―氢氧化钠缓冲溶液中的适用性,并以滴加的方式向体系中加入氧化剂NaClO溶液,探索两种缓冲体系中各自适宜的NaClO滴加速度。实验结果表明,碳酸钠―碳酸氢钠缓冲溶液和硼砂―氢氧化钠缓冲溶液都适用于TEMPO/NaClO/NaBr氧化体系。与传统方法相比,氧化纤维素的羧基含量可以提高20%～25%。使用缓冲溶液较单一氢氧化钠溶液对稳定pH值有更好的效果,并且方法简便。另外,硼砂―氢氧化钠缓冲溶液对NaClO滴加速度敏感,控制滴加速度可以有效地提高反应活性。     

Studies in chemically modified celluloses. VII. Periodate oxidation of cellulose dyed with reactive dyes

Periodate oxidation of cellulose and cellulose dyed with reactive dyes at 32°C in the presence and absence of various concentrations of alkali metal chlorides and sulfates has been studied. The extent of oxidative degradation undergone by cellulose and assessed in terms of copper number and cuprammonium fluidity of the oxidized dyeing, the oxygen consumption during the oxidation, and the rate constant were shown to be markedly increased by the presence of the salts in the oxidizing solution. Borohydride and chlorous acid treatment were used to further modify the periodate oxycelluloses, and the effect of these treatments on the properties was studied.     

Synthesis and applications of reactive carbohydrates. IV. Reactive finishes based on CMC and oxidized CMC copolymers

Carboxymethyl cellulose (CMC) and persulfate-oxidized CMC samples were assessed for acidic and reducing properties before and after graft copolymerization with acrylamide using K₂S₂O₈ as initiator. The acidic properties were expressed as milliequivalents of carboxyl groups/100 g CMC while the copper number was taken as a measure for the reducing properties. Results obtained signify that increasing the duration of persulfate oxidation from 15 to 60 min causes a significant enhancement in the carboxyl content of CMC particularly during the first 50 min of oxidation. The same holds true for the copper number, but no further significant enhancement could be observed after 30 min. The copolymerization reaction decreases substantially the acidic and reducing groups of the CMC substrates examined. The results showed also that oxidation of CMC increases its susceptibility toward grafting: The higher the extent of oxidation, the higher the extent of grafting. Methylolation of the polyacrylamide–CMC graft copolymers results in reactive finishes. The latter were applied either alone or together with etherified methylolated melamine to cotton fabric samples according to the conventional pad–dry–cure method. The treated samples withstand severe washing, suggesting the occurrence of chemical bonding between the methylolated CMC products and cotton.     

A facile method to increase dielectric properties of rutile TiO2

Investigations on dielectric properties of N₂-sintered TiO₂ ceramic samples followed by chemically oxidized in H₂O₂ aqueous solution with different time spans were performed in the frequency range of 10²–10⁶ Hz. The results reveal that the dielectric properties strongly depend on the lengths of oxidizing time. An optimum oxidizing time of 12–16?h gives rise to superior dielectric properties with the permittivity of 12589 and the loss factor of 0.34 at 100?Hz. The time effect is attributed to the formation of an insulating layer at grain boundaries. Chemical oxidation proves to be a facile strategy to manipulate the insulating layer and opens a new way to modify the dielectric permittivity of versatile oxides.     

Solution combustion synthesis of MnFeCoNiCu and (MnFeCoNiCu)3O4 high entropy materials and sintering thereof

Single-phase, fine-grained high entropy MnFeCoNiCu alloy and (MnFeCoNiCu)₃O₄ oxide powders have been pioneered by an in-situ solution combustion synthesis without post-treatment, utilizing alanine and glycine as organic reducers and ammonium nitrate as an auxiliary oxidizing agent. In order to regulate the porosity, homogeneity, phase composition and morphology of the obtained powders, multivariate studies on the influence of the following parameters were performed: type and quantity of reducers, pH of solution, ignition temperature, presence of oxidizing agent, etc.. It has been shown that the amount of the reducer holds the key for governing combustion temperature and speculation of lower and upper combustion limits in the systems under study. In turn, according to the temperature-time profiles of combustion, product microstructure, as well as phase composition of the product, the combustion area turned out to be divided into two sub-domains: volume combustion (with vigorous flaming) and self-propagation combustion. After thorough characterization, high entropy MnFeCoNiCu alloy and high entropy (MnFeCoNiCu)₃O₄ oxide powders were subjected to consolidation by spark plasma sintering and mechanical properties were evaluated. The influence of sintering temperature on the phase transformation of the products was revealed.     

Photodegradation of oxidized cotton cellulose

Different functional groups were systematically introduced into cotton cellulose and their effect on the stability of cotton toward photochemical degradation was examined. First, the cotton was progressively oxidized with sodium metaperiodate up to an oxygen consumption of 2.45 atom per 100 anhydroglucose units (agu) to form 2,3-dialdehyde cellulose. These products were reduced with potassium borohydride to obtain 2,3-dialcohol cellulose or were oxidized with sodium chlorite to obtain 2,3-dicarboxyl cellulose. The chemical modifications of the cellulose structure caused a drop in tensile strength of 34% for the dialdehyde of 2.45 oxygen consumption per 100 agu compared to 27% and 37% in the case of the dialcohol and the dicarboxyl celluloses, respectively. Upon irradiation with UV light, the breaking strengths of the dialcohol and the dicarboxyl celluloses were generally higher up to an oxygen consumption of 0.63 per 100 agu when compared to the dialdehyde and then fell more drastically. The tensile data of the exposed oxycelluloses correlated well with the reciprocal of their intrinsic viscosities (R² = 0.89). The primary alcohol groups appeared to accelerate the progress of the photochemical degradation. The photochemical effects of the carboxyl groups could not be differentiated from the autohydrolysis to which dicarboxyl cellulose is susceptible. No indication of a change in morphology was detected for any of the treatment types. © 1993 John Wiley & Sons, Inc.     

Effect of Modifiers on the Reactivity of Cr2O3/Al2O3 and Cr2O3/TiO2 Catalysts for the Oxidative Dehydrogenation of Propane

Chromium oxide supported on alumina and titania supports was modified with oxides of sodium, vanadium and molybdenum. The modified and unmodified chromium oxide catalysts were characterized by several techniques. The presence of surface chromium oxide and surface molybdenum and vanadium oxide species was detected in the unmodified and molybdenum and vanadium oxide modified supported chromium oxide catalysts. The reducibility (T_max and H/Cr ratio) of the surface chromium species was not affected for the vanadium and molybdenum oxide modified catalysts; however, the reducibility changed noticeably for sodium modified supported chromium oxide catalysts. Studies of the reactivity of the ODH of propane revealed the effect of modifiers on the reactivity properties of the surface chromium oxide species. The activity and propene selectivity decreased for sodium modified supported chromium oxide catalysts. However, the activity increased for vanadium oxide modified catalysts and was similar for molybdenum oxide modified catalysts irrespective of the support. The propene selectivity was higher for molybdenum oxide modified chromium oxide catalysts. However, the propene selectivity for vanadium oxide modified catalysts depends on the support since it appears that the inherent selectivity of the surface vanadium oxide species is reflected.     

Ceric salt-initiated graft copolymerization of methyl methacrylate on preoxidized cellulose samples

The effects of preoxidation of cellulose samples on graft copolymerization in the presence of ceric salt (Ce⁴⁺) as an initiator were investigated. The use of Ce⁴⁺ as oxidizing agent inhibited the formation of grafts, and the number of grafts decreased as the concentration of Ce⁴⁺ in the pretreatment rose. In contrast, the samples oxidized with periodic acid (HIO)₄ or hydrogen peroxide (H₂O₂) tended to show improved initiation characteristics and gave a larger number of grafts than the unoxidized samples. The reactivity of oxidized samples toward Ce⁴⁺ was examined and the following results were obtained: The part of easy reactivity decreased in the Ce⁴⁺-oxidized samples but it increased in the HIO₄-oxidized samples; on the other hand, it did not vary greatly in the H₂O₂-oxidized samples. Cellulose is probably oxidized to different states by different oxidizing agents, and the reactivity of Ce⁴⁺ toward these oxidized samples, as one index, seems to be reflected in the graft copolymerization. It was consequently recognized that the graft copolymerization characteristics of cellulose samples could be improved significantly by proper selection of oxidiation conditions.     

Dependence of the phase composition of the anodic layer on oxygen evolution and anodic corrosion of lead electrode in lead dioxide potential region

At potentials more positive than 1300 than 1300 mV with respect to a Hg/Hg₂SO₄ electrode the partial currents of lead corrosion and oxygen evolution in 7_n H₂SO₄ follow the Tafel's dependence. X-ray investigations and wet analyses of the anodic layer show that it consists of tet-PbO and α PbO₂. Besides, at potentials more negative than 1530 mV, β PbO₂ forms at the oxide/solution interface.It is established that lead anodic corrosion proceeds in two stages. During the first Pb is oxidized to tet-PbO. During the second stage tet-PbP is oxidized to PbO₂. If this process is performed in solid state, α PbO₂ forms. If PbO is dissolved and then oxidized, β PbO₂ crystals are formed. The oxidation of tet-PbO to α PbO₂ proceeds at different rates in the bulk of the oxide and at its surface with the solution. At potentials more positive than 1530 mV the oxidation of tet-PbO to α PbO₂ at the surface of the oxide with the solution leads to the dropping off of part of the oxide layer.The oxidation of Pb and PbO is carried out under the action of O atoms and O^? radicals which evolve at the oxide/solution interface and penetrating the oxide reach the metal/oxide surface.     

Alkalization mechanism of cellulose in hydroxypropylcellulose preparation process

The role of alkali treatment of cellulose in hydroxypropylcelluloe (HPC) preparation has been studied from the viewpoint of the selective distribution of sodium hydroxide between HPC and cellulose phases. Generally it is considered to be important to prepare uniform alkali cellulose, whose calculated value of composition is C₆H₁₀O₅NaOH. x H₂O, prior to etherification of cellulose. Therefore, enough alkali to obtain alkali cellulose is used by major manufacturers of cellulose ethers. However, HPC having good solution qualities and performance properties can be prepared even from partially alkalized cellulose by using 0.2–0.4 molar sodium hydroxide per anhydroglucose unit. The results obtained from a series of experiments indicate the following mechanism for the formation of HPC. Hydroxypropylation is initiated in the alkalized portion of cellulose if such portion and propylene oxide are present together. As the hydroxypropylation proceeds, the liberated sodium hydroxide in HPC phase migrates into the cellulose phase due to the shift of distribution equilibrium of sodium hydroxide; the above tendency is enhanced by byproducts such as propyleneglycol. Some newly alkalized portions are present and hydroxypropylation follows. These processes are repeated, and then all portions of cellulose are alkalized and hydroxypropylated.     

Preparation of magnetic iron-oxide nanoparticles by successive reduction-oxidation in reverse micelles: Effects of reducing agent and atmosphere

The chemical reduction of ferrous or ferric ions to metallic iron by a reducing agent such as sodium borohydride (NaBH₄), hydrazine (N₂H₄), or sodium phosphonate (NaH₂PO₂) and its subsequent oxidation were investigated for the preparation of iron-oxide nanoparticles in reverse micelles. The crystal structure and crystallinity of the oxide nanoparticles were found to depend on the oxidation potential of the reducing agent and the catalytic activity of the surface of deposited iron. In addition, it was demonstrated that the process used for oxidizing the metallic iron was critical for the formation of oxide nanoparticles, and that the enhancement of magnetic properties was achieved by providing a flow of oxygen during the oxidation process.     

ESR study of the chromium oxide-alkali metal oxide systems

An ESR investigation of chromium oxide samples containing lithium, sodium, potassium, rubidium and cesium oxides has been carried out. Two kinds of defect structures attributed to Cr³⁺ vacancies connected with occluded alkali metal ions and Cr⁶⁺ ions in the α-Cr₂O₃ lattice are discussed. The observed fine structure is interpreted by means of axially symmetric spin Hamiltonian and two different values for parameter D. The possibility for ACr₃O₈ structure formation is also considered. The connection between these defect structures and the catalytic properties of the chromium catalyst is considered.     

Study on silica coated chrome oxide green pigment and its performance

In this study, sodium silicate (Na₂SiO₃) was used as the precursor of SiO₂ to prepare Cr₂O₃/SiO₂ composite pigments. First, using cetyltrimethylammonium bromide to surface modification of the chromium oxide green pigment to make the surface of the particles positively charged. The negatively charged SiO₂ sol coats the surface of the pigment particles through electrostatic attraction. Then use HCl solution acid to precipitate SiO₂, and finally get Cr₂O₃/SiO₂ composite pigment. The structure of the modified pigment was analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size distribution analysis, and X-ray diffraction (XRD) characterization. The influence of silica coating on the tinting strength, dispersibility of pigment in water, and hiding power of the chromium oxide green pigment were further investigated. The results demonstrate that SEM, TEM, and particle size analysis shows that this experimental method can obtain a good silica coating. When the coating quantity is 30%, the full coating of chromium oxide green by silica is achieved. According to the XRD analysis before and after the chromium oxide green coating, as the reaction temperature increases, the intensity of the SiO₂ diffraction peak becomes larger and the crystallization effect becomes better. In the test of pigment performance, it can be concluded that the coloring power, hiding power, and dispersibility of Cr₂O₃/SiO₂ composite pigments are better than those of uncoated chromium oxide green pigments.     

Sibunit-based catalytic materials for the deep oxidation of organic ecotoxicants in aqueous solution: I. Surface properties of the oxidized sibunit samples

This article presents the first systematic study of the effect of oxidation under various conditions on the surface composition and catalytic properties of a carbon material of the Sibunit family (which is promising for oxidative treatment of industrial wastewater) in deep oxidation of organic ecotoxicants in aqueous solution. Modification of the surface properties and texture of Sibunit-4 by various oxidative treatment procedures using nitric acid, sodium hypochlorite, hydrogen peroxide, or oxygen as the oxidant is reported. The chemical state of the surface of oxidized Sibunit and its texture have been investigated by a combination of physical and chemical methods (XPS, acid-base titration with bases differing in strength, pH slurry measurements, determination of pH at the point of zero charge, and low-temperature N₂ adsorption). By using different oxidation procedures, it is possible to obtain oxidized Sibunit samples with variable concentrations and natures of oxygen-containing surface groups.     

Structural analysis of iron oxide coated n-silicon heterojunction photoanodes

The iron oxide/n-Si heterojunction electrode was investigated as a possible efficient photoanode with the iron oxide prepared under various conditions. The oxide structure was studied by RBS (Rutherford backscattering spectrometry) and XPS (X-ray photoelectron spectroscopy) techniques and the correlation between photocharacteristics and its oxide structure was discussed. The iron oxide prepared by thermal oxidation and reactive-evaporation methods. The photocurrent in a 0.2 mol dm^?3 KOH solution was found to strongly depend on the preparation of the iron oxide. The reactive-evaporated Fe₂O₃/n-Si electrode showed the most effective characteristics of photoresponse and the 60 min thermally oxidized Fe₂O₃/n-Si electrode gave the best results in the thermally oxidized oxide system. The compositional analysis by RBS indicated that the oxide is formed stoichiometrically as Fe₂O₃ and that small amounts of a heavy-mass and a tungsten impurity are present in the thermally oxidized and the reactive-evaporated oxides, respectively. The XPS analysis of the chemical state of the iron in the oxide showed that the chemical state of the iron oxide formed by thermal oxidation varies as a function of depth and that the iron oxide formed by reactive- evaporation is in a almost constant chemical state throughout the film.     

11C-Radiolabeling study of methanol decomposition on chromium modified SBA-15 silica

Chromium modified mesoporous SBA-15 silica was prepared by incipient wetness impregnation of the silica support with toluene solution of chromium acetylacetonate. Powder X-ray diffraction, Nitrogen physisorption, FTIR, UV?CVis and XPS techniques as well as temperature programmed reduction with hydrogen were used for the characterization of the obtained materials. The catalytic behaviour of the samples was tested in methanol decomposition to CO and hydrogen. Novel catalytic radio isotopic based method was applied for the elucidation of the role of different chromium surface species in the catalytic process. This approach is based on the selective coverage of the surface active sites with ¹¹C- and ¹²C-methanol and simultaneous observation of the products of their conversion. It was found that the supported active phase represents a complex mixture of chromium oxide nanoparticles (mainly Cr₂O₃, CrO₃) and anchored to the silica surface mono- and polychromate structures. It was established that these species differ significantly in their catalytic behavior during the methanol decomposition and the reaction mechanism of the formation of different products was discussed.