Reactions ofOrtho-quinones and the model compound 4-t-butyl-1,2-benzo-quinone in alkaline peroxide

The literature on the occurence and reactions of quinone structures in mechanical pulps has been reviewed. A study on the reactions of a quinone model compound (4-t-butyl-1,2-benzoquinone) in alkaline hydrogen peroxide has been reported, with detailed analysis of reaction products and kinetic phenomena. The study reveals that the diversity of products formed is much more complex than that obtained using an α,β-unsaturated aldehyde as a model, a simple second-order expression can be used to describe the kinetics. The kinetics representing chromophore removal for the two classes of model compound are compared with previously reported studies of kinetic phenomena during bleaching of mechanical pulps.     

The middle lamella remainders on the surface of various mechanical pulp fibres

The surfaces of various mechanical pulp fibres, including thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), and alkaline peroxide mechanical pulp (APMP) fibres, were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), and X‐ray photoelectron spectroscopy (XPS). With SEM and AFM, the middle lamella material was observed to be non‐fibrillar and patch‐like, while the fibre secondary wall was observed to have a micro‐fibrillar structure. It was found that after the first‐stage refiner, lignin‐rich middle lamella remainders were present on the fibre surface of all three pulps, although most of the fibre surfaces exhibited a micro‐fibrillar structure. After the final‐stage refining, large amounts of granules were present on the TMP fibre surface. In contrast, most middle lamella remainders were still visible on the surface of CTMP fibres after the final‐stage refining and even after peroxide bleaching. XPS results have confirmed that the non‐fibrillar surface material is the lignin‐rich middle lamella remainder that contribute to the high surface lignin concentration. Copyright © 2006 John Wiley & Sons, Ltd.     

Size-exclusion chromatography of the soluble lignin products from hydrogen peroxide brightening of mechanical pulps

The molar mass distributions and yields of soluble lignin degradation products formed during the alkaline hydrogen peroxide brightening of spruce mechanical pulp have been determined. These distributions depend on pH, the extent to which the pulp was subjected to solvent extractions, H₂O₂ concentration and reaction time. At and below pH 11, there is an excellent correlation between the brightness and b values on pulp and the amounts of residual peroxide and lignin derived products in solution. Brightening reactions lead primarily to products with intermediate molecular mass values between 1500 and 6000. During multistage brightening, the products formed are initially those with high ionizable phenolic hydroxyl and low carbohydrate contents, and carbonyl-containing moieties. NMR and UV spectroscopic analysis show that products formed include lignin and carbohydrate-containing species. The lignin-originating products from the later stages contain a lower relative concentration of ionizable phenolic groups and more visible-absorbing and carbohydrate-containing species. Brightness and b gains during multistage brightening were exponential with product yield.     

Characterization of hemicelluloses isolated with tetraacetylethylenediamine activated peroxide from ultrasound irradiated and alkali pre-treated wheat straw

Post-treatment with 2% H₂O₂-0.2% tetraacetylethylenediamine (TAED) at pH 11.8 for 12 h at 48 °C solubilized 27.1-28.1% of the original hemicelluloses and 43.3-46.2% of the original lignin from the ultrasound irradiated and alkali pre-treated wheat straw. It was found that the bleaching activator TAED forms peracetic acid with hydroperoxide anion in aqueous alkali and improves the brightness of the solubilized hemicelluloses. Xylose was a predominant sugar in all the hemicellulosic preparations, comprising 72.0-73.1% of the total sugars. Arabinose (13.4-13.9%) and glucose (6.7-7.9%) were detected as the second and third major neutral sugar constituents. In addition, the hemicelluloses also contained a noticeable amount of 4-O-methyl-d-glucuronic acid (6.0-7.0%) together with a small amount of galactose (4.3-4.9%) and minor quantities of rhamnose (1.5-1.8%) and mannose (0.5-0.8%). Furthermore, the hemicellulosic preparations obtained by TAED activated peroxide extraction showed a relatively lower content of associated lignin (3.8-4.4%), but a higher molecular weight (34,210-40,310 g mol⁻¹). No significant differences in the structural features such as oxidation of the hemicelluloses by TAED were observed as compared to those obtained by alkaline peroxide without bleaching activator.     

Formamidine sulfinic acid used as a bleaching chemical on softwood TMP

In this work, we showed that formamidine sulfinic acid, FAS, can be efficiently used as a bleaching chemical for softwood TMP. The bleaching reaction was very rapid. The brightness achieved is dependent on the temperature and the chemical charge applied. The highest brightness was obtained at pH 10, and the lowest yellowness at pH 12. FAS treated pulps are much more stable toward light-induced yellowing. In all cases, FAS bleaching at pH 12 results in much slower and less intense aging than all other pulps, either untreated or bleached with FAS at pH 8 or 10.     

Applications of ESR and CIDEP to mechanistic studies of lignin chemistry

Using time-resolved CIDEP and conventional ESR as well as optical studies much can be elucidated concerning the initial photochemical reactions of lignin model compounds such as α-guaiacoxyacetoveratrone and α-guaiacoxy-β-propioveratrone. The resulting transient radicals are models for intermediates in the processes of photoyellowing of lignin containing mechanical pulps and papers. Model solution experiments which produce evidence for the roles of free radicals in reductive bleaching of mechanical pulps and in the oxidative aging of papers are also described. The more thorough understanding of these complex processes allows the development of strategies to try to “protect” mechanical pulps and papers from light and thermally induced damage and to “repair” any damage which invevitably occurs.     

Alkaline hydrogen peroxide bleaching of cellulose

A closed system bleaching apparatus was designed to determine the kinetics and effects of various factors on alkaline hydrogen peroxide bleaching of textile cellulose fabrics. It was confirmed that perhydroxyl anion is the primary bleaching moiety in alkaline hydrogen peroxide systems. The use of the apparatus in the measurement of fabric color, waste oxygen, and the subsequent calculation of hydroxyl ion, and molecular hydrogen peroxide confirmed that pH and titration of 'free' hydrogen peroxide in alkaline bleaching systems are not good indicators of bleaching mechanism. The role of the cellulose itself in the chemical bleaching system was determined. The rate of bleaching on cotton fabric was shown to be a first order reaction in concentration of perhydroxyl anion at 60 and 90°C. An activation energy of 17kcal/mole was estimated. Decomposition of H₂O₂ into waste oxygen was found to be second order kinetics.     

Efficient total halogen-free photochemical bleaching of kraft pulps using alkaline hydrogen peroxide

Total halogen-free bleaching of kraft pulps was conducted by an oxidative photochemical process at room temperature using alkaline hydrogen peroxide. Selection of an appropriate wavelength of light was crucial for effective bleaching and avoiding degradation of cellulose. The wavelength of the light has to be selected so that the light is absorbed only by the colored compounds in the pulps and not by the bleaching reagents or the pulp itself. When a long-wavelength black-light fluorescent lamp was used in combination with aqueous hydrogen peroxide solution at pH 11, the bleaching efficiency for hardwood and softwood kraft pulps reached the same level as that obtained by conventional two-stage elemental chlorine-free processes.     

Sulfur-free dissolving pulps and their application for viscose and lyocell

In this study, the concept of multifunctional alkaline pulping has been approved to produce high-purity and high-yield dissolving pulps. The selective removal of hemicelluloses was achieved by either water autohydrolysis (PH) or alkaline extraction (E) both applied as pre-treatments prior to cooking. Alternatively, hemicelluloses were isolated after oxygen delignification in a process step denoted as cold caustic extraction (CCE). Eucalyptus globulus wood chips were used as the raw material for kraft and soda-AQ pulping. In all process modifications sulfur was successfully replaced by anthraquinone. By these modifications purified dissolving pulps were subjected to TCF bleaching and comprehensive viscose and lyocell application tests. All pulps met the specifications for dissolving pulps. Further more, CCE-pulps showed a significantly higher yield after final bleaching. Morphological changes such as ultrastructure of the preserved outer cell wall layers, specific surface area and lateral fibril aggregate dimension correlated with the reduced reactivity towards regular viscose processing. The residual xylan after alkali purification depicted a lower content of functional groups and a higher molecular weight and was obviously entrapped in the cellulose fibril aggregates which render the hemicelluloses more resistant to steeping in the standard viscose process. Simultaneously, the supramolecular structure of the cellulose is partly converted from cellulose I to cellulose II by the alkaline purification step which did not influence the pulps reactivity significantly. Nevertheless, these differences in pulp parameters did not affect the lyocell process due to the outstanding solubility of the pulps in NMMO. Laboratory spinning revealed good fiber strength for both, regular viscose and lyocell fibers. The high molecular weight xylan of the CCE-treated pulps even took part in fiber forming.     

Pyrolysis-GC/MS and TG/MS study of mediated laccase biodelignification of Eucalyptus globulus kraft pulp

Biobleaching studies using laccase mediator system (LMS) were carried out, under optimized conditions, on two unbleached Eucalyptus globulus kraft pulps, one produced by conventional way, with kappa number of 16.1, and another with kappa number of 14.5, obtained by modified kraft procedure with a high liquor/wood ratio and with black liquor replacement in the middle of the cooking. The pulp properties before and after LMS and alkaline extraction were evaluated in terms of kappa number, hexeneuronic acid content, viscosity, brightness and acid insoluble lignin content.The original milled wood sample and the kraft pulps were characterized by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetry/mass spectrometry (TG/MS). Eucalypt wood lignin produces guaiacol and syringol derivatives during pyrolysis. These lignin products can be detected with high sensitivity using the selected ion chromatograms even in the bleached pulp of low lignin content (about 0.5%). Py-GC/MS revealed that the lignin moieties were similarly altered during biobleaching as during pulping, which is exemplified by the preferential removal of aldehyde groups from the alkyl side groups. Semi-quantitative analysis of the pyrograms indicates that the lignin content of the biobleached pulps is reduced by about half in comparison with the unbleached pulps. The TG/MS results show that the hemicellulose content of wood was strongly modified during pulping resulting in higher thermal stability.     

Laccase from Trametes versicolor

The enzyme laccase was produced by the white-rot fungus Trametes versicolor in repeated batches cultures with immobilized mycelium. Two different culture conditions were used. Enzymes produced were evaluated regarding their stability a thigh temperatures (55°C and 65°C) and at alkaline conditions (pH 7.0 and pH 8.0) having in view the application of these enzymes in biobleaching of hardwood Kraft pulp. Biobleaching experiments were divided in two parts, enzymatic prebleaching followed by chemical bleaching. In the enzymatic prebleaching the enzyme laccase was used at two conditions of pH and temperature, whereas the reaction time was fixed at 1h in all pretreatments. In the chemical bleaching the DEDED and DEpDED sequences were used. The enzyme action was evaluated by Kappa number, viscosity, and brightness at the end of bleaching sequences. There were obtained values of Kappa numbers lower than control assays, viscosities compatible with industrial pulps, and brightness higher than controls, when pulps were pretreated for 1 h with laccase at pH 8.0 and 55°C.     

The effect of eucalypt pulp xylan content on its bleachability,refinability and drainability

Currently, bleached eucalypt pulps are largely used for printing and writing (P&W) and sanitary (tissue) paper grades. Among the many pulp quality requirements for P&W and tissue paper production the xylan content is one of the most significant. For P&W papers, increasing xylans improve pulp refinability and strength properties but negatively affect bulk and drainability. For tissue paper, xylans are purportedly advantageous during paper drying in the Yankee cylinder but negatively affect paper bulk and may increase dusting during paper manufacture. On the other hand, bleachability is a very important parameter for both P&W and tissue grade pulps since bleaching cost is the second most significant in eucalypt bleached kraft pulp production. The aim of this study was evaluating the influence of eucalyptus pulp xylan content on its bleachability, refinability and drainability. A sample of industrial unbleached eucalyptus kraft pulp containing 15.6?% xylans was treated with various alkali charges at room temperature in order to obtain materials with different xylan contents. The pulps were bleached to 90 % ISO brightness with the O–D_HT–(EP)–D sequence and evaluated for their refinability and drainability. By increasing the alkali concentration in the range of 10–70 g/L pulps of 14.5–5.9 % xylans were produced with no significant impact on cellulose crystallinity. The decrease of xylan content significantly decreased pulp bleaching chemical demand, water retention value and refinability and increased pulp drainability.     

Developments in the chemistry of tris(hydroxymethyl)phosphine

Tris(hydroxymethyl)phosphine, P(CH₂OH)₃, a water-soluble compound, has been known for about 50 years but development of its coordination chemistry has been slow and relatively recent. During some collaborative studies with a pulp and paper research institute on testing water-soluble catalysts for hydrogenation of lignin in pulp and the unsaturated functionalities in lignin model compounds, with the aim of bleaching pulps, we discovered new, in situ, Ru-P(CH₂OH)₃ hydrogenation catalysts. Interest in the coordination chemistry of this phosphine thus ensued, and this review covers this topic as well as the coordination chemistry of a diphosphine analogue, bis[bis(hydroxymethyl)phosphino]ethane, (HOCH₂)₂P(CH₂)₂P(CH₂OH)₂. The applications of the water-soluble metal complexes of these two phosphines in the areas of catalysis and medicinal drugs are also described. These phosphines, in the absence of metals, were found serendipitously to be effective bleaching agents for pulps (and also brightness stabilizing agents), and some relevant organo-phosphorus chemistry from our group is also briefly presented, particularly because of its possible significance in hydroformylation and hydrogenation processes catalyzed by metal–phosphine complexes.     

Chromophores from hexeneuronic acids: chemical behavior under peroxide bleaching conditions

Hexeneuronic acids (HexA) are a major cause of discoloration (yellowing/brightness reversion) in pulps from xylan-containing wood, being generated from the xylan’s 4-O-methylglucuronic acid residues. The HexA-derived chromophores, whose identification and structure confirmation have been described in the previous part of this series (Rosenau et al. in Cellulose, 2017), were subject to conditions of peroxide bleaching, i.e. treatment with hydrogen peroxide in alkaline medium. These chromophores, ladder-type oligomers of mixed aromatic-quinoid and mixed furanoid-benzoid character, are degraded relatively quickly to one major product, 2,5-dihydroxy-[1,4]-benzoquinone (DHBQ), and a minor component, 2,5-dihydroxyacetophenone (DHA). These two compounds, which have already been identified as two of the three key chromophores (besides 5,8-dihydroxy-[1,4]-naphthoquinone, DHNQ) in aged cellulosics, are potent chromophores themselves and are subsequently more slowly degraded to non-colored degradation products, according to pathways already described in previous parts of this series. The occurrence of DHBQ and DHA in the bleaching treatment of the HexA-derived chromophores establishes the link between HexA chemistry and the key chromophore classes of residual chromophores found in aged cellulosic materials.     

Direct analysis of lignin and lignin-like components from softwood kraft pulp by Py-GC/MS techniques

Analytical pyrolysis combined with gas chromatography/mass spectrometry was used to analyse the structure and quantity of aromatic components, mainly guaiacyl and hydroxyphenyl derivatives, directly from chemical pulps. The quantity of aromatic degradation products was determined using a new external calibration method. The external standard was analyzed similarly to the pulp sample, and the combined area of the degradation products formed, normalized to the sample amount, was used for calibration. The method was sensitive enough to detect aromatics from fully bleached softwood pulps at a concentration level of 0.4 wt.%.The effect of bleaching on lignin structures in softwood pulps was studied by following the changes in guaiacyl-type degradation product distribution. The residual lignin structures that had been modified during cooking were removed during the course of bleaching. The residual lignin in fully bleached pulps therefore was found to bear features characteristic of native lignin in addition to increased oxidation. A striking enrichment of hydroxyphenyl-type aromatic pyrolysis products was observed during bleaching. It is suggested that they are derived not only from lignin but also from other pulp components.     

Direct characterization of hydrogen peroxide bleached thermomechanical pulp using spectroscopic methods

The effects of thermomechanical pulp (TMP) bleaching with hydrogen peroxide under acidic and alkaline conditions were studied using different spectroscopic analytical methods. The results of hydroxyl radical determination in bleaching solutions, analyses of carbonyl and carboxyl groups contents in the pulp, and the cellulose fiber surface analysis by X-ray photoelectron spectroscopy (XPS) elucidate the chemistry of the hydrogen peroxide treatment. Diffuse reflectance laser flash photolysis (DRLFP) method showed the differences in the photochemical behavior that reflect the changes of the chromophoric system after the preliminary peroxide bleaching stage under acidic conditions. Fourier transform infrared (FTIR) spectroscopy confirmed the non-delignifying character of the bleaching process. Suppression of carbonyl and formation of carboxyl groups in the case of the two-stage peroxide bleaching performed in the presence of catalysts and stabilizers was also confirmed. FT-Raman studies showed the removal of coniferaldehyde groups after treatment under acidic and alkaline conditions.     

Sorption of spruce O-acetylated galactoglucomannans onto different pulp fibres

Sorption of spruce acetylated galactoglucomannans (GGM) onto different pulps, among which unbleached and peroxide-bleached mechanical pulps, and unbleached and bleached kraft (BK) pulps, was studied as a means of understanding the retention of acetylated GGMs in mechanical pulping and papermaking. The fibre surface coverage of lignin and carbohydrates was estimated by X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA). GGM sorption was clearly favoured on kraft pulps. Hardly any differences in sorption were, however, observed between unbleached and BK pulps, even if the surface coverage of lignin was lower on the bleached pulp. Neither thermomechanical pulp (TMP) nor chemithermomechanical pulp (CTMP) manufactured from spruce sorbed any acetylated GGMs. Peroxide bleaching of the pulp did not increase sorption. Only CTMP produced from aspen sorbed some GGMs. The anionic charge of neither chemical nor mechanical pulps influenced GGM sorption.     

Analytical pyrolysis study of biodelignification of cloned Eucalyptus globulus (EG) clone and Pinus pinaster Aiton kraft pulp and residual lignins

This study centred on the analysis of lignin in situ of cloned eucalypt and pine kraft pulps. Trametes versicolor laccase-violuric acid system (LMS) delignifications were performed on a softwood (Pinus pinaster) and a hardwood (Eucalyptus globulus) conventional kraft pulp with an initial kappa number of 34.5 and 15.5, respectively. The LMS treated pulps were then subjected to alkaline extraction stages (E). The kappa number data show that LMS is effective at biodelignifying both softwood and hardwood kraft pulps. However, under the conditions employed in this study, a greater level of biodelignification was obtained with LMS E. globulus (hardwood) than with LMS P. pinaster (softwood), but the amount of lignin removed was higher for the softwood pulp. The original milled wood samples, kraft pulps, biodelignified kraft pulps, and isolated residual lignin and milled wood lignins from the two wood samples have been characterized by pyrolysis-gas chromatography/mass spectrometry. The analysis of the pyrograms indicates that the lignin compositions of the two wood species and corresponding pulps are very different, as expected; however, the knowledge of the chemical mechanisms of delignification is very limited and requires additional work. Analytical pyrolysis is one the few degradative methods for the analysis of biopolymers that has shown a sufficient degree of success.     

Chemically-induced geometrical isomerization of stilbenes during peroxyoxalate chemiluminescence reaction: revisit to ‘photochemistry without light’

The chemically-induced isomerization of stilbenes during the peroxyoxalate chemiluminescence (PO-CL) reactions was reinvestigated. The PO-CL reactions using bis(2,4,6-trichlorophenyl) oxalate in the presence of several stilbenes (type A reaction) produced cis-stilbenes in 0–4% yields, which was dependent on the singlet excitation energy of the stilbenes. On the other hand, the PO-CL reactions of the oxalates, containing the stilbene moieties in the molecules (type B reaction), produced cis-stilbenes 0–9.3% yields, some of which were much more effective than the type A reactions considering the amount of the oxalate moiety as the energy supplier.     

Photodynamic action of a water-soluble hypocrellin derivative with enhanced absorptivity in the phototherapeutic window II. Photosensitized generation of active oxygen species

Cysteine-substituted hypocrellin B (Cys-HB) is a water-soluble perylenequinonoid derivative with significantly enhanced absorptivity in the range of wavelength longer than 600 nm. Electron paramagnetic resonance (EPR) measurements, quenching experiments and 9,10-diphenyl-anthracene bleaching studies were used to investigate the photodynamic action of Cys-HB in the presence of oxygen. Illumination of Cys-HB solution, in the presence of oxygen, generated singlet oxygen, superoxide anion radical, hydroxyl radical and hydrogen peroxide. The accumulation of active oxygen species was transformed into that of the semiquinone anion radical with the depletion of oxygen, detected by the spin counteraction of TEMPO radical formed via the reaction of TEMP with singlet oxygen produced by Cys-HB photosensitization. Oxygen content, Cys-HB concentration and reaction environment affected the transformation and the competition between the Type I and Type II reactions. Compared with hypocrellin B (HB), Cys-HB primarily remained similar and slightly lower capability of active oxygen-generation, confirmed to be a favorable phototherapeutic agent.