Christine Chirat

Effect of hydroxyl radicals on cellulose and pulp and their occurrence during ozone bleaching

The effects of hydroxyl radicals on cellulose and pulp are studied using gamma irradiation of water. It is shown that hydroxyl radicals can depolymerize cellulose severely. For a given drop in DPv ozone leads to the formation of more carbonyl groups on cellulose than hydroxyl radicals. Furthermore, hydroxyl radicals do not delignify kraft pulps significantly at room temperature, contrary to ozone. It is also shown that the presence of metal ions, such as iron and copper, or a rise in temperature increases the formation of hydroxyl radicals during ozonation, which is reflected by lower viscosities. The addition of manganese (II) on the other hand does not lead to the formation of hydroxyl radicals with ozone. Increasing the pH of ozonation does not yield significantly more hydroxyl radicals and the cellulose is less degraded at high pH ozonation than at low pH, for a given ozone charge. It is concluded that molecular ozone is mainly responsible for cellulose degradation during ozonation. Ozonation of unbleached pulp leads to the formation of considerable amounts of hydrogen peroxide. An additional degradation is due to the hydroxyl radicals generated by the peroxide formed during ozonation. This explains the effect of the metal ions and the difference between high and low consistency ozonations.

Studies on fluorescence of cellulosics

Abstract Steady-state fluorescence emission spectra of various celluloses were measured at an excitation wavelength of 320 nm. Various spectra recorded in the solid state were compared: (1) ECF bleached papers made of hardwood, the anhydroglucose units of which were chemically modified at C1 and C6 or C2 and C3 positions with carboxylic groups; (2) microcrystalline cellulose; (3) cotton linters; and (4) delignified sisal fibers (mercerized or not). Fluorescence emission was quite independent of the carboxylic acid content and average molecular weight (determined by viscosimetry) of the cellulose polymers. Microcrystalline cellulose (Avicel), cotton linters, and mercerized delignified sisal cellulose were acetylated in homogeneous medium (DMAc/LiCl as solvent system) to obtain soluble polymers in dichloromethane for comparison of spectra recorded in the solid and liquid states. Fluorescence of cellulose acetates in solution (CH2Cl2) and in the solid state was compared under similar experimental conditions to non-esterified celluloses in the solid state. The importance of the solid state for fluorescence emission could be demonstrated. Fluorophores are present in minute amounts in the polymer and their favorable energy transfer for excitation in the solid state likely enhances fluorescence emission. Among numerous fluorophores, dityrosine appeared to be a good candidate for fluorescence because it displayed emission in the fluorescence range of cellulose. Dityrosine is an amino acid involved in the lignification of non-woody plants. Mercerized sisal impregnated with tyrosine in the presence of peroxidase and hydrogen peroxide did not show enhanced emission, in contrast to para-hydroxycinnamic acid (coumaric acid), which is also involved in the lignification process at least for non-woody plants. The origin of cellulose fluorescence remains uncertain and appears to have several origins. This study clearly underlines the importance of the solid state for enhancing fluorophore emission.

Formation of carbonyl groups on cellulose during ozone treatment of pulp: consequences for pulp bleaching.

The formation of carbonyl groups during the ozone treatment (Z) of eucalyptus (Eucalyptus grandis and Eucalyptus urophylla hybrid) kraft pulps and their behaviors during subsequent alkaline stages were investigated by the CCOA method with carbazole-9-carboxylic acid [2-(2-aminooxethoxy)-ethoxy] amide (CCOA) as the carbonyl-selective fluorescence label. Several pulp samples with or without lignin and hexenuronic acids (hexA) were used to elucidate the effects of these components when present in unbleached kraft pulp. Both hexA and lignin increased the formation of carbonyl groups on cellulose and hemicellulose during ozonation. It was concluded that radicals are likely formed when ozone reacts with either lignin or hexA. These carbonyl groups were involved in cellulose depolymerization during subsequent alkaline extraction stages with sodium hydroxide (E) and alkaline hydrogen peroxide (P, in ZEP or ZP). Their numbers decreased after E but increased during P when H2O2 was not stabilized enough. Several ways to minimize the occurrence of carbonyl group formation are suggested.

Application of ESR spectroscopy in bleaching studies

Summary In this study several pulp components were characterized by ESR spectroscopy. Free radicals present in the pulp lignin gave a well-defined signal, and the residual lignin content during a bleaching sequence could be followed using this technique. Organic radicals on the pulp carbohydrates were also detected and could be related to the formation of chromophore structures in pulp. Further, ESR spectroscopy made it possible to characterize several transition metal ions in pulp. Special attention was paid to metal retention and the impact of a sodium borohydride treatment, and the formation and evolution of complexes between metal ions and pulp components was scrutinized.

Fractionation of lignosulfonates: comparison of ultrafiltration and ethanol solubility to obtain a set of fractions with distinct properties

Abstract Commercial lignosulfonates (LS), recovered from bisulfite pulping processes, are known for their wide polydispersity. In this study, LS were fractionated by ultrafiltration (UF) and based on their solubility in ethanol (EtOH) solutions. The combination of both methods leads to a set of fractions of varying properties, suitable for several applications. UF gives well-separated fractions, with low polydispersity, and increasing functionality (sulfonate and phenolic hydroxyl groups) with decreasing molecular weight (MW). EtOH solubility yields more polydisperse fractions but still appears to be an easy and rapid way to obtain fractions with increasing MW. The residual sugars in LS, resulting from incomplete removal of hemicelluloses, were found to be mainly in the form of low MW oligomers, which were not bound chemically to lignin. Thermal degradation of the different fractions was also compared, giving better comprehension of the influence of the structure and composition. A glass transition temperature (Tg) could be measured for some fractions and was correlated to their MW.

Comparative effect of ozone, chlorine dioxide, and hydrogen peroxide on lignin: Reactions affecting pulp colour in the final bleaching stage

Abstract Several lignin-like model compounds (vanillin, syringaldehyde, guaiacol, syringol, p-benzoquinone, naphthoquinone) and commercial softwood lignin were submitted to small charges of ozone, chlorine dioxide and hydrogen peroxide in aqueous solution under conditions simulating a final bleaching stage. In the case of ozone, the coloured quinone models were directly destroyed, whereas the phenolic models and lignin underwent a two-step decomposition mechanism: chromophores were formed at very low ozone charges, and then were destroyed with increasing ozone charge. Chlorine dioxide had hardly any effects on the quinone models, but formed coloured groups from the phenolic models and lignin. However, these were more intensely coloured and were only partially removed with higher ClO2 charges. As for hydrogen peroxide, the colour of lignin and naphthoquinone were directly removed, at least partially, but high H2O2 charges were necessary. Consequently, ozone seems to be the best reagent for final bleaching in which small chemical charges are applied. We suggest reaction mechanisms between ozone and the phenolic model compounds and p-benzoquinone that can explain the observations.

The chromophores remaining after bleaching to moderate brightness

Abstract A pine kraft pulp was bleached to 84% ISO brightness with an ECF DoEpD1 and a TCF OQ(PO)ZQ(PO) sequence. Both bleached pulps were subjected to ClO2, H2O2, and NaBH4. EPR analyses confirmed that the ECF pulp contained more quinone groups than the TCF pulp which contained more quinone–phenol groups. The colored compounds present at the end of the ECF bleaching sequence were less conjugated than the colored compounds present at the end of the TCF sequence. UV resonance Raman analyses performed on these pulps highlighted the differences between ECF and TCF bleached pulps, the latter containing more aromatics relative to C˭O groups. The bleachability of pulps impregnated with p‐benzoquinone, a quinone model, and alizarin, a quinone–phenol model, were compared with the ECF and TCF pulps. The quinone impregnated pulp behaved like the ECF pulp, whereas the quinone–phenol impregnated pulp behaved like the TCF pulp.

AFFINITY OF METAL IONS FOR KRAFT PULPS STUDIED BY ESR. INHIBITION OF THEIR CATALYTIC ACTION IN OXYGEN BLEACHING

ESR spectrum of an unbleached kraft pulp exhibits several signals which are attributed to phenoxy or semi-quinone radicals and also to paramagnetic metal ions such as FeIII and MnII. The same signals are visible on a totally bleached pulp even though their intensity is lower. Analysis of FeIII and MnII signals reveals that these ions form complexes with lignin or cellulose. Pulp treatment with NaBH4 is followed by ESR spectroscopy. Reduction of the metal ions to their lower valence levels is observed (FeIII → FeII or Fe0, MnII → Mn0). New complexes are formed with the pulp components. These observations are related to the protecting effect of the NaBH4 pre-treatment, observed during oxygen bleaching.

Biorefinery in a pulp mill: simultaneous production of cellulosic fibers from Eucalyptus globulus by soda-anthraquinone cooking and surface-active agents

Abstract Industrial Eucalyptus globulus wood chips were submitted to different autohydrolysis conditions followed by kraft cooking and soda-anthraquinone cooking. The autohydrolyzed wood chips were much easier to delignify than the control wood chips. Soda-anthraquinone cooking could be performed at a cooking temperature that was 20°C lower than that for the kraft cooking on control wood chips. Furthermore, the active alkali could be reduced. The resulting unbleached pulps reacted as well to oxygen delignification as the control pulps and could be further bleached to 90% ISO brightness with a D(EP)D sequence. The autohydrolysis liquors were investigated for their suitability as a source for the synthesis of alkylpolyxylosides (APX). These surface-active agents are synthesized through the reaction between the saccharides of the autohydrolysates and a fatty alcohol, with the former being the hydrophilic and the latter being the hydrophobic part of the molecule. The impact of the substances detected in autohydrolysates on the APX synthesis was studied. It was demonstrated that lignin dissolved during autohydrolysis should at least partially be removed before the production of APX.

Production of hemicellulose oligomers from softwood chips using autohydrolysis followed by an enzymatic post-hydrolysis

Abstract In the context of value added valorization of hemicelluloses (HCs), their soft extraction by autohydrolysis (AH) of softwood (SW) chips has been optimized via the temperature/time parameters (170°C/2 h, 170°C/1 h and 150°C/1 h). Two enzyme mixtures containing mainly a glucanase and a mannanase were used to decrease the degree of polymerization (DP) of the extracted HCs. Hydrolysates containing HCs were analyzed in terms of monomers and oligomers, molecular weight distribution (MWD) and chemical composition. The MW was strongly dependent on AH conditions: most of the water-soluble HCs with 1800 Da MW were obtained at 150°C/1 h. The parameters 170°C/2 h gave rise to MWs<1800 Da. Enzymatic hydrolysis (EH) reduced efficiently the DP of HCs, and the glucosidase was more efficient than the mannanase, but the former also hydrolyzed more oligomers into their monomeric components.