Dominique Lachenal

Structure and Reactivity of Spruce Mechanical Pulp Lignins. IV: 13C-NMR Spectral Studies of Isolated Lignins

Abstract 13C-NMR spectroscopy has been used to characterize the lignins extracted by an organosolv flow-through process from spruce TMP, bleached TMP (BTMP) and yellowed BTMP (YBTMP). In general, the 13C-NMR spectra of these lignins are similar to that of spruce MWL, indicating that the lignins underwent limited structural changes. Thus they may be an acceptable substrate for lignin structural study. However, these lignins appear to have a significant amount of condensed aromatic carbons assigned mainly to β-5, 5-5 and diarylmethane structures. These were likely formed during the organosolv cook, or initially present in TMP and extracted with this organosolv process. On the other hand, the hydrogen peroxide bleaching causes little change in the lignin overall structure. As a favorable effect, elimination of the coniferaldehyde structures can clearly be seen. The photoyellowing appears to result in the creation of aromatic carbonyl and carboxyl groups. This suggests that oxidative cleavage of Cα, Cβb bonds...

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.

Structure and Reactivity of Spruce Mechanical Pulp Lignins Part I. Bleaching and Photoyellowing of in situ Lignins

ABSTRACT This paper reports the structural investigation of a series of in situ spruce lignins in wood, thermomechanical pulp (TMP) and the corresponding bleached (BTMP) and photoyellowed (YBTMP) samples. This was achieved by using an original two-step degradative technique, thioacidolysis followed by Raney nickel desulfuration. The determination of thioacidolysis monomeric and dimeric products allowed an estimate of various lignin building units and interunit bonds, respectively. It was observed that the thermomechanical and bleaching treatments did not affect spruce lignin structure to an appreciable extent. On the contrary, the photoyellowing treatment caused marked structural changes in lignin, particularly when run in severe conditions. These changes were essentially a decrease in the amount of β-O-4 and β-1 interunit bonds and a relative increase in catechol units and in vanillin end-groups.

Eucalyptus globulus kraft pulp residual lignins. Part 1. Effects of extraction methods upon lignin structure.

Summary Residual lignins of Eucalyptus globulus kraft pulp were shown to be structurally different depending upon the type of extraction chosen. Enzymatic hydrolysis releases a residual lignin with a well preserved etherified structure but substantially contaminated by protein and carbohydrate moieties coming from the enzyme. Moreover, the extraction yield is poor and obviously it does not work as well as in the case of softwood kraft pulp. The residual lignin extracted by acid hydrolysis has less etherified units and more free phenolic functions than that of the enzymatic hydrolysis. It contains some unsaturated structures but no carbohydrate contaminants. Comparison with Milled Wood Lignin (MWL) treated under the acid hydrolysis conditions shows that ether splitting occurs during this extraction. The different residual lignin fractions were characterised by HPSEC, FT-IR, 13C NMR and sugar analysis.

Comparative Studies of Chlorine Dioxide Reactions with Muconic Acid Derivatives and Lignin Model Compounds

Abstract The reaction of chlorine dioxide with different types of lignin model compounds was investigated in order to compare the kinetics and to evaluate the amount of oxidant consumed by the different substrates. Complete reaction of lignin model compounds was observed at ClO2‐to‐substrate molar ratios of 0.9–1.2, which corresponds to an electron transfer varying between 5–6 equivalents per mole of substrate. Muconic acid derivatives also fully reacted, at a ClO2‐to‐substrate molar ratio of 1.2, with the oxidant consumption being about 4 equivalents per mole of substrate. The reaction of mixtures of phenolic, non‐phenolic, and muconic acid type substrates showed that the reaction rates of non‐phenolic and muconic acid type substrates were rather similar. This study suggests that further reaction between ClO2 and the primary lignin oxidation products, such as muconic acid type structures could be the cause of overconsumption of oxidant in a D stage.

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.

Structure and Reactivity of Spruce Mechanical Pulp Lignins Part III. Bleaching and Photoyellowing of Isolated Lignin Fractions

Abstract Organosolv lignin fractions were used as substrate for further investigations on the effect of bleaching and photoyellowing on lignin. These lignins were extracted from spruce TMP, bleached TMP (BTMP) and bleached and yellowed TMP (YBTMP). Furthermore, lignin fractions from TMP were also directly submitted to bleaching conditions and lignin fractions from BTMP to irradiation in the solid state. These lignin samples were all analyzed by thioacidolysis and FT-IR spectroscopy. The bleaching did not substantially change the lignin polymer but considerably destroyed some chromophoric structures such as coniferaldehyde end-groups. On the other hand, the photoyellowing resulted in marked degradations of lignin, shown by a decrease in the amount of β-O-4 and β-1 interunit bonds and a substantial increase in vanillin and vanillic acid end-groups and in catechol units.

19F NMR spectroscopy for the quantitative analysis of carbonyl groups in lignins

Summary The amount of carbonyl groups in an organosolv lignin was determined by 19F NMR spectroscopy after different derivatizations: derivatization with trifluoromethylphenylhydrazine, fluorobenzoylation of alcohol groups formed after NaBH4 reduction of lignin, fluorobenzoylation of phenolic hydroxyl groups after catalytic hydrogenation of lignin. Oximation was also used for comparison. Carbonyl group contents determined by 19F NMR after trifluoromethylphenylhydrazine derivatization were in good correlation with results from other methods. Trifluoromethylphenylhydrazine derivatization is a fast and easy technique, which provides a distinct quantification of aldehyde, ketone and quinone groups. Some round robin lignins (Milne et al. 1992), using trifluoromethylphenylhydrazine derivatization and modified oximation (Zakis 1994) provided significantly higher carbonyl group contents than the round robin test data, which had been measured either by classical oximation or by reduction-UV methods. Other recently reported carbonyl group estimations on round robin lignins (Faix et al. 1998; Ahvazi et al. 1999) were either in agreement or in disagreement with the herein-presented results. This indicates that carbonyl group analysis in lignins is difficult and that care must be taken when selecting an appropriate method.

Eucalyptus globulus kraft pulp residual lignin. Part 2. Modification of residual lignin structure in oxygen bleaching

Summary Residual lignins were isolated from unbleached and oxygen-bleached Eucalyptus kraft pulps by acid hydrolysis. The structural changes and degradation of residual lignin occurring during kraft pulping and oxygen bleaching were followed and identified by elemental analysis, residual carbohydrate analysis, molecular mass distribution, as well as qualitative and quantitative solution 13C NMR. The dissolved lignins in the kraft cooked and oxygen bleached liquors were also studied and compared with the corresponding residual lignins. Milled wood lignin treated under acid hydrolysis conditions served as a reference for the structural comparison. The results show that etherified syringyl structures were quite resistant towards degradation in the oxygen bleaching, causing little depolymerisation in residual lignin and a small increase in carboxylic acid content, but producing appreciable amounts of saturated aliphatic methylene groups.

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.