Yuan-Zong Lai

Estimation of Phenolic Hydroxyl Groups in Wood by a Periodate Oxidation Method

Abstract Oxidation of woodmeals with an excess of aqueous sodium periodate solution at 4°C released an amount of methanol approximately equivalent to the phenolic hydroxyl content of the lignin as determined by the aminolysis method. The periodate and aminolysis methods gave very comparable data for Norway spruce and aspen woodmeal samples.

Determination of Phenolic Hydroxyl Groups

The phenolic hydroxyl group is one of the most important functionalities affecting the physical and chemical properties of lignin polymers (Adler 1977). It plays a prominent role in commercial pulping and bleaching processes by virtue of its ability to promote the base-catalyzed cleavage of interunitary ether linkages and the oxidative degradation of lignin (Gierer 1985, 1986). The chemical reactivity of lignin in various modification processes is also profoundly influenced by its phenolic hydroxyl content (e.g., in the reaction with formaldehyde for the production of lignin adhesives) (Olivares et al. 1988). On the other hand, the phenolic hydroxyl group significantly contributes to the poor brightness stability of lignin-containing pulps, which seriously limits their more widespread utilization (Gellerstedt and Pettersson 1977, Tschirner and Dence 1988). Quantitative measurement of phenolic hydroxyl groups thus provides pertinent information relating to the structure and reactivity of lignin and to the mechanism and extent of lignin degradation.

Variation of the phenolic hydroxyl group content in wood lignins

SummaryThe phenolic hydroxyl group content of wood lignin has been determined in situ by a periodate oxidation method for four softwood and six hardwood species. Hardwood lignins, in contrast to softwood lignins, showed a significant variation among different species in this functional group content which decreased with an increase in the proportion of syringyl units in the wood lignin.

Variation of phenolic hydroxyl contents in unbleached kraft pulps

The content of phenolic hydroxyl groups in the unbleached kraft pulp residual lignin was determined in situ by periodate oxidation method. This functional group has been shown to increase steadily with the extent of kraft delignification throughout the entire pulping process.

The Distribution of Phenolic Hydroxyl Groups in Hardwood Lignins

Abstract Periodate oxidation, because of its high selectivity in the degradation of free-phenolic units, has been used along with the combined phenyl nucleus exchange and nitrobenzene oxidation techniques to estimate the distribution of phenolic hydroxyl groups in hardwood lignins. Analyses of the aspen and white birch wood lignin in situ indicate that the percentage of uncon-densed structures having a phenolic hydroxyl group was considerably higher for guaiacyl (∼20%) than for syringyl (−5%) units. Thus, the total phenolic hydroxyl groups of hardwood lignins are largely associated with the guaiacyl propane units.

The Chemical Aspects of Acidic Delignification Processes. 1. Role of Aryl-Ether Hydrolysis in Aspen

The significance of aryl-ether hydrolysis in the acidic delignification of aspen in both water and methanolic Systems was evaluated. The hydrolysis of ß-aryl ether units, in addition to the non-cyclic oc-aryl ethers, was shown to have a direct irapact on the initial rapid delignification up to about 30 and 60 % level for the water and methanolic Systems, respectively. Further delignification was also affected by other chemical reactions, e.g. lignin condensation.

Kinetics of base-catalyzed degradation of phenyl d-gluco-pyranosides

Abstract The kinetics of base-catalyzed cleavage of the glycosidic linkage in phenyl α- and β- d -glucopyranoside have been studied at various concentrations of base, and are interpreted in terms of formation of anionic species as reactive intermediates, accompanied by a minor mechanism of bimolecular, nucleophilic substitution. The data indicate that, in M sodium hydroxide solution, more than 80% of the cleavage reaction proceeds through an intramolecular-displacement process facilitated by the anchimeric assistance of the hydroxyl group at C-6, and C-2, for the α- and β- d -glycoside, respectively. However, the bimolecular substitution reaction becomes increasingly apparent as the alkalinity is raised above 1.5 M concentration of base.

Characteristics of Bagasse Lignin in Situ and in Alkaline Delignification

The structural features of depithed bagasse lignin and its changes in soda cooking have been characterized in situ by the nitrobenzene oxidation, phenyl nucleus exchange and periodate oxidation methods. The bulk of bagasse lignin (∼ 75%) can be readily delignified in alkali below 100°C, and does not appear to be resulted from the cleavage of β-aryl ether structures. Concerning the reactivity of uncondensed structures, the p-hydroxyphenyl units were most labile to alkaline treatments, whereas the guaiacyl and syringyl units were removed nearly at the same rate. The residual lignin appears to be rather condensed and to contain an appreciable amount of guaiacyl diphenylmethane-like structures.

The effect of wood species in sulfonation

The extent of neutral sulfonation for a variety of softwood and hardwood species was shown to be directly proportional to the phenolic hydroxyl group content of the wood lignin. Sulfonation of hardwoods, in contrast to softwoods, displayed a significant variation among different species.

Attempts to understand the nature of phenolic and etherified components of wood lignin

SummaryPeriodate oxidation, because of its high selectivity in degrading phenolic nuclei, has been combined with nitrobenzene oxidation and phenyl nucleus exchange techniques to investigate the nature of wood lignin in situ. For both softwood and hardwood, the phenolic and etherified components of wood lignin have been shown to differ significantly in chemical composition, and the etherified lignin structure appears to be substantially more condensed.