The acidic groups in wood and pulp as measured by ion exchange

Abstract

The ion‐exchange capacities were determined for a variety of wood samples consisting of both softwood and hardwood. The experiments revealed that considerably more acidic groups are present in heartwood than in sapwood. The capacity (carboxyl) values found for all samples were substantially lower than would be expected from the estimated uronic acid contents. This difference was more striking for hardwood than for softwood. After alkali treatment (pH 11.5, 64°C), the exchange capacity of the wood was increased considerably and this effect was more pronounced for hardwood than for softwood. This observation either indicates the presence of carboxylic acid estera and lactones or is attributable to an increased accessibility of carboxyls. Such an increase may occur in hardwood as a consequence of the deacetylation of xylan. In another series of experiments the effect of sulfonation was studied. Wood samples (Picea abies, Betula verrucosa, and Betula pubescens) were sulfonated at various pH values at 80 and 90°C. so that only small quantities of the wood substance were dissolved. In addition, the number of acidic groups was determined for a series of spruce and birch pulps with varying lignin contents. It was possible to distinguish between the carboxyl and sulfo groups by means of an ion‐exchange method developed for that purpose. The experiments revealed that the accessibility of carboxyl groups is considerably increased even after sulfonation at a low temperature. It also became apparent that most of the carboxyl groups are accessible even in pulps with relatively high lignin contents. The carboxyl values together with xylose analyses showed that the acidity of xylan remaining in pulp is substantially decreased during cooking. This decrease is more pronounced for birchwood than for sprucewood. Finally, it is evident from these experiments that sprucewood lignin is sulfonated to a markedly higher degree than birchwood lignin.

Investigations of Fresh and Biologically Decayed Birch

“…The high uronic acid content in the fresh birch in relation to the number of carboxylic acid groups can be explained by the fact that the ion-exchange method only measures free carboxylic acid groups, while the decarboxylation method also measures ester and lactone groups, which are known to occur in hardwoods (Sjöström et al 1965). The low content of 0.6% for uronic acids in the decayed wood compared to 4% in the fresh birch wood is noteworthy, because it has been shown (Theander 1987) that glucuronic acid by heating at 80°C gives dark reaction products.Thus it is possible that the low uronic acid content may partly explain the light colour of decayed wood.…”

Improved impregnation efficiency and pulp yield of softwood kraft pulp by high effective alkali charge in the impregnation stage

“…As already pointed out, an increased shives content is one of the results of inhomogenous alkali concentration in the chips. The deacetylation increases the ion exchange capacity of wood (Sjöström et al 1965), which is also manifested by an improved enzymatic hydrolysis due to the higher accessibility of the polysaccharide structures to enzymes (Kong et al 1992). The water retention ability of wood meal also increases with increasing degrees of deacetylation (Zanuttini et al 1998).…”

Rapeseed straw as a renewable source of hemicelluloses: Extraction, characterization and film formation

“…1) has been assigned to acetyl groups that have been cleaved from the chain due to deacetylation (Lundqvist et al, 2002;Teleman, Tenkanen, Jacobs, & Dahlman, 2002; Teleman, Lundqvist, Tjerneld, Stålbrand, Höije et al, 2005;Xu et al, 2010), and the signal is especially strong for extracts D and H which can be explained by higher degrees of deacetylation of hemicelluloses extracted under alkaline conditions. Sjöström et al (1965) and Zanuttini et al (1998) have demonstrated that during impregnation of wood in alkaline medium, deacetylation increases with increasing alkali. The mechanism of deacetylation during the extraction of oil-rapeseed straw should be similar to the deacetylation during impregnation in wood.…”

“…An overall lowering of pH was observed during extraction except for sample A ( Table 2). The decrease in pH is explained by the release of acetic acid formed due to cleavage of acetyl pendant groups present on the hemicellulose backbone (Sjöström, Janson, Haglund, & Enström, 1965;Zanuttini, Citroni, Martínez, & Marzocchi, 1998). The yield was calculated as the amount of freeze-dried material obtained divided by the amount of dry weight straw charged.…”