Behavior of xylans from Eucalyptus species. Part 1. The influence of structural features of eucalyptus xylans on their retention in kraft pulp

Abstract

4-O -methylglucuronoxylans were isolated from the wood and pulp of different Eucalyptus species ( E. dunni, E. grandis, E. nitens, E. urograndis and E. urophylla) and characterized by 1 H NMR spectroscopy and size-exclusion chromatography (SEC). The content of 4-O -methylglucuronic acid (MeGlcA) varied between the eucalyptus species from 1.90 to 2.80 expressed as MeGlcA/10 xyloses, whereas in the pulp, the total uronic acid content was approximately 1.02, from which the half was hexenuronic acid (HexA). From the MeGlcA linked to the xylan chain 9 -26 % are substituted in position O -2 by hexose units. The M w of the xylans extracted from wood ranged between 26.1 and 28.4 kDa. The retention of 4-O -methylglucuronoxylans (51.3 -58.7 % ) during kraft pulping was high, and the retention is related to the content of MeGlcA group, the content of MeGlcA group substituted at O -2, and the average molecular weight of the xylans. The parameters observed affect the overall pulp yield and pulp properties.

Retention and Structure of Xylans fromEucalyptus GlobulusGenotypes with Different Pulpwood Characteristics

“…[23] Studies with different Eucalyptus species showed that the lower degradation of xylans during kraft pulping could be associated with a higher molar mass or more branched chains of the polysaccharide with MeGlcA. [8,9] For our samples, MeGlcA per 10 xylose units (determined by acid methanolysis) were in the range of 1.03-1.37 and 0.79-0.87 for samples of groups I and II, respectively, which supports the previous results.…”

“…[7,17,21,22] The The degree of substitution (DS) of the xylose residues with MeGlcA and HexA ( Table 2) was calculated by the integration of the corresponding signals in the 1 H-NMR spectra, following the procedures described elsewhere. [7,9,17,21,22] Xylans isolated from wood holocellulose of samples from group I presented high DS with MeGlcA (1.4 to 1.9 MeGlcA/10 xylose units) in comparison with xylans from group II (0.9 to 1.3 MeGlcA/10 xylose units). As previously reported, these values are usually higher than those determined by acid methanolysis.…”

“…The relative content of methylglucuronic acids (MeGlcA) and hexenuronic acids (HexA) was determined by the integration of the corresponding signals in the 1 H-NMR spectra. [9,21,22] Size Exclusion Chromatography (SEC)…”

“…As previously reported, these values are usually higher than those determined by acid methanolysis. [9,24] According to Pinto et al [7] and Sixta, [11] the sidechain substituents of the xylans significantly hamper the alkaline degradation, contributing to the retention of xylans in pulp. The formation of HexA from MeGlcA during kraft pulping also stabilizes the reducing ends of xylan chains and contributes to their retention in pulps.…”

“…Xylans are less degraded and more stable than glucomannans during kraft delignification, which is attributed to the stabilizing effect of the methylglucuronic acid (MeGlcA) side-groups that stop the peeling reactions in the reducing ends of the xylan chains. [6][7][8][9] Xylans retention has also been associated with lignin-carbohydrate complexes (LCC) formed during pulping that can be retained or reprecipitated in pulps. [2,7,[10][11][12] Pinto et al [7] and Magaton et al [8,9] demonstrated that, for different Eucalyptus species, the retention of xylans in kraft pulps is affected by their degree of substitution with MeGlcA, alkali stability, and molar mass.…”