2990
Ind. Eng. Chem. Res. 2005, 44, 2990-2996
Evolution of Methylglucuronic and Hexenuronic Acid Contents of Eucalyptus globulus Pulp during Kraft Delignification Joa˜ o P. F. Sima˜ o, Ana P. V. Egas, Cristina M. S. G. Baptista, M. Grac¸ a Carvalho,* and Jose´ A. A. M. Castro† GEPSI-PSE Group, Department of Chemical Engineering, University of Coimbra, Po´ lo II-Pinhal de Marrocos, 3030-290 Coimbra, Portugal
The evolution of 4-O-methylglucuronic (GlcA) and hexenuronic (HexA) acids contents during kraft pulping of Eucalyptus globulus was investigated. Different operating conditions were considered in the range of 80-165 °C for temperature, 10-45 g of Na2O/100 g of oven-dried wood for effective alkali charge, and 15-75% for sulfidity, with a liquid-to-wood ratio of 8 L/kg. The GlcA content was about 5% in the native wood and decreased continuously throughout the cook. Up to 87% of this component was consumed, being dissolved together with the xylan chain or converted to HexA. The latter were detected for temperatures higher than 110 °C. The highest HexA values (∼40 mmol/kg of oven-dried pulp) were observed for delignification degrees close to those of industrial bleachable pulps, and the subsequent decrease in their content was only noticeable for lower lignin amounts. The temperature and the effective alkali charge were confirmed to have a strong influence on the HexA content profiles and on the degradation/ dissolution of GlcA, but the effect of sulfidity revealed to be negligible on any of them. Introduction Kraft pulping has a significant effect on the amount and structure of wood carbohydrates, especially hemicelluloses. For instance, more than 50% of the wood hemicelluloses can be dissolved in the kraft liquor. The removal of these carbohydrates from the wood solid matrix is due to dissolution of low molecular weight chains, end-initiated depolymerization (primary peeling), and alkaline hydrolysis of glycosidic bonds, which leads to secondary peeling and to the decrease in the polymerization degree.1 The latter fact increases the number of chains that can be potentially dissolved. The structure of the acidic hemicelluloses is also modified. Early in the cook, for instance, the arabinose side units of softwood arabinoglucuronoxylan and the acetyl side units of hardwood glucuronoxylan are extensively detached from the xylan chain. The 4-O-methylglucuronic acid side groups of the two native wood xylans (hereafter referred to as GlcA) also undergo alkaline dissolution along with the degradation/dissolution of the xylan chain. Additionally, part of the remaining GlcA is converted to 4-deoxy-4-hexenuronic acid groups (abbreviated to HexA) by the elimination of the 4-Omethoxyl group. The mechanism of these alkali-catalyzed reactions was proposed in 1963 by Clayton2 and confirmed in 1977 by Johansson and Samuelson3 using a dimeric compound model. However, the formation of HexA throughout the pulping process was identified only recently by Buchert et al.,4 and simultaneously, the decrease of the HexA content in the pulp was also detected. In alkaline conditions, this reduction is mainly due to two phenomena: the dissolution of HexA, along with the degradation of the xylan chain, and the degradation of HexA themselves, by alkaline splitting * To whom all correspondence should be addressed. Telephone: +351 239 798 700. Fax: +351 239 798 703. E-mail:
[email protected]. † Deceased.
of HexA from the xylan backbone.5,6 Thus, at each cooking time, the HexA content in pulps is a balance between formation and degradation/dissolution. Since temperature and alkaline profiles may vary with pulping technology and conditions, it is not surprising to find different methylglucuronic and hexenuronic acids profiles.4,7,8 Nevertheless, while GlcA content decreases continuously along the cooking, not depending upon wood species,2,9 there are significant differences between HexA content profiles in softwood and hardwood pulping.10 The formation of HexA in softwood pulping is very fast and occurs mostly in the heating-up period, and their degradation/dissolution overcomes their formation in the bulk phase of the cook.4,5,11 Nevertheless, the studies involving Eucalyptus globulus and other hardwoods6,10,12,13 revealed that, in the pulping process of most of these wood species, the rise in HexA content is very slow and the effects of the degradation/dissolution are not seen until the end of the cook. According to some researchers,10,13 this different behavior might be attributed to the milder conditions used in hardwoods pulping rather than to the chemical structure of the corresponding native wood xylans. Although HexA may prevent the xylan peeling degradation at high temperatures during kraft pulping, it is believed that these groups, even in a small amount (
