Progress and Challenges in the Isolation of Xylan from Eucalyptus Wood

There is an increasing interest in utilizing xylan from hardwood in the food and non-food area. The liberation of xylan from the cell wall matrix of wood is, however, highly restricted by the strong interactions with the lignin network. Xylan stability against alkaline pulping is particularly high in Eucalyptus globulus owing to its peculiar structure containing galactosyl and glucosyl units attached at O-2 of 4-O-methyl--D-glucuronic acid (MeGlcA). Nevertheless, alkaline pulping constitutes a suitable platform for the selective and efficient separation of xylan from cellulose and lignin. The use of Eucalytpusglobulus allows the entire replacement of hydrogen sulfide by anthraquinone in the cooking liquor during the delignification part of wood fractionation, which greatly facilitates the isolation of separated lignin and hemicelluloses fractions. More than 30% of the wood xylan can be isolated as polymer in relatively high purity by pre-alkaline extraction applying appropriate conditions, while subsequent alkaline pulping exertsonly a minor effect on pulp yield. The xylan yield may be increased to more than 60% by combining pre-alkaline extraction with cold alkaline post-extraction. The resulting hemicellulose-lean pulp constitutes a novel generation of dissolving pulps. Autohydrolysis prior to alkaline cooking represents a reliable concept to separate xylan, predominantly in oligomeric form, provided that lignin precipitation can be kept under control. The direct formation of monomeric xylose was accomplished by the addition of sulphuric acid or oxalic acid as catalysts which favour the cleavage of the glycosidic bonds of the xylo-oligosaccharides rather than dehydration and other side reactions. In contrast to neutral or alkaline pretreatments, acid pretreatments of wood render the remaining carbohydrate fraction very vulnerable against alkaline post-treatments such as kraft or sodaanthraquinone (SAQ) pulping.