Self Assembly of Hardwood Xylan -In Aqueous Solution and at Cellulosic Surfaces
Doctoral thesis, 2008
The climate change on our planet is an issue of great concern. The dignity of this issue was confirmed by the award of the international panel of climate change (IPCC) the Nobel peace prize. This problem arises from the way that we live our lives, how we produce energy, materials and chemicals from a fossil source. One solution to the problem is to more efficiently utilize the enormous source of biomass that is produced annually. Wood has traditionally been used as timber and as a raw material for pulp and paper production. In traditional pulping, lignin and heteropolysaccharides, so called hemicelluloses, are degraded, which is unfortunate since it is possible to use these polymers to create new renewable products. In order to increase the utilization of biomass, new innovative methods for the isolation and fractionation of wood components must be developed through better knowledge of wood polymer interactions, both chemically and physically.
The aim of this thesis was to isolate hemicellulose by mild conditions from birch and to study their assembly characteristics in both aqueous solutions and at cellulosic surfaces. The isolation of xylan, the main hemicellulose in hardwood, was optimized with regard to prehydrolysis conditions. The prehydrolysis was followed by a mild alkaline extraction. The best prehydrolysis conditions with regard to xylan yield, molecular weight and lowest weight loss (i.e. preservation of wood mass for subsequent pulp production) were achieved at 0.01M acetic acid, 140°C, for 1h. This fraction was composed mainly of glucuronoxylan but also of considerable amounts of lignin (12%). This fraction and other extracts (other prehydrolysis conditions) readsorbed onto fully bleached pulps to different extents (5-25%) depending on their composition and solubility. It was concluded that the adsorbed material consisted of more lignin and less glucuronic acid than the fraction in total. A lignin extraction was therefore performed to study the influence of lignin on agglomeration and adsorption. In total, 75% of the lignin was removed; this lignin (L) was essentially water soluble in the presence of xylan (X) (i.e. in form of the XL complex), but not on its own. This behavior suggests that xylan is capable of forming inclusion complexes with lignin that result in lignin solubility in water and that are based on secondary rather than primary (covalent) bonds. This was also seen in terms of agglomeration in aqueous solutions, where the amounts and size of agglomerates decreased by the removal of lignin. Finally, the tendency for adsorption onto model cellulose surfaces for xylan fractions containing different amounts of lignin was analyzed, and it was observed that lignin enhanced the adsorption process.
This work contributes to knowledge about glucuronoxylan and its interaction with lignin in aqueous solutions and their combined adsorption onto cellulosic surfaces. My intention is that this work will lead to the development of a novel pulping process that better utilizes wood. I also believe that my work can act as a new platform for an understanding of the agglomeration and adsorption mechanisms of glucuronoxylan. Greater knowledge in this field is of great importance for creating tailor made fibers with unique properties and new materials.
Lignin Carbohydrate complexes