Isolation of birch xylan as a part of pulping-based biorefinery

Abstract

This study combines various aspects of xylan isolation from birch wood as part of a pulping-based biorefinery concept. Acidic prehydrolysis and alkaline pre-extraction are the two processes used as the starting point of the work. Solutions of xylan with diverse macromolecular and chemical properties were obtained by applying different pre-treatments. Pre-extraction at high alkalinity produced water-insoluble xylan with a high molar mass and low polydispersity. Autohydrolysis at a mild intensity yielded a liquid phase containing a variety of xylooligosaccharides and xylan of low molar mass. The fragments were mainly acetylated and some of them carried 4-O-methylglucuronic acid substituents. Intensification of autohydrolysis promoted formation of monomeric xylose and its degradation products. The addition of oxalic acid increased the monomeric fraction even at mild prehydrolysis intensities. The properties of cellulose in the wood residue were affected to a smaller or greater extent depending on the type and intensity of the pre-treatment. After alkaline pre-extraction at low temperature and high alkalinity, the macromolecular properties of cellulose were barely affected. After mild prehydrolysis, cellulose was partly depolymerised without a notable yield loss. When more severe prehydrolysis conditions were applied, as required for an almost complete removal of hemicelluloses, both the degree of polymerisation and yield of cellulose were affected dramatically. An attempt to mitigate gradual cellulose degradation induced in the pulping stage by prehydrolysis was made with cotton linters as a cellulose substrate. Sodium borohydride and different types of anthraquinone (AQ) were able to convert a share of the reducing end groups to either alditol or aldonic acid moieties stable to alkaline peeling. Such stabilisation had a positive effect on the yield of cellulose. Stabilisation was also reflected in the decreased ratios between the peeling and stopping reaction rate constants. An improved model for cellulose degradation in alkaline environments was developed that took secondary peeling into account. Application of the stabilisation chemicals to birch wood resulted in a moderate yield increase and the preferred stabilisation of hemicelluloses. Aqueous-phase prehydrolysis of birch wood followed by alkaline pulping produced dissolving pulps of viscose and acetate quality without alkaline post-extraction. Changes of cellulose crystallite dimensions and specific surface area between microfibril aggregates in bleached pulps were observed as functions of prehydrolysis intensity. Mild oxalic acid prehydrolysis and alkaline pre-extraction were shown to be well suited the production of paper pulps, where the latter pre-treatment ensured excellent papermaking properties.