Influence of drying process on reactivity of cellulose and xylan in acetylation of willow (Salix schwerinii E. L. Wolf) kraft pulp monitored by HSQC-NMR spectroscopy

The acetylation behavior of xylan and cellulose in kraft pulp (KP) was investigated. Heteronuclear single quantum coherence-nuclear magnetic resonance (HSQC-NMR) spectroscopy was used to identify the positions at which the hydroxyl groups of polysaccharides were acetylated. X-ray diffraction analysis was used to determine the acetylation of cellulose in the crystal region. It was found that only xylan was obviously acetylated in the initial stages of the reaction. In the next stage of the acetylation, however, both the xylan and surface of the cellulose crystals reacted. Then, acetylation of the inner crystal region of the cellulose started before the completion of the xylan modification. To improve the reactivity of xylan in KP, the effect of drying or the dehydration of never-dried KP prior to acetylation was also investigated. The use of supercritical CO₂ (scCO₂) drying produced a KP with a higher specific surface area. This caused the xylan to exhibit a higher reactivity than that resulting from freeze drying and vacuum drying after solvent exchange. Furthermore, never-dry dehydration pretreatment produced xylan with the highest reactivity. This suggests that the cellulose–cellulose and/or cellulose–hemicellulose adhesion in the cell wall that arises as a result of drying reduces the reactivity of the xylan.