Role of moisture in esterification of wood and stability study of ultrathin lignocellulose nanofibers

Esterification of wood with maleic anhydride assists its mechanical fibrillation to realize ultrathin lignocellulose nanofibers (LCNFs) with thicknesses of 3 nm. We investigated the effects of the moisture content of raw wood and the esterification reaction time on the properties of the LCNFs. Increasing the moisture content of raw wood decreases both the amount of maleic acid ester and the lignin content in esterified wood. Moisture is required not only for a high degree of esterification but also for the removal of lignin to prepare ultrathin LCNFs suitable for mechanical fibrillation. 2D HSQC-NMR analysis reveals that cellulose, hemicellulose, and lignin are esterified. The maleic acid ester introduced on the LCNF surface tends to be hydrolyzed, resulting in the detachment of the maleic acid ester. Ester hydrolysis, which is facilitated under alkaline and high temperature conditions, converts the LCNF suspension into a gel. Finally, the stability against ultra violet (UV) light irradiation is examined. Since lignin absorbs UV light, the reduction in the viscosity of the LCNF suspension is lower than that of the pure cellulose nanofiber suspension. These results should contribute to the effective utilization of wood, which is the most abundant biomass.