Tailored and Integrated Production of Functional Cellulose Nanocrystals and Cellulose Nanofibrils via Sustainable Formic Acid Hydrolysis: Kinetic Study and Characterization

Cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) are of great interest to researchers due to their outstanding properties and wide application potentials. However, green and sustainable production of CNCs and CNFs is still challenging. In this work, the integrated and sustainable production of functional CNCs and CNFs was achieved by formic acids (FA) hydrolysis. Kinetic study for FA hydrolysis of cellulosic pulp was performed to investigate the hydrolysis mechanism. FA concentration of 80–98 wt %, reaction temperature of 70–100 °C, and reaction duration up to 24 h were employed to capture the feature of the coexistence of a diversity of reaction products, i.e., CNCs, cellulose solid residue (CSR), cellulose formate (CF), xylose, glucose, and furfural. The separated CSR was further fibrillated to CNFs by homogenization. It was found that the yield, morphology, crystallinity, thermal stability, and degree of esterification of CNCs and CNFs were significantly affected by hydrolysis conditions (particularly for acid concentration). Detailed characterization indicated that the as-prepared CNCs exhibited high thermal stability (maximal weight loss temperature of 375 °C) and high crystallinity index of 79%. Both the resultant CNCs and CNFs showed good dispersibility in dimethylacetamide due to the introduction of ester groups on cellulose surface during FA hydrolysis. More interestingly, the regenerated CF was also a kind of functional CNFs with more ester groups. These ester groups would enable the CNCs/CNFs to be potentially used in polymeric materials due to the hydrophobic surface. Therefore, this study provided fundamental knowledge for the sustainable and integrated production of thermally stable and functional CNCs and CNFs with tailored characteristics.