Highly thermal-stable and functional cellulose nanocrystals and nanofibrils produced using fully recyclable organic acids
Here we report the production of highly thermal stable and functional cellulose nanocrystals (CNC) and nanofibrils (CNF) by hydrolysis using concentrated organic acids. Due to their low water solubility, these solid organic acids can be easily recovered after hydrolysis reactions through crystallization at a lower or ambient temperature. When dicarboxylic acids were used, the resultant CNC surface contained carboxylic acid groups which facilitate functionalization and dispersion in aqueous processing. The carboxylic acid group content was 0.1–0.4 mmol g-1 for CNC produced from a bleached eucalyptus kraft pulp (BEP) using oxalic acid at concentrations of 50–70 wt%. The onset thermal degradation temperature for the CNC was increased to 322 °C from 274 °C for the feed BEP fibers, compared with 218 °C for CNC produced from the same feed fibers using conventional concentrated sulfuric acid hydrolysis. The low strength (high pKa) of organic acids also resulted in CNC with longer lengths of approximately 275–380 nm and higher crystallinity than those produced using mineral acids. Fibrous cellulosic solid residue (FCSR) collected from acid hydrolysis was an excellent feedstock for producing CNF through subsequent mechanical fibrillation with low energy input. The ability to recover organic acids using a conventional and commercially proven crystallization method makes these organic acids uniquely suitable for sustainable and green production of cellulose nanomaterials. The resultant CNC and CNF with high thermal stability and a large aspect ratio are excellent for bio-composite applications.