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 pK_a) 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.

We report the discovery of the hydrotropic properties of a recyclable aromatic acid, p-toluenesulfonic acid (p-TsOH), for potentially low-cost and efficient fractionation of wood through rapid and near-complete dissolution of lignin. Approximately 90% of poplar wood (NE222) lignin can be dissolved at 80°C in 20 min. Equivalent delignification using known hydrotropes, such as aromatic salts, can be achieved only at 150°C or higher for more than 10 hours or at 150°C for 2 hours with alkaline pulping. p-TsOH fractionated wood into two fractions: (i) a primarily cellulose-rich water-insoluble solid fraction that can be used for the production of high-value building blocks, such as dissolving pulp fibers, lignocellulosic nanomaterials, and/or sugars through subsequent enzymatic hydrolysis; and (ii) a spent acid liquor stream containing mainly dissolved lignin that can be easily precipitated as lignin nanoparticles by diluting the spent acid liquor to below the minimal hydrotrope concentration. Our nuclear magnetic resonance analyses of the dissolved lignin revealed that p-TsOH can depolymerize lignin via ether bond cleavage and can separate carbohydrate-free lignin from the wood. p-TsOH has a relatively low water solubility, which can facilitate efficient recovery using commercially proven crystallization technology by cooling the concentrated spent acid solution to ambient temperatures to achieve environmental sustainability through recycling of p-TsOH.