5.2.3.3.1 Other Types of AcidsDue to the low water solubility of solid organic acids, they can be easily recovered after acid hydrolysis through crystallization at ambient temperature [196]. Chen and Zhu [197] obtained carboxylated CNC (CCNC) and CNF from a bleached eucalyptus kraft pulp using oxalic acid atthe concentrations between 50 and 70 wt.%. They increased the onset thermal degradation temperature to 322C compared to 218C for CNC produced with traditional mineral acids. In addition, longer CNC and CNF were obtained due to the lower strength of organic acids.Phosphotungstic acid (PTA) was also used to produce CNC as an example of heteropoly acid, due to its relatively simple regeneration and notably less corrosive action than mineral acids [198]. They proposed the destruction of cellulose in acetic acid/PTA with H2O2.Obtained CNC formed stable hydrosols that after freezedrying tend to agglomerate into different structures depending on the concentration.Bian and Chen [199] used 60 wt.% maleic acid at 120C for 120 min to produce ligno-CNC (LCNC) andligno-CNF (LCNF) from two unbleached hardwood chemical pulps of different lignin contents (3.9% and 17.2%). According to their results, a low LCNC yield was obtained (lower than 6%), defibrillating the nonhydrolyzed material to CNF. A lower carboxylated and therefore charged LCNC and LCNF were obtained compared to lignin-free CNC and CNF, but obtaining a more hydrophobic and thermally stable material, which is more favorable for composite applications.