Theoretically, higher catalyst concentration can enhance the degree of hydrolysis of cellulose. Unfortunately, the hydrolysis process is not only occurs in the cellulose defect domains but also in its crystalline structure. The defective crystalline phase in cellulose chains is more easily decomposed to its water soluble oligomers in the presence of metal ion catalyst than that of crystalline phase owing to its tightly packed structure and strong hydrogen bonding . Thus, enhancing the hydrolysis degree of less-ordered cellulose phase by increasing the metal ion concentration benefits the production of high crystallinity nanocellulose. However, it is necessary to find an optimal metal ion concentration, as it is possible that some crystalline nanocellulose segments could gradually be hydrolyzed to further convert to liquid organic molecules as the secondary product when treated with an extremely high concentration of metal salt catalyst. Figure 1c reveals that when Cr(NO3)3 concentration increased from 0.2 to 1.2 M, the nanocellulose crystallinity progressively increased from 62.5% to 87.1% while maintaining reaction duration for 1.5 h, and a solid–liquid ratio of 1:30 at 80 °C. Simultaneously, the product yield decreased almost linearly with increasing concentration of metal salt. This implied that the successive degradation of defective crystalline allomorphs after the catalytic hydrolysis process induces the exposure of high crystalline segment in the nanocellulose, which leads to increasing crystallinity. However, it is worth noting that the increasing tendency of crystallinity index became slower when the metal salt concentration was higher than 0.8 M. Therefore, for better economic competence, 0.8 M metal salt catalyst was selected as the optimum concentration for hydrolysis.