Enhancing the acidity of the hydrolytic system would weaken and thus promote the breakdown of anhydro-glucopyranose units between cellulose polymeric chains, producing smaller cellulose units. Otherwise, there were insufficient hydronium ions present in the reaction system if the metal salt concentration was low. In addition, the Cr3+ metal ions could easily interact with the oxygen atoms of C–O–C glycosidic bonds between the glucose units, leading to the formation of the oxygen-chromium complex intermediate. Due to the adsorbed metal ions, the bonding energy of oxygen atoms and carbon atoms of pyranose rings in the intermediate were reduced because of the increase of its bond length and bond angle . Therefore, the activation energy of the hydrolysis process was reduced. However, as stated earlier, the operational conditions of the hydrolysis reaction must be optimized in order to prevent the carbonization process (formation of char).