Modified Fenton Oxidation of Cellulose Fibers for Cellulose Nanofibrils Preparation

A novel catalytic oxidation process based on the Fenton reaction (H2O2–FeSO4) was developed to pretreat cellulose fibers for the preparation of cellulose nanofibrils (CNF). In the so-called modified Fenton process, softwood bleached kraft pulp (SWBK) fibers were utilized as individual microreactors to carry out efficient in situ oxidation of cellulose chains, which in turn facilitated nanofibrillation of fibers in subsequent mechanical treatment. Ferrous ions were preloaded into the fiber cell wall by adsorption and diffusion, which initiated the catalytic oxidation of cellulose simultaneously inside the fiber cell wall structure when hydrogen peroxide was introduced. The C2, C3, and C6-hydroxyl groups on the glucosyl of cellulose chains were oxidized to carboxyl groups, which could enhance the separation of micro/nanofibrils by increasing the electrostatic repulsion of the fibrils. The carboxyl group content was found to increase from 39 to 56 mmol/kg after the oxidation pretreatment. The oxidation also caused breakage of the 1,4-β-d-glucoside bonds of cellulose chains and dramatically decreased the degree of polymerization (DP) of the cellulose macromolecules. The oxidized SWBK fibers were well dispersed into cellulose nanofibrils in the subsequent homogenization treatment. The obtained CNF had a uniform distribution of cellulose fibrils with an average diameter of less than 100 nm.