Carboxy group-rich cellulose fibers and nanofibrils from wood cellulose by aqueous catalytic oxidation with sodium dichloroisocyanurate

Aqueous catalytic oxidation with 2,2,6,6-tetramethylpiperidine-1-oxyl radical and sodium dichloroisocyanurate (NaDCC) at pH 9 and ~23 °C is an environmentally adaptable system for the position-selective oxidation of the C6–OH groups of crystalline wood cellulose microfibril surfaces to sodium C6-carboxylate groups without significant depolymerization or side reactions. Carboxy contents of the water-insoluble oxidized products increased from 0.02 to 0.06 mmol/g up to 1.1–1.5 mmol/g (i.e., increases of 18–75 times) by oxidation. Nevertheless, the fibrous morphologies and molar masses of the oxidized products were nearly the same as those of the original wood cellulose fibers. The water-insoluble oxidized products were then converted into nearly individualized cellulose nanofibrils with homogeneous widths and lengths of >1 μm via mechanical fibrillation in water. Consequently, the NaBr/NaClO-free catalytic oxidation system with NaDCC for wood cellulose fibers can be regarded as a green chemical process for the preparation of carboxy group-rich cellulose fibers and nanofibrils. Crystalline wood-cellulose microfibril models were proposed based on the homogeneous chemical and solid-state structures of the oxidized products. The abundant carboxy groups in the oxidized cellulose fibers/nanofibrils are possibly used as scaffolds to introduce various cationic compounds for further functionalization by a simple ion-exchange treatment in water.