Influence of Chemical and Enzymatic TEMPO-Mediated Oxidation on Chemical Structure and Nanofibrillation of Lignocellulose

Chemically pretreated lignocellulose (CL) from poplar wood and commercial thermomechanical pulp (TMP) were oxidized using a TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)/NaBr/NaClO system and a TEMPO/laccase/O2 system in water at pH 10 and 6.8, respectively. The formation rate of carboxylate groups was slower than that of delignification in the TEMPO/NaBr/NaClO oxidation of CLs. The carboxylate content of TEMPO/NaBr/NaClO-oxidized TMP rapidly achieved a stable value of 1.6 mmol/g along with continuous degradation of lignin. The yields and lignin contents of TEMPO/NaBr/NaClO-oxidized TMPs decreased from 95% to 70% and 17% to 5%, respectively. In contrast, the formation of carboxylate groups and delignification simultaneously occurred in TEMPO/laccase/O2 oxidation and resulted in less loss of lignin. The yields and lignin contents of oxidized TMPs containing 1.8 mmol/g carboxylate groups were 78% and 15%, respectively. The crystal structure of oxidized lignocellulose was minimally changed. The remaining lignin in TEMPO/NaBr/NaClO-oxidized TMP exhibited significant inhibition on the separation of nanofibers. However, TEMPO/laccase/O2-oxidized TMP exhibited high water retention values and good dispersion properties after sonication. When the TMP sample was oxidized 6 times via the TEMPO/laccase/O2 system and suffered sonication treatment, almost all lignin was well dispersed in water together with cellulose nanofibers. The obtained lignocellulose nanofibers exhibited the ability to prepare reduced Ag nanoparticles. The water contact angle of further prepared films was increased in comparison with that of pure cellulose nanofiber film.