Optimizing addition of NaClO in TEMPO-mediated oxidation of cellulose for less nanofiber degradation

Abstract

The TEMPO-mediated oxidation is one of the most popular methods to prepare cellulose nanofibers due to its easy operation, high oxidation efficiency and resulting high aspect ratio nanofibers with uniform size. Among TEMPO-mediated oxidation methods, the TEMPO/NaBr/NaClO (TBN) oxidation system is the most commonly used and has the highest oxidation efficiency. However, cellulose suffers from the degradation by NaClO in TBN oxidation, which can be directly reflected in the reduction of degree of polymerization of cellulose. In this paper, we explored the effect of the content of NaClO on oxidation and figured out an optimization method where NaClO was added periodically for TBN oxidation system (semi-batch process) that could efficiently oxidize hardwood bleached kraft pulp (HBKP) cellulose while reducing the degradation. This phenomenon is most obvious when the addition amount of NaClO is 10 mmol/g and the reaction time is 7.5 h, the carboxyl content of the TEMPO oxidized cellulose obtained by adding NaClO in different methods has no significant change (both are around 1.5 mmol/g), but the degree of polymerization of TEMPO oxidized cellulose obtained by adding NaClO periodically is 2.3 times that of TEMPO oxidized cellulose obtained by adding NaClO in one batch. This optimization method provides a feasible way for the adjustable preparation of nanofibers with high carboxyl content by maintaining relative high degree of polymerization.