Effect of coexisting salt on TEMPO-mediated oxidation of wood cellulose for preparation of nanocellulose

The influence of coexisting salt in 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation of wood cellulose in water at pH 10 for 100 min was investigated, in which Na2SO4 was partly used in place of NaBr in the conventional TEMPO/NaBr/NaClO oxidation system. The amount of NaBr could be reduced from 1 to 0.2 mmol/g-wood cellulose by adding 0.4 mmol/g Na2SO4. This introduced a carboxylate content of similar to 1.2 mmol/g, which is sufficient to prepare TEMPO-oxidized cellulose nanofibrils (TOCNs) by mechanical disintegration of the oxidized cellulose in water. When no NaBr was used and Na2SO4, Na2SO3, NaCl or CH3COONa was instead used as a coexisting salt in TEMPO/NaClO oxidation, the oxidized celluloses had carboxylate contents of 0.6-0.8 mmol/g, which are insufficient to prepare TOCNs with nanofibrillation yields > 55%. The viscosity-average degrees of polymerization of the resulting TEMPO-oxidized celluloses were in the range 420-450, indicating that depolymerization of cellulose is not governed by the carboxylate content of the TEMPO-oxidized cellulose but rather by the oxidation time.