Mechanism and kinetics of TEMPO mediated oxidation of cellulose by NaOCl was studied in order to simplify and intensify the manufacturing process for chemically oxidized nanofibrillar cellulose (NFC). Conversion of TEMPO into the nitrosonium cation (TEMPO+) was found to be a rate-determining step in the absence of added NaBr. The use of NaBr could be avoided by preactivation of TEMPO by HOCl or by addition of an equimolar amount of ClO2 which led to immediate and stoichiometric formation of TEMPO+. The initial oxidation rate of cellulose was independent of the (pre)activation method of TEMPO. The content of TEMPO+ during the oxidation was monitored by iodometric titration. At pH &lt; 10 TEMPO existed mostly as TEMPO+ until all NaOCl was consumed after which also TEMPO+ disappeared. In addition, oxidation at high pH in the absence of NaBr led to a low concentration of TEMPO+ during the reaction.<br/>After the initial fast reaction (NaOCl consumption of ca. 1.5 mol/kg), the oxidation was retarded and the further consumption of the oxidant depended on the amount of added NaBr. At this stage the unselective oxidation by HOBr was found to be important; direct oxidation by HOCl was much slower. Thus, although the presence of NaBr increased the consumption of NaOCl and led to a higher carboxylate content of cellulose, the oxidation without NaBr resulted in a more selective oxidation (higher DP of cellulose). Pretreatment of cellulose pulps by mild alkali (&lt; 1 M NaOH) increased the reactivity significantly. In part, the effect was caused by solubilisation of xylan although the direct effect of alkali on the accessibility of cellulose seemed to contribute as well. A carboxylate content of 1 mmol/kg was easy to achieve without using NaBr and the oxidized pulp could be easily disintegrated to form a strong aqueous gel.

Mechanistic and kinetic aspects of TEMPO mediated oxidation of cellulose

251st National Meeting (2016)

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