Rate-limiting steps in bromide-free TEMPO-mediated oxidation of cellulose—Quantification of the N-Oxoammonium cation by iodometric titration and UV–vis spectroscopy

• Published in: Applied catalysis. A, General General, 2015-09, Vol.505, p.532-538
• Main Authors: Pääkkönen, Timo, Bertinetto, Carlo, Pönni, Raili, Tummala, Gopi Krishna, Nuopponen, Markus, Vuorinen, Tapani
• Format: Article
• Language: English
• Subjects: Birch pulp; Iodometric titration; Multivariate curve resolution; Oxoammonium cation; TEMPO-mediated oxidation; UV–vis spectrometry
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/j.apcata.2015.07.024

[Display omitted] •Iodometric titration can be applied to determine TEMPO+.•UV–vis spectrometry with MCR can be utilized to analyze TEMPO and TEMPO+.•The conversion of TEMPO to TEMPO+ can be monitored with iodometric titration.•HOBr is a more effective activator than HOCl.•Preactivation of TEMPO enhances bromide-free TEMPO-mediated oxidation of pulp. A iodometric titration method was introduced to study the conversion of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) to the corresponding N-oxoammonium cation (TEMPO+) by hypochlorite in the absence and presence of bromide ion. The validity of the titration was verified with UV–vis spectroscopy combined with a multivariate curve resolution (MCR) algorithm to calculate the concentrations and spectral signatures of the pure components (i.e., TEMPO, Cl(+1) and TEMPO+). The formation of the oxoammonium cation was successfully followed during the activation of TEMPO by HOCl and HOBr. It was found that HOBr is a more effective activator for TEMPO than HOCl is. Moreover, the importance of a separate activation step for TEMPO with bromide-free TEMPO oxidations could be identified with this titration method. The content of TEMPO+ was also monitored during the TEMPO-mediated oxidation of a cellulosic pulp by hypochlorite in the absence and presence of bromide. It was found that the oxidation of the alcoholic groups by TEMPO+ was generally the rate-determining step and much slower than the regeneration of TEMPO+ through oxidation of the hydroxylamine by HOCl and HOBr. However, at high pH the latter reaction became rate-limiting.