Halogen-free “green and mild” process to generate N-oxoammonium cations for use in oxidation of hydroxyls and its quantitative analysis technique and optimization

Abstract

Oxoammonium cation (TEMPO+) is a powerful oxidant, generated from nitroxyl radicals (TEMPO). The compound has attracted vast attention as a promising selective oxidant in biology and materials fields. Although TEMPO is regarded as a “green” chemical, its use with halogenated co-oxidants has raised serious environmental concerns. In this study, a halogen-free process has been developed by using ozone as an effective, low-cost, and environmentally-friendly alternative in the generation of TEMPO+. An iodine titration combined with UV–Vis spectroscopy is employed to quantitatively monitor the reaction at a fast speed. The results showed that the conversion of TEMPO reached 70% when the ozone feed content and the reaction time were optimized at 25.60 mM and 45 min, at 4–5 °C, in H2SO4 solution. The TEMPO conversion rate is dependent on the reaction time. A fast reaction rate was observed in the first fifteen-min period and became slower in the subsequent period, depending on the ozone feed contents. The difference in concentrations of the TEMPO and the TEMPO+ decreases in the presence of high ozone feed contents, implying the high selectivity of ozone towards TEMPO oxidation to form TEMPO+ over the disproportionation in acid solutions. The developed “green and mild” process has a high potential for selective oxidation of primary alcohols to other high-value products. The optimized conditions were employed to prepare TEMPO+ for use in the oxidation of Quaternized chitosan (QC). Chemical structures of the resulting Chitosan polyampholyte (CPA) products were characterized, and the Degree of oxidation (DO) was determined. CPA with an OD value as high as 100% was obtained with 1.867 mmol of TEMPO+ per gram of QC.