Assessing the reactivity of cellulose by oxidation with 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxo-piperidinium cation under mild conditions

The accessibility and reactivity of cellulose are key parameters in its conversion into various products. Several indirect measures, such as water retention value (WRV), fiber saturation point (FSP) and specific surface area (SSA), are often used to characterize cellulosic samples for their reactivity. In this paper, we report on using oxidation with 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxo-piperidinium cation (4-AcNH-TEMPO⁺) as a probe reaction for the reactivity of cellulose in mild conditions (pH 9, room temperature). 4-AcNH-TEMPO⁺ is able to selectively convert hydroxymethyl groups into carboxylate groups. The time dependence of the conversion was monitored by iodometric quantification of the residual 4-AcNH-TEMPO⁺. Soluble substrates, such as 1-propanol and maltose, were quantitatively oxidized in ca. 1 min while 3–16% of cellulose was oxidized in ca. 15 min depending on its origin. Extrapolation of the slow residual oxidation to zero time allowed quantification of the easily reactive or accessible cellulose. The 4-AcNH-TEMPO⁺ reactivity was correlated with several pulp characteristics, including WRV, FSP, SSA, chemical composition, crystallinity, the pulping process and the drying history.