Electrocatalytic TEMPO-mediated oxidation of cellulose nanocrystals: gaining a mechanistic understanding

Electrocatalytic (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and 4-acetamido TEMPO-mediated oxidation were used in the regioselective functionalization of cellulose nanocrystals (CNCs) to achieve tunable carboxyl and aldehyde contents. Compared to conventional TEMPO-mediated oxidation that is optimized for the secondary oxidizing agents used, this electrocatalytic approach has no additional chemicals; however, the reaction mechanism remains unexplored. This work elucidated the pH-dependence of the electrocatalytic oxidation of uncharged cotton-derived CNCs and demonstrated that the oxoammonium form of TEMPO (TEMPO+) reacts faster with CNCs at higher pH. Counterintuitively, CNCs with the highest carboxyl content were produced at intermediate pH values, reaching an impressive degree of surface substitution of 0.47 - corresponding to 94 % of the theoretical limit. In alkaline conditions, the degree of carboxylation decreased, which was attributed to β-elimination. The rate-limiting step was determined to be the regeneration of TEMPO+ and switching from a non-divided electrochemical cell to a divided one mitigated the issue. The use of a divided cell also helps the cathodic production of H₂ gas, a desirable co-product formed at the counter electrode during the reaction. The electrocatalytically oxidized CNCs were colloidally stable, maintained their crystallinity/size, and had carboxyl contents similar to lab-made chemically-driven TEMPO-oxidized CNCs and five times higher than commercially-available carboxylated CNCs.