(Invited) Electrochemical TEMPO-Mediated Oxidation of Cellulose Nanocrystals: Gaining a Mechanistic Understanding of a New Non-Conventional Process
Authors: Rousseau Alexandra , Cranston Emily , Wilkinson David P. 
Published: December 2024

This research aims to address the limitations of traditional (2,2,6,6 tetramethylpiperidin-1-yl)oxyl (TEMPO)-mediated chemical oxidation by introducing electrochemical oxidation as a novel approach. Our interest lies in using electrochemical TEMPO-mediated oxidation for the carboxylation of cellulose nanocrystals (CNCs). CNCs are bio-based “rice-shaped” nanoparticles that have low density and high strength, making them useful in materials, medicine, electronics, and adhesives.1 The carboxylation of the native alcohol groups on the surface of CNCs is an important functionalization which imparts electrostatic charge to the nanoparticles – allowing them to form colloidally stable suspensions.2 These are crucial properties for CNC users because they allow for long-term stability and predictability of CNC performance.

In this research, an electrochemical approach is used to replace chemical oxidizing agents in TEMPO-mediated CNC oxidation. In conventional TEMPO-mediated oxidation, strong secondary oxidants, usually toxic halogenated compounds, are required to drive the overall reaction: (i) TEMPO reacts with an oxidizer to form the corresponding nitrosonium ion (TEMPO+), (ii) TEMPO+ oxidizes a substrate to form an oxidized product and generate a TEMPO hydroxylamine (TEMPOH), (iii) the secondary oxidants react with TEMPOH to regenerate the TEMPO radical, and the cycle continues.3

CNCs were electrochemically carboxylated at pH 4, 6, 8, 10, and 12 using TEMPO and 4-acetamido TEMPO to elucidate the impact of pH and catalyst type on product selectivity and CNC properties (charge, crystallinity, degree of polymerization, and size). Chronoamperometry reveals that in the presence of CNCs, higher pH increases the turnover frequency of TEMPO to TEMPO+. However, bulk electrolysis demonstrates that lower pH increases reaction conversion of alcohols to carboxylic acids. This interesting mismatch between reaction rate and product formation was further investigated using model compounds, such as glucose, to deconvolute the mechanism and improve oxidation efficiency and selectivity for desired products. The presented research supports the opportunity to replace secondary chemical oxidants with electrochemical oxidation.

(1) Vanderfleet, O. M.; Cranston, E. D. Production Routes to Tailor the Performance of Cellulose Nanocrystals. Nat. Rev. Mater. 2021, 6 (2), 124–144. https://doi.org/10.1038/s41578-020-00239-y.

(2) Antoniw, J. M.; Hallman, M. T.; Kiriakou, M. V.; Morse, T.; Cranston, E. D. Colloidal Stability Window for Carboxylated Cellulose Nanocrystals: Considerations for Handling, Characterization, and Formulation. Langmuir ACS J. Surf. Colloids 2023, 39 (30), 10321–10334. https://doi.org/10.1021/acs.langmuir.3c00319.

(3) De Nooy, A. E. J.; Besemer, A. C.; Van Bekkum, H. Highly Selective Tempo Mediated Oxidation of Primary Alcohol Groups in Polysaccharides. Recl. Trav. Chim. Pays-Bas 1994, 113 (3), 165–166. https://doi.org/10.1002/recl.19941130307.

Figure 1