Electromediated Oxidation of Hydrolysed Bacterial Cellulose

Abstract

The state-of-the-art method to isolate cellulose nanofibres employs a 2,2,6,6-Tetramethylpiperidinyloxyl (TEMPO)/NaBr/NaClO system, where the primary alcohols on the surface of cellulose microfibrils are oxidised. However, the large consumption of hypochlorite in this system makes it problematic for industrial implementation. Previously, electrochemical oxidation with a TEMPO catalyst has been reported for the preparation of cellulose nanofibres. Here, we present a study on the electromediated oxidation of bacterial cellulose (BC), the purest form of cellulose. BC has been used in a hydrolysed form, enabling the electromediated preparation of cellulose nanocrystals (CNCs), which is generally deemed even more costly than the preparation of cellulose nanofibres.In this work, the electromediated oxidation and cyclic voltammetry were performed in an electrochemical cell, which featured three electrodes: A Hg|Hg2SO4|K2SO4 reference electrode, a titanium net counter electrode, and a carbon foam working electrode, offering a high surface area. A static potential of 0.5 V was applied during the chronoamperometry and explored oxidation times were 3, 5, 9, 12 and 24 h.Using a TEMPO catalyst, the electromediated oxidation was applied to hydrolysed and non-hydrolysed BC fibres in a pH 10 buffer. The C6 primary hydroxyls of BC were successfully converted to sodium carboxylate groups. By the application of the TEMPO electro-mediated oxidation to hydrolysed BC for 9 h, high carboxylate and aldehyde concentrations of 1.4 and 0.3 mmol/g, respectively, were formed in the oxidized fibre. The carboxylate and aldehyde content was determined by a conductometric titration method. Surface morphologies after oxidation were investigated, showing alteration of the microfibril morphology for BC and the production of cellulose nanocrystals for hydrolysed BC. Hence, we present an effective and environmentally friendly route, which is particularly suitable for the preparation of cellulose nanocrystals.