Advancing sustainable high degree carboxylation of nanocellulose using dual polyacid ternary deep eutectic solvent

Abstract

The acid-based approach for extracting nanocellulose supplies prfotons essential to hydrolyse cellulose and catalyse topochemical functionalization. However, the hydrolytic kinetics are so rapid that they cause the cellulose to degrade into monomeric sugars intractably, limiting the functionalization capacity. Thus, we posit a dual polyacid-based ternary deep eutectic solvent (DP/TDES), with incorporation of oxalic and citric acids that shifted the pronounced reactivity contribution from hydrolysis to H-bonding disruption mechanism. This was achieved by interspersing the carboxyl- and hydroxyl- rich polyacids, leading to densification of H-bond bridges while maintaining ion mobility, which thus fortifying solvent-cellulose reaction. The resultant DP/TDES shows a higher H-bond donating ability (α_sβ_c = 0.779) than both cellulose cohesive (α_cβ_c = 0.762) and solvent self-association strength (α_sβ_s = 0.081) from solvatochromic analysis, signifying a cellulose dissolution ability independent of hydrolytic action. This intensified H-bond reactivity, combined with the capability of the dual polyacids to co-catalyze via Fischer-Speier and organocatalytic esterification, culminating in an improved degree of carboxylation on nanocellulose. This study achieved a 1.8-fold enhancement (0.822 mmol g^-1) of carboxyl content compared to its respective binary DESs of <0.4 mmol g^-1 while maintaining a high crystallinity index (78 %). Our study achieved high functionalization degree which comparable to TEMPO-mediated and ammonium persulfate-mediated oxidation while offering a cost-effective, green and safe alternative for production of highly functionalized nanocellulose production.

Keywords: Carboxylation; Fischer-Speier esterification; Nanocellulose; Organocatalytic esterification; Ternary deep eutectic solvent.