Production of cellulose nanocrystals using Brønsted Acidic Ionic Liquids (BAILs), based on anionic clusters with changes in the cation and anion's structure

Abstract

Nine Brønsted Acid Ionic Liquids (BAILs) based on two cationic bases (1-methylimidazolium and pyridinium) and anions derived from sulfuric (SA) and phosphoric (PA) acids, were used as solvents and acid catalysts for the hydrolysis of commercial cellulose to obtain cellulose nanocrystals (CNCs). The BAILs' effect on the reactions was evaluated as a function of the acidity and polarity of the solvents, determined by the Hammett acidity parameter (H₀) (determined experimentally) and by the β parameter of the Kamlet-Taft analysis (determined computationally using COSMOS-RS). Under the same experimental conditions, the change of solvent not only affects the reaction yield of the CNCs but also the final characteristics of the nanomaterials obtained. All CNCs derived from PA-BAILs show a high thermal stability compared to those obtained with SA-BAILs, with T_max values of ~345 °C and 271 °C respectively. Effective surface modification of the CNCs was assessed using X-ray photoelectron spectroscopy (XPS) analysis, Zeta potentials and surface charge density (SCD) determined by conductometric titration. This study provides a solid foundation for the design of ionic liquid-based systems aimed at producing surface-modified CNCs for advanced applications.

Keywords: Brønsted Acid Ionic Liquids; Cellulose nanocrystals; Surface modification.