Abstract

This work deals with the production of crystalline nanocellulose (CNC) through an acid hydrolysis reaction using protic ionic liquids (PILs) as solvents and catalysts. The PILs used belong to a series derived from a common cation, 1-methyl- imidazolium and anionic clusters of the general formula [(HSO4)(H2SO4)x]− (x = 0, 1, 2). These PILs were used for the first time to obtain CNC from two commercial cellulose samples and one cellulose from a corn husk source. The PILs were used under different time and temperature conditions to set the optimal parameters for extraction. The CNCs obtained were characterized by IR spectroscopy, X-ray diffraction spectroscopy, STEM and FESEM microscopy and thermal stability by thermogravimetric analysis (TGA). It was determined that the [Hmim][(HSO4)(H2SO4)x] with x = 1 presents the appropriate acidity, (measured by Hammett acidity function (H0) and acceptor number (AN) parameters), to extract CNC from two different commercial cellulose with a yield of ca 73 % and 60% (at 40 °C with 2–3 h of reaction); respectively. The experimental conditions used in this study (time and 40 °C) are milder than those reported when sulfuric acid and other ILs were used to obtain CNC. The [Hmim][(HSO4)(H2SO4)x] with x = 1 present an adequate acidity for the selective water-insoluble oligosaccharides release, while products resulting from 64% sulfuric acid (H0 = 0.65) at 40 °C were mainly water-soluble oligosaccharides. Additionally, the PILs were recycled 5 times maintaining their original structural and catalytic properties. The proposed process was also compared with a conventional acid hydrolysis using sulfuric acid (64 wt%), the nanomaterial obtained with the PILs always presented better thermal properties. This work contributes towards the use of new solvents-catalysts in the field of biomass waste recovery under mild reaction conditions.