Isolation and Characterization of Cellulose Nanocrystals from Different Lignocellulosic Residues: A Comparative Study

Recently, cellulose nanocrystals (CNCs) have captured the interest of researchers and industries. In this study, CNCs were isolated from four abundant lignocellulosic byproducts: teff (Eragrostis tef, Poaceae) straw, enset (Ensete ventricosum, Musaceae) fiber (EF), sugarcane (Saccharum officinarum, Poaceae) bagasse and coffee (Coffea arabica, Rubiaceae) hull (CH). Cellulose fibers were obtained using chlorine-free extraction with 5% sodium hydroxide pretreatment followed by delignification (with formic acid, acetic acid and hydrogen peroxide) and bleaching (with alkaline hydrogen peroxide). CNCs were then isolated following hydrolysis of the cellulose fibers with 64% sulfuric acid. The as-obtained CNCs were investigated and characterized in terms of yield, crystallinity, chemical functionality, morphology, particle size, zeta potential (ZP) and thermal stability. The CNCs displayed a typical crystal lattice of Iβ-type based on XRD patterns, d-spacings and Z-values. The highest yield (~ 70%), CrI (~ 86%), and crystal size (~ 6 nm) were observed in EF–CNCs, and the least in CH–CNCs (yield: ~ 25%, CrI: ~ 77%, crystal size: ~ 4 nm). FTIR spectra of all CNCs indicated typical chemical composition of cellulose. TEM observations revealed that the CNCs were needle-shaped nanoscale structures with different aspect ratios (17.32–36.67) and dimensions (average length: 154.28–193.06 nm; diameter: 5.16–11.79 nm), while the DLS measurements provided the hydrodynamic sizes, 96.96–184.90 nm. The thermal studies by TGA/DTG revealed the CNCs had a two-step decomposition process at Tmax 215–225 °C and 340–355 °C. This study showed that the CNCs isolated exhibited high crystallinity, aspect ratio, colloidal and thermal stability although differences were observed due to variations in cellulose sources.