Optimization of cellulose nanocrystal isolation from sorghum bicolor stem using deep eutectic solvents: a response surface methodology approach

Deep eutectic solvents (DESs) have attracted increasing attention as eco-friendly and efficient alternatives to conventional organic solvents. In this study, cellulose nanocrystals (CNCs) were successfully isolated from untreated sorghum bicolor stalk (SBS), a lignocellulosic biomass, using DES treatment combined with an ultrasonic process. The DES composed of choline chloride (ChCl):urea with a molar ratio of 1:2 was used to isolate CNCs. Response surface methodology (RSM) using central composite design (CCD) was employed to optimize the treatment conditions concerning solid loading, temperature, and time. The main goals were to achieve the highest CNC yield and crystallinity index (CrI). Under optimum treatment conditions (solid loading of 1.88%, time of 411 min, and temperature of 126 °C), CNC yield of 55.9 ± 1% and CrI of 45.05 ± 1% were obtained, which were close to the predicted values. The obtained sample was characterized using Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The results demonstrated that DES was an excellent solvent for the isolation of CNC from untreated SBS. The reusability of DES was evaluated by two runs, and a CNC yield of 52.0 ± 1% and CrI of 44.65 ± 1% were obtained without adding any excess chemicals. The recyclability of DES makes the whole treatment process feasible and amenable for large-scale production of CNC.