Isolation, characterization and optimization of Cellulose Nanocrystals using deep eutectic solvent treatment from rice straw

Cellulose nanocrystal (CNC) is a renewable nanomaterial that has been used in various applications due to its excellent physicochemical properties. The aim of this research study is to evaluate the performance of the alkaline DES in lignocellulose pulping, acidic DES in acid hydrolysis process as well as optimizing the yield of oxalic acid-choline chloride cellulose nanocrystal (OA-CNC). In this study, cellulose pulp and CNC were successfully extracted from the rice straw (RS) using a novel green approach. The cellulose pulp was obtained through alkaline deep eutectic solvent (DES) pulping of RS using an alkaline DES. CNC was further produced from the extracted cellulose pulp through acidic DES hydrolysis using an acidic DES. The 1:7 molar ratio of potassium carbonate-glycerol DES (K2CO3-Gly DES) was chosen as the potential alkaline pulping solvent. The screened pulping parameters were determined using the one-factor-at-a-time (OFAT) method. The results revealed that the pulping temperature of 140 °C, 100 min reaction time and 1:10 RS to K2CO3-Gly DES mass ratio produced the highest cellulose content of 73.8% for the unbleached DES treated pulp (UP). The Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) spectroscopy analyses confirmed that the non-cellulosic materials were removed from the RS after subjected to the alkaline DES pulping and bleaching process. The thermogravimetric analysis (TGA) indicated that the thermal stability of UP was comparable to that of bleached DES treated pulp (BP). The 1:1 molar ratio of oxalic acid-choline chloride DES (OA-ChCl DES) was prepared and chosen as the potential acidic solvent for the acidic DES hydrolysis process. According to the OFAT experiment, the temperature of 80 °C, 4 h reaction time and 1:10 BP to OA-ChCl DES mass ratio were obtained and further used for optimizing the acidic DES hydrolysis process through the face-centered central composite design (FCCCD). The optimal operating conditions were found as temperature (79.5 °C), reaction time (4 h) and BP to OA-ChCl DES mass ratio (1:12.64) at which 55.08% of the OA-CNC yield was achieved. The physicochemical properties of OA-CNC were compared with the sulphuric acid cellulose nanocrystal (SA-CNC) that prepared through sulphuric acid hydrolysis. The FTIR, EDX and zeta potential analyses confirmed the presence of negatively charged carboxyl groups on the OA-CNC surface and negatively charged sulphate groups on the SA-CNC surface after acidic DES hydrolysis and sulphuric acid hydrolysis, respectively. The TGA results revealed that the OA-CNC has a higher thermal stability than the SA-CNC. The field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and particle size analyses proved that the OA-CNC was in nano-sized range. These findings have demonstrated a novel green approach for the preparation of OA-CNC using alkaline DES pulping and acidic DES hydrolysis.