Synthesis of carboxylic cellulose nanocrystals from yellow thatching grass (Hyparrhenia filipendula) via citric acid hydrolysis

Abstract

Grass is an abundant and renewable source of cellulose, which makes it a sustainable and cost-effective source for producing cellulose nanocrystals. Moreover, the extraction of cellulose nanocrystals from grass provides a value-added product from an otherwise low-value agricultural waste material, which can contribute to the development of a circular economy. In this study, cellulose nanocrystals (CNCs) were extracted from Hyparrhenia filipendula via citric acid hydrolysis. The Hyparrhenia filipendula stems were pre-processed through mechanical size reduction and Soxhlet extraction. The extractive-free stems were fractionated using two solvents: 10 wt% (w/v) sodium hydroxide (NaOH) and deep eutectic solvent of ethylene glycol:citric acid (1:2 molar ratio). The fractionated samples were bleached with acidified sodium chlorite and hydrolyzed with 80 wt% citric acid for 4 h at 120°C in a Parr reactor. The samples obtained at each treatment stage were characterized using standard scientific procedures for chemical composition, morphology, elemental composition, crystallinity, and thermal stability. The results show that CNCs were successfully extracted from Hyparrhenia filipendula via citric acid hydrolysis. The surface morphology of alkali fractionated CNCs was needle-like, whereas the surface morphology of DES fractionated CNCs was rod-like. The alkali fractionated and hydrolyzed sample, NaCNC, had the highest cellulose purity (91%), as well as the highest thermal stability. The FTIR analysis proved the removal of non-cellulosic components in the CNCs, except for the unbleached CNCs that had significant quantities of hemicellulose and lignin. The XRD analysis revealed the presence of characteristic cellulose Iβ in the CNCs, with the UNNaCNC sample (NaOH fractionated, unbleached, acid hydrolyzed sample) having the highest crystallinity index of 81% and the largest crystallite size of 4.20 nm. The properties of the CNCs obtained in this study are comparable to CNCs derived from previously reported lignocellulosic materials. The CNCs from Hyparrhenia filipendula therefore have a wide range of potential applications.