Cellulose Isolated from Giant Miscanthus by Nitric Acid Treatment with Acid Recycling

Stems of giant miscanthus―a perennial grass crop―were investigated in this study. The Kürschner cellulose content was determined to be 52.8 wt%, indicating that this feedstock holds considerable promise for cellulose isolation. For the first time, cellulose was isolated from giant miscanthus stems via a modified nitric acid process involving treatment of the feedstock with nitric acid at the first stage with fivefold acid recycling. The isolated cellulose samples were characterized by consistently high and mutually comparable quality. A high α-cellulose content of 92.0–94.0 wt% and a high degree of polymerization of 1750–1850 were achieved, along with minor contents of non-cellulosics: 0.41–1.06 wt% acid-insoluble lignin, 3.12–3.82 wt% pentosans, and 0.13–0.35 wt% ash. This method ensures the isolation of high-quality cellulose in a 36% yield. At the same time, the content of non-cellulosics were reduced manifold, specifically: lignin by 24.6–63.7 times, pentosans by 4.0–6.2 times, and ash by 5.9–15.9 times, which corresponds to their extraction degree of 98%, 81% and 90% on average, respectively. A pooled cellulose sample was characterized by SEM, IR spectroscopy, and TGA/DTA/DTG. The obtained results justify the use of the modified nitric acid method with fivefold recycling of the spent nitric acid solution, which would reduce reagent and water consumption without compromising the quality attributes of the resulting cellulose. The practical value lies in the experimental demonstration that high-polymer cellulose can be obtained from giant miscanthus with a degree of polymerization reaching 1850, which significantly expands the potential applications of cellulose and its chemical modification products.