Saccharification of Miscanthus $ Giganteus With Dilute Nitric Acid

Miscanthus × giganteus (M. giganteus), a C4 perennial grass, is a potential renewable resource for energy and chemicals. M. giganteus contains 41.5% cellulose, 24.5% hemicellulose, 27.0% lignin, 7% others (4% ash and 3% extractables). Hydrolysis of hemicellulose and cellulose gives sugars that can be converted to a biofuel or to useful chemicals. Like similar agricultural materials, M. giganteus contains lignin that makes hydrolysis difficult.
We investigated a two-step dilute nitric acid process at advanced temperatures to obtain sugars from M. giganteus. The two-step process removes hemicellulose using dilute nitric acid at 120-160 °C for 10 to 40 min whereas cellulose hydrolysis is conducted at higher temperature, 170-200 °C for less than 6 min. Nitric acid is a better choice than sulfuric acid due to its compatibility with stainless steel. It can easily be neutralized with ammonia to produce ammonium nitrate, a useful nutrient for microorganisms used in downstream fermentation. We present the best conditions for dissolving about 75% polysaccharides (cellulose and hemicellulose together) keeping degradation minimal. We also present chemical analysis of the solid and liquid phase. In the first step, more than 90% of hemicellulose was transferred to the liquid phase and overall for both steps, more than 70% of cellulose was transferred to the liquid phase.

For better understanding of the process, we investigated the effects of various conditions by varying the temperatures, acid concentration, solid-to-liquid ratio and residence time. High-performance liquid chromatography was used to determine any degradation of sugars in the liquid phase; we observed small amounts of furfural and 5-(hydroxymethyl)furfural. In addition, the recovered solid M. giganteus was characterized by Fourier-transform infrared spectroscopy and by two-dimensional 13C-1H heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy, scanning-electron microscopy, attenuated total reflection-infrared spectroscopy and X-ray diffraction. 2D-NMR analysis shows that a higher temperature enhances the removal of hemicellulose. Dilute nitric acid pretreatment reduced 9% lignin in M. giganteus at 140 °C and 20 min while it decreased 27% lignin at 160 °C and 10 min. A substantial amount of lignin side-chains, mostly β-O-4’ aryl ether and 5-5’/α-O-1’ dibenzodioxin, were cleaved. As a result, the molecular weight of lignin in recovered residues is lower than that in native lignin.