Solid-state NMR characterization of switchgrass cellulose after dilute acid pretreatment

Background: Switchgrass (Panicum virgatum L.) is being considered as a potential feedstock for bioethanol production and extensive research is ongoing to establish the optimum pretreatment conditions for switchgrass. Cellulose comprises the most abundant biopolymer in the biosphere and its crystalline/ultrastructure is considered to be related to biomass recalcitrance. This study investigates the effects of dilute acid pretreatment on ultrastructural features of switchgrass cellulose. Results: In this study, switchgrass was pretreated in a pilot-scale reactor at 190°C (0.05 g sulfuric acid per gram of dry switchgrass) with 25% total solid loading and a reactor residence of 1 min. Cellulose was isolated from the pretreated and untreated switchgrass. The impact of pretreatment on the ultrastructure of cellulose was determined by solid-state cross polarization/magic angle spinning ¹³C NMR spectroscopy. Switchgrass demonstrated a preferable degradation of amorphous cellulose regions during dilute acid pretreatment. The pretreated switchgrass cellulose showed an 18% increase in crystallinity index when compared with the untreated switchgrass. Line-fitting analysis of the C-4 region of ¹³C NMR spectra revealed that the relative proportion of crystalline and paracrystalline celluloses in switchgrass was observed to increase after dilute acid pretreatment, accompanied with a concurrent decrease of the relative abundance of fibril surface cellulose. Conclusion: After dilute acid pretreatment, most of the hemicellulose in switchgrass was removed. The amorphous cellulose regions in switchgrass were degraded preferably during dilute acid pretreatment and the cellulose crystallinity index of pretreated switchgrass increased. Pretreated switchgrass had an increase in relative proportion of crystalline and paracrystalline cellulose in comparison to the starting material.