Understanding Multiscale Structural Changes during Dilute Acid Pretreatment of Switchgrass and Poplar

Biofuels produced from lignocellulosic biomass hold great promise as a renewable alternative energy and fuel source. To realize a cost and energy efficient approach, a fundamental understanding of the deconstruction process is critically necessary to reduce biomass recalcitrance. Herein, the structural and morphological changes over multiple scales (5-6000 Å) in herbaceous (switchgrass) and woody (hybrid poplar) biomass during dilute sulfuric acid pretreatment were explored using neutron scattering and X-ray diffraction. Switchgrass undergoes a larger increase (20-84 Å) in the average diameter of the crystalline core of the elementary cellulose fibril than hybrid poplar (19-50 Å). Switchgrass initially forms lignin aggregates with an average size of 90 Å that coalesce to 200 Å, which is double that observed for hybrid poplar, 55-130 Å. Switchgrass shows a smooth-to-rough transition in the cell wall surface morphology unlike the diffuse-to-smooth transition of hybrid poplar. Yet, switchgrass and hybrid poplar pretreated under the same experimental conditions result in pretreated switchgrass producing higher glucose yields (∼76 wt %) than pretreated hybrid poplar (∼60 wt %). This observation shows that other aspects like cellulose allomorph transitions, cellulose accessibility, cellular biopolymer spatial distribution, and enzyme-substrate interactions may be more critical in governing the enzymatic hydrolysis efficiency.