When the order matters: Impacts of lignin removal and xylan conformation on the physical structure and enzymatic hydrolysis of sugarcane bagasse

Dilute acid and alkaline pretreatments are commonly used to fractionate lignocellulosic biomass for its conversion into biofuels and renewable materials. In the current work, we compare the structure and morphology of one and two-step alkaline and acid pretreated sugarcane bagasse (SCB) and differences in their enzymatic hydrolysis rates and yields. We used physical techniques such as nuclear magnetic resonance (NMR) and X-ray diffraction to shed light on physical and morphological changes introduced by the pretreatments in SCB. We also applied solid state NMR procedures which allowed us to separate xylan structural conformations inside pretreated plant biomass samples. Our results reveal that practically all xylan in two-step “alkaline first” (NaOH 1% + H2SO4 1%) pretreated samples adopts two-fold ribbon-like conformation, tightly associated with crystalline cellulose. On the contrary, a main part of xylan in the SCB samples after “acid first” (H2SO4 1% + NaOH 1%) combined pretreatment has a three-fold screw conformation characteristic of xylan bound to lignin. Modifications in the biomass composition, physical structure and xylan conformation introduced by the alternative pretreatments contribute to the observed differences in the enzymatic hydrolysis rates and yields. These results could be relevant for analysis and optimization of pretreatments and enzymatic hydrolysis of lignocellulosic biomass in general. [Display omitted] •An order of acid/alkaline pretreatments has a strong effect on xylan conformation in the remaining solid fraction.•Most of xylan in “acid first” pretreated SCB is in lignin-bound three-fold conformation.•Almost all xylan in “alkali first” pretreated biomass is associated with crystalline cellulose.•Cellulose-bound two-fold xylan slows down enzymatic conversion of cellulose.•The final yields of xylose and glucose were higher in “alkali first” pretreated SCB.