Flowthrough Pretreatment of Softwood under Water-Only and Alkali Conditions

Flowthrough pretreatment provides valuable insight into the fundamentals of deconstruction of plant biomass. In this study, the potential softwood degradation pathways under water-only and alkali conditions were determined by elucidating the deconstructed biomass-derived products at 0–270 °C for 2–10 min at a flow rate of 25 mL/min with water-only or alkaline at the initial pH of 8, 9, 11, and 12, respectively. The results indicated that the initial pH value was a convenient indicator along with the severity parameter to control the biomass degradation through the pH < 9 and the pH ≥ 9 pathways. Up to 100% of hemisugars, 90% of cellulose, and 70% of lignin were derived from softwood under pH < 9 at severity parameter log R0 around 5.5, respectively. On the contrary, at pH 12, the degradation resulted in pretreated hydrolysate rich in monomeric and oligomeric phenolic products as well as glycolic acid, acetic acid, and formic acid from carbohydrates. The two-dimensional 1H–13C heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance (NMR) analysis revealed that nonoxidative degradation at initial pH values lower than 9 mainly cleaved the majority of β-O-4 and preserved most β–β and β-5/α-O-4 linkages in lignin, whereas at pH 12, substantial original aromatic ring structures survived further degradation.