N-heterocycle-based deep eutectic solvent-driven lignocellulosic biomass valorization: Efficient extraction of lignin facilitates enzymatic hydrolysis to produce bioethanol and butyric acid from corn stover

The existence of lignin in lignocellulose largely hinders the accessibility of polysaccharides, leading to lower saccharification yield and substrate fermentability. Therefore, extraction of lignin by pretreatment is an efficient strategy for biomass valorization. The non-uniform electron density distribution in N-heterocycles affects interactions with electron-rich lignin, which is beneficial for lignin extraction. Based on this, N-heterocyclic deep eutectic solvents (DESs) were prepared using pyrazole and organic acids. Lignin extraction yield of 94.27% was achieved when pretreating corn stover at 145 °C for 180 min by pyrazole-formic acid DES (Py-FA), and >85% glucan was retained. Glucose yield of DES-pretreated substrate reached ∼100% by enzymolysis. A high correlation (R2 > 0.91) between lignin removal and hydrolysis yield was observed in N-heterocycle-based DES pretreatment. Also, Py-FA DES-extracted lignin had comparable thermostability within 200–500 °C to sodium lignosulfonate. Finally, corn stover hydrolysates were applied as feedstock for bioethanol and butyric acid production. Consequently, ethanol production and yield reached 31.77 g/L and 0.38 g/g from real hydrolysates by Saccharomyces cerevisiae. Up to 9.48 g/L butyric acid was biosynthesized by Clostridium tyrobutyricum with yield of 0.43 g/g, ∼88% of theoretical yield from glucose. Overall, N-heterocycle-based DES pretreatment provides an efficient and robust strategy to produce value-added products from lignocellulose.