Lignocellulosic biomass conversion by sequential combination of organic acid and base treatments

The present work aims at exploring a new hydrolysis approach using sequential combinations of organic acid [oxalic acid (OA)] and base [tetramethylammonium hydroxide (TMAH)] to convert lignocellulosic biomass at mild conditions (150-215 °C, 1-2 h, and 1 atm N₂) using a spruce wood sample in a batch reactor. In the single-step tests, hydrolysis with OA can convert biomass to yield sugars and furanic compounds. Hydrolysis with TMAH mainly depolymerizes lignin to produce phenolic compounds but also converts some polysaccharides in biomass possibly by the peeling reaction and/or alkaline hydrolysis. A sequential combination of OA and TMAH can significantly increase the conversion of the biomass compared to single- or two-step reactions with either the acid or the base. The analysis of liquid products by chemical derivatization followed by gas chromatography equipped with a flame ionization detector (GC-FID) and the characterization of the solid residues by pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and ¹³C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) reveal strong synergetic effects from the sequential combination of acid and base. In single-step reactions, OA serves as a proton donor to cleave the glycosidic bond between polysaccharide monomers, while TMAH reacts more selectively to lignin than polysaccharides. In two-step tests, the reaction with OA in the first step significantly enhances the conversion with TMAH in the second step by almost twice compared to the corresponding reaction with base alone. GC-FID analysis of the liquid sample shows relatively higher conversion of carbohydrates in the biomass with base in the second step. Py-GC-MS and ¹³C CPMAS NMR analysis also reveals that acid treatment in the first step enables base to convert more cellulose in the second step.