Sequential fractionation of the lignocellulosic components in hardwood based on steam explosion and hydrotropic extraction

Abstract

BACKGROUND: The forest biorefinery plays an important part in the evolving circular bioeconomy due to its capacity to produce a portfolio of bio-based and sustainable fuels, chemicals, and materials. To tap into its true potential, more efficient and environmentally benign methods are needed to fractionate woody biomass into its main components (cellulose, hemicellulose, and lignin) without reducing their potential for valorization. This work presents a sequential fractionation method for hardwood based on steam pretreatment (STEX) and hydrotropic extraction (HEX) with sodium xylene sulfonate. By prehydrolyzing the hemicellulose (STEX) and subsequently extract the lignin from the cellulose fraction (HEX), the major wood components can be recovered in separate process streams and be further valorized. RESULTS: Using autocatalyzed STEX and HEX, hemicellulose (> 70%) and lignin (~ 50%) were successfully fractionated and recovered in separate liquid streams and cellulose preserved (99%) and enriched (~ twofold) in the retained solids. Investigation of pretreatment conditions during HEX showed only incremental effects of temperature (150-190 °C) and hold-up time (2-8 h) variations on the fractionation efficiency. The hydrolyzability of the cellulose-rich solids was analyzed and showed higher cellulose conversion when treated with the combined process (47%) than with HEX alone (29%), but was inferior to STEX alone (75%). Protein adsorption and surface structure analysis suggested decreased accessibility due to the collapse of the fibrillose cellulose structure and an increasingly hydrophobic lignin as potential reasons. CONCLUSION: This work shows the potential of sequential STEX and HEX to fractionate and isolate cellulose, hemicellulose, and a sulfur-free lignin in separate product streams, in an efficient, sustainable, and scalable process.