Cellulose solvent-based pretreatment for enhanced second-generation biofuel production: a review
Cellulose, in addition to hemicellulose and lignin, makes the major fraction of lignocellulosic biomass – the only sustainable feedstock to meet the long-term sustainable energy need of the world. Cellulose is soluble in a number of solvents, e.g., concentrated phosphoric acid (CPA), N-methylmorpholine-N-oxide (NMMO), and ionic liquids (ILs), and can be regenerated by an anti-solvent without major derivatization for various applications. For one, the regenerated and much less crystalline cellulose is highly reactive for its biological conversion to sugars, fuels, and chemicals mediated by enzymes and/or microbes. This ability can be used as a core pretreatment step for improved bioprocessing of lignocelluloses. In this comprehensive review, cellulose solvent-based lignocellulosic fractionation technologies for enhanced enzymatic hydrolysis to improve biofuel and renewable chemical production are reviewed. The first part is focused on the background information of lignocellulosic biomass, lignocellulosic derived biogas, biohydrogen, and ethanol as well as acetone, butanol, and ethanol (ABE) production, and enzymatic hydrolysis. In the second part, the conditions for pretreatments applying CPA, NMMO, and IL solvents, improvements in enzymatic hydrolysis rates and yields for solids resulting from application of these pretreatments, and the features of the lignocellulosic structure affecting the improved bioprocessing have been thoroughly reviewed.