CO2-Enabled Biomass Fractionation/Depolymerization: A Highly Versatile Pre-Step for Downstream Processing

Lignocellulosic biomass is inevitably subject to fractionation and depolymerization processes for enhanced selectivity toward specific products, in most cases prior to catalytic upgrading of the three main fractions-cellulose, hemicellulose, and lignin. Among the developed pretreatment techniques, CO2-assisted biomass processing exhibits some unique advantages such as the lowest critical temperature (31.0 degrees C) with moderate critical pressure, low cost, nontoxicity, nonflammability, ready availability, and the addition of acidity, alongside easy recovery by pressure release. This Review showcases progress in the study of sub- or supercritical CO2-mediated thermal processing of lignocellulosic biomass-the key pre-step for downstream conversion processes. The auxo-action of CO2 in biomass pretreatment and fractionation, along with the involved variables, direct degradation of untreated biomass in CO2 by gasification, pyrolysis, and liquefaction with relevant conversion mechanisms, and CO2-enabled depolymerization of lignocellulosic fractions with representative reaction pathways are summarized. Moreover, future prospects for the practical application of CO2-assisted up- and downstream biomass-to-bioproduct conversion are also briefly discussed.