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

Abstract

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 °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.

Advances in Pretreatment of Straw Biomass for Sugar Production

“…Moreover, Sc-CO 2 is based on the application of CO 2 as a supercritical fluid. The fluid exhibits "gas-like" mass transfer characteristics and "liquid-like" solvating power (Li et al, 2020;Ramezani et al, 2020). Besides, CO 2 is non-flammable and non-toxic, which could be adjusted to supply a higher diffusion co-efficiency, strong solubility, excellent diffusivity, and strong recoverability (Rezakazemi et al, 2017;Rezakazemi et al, 2019).…”

“…P c : critical pressure; T c : critical temperature. Reproduced with permission from Li et al (2020) .…”

A CO2-Mediated Conjugate Cyanide Addition to Chalcones

“…The importance of CO 2 , however, cannot be neglected due to the severe side products' formation in the absence of CO 2 . This phenomenon-a cleaner reaction mixture under CO 2 -has been sporadically revealed in synthetic chemistry [29,30]. Further investigation will pave the way to understand the true capacity of CO 2 in organic synthesis, potentially providing new ways of mitigating anthropogenic CO 2 via CO 2 capture and sequestration in a catalytic manner.…”

Conversion of Lignocellulosic Biomass to Reducing Sugars in High Pressure and Supercritical Fluids: Greener Alternative for Biorefining of Renewables

“…to commodity and value‐added products (e.g., 2,5‐dimethylfuran). [ 46,47 ] In this review, we particularly focused on processing agricultural and woody biomass in sub/scCO 2 and sub/scH 2 O for derivation of sugars. Significant effort was put into collecting and processing the sugar yield data in the form of consistent units (e.g., sugar released per total hydrolysable sugars) for comparing the performance of various sub/supercritical fluid‐based systems.…”