Lignin-First Approach to Biorefining: Utilizing Fenton’s Reagent and Supercritical Ethanol for the Production of Phenolics and Sugars

Selective lignin depolymerization (SLD) has emerged as a value-added method of pretreatment for lignocellulosic biorefining, in which lignin is depolymerized into valuable phenolic monomers prior to utilization of the hemicellulose and cellulose. Herein, we report a biomimetic Fenton catalyzed SLD process, converting sweet sorghum bagasse into an organic oil that is rich in phenolic monomers and a solid carbohydrate that is favorable for enzymatic hydrolysis into sugars. Initially, the feedstock’s molecular structure was modified through iron chelation and free radical oxidation via Fenton’s reagent (Fe³⁺, H₂O₂). The lignin component of the modified feedstock was then selectively depolymerized in supercritical ethanol (250 °C, 6.5 MPa) under nitrogen to produce a phenolic oil, with a maximum yield of 75.8 wt %. Six valuable phenolic monomers were detected in this oil, with a maximum cumulative yield of 19.1 wt %. The solid carbohydrate obtained after the SLD process was enzymatically hydrolyzed to liberate 62.7 and 79.9 wt % of the initial 5- and 6-carbon polysaccharides within 24 h, respectively, indicating the majority of the hemicellulose and cellulose were preserved during the SLD process. Fenton modification not only increased the yields of phenolic monomers, particularly ethyl-p-coumarate and ethyl-ferulate, but also enhanced enzymatic hydrolysis.