Acid-Catalyzed Glycerol Pretreatment of Sugarcane Bagasse: Understanding the Properties of Lignin and Its Effects on Enzymatic Hydrolysis

In this study, lignin from acid-catalyzed glycerol (AG) pretreatment of sugarcane bagasse was recovered and characterized. Its effects on enzymatic hydrolysis and cellulase recycling were then investigated. Prior to lignin recovery, a two-step dilute acid and AG pretreatment was used to deconstruct sugarcane bagasse, which led to a glucan enzymatic digestibility of 99%, a glucose yield of 91%, and a xylose yield of 67%. Following enzymatic hydrolysis, lignin-rich residues were recovered by simple filtration at a lignin yield of 63% and a lignin purity of 90%. Two-dimensional heteronuclear single quantum correlation nuclear magnetic resonance analysis showed that glycerol had modified the bagasse lignin through α-etherification of β-aryl ethers and γ-esterification of hydroxycinnamic acids, generating a novel lignin structure. 31P NMR analysis showed that the recovered lignin had a high number of aliphatic hydroxyl groups suggesting that it is highly hydrophilic in nature. As a result, the AG lignin did not inhibit enzymatic hydrolysis of pretreated bagasse, and cellulases adsorbed onto lignin-rich solid residues were successfully recycled three times, leading to an average glucan digestibility of 93% (for a total of four batches) at an average cellulase dosage of only 4.1 FPU/g glucan. This study provides new and important information on AG pretreatment, which is critical toward the development of biorefinery processes based on this pretreatment.