Strategic organosolv pretreatment toward energy-efficient sugar and lignin utilization in lignocellulose biorefinery

Abstract

Biorefinery to convert lignocellulosic biomass into bioenergy and chemicals has become inevitable for the sustainable replacement of the petroleum refinery. However, the progress of the technology development has been limited due to the significant variation of biomass recalcitrance and product values, especially lignin, after pretreatment. Therefore, the modern pretreatment concept has broadened its target from cellulose and hemicellulose utilization to lignin valorization. The main technical gaps are due to the side effects of lignin degradation and condensation during thermochemical processes. In this dissertation, three technical routes using different organosolv were designed and tested to facilitate lignin valorization in biorefinery process. N-Methyl-2-pyrrolidone (NMP) was selected first (Chapter-5) for its highest lignin soluble potential and low-cost. The pretreatment resulted in complete removal of hemicellulose, more than 90% lignin removal, and 98% cellulose digestibility using commercial enzyme. 2D-HSQC NMR analysis showed that the fractionated lignin contained 16.4% of β-o-4 linkage, which was more than double of the conventional ethanol pretreatment. To better separate the lignin and hemicelluloses, staged organosolv (OS)-dilute acid (DA) process (Chapter-6) was developed to successfully facilitate high reactive lignin fractionation and solvent recovery. The staged pretreatment yielded net positive energy (1.5 MJ/kg-wood) in biorefinery conversion of palm tree biomass to bioethanol. Finally, pentanol-water biphasic pretreatment approach (Chapter-7) simplified the fractionation of lignin, hemicelluloses, and cellulose in one-pot, while achieving 70.3% of lignin removal and more than 90% enzyme digestibility of substrates. Biphasic pretreatment process preserved approximately 42.5% of β-o-4 linkages due to the physical separation of lignin from acidic environment, resulting in net positive energy of 1.1 MJ/kg of wood residue.