Towards lignin valorization from diverse biomass feedstocks

Abstract

The biorefinery concept shows great promise for renewable energy and bioproducts. However, achieving economic viability remains a key challenge, particularly in realizing high- value applications for lignin. In this dissertation we investigated three key aspects of processing that affect lignin valorization: extraction, recovery, and solubilization. In Chapter Two, lignin was extracted from biorefinery hydrolysis residue using organosolv, alkaline, and ionic liquid methods and recovery yields were determined for each approach. Recovered lignin solubility was then screened with a range of organic solvents, and the fractional solubility of lignin was successfully predicted by using a novel method, which assumes lignin exhibits a distribution of solubility parameters, given its polydispersity. In Chapter Three, the alkaline extraction of lignin from corn stover and hybrid poplar was scaled and implemented to a 20-L scale by prioritizing the purity of lignin and throughput of lignin production. Lignin was recovered from the pretreatment black liquor by precipitation. The final lignin products were sent to collaborators for research on lignin fractionation and potential high-value applications. Due to inconsistent physical properties, especially color, we conducted a variable screening to determine which recovery condition impacts the physical properties of lignin. The filtering temperature had a significant impact on lignin recovery. In Chapter Four, the filtering temperature during lignin recovery was investigated in detail. A transition temperature was identified for lignin recovery marking a change in processability and color, from dark, slow-filtering lignin to light, easily filterable lignin. Chemical and physical differences between dark and light lignins were identified, such as particle morphology, surface roughness, chemical composition, and solubility. Finally, we proposed a theoretical framework based on colloid science to explain the observed differences by the kinetics of lignin aggregation.