(66g) Monomers and Biocrude from Hydrothermal Liquefaction of Solvent-Fractionated Lignin

Lignin is the second most abundant biological polymer; a natural aromatic biomacromolecule that serves to bind lignocellulose. Together with cellulose and hemicellulose, lignin serves as one of the three major structural biopolymers comprising about 15–40 wt% of lignocellulosic biomass. Its phenolic structure makes it a potential renewable source for organic compounds, especially those containing electron rich aromatic rings. However, lignin’s highly recalcitrant nature creates obstacles in its utilization as a renewable alternative source. In contrast to well-studied processes to convert cellulose and hemicellulose, lignin has often been categorized as “waste” with a predicted annual production of ̴ 62 M dry tons/year.

Co-solvent enhanced lignocellulosic fractionation (CELF) is a novel biomass pretreatment technique involving dilute acid treatment of biomass in a THF-water mixture and yields a clean lignin byproduct open for valorization. Lignin obtained from this pretreatment process, denoted as CELF lignin, has relatively low molecular weight, few phenolic hydroxyl groups and low content of aryl ether bonds suggesting extensive cleavage of β-O-4 interunit linkages (most dominant) after CELF pretreatment. Hydrothermal liquefaction (HTL) is a promising thermochemical technology for converting lignin and other waste streams in near critical water (T = 240–370 °C and P = 10–30 MPa) into valuable fuel and chemicals. HTL employs hot pressurized water, considered as one of the greenest solvents, favorable for promoting reactions without catalysts. Subcritical water in the form of OH^- and H⁺ ions can dissolve and catalyze lignin fragments into phenolic products. From the CELF process, a warm and wet lignin-rich precipitate is generated from a continuous flow system and HTL is ideal for utilizing wet substrates. Hence, we avoid the energy intensive lignin drying step. CELF lignin has thus far never been used as a feed for HTL and the unusual structure of CELF lignin could provide opportunities to further understand the thermal chemistry of lignin depolymerization.

Five different CELF lignin types differentiated by their feed source ranging from agricultural feedstock like pine, corn stover and sugarcane bagasse to hardwoods like poplar and maple wood were selected for this study. Our goal was to characterize, in molecular detail, the structure of CELF lignin and evaluate its thermal reactivity before evaluating HTL and its products for these five CELF lignin. ¹³C NMR on biomass feeds and their CELF lignin counterparts show significant reduction of aromatic C-O and COO linkers. Almost negligible (<2%) amount of polysaccharides were observed for all CELF lignin samples indicating a clean lignin byproduct obtained via CELF pretreatment. Also, NMR based quantification of lignin monomers provided S/G ratios whose influence was later investigated for downstream HTL degradation. Thermogravimetric analysis (TGA) of CELF lignin samples depict a narrow decomposition temperature range between 300-350 °C. HTL of CELF lignin was performed at 300 °C, 15 MPa for 1 hour with a solid loading of 10 wt%, yielding gaseous, aqueous and char phase products. Biocrude was extracted from the char phase via solvent extraction using acetone. Hardwood derived CELF lignin (poplar and maple) produced the highest biocrude yield of 52 wt%, while bagasse, corn stover and pine CELF lignin had yields in between 40-43 wt%. Major monomers identified in the biocrude using GC-MS included guaiacol, ethylphenol, cresol, ethylguaiacol, syringol, butylated hydroxytoluene, propylguaiacol, and trimethoxybenzene. After quantifying these monomers with FID, Sugarcane bagasse derived CELF lignin produced the highest with 21 mg of monomers per gram of lignin, while pine had the lowest amount with almost no syringol. Thus, HTL of CELF lignin can produce value-added monomers and a biofuel precursor in the form of upgradable biocrude. Understanding the structure and thermal degradation of this novel CELF ligbn nin could also provide insights into feedstock influence and other significant parameters for degradation of all other lignin sources.