Structure and thermal reactivity of kraft and co-solvent fractionated lignin processed under Hydrothermal Liquefaction

Shrestha, Ronish

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Structure and thermal reactivity of kraft and co-solvent fractionated lignin processed under Hydrothermal Liquefaction

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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 aromatic organic compounds, 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. While Kraft lignin extracted from kraft paper pulping process is the most abundant form of lignin in the word, co-solvent enhanced lignocellulosic fractionation (CELF) is a novel biomass pretreatment technique involving dilute acid treatment of biomass in a THF-water mixture which yields a clean lignin byproduct (termed as CELF lignin) open for valorization. Hydrothermal liquefaction (HTL) is a promising thermochemical technology for converting lignin and other waste streams in near critical water 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. Four different Kraft lignin and five different CELF lignin types were studied differentiated by their feed source ranging from softwood, agricultural residue/grasses and hardwood. 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 can provide a plethora of information regarding lignin structure and its fundamental subunits. Thermogravimetric analysis (TGA) of CELF lignin samples depict a narrow decomposition temperature range between 360-370 °C. After HTL, hardwood derived CELF lignin (poplar) produced the highest biocrude yield of 63 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. Quantifying these monomers with FID showed Sugarcane bagasse and corn stover derived CELF lignin produced the highest with 21 mg of monomers per gram of lignin. Thus, HTL of CELF lignin can produce value-added monomers and a biofuel precursor in the form of upgradable biocrude. Better understanding the structure and thermal degradation of Kraft lignin and this novel CELF lignin could provide insights into feedstock influence and other significant parameters for degradation of lignin and open avenues for its valorization.

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