Sequential Extraction and Characterization of Lignin-Derived Compounds from Thermochemically Processed Biorefinery Lignins

As the most abundant source of renewable aromatic compounds on the planet, lignin is an attractive feedstock for producing a range of chemicals and products that are currently derived from petroleum. Despite its great potential, separation of lignin depolymerization products remains one of the main obstacles toward cost-effective lignin valorization. Two lignin-rich streams, residues from enzymatic hydrolysis of the dilute acid and alkaline-pretreated corn stover, were depolymerized via pyrolysis using induction heating and catalytic transfer hydrogenolysis (CTH), respectively. Differences in phenolic compounds from gas chromatography-mass spectrometry and gel permeation chromatography analyses suggest that both pretreatment conditions and lignin depolymerization methods affected the product distribution. CTH lignin oils contain less polar compounds as compared to pyrolysis lignin oils, probably due to saturation of the derived compounds as a result of the reductive chemistry. The resulting liquid oils were subjected to sequential liquid-liquid extraction using a series of solvents with different polarities: hexane, petroleum ether, chloroform, and ethyl acetate. Sequential extraction fractionated lignin-derived oil into groups of different compounds depending on the solvent polarities. This study provides a better understanding of how the lignin source and processing method affect the depolymerization products and provides a possible way to fractionate lignin-derived compounds.