Sequential Fractionation of Lignin-derived Pyrolysis Oil via Extraction with a Combination of Water and Organic Solvents

Alkali lignin pyrolysis oil (PO) was subjected to a sequential extraction method in which PO was separated into water soluble (WS), water insoluble (WI), methanol soluble (MS), methanol insoluble (MI), dichloromethane soluble (DS), and dichloromethane insoluble (DI) subfractions via combinations of water and organic solvents (water, methanol, and dichloromethane). Several analytical techniques were used to characterize PO and its subfractions. Elemental analysis showed that the higher heating value (HHV) of DS was highest, and there was higher aromaticity in the MI fraction, while the DI fraction had lower aromaticity. Interestingly, gas chromatography–mass spectrometry (GC–MS) and positive ion electrospray high resolution mass spectrometry (ESI+–HRMS) analyses showed that both the MI and DI subfractions were lignin-derived oligomers, which accounted for 25.0% of the total mass of the original PO. Additionally, the oligomers in the PO were mainly composed of dimers to pentamers. The secondary tandem MS/MS (MS2) experiments revealed that the m/z 360 is a dimer linked by coniferyl alcohol and a ferulate linkage, and no further fragmentation was observed in both the m/z 475 and 701 due to the rigidity of these structural motifs.