Using 2D NMR to characterize the structure of the low and high molecular weight fractions of bio-oil obtained from LignoBoost™ kraft lignin depolymerized in subcritical water

Abstract

In this work a multilevel analysis approach have been used for characterization of LignoBoost™ kraft lignin and bio-oil produced from LignoBoost™ kraft lignin using a process based on subcritical water (350 °C, 25 MPa). LignoBoost™ kraft lignin and the different fractions of the bio-oil (light oil, heavy oil and suspended solids) was characterized with high field NMR (18.8 T, 2D13C, 1H-HSQC NMR and 13C-NMR), GPC, GC-MS and elemental composition to improve understanding of the subcritical process. By using high resolution 2D HSQC NMR it was possible determine the chemical structures both on low and high molecular weight fractions of the bio-oil. It was confirmed that the signals from the aliphatic lignin inter-unit linkages, i.e. β-O-4′, β-β′, β-1′ and β-5′, had disappeared from all of the bio-oil fractions studied. This means that both the aliphatic carbon-oxygen (CO) and to some extent carbon-carbon (CC) bonds in LignoBoost™ kraft lignin have been cleaved and an effective depolymerization has occurred. However, re-polymerization into higher molecular weight (Mw) fractions takes place simultaneously. These higher Mw fractions (heavy oil and suspended solids) were found to be re-polymerized macromolecules, with new structural networks based on guaiacol/disubstituted aromatic ethers and polyaromatic hydrocarbon structures bound tightly together.