Comprehensive Characterization of Lignin Bio-Oils from Alkali and Kraft Lignin with Integrated ¹H NMR Spectral Modeling

The characterization of fast pyrolysis bio-oils is essential for advancing their valorization and utilization as a biobased commodity. Within these bio-oils, lignin-derived oligomers represent a valuable yet chemically challenging and largely unknown fraction, with significant potential for conversion into high-value chemicals and fuels. In this work, a multidisciplinary analytical approach was employed to achieve detailed insights into the composition of bio-oils obtained via fast pyrolysis of Kraft and alkali lignin. Fourier-transform infrared spectroscopy (FTIR), gas chromatography–mass spectrometry (GC-MS), nuclear magnetic resonance (NMR), and ultrahigh-resolution mass spectrometry (UHRMS) were combined to unravel the molecular complexity of these bio-oils. FTIR revealed distinct functional group distributions, while GC-MS characterized the volatile components and identified small oligomeric structures in both bio-oils. The 2D HSQC NMR spectra identified interunit linkages originating from the lignin feedstock, such as β-O-4 and 4-O-5, and highlighted structural differences between the lignin bio-oils. UHRMS results are consistent with these findings, with higher intensities observed for monomers and dimers, followed by fewer trimers. Tetramers were only detected in the Kraft lignin bio-oil. The experimental findings were complemented by modeling the ¹H NMR spectra using a surrogate mixture composed of representative lignin model molecules, selected based on the characterization results. This integration of experimental characterization with modeling helps to better account for the unresolved heavy fraction. Together, these combined analytical and modeling approaches enhance the understanding of lignin-derived bio-oils, offering a comprehensive basis for interpreting their composition.