Investigating reaction pathways for formic acid and lignin at HTL conditions using 13C-labeled formic acid and 13C NMR

Abstract

The lignin to liquid (LtL) solvolysis is a type of hydrothermal liquefaction (HTL) that utilizes formic acid to facilitate the desired hydrodeoxygenation process to depolymerize lignin. Formic acid has been chosen due to its hydrogen donor ability, and it has proven to be a more efficient hydrogen donor than hydrogen gas and isopropanol. Based on previous experiments where the amount of carbon in the solid phase and organic phase together exceeded 100% relative to carbon in the lignin starting material, we set up experiments with 13C-labeled formic acid in order to investigate where the carbon form formic acid is incorporated. All the experiments were performed in duplicates, one with 13C-labeled formic acid and one with standard formic acid. The bio-oils from the experiments were analyzed by Nuclear Magnetic Resonance (NMR) and the spectra were compared. The 13C NMR spectra showed significant differences between oils made with standard formic acid and oils made with 13C-labeled formic acid, located in the section between δ 170 ppm and δ 180 ppm in the NMR spectra. The signal in this range arise from carbonyl bonds and shows that the carbon from formic acid ends up in the carbonyl structures of the bio-oil in compounds such as aromatic acids. This paper has also shown that the amount of formic acid plays a minor role in product composition compared to reaction temperature, which influences the product composition on a functional group level.