Subcritical Water Reactions of a Hardwood Derived Organosolv Lignin with Nitrogen, Hydrogen, Carbon Monoxide, and Carbon Dioxide Gases

Abstract

Subcritical H2O at 365 °C is considered for lignin conversion, because H2O exhibits unusual properties at higher temperatures (i.e., decreased ion product and static dielectric constant), such that there is a high solubility for organic compounds. This high solubility for organic compounds is expected to apply to lignin for its conversion into high value transportation fuels, which may prove the effectiveness of integrated biorefineries. Experiments were conducted with hardwood derived Organosolv lignin, subcritical H2O (defined here as H2O at 365 °C and autogenous pressure), and various industrial gases (N2, H2, CO, and CO2 at a cold pressure of 500 psi) for 30 min to determine both lignin’s potential to generate value-added products (e.g., monomer compounds and methanol) without the need for a catalyst and the roles (if any) of the H2O and the gases in the reactions. The behavior of H2O at temperature (365 °C) and pressure within this research is expected to be similar to the behavior of supercritical H2O (374 °C and 3205 psi), without the need to maintain supercritical conditions. Different characterization techniques were used for the products collected including primarily gas chromatography with flame ionization detection and thermal conductivity detection (GC/FID-TCD) of the evolved gases, GC/MS analysis of the organic liquids, solid phase microextraction analysis of the recovered H2O, and solid state 13C NMR analysis of the solid residues. The reactor pressure at temperature was shown to influence the outcome of products, and the highest conversions (≈54–62%) were obtained when adding gas. The collected solids from the N2, H2, and CO reactions appeared to be the most reacted (i.e., the most changed from the unreacted lignin) according to solid state 13C NMR analysis, and the widest variety of products (methoxy-substituted phenolic compounds) were also obtained when using CO, according to GC/MS analysis.