Subcritical Water Reactions of Lignin-Related Model Compounds with Nitrogen, Hydrogen, Carbon Monoxide, and Carbon Dioxide Gases

Abstract

Experiments using subcritical H2O (365 °C) and various industrial gases (500 psi cold pressure of N2, H2, CO, or CO2) have been considered previously for the conversion of Organosolv lignin (representative of lignin-derived compounds generated from the production of bioethanol from woody biomass sources) into value-added products. The findings indicated that subcritical H2O can be used to depolymerize lignin into GC amenable compounds with or without added gas pressures, though adding gas further modified the types of products generated. However, the roles of the combined effect of subcritical H2O and the gases were not confirmed, as the reaction chemistry for lignin depolymerization at these conditions is not intuitive. Thus, experiments with subcritical H2O, the aforementioned industrial gases and lignin-related model compounds (i.e., aromatic aldehydes represented by vanillin and syringaldehyde, aromatic ketones represented by acetovanillone and acetosyringone, and aromatic ethers represented by dibenzyl ether and 2-phenethyl phenyl ether), were completed to study this. From these results, the suggested reaction pathway of Organosolv lignin reactions in subcritical H2O with and without added pressure appears to proceed in two stages. The first stage appears to be a hydrolysis reaction that breaks the ether linkages between lignin aromatic monomers with typical methoxy and hydroxyl functional groups and either an aldehyde or ketone (i.e., syringaldehyde, acetophenone). A second stage appears to remove the aldehyde/ketone so that the main products are methoxy phenols.