Elucidating the reaction mechanism of hydrodeoxygenation of lignin monomers with ether function groups in systems with nonpolar solvents

Abstract: Hydrodeoxygenation (HDO) offers an effective pathway for the synthesis of hydrocarbons from phenolic monomers after lignin depolymerization. However, problems arise through attempting to remove the oxygen atom without saturating the aromatic group. While the HDO of these monomers can be hydrogenated by transition metals and chemically dehydrated using solid acid catalysts at high temperatures, this results in unwanted reactions that saturates the aromatic group. Our previous work on HDO of phenol and cresol showed that selectivity of aromatic hydrocarbons improves in reactions with nonpolar solvents under Pd/C or Pt/C catalysts.It was hypothesized that the presence a nonpolar solvent favors a keto-tautomer mechanis. However, little is known if this reaction mechanism would work with lignin-based monomers with ether groups like guaiacol and anisole. In this study, we investigated possible reaction mechanisms explaining how the oxygen from guaiacol and anisole are removed through HDO under a nonpolar solvent. All reactions were conducted at a constant temperature of 250°C under 4.14 MPa of hydrogen gas in a batch reactor for a period of 4 hours. Hexadecane was used as the nonpolar solvent and Pd/C was added as catalyst. A mixture of two catalysts, namely alumina oxide (Al₂O₃) and Pd/C, was also used to investigate the role of a solid acid catalyst in the production of aromatic hydrocarbons. Methyl 1,2-cyclohexanediol decreased significantly when Al₂O₃ was added along with Pd/C, supporting the catalytic dehydration of methyl 1,2-cyclohexanediol. The high concentration of methyl 1,2-cyclohexanediol in the product after HDO of guaiacol under Pd/C infers that the removal of oxygen via keto-tautomer was not favored at the conditions of the study.