In-situ hydrodeoxygenation of a lignin-derived monomer using Ar dielectric barrier discharge plasma: From conversion performance to mechanism analysis

Bio-oil derived from lignin biomass serves as a promising alternative to fossil fuels. However, due to its high oxygen content and low energy density, it requires hydrodeoxygenation (HDO) to be viable as a biofuel. Non-thermal plasma, as an innovative molecular activation method, enables HDO of bio-oil under ambient conditions without catalysts. Nevertheless, the relationship between operating conditions, plasma characteristics, and product distribution remains unclear, necessitating elucidation of the underlying reaction mechanisms. Herein, we present an in-situ hydrogenation approach for the plasma-assisted conversion of a lignin monomer (guaiacol) using Ar dielectric barrier discharge plasma without an external hydrogen source. By integrating conversion experiments with reactive molecular dynamics simulations, we reveal the mechanisms governing the effects of temperature and H radical on guaiacol conversion. Results show that increasing temperature promotes demethoxylation of guaiacol, yielding cresol and phenol. Excessively high temperatures inhibit dehydroxylation while facilitating O-CH₃ bond cleavage, leading to increased formation of undesired catechol. Around 400 K represents an optimal reaction temperature. As the applied voltage increases, the concentrations of desired liquid products (cresol, phenol, anisole) first rise then decline. This occurs because while H radical concentration progressively rises with the voltage, H radical-mediated deoxygenation efficiency peaks and subsequently decreases. Thus, maintaining an optimal H radical concentration range enhances conversion efficiency. Overall, the revealed interaction mechanisms between plasma-generated H radicals and guaiacol provide novel insights and guiding principles for future bio-oil upgrading.