Selective Clipping of a Lignin-Derived Monomer by Plasma for Bio-Oil Upgrading

Oxygen atom removal is the key to improving the heating value and thermal stability of biomass-derived bio-oils. Nonthermal plasma provides a highly promising method for bio-oil upgrading under ambient conditions without catalysts. However, the diversity of products formed in plasma processing makes it necessary to understand the interaction mechanisms and reaction pathways. Here, we present a new approach for selectively clipping a lignin-derived monomer (guaiacol) by nonthermal plasma and propose a product selection mechanism governing the relationship between the product distribution and mean electron energy. Density functional theory calculations and a one-dimensional plasma fluid model show that the gas temperature determines whether the reaction is directed toward aromatic ring hydrogenation or hydrodeoxygenation. Increasing the voltage amplitude or decreasing the pulse rise time increases the mean electron energy, which facilitates removing the oxygenated functional groups. The main products can be adjusted from catechol to cresol and phenol and then to benzene, toluene, and xylene. The work shows the correlation of the product distribution with the mean electron energy and analyzes the underlying reaction pathways, guiding the practical application of plasma-enabled bio-oil conversion.