Hydropyrolysis vapor upgrading reaction pathways of guaiacyl phenolic compounds: Hydrogen transfer mechanisms

Hydropyrolysis vapor upgrading (HP-VU) is a promising route for producing high-quality fuels from biomass, yet the migration of external hydrogen within the reaction network remains unclear. In this study, the reaction pathways of three guaiacyl phenolic compounds were investigated over a NiMo/γ-Al2O3 catalyst during HP-VU. Product analysis coupled with D2 isotopic tracing were employed to elucidate hydrogen transfer mechanisms. It was found that during hydropyrolysis, hydrogen radicals from H2 dissociation preferentially attack the aromatic carbon bearing methoxy group, promoting the removal of methoxy group and creating internal methyl groups for alkylation. Subsequently, during catalytic vapor upgrading, external hydrogen moves between metal and acid sites in the form of Ni–H species. These species drive two competing pathways: direct deoxygenation to aromatics and hydrogenation–dehydration to cycloalkanes. Under D2 atmosphere, a pronounced kinetic isotope effect suppressed the direct deoxygenation route, thereby enhancing the selectivity to cyclohexane and methylcyclohexane. Deep deuterium incorporation, the presence of D2O, and deuterated light hydrocarbons confirmed that extensive H/D exchange and hydrogen migration occurred on the catalyst surface. These findings offer new insight into hydrogenation and deoxygenation pathways of lignin-derived compounds during HP-VU.