Production of high-carbon-number hydrocarbon bio-aviation fuels via catalytic hydrogenation of vanillin and non-catalytic condensation: a mechanistic study with DFT and experimental insights

Lignocellulose or lignin present significant potential as sustainable feedstocks to replace petroleum-derived resources through catalytic upgrading. Hydrodeoxygenation of phenolic molecules derived from lignocellulose or lignin can produce cycloalkanes, but often forms low-carbon-number hydrocarbons, which are more suitable for gasoline rather than high-carbon-number diesel or aviation fuels. This study investigates the production of high-carbon-number hydrocarbons in the aviation fuel range from lignin-derived compounds, using vanillin as a model. A two-step process was performed to achieve this: selective hydrogenation of vanillin to vanillyl alcohol and creosol using 1 wt% ruthenium on carbon, followed by non-catalytic condensation and subsequent hydrodeoxygenation of the condensates to cycloalkanes using 3 wt% ruthenium on HZSM-5. This process yielded C14 aviation fuel precursor (19%) and C14 deoxygenated hydrocarbon (5%) whereas the one-step process without the condensation step did not yield any C14 compounds. The reaction pathway was elucidated through density functional theory calculations and control experiments with intermediates, providing insights into the mechanisms of upgrading lignin-derived compounds for sustainable aviation fuel production.