Evaluation of anisole hydrodeoxygenation reaction pathways over a Ni/Al2O3 catalyst

Anisole is a model molecule for studying hydrodeoxygenation (HDO) of lignin-derived oxygenates. Here we elucidate its HDO pathway over 10 % Ni/Al₂O₃ catalyst. Adsorption experiments showed that anisole is adsorbed on the acidic sites of the Al₂O₃. Anisole adsorption at 200–300 °C is reactive in nature, and results in its demethylation. The catalyst was tested at 100–300 °C, 5–40 bar H₂ pressure. Conversion of 78 % was obtained at 5 bar and 300 °C, restricted by hydrogenation-dehydrogenation equilibrium. HDO mainly starts through the ring-hydrogenation pathway. This is followed by demethoxylation beyond 180 °C. At 5–12 bar, cyclohexane dehydrogenates to benzene. This was confirmed by conducting an HDO experiment with methoxycyclohexane. At lower pressure deoxygenation is favored; and demethylation is accompanied with methylation of the aromatic ring, for temperature >260 °C. Investigation of the initial reaction stages showed that anisole HDO on Ni/Al₂O₃ catalyst proceeds via two independent pathways i.e., reactive adsorption/(de)methylation and  aromatic ring hydrogenation.