Efficient hydrodeoxygenation of lignin-derived phenolic compounds under acid-free conditions over carbon-supported NiMo catalysts

High-quality liquid biofuels can be produced from renewable lignin-derived phenolic compounds through the efficient hydrodeoxygenation (HDO) process, in which the traditional catalysts usually include metal site and acid site that catalyzes hydrogenation and deoxygenation procedures respectively. This work presented a novel acid-free NixMoyN/C catalyst from the perspective of green chemistry, providing a new pathway for HDO of lignin-derived phenolic compounds that involves hydrogenation, deoxygenation and hydrogenolysis at the same time. A series of NixMoyN/C catalysts were prepared by varying Ni and Mo molar ratio, where the Ni1Mo3N/C catalyst showed the best HDO performance. Guaiacol was completely converted at 260 °C after 4 h, with 95.8% cyclohexane selectivity. And a small amount of benzene can be obtained as a valuable fuel additive by-product by altering the conventional HDO reaction path. By shortening the reaction time, benzene can be obtained as the intermediate products with a relative high selectivity. Based on the characterizations of XRD, BET, SEM, TEM, XPS, H2-TPD and EPR, the results demonstrate that the multiple active components of Ni1Mo3N/C catalyst allow it to efficiently catalyze the hydrogen activation and C-O bond cleavage even under acid-free condition. The existence of the active phase of Ni, Ni2Mo3N and β-Mo2C, as well as the interaction between Ni and Mo metals, contributes together to achieve an efficient HDO performance. Not only for the various phenolic model compounds, the feasibility of Ni1Mo3N/C catalysts for upgrading raw lignin-oil was also demonstrated, with the hydrocarbon content increasing from 5.7% to 88.4%. Notably, arenes account for 18.2% in the hydrocarbon products, which confirms the occurrence of hydrogenolysis in the catalytic process. This work provides a novel route for the conversion of lignin-derived phenolic compounds to the production of high-quality hydrocarbon liquid biofuels, especially the direct production of arene component.