Tuning surface-active sites of Ru catalysts for the selective deoxygenation of lignin monomers to fuels and chemicals

Cycloalkanes and cyclohexanols find diverse applications, including sustainable aviation fuel, fuel additives, and value-added chemicals. These compounds can be produced via complete or selective hydrodeoxygenation (HDO) of lignin-derived phenolic monomers. In this study, a urea-assisted method was employed to control the size of Ru nanoparticles (NPs) and tune the surface-active sites on Ru metal supported on CN_x. By adjusting the Ru-to-urea molar ratio, both the size of Ru NPs and the metallic-to-oxide ratio of Ru were controlled, along with the incorporation of N atoms into the RuO₂ surface coordination. In the absence of urea, the Ru/CN_x(1:0) catalyst, containing 4.8 nm Ru NPs, achieved complete HDO of 4-propyl guaiacol to n-propyl cyclohexane through ring saturation, followed by hydro-demethoxylation of the C–OCH₃ group, and hydrogenolysis of the–OH group, with a 97.0% yield at an initial H₂ pressure of 1.5 MPa, 200 °C, and 20 h in water. The final hydrogenolysis step was inhibited over the urea-assisted catalysts. The Ru/CN_x(1:8) catalyst, containing 3.1 nm Ru NPs, achieved a maximum selectivity of 67.6% for n-propyl cyclohexanol, along with complete conversion of 4-propyl guaiacol. Similar trends were observed in the product distribution when upgrading reductive catalytic fractionation bio-oil over Ru/CN_x(1:0) and Ru/CN_x(1:8). (Figure presented.)