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

Abstract

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 CNx. 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/CNx(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/CNx(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/CNx(1:0) and Ru/CNx(1:8).