High Yield of Hydrocarbons from Catalytic Hydrodenitrogenation of Indole under Hydrothermal Conditions

We report herein on a new method to achieve a high yield of hydrocarbons from hydrothermal catalytic hydrodenitrogenation (HDN) of indole, which is higher than that from conventional pyrolysis methods. The main hydrocarbon products were aromatic hydrocarbons and alkanes, which are similar to fossil oils to be used as liquid fuels in the future. Catalyst screening experiments show that noble metal catalysts (e.g., 5 wt % Pt, Pd, or Ru) supported on porous solids (e.g., carbon, Al₂O₃) enhanced the conversion of indole to hydrocarbons under hydrothermal condition. Of those different catalysts, Pd/γ-Al₂O₃ shows the greatest influence on the yield of hydrocarbons, which we focus on the catalyst Pd/γ-Al₂O₃ in more details. On the basis of the Pd/γ-Al₂O₃ catalyst, the effects of time, temperature, and H₂ pressure on the hydrocarbons were discussed. HDN of the indole reaction at 450 °C, 0.015 g/cm³ water density, 5 MPa H₂, and 50 wt % Pd/γ-Al₂O₃ loading led to a maximum yield (51 mol %) of hydrocarbons at 120 min. It proposes the mechanism to acquire hydrocarbons from hydrogenational indole denitrogenation, which experiences two different pathways, as (1) indole is directly hydrodenitrogenated into hydrocarbons, and (2) intermediate oxygenated products from hydrolysis of partly hydrodenitrogenated indole were hydrodeoxygenated to removal of O to acquire hydrocarbons. The factors for deactivation of catalyst under hydrothermal condition are also discussed by the results from characterizing the surface, bulk structure, and microscopy experiments.