Production of cleaner waste tire pyrolysis oil by removal of polycyclic aromatic hydrocarbons via hydrogenation over Ni‐based catalysts

This research aimed to improve the quality of waste tire pyrolysis oil (WTPO) via hydrogenation to eliminate polycyclic aromatic hydrocarbons (PAHs), which are classified as toxic compounds or precursors to induce the formation of particulate matter. Given the presence of organosulfurous compounds in WTPO, this research was consisted of two steps. First, the hydrogenation of naphthalene, a model PAH compound, was used to screen for suitable promoters [molybdenum (Mo), tungstate (W), and platinum (Pt)] for nickel-supported gamma-alumina (Ni/γ-Al2O3) catalyst to provide a high sulfur tolerance and high PAH hydrogenation efficiency. Second, the obtained suitable catalysts were applied for the hydrogenation of 50 wt% WTPO solution in decane containing 15 982 ppm PAHs and a 0.75 wt% sulfur content. Although the NiPt/γ-Al2O3 catalyst exhibited the highest performance with 83.8% naphthalene conversion under 40 bar initial H2 pressure at 350°C for 6 hours, it was less tolerant to the presence of thiophene, one of the organosulfurous compounds found in WTPO. Thus, the NiMo/γ-Al2O3 and NiW/γ-Al2O3 catalysts were selected for WTPO hydrogenation. The PAHs hydrogenation and hydrodesulfurization occurred simultaneously in both catalytic systems. Under 50 bar initial H2 pressure at 350°C, the NiMo/γ-Al2O3 catalyst had a higher ability for hydrodesulfurization (98.4% maximum sulfur removal) than hydrogenation of PAHs (66.0% removal of PAHs), whereas NiW/γ-Al2O3, with lower acidity, showed a preference for hydrogenation of PAHs (71.1% removal of PAHs) with a lower hydroisomerization efficiency (74.2% sulfur removal). Moreover, the chemical structure of PAHs affected the ability to be eliminated by hydrogenation. PAHs having unsubstituted rings could be easily hydrogenated, while the removal of PAHs containing alkyl substituents in both aromatic rings required severe reaction conditions due to their high steric hindrance for adsorption onto the catalyst active sites.