Mechanistic study on hydrotreating of N/S-heterocyclic compounds in waste tire oil: Insights from benzothiazole and D-limonene

Pyrolysis represents a promising strategy for the value-added utilization of waste tires, however, the high nitrogen and sulfur content in the resulting pyrolytic oil poses significant challenges for its downstream application. Consequently, hydrotreatment is essential for the effective upgrading of this complex feedstock. Understanding of the hydrogenation desulfurization mechanism for heterocyclic compounds remains insufficient. In this study, the hydrodesulfurization (HDS) mechanisms of nitrogen- and sulfur-containing heterocyclic compounds were investigated using model compounds of D-limonene and benzothiazole through a combination of catalytic hydrogenation experiments and density functional theory (DFT) calculations. Catalytic hydrogenation experiments performed under different catalysts (Ni-Zn and Ni-Mo), temperatures, and H2 pressures revealed: (1) D-Limonene underwent processes of polymerization, isomerization, dehydrogenation, and hydrogenation, along with cross-reactions involving heteroatomic compounds; (2) Benzothiazole desulfurization achieved a conversion of 96.38 % under Ni/Mo catalysis at 300 °C and 3.0 MPa H2, producing N-methylaniline and aniline as key products. DFT simulations revealed that H2 undergoes dissociative adsorption on the NiMoO4(110) surface. The adsorption of benzothiazole was found to be endothermic and promoted the cleavage of the Csp3-S bond, with sulfur showing preferential adsorption on the catalyst surface. Based on these findings, a sequential HDS reaction pathway was proposed: Benzothiazole→C=N saturation→Dihydrobenzothiazole→Csp3-S cleavage→2-(Methylamino)thiophenol→Csp2-S cleavage→N-Methylaniline→Csp3-N cleavage→Aniline. This study provides mechanistic insights and theoretical guidance for the catalytic upgrading of waste tire pyrolytic oils.