Hydrogenation liquefaction behavior and catalytic mechanism of Naomaohu coal-derived asphaltenes over different catalysts

To elucidate the conversion mechanism of Naomaohu coal-derived asphaltenes (NMH-ASP) in the high-temperature stage during the two-stage liquefaction (TSL) process, various catalysts, including Fe₂O₃, α-FeOOH, iron stearate (FeSA) and NiMo/γ-Al₂O₃, were presulfurized at low-temperature to reveal the effect of their active phase on liquefaction performance and products selectivity. The results showed that NMH-ASP conversion and oil yield dropped in the following order at 430 °C for 60 min: NiMo/γ-Al₂O₃ (85.32 wt%; 71.20 wt%) > FeSA (70.97 wt%; 63.71 wt%) > α-FeOOH (68.79 wt%; 61.50 wt%) > Fe₂O₃ (63.46 wt%; 54.76 wt%). The NiMoS phase from NiMo/γ-Al₂O₃ sulfurization exhibited superior H₂ activation ability, facilitating aromatic-ring hydrogenation and the cleavage of C_al–O and C_ar–O bonds, resulting in deep deoxygenation. Consequently, the liquefied oil showed the highest cycloalkane content (10.39 %) and the lowest oxygen-containing compounds (1.89 %), along with higher CO₂ content (1.50 wt%) in the gas. The Fe_1-xS phase of FeSA showed smaller, more uniform crystallite sizes, promoting the cleavage of aliphatic side chains and C_al–C_al bonds, leading to increased formation of chain alkanes (20.82 %) and C₁–C₄ gases (2.34 wt%). α-FeOOH-derived Fe_1-xS enhanced solvent hydrogen transfer to free radical fragments and promoted C_al–C_ar dealkylation, resulting in a higher benzene series content (29.62 %). However, the Fe_1-xS phase of Fe₂O₃ exhibited lower efficiency in C_ar–O bond cleavage, leading to the accumulation of phenols (8.21 %) and a higher content of the naphthalene series (43.49 %) due to insufficient aromatic-ring hydrogenation. This work provides theoretical guidance for catalyst design and process optimization in TSL.