Synergistic regulation of dual thresholds: Hydroxyl occupancy and carbon species in the targeted catalytic hydrocracking of lignite

Abstract

The preparation of carbon-based catalyst supports from low-grade lignite, followed by its self-catalytic hydrocracking to produce lignite-derived aromatic compounds, is crucial for achieving efficient lignite utilization and expanding aromatic compound sources. During lignite catalytic hydrocracking, the uncontrolled diffusive migration of active hydrogen (H*) and insufficient protection of aromatic rings reduce the yield of aromatic compounds. Using Heishan lignite as the raw material, we employed a relay process of thermal-induced self-assembly and oxidation-mediated carbon distortion for fabricating the Co₃O₄/AC&GC-S-600 composite material, which features an optimal hydroxyl occupancy ratio and a balanced combination of amorphous carbon/graphite carbon (AC/GC) types. The optimal hydroxyl occupancy threshold balances Co₃O₄ dispersion and hydrophilicity, facilitating the efficient formation and directional migration of H* through synergistic interaction between Co₃O₄ and H₂O. The adequate threshold of AC/GC ratio regulates substrate adsorption, activation, and inhibition of aromatic hydrogenation, enhancing selective C–O bridge bond cleavage and preserving the unsaturation of aromatic rings. The synergistic regulation of dual thresholds promotes the rapid formation and the directional migration of H*, enabling efficient conversion of Heishan lignite to aromatic compounds (45.8% conversion, 80.8% selectivity) under mild conditions. Characterization techniques, experimental analyses, kinetic studies, and density functional theory calculations confirm that thermally induced self-assembly and oxidation-mediated carbon distortion are crucial for redistributing the carbon framework of the Heishan lignite matrix and regulating the optimal dual thresholds of hydroxyl occupancy and AC/GC types, which are essential for the catalytic hydrocracking of Heishan lignite to aromatic compounds. This strategy provides a scientific basis for selective catalytic production of lignite-based aromatic compounds, thereby supporting its clean and efficient utilization.