Abstract: A green and efficient self-combustion depolymerization (SCD) method is proposed to activate coal gangue into highly reactive silica and alumina species and to construct catalytic cracking catalysts with hierarchical porous structure by direct method. X-ray diffraction, differential thermal analysis, nuclear magnetic resonance and other methods were used to investigate the coal gangue activated by SCD method and the hierarchical pore catalysts synthesized by hydrothermal synthesis, elucidating the crystal growth law of gangue-based zeolies and the association with the activated silica and alumina species in the gangue. The results show that SCD properties of the gangue are conducive to its efficient depolymerisation and activation into amorphous highly active silica-alumina materials, which promotes its transcrystallisation into zeolite components. The prepared catalysts were enriched with 4–100 nm pore structure. ACE evaluation results showed that the hierarchical pore catalysts had excellent activity and stability compared with conventional FCC catalysts, with an increase in (gasoline + LPG) yield and a significant decrease in coke yield, which is attributed to the formation of abundant macropores on the surface of the microspheres during the specific “self-combustion depolymerization-alkali solubilisation-crystallisation” process, giving the microspheres an ideal pore size distribution. Furthermore, the uniform growth of zeolites on the inner and outer surfaces of the microspheres has been demonstrated to enhance the utilization rate of zeolties, thereby facilitating multi-directional diffusion paths, which results in a hierarchical porous structure catalyst with optimal accessibility to the active center, thus effectively improving the heavy oil cracking activity and decreasing coke yield.

Key words: coal gangue, self-combustion depolymerization, hierarchical porous structure, catalytic cracking, in-situ crystallization, kaolinite