Hydrocracking of polycyclic aromatic hydrocarbons (PAHs) to produce benzene, toluene, and xylene (BTX) represents an effective route for upgrading light cycle oil (LCO). However, this process tends to generate large amounts of alkylbenzenes with long or multiple side chains, making it difficult for a single catalyst to achieve a high BTX selectivity. Herein, composite zeolite catalysts and graded catalyst systems were designed and prepared for the hydrocracking of 1-methylnaphthalene (1-MN) to BTX at a temperature of 420 °C and a pressure of 6 MPa. The NiW/BY-20 catalyst (Beta-USY zeolite composite) promotes the ring-opening cracking of monocyclic aromatics and the side-chain cleavage of alkylbenzenes, increasing the BTX selectivity and yield to 37.4% and 36.4%, respectively. The NiW/BZ-20 catalyst (Beta-ZSM-5 zeolite composite), with its strong acidity and high dealkylation capability, further improves the BTX selectivity and yield to 38.1% and 37.3%, respectively. These results indicate that composite zeolite catalysts can synergistically integrate the catalytic properties of zeolites with distinct topologies, enabling a relay catalysis pathway. Further investigations reveal that the graded catalyst system optimizes the reaction process; the B-Y catalyst (NiW/Beta in the upper bed, NiW/USY in the lower bed) increases the BTX selectivity and yield to 38.5% and 38.3%, respectively. The optimized B-Y-Z three-bed graded catalyst (upper: NiW/Beta, middle: NiW/USY, and lower: NiW/ZSM-5) exhibits excellent compatibility with the reaction pathway for 1-MN hydrocracking to BTX. It enhances the tandem catalytic effect while maintaining high hydrocracking activity, achieving BTX selectivity and yield of 41.3% and 40.2%, respectively, with alkylbenzene selectivity reduced by 3.3%. This work demonstrates that the synergistic design of composite zeolites and graded catalysts can effectively boost the efficient and targeted conversion of PAHs to BTX, providing valuable insights for the construction of bifunctional catalytic systems tailored for multistep tandem reactions. We propose a strategy that combines zeolite composite modification with different topologies and catalyst grading to developing efficient hydrocracking catalyst. The B-Y-Z three bed graded catalyst exhibits excellent compatibility with the reaction pathway for 1-MN hydrocracking to BTX. It enhances the tandem catalytic effect while maintaining high hydrocracking activity, achieving BTX selectivity of 41.3%, with alkylbenzene selectivity reduced by 3.3%. • Beta zeolite catalysts modified by compositing with USY and ZSM-5 achieve high BTX selectivity by integrating the advantages of different zeolite types. • The reaction pathway of 1-MN conversion to BTX in the graded catalytic system and the corresponding catalyst structure-activity relationship were elucidated. • The B-Y-Z three-bed graded catalyst enhances tandem relay catalysis between different zeolite beds, achieving the highest BTX selectivity.
Hu et al. (Mon,) studied this question.