Effect of Phosphorus and Transition Metal-Modified ZSM-5 on Catalytic Pyrolysis of C7 Hydrocarbons with Different Structures
Phosphorus and transition metal-modified ZSM-5 zeolites were prepared via incipient wetness impregnation and characterized by XRD, N2 adsorption-desorption, and pyridine-IR spectroscopy. The effects of modified zeolites on catalytic pyrolysis of n-heptane, 3-methylhexane, and methylcyclohexane were systematically investigated, revealing the structure-activity relationship between acid properties and catalytic performance. Results indicate that phosphorus and transition metal modification can regulate the L/B acid ratio of ZSM-5, which significantly influences product distribution. An excessively high L/B acid ratio promotes hydrogen and coke formation, leading to pore blockage and reduced conversion, whereas an appropriate L/B acid ratio facilitates mild dehydrogenation, enhancing ethylene and propylene yields. Reactant conversion followed the order: n-heptane > 3-methylhexane > methylcyclohexane. Efficient conversion of 3-methylhexane and methylcyclohexane requires catalysts with high L acid amount, strong Brønsted acid amount, optimized L/B acid ratio, as well as high specific surface area and micropore specific surface area. This study provides critical insights for designing high-efficiency alkane pyrolysis catalysts.