Preparation, Characterization And Catalytic Performance Of Nanocrystalline ZSM-5with Hydrophobic Surface

Endothermic hydrocarbon fuels not only offer the required power capacity, but also absorb the heat of the high temperature components of hypersonic aircraft through both physical heat-absorbing and chemical reaction. Catalytic cracking reaction is an effective way to improve the cooling capacity of endothermic hydrocarbon fuels in low temperature region. Among the catalytic processes, quasi-homogeneous catalytic cracking is carried out by dispersing catalyst to hydrocarbon fuels, during which flowing and heating resistance is decreased,and coke deposits within the reactor is decreased. Corresponding to the quasi-homogeneous catalytic cracking of hydrocarbon fuels, the preparation, characterization and catalytic activity evaluation of hydrophobic nanoscale zeolite was investgated in this paper.Hexadecyltrimethoxysilane was used as the modification agent, the influencing factors for the preparation of hydrophobic ZSM-5nanocrystals with a Si/Al ratio of25was studied such as hydrothermal temperature, hydrothermal time and the concentration of silane. ZSM-5zeolite with high crystallinity and dispersity in n-dodecane was prepared with the hydrothermal temperature165℃, hydrothermal time9h and hexadecyltrimethoxysilane in a proportion of10mol%in regards to the content of the silane.Nanocrystalline ZSM-5(m-n) with hydrophobic surfaces of different ratios (m) of silica to aluminia and carbon chain lengths (n) was prepared, which was characterized by X-ray diffraction,29Si magic angle spinning nuclear magnetic resonance, frustrated infrared spectrum, transmission electron microscope and thermogravimetric analysis. For nanocrystalline ZSM-5with hydrophobic surfaces, the amount of the long chain organic group chemically bonded to the surface and the thermal stability in hydrocarbon fuels deceased as the Si/Al ratio increased. The amount of the long chain organic group chemically bonded to the surface increased and the thermal stability in hydrocarbon fuels increased as the carbon chain length increased.The catalytic cracking of n-dodecane was carried out in a tubular reactor to study the catalytic activities of the hydrocarbon fuels dispersible nanocrystalline ZSM-5. The conversion and yield of gas products of n-dodecane catalyzed by samples of different Si/Al ratios and carbon chain lengths were compared, and the activity parameter of the samples was also calculated. The results show that the catalytic activity of nanocrystalline ZSM-5decreased as the Si/Al ratio increased, the catalytic activity increased as the length of the carbon chain increased.For the quasi-homogeneous catalytic cracking of n-dodecane using nanocrystalline ZSM-5with hydrophobic surfaces, ZSM-5(20-16) was optimal when the activity parameter, gas yield and thermal dispersion stability were comprehensive considered.