Study On The Application Of Mesoporous Molecular Sieves For Catalytic Cracking

The discovery of the M41S family of siliceous solids has attracted more attention, for their uniform mesopore array, larger surface area and pore volume, higher thermal and chemical stability compared with classical microporous molecular sieves. The large mesopores can provide the potential for its use as catalysts for bulky molecules. Microporous molecular sieves are widely used in acid-catalyzed reactions. They often suffer from diffusion limitations when applied to petrochemicals and fine chemical synthesis. Hence, there have been an increasing demand for aluminosilicate molecular sieves with a pore diameter in the mesopore range used in acid-catalyzed reactions, particularly in the treatment of heavier feeds and the synthesis of large molecules for producing fine chemicals.The incorporation of Al is particularly important as it gives rise to solid acid catalysts with acid sites associated with the presence of Al in the framework position. The Al-containing mesoporous MCM-41 catalysts can be synthesized by both direct and post-synthesis methods with a wide range of framework Si/Al ratios. Previous investigations have reported the synthesis and characterization of these Al-containing materials. But fewer are in comparative studies on- the synthesis, characterization and catalytic cracking performances of different Al-containing mesoporous materials. Therefore, the main contents of my thesis were summarized as follows:The present investigation was a comparative study on the characterization of different mesoporous aluminosilicate materials with the same Si/Al molar ratio of 7.5 by post-synthesis method and their catalytic cracking activities in the 1, 3, 5-triisopropylbenzene and cumnen cracking. By comparision, A1SBA-15 shows excellent acid property, hydrothermal stability, and efficient catalytic cracking performance; MSU is the secondary in hydrothermal stability; MCM-48 is the worst inhydrothermal stability.Nanometer bimodal mesoporous aluminosilicate (abbreviated as NBMAS) was synthesized by sol-gel method. The important information about the textural characters, and acidity explained its catalytic cracking performance in the 1, 3, 5-triisopropylbenzen and cumene cracking. The catalytic activity of NBMAS is superior to that of AlMCM-41 directly hydrothermal synthesized, amorphous SiO2-Al2O3 and microporous HZSM-5.We also present another route to the production of mesoporous aluminosilicate nano-particles with particle size about 40-50 nm (denoted as Z-1), and the experimental results show that the material with wormlike but uniform mesostructured interiors can be routinely obtained by a simple sol-gel method at low temperature. The NH3-TPD showed that the acidity strength of Z-1 was similar to that of AlMCM-41, but the amount of medium-acid of Z-1 was more than that of AlMCM-41 prepared by post-synthesis method. In the catalytic cracking of cumene and 1, 3, 5-triisopropylbenzene, Z-1 exhibited higher cataclytic acitivity, selectivity and steam stability (100 % water vapor, 800 C, 2 h) compared with AlMCM-41.The enhancement of hydrothermal stability and acid strength of mesoporous molecular sieves was limited by optimizing synthesis method. If adjusting reaction conditions properly, the catalytic cracking reaction can be improved. Therefore, we have gone further into the investigation of the effect of temperature and mole ratio of catalyst to oil on the catalytic cracking reaction, such as 1, 3, 5-triisopropylbenzene and cumene cracking. We can concluded that the conversions of 1, 3, 5-triisopropyl benzene and cumene cracking increased with higher reaction temperature and mole ratio and the selectivities of light molecule products increased simultaneously with their conversions.