Mechanistic And Kinetic Study On The Catalytic Cracking Of Light Hydrocarbons

The petrochemical industry is the foundation of the modern society. Basic chemicals such as ethylene, propylene, and other light olefins are currently manufactured mainly by thermal cracking of naphtha, i.e., so-called steam cracking or steam pyrolysis. However, the current steam pyrolysis process, taking place at the high temperature, consumes as much as 40% of the energy needed by the entire petrochemical industry. Global increasing environmental issues and the drying fossil fuel energy have stimulated the development of processes of energy saving and emission reduction. For this purpose, the research on the catalytic cracking to replace the steam cracking of light hydrocarbons has attracted more and more scientific interests. The catalytic cracking can not only reduce the reaction temperature about 50～200℃but also control the products distribution of light olefins easily. Hence, the R&D of proper catalyst plays the key role in the catalytic cracking.The present work compared H0108 and P57 catalysts which represent two generations of the catalytic cracking catalysts synthesized and provided by Shanghai Research Institute of Petro-chemical Technology. The pure zeolites catalyst H0108, corresponding to the first catalyst generation, showed high catalytic activity but low hydrothermal stability in the reaction. The hydrothermal treatment can easily change the active sites and porous channel structures of H0108. The in-situ Diffuse Reflectance Infrared Fourier transform Spectroscopy (DRIFTS) was used to analyze the cracking mechanisms of fresh and the hydrothermal treated H0108. And a brief model of the catalytic cracking of n-heptane in micropores and mesopores was proposed.The composite catalyst P57 constitutes of the equal amount of zeolites and Al2O3, which exhitied high hydrothermal stability and mild catalytic activity. The phase pattern, activity, products distribution, acidity and stability were analyzed by various characteristic methods. Different feedstocks were employed to study the catalytic mechanisms and reaction pathways at different temperatures. Comparisons of the structures and natures between H0108 and P57 were also studied. Besides, the mechanisms of the catalytic cracking of n-heptane over H0108 and P57 were discussed.Besides the activity and the reaction mechanism, the deactivation and the regeneration of these two catalysts were studied. The coke was the main cause of the deactivation over P57. For H0108, its deactivation is not only due to the coke but also the framework dealumination. Moreover, in situ FTIR was employed to construct the mechanistic model and the kinetic model of the freamework dealumination of H0108. Finally, the types of coke and the kinetic models of the regeneration by the coke combustion over the deactivated H0108 and P57 were studied by TG-DTG.