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About 80% of the gasoline pool as a whole in China for supplying the domestic market at current stage directly originates from FCC units. Obviously, FCC gasoline quality is critical for refiners to meet the nations more and more stringent gasoline specifications. FCC process is expected to produce gasoline with reduced contents of olefins, aromatics, benzene, sulfur, and, contradictorily, still with high octane number. Catalytic cracking process involves a series of acid catalyzed reactions. Bronsted acid sites dominate the surface of the catalyst used for FCC process. All the reactions of hydrocarbons in FCC process are based on carbonium ions of penta coordinated, or carbenium ions of tri coordinated. The monomolecular beta scission mechanism for alkane cracking explains that the cracking of carbon carbon bonding occurs at the beta position to the carbon atom bearing positive charge, and hence forms two small hydrocarbon molecules: one alkane molecule and one olefin molecule. The molar ratio of alkane to olefin for the primary cracking product will be 1 and it will be less than 1 if the cracking reaction proceeds. However, it is proved that bimolecular reaction pathways exist between surface carbenium ions and the feed molecules. The products of this bimolecular disproportionation reaction could be an alkane molecule and a newly formed carbenium ion. The better understanding of the reaction chemistry of FCC process based on monomolecular pathways and bimolecular pathways should be the basis for searching approaches to the improvement of FCC gasoline quality. In the complicated reaction scheme of the FCC process, the isomerization reaction leading to the formation of iso alkanes is obviously a target reaction, which favors both olefin reduction and octane enhancement. The cracking of small paraffin molecules, due to its limited number of reaction pathways and products, has been used to investigate cracking mechanism. In the present work the cracking of n hexane on different zeolites has been studied. Catalytic experiments at atmospheric pressure were carried out in a continuous fixed bed reactor. By means of measuring the initial rates for the cracking of n hexane over two series zeolites, Y and Beta, a correlation between the Bronsted acid sites and the initial rates has been found. The results show that the initiation steps of n hexane cracking occur on the Bronsted acid sites, Lewis acid sites are inactive for these steps. Acid site density and acid site strength of zeolite determine the activity of n hexane cracking. The role of acid site density predominates in most cases, however, the acid site strength also plays an important role in some instances. Meanwhile, similar intrinsic catalytic activities of different zeolites have been found. A well correlated linear relationship exists between acid site density of zeolite and the initial formation rates of some primary products. For zeolites with different structural pore diameters, their acid site density does not show significant effects on the cracking chain length for n hexane. The results indicate that bimolecular reactions in the cracking of n hexane proceed via Rideal mechanism. The reaction scheme of n hexane can be explained by chain mechanism. A parameter “Cracking chain length” (CCL) has been proposed, and some elementary steps in cracking procedures have been scrutinized. The effects of zeolite structure, acid site density, and reaction temperature on the mechanism of n hexane cracking and CCL have been studied. The cracking of n hexane proceeded via the chain reactions includes three steps: initiation, propagation and termination. The chain initiation proceeds through monomolecular protolytic cracking mechanism. All the carbenium ions formed with carbon number above 4 are mainly tertiary carbenium ions. The chain process of cracking is propagated by bimolecular reactions, namely disproportionation and hydrogen transfer reactions. All the reactions occurred in the chain termination step are
About 80% of the gasoline pool as a whole in China for supplying the domestic market at current stage directly originates from FCC units. Obviously, FCC gasoline quality is critical for refiners to meet the nation’s more and more stringent gasoline specifications. FCC process is expected to produce gasoline with reduced contents of olefins, aromatics, benzene, sulfur, and, contradictorily, still with high octane number. Catalytic cracking process involves a series of acid catalyzed reactions. Bronsted acid sites dominate the surface of the catalyst used for FCC process. All the reactions of hydrocarbons in FCC process are based on carbonium ions of penta coordinated, or carbenium ions of tri coordinated. The monomolecular beta scission mechanism for alkane cracking explains that the cracking of carbon-carbon bonding occurs at the beta position to the carbon atom bearing positive charge, and hence forms two small hydrocarbon molecules: one alkane molecule and one olefin molecule. The molar ratio of alkane to olefin for the primary cracking product will be 1 and it will be less than 1 if the cracking reaction proceeds. However, it is proved that bimolecular reaction pathways exist between surface carbenium ions and the feed molecules. The products of the bimolecular disproportionation reaction could be an alkane molecule and a newly formed carbenium ion. The better understanding of the reaction chemistry of FCC process based on monomolecular pathways and bimolecular pathways should be the basis for searching approaches to the improvement of FCC gasoline quality. In the complicated reaction scheme of the FCC process, the isomerization reaction leading to the formation of iso alkanes is obviously a target reaction, which favors both olefin reduction and octane enhancement. The cracking of small paraffin molecules, due to its limited number of reaction pathways and products , has been used to investigate cracking mechanism. In the present work the cracking ofCatalytic experiments at atmospheric pressure were carried out in a continuous fixed bed reactor. By means of measuring the initial rates for the cracking of n hexane over two series zeolites, Y and Beta, a correlation between the The results show that the initiation steps of n hexane cracking occur on the Bronsted acid sites, Lewis acid sites are inactive for these steps. Acid site density and acid site strength of zeolite determine the activity The role of acid site density predominates in most cases, however, the acid site strength also plays an important role in some instances. Meanwhile, similar intrinsic catalytic activities of different zeolites have been found. A well correlated linear relationship exists between acid site density of zeolite and the initial formation rates of some primary products. For zeolites with different st ructural pore diameters, their acid site density does not show significant effects on the cracking chain length for n hexane. The results indicate that bimolecular reactions in the cracking of n hexane proceed via Rideal mechanism. The reaction scheme of n hexane can be explained by chain A parameter “Cracking chain length” (CCL) has been proposed, and some elementary steps in cracking procedures have been scrutinized. The effects of zeolite structure, acid site density, and reaction temperature on the mechanism of n hexane cracking and CCL have has studied. The cracking of nhex proceeded via the chain reactions includes three steps: initiation, propagation and termination. The chain initiation proceeds through the monomolecular protolytic cracking mechanism. All the carbenium ions formed with carbon number above 4 are mainly tertiary carbenium ions. The chain process of cracking is propagated by bimolecular reactions, namely disproportionationand hydrogen transfer reactions. All the reactions occurred in the chain termination step are