Hexadecane, catalytic cracking

Figure 1. Product distribution of catalytic cracking reaction using n-hexadecane,...

Fig. 6.3 Molar carbon number distributions in catalytic cracking and hydrocracking of n-hexadecane at 50% conversion (24)...

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

Hexadecane cracking activity correlates with total aluminum content USY materials are more active than AFS materials before and after steaming. Extraframework aluminum contributes to catalytic cracking activity. 

Figure 2.1-2 Catalytic cracking of n-hexadecane. Solid line ejqrerimental products, 24 per cent conversion over alumina-zirconia-sUica at 500 C. Dotted line Calculated products, car-bonium ion mechanism (from Greensfelder, Voge, mdGood 2 f. 

Catalytic Cracking of Hexadecane Temperature 500°C. Catalyst silica-alumina-zirconia Pressure atmospheric Process Period 1 Hour... 

Nace, D.M., "Catalytic cracking over crystalline aluminosilicates. I. Instantaneous rate measurements for hexadecane cracking", Ind. Eng. Chem. Proc. Des. Dev., 8(1), 24-31 (1969a). 

The structural features of dealuminated zeolite samples were characterized using X-ray powder diffraction, porosimetry and solid-state NMR measurements. Hexadecane cracking was used as a probe reaction to investigate catalytic properties of pure zeolites. 

Catalytic Properties. Catalytic properties were determined for both calcined and steamed zeolites using hexadecane cracking as a test reaction. Hexadecane cracking provides information on zeolite activity and selectivity which can be used to estimate octane production. Data were obtained for both calcined and steamed zeolites at hexadecane conversions from 30% to 70% multiple runs were made for each catalyst. 

Catalyst Performance Relationships. Hexadecane cracking activity of AFS and USY zeolites, when corrected for deactivation effects, shows little or no dependence on framework composition. Rather, as shown in Figure 6, activity appears to be a function of total aluminum content independent of the method of dealumination. This result implies that hexadecane cracking occurs over both framework and extraframework acid sites and that it is the total number of such sites which determines catalytic activity. Hence, extraframework material in the USY samples makes a significant contribution to catalyst activity as reported by others(18.19). 

Through the use of hexadecane cracking alone, we have been unable in this work to elucidate the role of mesoporosity in the catalytic behavior of calcined or steamed zeolites. Steamed AFS and USY zeolites show differences in mesoporosity but exhibit similar catalytic performance. While mesoporosity may affect diffusion in actual FCC catalysts, larger molecules than hexadecane will be required to determine mesopore effects. 

Other paraffins have also been used for measuring acidity. Neopentane is an attractive compound since protonation of a C-C bond is the preferred primary step for cracking. Corma and coworkers and Guisnet and coworkers used n-heptane in studies of H-Y. Klyachko et al. used octane to characterize the acidities of mordenites and ZSM-5 zeolites. The catalytic activity correlated well with their acidities as determined by calorimetric measurements of the heats of adsorption of ammonia. Higher paraffins such as hexadecane have also been used, but their utility is questionable due to the increased number of secondary reactions that can occur. ... 

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