Influence of framework aluminum gradients on the catalytic activity of Y zeolites: Cracking of gas-oil on Y zeolites dealuminated by different procedures

The catalytic cracking of gas-oil has been studied in a series of REHY and HY zeolite catalysts dealuminated by “deep bed” calcination, steaming, and SiCl4 treatment. The samples have been characterized by X-ray diffraction, pyridine adsorption, and X-ray photoelectron spectroscopy (XPS). It has been found that the framework Si Al ratio determined by XPS (surface Si Al and that determined from unit cell dimension measurements (bulk Si Al ) differ, indicating a gradient in the concentration of the framework aluminum normal to the surface of the zeolite crystallites. When the gas-oil cracking activity is plotted versus unit cell dimension, three different volcano curves are obtained, with activity maxima located at about 24.59, 24.35, and 24.33 Å for deep bed, steamed, and SiCl4-treated zeolites, respectively. These results, together with the behavior of the selectivity curves to gasoline, light cycle oil (LCO), gases, coke, and butene butane ratio, are discussed on the basis of Si Al gradients. Comparison of the cracking behavior of a short molecule such as n-heptane with that of gas-oil, in relation to the acidity of the samples measured by pyridine adsorption, supports the idea that gas-oil does not penetrate the zeolite deeply and, therefore, that the activity of the catalyst is controlled by the framework Si Al ratio close to the external surface, instead of the bulk Si Al ratio of the zeolite crystallites.