(114e) Hydrocracking of a Heavy Feedstock Using Modified USY Zeolite-Based Catalysts

Currently, refiners are facing several important challenges. They have to process lower quality feeds because of the depletion of conventional lighter crudes and, at the same time, to fulfill more stringent fuel quality specifications and satisfy the growing demand for middle distillates. In this scenario, hydrocracking technology has proven its usefulness because it is an extremely versatile refinery process. The most appealing example is that it can be used to produce high-quality diesel from even the most difficult-to-process feeds.

The catalyst plays a pivotal role in hydrocracking technology. The product distribution depends strongly on the nature of the acidic component. For production of middle distillates, amorphous silica-alumina and faujasite (Y) zeolite in its ultra stable form are the main acidic components in practical use. Comparatively, USY zeolite offers higher acidity and better stability, but it may impose diffusional limitations and lower middle distillates selectivity than amorphous silica-alumina. Nowadays, efforts are directed towards development of a new generation of zeolite Y-based catalysts with optimum number and strength of acid sites and suitable pore structure with high activity tailored to desired selectivity.

Despite many studies often using model feedstocks, there is still a lack of systematic studies using real feedstocks. Also, the role of extraframework aluminum species, the relation between Brønsted/Lewis sites and the effect of the method employed to create mesoporosity and/or to dealuminate the zeolite has not been systematically investigated. The present work reports on such parameters related to a USY zeolite for the hydrocracking conversion and yield to middle distillates of a heavy feedstock. Chemical treatments were performed with a chelating agent for selectively extracting extraframework aluminum species and with ammonium nitrate to create mesoporosity. The properties of the solids were characterized by means of ²⁷Al-MAS-NMR, ²⁹Si-MAS-NMR, XRD, XPS, TEM, N₂ adsorption, H₂-TPR, ammonia TPD, Pyr-IR, and elemental analysis. The catalysts were tested in a tubular reaction system at 350 °C, 1500 psig, H₂/feed: 1250 NL/L, LHSV: 1h^-1 with a pretreated vacuum gas oil.

Overall activity and middle distillates yield was enhanced by the modifications performed to the USY zeolite. However, a progressive deactivation of the catalysts was observed, presumably due to coke formation. Although mesoporosity was significantly enhanced in all treatments, it will be shown that the activity most significantly depends on the density and strength of acid sites. ²⁷Al-MAS-NMR results evidenced that the treatment with the chelating agent caused partial reinsertion of some extraframework aluminum species, increasing in that way the density of Brønsted acid sites and favoring the cracking activity. According to the textural properties, ammonium nitrate treatments were effective to generate mesoporosity, but mass transfer limitations for molecules in the real feedstock seem to be still present.