On the effect of EDTA treatment on the acidic properties of USY zeolite and its performance in vacuum gas oil hydrocracking

Abstract

USY zeolite catalysts were chemically modified by treatment with aqueous solutions of sodium ethylenediaminetetraacetic acid (EDTA) to study the effect of removal of extraframework aluminum species (EFAl) on hydrocracking performance. The textural, structural and acidic properties were characterized by XRD, elemental analysis, XPS, N2 physisorption, HRTEM, 29Si and 27Al solid-state NMR, ammonia TPD and IR spectroscopy of adsorbed pyridine. The acid catalytic activity of the zeolites was measured by the conversion of propane and 1,3,5-tri-isopropylbenzene. EDTA is a selective agent for the extraction of EFAl from USY preserving the framework Al content. Concomitant with EFAl removal, EDTA enhances the textural properties and changes bulk and surface properties, in particular the Al speciation. Hydrocracking catalysts were prepared from the modified USY zeolites and alumina loaded with a P-promoted NiMo-sulfide phase. Their performance was evaluated in the hydrocracking of a heavy vacuum gas oil under conditions close to the industrial practice. The most active hydrocracking catalyst is obtained by removal of a relatively small amount of EFAl from the USY zeolite. The mild EDTA treatment to achieve this selectively extracts amorphous EFAl species from the mesopore system, resulting in better access to acid sites and an increase of the concentration of strong Brønsted acid sites. Severe EDTA treatment resulted in lower hydrocracking activity which was ascribed to progressive leaching of EFAl synergistically involved in the cracking catalysis. The present study therefore points out the importance of the presence of some EFAl species for the hydrocracking of real feedstocks. This work also shows that even with supports with increased mesoporosity and stronger acid character, higher hydrocracking activity of a real feedstock is not guaranteed.