Synthesis of small crystal NiY zeolites and their catalytic performance in hydrocracking

Abstract

A series of small crystal Y-xNi zeolites with different amounts of Ni doping were synthesized by in-situ introducing the Ni precursors during the synthesis, through which the active Ni metal was incorporated into the framework of the Y zeolites. With the mechanical mixture of Y-xNi zeolites and amorphous silica-alumina (ASA) as the support, a series of Cat-xNi catalysts were prepared through loading the Ni and W components by incipient wet impregnation and the catalytic performance of Cat-xNi in the hydrocracking of n-hexadecane was then investigated. In addition, the effect of Ni doping on the physicochemical properties of Y zeolite and Cat-xNi catalysts was elucidated with the help of scanning electron microscopy (SEM), X-ray diffraction (XRD), N₂-adsorption desorption, NH₃ temperature programmed desorption (NH₃-TPD), H₂ temperature programmed reduction (H₂-TPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and so on. The results indicate that Ni can replace Al to enter the framework of the Y zeolites. The incorporation of appropriate amount of Ni into the Y zeolites can increase their relative crystallinity and the number of Br⊘nsted and Lewis acid sites; however, excessive Ni incorporation is detrimental to the crystallization of Y zeolite and excessive non-framework Ni species will cover the surface Br⊘nsted acid sites. Moreover, Ni doping can weaken the metal-support interaction, increase the sulfation extent of the active metal and the stacking number and dispersion of the active NiWS phase, and then improve the matching between the metal and acid sites on the Cat-xNi catalysts. As a results, in comparison with the counterpart Cat-0Ni catalyst, the Cat-xNi catalysts display more Br⊘nsted acid sites and active NiWS sites as well as improved the synergy between the metal sites and acid sites, which can enhance the conversion of n-hexadecane whereas inhibit the over-cracking, and then booster the yield of the middle distillate products (C₈–C₁₂). In particular, for the n-hexadecane hydrocracking at 360 °C, the Cat-0.2Ni catalyst exhibits a C₈–C₁₂ product yield of 65.4%, with a much higher n-C₁₆ conversion than the Cat-0Ni counterpart. All these suggest that the pre-impregnation of active metal Ni on the Y zeolites can effectively regulate the balance between the hydrogenation and cracking performance and improve the catalytic activity and the yield of middle distillate products in the hydrocracking of paraffins.