A core–shell Y zeolite with a mono-crystalline core and a loosely aggregated polycrystalline shell: a hierarchical cracking catalyst for large reactants†
Crude oil is very difficult to refine because of its large and very heavy oil molecules. The increased external surface areas of the engineered catalysts used in FCC contribute to promoting the preliminary cracking of heavy or extra-heavy oil molecules. Core–shell zeolites with a commercial Y zeolite core and a polycrystalline shell composed of loosely aggregated Y nanocrystals are designed and synthesized. Compared to a commercial Y zeolite catalyst, the as-synthesized core–shell Y zeolite with a high percentage of silicon in the shell exhibits excellent catalytic performance during the catalytic cracking of tri-isopropylbenzene, n-octane, and their physical mixture because of the increased external surface area and high hydrothermal stability. The results verify that large reactants can undergo a hierarchical cracking process over the as-prepared core–shell Y zeolite catalyst: the reactants can be pre-cracked on the accessible external surfaces and the pre-cracked products can be further deeply cracked by the strong acid centers in the confined microporous channels.