Core-shell structure catalysts fabricated demonstrated a volcanic trend in olefins yields with the increase of shell thickness. Controlling shell thickness to 150 nm could manifest the best performance with 27.9 % higher in olefins yields and dramatically improved catalyst lifetime compared with the core catalyst ascribed to passivation of external acid sites by protective shell outside the core to inhibit side reactions considerably.

Abstract

A variety of core-shell structure catalysts with different shell thicknesses were fabricated by hydrothermal coating. Multiple characterization technologies including XRD, SEM, N₂ adsorption-desorption, NH₃-TPD and IR were employed to explore the physicochemical properties of all the samples, demonstrating that the shell layer was successfully introduced on the exterior of the core. The results of hexane cracking disclosed that with the increase of shell thickness, olefins selectivities increased continuously accompanied by a minor decrease of hexane conversion. Only when controlling the shell thickness to 150 nm could manifest the best performance with light olefins yields of 51.1 %, much higher than those of 23.2 % on the core catalyst. On the other hand, the catalyst lifetime was greatly improved, which presented a decrease of only 2.8 % in hexane conversion, relative to 9.7 % on the core catalyst during the reaction period of 80 h. Above exciting phenomenon was attributed to the growth of protective shell outside the core, which passivated the external acid sites greatly, thus side reactions were inhibited dramatically. It is firmly believed that our work provided a novel reference for the production of light oil cracking catalyst in industry, which will make up for the shortage of olefins considerably.