Core–Shell-Structured Ni/ZSM-5@Silicalite-1 Zeolite Catalyst with a High Catalytic Performance for Ethylene to Propylene Reaction

The catalytic conversion of ethylene to propylene (ETP) has been proposed as a novel and attractive process to meet the increasing demand for propylene. Several ETP catalysts have been studied and developed, and catalysts based on Ni are viewed as promising options due to their cost-effectiveness and high levels of activity. Nevertheless, catalysts based on Ni are prone to deterioration due to the aggregation of Ni particles and the generation of carbon nanotubes (CNT) during the ETP process. As a solution to these drawbacks, we synthesized a Ni-encapsulated core–shell-structured zeolite catalyst by overgrowth of silicalite-1 shell layers on Ni-loaded ZSM-5. The silicalite-1 shell layers inhibited the aggregation of Ni nanoparticles and the formation of CNT, leading to an improved catalytic performance. The best sample among the synthesized catalysts exhibited a maximum propylene yield of 28.3 C-mol % and butene yield of 17.8 C-mol % in the ETP reaction test. The maximum combined propylene and butene selectivity was 69.2%. To support the above result, transmission electron microscopy observation of the spent catalysts revealed that the core–shell structure prevented Ni aggregation and the formation of CNT. In addition, the reduction in the amount of coke deposition (mainly CNT) for the core–shell catalyst after the reaction was measured by TG analysis. For the spent catalyst without the silicalite-1 shell layers, the coke weight in the catalyst weight was 1.7%, whereas it was 0.7% for the catalyst with the silicalite-1 shell layers. Furthermore, reaction tests with different contact times verified the roles of the acidic sites of the ZSM-5 support and Ni species in the ETP reaction, and the reaction pathway was also identified.