Metal-assisted low-temperature cracking of n-hexane over Rh-encapsulated ZSM-5 catalysts

Abstract

Naphtha cracking is the most common process for producing light olefins such as ethylene, propylene, and butene. This process consumes enormous amounts of energy, so decreasing the energy consumption and operating temperature is an urgent issue. To decrease the reaction temperature for naphtha cracking, we focused on the metal-assisted cracking reaction, in which paraffin is first dehydrogenated into the corresponding olefin on a metal catalyst, and the produced olefin is then decomposed into light olefins. To effectively realize metal-assisted cracking, we considered metal-encapsulated zeolite catalysts to be useful. In metal-encapsulated zeolite catalysts, the dehydrogenation reaction proceeds inside the zeolite particles, and the dehydrogenated intermediates can access the solid-acid sites frequently. In this study, Rh nanoparticle encapsulated ZSM-5 catalysts (Rh@ZSM-5) were employed for the metal-assisted cracking of n-hexane. Rh@ZSM-5 exhibited significantly high activity for n-hexane cracking below 450 °C owing to the metal encapsulation structure and close proximity between the metal and solid-acid sites. Furthermore, the effects of reaction conditions, reaction temperature, amounts of metal and solid-acid sites, and contact time on metal-assisted n-hexane cracking over the Rh@ZSM-5 catalysts were investigated. The highest light-olefin yield of 38.6 carbon mol% was achieved by the conversion of n-hexane over Rh@ZSM-5 using 0.3 wt% Rh loading, an Si/Al ratio of 100, temperature of 450 °C, reaction time of 0.5 h, and W/F of 2 h.