Catalytic conversion of rapeseed oil for the production of raw chemicals, fuels and carbon nanotubes over Ni-modified nanocrystalline and hierarchical ZSM-5

Abstract

Catalytic conversion of rapeseed oil has been investigated over nanocrystalline and hierarchical ZSM-5 zeolites aimed to the production of both raw chemicals and fuels. Ni has been added to some of the samples in order to induce bifunctional properties in the catalysts. Almost total rapeseed conversion and deoxygenation degree were achieved with all the catalysts. The main products were gaseous olefins (mostly ethylene and propylene) and aromatic compounds (benzene, toluene and xylenes), which have high interest as raw chemicals. Likewise, high hydrogen contents are found in the gaseous stream since the operating conditions employed (high reaction temperature and atmospheric pressure) favor the occurrence of dehydrogenation reactions. The introduction of hierarchical porosity in ZSM-5 increases the production of light olefins at the expense of aromatic compounds, showing the close relationship between these two types of hydrocarbons. Hierarchical zeolites exhibit enhanced accessibility and diffusion rates, which hinders secondary reactions of the primary cracking products. Ni-containing catalysts show an enhanced dehydrogenation activity as denoted by the increased hydrogen, light olefins and coke production. Important changes were observed in the catalytic activity along time on stream, with a progressive increase in the fraction of aliphatic hydrocarbons. Interestingly, the coke formed over the Ni-containing ZSM-5 samples is in the form of carbon nanotubes, which grow causing a segregation of the Ni particle from the zeolite support. This effect is attenuated in the hierarchical zeolites because the secondary porosity stabilizes the metal particles. In summary, rapeseed oil can be transformed into valuable products (hydrogen, light olefins, aromatic hydrocarbons and carbon nanotubes) by using tailored Ni-containing nanocrystalline and hierarchical ZSM-5 zeolites.