The Role of Carbon in the Catalytic Isomerisation-Cracking of n-Alkanes

The isomerisation-cracking of n-alkanes is one of the important processes in the refining and petrochemical industries. It is considered as a sustainable process where low value hydrocarbons are upgraded into value added products. Zeolites, and metal modified zeolites, are widely utilised as solid acid catalysts in such reactions, with metals such as platinum providing the functionality to effectively dissociate hydrogen. Catalyst deactivation by coke formation is however a crucial concern associated with hydrocarbon transformations over such catalysts. Nevertheless, recent studies have shown that these carbonaceous deposits, in addition to being implicated in catalyst deactivation, may have a beneficial or an active role in enhancing the catalytic performance. The main focus of this thesis is to understand the role of coke in the catalytic isomerisation-cracking of n-alkanes and to explore how a controlled pre-coking treatment can be used to enhance catalytic performance. Specifically, this thesis investigates the role of carbonaceous deposits in the isomerisation-cracking of n-pentane and n-hexadecane over the metal modified zeolite Pt/H-Beta.
Carbon laid down during n-pentane isomerisation over Pt/H-Beta is shown to as a catalytically beneficial species by control coking reactions and enhancing the selectivity to iso-pentane. These carbonaceous species are predominantly composed of polyaromatic compounds in graphitic-like structures. In n-hexadecane conversion over Pt/H-Beta, cracking is favoured over isomerisation and carbonaceous deposits are formed rapidly causing catalyst pore blockage. Reactions over Pt/H-Beta modified by desilication revealed that coke formation is due to the retention of reaction intermediates and/or products inside the pores, resulting in improved selectivity to iso-hexadecane production over the modified catalyst, with carbon deposition significantly reduced.
n-Hexadecane conversion performed over Pt/H-Beta pre-coked with n-pentane or toluene as paraffinic and aromatic precursors respectively, showed that selectivity to short-chained alkanes (pentane, hexane, and heptane) can be enhanced by tailored-carbon deposition. These products are highly in demand in petrochemical industry for the production of commercial fuels and solvents. Additionally, the rate of coke formation is remarkably reduced, in particular over toluene pre-coked Pt/H-Beta
where the internal sites are blocked by carbonaceous species. In general, tailored carbon deposition can be considered as a useful approach to maximise the production of short-chained hydrocarbons from heavy feedstocks and minimise the coking rate.