Multi-functionality of Ga/ZSM-5 catalysts during anaerobic and aerobic aromatisation of n-decane

Abstract

Zeolite catalysts have been evaluated for the gas-phase conversion of decane, to study new routes for upgrading intermediate-length straight chain hydrocarbons. For a gas-feed of dilute n-decane in an inert carrier, at a contact time of 4 s, the initial activity of ZSM-5 and Ga/ZSM-5 was consistently high (>95% conversion) over the temperature range 300–460 °C. The parent zeolite produced almost equal yields of cracked hydrocarbons and aromatics, while the Ga-modified zeolite produced predominantly BTX and other heavier aromatics. This difference in product distribution is consistent with the short-chain alkanes formed within the internal pore structure of the zeolite being intermediates in a Cyclar-type aromatisation mechanism, while the direct conversion of decane to heavy (C₁₀–C₁₅) aromatics occurs at the unconstrained external acid sites. Under aerobic conditions, the rate of CO formation was negligible and CO₂ was barely detectable over either the parent or the Ga-modified zeolite, even though all the O₂ was consumed. The ability of Ga/ZSM-5 to catalyse selective oxidation, of the H₂ released during the dehydrogenation steps, thus provides the prospect of the aromatisation reaction being operated autothermally. Although the external sites are preferentially blocked by carbon retention, rapid deactivation did not occur until after 65 h on line (under either anaerobic or aerobic conditions) when blocking of the internal pore structure became limiting.