A Critical View on Direct Syngas to Aromatics over Combined Zinc Oxide on Zirconia and H-ZSM-5 Catalysts

ZnO supported on zirconia (ZnO/ZrO₂) catalyses methanol synthesis with high selectivity and stability at temperatures >300 °C, where methanol dehydration to hydrocarbons can also occur. When combined with ZSM-5 a CO₂ conversion of 14-15 % with an aromatics selectivity of 73-76 % within the hydrocarbons has been reported for a direct syngas to aromatics process [1,2]. The aromatization on Brønsted acid sites within the zeolite pores involves cyclization and hydrogen transfer reactions. The broadly accepted hydrocarbon dual cycle mechanism states that olefins act as hydrogen acceptors in bimolecular hydrogen transfer reactions that produce alkanes and aromatics. This co-production of alkanes limits the selectivity to aromatics. Promoting the zeolite with transition metals can favour the aromatization reaction via an additional dehydrogenation functionality [3]. However, the methanol synthesis over ZnO/ZrO₂ is favoured at high pressure and high hydrogen concentration; by contrast the methanol- to-aromatics (MTA) process, occurs with no hydrogen in the feed to promote the hydrogen transfer reactions. This work investigates the combination of ZnO/ZrO₂ with ZSM-5 of
various acid site densities, for direct conversion of syngas to aromatics. The mixing ratio and operating conditions were optimized to increase the CO_x conversion and aromatics selectivity.