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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 COx conversion and aromatics selectivity.

Direct conversion of synthesis gas to olefins (STO) is an attractive way of producing large volume base chemicals currently almost exclusively obtained from crude oil. It is a simplification of a two-step process of methanol synthesis followed by methanol to olefins (MTO), avoiding methanol condensation and purification and having fewer recycle streams. Recent literature reports that a combination of different mixed metal oxides or supported metal oxides, such as ZnAl2O4, ZnGa2O4, ZnCr2O4, ZnO/ZrO2 or In2O3/ZrO2, can catalyze methanol synthesis at temperatures >380 °C where methanol dehydration to olefins can occur over narrow pore zeolites SSZ-13 or SAPO-34 [1-3]. However, literature is typically focusing on one type of each component. Here we report a comprehensive comparison between ZnAl2O4, ZnGa2O4 and ZnO-ZrO2 catalysts for methanol synthesis and for syngas to olefines when combined with SAPO-34. This includes fundamental characterization using BET, ICP and XRD and in-depth characterization using operando XAS and DRIFTS.

It has been estimated that 302 million tons of plastic waste were generated worldwide in 2015 [1]. Unfortunately, there is still a need for burning plastic waste to avoid it being disposed on landfills where no value of the products is regained. When incinerating a plastic product, the fraction of the energy that went into producing the plastic can be recovered as electricity and heat. Closing the carbon loop from plastic incineration by capturing and recycling CO2 to produce new plastic will minimize the climate impact of the current emissions and reduce the need for fossil resources in plastic production. Promoted Fisher-Tropsch catalysts can convert CO2 and H2 into ethylene and propylene with yields up to 60% [2]. This yield is close to the theoretical limit by the Anderson-Schulz-Flory distribution, which is still considered to be a limitation for the Fisher-Tropsch process. An alternative route for converting CO2 into hydrocarbon products is the combination of methanol synthesis and the methanol to hydrocarbon reaction. The methanol synthesis is equilibrium limited and a strategy to overcome this is to combine a methanol synthesis catalyst with a zeolite catalyst within one reactor [3,4]. In the temperature range of 300 to 420 °C, necessary for the zeolite to be active, the traditional Cu/ZnO/Al2O3 methanol synthesis catalyst cannot be used. At these temperatures, severe sintering of the metallic copper deactivates the catalyst. Furthermore, the hydrogen spill-over effect for the metallic copper results in the hydrogenation of the olefins formed in the zeolite [5]. To overcome these limitations, metal oxides capable of converting CO2 and H2 to methanol at relatively high temperatures have been synthesized and tested.