Catalytic Activity of Tandem Catalysts Based on ZnGa₂O₄ and MFI Zeolite in CO₂ Hydrogenation

A series of tandem catalysts based on a ZnGa₂O₄ spinel and on MFI-type acidic components with varying SiO₂/Al₂O₃ ratios were prepared by mechanical mixing at a spinel to MFI weight ratio of 2 : 1. These catalysts were tested in the conversion of carbon dioxide to olefins at 380°C and 27 bar. The hydrogenating component (ZnGa₂O₄) was prepared by co-precipitation of the corresponding hydroxides followed by heat treatment. The zeolites were synthesized by steam-assisted crystallization. A reference catalyst was prepared from ZnGa₂O₄ and a hydrothermally synthesized commercial zeolite CBV28014. The physicochemical properties of the catalysts and their components were characterized by low-temperature nitrogen adsorption–desorption, chemical analysis, NH₃-TPD, H₂-TPR, XRD, and SEM. The sample with the highest SiO₂/Al₂O₃ ratio (250) achieved the highest CO₂ conversion; this parameter dropped as the aluminum content in the samples increased. The highest selectivity towards light olefins was observed for the catalyst with SiO₂/Al₂O₃ = 172 in the acidic component, likely because this zeolite had an optimum acidity for the production of light olefins.