CO2 hydrogenation to light olefins over mixed Fe-Co-K-Al oxides catalysts prepared via precipitation and reduction methods

Abstract

A series of Fe-Co-K-Al oxides catalysts were prepared by precipitation-reduction method using NH4OH as precipitating agent and NaBH4 as reducing agent. The effect of preparation methods including one-pot preparation (C1), without the addition of NH4OH (C2), precipitation followed by reduction (C3) and doubling NaBH4 content of C3 (C4) on the physicochemical properties of the catalysts as well as their catalytic CO2 hydrogenation to light olefins was investigated. The preparation methods had a significant influence on phase, reducibility and the amount and strength of CO2 and H2 adsorption with the catalysts surface. The structure–activity relationships indicated that the CO2 conversion was proportional to the amount of medium CO2 adsorption sites, while the O/P ratio increased with decreasing the amount of weak H2 adsorption sites. The C3 catalyst exhibited the highest light olefins yield of 16.58% at 350 °C and 20 bar. This was attributed to a significant reduction of weak adsorption of H2, while maintaining a high level of medium CO2 adsorption, inhibiting the hydrogenation of olefins to paraffins products and providing the relatively high CO2 conversion, respectively. Replacing the reducing gas from H2 to a gas mixture containing H2 and CO2 of the C3 catalyst (C5) was unable to reduce the iron species to metallic Fe, resulting in the formation of Fe3C which was less active for the production of hydrocarbon compared to Fe5C2 phase formed in the C3 catalyst.