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

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.