Effects of Zn and Mn Promotion in Fe-Based Catalysts Used for CO_x Hydrogenation to Long-Chain Hydrocarbons

The promoting effect of Mn and Zn on the performance of Fe-based catalysts has been comparatively investigated in the CO_x hydrogenation to heavy hydrocarbons in the presence of H₂-deficient streams. To this scope, two catalysts have been prepared by coprecipitation, followed by impregnation with Cu and K, and tested at 220 °C and 30 bar_g after an activation treatment with syngas. Both catalysts have been found to be active and selective to long-chain hydrocarbons in the presence of either H₂/CO or H₂/CO₂ mixtures. Despite lower catalyst reducibility, the presence of Zn has resulted in higher CO_x conversion rates. Furthermore, the Zn-promoted catalyst converted CO_x into heavier and less-saturated hydrocarbons. These results are consistent with a role of Zn in promoting the catalyst basicity, which is a key property to keep low the superficial H/C ratio and to slow chain termination reactions as well as secondary olefin hydrogenations.