Performance of K-Fe₃O₄/Ni-AlMCM-41 tandem catalyst for CO₂ hydrogenation to long-chain hydrocarbons

The K-Fe₃O₄ and Ni-AlMCM-41 catalysts were first prepared by a solvothermal and ion-exchange methods, respectively; they are then assembled to K-Fe₃O₄/Ni-AlMCM-41 tandem catalyst for CO₂ hydrogenation to long-chain hydrocarbons. The catalyst samples were characterized in detail by means of XRD, SEM, TEM, NH₃-TPD, olefin-TPD, ICP-OES, XRF and XPS; the effect of potassium modification and Si/Al ratio on the performance of Fe₃O₄/Ni-AlMCM-41 tandem catalyst in the hydrogenation of CO₂ was investigated. The results illustrate that the Fe₃O₄ component has a uniform spherical particles in the size range of 400–800 nm, whilst the Ni-AlMCM-41 component displays mesoporous structure, dominantly with weak acid sites on the surface. CO₂ is first converted to gaseous products rich in light olefins over the K-Fe₃O₄ catalyst and the light olefins is then transformed to long-chain hydrocarbons by a series of oligomerization and hydrogenation reactions over the acid sites of Ni-AlMCM-41. Appropriate content of potassium can improve the selectivity to light olefins over the Fe₃O₄ catalyst in the first stage. In particular, for CO₂ hydrogenation under 2 MPa and with a space velocity of 1000 h⁻¹ and a H₂/CO₂ ratio of 3, when 0.5%K-Fe₃O₄ (320 °C) is connected with Ni-AlMCM-41(Si/Al = 50) (250 °C), the conversion of CO₂ reaches 32.9% and the selectivities to CO, CH₄, and long-chain hydrocarbons are 7.1%, 10.9% and 49.8%, respectively. That is, the selectivity to long-chain hydrocarbons over the K-Fe₃O₄/Ni-AlMCM-41 catalyst (49.8%) is much higher than that over the single K-Fe₃O₄ catalyst (12.2%).