Novel bifunctional catalysts based on crystalline multi-oxide matrices containing iron ions for CO2 hydrogenation to liquid fuels and chemicals

Abstract

Seven solid mono-, bi- and tri-metallic oxide matrices where Fe(2+,3+) ions are distributed in different chemical/spatial environments were synthesized and characterized by XRD, N₂-adsorption and EDAX methods. After basification with potassium, all matrices were activated by carburization or reduction–carburization under conditions selected based on the TPC/TPR spectra, tailoring the carburization extent of iron. The performances of the activated Fe-based catalysts with respect to CO₂ conversion and C₅₊ selectivity were measured in a fixed-bed reactor under standard conditions in transient and continuous operation modes in units containing one or three reactors in series with water separations between the reactors. The catalysts were characterized by XRD, N₂-adsorption, HRTEM-EELS and XPS before and after steady-state operation in the reactors. It was found that the rate of CO₂ conversion is not limited by thermodynamic equilibrium but is strongly restricted by water inhibition and it depends on the nature of the Fe-oxide precursor. The ratio between the FTS and RWGS rates, which determines the C₅₊ hydrocarbons productivity, is strongly affected by the nature of the Fe-oxide matrix. The catalysts derived from the Fe–Al–O spinel and Fe–Ba–hexaaluminate precursors displayed the best balance of the two functions R_FTS/R_RWGS = 0.77–0.78. They were followed by magnetite, CuFe–delafossite, K–ferrite, Fe–La–hexaaluminate and LaFe–perovskite with a gradual lowering of R_FTS/R_RWGS from 0.60 to 0.15 and a gradual decrease in the C₅₊ productivity. The active sites that enhance the RWGS reaction are located on the surface of the Fe-oxide phases, while the FTS and methanation reactions occur on the surface of the Fe-carbide phases.