CO₂ hydrogenation to lower olefins on a high surface area K-promoted bulk Fe-catalyst

A high surface area K-promoted iron-based catalyst is prepared by thermal decomposition of ammonium glycolate complexes, followed by impregnation with an aqueous solution of potassium carbonate, drying and calcination. It is found that the duration of the calcination has a dramatic effect on the textural and structural properties of the obtained material, as well as on the iron oxidation state. Fast calcinations allow to synthetize materials isostructural with Fe₃O₄/γ-Fe₂O₃ that, after a carburization treatment with CO/H₂ mixtures, are highly active in the CO₂ hydrogenation to lower (C₂–C₄) olefins at mild process conditions (300 °C and 5 barg). Interestingly, the decrease of the operating pressure leads to a shift from the “hydrocarbon synthesis” regime to reverse water gas shift regime. The selectivity to lower olefins is instead maximised at mild process conditions due to the moderate chain growth probability and the slow rate of olefin secondary hydrogenations, which dominate at higher pressure and temperatures, respectively. The former process is activated by type II catalytic sites (iron carbides), while the latter occurs on type III catalytic sites (Fe⁰). Type I catalytic sites also exist (Fe₃O₄) and are responsible for the RWGS activity of the catalyst. Collected data suggest that CO is the primary product of CO₂ hydrogenation, while hydrocarbons are formed via CO hydrogenation, following a Fischer-Tropsch type mechanism. Eventually, the performances of the prepared catalyst are compared with those of K-promoted reference model samples based on commercial α-Fe₂O₃ and Fe₃O₄ powders. It is found that, due to a better carburization and the higher surface area, the catalyst prepared by fast decomposition of ammonium glycolate complexes is more active than the reference materials in terms of both CO₂ conversion and C₂–C₄ olefins selectivity.