Topotactic transformation of metal–organic frameworks to iron-based catalysts for the direct hydrogenation of CO2 to olefins

Abstract

Iron-based catalysts are promising for direct hydrogenation reactions of carbon dioxide (CO₂) to olefins. In this work, a series of iron-based catalysts (MFe/C) have been fabricated by pyrolysis of metal–organic frameworks (Fe-MIL-88B) modified with a second transition metal M (M = Zn, Ni, Mn) for exploring the promoter effects on the selectivity of CO₂ hydrogenation. Combined with Mössbauer spectroscopy and other characterization techniques, it is observed that the promoters play a significant role in tuning the phase transition and the content of different Fe sites within Fe₅C₂ of the MFe/C catalyst, which in turn impacts the reactivity for CO₂ hydrogenation. Specifically, the introduction of Mn favored the formation of Fe₅C₂, which enhances C–C coupling and inhibits the secondary hydrogenation of alkenes, realizing an optimal catalytic performance with a CO₂ conversion of 37.60%, the ratio of olefin to paraffin (O/P) of 2.38 and a C₅⁺ space–time yield of around 0.27 mol g_cat⁻¹ h⁻¹ under the reaction conditions H₂/CO₂ = 3, 400 °C, 2.0 MPa, and 12 000 mL h⁻¹ g_cat⁻¹. Furthermore, it is found that the content of specific Fe sites within Fe₅C₂ (sites II and III) is positively correlated to O/P and olefin selectivity. The results of this work provide new insights into the influence of different metal promoters on the active sites of Fe₅C₂ for CO₂ hydrogenation.