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

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 Mossbauer 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 C-5(+) space-time yield of around 0.27 mol g(cat)(-1) h(-1) under the reaction conditions H-2/CO2 = 3, 400 degrees 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.