Iron-Substituted Zn-Al LDH-Derived Mixed Oxides: Tailoring Structure, Composition and Redox Properties for Efficient CO₂-to-Olefins Conversion

Herein, CO₂ hydrogenation to light olefins via the modified Fischer-Tropsch synthesis (mFTS) route was thoroughly explored over Zn-Fe-Al mixed oxides, derived by calcination of layered double hydroxides (LDHs). Two LDH synthesis strategies were employed: progressive substitution of Zn by divalent iron (Zn+Fe/Al series) or Al by trivalent iron (Zn/Al+Fe series), keeping the total divalent to trivalent ions ratio constant. Complementary characterization revealed that the elemental composition of the parent LDH and derived mixed oxides profoundly affected their structural and textural properties, with notable implications for catalytic performance. CO-pretreatment of the mixed oxides induced iron carbide phase formation, the extent of which strongly depended on the substitution route and iron content. Still, the predominant phases were spinel ZnFe₂O₄ (Zn+Fe/Al) and ZnO (Zn/Al+Fe). Both series exhibited similar CO₂ conversion (∼40%) at 340°C, 20 bar, with a small superiority of the Zn/Al+Fe catalysts at lower Fe contents. However, the Zn+Fe/Al series was substantially more selective towards light olefins. Among all catalysts, Zn+Fe(40)/Al exhibited the highest light olefins selectivity (26.7%) and yield (10.6%) and maintained stable performance over 72 h on stream due to a balanced contribution of iron carbide phase and ZnFe₂O₄ phase, which exhibited low crystallite size, enhanced porosity, and basicity.