Mechanistic Insights into Synergistic Enhancement of Linear α-Olefins Selectivity over Ca–Na Modified Fe₅C₂–ZnO Catalysts in CO Hydrogenation

Selective synthesis of long-chain linear α-olefins (LAOs) from syngas remains a fundamental challenge due to competitive hydrogenation and water–gas shift (WGS) reactions. Herein, we report that CaO- and Na2O-promoted Fe5C2–ZnO catalyst (FeZnCaNa) demonstrates prominent performance and stability in CO hydrogenation to LAOs, which achieved 97.4% of CO conversion, 73.7% of LAOs in C4+ olefins with LAOs space-time yield of 304.4 mg·gcat–1·h–1 at 320 °C, 2.0 MPa. Comparative studies of modification with other alkaline earth metals (Mg, Sr, and Ba) underscored the unique promotional role of Ca in enhancing LAOs yield. The characterizations revealed that CaO accelerated the transformation of ZnFe2O4 into dispersed χ-Fe5C2 domains anchored at the ZnO interface. These iron carbide domains served as the principal active sites for the CO dissociation and C–C coupling. The Ca–Na–ZnO matrix modulated surface basicity, facilitating olefin desorption, and suppressing secondary hydrogenation. The ratio of olefin/paraffin attained 4.5, 58.6% of α-olefins in hydrocarbons, and suppressed CH4 selectivity to 8.1%. In situ spectroscopic analyses further explained that CaO–Na2O incorporation promoted CHx formation, thereby accelerating chain propagation. The catalyst also exhibited a reduced CO2 selectivity of 30.5%, attributed to the attenuated WGS activity resulting from decreased H2O adsorption during hydrocarbon formation. This study uncovers a dual-site mechanism, offering insights for designing efficient Fe-based catalysts for viable syngas-to-olefins conversion.