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 Na 2 O-promoted Fe 5 C 2 –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 C 4+ olefins with LAOs space-time yield of 304.4 mg·g cat –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 ZnFe 2 O 4 into dispersed χ-Fe 5 C 2 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 CH 4 selectivity to 8.1%. In situ spectroscopic analyses further explained that CaO–Na 2 O incorporation promoted CH x formation, thereby accelerating chain propagation. The catalyst also exhibited a reduced CO 2 selectivity of 30.5%, attributed to the attenuated WGS activity resulting from decreased H 2 O 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.