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

Abstract

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·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 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 CH_x 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.