Application of Fe/ZrO2 Catalysts Modified with Different Organic Acids in Fischer–Tropsch Synthesis

Organic acid modification can significantly enhance the performance of iron-based catalysts in Fischer–Tropsch synthesis (FTS) by regulating the quantity and type of functional groups on the catalyst surface. In this study, efficient Fe/ZrO2 catalysts were prepared using impregnation and environmentally friendly solid-state synthesis methods, and the effects of different organic acids on catalytic performance were systematically investigated. The catalysts were rigorously evaluated under industrially relevant FTS conditions, with reaction temperatures set at 270 °C for the impregnation method and 250 °C for the solid-state grinding method under 20 bar and WHSV = 5400 mL/(gcat·h). Comprehensive characterization techniques were employed to reveal the complex relationship between catalytic activity and structural characteristics. We systematically studied the effects of organic acids with different quantities of carboxyl and hydroxyl groups on improving FTS catalytic performance. The results show that citric acid (CA) significantly enhanced the catalytic activity in the CA-Fe/ZrO2 system. At a 6% Fe loading, a CO conversion of 39.89% was achieved at 270 °C, while maintaining a relatively low CH4 selectivity. The hydroxyl and carboxyl groups of CA can form stable complexes with metal ions, which improves the dispersion of iron, enhances the reducibility of iron oxides, promotes the formation of active Hägg iron carbide (χ-Fe5C2), facilitates chain growth, and ultimately boosts the production of valuable C5+ hydrocarbons. These findings highlight the potential of CA in optimizing iron-based FTS catalysts, offering important theoretical insights and practical guidance for future industrial applications.