Effect of Zn promoter on precipitated iron catalyst for linear alpha olefin production via high-temperature Fischer-Tropsch synthesis: Modulating carbon chemical potential

Abstract

The carbon chemical potential (μ_c) affects the active phase formation of the Fischer-Tropsch synthesis (FTS) catalyst, which determines catalytic performance and stability. Herein, we report a Zn-promoted precipitated iron-based (Zn-P-Fe) catalyst with enhanced linear alpha-olefin (LAO) yields and stability for high-temperature FTS. This performance improvement is obtained by modulating the μ_c on the catalyst surface without changing the structural properties. An optimized 10Zn-P-Fe catalyst exhibits a remarkably high C₆-C₈ LAO selectivity in total hydrocarbon (12.1 %) at high CO Conversion (89.8 %) with 114 h stability under HT-FTS conditions of 305 ℃, 1.5 MPa and H₂/CO = 1, outperforming a 0Zn-P-Fe catalyst and other catalysts previously reported in the literature. An analysis of spent catalysts reveals that the superior activity of the 10Zn-P-Fe catalyst can be ascribed to high coke resistance during the reaction. The primary role of the Zn promoter is to reduce μ_c, which hinders the transformation of the iron carbide phase from Fe₅C₂ to Fe₇C₃ and coke formation during the reaction, on the catalyst surface. However, introducing an excessive amount of Zn promoter induces successive olefin hydrogenation on the catalytic surface, resulting in deteriorated olefin selectivity. Therefore, a key factor is maintaining the proper μ_c by introducing appropriate amounts of Zn promoter.