Sodium-promoted iron catalysts prepared on different supports for high temperature Fischer–Tropsch synthesis

The paper addresses the effect of sodium promotion on the structure and catalytic performance of iron catalysts supported on alumina, silica, CMK-3 and carbon nanotubes in high temperature Fischer-Tropsch synthesis giving general rules for the selection of supports for selective synthesis of olefins in the presence of promoter. Our results show that the effect of sodium promotion strongly depends on the support. In silica and CMK-3 carbon support, iron species interact with sodium with formation of inactive sodium-iron mixed oxides or carbides which results in higher olefin selectivity and chain growth probability but lower Fischer-Tropsch reaction rate. In alumina supported catalysts, sodium promotion preferentially leads to formation of sodium aluminate with a relatively small effect in Fischer–Tropsch synthesis. The effect of promotion with sodium is more pronounced on carbon nanotubes which contain iron carbide species stabilized by encapsulation in the carbon matrix. The sodium promoted catalysts supported on carbon nanotubes exhibit higher selectivity to both light and long-chain olefins.