High-temperature Fischer–Tropsch synthesis over the Li-promoted FeMnMgOx catalysts

Abstract

Fischer–Tropsch-to-Olefins (FTO) over iron-based catalysts is a promising process for the production of light olefins from a non-petroleum source. However, the catalytic mechanism is unclear because of the variation in the iron phase during the reduction and activation processes and the complex iron phase composition under the reaction conditions. A series of Li-promoted FeMnMgOx catalysts is synthesized using co-precipitation and impregnation methods, after which the effect of promoters on the evolution of the iron phase, chemisorption, and FTO performance are analyzed. The results demonstrate that Mg enhances dissociative CO adsorption and hydrogen adsorption, improving the FTO activity. Mg facilitates the dispersion of α-Fe2O3 and suppresses the carbonization, which weakens the chain propagation, encourages the formation of short-chain hydrocarbons, and increases the selectivity of light olefins. Li also enhances dissociative CO adsorption and increases CO conversion. Moreover, Li facilitates the formation of the ε'-Fe2.2C phase, which reveals a higher FTS activity and better C–C coupling ability than the χ-Fe5C2 phase. This results in a decrease in CO2 and increase in light olefins. At the optimal Li loading, the FeMnMg-0.75Li sample displays the highest space time yield of light olefins at 355 g/(kgcat·h).