Effect of sodium incorporation into Fe-Zn catalyst for Fischer- Tropsch synthesis to light olefins

Abstract

The effects of precipitant and solid basicity on the Fischer-Tropsch Synthesis (FTS) activity of the sodium doped zinc ferrite catalysts were investigated. Olefin selectivity and stability of the catalysts were interrelated with the solid basicity which varied with the type of the precipitant, the amount of alkali promoter and its incorporation way into the catalyst matrix. The type of the precipitant and ionic medium of hydrolysis were seen to affect the textural properties of zinc ferrites. A cubic spinel phase of ZnFe2O4 were observed for all catalysts. Sodium promoter significantly modified the reduction behavior of the samples as well. Since the ionic strength was believed to have an effect on the solubility constants of Fe(OH)3 and Zn(OH)2, the purity of ZnFe2O4 nanoparticles would be expected to change as to be noticed from their different TPR profiles. All zinc ferrite samples had a much smaller crystal size than the precipitated iron, showing the structural promoter effect of zinc. In the study, the basicity of the catalysts was also correlated with FTS activity. All alkali promoted zinc ferrite catalysts have shown high olefin selectivity of about 50%. Although a basicity of certain extent is a must to attain high olefin selectivity, coking seems inevitable at high basicity. Therefore, an optimum interaction strength of the surface for CO dissociation appeared to exist and it might be tuned by surface basicity.