Alternative Fe7C3/ZrO2 carbides for stable and selective olefins production via CO2-FT process: Co-operative effect of alkali-alkaline earth promoters

Abstract

Improved light olefins yield via catalytic CO₂ conversions supports circular carbon economy and also addresses global climate change effects to some extent. Present work demonstrates stable orthorhombic Fe₇C₃ formation on commercial ZrO₂ by using citric acid chelation method and its subsequent pretreatment under CO/H₂(g) = 0.93 studied in CO₂-FT process. For the first time, co-operative effect of alkaline earth promoters (AEP= Mg or Ca or Ba) with alkali metal (K) was investigated in detail on the extent of Fe⁰/Fe₇C₃ formation in FeZnK-AEP/ZrO₂ catalysts. Significant enhancement in the textural properties and iron oxide reduction were found in K-AEP duo catalysts. Essentially, XRD and H₂-TPD studies revealed improved metallic iron phase in Mg-K or Ca-K duo which resulted in greater light paraffins formation. However, Ba-K duo enhanced the carbidization of reduced iron species in the catalyst as established through Mossbauer data wherein Fe_xC_y/Fe₃O₄ ratio was 1.2 times higher than in reference FeZnK/ZrO₂ catalyst. Among the K-AEP duos, the decreasing order of light olefins space time yield found as: FeZnK-Mg/ZrO₂ (6.75 mmol g_cat⁻¹ h⁻¹) < FeZnK/ZrO₂ (7.39 mmol g_cat⁻¹ h⁻¹) < FeZnK-Ca/ZrO₂ (7.53 mmol g_cat⁻¹ h⁻¹) < FeZnK-Ba/ZrO₂ (9.01 mmol g_cat⁻¹ h⁻¹). The greater light olefins yield was associated with surface enrichment of Fe-C species, optimized basicity and H₂ activation on FeZnK-Ba/ZrO₂ respectively noticed through XPS, CO₂ and H₂-TPD results. Therefore, FeZnK-Ba/ZrO₂ with 20h of consistent activity can serve as an alternative catalyst with an alternative active phase of Fe₇C₃ in CO₂-FT studies, unlike literature heavily loaded with Fe₃C and Fe₅C₂ bulk active phases.