Effect of alkali promotion on the catalytic performance of the synergetic K/Fe3O4-Fe5C2/Al2O3 catalyst for CO2 hydrogenation to light paraffins and olefines

Abstract

A series of K/α-Fe₂O₃/γ-Al₂O₃ catalysts with high specific surface area (150–300 m²/g) and various levels of alkali promotion (0–11 wt% K) were synthesized using the reverse microemulsion method. The capacity of the catalysts to transform CO₂ into light olefins and paraffins via direct hydrogenation at moderate pressures was examined. The effect of potassium loading was investigated in the 300–500 °C range at 11 bar, evaluating catalytic performance in terms of CO₂ conversion, C2+ selectivity, and space time yield (STY). Reaction tests showed that 7.8 wt% K loading provides the highest CO₂ conversion and C2+ selectivity, of 45 % and 46 %, respectively, with a space time yield of 6.5 mmol g⁻¹ h⁻¹ at 11 bar and 400 °C. The catalyst without alkali promotion provided only 24 % CO₂ conversion, 12 % C2 + selectivity, and a space time yield of 0.87 mmol g⁻¹ h⁻¹. Under reaction conditions α-Fe₂O₃ was converted to a mixture of Fe₃O₄ and χ-Fe₅C₂ nanoparticles that act synergistically to reduce and hydrogenate CO₂. Alkali promotion notably enhanced the formation of χ-Fe₅C₂, leading to higher selectivity to C2 + hydrocarbons.