Sodium-Mediated Bimetallic Fe–Ni Catalyst Boosts Stable and Selective Production of Light Aromatics over HZSM‑5 Zeolite

Conversion of syngas into aromatics via the Fischer–Tropsch (FT) route provides a promising way to supply the value-added chemicals. However, it is still a great challenge to achieve controllable aromatics selectivity with good stability. Herein, we report the Na-mediated bimetallic Fe–Ni catalyst simply particle mixed with HZSM-5, which can boost the stable and selective production of aromatics from the syngas. In detail, FeNiO_x(5:1)-0.41Na exhibits quite stable catalytic activity with notably higher selectivity to light α-olefins (C₂⁼–C₄⁼) compared to α-Fe₂O₃-0.75Na. Various characterizations suggest that an appropriate addition of Ni with Na substantially regulates iron carbide formation and suppresses carbon deposition owing to the electron donation of Fe to Ni reducing CO adsorption and dissociation. Interestingly, extra Na results in remarkably increased selectivity to C₁^o–C₄^o, despite the fact that Na shows the ability to suppress hydrogenation. Both characterizations and density functional theory (DFT) calculations demonstrate that the addition of Na weakens the Fe–Ni interaction and reduces electron transfer from Fe to Ni, which promotes C₁^o–C₄^o formation as Ni tends to reflect its intrinsic catalysis. DFT calculations confirm that the adsorbed CH₂ species at the Ni–Fe interface prefers to undergo coupling to form C₂H₄, and the formed C₂H₄ spontaneously desorbs from the surface, indicating that the formation of lower olefins is favorable and further chain growth is inhibited. This could be attributed to the fact that the accumulated electron at the Ni–Fe interface weakens the binding strength of CH₂ and C₂H₄ species because of electronic repulsion. Furthermore, the effects of FT product distribution on aromatics formation were also investigated. FeMnO_x(5:1)-0.40Na produces longer olefin-rich hydrocarbons in C₅₊ owing to oxygen vacancy-assisted CO dissociation, leading to highly branched monocyclic aromatics and faster HZSM-5 deactivation because of increased coking from the isomerization–hydrocracking of long hydrocarbons. In contrast, FeNiO_x(5:1)-0.41Na-HZSM-5 produces major light aromatics about 65.7% including toluene, ethyl-benzene, and xylene in total aromatics with about 98.6% aromatics in the liquid phase, which benefits from the fact that FeNiO_x(5:1)-0.41Na makes shorter hydrocarbons in C₅₊ and/or Ni metal has a strong ability of H₂ dissociation to provide atomic H spillover onto HZSM-5. Thus, HZSM-5 also shows improved stability. Note that the mixed catalysts with a high Na content indicate a substantial Na⁺ migration to increase light α-olefins in the gas phase owing to suppress H-transfer reaction. This study provides deep insights into how to develop stable and selective iron-based catalysts for the production of FT products and its further conversion into value-added light aromatics.