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, FeNiOx(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 C1o–C4o, 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 C1o–C4o 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. FeMnOx(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, FeNiOx(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 FeNiOx(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.