Microfibrous-structured Al-fiber@ns-Al₂O₃ core-shell composite functionalized by Fe-Mn-K via surface impregnation combustion: As-burnt catalysts for synthesis of light olefins from syngas

Promising microfibrous-structured Al-fiber@ns-Al₂O₃@Fe-Mn-K catalysts are developed for the mass/heat-transfer limited Fischer-Tropsch synthesis of light olefins. The Al-fiber@ns-Al₂O₃ core-shell composites, engineered on the nano- to macro-scale, are first prepared by endogenously growing thin shell (∼0.5 μm) nanosheet γ-Al₂O₃ (ns-Al₂O₃) onto the 3-dimentional microfibrous-structured network consisting of 10 vol% 60 μm Al-fiber and 90 vol% voidage. After modification by K through an impregnation method, the Al-fiber@ns-Al₂O₃ composites are functionalized with nano-structured Fe and Mn active components via a surface impregnation combustion method. The effect of combustion atmospheres (air, N₂, and N₂ followed by air (N₂-air)) on the catalyst performance is investigated. The as-burnt catalyst obtained under air delivers the highest iron time yield of 206.0 μmol_CO g_Fe^-1 s^-1 at 89.6% CO conversion with 42.1%C selectivity to C₂-C₄ olefins (350°C, 4.0 MPa, 10 000 mL (g^-1 h^-1)), while the other two as-burnt catalysts under N₂ and N₂-air yield relatively low CO conversions of 58-67%. Combustion under air is helpful to form 6 nm Fe-Mn-K oxide particles with better reducibility and carbonization properties thereby leading to high performance. In contrast, under either N₂ or N₂-air atmosphere, smaller oxide particles (3-4 nm) are formed but suffer from deteriorated reducibility and carbonization properties due to the strong support-metal interaction. Such as-burnt catalysts obtained under air also demonstrate promising stability.