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

Abstract

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.