Hierarchical hollow nanostructured core@shell recyclable catalysts γ-Fe2O3@LDH@Au25-x for highly efficient alcohol oxidation

Abstract

A series of honeycomb-like hierarchical hollow nanostructured magnetic catalysts, γ-Fe₂O₃@M₃Al-LDH@Au₂₅-x (x represents determined mass of gold in wt%), have been successfully fabricated by modified electrostatic adsorption of water-soluble captopril-capped Au₂₅ nanoclusters (Au₂₅Capt₁₈) on pre-prepared magnetic supports, Fe₃O₄@M₃Al-LDH (M = Ni, Mg, or Cu/Mg (0.5/2.5), LDH: layered double hydroxide) followed by proper calcination. The characterization results show that the low Au-loading samples, γ-Fe₂O₃@Ni₃Al-LDH@Au₂₅-x (x = 0.053, 0.11), show nearly atomically precise Au₂₅ nanoclusters (Au₂₅NCs) (∼1.4 ± 0.3–1.9 ± 0.6 nm) dispersed on the surface of the Ni₃Al-LDH shell, while the catalysts, γ-Fe₂O₃@M₃Al-LDH@Au₂₅-x (x ∼ 0.2), display slightly aggregated Au₂₅NCs on the LDH shell with sizes of ∼3.0 ± 1.3, 3.3 ± 1.2, and 4.2 ± 1.5 nm for Ni-, Mg-, and CuMg-based catalysts, respectively, related to varied synergy between Au₂₅NCs and magnetic LDH supports at slightly higher Au loadings. All the γ-Fe₂O₃@M₃Al-LDH@Au₂₅-x catalysts exhibit much higher catalytic activity for the oxidation of 1-phenylethanol under atmospheric O₂, without basic additives, in toluene than Fe₃O₄@Mg₃Al-LDH@Au prepared by a traditional deposition–precipitation method. The catalyst, γ-Fe₂O₃@Ni₃Al-LDH@Au₂₅-x, presents even higher alcohol oxidation activity than γ-Fe₂O₃@Mg₃Al-LDH@Au₂₅-0.21 and γ-Fe₂O₃@Cu_0.5Mg_2.5Al-LDH@Au₂₅-0.2 and, particularly, γ-Fe₂O₃@Ni₃Al-LDH@Au₂₅-0.053 exhibits the highest activity (TOF: 112 498 h⁻¹) for the aerobic oxidation of 1-phenylethanol under solvent-free conditions and can be applied for a variety of alcohols, this being mainly attributed to its nearly atomically precise Au₂₅NCs and remarkable Au₂₅NCs–LDH–magnetic core three-phase synergistic interaction, along with its abundant Ni-OH sites. Moreover, γ-Fe₂O₃@Ni₃Al-LDH@Au₂₅-0.053 can be easily separated, by simply applying an external magnetic field, and reused for more than ten runs without significant loss of activity, giving the magnetic catalysts long-term stability. These observations make Fe₃O₄@LDH a universal green platform to support other noble or non-precious metal nanoparticles by using a metal nanocluster precursor method to construct desired metal-loaded ecofriendly catalysts that can be applied in a broad range of heterogeneous catalysis processes.