Development of Ruthenium−Hydroxyapatite-Encapsulated Superparamagnetic γ-Fe₂O₃ Nanocrystallites as an Efficient Oxidation Catalyst by Molecular Oxygen

Magnetic γ-Fe 2 O 3 nanocrystallites dispersed in a hydroxyapatite matrix (HAP-γ-Fe 2 O 3 ) have been synthesized for the first time as a new catalyst support. The cation-exchange ability of the external HAP surface enables an equimolar substitution of Ru for Ca to form a catalytically active center (RuHAP-γ-Fe 2 O 3 ). Characterization by several spectroscopic methods demonstrated the formation of γ-Fe 2 O 3 nanocrystallites within the HAP matrix having a mean diameter of 8.0 nm (σ = 18.4 Å, σ/ d = 21.7%). The Ru species was proven to exist in the monomeric form in a highly dispersed manner. The RuHAP-γ-Fe 2 O 3 exhibits superior catalytic activity for the oxidation of various alcohols to the corresponding carbonyl compounds using molecular oxygen as a primary oxidant. The magnetic properties of the RuHAP-γ-Fe 2 O 3 provided a convenient route for separation of the catalyst from the reaction mixture by application of an external permanent magnet. No metal leaching was detected, and the spent catalyst could be recycled without appreciable loss of catalytic activity.