Dioxygen oxidation of 1-phenylethanol with gold nanoparticles and N-hydroxyphthalimide in ionic liquid

Gold nanoparticles (Au-NPs) of 8 nm average diameter were obtained by thermal reduction under nitrogen from KAuCl4 in the presence of n-butylimidazol dispersed in the ionic liquid (IL) 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIm+BF4−). Characterization of the Au-NP was done by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Catalytic activities of the Au-NP/IL dispersion were evaluated in the oxidation of 1-phenylethanol at 100 and 160 °C under 4 bar pressure of dioxygen in a base-free system. Au-NP in combination with the radical initiator N-hydroxyphthalimide (NHPI) showed good conversion and selectivity for the oxidation of 1-phenylethanol to acetophenone through formation of an α-hydroxy carbon radical. The concomitant side products di(1-phenylethyl)ether and di(1-phenylethyl)peroxide were rationalized by an equilibrium due to the IL matrix of the α-hydroxy carbon radical with the 1-phenylethoxy radical. Maximum turnover number was ∼5200 based on the total number of moles of gold but a factor of about six larger, TON ≈ 31 300, when only considering the Au-NP surface atoms. The fraction (NS/NT) of exposed surface atoms (NS ≈ 2560) for an average 8 nm Au-NP (having NT ≈ 15 800 atoms in a ∼17-shell icosahedral or cuboctahedral particle) was estimated here as 0.16.

Gold nanoparticles (Au-NPs) in combination with the radical initiator N-hydroxyphthalimide (NHPI) showed good conversion and selectivity for the oxidation of 1-phenylethanol to acetophenone with concomitant side products di(1-phenylethyl)ether and di(1-phenylethyl)peroxide which can be rationalized through the radical mechanism in the IL matrix.Figure optionsDownload high-quality image (159 K)Download as PowerPoint slideHighlights
► Oxidation catalysis of alcohol with gold nanoparticles (Au-NPs) in ionic liquid (IL).
► Good substrate conversion by Au-NPs together with radical initiator NHPI.
► Mechanistic rationalization of side products.
► α-Hydroxy carbon radical equilibrium with 1-phenylethoxy radical due to IL matrix.
► Au-NP analysis for total and surface atom number.