Silver Nanoparticles Densely Grafted with Nitroxides as a Recyclable Green Catalyst in the Selective Oxidation of Alcohols

Abstract

The selective oxidation of alcohols, leading to appropriate aldehydes, is widely recognised as one of the most important reactions in organic synthesis. With ever-increasing environmental concerns, much attention has been directed toward developing catalytic protocols that use molecular oxygen as an oxidant. An ideal green oxidation process should employ a highly active, selective and recyclable catalyst that can work with oxygen under mild conditions. This paper presents a successful application of densely grafted silver nanostructures with stable nitroxide radicals (N-AgNPs) as an effective, easily-recovered and regenerable catalyst for the selective oxidation of alcohols. The fabricated ultra-small and narrow dispersive silver nanoparticles have been fully characterised using physicochemical methods (TEM, DLS, XPS, TGA). N-AgNPs have been successfully applied to oxidise several model alcohols: benzyl alcohol, 4-pyridinemethanol, furfuryl alcohol, 1-phenyl ethanol, n-heptanol and allyl alcohol under mild conditions using oxygen as a stoichiometric oxidant. Notably, the fabricated nitroxide grafted silver nanoparticles (N-AgNPs) were reused more than ten times in the oxidation of a series of primary alcohols to corresponding aldehydes under mild conditions with very high yields and a selectivity close to 100%.

Keywords: catalysis; nitroxides; silver nanoparticles; surface modification.

Scheme 1

Synthesis of silver nanoparticles stabilised by nitroxide ligand (N-AgNPs).

Figure 1

UV-vis spectrum of N-AgNPs recorded in DMF solution (a), TEM micrograph of N-AgNPs with the histogram prepared on the base of this micrograph as the inset (b), the size distribution of the nanoparticles determined from DLS (c).

Figure 2

XPS spectra: Ag 3d (a), S 2p (b) and N 1s (c) of N-AgNPs. The values of BDE (eV) are presented in brackets.

Figure 3

Structures of six representative alcohols: benzyl alcohol (a), 1-phenylethanol (b), 4-pyridinemethanol (c), furfuryl alcohol (d), n-heptanol (e) and allylic alcohol (f), used in the catalytic studies.

Figure 4

The yields of benzaldehyde in six consecutive cycles using N-AgNPs (performed according to the description of Table 3 Experiment 1).

Figure 5

The yields of benzaldehyde, 4-pyridinecarboxaldehyde and furfural in eleven consecutive cycles using N-AgNPs (performed according to the description in Table 4).

Figure 6

TEM images of N-AgNPs used in the first cycle of oxidation benzyl alcohol (a) separated after 2nd cycle (b) 4th (c) and 11th (d) performed under conditions described in Table 4.