Catalytic oxidation of cyclohexene by supported gold nanoclusters synthesized in a two-liquid phases system containing eucalyptus essential oil

Modulation of the Activity of Gold Clusters Immobilized on Functionalized Mesoporous Materials in the Oxidation of Cyclohexene via the Functional Group. The Case of Aminopropyl Moiety

“…The BE of the two peaks show in both cases a negative shift in the range 0.9-1.5 eV respect to the value of 84.0 eV corresponding to the Au 4f 7/2 of metallic Au foils. This shift is opposite to the positive BE shift reported for Au clusters synthesized by the same procedure here described but immobilized on SBA-15 functionalized with mercaptopropyl groups [15,16]. This result evidences that, in this case, the small Au entities are strongly sensitive to the nature of the functional groups, and in particular to the transfer of electron density between these groups and the gold entities.…”

“…This points out to a very selective immobilization of gold entities smaller than AuNPs (< 2 nm). This selective uptake of small gold entities has also been described for SBA-15 functionalized with thiol groups [14][15][16]. Spectral features of the UV-vis spectra in the 250-450 nm range have been reported for small Au clusters, which are dependent upon the cluster size and charge, and the specific nature of the ligands attached to the gold atoms at the cluster surface [25][26][27][28].…”

“…A further clue to elucidate the reason for these differences is provided by the UV-vis spectra of the catalysts recovered after reaction. It has been shown elsewhere [14][15][16] that the gold clusters immobilized on -SO 3 − groups aggregate during the reaction, and the spent catalyst showed the SPR characteristic of AuNPs. Contrary to this observation, this band is absent in the spectra of used catalysts reported in this work, except in the one with the highest Au content (0.11-Au-17d), Figure 4.…”

“…However, the C/N molar ra r than that corresponding to the propylamine backbone (C/N = 3). In this regard, it has n elsewhere [16] that the treatment with hot ethanol required to remove the surfactant prod same time the etherification of some Si-OH of the support to form Si-O-CH2-CH3. A direct evidence of the anchoring of n-aminopropyl moieties in the solid is provided by MAS NMR.…”

“…The resulting materials have shown to be active catalysts for the selective oxidation of cyclohexene with molecular oxygen in liquid phase at mild conditions and atmospheric pressure [14,15]. Eucalyptus essential oil can also be used instead of rosemary oil [16] and it is worth noticing that in neither case were AuNPs found in the as-prepared, fresh catalysts. However, it has been realized that, as a function of specific preparation conditions of the gold entities, the contact of the Au-containing oil phase with the SH-bearing support required to immobilize gold may lead to the oxidation of the thiol to sulphonic groups.…”