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

Abstract

Gold nanoclusters (d < 1 nm) have been synthesized in a two-liquid phases system, the aqueous phase being formed by dissolving gold in a solution of ammonium chloride in concentrated nitric acid (1: 4, w/w), while eucalyptus essential oil is the organic phase. The mesoporous SBA-15 material functionalized with mercaptopropyl groups (after surfactant removal: dp = 6.3 nm, 3.5 wt% S) was contacted with aliquots of the Au-containing essential oil phase taken at 1, 3, 6 and 8 days of contact time between both phases. In this way, gold was immobilized on the support, ranging from 0.4 wt% for the 1-day sample, to 2.7 wt% in the 3-days material. UV–vis spectra show the presence of gold nanoclusters in these samples, but the surface plasmon resonance at 520 cm−1, characteristic of Au nanoparticles, was not detected save for the 3-days sample. 13C MAS NMR and TG evidence that the thiol groups of the support remain mostly unaltered for the 1-day sample, but oxidation to sulfonic acid groups becomes apparent for contact time > 3 days, and reaches nearly 60 % of the total sulphur species after 8 days of contact time as estimated from XPS analysis. The Au-SH-bearing catalyst is inactive for cyclohexene oxidation with molecular oxygen in liquid phase, but those having sulfonic groups are active and selective for its allylic oxidation. It has been found for the 8-day catalyst that the gold nanoclusters partially evolve spontaneously in the reaction medium to form gold nanoparticles, and this agglomeration process parallels the increase in catalyst activity.