Gold nanoclusters prepared from an eighteenth century two-phases procedure supported on thiol-containing SBA-15 for liquid phase oxidation of cyclohexene with molecular oxygen

Abstract

The present work describes the incorporation of finely divided and well-dispersed Au species into SBA-15 mesoporous silica functionalized with mercaptopropyl groups, and the catalytic activity of the resulting Au-materials in the liquid phase oxidation of cyclohexene with molecular oxygen at atmospheric pressure. The gold entities have been prepared through a two-phase system described in the eighteenth century involving a solution of gold in a mixture of nitric acid and ammonium chloride (aqua regia) and rosemary oil. Gold nanoclusters, which present a disordered structure and sizes below 3nm, and even single gold atoms which may be anchored/stabilized through sulphur groups are found immobilized on the mesoporous support, but no gold nanoparticles (with well-defined structure and usually over ∼5nm) are detected. Such observations have been carried out by Cs-corrected STEM-HAADF where the visualization of bright spots is attributable to these individual entities. These materials are active in the oxidation of cyclohexene, but their relative activity is governed by the synthesis conditions. Partial aggregation of the gold entities into nanoparticles has been observed during the reaction, which are mainly responsible for the catalytic activity. The allylic oxidation of the cyclohexene ring over the double bond epoxidation is strongly favored, and at high conversion the two dominants products are 2-cyclohexen-1-one and 2-cyclohexen-1-ol.