Biosynthesis of Gold Clusters and Nanoparticles by Using Extracts of Mexican Plants and Evaluation of Their Catalytic Activity in Oxidation Reactions

Abstract

This work shows the biosynthesis of gold clusters and nanoparticles (AuNPs) from a solution of gold in a mixture of ammonium chloride and nitric acid treated with two different Mexican plants extracts, Toad Leaf (Eryngium heterophyllum, SF) and Cuachalalate (Amphipterygium adstringens, CF) as bioreductants. Subsequently, the gold entities were supported on the ordered mesoporous material SBA-15 functionalized with mercaptopropyl groups (1.6 wt% of sulphur). The gold contents were 0.7 wt% and 1.1 wt% for the CF and SF samples, respectively. UV–vis spectroscopy and TEM show the presence of gold nanoclusters in both samples, while very minor amount of AuNPs is detected only in the sample with the highest Au content (SF). The Au/SBA-15 materials are active in the aerobic oxidation of cyclohexene at 65 °C and atmospheric pressure. Two of the catalytic reaction products come from the addition of oxygen to the double bond of cyclohexene: cyclohexene epoxide and cyclohexanediol, while the other three come from allylic oxidation of the ring, which is the major reaction pathway: 2-cyclohexen-1-ol, 2-cyclohexen-1-one and 2-cyclohexenyl hydroperoxide, being the two first the major stable reaction products, while the enone/enol ratio is nearly 2. It has been found that the gold nanoclusters initially present in the catalysts evolve toward AuNPs during catalytic reaction.