Biosynthesis of Gold Clusters and Nanoparticles by Using Extracts of Mexican Plants and Evaluation of Their Catalytic Ac
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Biosynthesis of Gold Clusters and Nanoparticles by Using Extracts of Mexican Plants and Evaluation of Their Catalytic Activity in Oxidation Reactions Julia Aguilar‑Pliego1,2 · Ramón Zárraga Nuñez3 · Javier Agundez1 · Ramón de la Serna Valdés1 · Joaquín Pérez‑Pariente1 Received: 20 September 2020 / Accepted: 2 October 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
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. Graphic Abstract
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Keywords Gold nanoparticles · Gold clusters · Biosynthesis · Mexican plants extracts · Toad leaf · Cuachalalate · Cyclohexene · Oxidation reactions
1 Introduction The importance of knowing how to prepare gold nanoparticles (AuNPs) with different sizes and shapes resides in their potential application in materials science, medicine, biological processes and catalysis [1–3]. In the case of heterogeneous catalysis, the catalytic activity and selectivity of these nanoparticles depend on their size and shape, the chemical medium to which they are exposed during reaction and, if they are immobilized on a support, the interaction between the nanoparticles and the support [4]. A convenient method to prepare metal nanoparticles is the use of plant extracts, as they contain a complex mixture of biomolecules, resulting from the plant metabolism, which are able to reduce the precursor metal salt to metal nanoparticles while acting as the same time as capping agents that would protect the nascent metal aggregates from excessive growing. This overall process is called bioreduction [5–8]. Typically, bioreduction by plant extract involves m
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