Catalysis Today 235 (2014) 72–78 Contents lists available at ScienceDirect Catalysis Today j o ur na l ho me page: www.elsevier.com/locate/cattod Atomically well-defined Au 25 (SR) 17/18 nanoclusters deposited on silica supports for the aerobic epoxidation of trans-stilbene Christophe Lavenn, Aude Demessence ∗ , Alain Tuel Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR 5256, CNRS/Lyon 1 University, 2 avenue Albert Einstein, 69626 Villeurbanne, France a r t i c l e i n f o Article history: Received 20 November 2013 Received in revised form 13 February 2014 Accepted 25 February 2014 Available online 27 March 2014 Keywords: Gold clusters Thiolate ligands Aerobic oxidation Alkene a b s t r a c t Two atomically well-defined thiolate gold nanoclusters, Au 25 (SC 2 H 4 Ph) 18 and Au 25 (SPhNH 2 ) 17 , having the same gold core but different thiolate ligands have been deposited on silica supports and the obtained catalysts have been tested in the aerobic oxidation of trans-stilbene. By properly selecting the support to maximize cluster/surface interactions, it was possible to prepare materials with small nanoparticles and very narrow particle size distributions. Au 25 (SPhNH 2 ) 17 @SBA-15 catalyst showed an activity and a trans-stilbene epoxide selectivity comparable to the best heterogeneous catalysts. Elimination of the thiolate molecules by calcination did not significantly modify the particle size but decreased the activity, pointing out the critical role of the ligands in the reaction. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Since Haruta’s work on CO oxidation catalyzed by gold nanoparticles [1], those materials have been widely used as catalysts in various catalytic reactions such as alcohols and alde- hydes oxidation, alkenes epoxidation, hydrochlorination of ethyne, carbon–carbon bond formation, and so on [2]. When reactions can be performed in the presence of non-polluting oxidants, typically oxygen from the air, they are particularly important for industry. Aerobic oxidations can be carried out over supported gold nanopar- ticles [3] as well as colloids [4]. Among these oxidative reactions, selective alkenes epoxidation remains an important challenge in fine chemicals area and efficient catalysts are still needed. Gold nanoparticles deposited on different supports can efficiently cat- alyze the aerobic epoxidation of alkenes [5–7] but despite extensive studies on the subject, the understanding of the role of particles diameter, their surface, the effect of the support and the difficulty to obtain monodisperse particles with a narrow particle size distribu- tions (PSD) makes difficult to rationalize the mechanisms involved in this reaction. Over the past decade, atomically well-defined thiolate gold clusters, defined as Au n (SR) m , where n and m are number of gold atoms and surrounding thiolate-based molecules, respec- tively, have attracted intense research interests due to their ∗ Corresponding author. Tel.: +33 472 445 322; fax: +33 472 445 399. E-mail address: aude.demessence@ircelyon.univ-lyon1.fr (A. Demessence). unique electronic, magnetic and optical properties [8]. These atom- ically and structurally well-defined thiolate gold clusters have emerged as a new promising catalysts class to understand the rela- tionship between the size, the structure of the nanoparticles and their catalytic activity [9]. Indeed, their atomically well defined composition and known structure can lead to a precise under- standing of size, structural and electronic effects on the catalytic mechanisms. In addition, although sulfur species are considered as a poison in catalytic reactions such as the oxidation of styrene [10], it has been shown that thiolate gold clusters can be active, even in the pres- ence of thiolate groups, for the oxidation of CO [11] or sulfide [12], the reduction of nitrophenol [13] or the selective hydrogenation of ketones and aldehydes [14,15]. For the epoxidation of alkenes, thiolate gold clusters have been tested on styrene [14], and on both trans- and cis-stilbene substrates [16]. The conversion of styrene, with Au 25 (SC 2 H 4 Ph) 18 clusters as the catalyst, in the presence of O 2 , was of ca. 18 and 27% at 24 h, depending on whether clusters were supported or not on silica [14]. The major product in this reac- tion was the benzaldehyde (selectivity ∼70%). On the contrary, for the trans-stilbene epoxidation with Au 25 (SC 2 H 4 Ph) 18 @SiO 2 and an excess of tert-butyl hydroperoxide used as the oxidant, the main product was the trans-epoxide (selectivity 94%) with a 51% conver- sion at 24 h [16]. In the present work, we have studied the effects of the pres- ence of thiolate ligands around gold nanoclusters, their nature and the type of silica on which they are supported in the aer- obic epoxidation of trans-stilbene. Reactions are carried out in http://dx.doi.org/10.1016/j.cattod.2014.02.045 0920-5861/© 2014 Elsevier B.V. All rights reserved.