Efficient and selective epoxidation of styrene with TBHP catalyzed by Au25clusters on hydroxyapatite

Abstract

Au 25 clusters supported on hydroxyapatite oxidized styrene in toluene with 100% conversion and 92% selectivity to the epoxide, under optimum conditions and using anhydrous tert-butyl hydroperoxide (TBHP) as an oxidant. 10In 1998, Haruta and coworkers demonstrated for the first time that supported Au nanoparticles (AuNPs) are effective for the direct epoxidation of propene using molecular oxygen. 1 the yield of the epoxide is not high (~5%). 4 This work hints at the possibility that there is a optimal cluster size for the efficient and selective oxidation of alkenes. Here we focus on the epoxidation of styrene and aim to systematically study the effect on the catalytic activity of the size of Au clusters that 25 are immobilized on hydroxyapatite (HAP; Ca 10 (PO 4 ) 6 (OH) 2 ). 8 We use tert-butyl hydroperoxide (TBHP) as an oxidant since it is known to be more effective than O 2 for alkene oxidations. [5][6][7] We describe the synthesis of HAP-supported Au 25 clusters; they show a higher catalytic performance 30 (100% conversion and 92% selectivity to the epoxide) than larger analogs.The preparation of HAP-supported Au 25 catalysts is depicted in Scheme 1. The basic idea is to use as a precursor ligand-protected Au clusters with a well-defined core size. 4 18 clusters were found to be adsorbed completely on HAP as evidenced by the colorless filtrate (ESI, † Fig. S1). The Au loading of the composites thus obtained is 0.5 wt% (ESI, † Table S1), so the composite is Fig. S3). The diameter of the Au core is comparable to that of Au 25 (~0.9 nm), assuming a spherical shape. 80The 0.5Au 25 :SG-HAP composite was then incubated for removal of the thiolate ligands.The results of thermogravimetric analysis revealed a 49% weight loss of Au 25 (SG) 18 after calcination at 300 C in vacuo for 2 h (ESI, † Fig. S4); this weight corresponded to the calculated weight of 85 the GS ligands in Au 25 (SG) 18 (53%). Thus, 0.5Au 25 :SG-HAP was calcined under the above mentioned conditions to remove the GS ligands. This removal was confirmed by inductively coupled plasma (ICP) measurements. After calcination, the sulfur content reduced from 0.058 wt% to 0.005 wt%, which 90 was comparable to the amount of sulfur impurity in HAP. An HAADF-STEM image of the resulting Au 25 -HAP shows that most of the clusters are 1.2-1.5 nm in size (Fig. 1A).

Development of novel supported gold catalysts: A materials perspective

“…However, metal salts have been seldom used as supports for making gold catalysts [73,[133][134][135][136][137][138][139][140][141][142][143][144]. This is so, not only because the bulk of gold catalysis papers have been following the classic work of Haruta and co-workers who used metal oxide supports [15][16][17], but also because compared with metal salts, metal oxides are more frequently encountered in heterogeneous catalysis.…”

Size- and Structure-specificity in Catalysis by Gold Clusters

“…48 Extensive investigation was focused on Au 25 (SG) 18 (SG: glutathionate) supported on SiO 2 by the group of Jin, 4951 while the group of Tsukuda focused on thiolate-ligand-free Au 25 clusters supported on HAP. 52 In the styrene epoxidation with tert-butyl hydroperoxide (TBHP) in toluene solvent at 353 K, Au 25 supported on HAP produced styrene epoxide with selectivity of 92% at 100% Figure 8. Size specificity in the resistivity of gold nanoparticles and clusters against oxidation by oxygen plasma.…”

The impacts of nanotechnology on catalysis by precious metal nanoparticles

“…The non-metallic behavior of Au n (SR) m nanoclusters is of interest to catalysis [45,49,117,118] . These atomically precise Au n (SR) m nanoclusters are expected to become a promising class of model catalysts.…”