Thiolate-Mediated Selectivity Control in Aerobic Alcohol Oxidation by Porous Carbon-Supported Au₂₅ Clusters

Abstract

Supported Au25 clusters were prepared through the calcination of Au25(SC12H25)18 on hierarchically porous carbon nanosheets under vacuum at temperatures in the range of 400–500 °C for 2–4 h. TEM and EXAFS analyses revealed that the thiolate coverage on Au25 gradually decreased with increasing calcination temperature and period and became negligibly small when the calcination temperature exceeded 500 °C. The catalysis of these Au25 clusters was studied for the aerobic oxidation of benzyl alcohol. Interestingly, the selectivity for benzaldehyde formation was remarkably improved with the increase in the amount of residual thiolates on Au25, while the activity was reduced. This observation is attributed to the dual roles of the thiolates: the reduction of the oxidation ability of Au25 by electron withdrawal and the inhibition of the esterification reaction on the cluster surface by site isolation.

Phosphine/phenylacetylide-ligated Au clusters for multicomponent coupling reactions

“…Such a speculation is reasonable, as Au 25 (PPh 3 ) 10 (PA) 5 X 2 was synthesized via a ligand exchange process in which some of the phosphine groups were replaced with phenylacetylide in the presence of amine (e.g., pyridine) [28]. The ligand removal phenomenon has already been reported [48][49][50][51][52][53][54]. For example, one phosphine ligand (e.g., PPh 3 and PPh 2 Py (diphenyl-2-pyridylphosphine)) of Au 11 (PPh 3 ) 7 Cl 3 and Au 11 (PPh 2 Py) 7 Cl 3 can leave the clusters in the presence of amine to provide an active site for the selective hydrogenation [55].…”

Engineering Surface Ligands of Noble Metal Nanocatalysts in Tuning the Product Selectivity

“…The reports on ligand-modulated selective oxidation using Au NPs are few. Yoskamtorn et al [24] reported that the surface ligands of supported Au 25 nanoclusters had a profound influence on the product selectivity of aerobic benzyl alcohol oxidation reaction (Figure 3). By reducing surface ligands through high-temperature calcination at 400-500 • C, as-resulted Au nanocatalysts exhibited a continuous decrease in selectivity to benzaldehyde from 100% to 15%, along with the increase of TOF.…”

“…A high selectivity to the hydrogenation of aldehyde groups (close to 100%) was attributed to the formation of -S(R)-Au-S(R)-Au-S(R) staple-like motifs, which resulted in the strong interaction of 4-nitrobenzaldehyde with C=O bonds. Reprinted with permission from [24]. Copyright American Chemical Society, 2014.…”

“…In the presence of excess surface ligands (essentially necessary during the wet-chemical synthesis), ligands can form a densely packed self-assembled monolayer (SAM) in the thickness of 0.5-3 nm by maximizing van der Waals interactions of alkyl chains [18]. The surface ligands are demonstrated to be detrimental to the catalytic activity in many examples [20][21][22][23][24][25][26][27]. The formation of SAMs is often considered to block the surface accessibility, leading to a decrease in the accessibility of catalytic sites [20,23], although there are many reported examples showing that surface ligands do not depress catalytic activity of metal NPs [28][29][30][31].…”

Active metal (cadmium) doping enhanced the stability of inert metal (gold) nanocluster under O2 atmosphere and the catalysis activity of benzyl alcohol oxidation

“…In the past few years, significant advances have been made in the synthesis and property studies of these doping nanoclusters [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Among homogold nanoclusters, Au 25 (SR) 18 nanocluster has been well studied due to its stable structure among three different charges (−1/0/+1) [22][23][24][25][26], which led to the exploration of a wide range of catalytic pathways facilitated by Au 25 for organic reactions, such as oxidation [27][28][29], hydrogenation [30][31][32], electronic transfer reactions [26,33], and electrocatalysis [34,35]. Recent research implies that doping foreign atoms into this 25 noble metal system can largely affect the stability and the catalytic activity compared to the homogold counterpart [7][8][9][10][17][18][19].…”