Aqueous-phase synthesis of benzyl ester over partially thiolated Au25 nanocluster catalysts: improving selectivity with a hydrophobic carbon support

Abstract

The aqueous-phase synthesis of esters via oxidative coupling of alcohols catalyzed by carbon-supported gold catalysts is one of the environmentally benign approaches, but remains challenging. To understand the role of the carbon support in maximizing the selectivity to esters, we have here deposited partially thiolated Au₂₅ nanoclusters as common reactive centers on three types of carbon supports: commercially available porous carbon (CNovel), carbon mesosponge (CMS), and graphene mesosponge (GMS). These carbons have similar porous structures but significantly different amounts of functional groups. When GMS with the fewest functional groups was used as a support, the Au₂₅ nanocluster catalysts exhibited the highest selectivity towards benzyl benzoate in the oxidation of benzyl alcohol under aqueous conditions, with a maximum yield of 67% under the optimized conditions. Mechanistic studies revealed that the carbon supports, due to their hydrophobic nature, played two crucial roles in the unique selectivity: (i) protection of the intermediate benzaldehyde from nucleophilic attack by hydroxy ions while facilitating the attack by alkoxide and (ii) adsorption of benzyl benzoates on the support to prevent hydrolysis.