Influence of Surface Functionalities on Au/C Catalysts for Oxidative Homocoupling of Phenylboronic Acid

The formation of C–C bonds through coupling reactions is an important industrial process. The ability of Au to catalyze such reactions has been reported, with both homogeneous and heterogeneous catalyst examples. Previous work has shown that carbon-supported cationic and nanoparticulate Au are active for the homocoupling of phenylboronic acid to biphenyl. However, the stability of supported cationic Au is short-lived, and the formed nanoparticles were suggested to be the active species. Through the synthesis of two types of supported cationic Au catalysts, utilizing either aqua regia or acetone solvents, we show that both catalysts develop nanoparticulate Au species early in the reaction; however, only the aqua regia prepared catalyst is active. We ascribe the activity of the aqua regia prepared Au catalyst to excess Cl and the presence of C–Cl surface species in combination with Au. Carbon treated with aqua regia was inactive; however, when used as a support for Au deposited with acetone or via a sol immobilization method, activity was comparable to the aqua regia prepared catalyst. The role of C–Cl and Au nanoparticles is discussed with respect to their correlation to the biphenyl yield, which is shown to be significant only when the C–Cl species are present on the catalyst.