Influence of the support during homocoupling of phenylboronic acid catalyzed by supported gold

Gold or palladium nanoparticles with an average diameter of 10 and 15 nm, respectively, were deposited on the surface of CeO2, TiO2, ZrO2, and SiO2 powder supports by deposition-precipitation using a solution of HAuCl4 or Pd(NO3)(2). The supports and catalysts were characterized by N-2 adsorption, XRD, TEM, and FTIR spectroscopy. The influence of the crystallite size, degree of hydroxylation, and crystallinity of the supports on the conversion and selectivity during the Suzuki homocoupling of phenylboronic acid were investigated. Under the same reaction condition, the supported palladium catalysts are less active than the supported gold catalysts. The palladium catalysts show a selectivity of about 75% towards biphenyl with formation of benzene and phenol as byproducts. However, when supported gold is used for the reaction, a selectivity of 100% towards biphenyl is obtained in all cases except when SiO2 is used as support. The activity of the supported gold catalysts increases as the size of the crystallite support particles decreases and as the initial surface coverage of OH groups in the supports increases. The results confirm a previous model suggesting the participation of hydroxyl groups in the catalytic reaction through the dissociation of molecular water on the surface of the support to restore the surface hydroxyl groups. (c) 2008 Elsevier B.V. All rights reserved.