Gold nanoparticles deposited on mesoporous alumina for epoxidation of styrene: Effects of the surface basicity of the supports

Abstract

Three kinds of mesoporous alumina (denoted as meso-Al 2O 3- x ( x = a, b, c)) supports with different surface basicity were synthesized by different structure-direct agent and different assembly pathway, respectively. The surface basicity of the meso-Al 2O 3- x and the commercial γ-Al 2O 3 was measured by means of CO 2-TPD and IR spectroscopy. The profiles of CO 2-TPD and IR spectra indicated that the mesoporous alumina owned more abundant surface basic sites than that of γ-Al 2O 3. Au nanoparticles deposited on the above supports via the method of homogeneous deposition precipitation (HDP) using urea as the precipitation agent. The supported Au catalysts were characterized by N 2 adsorption–desorption isotherms, TEM, XPS and testing the catalytic performances in the epoxidation of styrene. The dispersion and average size of the gold particles are dependent of the number of surface basic sites on the supports. Transmission electron microscopy (TEM) observations show a homogeneous distribution of gold particles lower than 4 nm on meso-Al 2O 3-b and meso-Al 2O 3-c, which owned more basic sites on the surface. XPS spectra reveal that only metallic state of Au is presented on the supports, which is independent of surface properties. The different Al 2O 3 supported Au catalysts were employed as highly active/selective and reusable catalysts for the epoxidation of styrene by anhydrous t-butyl hydroperoxide (TBHP). Au nanoparticles as well as the surface basic sites of the support are responsible for the epoxidation.