Epoxidation of styrene by anhydrous t-butyl hydroperoxide over gold supported on Al 2O 3, Ga 2O 3, In 2O 3 and Tl 2O 3

Abstract

Group IIIa metal (viz. Al, Ga, In and Tl) oxide-supported gold nanoparticles, prepared by deposition–precipitation (DP) using NaOH and by homogeneous deposition–precipitation (HDP) using urea, were employed as highly active/selective and reusable catalysts for the epoxidation of styrene (at 82 °C) by anhydrous t-butyl hydroperoxide (TBHP). The loading and average particle size of gold, when it was deposited on the Gr. IIIa metal oxides by the HDP, was found to increase in the following order: Al 2O 3 (6.36 wt.%, 4.1 ± 2.3 nm) < Ga 2O 3 (6.43 wt.%, 6.67 ± 3.7 nm) < In 2O 3 (6.81 wt.%, 15.6 ± 6.3 nm) < Tl 2O 3 (8.00 wt.%, 24.4 ± 13.1 nm). Both the activity and the epoxide selectivity of the supported gold catalysts are in the following order: Au/Al 2O 3 < Au/Ga 2O 3 < Au/In 2O 3 < Au/Tl 2O 3. Using the DP method, the Au loading on Al 2O 3 and Tl 2O 3 was found to be 2.10 and 5.42 wt.%, respectively, such values are much lower than those achieved by the HDP method. The Al 2O 3, Ga 2O 3 and In 2O 3 supports alone showed little or no epoxidation activity/selectivity. However, the Tl 2O 3 support alone showed the activity and epoxide selectivity comparable to that of the Au/Tl 2O 3 catalysts. The surface species on the Au/Al 2O 3 were found to be only metallic gold (Au 0) ones and Al in single oxidation state (Al 3+). However, the surface species on the Au/In 2O 3 were both the metallic and ionic gold (Au 0 and Au 3+) and the indium in two oxidation states (In 3+and In 2+ or In 1+). The gold loading and both the epoxidation activity and selectivity are increased with increasing the basicity and/or reduction potential of the support used in the supported Au catalysts.