Epoxidation of styrene by anhydrous t-butyl hydroperoxide over gold supported on Al₂O₃, Ga₂O₃, In₂O₃ and Tl₂O₃

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: Al₂O₃ (6.36 wt.%, 4.1 ± 2.3 nm) < Ga₂O₃ (6.43 wt.%, 6.67 ± 3.7 nm) < In₂O₃ (6.81 wt.%, 15.6 ± 6.3 nm) < Tl₂O₃ (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/Al₂O₃ < Au/Ga₂O₃ < Au/In₂O₃ < Au/Tl₂O₃. Using the DP method, the Au loading on Al₂O₃ and Tl₂O₃ was found to be 2.10 and 5.42 wt.%, respectively, such values are much lower than those achieved by the HDP method. The Al₂O₃, Ga₂O₃ and In₂O₃ supports alone showed little or no epoxidation activity/selectivity. However, the Tl₂O₃ support alone showed the activity and epoxide selectivity comparable to that of the Au/Tl₂O₃ catalysts. The surface species on the Au/Al₂O₃ were found to be only metallic gold (Au⁰) ones and Al in single oxidation state (Al³⁺). However, the surface species on the Au/In₂O₃ were both the metallic and ionic gold (Au⁰ and Au³⁺) and the indium in two oxidation states (In³⁺and In²⁺ or In¹⁺). 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.