Confinement of Au3+-rich clusters by using Silicalite-1 for selective solvent-free oxidation of toluene

Abstract

The selective oxidation of primary carbon–hydrogen bonds in the methyl group of toluene to corresponding oxygenates is of immense significance. This transformation, however, remains challenging and often requires either extensive optimization of the present chemical operating processes or, more importantly, the design and development of novel catalysts. In this paper, we achieve the confinement of uniform Au nanoparticles by using Silicalite-1 (Au@Silicalite-1) via an improved impregnation method for solvent-free oxidation of toluene by oxygen to benzaldehyde. In situ Ar⁺ etching XPS experiments revealed that in the evolution of Au⁰ and Au³⁺ species in a single Au@Silicalite-1 crystal the proportion of the Au³⁺ species becomes enriched from 0 on the surface to >80% in the center. In the synergetic catalysis between Au⁰ and Au³⁺ species, it was observed that the electron-rich Au⁰ species activated oxygen to form superoxide anion radicals, while the electron-deficient Au³⁺ species promoted activation of the primary carbon–hydrogen bond in toluene and enabled efficient desorption of the desired product benzaldehyde with selectivity >90%. For comparison, Silicalite-1 or SiO₂ supported Au⁰-rich catalysts exhibited strong adsorption of benzaldehyde, generating overoxidized benzoic acid as the main product.