Confinement of Au3+-Rich Clusters by Silicalite-1 for Selective Solvent-Free Oxidation of Toluene

Selective oxidation of the 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 process or, more importantly, the design and development of novel catalysts. In this paper, we develop the confinement of uniform Au nanoparticles by 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 the evolution of Au0 and Au3+ species in a single Au@Silicalite-1 crystal that the proportion of Au3+ species become enriched from 0 in the surface to >80% in the center. The synergetic catalysis between Au0 and Au3+ species was observed that electron-rich Au0 species activated the oxygen into superoxide anion radical, 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 showed strong adsorption of benzaldehyde, generating overoxidized benzoic acid as the main product.