Controlled Synthesis of Au Nanoparticles in the Nanocages of SBA-16: Improved Activity and Enhanced Recyclability for the Oxidative Esterification of Alcohols

Au nanoparticles with different sizes were introduced into the nanocages of a mesoporous material SBA-16 with the aid of chemical modification, leading to new Au-supported catalysts Au/SBA-16. These catalysts were characterized with Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microspectroscopy (TEM), N-2 sorption, and X-ray photoelectron microspectroscopy (XPS). These results revealed that uniform Au nanoparticles with sizes of a few nanometers were successfully positioned inside the nanocages of SBA-16. Such catalysts were catalytically active in the oxidative esterification of various alcohols even including less reactive straight-chain alcohols. It was found that the activity of this catalyst strongly depended on the Au loading, and the Au loading of S wt % (corresponding to Au particles of 2-3 nm in sizes) led to the highest activity. Its activity was much higher than those of the analogous catalysts prepared from commercial silica gel as well as SBA-15. Furthermore, Au/SBA-16 could be reused at least eight reaction cycles without significant decrease in activity and selectivity. Its recyclability was much superior to that of the catalyst derived from commercial silica gel. The underlying reason may be that the unique nanostructure of SBA-16 can effectively prevent the growth of Au nanoparticles into less active, larger particles, as evidenced from TEM investigations. This study not only supplies a new, active, recoverable catalyst for the green transformations of alcohols to esters but also demonstrates that the three-dimensional mesoporous cage-like material SBA-16 has a superior ability in reducing the diffusion resistance and stabilizing metal nanoparticles against growth.