Eleonora Monti

Supported on activated carbon gold colloidal nanoparticles have been prepared in the presence of the stabilizing polymer PVA and PVP (polyvinylalcohol, polyvinylpyrrolidone). The effect of the polymer to the Au weight ratio was investigated, for synthesizing gold nanoparticles with different particle size and particle size distribution. By varying the polymer/Au wt/wt ratio, gold nanoparticles with mean diameters from 3 to 8 nm were synthesized. The synthesized Au catalysts were studied in the liquid phase oxidation of 1,6-hexanediol (HDO) to adipic acid under base and base-free conditions. A range of experimental parameters were varied for the optimization of reaction conditions and the most promising Au catalysts were further evaluated in terms of catalytic performance. We demonstrated that the influence of choice of polymer, tuning the polymer to Au weight ratio, HDO to Au molar ratio, and use of basic conditions have an important influence in terms of catalytic activity and selectivity to adipic acid. The highest yield to adipic acid was obtained using Au-PVA catalysts (40% at 110 °C under base free conditions), however, at low HDO to Au molar ratio and lower carbon balance (70–80%). On the contrary, at higher HDO to Au molar ratio, and under basic conditions, the yield was in the range of 18–20% with a significant improvement in terms of carbon balance (88–100%).

The effect of polyvinyl alcohol (PVA) stabilizers and gold nanoparticles supported on active carbon (AuNPs/AC) was investigated in this article. Polymers with different molecular weights and hydrolysis degrees have been synthesized and used, like the stabilizing agent of Au nano-catalysts obtained by the sol-immobilization method. The reduction of 4-nitrophenol with NaBH₄ has been used as a model reaction to investigate the catalytic activity of synthesized Au/AC catalysts. In addition, we report several characterization techniques such as ultraviolet-visible spectroscopy (UV-Vis), dynamic light scattering (DLS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) in order to correlate the properties of the polymer with the metal nanoparticle size and the catalytic activity. A volcano plot was observed linking the catalytic performance with hydrolysis degree and the maximum of the curve was identified at a value of 60%. The Au:PVA-60 weight ratio was changed in order to explain how the amount of the polymer can influence catalytic properties. The effect of nitroaromatic ring substituents on the catalytic mechanism was examined by the Hammett theory. Moreover, the reusability of the catalyst was investigated, with little to no decrease in activity observed over five catalytic cycles. Morphological and kinetic studies reported in this paper reveal the effect of the PVA polymeric stabilizer properties on the size and catalytic activity of supported gold nanoparticles.