Exploring bulk and colloidal Mg/Al hydrotalcite–Au nanoparticles hybrid materials in aerobic olefin epoxidation

Au nanoparticles (AuNPs) were immobilized on S-containing amino acids (cysteine and methionine) Mg/Al hydrotalcite clay in both bulk and exfoliated (colloidal) forms. The Au nanoparticles were prepared by a biomimetic method using an anti-oxidant tea extract to reduce the Au salt solution. The performance of bulk and exfoliated clays with Au nanoparticles in aerobic olefin epoxidation was assessed with very interesting results. Both catalysts were very active towards the epoxide products; selectivity was found to be dependent on the catalyst form. The exfoliated catalysts yielded higher product stereoselectivity in the case of limonene epoxidation. These results can be explained by the existence of a confined environment around the AuNPs structures provided by the clay nanosheets wrapping the nanoparticles, modulating how substrates interact with the catalytic active centres. Based on experimental data mechanistic proposals were also rationalized for these catalysts showing how they assist the catalytic process.