Phenol acetylation under mild conditions catalyzed by gold nanoparticles supported on functional pre-acidified sepiolite

Abstract

Gold nanoparticles (AuNPs) were prepared from a gold hydrosol and deposited at the external surfaces of sepiolite previously functionalized by aminopropyltriethoxysilane (APTES). The grafting of APTES on the sepiolite fibers could be demonstrated by 29Si solid state NMR spectroscopy. The effect of a pre-acidification treatment of the sepiolite samples before the silanation reaction and the AuNP deposition was studied, particularly with respect to the size and the dispersion of AuNPs on the sepiolite surfaces. Very well dispersed nanoparticles with a very small size of 2–3 nm were obtained in the case of the sepiolite samples pre-treated with HCl under soft conditions (100% of AuNPs were under 5 nm in diameter, and 91% under 3 nm). The AuNPs were mainly aligned parallel to the sepiolite fiber axis, plausibly along the edges of the mineral rich in silanol groups. In strong contrast, in the case of sepiolite samples not submitted to a pre-acidification treatment, the deposition of AuNPs was not homogeneous. The reaction of acetylation of phenol derivatives was chosen to test the catalytic activity of the material resulting from the deposition of AuNPs on functionalized sepiolite. The catalytic reaction was done under soft conditions and organic solvent-free conditions, with acetic anhydride as acetyl donor. Reactions were done either using conventional heating or microwave irradiation. It was found that yields typically in the range 90+% were obtained, with dramatic reduction of the reaction time when microwave irradiation was used.