Organic transformations catalyzed by gold nanoparticles supported on functionalized silica

Recently, we presented the preparation, without any need of additional reducing and/or stabilizing agents, of a variety of stable silica-supported gold nanoparticles (AuNPs) by using as the only reactants an aqueous solution of chloroauric acid (HAuCl4) and different functionalized-silica supports. We started using commercial polyethyleneimine-functionalized silica beads (SiO2-PEI)[1] and we went on preparing silica nanoparticles with alkynyl carbamate moieties (Au/SiO2@Yne)[2] later post-functionalized by thiol-yne radical chemistry to give the amino-sulfide-branched silica (SiO2@AeThio).[3] We now present the latest results concerning the synthesis, characterization and catalytic application of AuNPs supported on functionalized silica. The main purpose of our work is to obtain a material with the right features for the application as catalyst in a continuous flow system. Indeed, flow chemistry is recognized by the Green Chemistry Institute (GCI) as a key area for research activities in the aim of developing sustainable manufacturing.[4] The main advantages of a flow reactor are: i) the use of small volumes, ii) the fast heating and mass transfer, iii) the very efficient thermal management, iv) the high pressure stability, v) the possibility to reduce the quantity of solvents and even realize solvent-free reactions. The silica-supported AuNPs have been prepared by the simple grafting of commercial available silica with the alkyne terminated organosilane [3-(2-propynylcarbamate)propyl]triethoxysilane (PPTEOS), followed by the addition of HAuCl4 to obtain the in situ formation of the target AuNPs-anchored systems. The resulting material (Au/SiO2@Yne) has been characterized with various techniques, such as atomic absorption spectroscopy (AAS), transmission electron microscopy (TEM), total reflectance fourier transformed infrared spectroscopy (ATR-FT-IR) and thermogravimetric analysis (TGA). Afterwards, Au/SiO2@Yne has been employed as heterogeneous catalyst in the oxidation of alcohols to their corresponding carbonyl compounds, which is one of the most important processes for the production of fine and specialty chemicals. The reaction conditions have been chosen with particular attention to the green aspect, therefore the oxidation have been carried out using a mixture of tert-butanol and water as solvent and hydrogen peroxide as the oxidant agent. The catalyst have shown a remarkable activity in the oxidation of a series of secondary alcohols in the batch system, and even more successful results have been observed when the reaction was performed in continuous flow conditions. References [1]. “Straightforward Synthesis of Gold Nanoparticles Supported on Commercial Silica-Polyethyleneimine Beads”, S. Fazzini , D. Nanni , B. Ballarin , M. C. Cassani , M. Giorgetti , C. Maccato, A. Trapananti , G. Aquilanti, S. I. Ahmed, J. Phys. Chem. C, 2012, 116, 25434−25443. [2]. “Novel Synthesis of Gold Nanoparticles Supported on Alkyne-Functionalized Nanosilica”, S. Fazzini, M.C. Cassani, B. Ballarin, E. Boanini, J.-S. Girardon, A.-S. Mamede, A. Mignani, D. Nanni, J. Phys. Chem. C, 2014, 118, 24538-24547. [3]. “Functionalizaton of Silica through Thiol-Yne Radical Chemistry: a Catalytic System based on Gold Nanoparticles Supported on Amino-Sulfide-Branched Silica”, M. C. Cassani, B. Ballarin, D. Barreca, E. Boanini, E. Bonansegna, G. Carraro, S. Fazzini, A. Mignani, D. Nanni, D. Pinelli, RSC Adv., 2016, 6, 25780-25788.[ [4]. “Continuous process technology: a tool for sustainable production”, C. Wiles, P. Watts, Green Chem., 2014, 16, 55-62.