Síntese mecanoquímica nanopartículas de óxido de cobre (II) para a redução catalítica de nitroarenos em meio aquoso

Abstract

Mechanochemistry was recently cited by the International Union of Pure and Applied Chemistry (IUPAC) as one of the ten chemical innovations with the potential for the development of more sustainable processes. The principle of the mechanochemical synthesis, through ball milling, is to promote chemical reactions from the energy released by the impacts between grinding balls. It results in an alternative one-pot method that follows the green chemistry principles, especially due to the non-use of solvents. In this work, mechanochemistry was the technique employed to synthesize copper (II) oxide nanoparticles (CuONPs), aiming for a fast and operationally simplified procedure in a dry medium. The synthesis was promoted by mechanical energy generated in a high energy ball mill that combines vibration and circular movements. The synthesis was carried out using copper (II) hydroxide as the copper precursor and sodium chloride as a diluent agent. The effects of rotation speed, milling time, and charge ratio (ball-to-powder weight ratio) over the final characteristics of the CuONPs were evaluated based on a two-level full factorial design, totalizing eight experiments. It was observed that, under the imposed experimental conditions, none factor showed statistically significant influence over the hydrodynamic diameter of the particles. Thus, it was established the combination of the following factors: rotation speed of 1000 rpm, milling time of 20 min, and the charge ratio of 40:1 as the best milling conditions for CuONPs synthesis. Under these conditions, an average hydrodynamic size of 82 ± 34 nm was obtained. Also, by microscopy techniques, it was observed a majority spherical morphology with a particle size of 26 ± 12 nm. The crystalline structure was identified as monoclinic with a single phase of copper (II) oxide. It shows a crystallite size of 11 nm and a microstrain of 0,012. Also, the CuONPs presented good thermal stability according to thermogravimetric analysis, a surface area of 68 ± 10 m2 g-1, which corroborates with the nanometric scale, and band gap of 2.0 eV. The synthesized CuONPs were applied as a catalyst in nitroarenes reduction to its corresponding aminoarenes in an aqueous medium. The conversion and selectivity reached values up to >99% for eight substrates, with reaction times ranging between 10 and 60 minutes. Therefore, according to our goals, the mechanochemical synthesis promoted, in an effective way, the synthesis of CuONPs with good catalytic properties and possibilities of scalability and reuse for five consecutive runs without loss on conversion or selectivity.