Synthesis, characterization and study of catalytical properties of mixed layered hydroxides containing Cu(II)/Mg(II)/Al(III) / Síntese, caracterização e estudo de propriedades catalíticas de hidróxidos mistos lamelares contendo Cu(II)/Mg(II)/AI(III)

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

In this work it was investigated the preparation and spectroscopic characterization of layered hydroxides of composition [Mg3-xCuxAl(OH)8](CO3)0.5.yH2O (x = 0.3; 0.6; 0.9; 1.2; 1.5; 1.8; 2.1; 2.4; 2.7 and 3.0), as well the reactivity of these materials towards phenol hydroxylation, using hydrogen peroxide as oxidizing agent. The solids had been prepared through the co-precipitation method and characterized by X ray diffractometry, elemental analysis (C, H, N and metals), thermogravimetric analysis, surface area measurements, and also by the following spectroscopic techniques: infrared vibrational absortion (FTIR), Raman scattering, vis-NIR electronic absorption and electron paramagnetic resonance (EPR). It was observed that the increase of copper(II) content in the samples favours the CuO formation. Materials of blue coloration containing only one crystalline phase were obtained by the co-precipitation method in the pH range 10 – 8, and were submitted to aging and drying processes at room temperature. As the amount of Cu2+ ions increases in the layered hydroxide material, it was observed a decrease in crystallinity and thermal stability. EPR spectra are anisotropic (parameters g// and g? well defined), indicating that copper(II) ions are in sites with axial symmetry. As the amount of Cu2+ in the samples increases, EPR signal decreases, what can be interpreted as result of an antiferromagnetic coupling between copper ions. FTIR data suggests that the layered hydroxides gave two types of cabornate ions. Moreover, the symmetry lowering phenomena of CO32- anions during the heating process was observed, originating, according to the temperature and sample composition, materials of varied coloration. This lowering of symmetry can be due to coordination (or grafting) of carbonate ions to the metal ions in the layers. Vis-NIR electronic spectra of green solids obtained by heating show absorption in the region below 400 nm, besides bands attributed to d-d transitions (700-800 nm), that can be attributed to O2- ->Cu2+ charge transfer transition. Despite the X ray diffraction peaks of low intensities attributed to spinel phases, the ochre coloration observed can be due to the presence of CuAl2O4 (brown solid). For gray and black samples, X ray diffractograms shows intense peaks attributed to CuO. Reactivity tests about phenol hydroxylation involved the variation of experimental parameters as temperature, time, substrate concentration and influence of light, keeping constant the phenol/Cu2+ molar ratio. The organic substrate was oxidized to cathecol, hydroquinone and p-benzoquinone and these products were quantified by deconvolution of the electronic spectra. As expected, the material synthesized without copper(II) ions does not show catalytic activity for the studied reaction.

ASSUNTO(S)

hdróxidos mistos lamelares mixed layered hydroxides

ACESSO AO ARTIGO

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