Synthesis and catalytic properties of nanostructured aluminas obtained by sol-gel method

Alumina was synthesized by using the sol-gel method. In order to study the aging effect gels, they were aged for 1, 14, 30, and 50 days. Samples were characterized with X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) with pyridine adsorption, nitrogen physisorption, and by their catalytic activity for cyclohexene isomerization. XRD of boehmite samples did not show the (020) reflection, indicating that the double layers characteristic of its structure did not correlate between each other, and the unit cell was not built. The diffraction pattern showed two new atom ordering at low angle regions corresponding to distances of 1.3 and 3 nm. When samples were transformed into gamma-alumina the atom ordering of 3 nm was maintained in all samples. In the one aged for 30 days, two additional diffraction peaks were observed, which correspond to distances between 5 and 6 nm. The specific surface area of the samples aged for 1, 14, and 30 days remained around 350 m(2)/g. This area, however, was 307 m(2)/g for the sample aged for 50 days. The highest catalytic activity for cyclohexene isomerization was obtained with the sample aged for 30 days, whereas the lowest activity was obtained with the sample aged for 1-day. The intrinsic activity correlated linearly with the Lewis acid site density, which at the same time correlated with the pore volume and size and the largest atom ordering distance, suggesting that the catalytic cyclohexene isomerization reaction on aluminas depends on the Lewis acid site density, which is favored with the aging time treatment. (C) 2004 Elsevier B.V. All rights reserved.