Influência de condições de síntese de sílica sol-gel sobre sua estrutura mesoporosa e comportamento de adsorção de azul metileno

Abstract

The present work shows simple synthesis routes for the preparation of sol-gel silica samples with adsorbent properties. For the obtained samples, the pore structure and the methylene blue (MB) organic dye adsorption capacity were evaluated. The synthesized materials wereexamined by a number of characterization techniques, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (DRX), nitrogen adsorption and desorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the first condition of preparation, it was shown that the pore structure and the adsorption capacity of the silicas can be altered by adjusting the pH of the sol. The results showed that the samples obtained from sols with low pH had a small pore size, whereas the one prepared in pH close to neutral, presented a larger pore size favoring the adsorption capacity. In the second condition, with the H2O:TEOS molar ratio increase there was an increase of the specific surface area and a reduction in the porosity of the material, but, in the adsorption tests with the dye, no difference was observed. Among the heat treated silicas, one of them had itsstructural properties significantly affected which decreased the dye adsorption capacity. In the last evaluated condition, it was observed that the F127 addition, as a pore organizer agent, gave rise to two distinct mesoporous morphologies, one with disordered pore networks andthe other with a disordered pore network and wormlike appearance. It is suggested that the formation of the disordered structures is associated with the lack of water in the initial sol-gel solution. It has also been noted that the use of F127 in the preparation of the samples, whichare acid free, did not lead to materials with high surface areas and adsorption capacities. On the other hand, the use of F127 in the preparation of samples, which contain acid, led to materials with improved surface areas and adsorption capacities. In general, the kinetic model of pseudo-second order was the best model to represent the experimental kinetic data of the investigated samples, indicating a high interaction between the silica surface and the dye molecules. Therefore, the samples with the highest adsorption capacities were those onesprepared without the use of acidic catalyst, heat treatment or F127, indicating a cheaper and greener synthesis.