Nanoesferas de sílica - otimização das condições de síntese e estudo de propriedades.

Abstract

The restriction in the application of molecular sieves in adsorption or catalytic process involving molecules with diameters higher than 0.8 nm, have lead to the study of solids possessing higher pore size. In that sense, pre - established porous solids might be obtained from the agglomeration of mono - disperse nanospheres, with the resulting porous system being a consequence of the nanospheres diameter and of their physical and chemical properties. In this work, silica-nanospheres were prepared through the tetraethyl-orto-silicate (TEOS) hydrolysis catalyzed by ammonia and in the presence of water and ethanol. In a first step it was developed an optimization of nanospheres synthesis conditions as a function of the temperature and the water and ammonia concentrations. In a later step, solids were prepared from the agglome ration of nanospheres by the liquid medium evaporation or centrifugation. The nanospheres diameter was determined using dynamic light scattering or scanning electron microscopy. The characterization of the agglomerates was realized by scanning and transmission electron microscopy, infrared spectroscopy, thermogravimetric analysis and adsorption/desorption measurements. The results showed that it is possible to synthesize spherical mono-disperse nanospheres with predefined diameter (40 180 nm) by controlling the temperature and water and ammonia concentrations in the synthesis medium. The preparation of nanospheres agglomerates by centrifugation leads to solids with regular array and body centered cubic or hexagonally compact organization. This type of arrangement for nanospheres smaller than 250 nm was favoured by the application of higher centrifugation rates and the obtained solids presented specific surface area higher than the calculated from the obtained diameter, which was attributed to the probable presence of microporous in the nanospheres. Nevertheless of the regular structure of nanospheres agglomerates, the thermogravimetric and infrared spectroscopy data showed that the nanospheres are linked by few Si-O-Si bonds, being the most of them linked by hydrogen bonds, this justifying its low mechanical strength.