Estudo exploratório da síntese de catalisadores à base de óxido de nióbio usando surfactantes por rota sol-gel

Abstract

Most industrial processes for the synthesis of chemical substances involve the use of catalysts, and the replacement of homogeneous catalysts with heterogeneous ones, which are easier to separate and reuse, has been a global trend. In this context, it is necessary to develop solids with peculiar catalytic properties, and, as Brazil is the world's largest producer of niobium, developing technology for the synthesis of catalysts containing this element is strategic for the nation. Niobium oxide (niobia) is an acidic solid, but the commercial material has a modest specific area, which may limit its use as a catalyst. One strategy used to obtain materials with a high area involves the use of routes that produce materials with a more open pore structure, such as sol-gel synthesis. There are only a few studies on the synthesis of mesoporous niobia obtained from surfactants, and the niobium precursors used are expensive and sensitive to air, making the process more expensive. Based on the above, the objective of this work is to investigate the synthesis of niobium-based solid acid catalysts via the sol-gel route, using amines as structure drivers, to generate materials with better textural properties than commercial ones. As an innovation, the niobium oxaloammonium complex was used, which is cheaper and easier to handle than niobium pentachloride and niobium pentaethoxide. The hydrothermal synthesis was carried out in an autoclave, using tetradecylamine and dodecylamine as structure drivers, and their removal was investigated. To increase acidity, three materials were sulfated with sulfuric acid. The materials involved were characterized by nitrogen adsorption, X-ray diffraction, thermogravimetry and infrared absorption spectroscopy. Some materials were tested in the esterification of acetic acid with ethanol at 1 atm and 70 °C, as it is a simple model reaction that fits among those used in the processing of bio-oil. The results indicated that it was possible to synthesize a mesoporous niobia from the ammoniacal niobium complex using tetradecylamine, although the surfactant removal procedure was critical. However, when dodecylamine is used, the results show that the same porous structure was not obtained, despite the areas being larger than that of commercial niobia, and surfactant removal is more efficient by combining acid washing and heat treatment. The synthesized niobias did not show activity in the esterification of acetic acid under the conditions used, but the sulfated materials were active and one of the catalysts had a conversion of almost 40% after 2 h of reaction, even using 0.5% by mass of the catalyst, opening the possibility of using these materials as acid catalysts.