Influência do método de preparação nas propriedades do óxido de ferro suportado. / Influence of preparation method on properties of iron oxide supported.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

Styrene is one of the most important basic chemicals to produce valuable commodities such as polystyrene, acrylonitrile-butadiene-styrene and styrene-butadiene-styrene. It is commercially produced by the dehydrogenation of ethylbenzene in the presence of large amounts of steam at high temperatures of 600-700 oC. This process is thermodynamically limited and energy consuming. The use of an oxidant, such oxygen, allows to overcome the thermodynamic limitation and consequently to operate at lower temperatures with an exothermic reaction. However, a significant loss of styrene selectivity is found and thus another oxidant been for a long time. The use of carbon dioxide emerges as a potential oxidant besides the convenience of use a global warming gas. In order to find an alternative catalyst to this reaction, the effect of the preparation method on the properties of supported iron oxide was studied in this work. Samples were prepared by adding iron oxides on lanthana, niobia, titania, magnesia and zirconia by two methods: the impregnation of iron nitrate and the deposition of iron nanoparticles previously prepared. The solids were calcined at 600C for 4 h and characterized by chemical analysis, thermogravimetry, differential thermal analysis, X-ray diffraction, specific surface area and porosity measurements and temperature-programmed reduction. The catalysts were evaluated in the ethylbenzene dehydrogenation in the carbon monoxide presence using a microreactor operating at 1 atm and at 600 oC and a carbon dioxide to ethylbenzene molar ratio of 10. After the tests, the samples were characterized by X-ray diffraction and surface area and porosity measurements. It was found that the amount of incorporated iron oxide depends on the kind of the support and on the method used to add the iron compound. Hematite was found in fresh catalysts. During the ethylbenzene dehydrogenation, hematite changed to magnetite but the supports did not change. Both the supports and the catalysts were found to be macroporous with limited mesoporosity. The surface area changed due to iron oxide addition and to the preparation method. The deposition of the nanoparticles led to an increase of the specific surface area, regardless the kind of the support, a fact which was assigned to the small size of the particles. The specific surface areas were kept constant during the reaction. The solids showed different resistance against reduction depending on the support and on the method of adding the iron compound. As whole, the impregnation of iron nitrate produced less reducible solids. The supports were found to be catalytically active in the ethylbenzene dehydrogenation in the presence of carbon dioxide and also were selective to styrene. The addition of iron compounds improved these properties and the deposition of nanoparticles improved them even more. The magnesia-supported iron oxide, prepared by the deposition of nanoparticles, was the most active (2.8 x 10-3 mol.g-1.h-1) and selective catalyst to styrene (96%) in the ethylbenzene dehydrogenation in the presence of carbon dioxide. Its activity and selectivity were higher than a commercial catalyst based in iron, chromium and potassium oxide (a=1.2 x 10-3 mol. g-1. h-1 and S=90%) and then the catalyst is promising to the reaction.

ASSUNTO(S)

desidrogenação do etilbenzeno catalisadores de óxido de ferro suportados catalysts iron oxide supported dehydrogenation ethylbezene quimica

ACESSO AO ARTIGO

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