Efeito das condições de preparação sobre as propriedades do óxido de zircônio sulfatado contendo ferro.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

Sulfated oxide-based catalysts have found several industrial applications mainly in reactions which demand for strong acid sites. In order to develop methods to get acidic oxides, the effect of the preparation methods on the properties of iron-doped zirconia oxides was studied in this work.The precursors were prepared by process sol-gel from iron nitrate and zirconium oxychloride at room temperature. After drying, the solids were impregnated with aqueous solutions of sulfuric acid (0.25 and 0.5mol.L-1), followed by calcination at 500 and 550 C. Samples with Fe/Zr= 0.2; 0.4 and 0.8 molar ratio were produced besides pure iron oxide and zirconia. The solids were characterized by chemical analysis, Fourier transformer infrared spectroscopy, X-ray diffraction, thermal analysis (DTA, TG), specific surface area measurements, temperature programmed reduction, acidity measurements (ammonia TPD) and X-ray photoelectron spectroscopy.Both tetragonal and monoclinic phases were produced in the zirconium-based samples whereas hematite was found in the iron oxide-based samples. By sulfating the solids, however, the monoclinic phase disappeared while the tetragonal phase was stabilized. On the other hand, hematite was not affected by the sulfate ions. The presence of iron in zirconia also stabilized the tetragonal phase regardless the presence of sulfate species. When the amount of iron in solids reached Fe/Zr=0.8, hematite segregated and co-existed with the tetragonal phase. The specific surface areas of zirconia increased by the addition of iron and were affected by the sulfate ions but no regular tendency was noted as a function of the preparation variables. Zirconia showed a higher ability in accepting and stabilizing the sulfate ions in its structure as compared to iron oxide. The zirconia was stable in reducing conditions in the range of 25-1000oC but after sulfating the solids lost sulfate species above 500oC. Iron oxides, however, underwent reduction above 350oC regardless the presence of sulfate ions. Pure zirconia was more acidic than pure hematite. The addition of iron to zirconia did not significantly affect the number of active sites, except in the sample with Fe/Zr(molar)= 0.2 calcined at 500oC and treated with a 0.25mol.L-1 sulfuric acid solution. As a whole, the most acidic solids area were produced by using these conditions. By sulfating zirconia one gets only strong acidic sites. However, the addition of iron generates moderate acidic sites and this effect increased with the amount of iron in solids. On the other hand, iron oxide itself produced acidic sites with different strengths. The superficial composition also changed with the kind of solid and with the preparation method. In general the iron and sulfur contents were lower on the surface than in the bulk. It was also noted that the amount of iron on the solid surface decreased due to sulfation. However, solids with higher amounts of iron were able to retain more sulfur on the surface. The best condition to get acidic zirconia-based solids is to prepare iron-doped zirconium hydroxide (Fe/Zr=0.2), followed by impregnation with a 0.25mol.L-1 sulfuric acid solution and calcination at 500oC. This material has high specific surface area (223 m2.g-1) being promising for catalytic applications.

ASSUNTO(S)

óxido sulfatado sulfated irom oxide quimica sulfated solid iron-doped zirconia óxido de zircônio dopado com ferro sulfated oxide óxido de ferro sólido sulfatado

ACESSO AO ARTIGO

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