Instituto de Química
URI Permanente desta comunidadehttps://repositorio.ufrn.br/handle/123456789/31488
Navegar
Navegando Instituto de Química por Autor "Alves, Maria do Carmo Martins"
Agora exibindo 1 - 2 de 2
- Resultados por página
- Opções de Ordenação
Artigo Effect of textural characteristics of supported metallocenes on ethylene polymerization(Springer, 2010-01-06) Silveira, Fernando; Brambilla, Rodrigo; Silveira, Nadya Pesce da; Alves, Maria do Carmo Martins; Stedile, Fernanda C.; Pergher, Sibele Berenice Castellã; Santos, João Henrique Zimnoch dosCatalyst particle size is an important parameter in the polymer industry. A series of supports with different textural properties (silicas prepared by different routes), surface sites (alumino-silicate, silica-zirconia, magnesia-silica supports, and methylaluminoxane-modified silica) and crystallinity (amorphous, lamellar, and crystalline materials) were investigated in the sequential grafting reaction of two metallocene catalysts, Cp2ZrCl2 and (n-BuCp)2ZrCl2, in a 1:3 ratio. The catalyst systems were analyzed by Rutherford backscattering spectrometry, atomic force microscopy, small angle X-ray scattering, extended X-ray absorption fine structure spectroscopy, and nitrogen adsorption. All of the systems were shown to actively polymerize ethylene when methylaluminoxane was used as the cocatalyst. Experimental results are discussed in terms of the relationships between the radius of gyration of secondary particles and catalyst activity. A radius of gyration in the range of 8–9 nm was shown to be associated with higher catalyst activity.Artigo Microporous and mesoporous supports and their effect on the performance of supported metallocene catalysts(Elsevier, 2010-01-15) Silveira, Fernando Tobias; Alves, Maria do Carmo Martins; Stedile, Fernanda Chiarello; Pergher, Sibele Berenice Castellã; Santos, João Henrique Zimnoch dosA series of hybrid-supported catalysts was prepared by sequentially grafting Cp2ZrCl2 and (nBuCp)2ZrCl2 (1:3 ratio) onto synthesized silica–zirconia xerogel, alumino-silicates, alumina, chrysotile and commercial MAO (methylaluminoxane)-modified silica. Supported catalysts were characterized by Rutherford backscattering spectrometry, atomic force microscopy, extended X-ray absorption fine structure spectroscopy, X-ray diffraction and nitrogen adsorption. The grafted metal content was between 0.21 and 1.00 wt.% Zr/SiO2 or Zr/Al2O3. All of the systems were shown to be active in ethylene polymerization with methylaluminoxane as the cocatalyst. The catalyst activity and polymer molecular weight depended on the effects of the textural characteristics of the supports on the structure of the generated supported catalyst species. The highest activity in ethylene polymerization (ca. 6560 kgPE molZr−1 h−1) was reached with the supported catalyst using commercial MAO-modified silica and also presented the lowest surface roughness. Correlation between EXAFS data and polymer characteristics was extracted: the production of polyethylenes with higher molecular weight was associated with the reduction in the interatomic Zr–O distance of the supported catalysts, which was shown to be dependent on the nature of the support.