Síntese de nanopartículas de cobalto suportadas em peneiras moleculares mesoporosas para a síntese de Fischer-Tropsch

• Ano de defesa: 2011
• Autor(a) principal: Zola, Andréia Soares
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Estadual de Maringá; Brasil; Departamento de Engenharia Química; Programa de Pós-Graduação em Engenharia Química; UEM; Maringá, PR; Centro de Tecnologia
• Palavras-chave em Português: Síntese de Fischer-Tropsch; Nanopartículas; Cobalto; MCM-41; Brasil.; Engenharias; Engenharia Química
• Link de acesso: http://repositorio.uem.br:8080/jspui/handle/1/3679

The Fischer-Tropsch synthesis (FTS) is the hydrogenation of carbon monoxide carbon using metal catalysts. To obtain products in the range of diesel, the most used metal is cobalt. Besides addition to a higher selectivity to long chain hydrocarbons, the catalysts should provide a good metallic dispersion. In this sense, nano catalysts supported on molecular sieves mesoporous could offer a good metal dispersion, and direct the formation of products with longer chains. Thus, this work aimed at preparation and evaluation of cobalt nanoparticles (Co- NPs) in the FTS. For this, the incorporation of nanoparticles into MCM-41 mesoporous molecular sieves was performed at different times of synthesis and by incipient wetness impregnation. Also samples were prepared by adding the metal precursor during the synthesis of porous matrix, on non-porous silica and by the wet impregnation method for comparison. Samples were prepared, too, with the silylated supports. Initially, cobalt nanoparticles were prepared by three methods in the literature: decomposition of organometallic, polyol and borohydride reduction. We opted for the use of nanoparticles prepared by decomposition of organometallic, which showed a narrower size distribution. The X-ray diffraction indicated the predominance of Co3O4 in samples prepared by wet impregnation and suggested a reduction in the organization of mesoporous channels when the precursor is added during MCM-41 synthesis and after the silanization of the support. The N2 adsorption/desorption isotherms of MCM-41 samples showed that the incorporation of cobalt leads to a small pore blockage. Analysis of Temperature Programmed Reduction (TPR) showed that the process of silanization does not anticipate the reduction process, suggesting that these samples have a strong metal-support interaction. Analysis of Temperature Programmed Desorption showed that only the samples prepared by wet impregnation on silica showed predominance of species less strongly linked to the support. DRS analysis showed the predominance of cobalt oxides in the samples prepared by wet impregnation and a little oxidation in the samples prepared by incipient wetness impregnation of Co- NPs. Thus, there is the possibility of formation of silicates during the analysis of RTP and DTP, due to the presence of moisture in the gas line or the samples themselves. The samples prepared by the addition of cobalt during the synthesis of MCM-41 showed only peaks associated with cobalt silicate. The High Resolution Transmission Electron Microscopy showed the presence of small particles of cobalt, from about 2 nm, probably formed due to the reduced size of the pores of the support. Analysis of X-rays Absorption Spectroscopy (TPR/XANES) indicated the predominance of Co3O4 in all the samples prepared by impregnation. The curves showed that the transition between Co3O4 and CoO in the samples prepared on MCM-41 occurs very slowly, indicating a strong metal-support interaction and agreeing with the previous results. The samples prepared on non-porous silica, in turn, presented species with weaker interactions. Evaluation of catalysts in the Fischer-Tropsch Synthesis indicates that samples prepared by wet impregnation show higher overall conversion of carbon monoxide. In particular, Co/MCM-41 and Co/SiO2(sil) samples showed the best results, indicating that the decrease in porosity of MCM-41 after silanization is an important factor in relation to its performance in the reaction. The product distribution of the Co/SiO2 sample was very similar to catalyst standard, optimized for the production of long chain hydrocarbons. The factor of chain growth ((α)) of samples prepared by impregnation values range from 0.72 to 0.83. The results may be related to the percentage of species Con+, consisting of oxides with stronger interaction with the support, reducible between 400-500 °C. The conversion of CO decreases linearly with the increase of these species, while the value of α increases exponentially with the percentage of species Con+. These results indicate that the good performance of a catalyst for FTS depends on several interrelated factors, and that the choice of support appears to be the most important factor in the preparation and optimization of catalysts for FTS.