Avaliação do desempenho de catalisadores do tipo Pt-Re-Sn/Al2O3 na reforma de n-octano.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

Pt-Re-Sn/Al2O3-Cl catalysts were evaluated in the n-octane reforming to produce aromatics specially xylenes and were compared with the correspondent monometallic(Pt/Al2O3) and bimetallic (Pt-Re and Pt-Sn/Al2O3) ones. The samples were prepared by coimpregnation and successive impregnations of alumina, with the solutions of the metallic precursors. The solids were then dried (120oC), calcined (500oC, 4h, air) and reduced (500oC, 4h, H2). In the case of the successive impregnations, the solids were calcined and reduced after each metal addition. The rhenium containing catalysts were previously sulfided with carbon disulfide. The following samples were prepared (0.3% w/w of each metal): Pt/Al2O3, Re/Al2O3 and Sn/Al2O3; (Pt+Re)/Al2O3,(Pt+Sn)/Al2O3, (Re+Sn)/Al2O3 and (Pt+Re+Sn)/Al2O3, prepared by coimpregnation;Pt-Re/Al2O3, Pt-Sn/Al2O3, Pt-Re-Sn/Al2O3, Pt-Sn-Re/Al2O3, Re-Pt-Sn/Al2O3,Re-Sn-Pt/Al2O3, Sn-Pt-Re/Al2O3, Sn-Re-Pt/Al2O3, prepared by successive impregnations. The acid and metal sites of the catalysts were evaluated through the model reactions of n-pentane isomerization and cyclohexane dehydrogenation, respectively. The catalysts thiotolerance was evaluated in the cyclohexane dehydrogenation. The samples were also characterized by elementary analysis, temperature programmed reduction (TPR), Fourier transformer infrared spectroscopy (FTIR) of CO adsorbed, H2 chemisorption, transmission electron microscopy (TEM)and electron diffraction (ED). After the tests of n-pentane isomerization and n-octane reforming, the samples were analyzed by temperature programmed oxidation (TPO).It was found that the dehydrogenation activity of platinum was reduced by rhenium and/or tin and this effect was stronger with tin. The activation energies in the cyclohexane dehydrogenation were more affected by tin, that modified the active sites, producing other sites with lower activity. Both metals can alloy to platinum, as shown by TPR and ED experiments. These metals turned platinum richer in electrons decreasing its chemisorption capacity. This electronic effect was confirmed by FTIR of adsorbed CO. The electronic enrichment of platinum also affected the thiotolerance since the Pt-S bond became stronger. Therefore, the bimetallic catalysts showed a lower thiotolerance than the monometallic one, and the trimetallics were even more susceptible to poisoning by sulfur, due to the simultaneous action of the two metals on platinum. In the samples prepared by coimpregnation, the poisoning was more severe, because of the best contact among the metals. The acid function was also affected by the presence of tin and rhenium, mainly as oxides. In the trimetallic catalysts, positive effects were found when tin was added first, because tin oxide reduced the amount of stronger acidic sites of alumina, producing a more convenient acidity to the isomerization and cyclization reactions. Therefore, the Sn-Pt-Re-S/Al2O3 catalyst was the most promising sample in n-octane reforming, with the highest selectivity to xylenes, the highest stability and the smallest production of gases. The trimetallic coimpregnated catalyst showed a similar performance and has the additional advantage of not having to be previously sulfided.

ASSUNTO(S)

n-octane reforming desativação de catalisadores pt-re-sn/al2o3 pt-re-sn/al2o3 catalysts deactivation fisico-quimica reforma de n-octano

ACESSO AO ARTIGO

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