Efeito da concentração de irídio sobre as propriedades de catalisadores do tipo Pt-Ge-Ir/Al2O3. / Effect of concentration of iridium on the properties of type catalysts Pt-Ge-Ir/Al2O3.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The effect of iridium content on the activity, selectivity and stability of platinum, germanium and iridium supported in alumina catalysts for oil naphta catalytic reforming was studied in this work. Samples were prepared by the impregnation of the metallic salts (hexachloroplatinic acid, hexachoroiridic acid and germanium chloride) over g-alumina in order to produce 0.3 % platinum and germanium and different iridium contents (0.03; 0.06; 0.09 and 0.15 %). The solids obtained were characterized by means of thermoprogramed reduction, Fourier transform infrared spectroscopy of carbon monoxide adsorbed, piridine thermoprogramed dessorption, cyclohexane dehydrogenation and n-pentane isomerization in order to evaluate the metallic and acid properties, respectively. The performance was evaluated in n-octane reforming. The catalysts used in this reactin were evaluated by means of thermoprogramed oxidation. The results showed that iridium catalyzes platinum reduction whereas germanium has an inhibiting effect and those effects depend on the iridium content in the solids, due to the different interactions among metals and with the support. Both metals modify the electronic properties of platinum, by making it richer (iridium) or more defficient (germanium) in electrons. However, for the trimetallic catalysts, the germanium effect is significant only at higher iridium concentrations (0.15 %), which promotes germanium reduction, producing solids with platinum species in different electronic states. Germanium presence reduces weak acid sites concentration and increases the ones with moderate and strong acidity while iridium produces changes in the concentration of sites of different acidic strengths, depending on its solids content. The catalytic activity of metallic sites in cyclohexane dehydrogenation decreases due to germanium while iridium (in contents higher than 0.03 %) leads to an increase due to its dehydrogenating activity and this effect increases with the iridium content in the solids. In the trimetallic catalysts the activity of metallic sites is lower than in the monometallic platinum system due to the simultaneous effect of both metals which can produce alloys with platinum and/or can cover its active sites thus decreasing its dehydrogenation activity. Iridium and germanium addition to platinum catalysts supported on alumina increases their activity over n-pentane isomerization and decreases hydrogenolisis activity. For the trimetallic catalysts, the selectivity to aromatics is lower than that of bimetallic systems and o-xylene is the most favored isomer. Germanium increases the catalysts stability while iridium performs the opposite role. The production of aromatics decreases with the increase of iridium content in the catalyst while the opposite behavior is observed regarding the selectivity to isomer products. The ratio between the selectivity to isomer and aromatics increases with iridium content so that if a reformate product richer in aromatics is desired, catalysts with low iridium contents should be used. On the other hand, a catalyst with higher iridium content produces a reformate richer in isomer compunds and therefore more suitable to be used as fuel.

ASSUNTO(S)

pt-ge-ir quimica catalytic reforming of naphtha reforma catalítica de nafta catalisadores bifuncionais catalisadores trimetálicos pt-ge-go bifunctional catalysts trimetallic catalyst

ACESSO AO ARTIGO

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