Effect of Oxidation Step on the Properties of Germanium-Containing Catalysts for Naphtha Reforming

M.C. RANGEL; K.C.S. CONCEIÇÃO; J.M. GRAU; C.L. PIECK; V.A. MAZZIERI; J.L.G. FIERRO; L.S. CARVALHO

Germanium: Characteristics, Sources and Applications

Nova Science Publishers

Lugar: New York; Año: 2013; p. 141 - 168

Germanium is widely used as component in electronic and optical devices and has found several applications in metallurgy, chemotherapy, nanotechnology and catalysis. Germanium oxide, for instance, is used as polymerization catalysts to produce polyethylene terephthalate. Germanium has also been found to catalyze other reactions, such as n-butane dehydrogenation, selective hydrogenation of citral and catalytic naphtha reforming. This process is the most important commercial one to obtain high octane gasoline and high value petrochemicals, such as benzene, toluene and xylenes. Several changes on the operation variables as well as on the catalysts were performed over the years, improving the process. The first generation of reforming catalysts was alumina-supported platinum which later gave rise to both second and third generation of catalysts by the addition of a second and of a third metal, respectively. Germanium has played an important role for both bimetallic and trimetallic catalysts leading to more selective and stable catalysts, which were also resistant against deactivation by coke. For trimetallic platinum-based catalysts, germanium delays platinum sintering decreasing the production of methane and light alkanes and improving the stability and selectivity to isomers reducing the aromatics formation. All these properties are strongly influenced by the preparation methods, including the calcination and reduction steps as well as the order of the metal addition. In order to improve the properties of Ge-Pt-Re/Al2O3 catalysts, the effect of heat treatment on their surface properties was studied in this work. Catalysts were prepared by successive impregnation of commercial -alumina with solutions of metal precursors. The impregnated solids were oxidized (O) with air and reduced (R) with hydrogen at 500 °C, after the addition of germanium, only reduced after the addition of platinum and only oxidized or reduced after the addition of rhenium. Monometallic, bimetallic and trimetallic catalysts were characterized by temperature programmed reduction, temperature programmed desorption, X-ray photoelectron spectroscopy, cyclohexane dehydrogenation, cyclopentane hydrogenolysis and n-hexane isomerization and evaluated in naphtha reforming. For trimetallic samples, the solid oxidized after the addition of rhenium (GeO,RPtRReO) showed the strongest interaction among the metals and with the support. The solid oxidized after rhenium addition (GeO,RPtRReO) presented a decrease in platinum and germanium on the surface, after activation, while the opposite occurred with the solid not previously reduced (GeO,RPtRReR). The monometallic and the PtRReO samples were more active in cyclohexane conversion while the GeO,RPtRReR was the least active one, this tendency is closely related platinum amount on the surface. For the trimetallic sample, it is probably that some germanium was covering or alloyed to platinum, inhibiting its dehydrogenating capacity. The samples oxidized after impregnating rhenium showed higher dehydrogenation activity (desirable) and lower hydrogenolysis activity (undesirable), compared to those doubly reduced after rhenium addition. Therefore, the addition of germanium and the oxidation step favors platinum dispersion for trimetallic samples. We can conclude that different ways of heating during catalysts preparation led to different metallic distributions on the surface, casing different catalytic activity.