Inoculação de cobalto em sílica mesoporosa do tipo MCM-41.

Abstract

The synthesis of mesoporous MCM-41 like materials have gained the interest of numerous research groups, due to their highly ordered structure of regular mesopores and high specific surface area, which results in a high potential as catalyst, catalyst support or as adsorbent in chemical processes. During their synthesis process the textural properties (porosity, surface area etc.) of these materials can be varied, thus allowing the tailor-made preparation of catalysts and adsorbents. If they are applied as support of catalytically active metals the interaction between the metal and the support influences significantly the catalytic properties. In this study the inoculation of a cobalt precursor (Co(NO3)2×6 H2O) into mesoporous silica obtained by deboronation of MCM-41 analogous borosilicates is investigated. The deboronation of this mesoporous structures allows enhancing the number of silanol groups on the surface of the mesoporous silica and permits the formation of catalysts with a stronger interaction between the support and the metallic species. The characterisation by X-ray diffraction (XRD), infrared spectroscopy (IR) and temperature programmed reduction (TPR) of the synthesised borosilicates, the deboronated silica and after the inoculation of cobalt, showed that the incorporation of boron into the structure is favoured in the presence of acetone. It is also observed that enhancing the ethanol concentration in the synthesis gels results during the synthesis in the precipitation of borates, which occasionally may inhibit the formation of the mesoporous MCM-41 structure. Furthermore it was found that the kind of the formed metallic species depends on the surface properties of the silica precursor, rather than on its textural properties. When cobalt nitrate is inoculated into highly hydrophilic mesoporous silicate part of the nitrate is not transformed into cobalt oxide after calcination at 550 °C for 5 h. In these cases TPR showed a significant part of the cobalt is present as extrastructural nitrate. When cobalt nitrate was inoculated into mesoprous silica with low hidrophilicity tPR combined with IR spectroscopy reveals that after calcination at 550 °C cobalt is present as tricobalttetraoxide at the mesopore surface. In all samples also cobalt species strongly interacting with silanol and silanolate groups at the mesopore surface are observed. Especially when cobalt was inoculated into highly hydrophilic silica, prepared by deboronation of borosilicate with high boron content in its structure a large number of different cobalt species coordinated to the silica surface was observed. It is known that cobalt with strong interaction with the support surface is active in the reduction of NOx or the oxidation of hydrocarbons, while cobalt oxides precipitated on the mesopore surface are precursors for catalysts active in Fischer- Tropsch synthesis. Therefore the results of this study may indicate suggestions for the preparation of tailor-made catalysts for different technical processes.