Physicochemical and catalytic properties of iron-doped silica—the effect of preparation and pretreatment methods
Introduction
Silica, besides alumina and zeolites, is often used as support for various transition metals. It has been also applied for iron oxide loading, for example, [1], [2], [3] and the obtained catalysts exhibit high activity in the gas-phase and liquid-phase oxidation processes. Among others Fe/SiO2 catalysts have been applied for partial oxidation of methane to formaldehyde [3], or partial oxidation of hydrogen sulfide to sulfur [4], [5], [6]. Recently, they have also been successfully tested in the liquid-phase hydroxylation of phenol [2]. Depending on the reaction phase (gas or liquid), various features of the active species influence the activity and selectivity. For instance in the liquid-phase oxidation processes the strong anchoring of Fe species on the support surface is necessary to avoid leaching of the iron-active phase. Partial oxidation of hydrocarbons in the gas phase usually occurs according to a radical mechanism and therefore, a good catalyst for this process should exhibit good electron-transfer properties. It is a general agreement that the reducibility of iron-active species plays a key role in the oxidation processes. It is influenced by various factors already discussed in the literature, such as the particle size of metal or oxides, concentration of iron species, and a type of oxides formed on the support. These features depend on the preparation methods as well as the atmosphere and temperature of the pretreatment of the materials before the catalytic reaction. Therefore, in this work we have applied three various iron-supporting procedures: chemical vapor deposition (CVD), and two various impregnations (wet and wetness). The aim of our study was the estimation of iron species formed on the prepared and calcined materials and species that is generated on the iron-modified silica surface after various activation conditions. The relationship between the nature of iron species and the catalytic activity has been shown.
Section snippets
Preparation of the catalysts
The parent material was silica (Ventron, 99.8%). The Fe modification was carried out in three ways.
Characterization of the calcined materials
Table 1 presents the fundamental data concerning the catalysts used in this study. The surface area of the iron-doped silica changed a little upon impregnation A and B (1.6 wt%). A higher decrease of the surface area is noted for the sample containing 2.9 wt% of iron and prepared via impregnation B and for that obtained with the CVD method. It illustrates that not only the amount of Fe species but also the technique of silica doping have an influence on the surface area of the final material.
Discussion
Recently, many investigations have been devoted to the characterization and catalytic use of Fe–silicalite. Depending on the desired catalytic application the location of iron in the zeolite framework can be useful or not. It is well known that Fe becomes active in oxidation reactions only after dislodgement from the framework position [26]. Taking that into account one can expect that grafting of iron species on a silica surface would be a useful method for production of active catalysts. The
Acknowledgements
This work was partially supported by the Polish Research Foundation—KBN Grant 3 T09A 102 19.
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