CHARACTERIZATION AND APPLICATIONS OF Fe-MESOPOROUS SILICA COMPOSITES

R. C. MERCADER; N. A. FELLENZ; L. A. CANO; F. J. BENGOA; S. G. MARCHETTI

Conferencia; 13th Latin American Conference on the Applications of the Mössbauer Effect; 2012

Over the last two decades a new family of mesoporous materials having a uniform arrangement of channels has been developed and extensively investigated. Different syntheses conditions lead to solids with diverse arrangement and dimensions of their pores, e.g., MCM-41, MCM-48, or SBA-15. When these matrices are loaded with iron salts, after suitable treatments, the solids obtained (made up of iron oxides or FeO beads embedded in the pores of nanometric dimensions) can be useful for multiple purposes. E.g., because of the magnetic properties of maghemite, γ Fe2O3, or magnetite, Fe3O4, the solids loaded with these oxides display the more promising properties for biomedical applications or high-density magnetic recording media. In addition, the matrices have a very large specific area (of the order of 1000 m2/g) and very uniform pore shape and dimensions. Hence, the solids are also prospective remarkable catalysts for numerous reactions. The hardest difficulty in the synthesis of these solids is to embed the Fe oxides in the channels and subsequently reduce them with hydrogen, CO, or a mixture of both, without the structure collapsing over the procedure. This must be done along very careful steps that depend heavily on the right choice of numerous operative variables. In previous works, we have already pointed out that the usual procedures to load these supports with iron salts lead to non-homogeneous filling of the pores with hematite. Since hematite is the precursor of the more promising magnetic oxides or metallic iron beads, we have tried several routes to achieve a uniform filling of the channels. One of the attempted routes was to perform a silylation treatment to the matrix in order to change the hydrophilicity of the pore walls and use an organic solution instead of the aqueous one that we use customarily. The result was an interesting solid that had a fraction of Fe3O4 nanoparticles that behaved like a paramagnet down to liquid helium temperatures but we didn?t get any proof that the pores filling uniformity had been improved. To investigate the still not fully known features of the composite preparation, we have studied recently the best conditions for using the materials as catalysts for the Fischer-Tropsch reaction to produce hydrocarbons from synthesis gas (mixture of CO and H2) after the reduction of the Fe oxides embedded in the channels of MCM-41 and SBA-15 matrices. Notwithstanding that the modification of the hydrophilicity of the matrix walls by a silylation treatment was not a successful method to homogenize the pores? filling with the iron species, it led, however, to a great improvement in the catalytic properties of the solid. In this talk, we will show how Mössbauer spectroscopy has been instrumental in the careful characterization of both systems, Fe/MCM-41 and Fe/SBA-15, allowing the relatively simple determination of the iron species formed after different impregnating methods and calcination atmospheres in the quest to obtain nanotubes and nanowires evenly embedded in the matrices pores. We will show that although the achievement of the original goals cannot be asserted unambiguously, the routes followed allowed obtaining excellent Fischer-Tropsch catalysts. In this respect, Mössbauer spectroscopy has enabled us determining the Fe species existing over the ?working? catalysts non-destructively and in the same atmosphere as the catalytic reactor. In this way, we could prepare catalytic solids capable of producing interesting distributions of hydrocarbons. The catalytic solids were tested not only in laboratory conditions but also in typical industrial operative conditions of 20 atm and 270 °C.