Sintese e caracterização de ferrissilicatos mesoporosos

AUTOR(ES)
DATA DE PUBLICAÇÃO

2002

RESUMO

MCM-41 structured mesoporous ferrisilicates were synthesized by polymerization of sodium silicate over supramolecular arrays of CTABr, used as structure-directing agent. Alternatively, tetramethylammonium silicate was used as source of silicate. The condensation of the silicate was stimulated by decreasing the pH with one of the following acids: hydrofluoric, hydrochloric, acetic, nitric and hydroiodic. The influence of the following reaction parameters was studied: CTABr/silicate molar ratio, the nature of acids used to polimerize the silicate and silicate source. Si-NMR studies of the solution, during the previous stages of the synthesis, under variable concentrations of structure-directing agent and Fe(III), showed that the quantity of CTABr influences the velocity of precipitation of silicate, specially the less basic species and that the Fe(lll) modified the basicity of these species which react with the more polimerized ones with high charge density. The X-ray diffraction showed that the obtained structure depends on the changed parameter. In the systems studied, several phases were synthesized: the hexagonal structure, the desordered mesoporous, phases mixtures, magadiite and amorphous materiaIs. Independentily of the silicate source, the best CTABr/silicate molar ratio to prepare the hexagonal structure was 0,11. The mesoporosity of materiaIs was confirmed by nitrogen adsorption at 77K. Electron paramagnetic resonance and UV-Vis spectroscopies indicated that Fe(lll) was in tetrahedral (framework) and octahedral positions (extra framework). The pyridine adsorption showed that tetrahedral Fe(lll) forms weak Brönsted sites that was confirmed by acid catalysis. The mesoporous ferrisilicates were more active for benzene alkylation with benzile chloride, which indicates expressive quantities of extra framework Fe(III).

ASSUNTO(S)

polimerização minerais silicatados reações quimicas

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