Insight into the properties of Fe oxide present in high concentrations on mesoporous silica

摘要

Oxide catalysts containing highly dispersed Fe phases supported on a mesoporous, high surface area silica, with iron in a wide range of concentration (4 < Fe_2O_3 mass% < 17), are presented. A suite of techniques was employed to determine the structural, morphologic, surface, electronic, acidic, and red-ox properties of the samples. Although the samples have great variation of the iron loading, they maintained good Fe-dispersion and low metal aggregation, even those with the highest concentrations of iron. By increasing the Fe-loading, the DR-UV-vis spectra showed that the band centered at 360 nm (low nuclearity 2d-Fe oxo entities) became more intense, while the band at 500 nm (typical of 3d-Fe_2O_3 nanoparticles) remained of low intensity and quite constant. Formation of significant amount of isolated Fe~(3+) centers (band at ca. 230 nm) were also identified, in agreement with EPR evidences. The titrated amount of surface acid sites increased with the Fe loading, because of the increased amount of FeO_x species, acting as Lewis acid sites. The test reaction of isomerization of α-pinene oxide revealed the prominent presence of Lewis acid sites on all the samples with main formation of the α-campholenic aldehyde product. The selectivity to α-campholenic aldehyde was around 53% for all the catalysts, independent of the Fe-loading. However, productivity to α-campholenic aldehyde increased with Fe-concentration, because of the increase of reaction rates as higher the Fe content was. Active acid Fe-sites could be associated with isolated Fe centers and in particular with low nuclearity 2d-Fe oxo entities.