Structural and electronic promotion with alkali cations of silica-supported Fe(III) sites for alkane oxidation

Promoters and precursors can control oxide phase, dispersion, and per-site reactivity of supported oxide catalysts. Previously, dispersed FeOx–SiO2 resulted from Fe3+ ethylenediaminetetraacetate (FeEDTA−) precursors, with NaFeEDTA giving enhanced dispersion and oxidation rates vs. NH4FeEDTA. Here, catalysts were synthesized by sequential alkali deposition and Fe3+ impregnation. At up to 0.9 Fe nm−2 from NH4FeEDTA and equimolar alkali, UV–visible and H2 TPR were consistent with isolated Fe3+ and small FeOx clusters. Omitting alkali, using Fe(NO3)3, or using Fe/alkali >1 gave evidence of larger agglomerates. For Fe/alkali ≤1 on non-porous SiO2, initial turnover frequencies in adamantane oxidation using H2O2 were independent of surface density. TOF increased as 6.3, 8.8, 15.4, and 20.9 (±0.3) ks−1 for Li+, Na+, K+, and Cs+, respectively, increasingly linearly with decreasing electronegativity. These results give a synthesis–structure–function taxonomy with alkali as an electronic and structural promoter of dispersed FeOx species for alkane selective oxidation.

Use of stoichiometric or greater alkali promoters and large, chelating ligands like ethylenediaminetetraacetate (EDTA) results in highly dispersed FeOx on silica, with high rates independent of surface density in adamantane selective oxidation with H2O2. This work amplifies the set of synthesis–structure–function relations for supported FeOx catalysts.Figure optionsDownload high-quality image (83 K)Download as PowerPoint slideHighlights
► High dispersion FeOx–SiO2 from stoichiometric alkali and NH4Fe(III)EDTA.
► Adamantane oxidation rates with H2O2 independent of surface density.
► TOF increased >3× with decreasing Sanderson electronegativity of alkali.
► Alkali as both structural and electronic promoter.
► Absence of crystalline Fe2O3 insufficient predictor of high catalytic activity.