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

Abstract

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 0.9Fenm−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.