Surface speciation and alkane oxidation with highly dispersed Fe(III) sites on silica

Abstract

When highly dispersed, supported Fe oxides are selective alkane oxidation catalysts, but new syntheses are required to reliably produce such materials. Here, highly dispersed, supported Fe3+ catalysts are prepared via incipient wetness impregnation of SiO2 with aqueous Fe complexes of ethylenediaminetetraacetic acid (FeEDTA), followed by calcination. With Na+ countercations, UV–visible diffuse reflectance spectra are entirely below 300nm and H2 temperature-programmed reduction only shows reduction at ∼630°C for all loadings up to 2.15wt%, the maximum loading for a single impregnation cycle. These characteristics indicate isolated sites not seen for Fe(NO3)3 precursors even at 0.3wt%. NH4+ countercations lead to amorphous oxide oligomers and a minority species with unusual reducibility at 310°C. Na+ countercations produce ‘single-site’ behavior in adamantane oxidation using H2O2 with a specific turnover frequency of 9.2±0.8ks−1, constant for all Fe loadings and approximately 10 times higher than that of other well-dispersed Fe/SiO2 materials. Similar turnover frequencies are obtained when counting only the highly reducible species on the NH4+-derived catalyst, allowing these sites to be tentatively assigned as small, undercoordinated clusters that are both easily reduced and participate in alkane oxidation, reminiscent of Fe-exchanged MFI zeolites.