The Low-Temperature Oxidation of Propane by using H2O2 and Fe/ZSM-5 Catalysts: Insights into the Active Site and Enhancement of Catalytic Turnover Frequencies

Fe-containing ZSM-5 catalysts are reported to be efficient catalysts for the partial oxidation of propane to oxygenated products at reaction temperatures as low as 50 °C in an aqueous phase reaction when using the green oxidant H2O2. It was previously proposed that extra framework Fe species at the exchange sites of the zeolite are responsible for activation of both the alkane and hydrogen peroxide. Through a systematic study of the influence of framework topology and exchange properties, it is now shown that this high catalytic activity is specific to the MFI-type Brønsted acidic zeolite ZSM-5. Furthermore, through a simple aqueous acid washing treatment, leaching of approximately 77 % of iron present within a Fe/ZSM-5 catalyst only caused the relative propane conversion to decrease by 17 %; implying that most of the initially loaded Fe does not actually contribute to the catalytic activity. This small change in conversion after ‘excess’ Fe removal, amounts to a three-fold increase in turnover frequency (TOF) (Fe) from 66 h−1 to 232 h−1 compared with the parent Fe/ZSM-5 catalyst. By comparing these samples, it is shown by NH3 temperature-programmed desorption, 27Al magic angle spinning NMR spectroscopy, X-ray photoelectron spectroscopy and TEM analysis that surface iron oxide species are effectively spectators in the oxidation of propane with H2O2. This provides further insight as to the location and true nature of the catalytically active Fe species.