Aqueous-Phase Methane Oxidation over Fe-MFI Zeolites; Promotion through Isomorphous Framework Substitution

Abstract

Fe and Cu-containing zeolites have recently been shown to be efficient catalysts for the one-step selective transformation of methane into methanol in an aqueous medium at only 50 oC, using H2O2 as green oxidant. Previously, we have observed that Fe species alone are capable of catalysing this highly selective transformation. However, further catalytic testing and spectroscopic investigations demonstrates that although these extra-framework Fe species are the active component of the catalyst, significant promotion is observed upon the incorporation of other trivalent cations e.g. Al3+, Ga3+, into the MFI-framework. Whilst these additional framework species do not constitute active catalytic centres, promotion is observed upon their incorporation as they (1) facilitate the extraction of Fe from the zeolite framework, and hence increase the formation of the active Fe species, and (2) provide an associated negatively-charged framework, which is capable of stabilizing and maintaining the dispersion of the cationic extra-framework Fe species responsible for catalytic activity. By understanding these phenomena, and subsequently controlling the overall composition of the catalyst (Fe and Al), we have subsequently been able to prepare a catalyst of equal intrinsic activity (i.e. TOF) but five-times higher productivity (i.e. space-time-yield) to the best catalysts reported for this reaction to date.