Maximizing Active Fe Species in ZSM-5 Zeolite Using Organic-Template-Free Synthesis for Efficient Selective Methane Oxidation.

The selective oxidation of CH 4 in the aqueous phase to produce valuable chemicals has attracted considerable attention due to its mild reaction conditions and simple process. As the most widely studied catalyst for this reaction, Fe-ZSM-5 demonstrates high intrinsic activity and selectivity; however, Fe-ZSM-5 prepared using conventional methods has a limited number of active Fe sites, resulting in low CH 4 conversion per unit mass of the catalyst. This study reports a facile organic-template-free synthesis strategy that enables the incorporation of more Fe into the zeolite framework with a higher dispersion degree compared to conventional synthesis methods. Because framework Fe incorporated in this way is more readily transformed into isolated extra-framework Fe species under thermal treatment, the overall effect is that Fe-ZSM-5 prepared using this method (Fe-HZ5-TF) has 3 times as many catalytically active sites as conventional Fe-ZSM-5. When used for the selective oxidation of CH 4 with 0.5 M H 2 O 2 at 75 °C, Fe-HZ5-TF produced a high C 1 oxygenate yield of 109.4 mmol g cat -1 h -1 (a HCOOH selectivity of 91.1%), surpassing other catalysts reported to date. Spectroscopic characterization and density functional theory calculations revealed that the active sites in Fe-HZ5-TF are mononuclear Fe species in the form of [(H 2 O) 3 Fe(IV)═O] 2+ bound to Al pairs in the zeolite framework. This differs from conventional Fe-ZSM-5, where binuclear Fe acts as the active site. Analysis of the catalyst and product evolution during the reaction suggests a radical-driven pathway to explain CH 4 activation at the mononuclear Fe site and subsequent conversion to C 1 oxygenates.