Insights into Fe Species Structure‐Performance Relationship for Direct Methane Conversion toward Oxygenates over Fe‐MOR Catalysts

Abstract

Direct oxidation of methane to value-added products under mild conditions remains a grand challenge due to the problem of selective CH 4 activation. Here we report a Fe-MOR-F catalyst prepared via freeze-drying method, indicating excellent catalytic performance for the direct partial oxidation of methane using H 2 O 2 under mild conditions. Various characterizations including XRD, NH 3 -TPD, H 2 -TPR, UV-Vis, XPS, and EPR studies revealed that Fe-MOR-F catalyst possesses more highly dispersed monomeric and dimeric iron species, which is strongly correlated to its high catalytic activity for direct methane oxidation. The DFT calculations indicate the highly dispersed monomeric and dimeric iron species can effectively homoge-nize H 2 O 2 into active hydroxyl groups, while iron clusters can decompose H 2 O 2 into inactive O 2 . The EPR experiments show more •OH was generated on Fe-MOR-F catalyst, which is in line with DFT calculation results. These structural characteristics endow Fe-MOR-F catalyst with superior catalytic performance for direct methane oxidation in terms of oxygenates yield (9.8 mol kg cat À 1 h À 1 ) and selectivity (91.8 %) at 80 °C. This work not only highlights the excellent catalytic performance of methane activation on the highly dispersed monomeric and dimeric iron species, but also promotes the potential application by freeze-drying method for the preparation of the highly dispersed active species.

Single Atom Catalysts for Selective Methane Oxidation to Oxygenates

“…On the contrary, Fang et al, based on DFT and spectroscopic analysis, demonstrated that monomeric species can form •OH and •OOH resulting in the formation of CH 3 COOH, while dimeric species produce CH 3 OH via •OH radicals. 237 Yu et al also showed, three synthesis routes named incipient wetness impregnation (IWI), liquid ion-exchange (IE), and solid-state ion-exchange (SSIE) for the synthesis of Fe-ZSM-5 among which SSIE had the highest (71%) population of monomeric species as determined by 57 Fe Mossbauer spectroscopy. 238 Compared to IWI and IE samples, the Fe/ZSM-5 SSE yielded the highest CH 3 OH productivity and selectivity demonstrating the direct role of monomeric species.…”

“…Many heterogeneous Fe-based zeolite supported catalysts such as Fe-ZSM-5 230−232 and Fe@MFI 233 have been reported for the CH 4 OR. 187 Like Cu-based zeolites, the nature of catalytic centers remains elusive and monomeric, and 237 Yu et al also showed, three synthesis routes named incipient wetness impregnation (IWI), liquid ion-exchange (IE), and solid-state ion-exchange (SSIE) for the synthesis of Fe-ZSM-5 among which SSIE had the highest (71%) population of monomeric species as determined by 57 Fe Mossbauer spectroscopy. 238 Compared to IWI and IE samples, the Fe/ZSM-5 SSE yielded the highest CH 3 OH productivity and selectivity demonstrating the direct role of monomeric species.…”

“…CH 4 and H 2 O 2 compete for the same Fe sites, and due to favorable oxidation of H 2 O 2 over CH 4 , the product yield remains low. On the contrary, Fang et al, based on DFT and spectroscopic analysis, demonstrated that monomeric species can form •OH and •OOH resulting in the formation of CH 3 COOH, while dimeric species produce CH 3 OH via •OH radicals . Yu et al also showed, three synthesis routes named incipient wetness impregnation (IWI), liquid ion-exchange (IE), and solid-state ion-exchange (SSIE) for the synthesis of Fe-ZSM-5 among which SSIE had the highest (71%) population of monomeric species as determined by 57 Fe Mössbauer spectroscopy .…”