Interpreting the Oxidative Catalytic Activity in Iron-Substituted Ferrierites Using in Situ Mössbauer Spectroscopy

Abstract

Iron- and iron + aluminum-substituted ferrierites (Fe−FER and Fe+Al−FER) have been characterized and their properties compared. Fe−FER exhibits excellent catalytic performance in mild oxidation processes, whereas Fe+Al−FER is hardly active. To specify the source of catalytic activity, in situ Mössbauer studies were performed to distinguish the various iron-containing species. Upon treatment in hydrogen performed on Fe−FER, the temporary presence of the Fe2+ state was evidenced in tetrahedral framework sites, and removal of iron did not occur in significant amounts from these lattice positions. In contrast, similar treatment on Al+Fe−FER results in permanent Fe3+ to Fe2+ reduction and in removal of iron in considerable amounts from the framework sites. From comparison of the behavior of samples and additional analysis of data, it is suggested that dinuclear Felattice−O−Feextralattice centers are present in Fe−FER and that they have a primary role in mild oxidations.

Changes in states of substituted iron in microporous (MFI analogue) and mesoporous (MCM-41) hosts exposed to redox conditions

“…A slight oxidation and an evacuation restores a higher percentage for the Bronsted acidic protonated component, i.e. a part of the dinuclear centre is probably decomposed in an opposite process and neutral FeO(OH) may leave the vicinity of FW iron: It might be mentioned that in this interpretation the Fe 3+ $ Fe 2+ redox cycle is restricted to proceed only on the EFW part of the dinuclear component, in a certain contrast to the explanation presented in our earlier report [12], where the redox cycle was supposed to proceed on both, FW and EFW types of iron. Thus, dynamic formation and decomposition of dinuclear centres can be proposed in the evacuation-reductionoxidation-evacuation cycle, combined with charge transfer and migration of neutral components.…”

“…After drying, the sample was calcined at 753 K. The synthesis is described in more detail in Ref. [12].…”

Mössbauer spectroscopy in heterogeneous catalysis: A brief overview and recent study on paramagnetic features detected in catalytic centres of Fe-ZSM-5

“…Features of relaxation phenomena also appear in a part of Mössbauer spectra in all of these reports. We contributed to the study of active centres in zeolites, among others with demonstrating the appearance of dinuclear centres [24,25]. Recently the contributions of the framework (FW) and extra-framework (EFW) iron species were distinguished in Fe-ZSM-5 with a specific preparation method.…”

Synthesis and Characterization of Iron-Rich Highly Ordered Mesoporous Fe-MCM-41

“…Moreover, their loading in the silica matrix can affect the overall acid strength, ion-exchange capacity, and catalytic performances extensively. Crystalline microporous ferrisilicates such as ZSM-5, -11, -beta, ferrite, etc., as well as amorphous mesoporous iron silicates, , have been known for decades, and these materials have some unique characteristic features. Among the trivalent heteroelements, the incorporation of tetrahedral Fe 3+ into a silica network is known for the generation of very strong acid sites .…”