Structure and nuclearity of active sites in Fe-zeolites: comparison with iron sites in enzymes and homogeneous catalysts

Abstract

Fe-ZSM-5 and Fe-silicalite zeolites efficiently catalyse several oxidation reactions which find close analogues in the oxidation reactions catalyzed by homogeneous and enzymatic compounds. The iron centres are highly dispersed in the crystalline matrix and on highly diluted samples, mononuclear and dinuclear structures are expected to become predominant. The crystalline and robust character of the MFI framework has allowed to hypothesize that the catalytic sites are located in well defined crystallographic positions. For this reason these catalysts have been considered as the closest and best defined heterogeneous counterparts of heme and non heme iron complexes and of Fenton type Fe(2+) homogeneous counterparts. On this basis, an analogy with the methane monooxygenase has been advanced several times. In this review we have examined the abundant literature on the subject and summarized the most widely accepted views on the structure, nuclearity and catalytic activity of the iron species. By comparing the results obtained with the various characterization techniques, we conclude that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view. The relative concentration of these species changes with thermal treatments, preparation procedures and loading. Only at lowest loadings the catalytically active species become the dominant fraction of the iron species. On the basis of the spectroscopic titration of the active sites by using NO as a probe, we conclude that the active species on very diluted samples are isolated and highly coordinatively unsaturated Fe(2+) grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe(2+) presumably located on small clusters is also obtained. The nitrosyl species formed upon dosing NO from the gas phase on activated Fe-ZSM-5 and Fe-silicalite, have been analyzed in detail and the similarities and differences with the cationic, heme and non heme homogeneous counterparts have been evidenced. The same has been done for the oxygen species formed by N(2)O decomposition on isolated sites, whose properties are more similar to those of the (FeO)(2+) in cationic complexes (included the [(H(2)O)(5)FeO](2+)"brown ring" complex active in Fenton reaction) than to those of ferryl groups in heme and non heme counterparts.

A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia

“…9,10 In general, such catalysts also have Brønsted acidity, either since the standard procedures for Cu or Fe exchange do not result in a complete exchange or new protonic sites are formed as a result of a reduction of the metal ions. 11 It is argued that the exchanged Cu or Fe ions facilitate the SCR reaction 9,12-14 by oxidizing the NO to NO 2 , while the SCR reaction takes place elsewhere in the zeolite, 15,16 which could be on the ammonia adsorbed on the Brønsted sites. If the Brønsted sites are located close to the Cu (or Fe) ions in the zeolite, it is conceivable that an NO 2 molecule on a Cu ion can interact directly with an NH 3 molecule on a neighboring Brønsted site, which would make a reaction possible without the need for desorption of NO 2 .…”

Nitrous Oxide Decomposition on the Binuclear [FeII(μ-O)(μ-OH)FeII] Center in Fe-ZSM-5 Zeolite

“…A lot of studies have been recently devoted to Fe-ZSM-5, which is considered as a model catalyst that helps understands catalytic reactions by these materials. 8 Fe-ZSM-5 was shown to be very efficient not only in the N 2 O decomposition but also in the oxidation of alkenes and aromatics using N 2 O as oxidant. 9,10 According to several authors, it has been concluded that the common feature of these two reactions catalyzed by Fe-ZSM-5 is the formation of adsorbed oxygen atoms called "R-oxygen" or "O ads " (eq 1).…”

Theoretical evidence of the observed kinetic order dependence on temperature during the N2O decomposition over Fe-ZSM-5

“…Based on kinetics, spectroscopic and modeling studies, mono-and binuclear iron-oxo species have been proposed as the potential active sites rather than clustered oxide species. 5,6 Modeling approaches play a crucial role due to the unusually low content of active iron species (around 3 Â 10 18 sites g À1 ) 7 preventing univocal spectroscopic experimental assignments. Indeed a detailed knowledge of a mechanism occurring between reactants and solids can be reached by combining energetic and kinetic of elementary steps with a macroscopic description of the reaction.…”