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

Structure and nuclearity of active sites in Fe-zeolites: comparison with

iron sites in enzymes and homogeneous catalysts

Adriano Zecchina,*a Mickael Rivallan,ab Gloria Berlier,a Carlo Lambertia and

Gabriele Ricchiardia

Received 6th March 2007, Accepted 12th April 2007

First published as an Advance Article on the web 16th May 2007

DOI: 10.1039/b703445h

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 Fe2+ 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 Fe2+

grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe2+

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 N2O

decomposition on isolated sites, whose properties are more similar to those of the (FeO)2+ in

cationic complexes (included the [(H2O)5FeO]2+ ‘‘brown ring’’ complex active in Fenton reaction)

than to those of ferryl groups in heme and non heme counterparts.

1. Introduction

Several gaps are encountered in heterogeneous catalysis: some

‘‘internal’’ and some other ‘‘external’’ (because they concern

the relations with other branches of catalysis like homoge-

neous and enzymatic catalysis).

Concerning the ‘‘internal gaps’’, the difference in the experi-

mental conditions employed in surface characterization and in

catalytic reactions can be considered as a ‘‘pressure gap’’.

Besides, the discrepancy between the composition of model

surfaces (investigated with surface science approaches) and

that involved in real catalysis is the ‘‘material gap’’.1–3

As for the ‘‘external’’ gaps, special mention must be made of

the gap between heterogeneous and homogeneous catalysis, i.e.

between branches of science which, although both devoted to

the study of the common function of promoting the assembly of

molecules, are often supposed to deal, on one hand, with well

defined systems (homogeneous complexes) and, on the other

hand, with ill defined and disordered materials difficult to

characterize (heterogeneous catalysts). Another similar gap

can be hypothesized to hold for the relations between hetero-

geneous and enzymatic catalysis, the latter showing an enor-

mous selectivity and activity advantage over the former.

A further gap, which cannot be classified either internally or

externally (because it is common to all catalysis branches) is

that between existing precursor structures (either constituted

by the structures of surface sites or by the structure of

homogeneous complexes) and the active sites really operating

under reaction conditions.

Scientists involved in heterogeneous catalysis and surface

characterization have been challenged by these gaps. One of

aUniversita di Torino, NIS Centre of Excellence, University ofTorino, Dipartimento di Chimica Inorganica, Fisica e dei Materiali,Via P. Giuria 7, 10125 Torino, Italy. E-mail:[email protected]; Fax: +39-011-6707855; Tel: +39-011-6707860

bUniversite de Rennes 1, Sciences Chimiques de Rennes, UMR-6226,Inorganic Materials: Soft Chemistry and Reactivity of Solids,F-35042 Rennes, France

This journal is �c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 3483–3499 | 3483

INVITED ARTICLE www.rsc.org/pccp | Physical Chemistry Chemical Physics

the most effective efforts to bridge the selectivity gap with

respect to enzymes is the development of microporous cata-

lysts. Crystalline microporous materials like zeolites, charac-

terized by an extraordinary variety of crystalline structures, by

a great ability to exchange cationic species, by the controlled

nuclearity of the active sites, and by the presence of accessible

nanosized cavities, have appeared as an ideal playground. It is

outside the scope of this review to give a general view of the

catalytic and structural properties of the zeolites and the

interested reader is referred to some reviews in the field and

to the references cited therein.4–10

Similarly, the fundamental studies involving the structure,

distribution and reactivity of metal centres like Al3+,11–13

Ga3+,14–17 B3+,17 Ti4+,18–22 Ge4+,23,24 Fe3+25 in zeolite

frameworks and of metal centres in extraframework positions

(Cu+,26–28 Cu2+,29–32 Rh+,33,34 Ni2+,35 Zn2+,36 Co2+ 37),

being outside the scope of the review, will not be dealt with in

detail.

In this review we shall concentrate only on zeolitic materials

containing Fe sites (with special emphasis on Fe-silicalite and

Fe-ZSM-5) and on the possible links between the chemistry of

Fe2+ and Fe3+ ions in extraframework position and the

chemistry of homogeneous analogues, like iron based metallo-

enzymes38–41 and other homogeneous Fe2+, 42 Fe3+ 43 and

(FeO)2+ complexes.44–48

The choice to limit the comparison only between Fe-based

heterogeneous and homogeneous systems is not decreasing the

generality of the approach because:

(i) Fe-zeolites are active in several important reactions (vide

infra) which find close analogues in a reaction catalyzed by

homogeneous and enzymatic complexes.49,50 For this reason

they are ideal solids for discussing analogies and differences

between heterogeneous and homogeneous catalysts and pos-

sibly for finding a link with enzymatic catalysis.

(ii) The crystalline structure of zeolites is expected to greatly

increase the insight into the structure of the catalytic centres (a

property lacking when amorphous supports are considered

and which is at the origin of many characterization difficulties)

and hence to help the characterization of the species formed by

interaction with incoming reactants.

(iii) Fe-zeolites are a family of catalysts where very diluted

metal ions appear to be extremely active, suggesting a com-

parison with the same ions in enzymes.

(iv) The ability of zeolites to exchange or incorporate single

cations in well defined crystallographic positions should allow

to control the nuclearity and structure of the metal centre, so

favouring the investigation of the analogies between homo-

geneous, enzymatic and heterogeneous counterparts.

(v) The known structure of the channels and cavities where

the sites are located is expected to facilitate the comprehension

of the size and shape effects influencing selectivity, a property

very pronounced in enzymatic catalysts.

(vi) Fe-ZSM-5 and Fe-silicalite have been extensively stu-

died and hence the number of experimental data useful to

perform a comparison with the homogeneous and enzymatic

compounds is abundant. However in spite of abundant studies

the exact structure of the active sites in iron zeolites remains

unclear, although some well supported hypothesis can be

made.

In conclusion, Fe-silicalite and Fe-ZSM-5 can be assumed

as models of catalysts in which diluted single sites of unknown

structure display enzyme-like activity. Therefore, we believe

that a careful comparison of the manifestations of iron ions in

zeolites, homogeneous catalysts and enzymes can bring some

contributions to our understanding of these catalysts.

2. The reactions catalysed by Fe-ZSM-5 and

Fe-silicalite

Fe-ZSM-5 and Fe-silicalite are active catalysts in many rele-

vant oxidation reactions, which find close analogues in homo-

geneous and enzymatic catalysis. For this reason in the

following the most important reactions are presented together

with information on surface sites obtained from catalytic

experiments.

2.1. N2O decomposition into N2 and O2

This simple reaction

2N2O! 2N2 þO2 ð1Þ

has been extensively studied.51–63 It is thought that the reac-

tion is proceeding via the initial deposition of one oxygen atom

per iron atom (the famous ‘‘a oxygen’’).64 This species (or

family of species) is active in many oxidation reactions

(vide infra).

As for the reactivity of ‘‘a oxygen’’ species, it must be taken

into account that not all the oxygen species formed by N2O

decomposition show the same activity. For instance it has been

found that only a part of them are reacting with CO at

523 K.61

Because of its simplicity, the N2O decomposition reaction

has been widely considered as ideal for basic studies on the

mechanism of active oxygen formation and migration on

Fe-zeolites. This reaction has also been proposed for the

titration of the surface active iron species.

The most frequently proposed mechanisms are:

(i) N2O + * - N2 + *O then *O + O - O2 + 2 *, see

ref. 54 and 61

(ii) N2O + * - N2 + *O then *O + N2O - N2 + O*O

O*O - *O2 - * + O2, see ref. 56

(iii) N2O + * - N2 + *O then *O + N2O - N2 + *O2

N2O + *O2 - *O3 + N2 then *O3 - *O + O2, see ref. 65

In mechanism (i), the migration of oxygen from one active

site (*) to the other is the rate determining step.54,61 This

reaction mechanism needs the active participation of at least

two iron centres not necessarily located in adjacent position.

In mechanisms (ii)56 and (iii)65 N2O decomposition and

oxygen evolution occur at the same isolated sites (*). Mechan-

isms (i)–(iii) are all in agreement with transient response

experiments, showing that N2 appears before O2 upon direct

N2O decomposition pulses in the 773–848 K interval (Fig. 1).

This is because in all cases the global decomposition reaction is

limited by reaction pathways leading to gas phase oxygen.

More recent accurate temporal analysis of products56 in-

dicates that mechanism (ii) is the most likely one.

From the point of view of the oxygen species formed on iron

active sites (*), the three mechanisms are associated with

3484 | Phys. Chem. Chem. Phys., 2007, 9, 3483–3499 This journal is �c the Owner Societies 2007

increasingly complex oxygen species: *O for (i); *O, O*O and

*O2 for (ii); and *O, *O2 and *O3 for (iii).

Other very relevant results coming from these catalytic

experiments are:

(a) Fe-ZSM-5 is always more active than Fe-silicalite con-

taining the same amount of Fe.56,61–63

(b) The active sites are formed during activation in inert

atmosphere.56,61–63 There is a widespread agreement that after

this treatment a large fraction of iron is in the divalent state.

(c) The activity augments with increasing activation

temperature in inert gas; the same happens with the number

of active sites.56,61–63

(d) In zeolites with low Fe content, the number of Fe sites

where adsorbed oxygen species are formed is roughly identical

in Fe-ZSM-5 and Fe-silicalite.56,61–63

(e) The activity (calculated per Fe centre) increases with

dilution.61 From this, it can be safely concluded that the active

sites contain a very small number of Fe atoms or, more likely,

a single atom. From this it is also inferred that the concentra-

tion of clustered species becomes relevant at the highest Fe

contents and that their catalytic activity in N2O decomposition

is lower than that of isolated species. The debate concerning

the nuclearity of the catalytic sites (mono- or dinuclear) is still

alive. The number of ‘‘a-sites’’ grows with the concentration,

this growth being smaller than that of clustered species.66

(f) All other factors being equal, the number of active sites is

maximum on catalysts formed by high temperature activation

of isomorphously substituted zeolites,61 as compared for ex-

ample with catalysts obtained with post synthesis impregna-

tion.

(g) An interesting point is the positive effect of NO,67–71

which is enhancing the decomposition rate of N2O. This

observation has relevant mechanistic implications.

2.2. N2O reduction by CO

This simple reaction

N2Oþ CO! N2 þ CO2 ð2Þ

is important mainly for mechanistic investigations.61,72,73 Its

simplicity is associated also with the fact that the product

(CO2) is not adsorbed on the catalyst surface, a fact which

simplifies the kinetic investigations and surface characteriza-

tion. From transient response studies it has been concluded

that not all the centres carrying adsorbed oxygen species are

active at 523 K and that the species active in CO oxidation are

also active in benzene hydroxylation.

2.3. Hydroxylation of benzene to phenol with N2O

Among the reactions catalyzed by Fe-zeolites, the reaction:

C6H6 þN2O! C6H5OHþN2 ð3Þ

is certainly the most studied51,61,64,74–85 because of its potential

industrial application.86 This reaction is also very important

Fig. 1 Normalized O2 and N2 transient responses upon direct N2O decomposition over iron-containing zeolites, showing that N2 appears before

O2.. Reprinted with permission from ref. 56. Copyright 2006 American Chemical Society.


because it can be conducted on Fe-ZSM-5 with a selectivity

near to 100% (a fact which is not common in catalysis).

Perhaps the most relevant conclusion coming from kinetic

experiments is that the rate limiting step of this reaction does

not involve the cleavage of the C–H bond.85 The reaction

reported in Scheme 1 has been proposed:(where a dimeric

species is involved). This mechanism, which is different from

that hypothesized for hydrocarbon hydroxylation and oxida-

tion reactions (vide infra), has been explained also on the basis

of theoretical calculations involving a single metal atom.87

2.4. Oxidation of methane and higher hydrocarbons with N2O

Due to its simplicity and its potential practical interest, the

reaction

CH4 þN2O! CH3OHþN2 ð4Þ

has received great attention.88–94 Unfortunately, unlike the

oxidation of CO by N2O, which generates a product that is not

adsorbed on the surface, in this case the methanol formed by

interaction of methane with adsorbed oxygen, remains

strongly adsorbed with formation of �OCH3. This fact is

limiting the practical application to methanol synthesis. When

the reaction is conducted at 723 K,91 oxidation products like

CO and CO2 are appearing in the gas phase. Similar oxidation

products are formed during oxidation of higher hydrocar-

bons.56,92,95–98

The formation of adsorbed methanol has stimulated the

hypothesis that the active site contains a pair of Fe centres like

in methane monooxygenase (MMO).99

2.5. Selective catalytic reduction (SCR) of NO by ammonia

and hydrocarbons

This reaction is of considerable practical importance and

is supposed to be catalysed by the same sites discussed

before.100–104 The catalytic reduction of nitrogen oxides can

be carried out selectively by using ammonia or urea. SCR of

NO by hydrocarbons is another important reaction which is

believed to be the most promising way to eliminate NO from

traffic exhausts.105,106

Some mechanistic studies were reported, showing that

nitrile groups derived from nitroso compounds as intermedi-

ates are formed when hydrocarbons are used as reduc-

tants.107–110

Most of the catalysts used in SCR with hydrocarbons were

initially prepared by post-synthesis iron exchange. The method

based on FeCl3 sublimation has been extensively used by the

group of Sachtler, as testified by copious scientific produc-

tion111–116 and by van Santen.117 The use of MFI zeolites

exchanged with FeCl3118 or FeCl2 was also reported for the

SCR of NO with ammonia.119–121

Generally, these preparation methods lead to high Fe load-

ings, so that many authors suggested that dimeric or clustered

Fe ions were responsible for the catalytic activ-

ity.104,108,111,112,122 However, careful titration of active sites

showed that the active sites in these samples usually represent

a very small fraction of total iron.123 The more recent

improvement appears to be the preparation of diluted samples,

because there is a compelling evidence that isolated sites are

the active centres, while clusters were found to be detrimental

to the catalytic activity (favoring total oxidation of the

reductant)124–126 or inactive.100

In conclusion, from the above-described results it is inferred

that the same sites active in the model N2O decomposition

reaction are also likely to be active in reduction of NO.

2.6. Oxidation of substrates with H2O2

Iron based zeolites (Fe-ZSM-5, Fe-silicalite and Fe-TS-1) have

been tested as Fenton-type catalysts in oxidation reactions

in solution.127,128 Iron was introduced by ion exchange or

directly during zeolite synthesis. Prior to their use in catalytic

tests, the samples were calcined at 773 K in air. Although the

results are somewhat influenced by leaching, all samples show

activity in phenol and propionic acid degradation with H2O2.

The reaction mechanism is supposed to be the same for the

Fenton reaction in solution.

3. Preparation methods

Several methods have been proposed for zeolite functionalisa-

tion with iron. The different procedures were initially designed

to insert Fex+ ions in definite structural position of the zeolite

framework with the hope to obtain crystalline and well defined

catalysts. In the following a concise description of the methods

will be given.

3.1. Exchange of H-ZSM-5 with Fe2+ and Fe3+ salts

Even if some authors reported solid state ion exchange synth-

esis using FeCl2 salts,122,129,130 the vast majority of studies on

iron loading methods deal with exchange in aqueous solu-

tions.126 The most frequently used salts are FeCl2,129,131

FeSO4,111,122,131,132 (NH4)2Fe(SO4)2,

133 FeCl3134 and

Fe(NO3)3.61,122,133,135 Since ferric salts have some tendency

to form dinuclear clusters in solution and then to precipitate as

hydroxides and because of insufficient local charge balance in

the MFI framework, ferrous salts are preferred. However, due

to the low stability of the ferrous state, synthesis in controlled

atmosphere or in the presence of reducing agents is often used

to avoid oxidation of Fe2+ to Fe3+.106 The final iron content

inside the MFI edifice is usually less than 1 wt%. To transform

the system into an active catalyst, it is calcined in air at 773 K

and activated in vacuo or in inert flow at T 4 773 K.

3.2. Exchange of H-ZSM-5 with Fe3+ oxalate

In the past some methods based on organic salts like Fe3+

acetylacetonate, Fe2+ oxalate106,136 have been reported. Even

if problems of precipitation during synthesis were observed,

some interesting innovations have been discovered. In

Scheme 1


particular, Nechita et al. have recently proposed an exchange

method using [Fe(COO)2]+1 cations.137 This method has the

advantage, with respect to the famous report of Feng and Hall

about samples prepared by ferrous oxalate,106 to be reprodu-

cible also on a large scale. The resulting Fe-containing zeolites

(o0.5 wt%) exhibit a remarkable iron dispersion. Also in this

case its transformation into an active catalyst requires calcina-

tion and activation.

3.3. Impregnation of silicalite with Fe2+

and Fe3+

salts

The impregnation method has been used to introduce Fe in

Al-free silicalite (i.e. a structure not containing exchangeable

cations). It was observed that, also in the absence of Brønsted

sites, some Fe could be introduced in the silicalite channels

and that the interaction with silanol was the main mechanism

of the anchoring process as evidenced by FTIR spectroscopy

(unpublished results from this laboratory). This interaction,

leading to formation of grafted Fex+(O�Si)x species is im-

portant because the resulting samples are catalytically active in

oxidation reactions. In other words, grafted sites in a fully

siliceous framework must also be considered as active sites.138

3.4. Sublimation of FeCl3 into H-ZSM-5

Ion-exchange can also be done in vapour phase. This kind of

exchange, also called chemical vapour deposition (CVD), has

been extensively reported in literature as testified by the

number of papers.111,129,139–149 The CVD process uses iron

salts that are volatile at elevated temperatures like FeCl3. This

method offers the possibility, after elimination of the FeCl3excess by means of successive high temperature treatment, to

obtain fully and over-exchanged zeolites. The final iron con-

tent is generally higher compared to other ion exchange

techniques (3.0–5.0 wt% in Fe). The system is then calcined

in air at 773 K and activated in inert gas at T 4 773 K to

transform into an active catalyst.

3.5. Isomorphously substituted Fe-ZSM-5 and Fe-silicalite

In isomorphously substituted Fe-MFI catalysts, iron is gen-

erally incorporated into the lattice during hydrothermal synth-

esis.82,150 The amount of iron that can be introduced in MFI

framework is very limited so that this technique usually leads

to diluted samples (less than 0.6 wt%). After synthesis, various

treatments can convert the as-synthesized sample into the

H-form. In this preparation the Fe3+ ion is located in a

tetrahedral position on the lattice.151–153 The method allows

to prepare both isomorphously substituted Fe-ZSM-5 and

Fe-silicalite. The active form is reached after a thermal treat-

ment at high temperature (more than 773 K) during which the

majority of the iron is dislodged from framework positions

due to the low stability of Fe in the framework. Many studies

were devoted to understanding the migration phenomenon of

Fe.61,69,72,153–157

4. The activation procedures

To summarize, it can be concluded that the methods described

in sections 3.1, 3.2 and 3.4 give iron species in extraframework

position, a method as in section 3.3 gives grafted species, while

isomorphous substitution (section 3.5) produces Fe3+ in

framework positions. The samples as prepared are not active.

To induce activity, the samples have to be calcined at TE 773 K

in air and treated in a flow of inert gas (or in vacuo) at

T 4 773 K.

High temperature treatments in air and inert flow or in

vacuo, of samples prepared by employing methods described in

sections 3.1, 3.2 and 3.4 can induce migration of Fe from the

initial position with subsequent clustering and grafting. Ana-

logously it has been demonstrated that high temperature

treatments of isomorphously substituted zeolites (section 3.5)

induces the removal of Fe3+ from the framework to extra-

framework positions, with subsequent migration and graft-

ing.151–153 All these processes are highly dependent upon the

presence of H2O impurities in the channels, which are known

to favour both hydrolysis of SiOSi, SiOAl, SiOFe and FeOFe

bridges (with formation of hydroxylated sites suitable for

anchoring migrating species) and clustering of extraframe-

work species.158 It is so concluded that the final (active) state

of the catalyst is completely different from that of the starting

material.

Perhaps method 3.5 appears to be the most convenient as it

allows to start from the best defined precursors, with higher Fe

dilution and because migration of iron can be better con-

trolled.61,72,153,159

A general scheme involving dislodgment, migration, graft-

ing and clustering is reported in Fig. 2a, giving a pictorial

representation of the structure of the Fe2+ species formed by

migration into extraframework positions and successive graft-

ing to different reactive centers located on the walls of the

zeolite channels. Two structures with regard to different

anchoring sites have been considered: Brønsted sites (structure

a) associated with residual M3+ in the lattice (for Fe-ZSM-5

M = Al or Fe; for Fe-silicalite M = Fe) and silanol nests

(structure b).

The Fex+ species resulting from this complex mechanism

form a distribution of isolated, dimeric or clustered sites, and

Fe2O3 particles as graphically represented in Fig. 2b.

From these considerations it can be concluded that what-

ever is the origin of the samples, the successive thermal

treatments at high temperature needed to induce catalytic

activity, tend to modify the catalysts in such a way that

the resulting distribution of sites is not dependent much

on the preparation method. Hereafter we assume the rea-

listic view that this distribution is substantially unknown.

The consequence of this assumption is that sensitive and

appropriate characterization methods are needed. On the

basis of the scheme presented in Fig. 2a it can be expected

that after high temperature thermal treatments, the following

groups of atoms can be present on the samples in various

proportions:

Fe-silicalite

� isolated Fe2+ and Fe3+ chemically anchored to the frame-

work via SiOFe bridges;

� species containing Fe3+OFe3+, Fe2+OFe2+ and

Fe3+OFe2+ ion pairs, oligomeric, clustered sites and Fe2O3

particles chemically anchored to the framework via SiOFe

bridges.


Fe-ZSM-5

� isolated Fe2+ and Fe3+ chemically anchored to Al-free and

Al-containing regions via SiOFe, AlOFe bridges;

� dinuclear Fe3+OFe3+, Fe2+OFe2+, and Fe3+OFe2+ sites

in oligomeric, clustered sites and Fe2O3 particles chemically

anchored to the framework via SiOFe or AlOFe bridges.

In both cases, the ratio between the concentration of the

various species is expected to depend upon the metal loading

and the activation temperature.

In order to give a picture of the real situation, several

physical techniques have been used. In the following the

results of some of them will be critically discussed.

5. Titration and characterization of exposed Fex+

centres with the IR spectroscopy of NO probe: the

problem of sensitivity

In the previous paragraphs we have discussed how the titra-

tion of the active sites can be made by means of the N2O

decomposition reaction. This is not only a very sensitive

method but also involves only the active centres at each given

temperature. We have also seen that the titration of the active

sites is not the only result of catalytic experiments. In fact

indirect information on site isolation and on valence state has

also been obtained.

In this paragraph we will describe another informative and

extremely sensitive technique designed to explore the structure

and reactivity of surface sites. This method is based on the use

of NO as a titrant of the exposed sites and is made possible by

the well known and exceptional affinity of NO towards Fe2+

and Fe3+ centres, leading to a formation of Fex+(NO)nnitrosyls (n = 1, 2, 3) characterized by extremely intense

n(NO) bands.69,160–162 A useful feature of this method is

related to the fact that upon changing the NO equilibrium

pressure, the full range of coverages can be explored. Under

these conditions all surface centres are titrated. Of course the

method reveals only exposed centres. Fex+ ions located in

subsurface position (for instance, in Fe2O3 particles) or buried

into the support lattice cannot be explored. This is not a

Fig. 2 Schematic representation of (a) possible mechanisms leading to the Fe migration from framework to extraframework positions (adapted

from ref. 159) and (b) pictorial representation of the final species: Fe2O3 particles, FexOy clusters, grafted Fe2+ ions and Fe3+ in framework

position.


negative factor since we are interested only in the species

accessible to reactants. A further advantage of the method

lies in the fact that the stretching frequency of formed nitrosyls

strongly depends upon the valence state of the adsorbing

centre (Fe2+ or Fe3+). Furthermore, as the sites with single

or multiple coordinative unsaturation can show mono- or

polyaddition and form Fex+(NO), Fex+(NO)2 and Fex+(NO)3complexes upon increasing the equilibrium pressure, it is

evident that the method is also informative about the structure

(ligands sphere) of the adsorbing sites.153,158,161 In the follow-

ing we shall show that while Fe3+ forms only mononitrosyls,

isolated Fe2+ can bind up to three NO molecules.

An example of the results of this titration method obtained

on two samples of different origin and composition (Fe-ZSM-5

ex-oxalate, prepared by the ferric oxalate method and Fe-

silicalite prepared by thermal removal of Fe3+ from a tetra-

hedral position of isomorphously substituted silicalite) and

activated in the same way (in vacuum at 773 K) is illustrated in

Fig. 3. On the basis of our experience, this figure is represen-

tative of all Fe-zeolites.

Without discussing in detail the attribution of the various

bands two facts are evident. Firstly, the spectra are very

similar. This similarity demonstrates that after the treatment

(activation) at high temperature, the distribution of Fe2+ and

Fe3+ sites is similar in Fe-ZSM-5 and Fe-silicalite. This result

is quite surprising because one of the two matrices (Fe-ZSM-5)

contains Al ions (the framework) while the other does not

(Fe-silicalite). In the following we shall however draw atten-

tion to some small but significant differences which are

influenced by the Al content (see discussion of Fig. 6).

Secondly, when the spectra are normalized with respect to

the Fe content, it comes out that the intensity of the NO

spectra on Fe-ZSM-5 is about 2.5 times larger than that of Fe-

silicalite. From this it is inferred that on Fe-silicalite a larger

fraction of Fe cannot be titrated by NO. This fact is a clear

indication of clustering. An indirect deduction is that the

presence of Al in the structure is favouring dispersion.

Concerning the sensitivity of the method, Fig. 4 reports the

normalized IR spectra of NO adsorbed on Fe-ZSM-5 samples

prepared by different methodologies137 and containing different

Fe loading content down to 0.08 wt% Fe. It is a fact that the IR

spectra of NO adsorbed on samples containing less than 0.1%Fe

can be clearly detected. This result highlights the high sensitivity

of the method. A second striking aspect is related to the

impressive decrease of the fraction of surface Fe sites (propor-

tional to the nitrosyl band intensity) with increasing Fe loading.

Fig. 3 FTIR spectra (background subtracted) of NO dosed at RT on

Fe-ZSM-5 (post synthesis exchange by ferric oxalate, left part) and Fe-

silicalite (isomorphous substitution, right part) samples previously

activated in vacuum at 773 K. Spectra were collected by reducing

NO equilibrium pressure (PNO) from 15 Torr (dashed spectrum) to

10�3 Torr (dotted spectrum). Unpublished.

Fig. 4 FTIR spectra (background subtracted) of NO dosed at room temperature (decreasing PNO from 15 Torr, dashed line spectrum, to 10�3

Torr, dotted line spectrum) on Fe-ZSM-5 samples obtained with different preparation methods and iron content, previously activated in vacuo at

773 K. Spectra were normalized with respect to the pellet thickness and with respect to the iron content. Top spectrum of part (a): 15 Torr of NO

on Fe2O3 sample (ex-goethite) outgassed and oxidized at 773 K. Springer Catal. Lett., 2005, 103, 33, Nechita et al., Fig. 3. Copyright 2005. With

kind permission from Springer Science and Business Media.


From the same figure it can also be clearly inferred that the

normalized intensity grows in the order: 0.08 4 0.38 4 3.30

wt%. If we assume, quite arbitrarily, that the spectra of NO on

the most diluted sample are due mainly to atomically dispersed

species (which is not: vide infra), it is deduced that even in the

0.38 wt% sample, clusters and Fe2O3 particles are present and

that aggregation is dominating in the sample containing

3.3 wt% Fe. In relation to the problem of formation of

Fe2O3 particles on the 3.3 wt% sample, in the inset of Fig. 4

the peak of NO adsorbed on a pure Fe2O3 sample is reported

for the sake of comparison. The complete coincidence of the

frequency of this peak with the frequency of one of the peaks

observed on the various samples is again a clear indication

that iron oxide particles are formed.

The detailed assignment of the IR peaks shown in Fig. 3 and

4 is schematically illustrated in Fig. 5b. This assignment has

been thoroughly discussed in ref. 137. The stretching frequen-

cies of the Fex+(NO)n peaks (x = 2, 3 and n = 1, 2, 3) are

strongly influenced by the iron oxidation state and by the

electron donating ability of the surrounding ligands.42,163

Comparison with homogeneous metal–organic complexes

shows that stretching frequencies in the 1800–1900 cm�1 range

are normally observed in Fe2+ cationic complexes

[FeLn(NO)]+X�42,163,164 and [Fe(H2O)5NO]2+,165 (Fig. 5a)

while bands in a lower frequency range (1600–1700 cm�1)

are found when Fe2+(NO) groups are bonded to strong

electron donor ligands such as porphyrins,166–168 imidazole169

or dithiolene.163 From this it is inferred that NO probing

Fe-ZSM-5 and Fe-silicalite reveals the presence of Fe2+ with

reduced d–p-bonding ability.

From the pressure dependence of the peaks the reversible

Fe2+(NO) $ Fe2+(NO)2 $ Fe2+(NO)3 transformation is

also documented for a large fraction of sites. Notice that the

Fe2+(NO) and Fe2+(NO)3 complexes are EPR active.153,170

From all the data reported in Fig. 3, 4 and 5b the following

conclusions are safely derived. (i) A fraction of Fe2+ sites

(very relevant in the most diluted samples) adsorbs up to 3 NO

ligands. This means that the adsorbing sites are highly un-

saturated (i.e. the coordination sphere is incomplete). Com-

plexes with the same stoichiometry are obtained on both

Al-containing and Al-free samples (Fig. 3). (ii) The Fe2+ in

zeolites sites are characterized by reduced d–p back-donation

ability and are therefore different from those present in heme

and non-heme organic complexes. (iii) As the frequency of the

associated peaks is not changing with coverage it is also

concluded that there is no dipole–dipole coupling2,3 or solvent

type effects between the NO oscillators. In other words, the

Fe2+ sites behave as isolated centres. (iv) A minor fraction of

Fe2+ sites inserts only one NO ligand into its coordination

sphere. Whether this site is isolated, paired or located at the

surface of a small cluster is difficult to infer. Considering the

tendency of dislodged iron to form clusters, we are more in

favor of the second hypothesis. (v) A fraction of sites is in the

trivalent state and it inserts only one NO ligand. The similarity

of the stretching frequency of these species with that measured

for the Fe2O3/NO system (see vertically shifted curve in

Fig. 4a) suggests that the involved sites are located on nano-

particles formed by aggregation.

In conclusion, the picture emerging for all investigated

samples with a Fe concentration in the 0.08–3.3% range is

that in all cases the nature of accessible sites is not dramati-

cally changing with passing from one sample to the other.

What is really changing upon increasing the Fe concentration

is the sites distribution and the fraction of non accessible sites

(located in oxide particles and clusters) which become defi-

nitely predominant in the sample containing 3.3 wt% Fe.

On the basis of these results we are forced to consider the

following questions: (i) Why is Fe-ZSM-5 always more active

than Fe-silicalite in N2O decomposition? (ii) Is this greater

activity related to the superior dispersion of sites (plausibly

containing a single Fe2+) observed on Fe-ZSM-5? (iii) Is the

titration method sufficiently sensitive to explore the really

active sites? (iv) Which is the real structure (coordination

sphere) of the highly coordinatively unsaturated Fe2+ sites?

(v) If grafted Fe2+ species are responsible for the catalytic

activity, why Fe/silica and Fe/MCM41 are less active?138

To answer these questions the results illustrated in Fig. 6 are

of extreme utility. The analysis of the results of a large number

of NO titration experiments on Fe-ZSM-5 and Fe-silicalite

clearly demonstrates that:

(a) The frequency of the NO stretching modes of the

equivalent Fe2+(NO)n complexes is statistically higher on

Fig. 5 (a) Frequencies of n(NO) in different Fe nitrosyl complexes

formed in mononuclear homogeneous systems or on supported oxides,

as detailed in ref. 137. Fe2+(NO) heme and non-heme nitrosyls absorb

at lower frequencies as indicated by the arrow. (b) FTIR spectra

(background subtracted) of NO dosed at room temperature (decreas-

ing PNO from 15 Torr, dashed line spectrum to 10�3 Torr, dotted line

spectrum) on Fe-ZSM-5oxa sample previously activated in vacuum at

773 K. Springer Catal. Lett., 2005, 103, 33, Nechita et al., Fig. 2.

Copyright 2005. With kind permission from Springer Science and

Business Media.


Fe-ZSM-5 than on Fe-silicalite. As the n(NO) is influenced by

the d–p back-bonding ability of the Fe2+ centre, it is con-

cluded that the presence of Al3+ decreases the electron density

at the Fe centre. This is plausible only if the Al is in the first or

second coordination sphere of T centres surrounding the

anchored Fe2+.

(b) The n(NO) frequencies are scattered: this fact, which

cannot be explained in terms of instrumental conditions,

means that we are dealing with families of similar species

instead of single species.

Considering that the intensity of the n(NO) bands is always

considerably higher on Fe-ZSM-5 than on Fe-silicalite, we are

able to definitely answer some of the questions advanced

before because it can be concluded that the presence of Al

favours both the dispersion of (isolated) centres and modifies

their electronic structure (thus influencing also their reactiv-

ity). Furthermore, the scattering of data is a strong evidence

of sites’ heterogeneity. It is evident that these results fully

confirm the scheme illustrated in Fig. 2a, showing the possible

mechanisms leading to Fe migration from the framework to

extraframework positions and the resulting distribution of

different Fe sites. As for the question of the smaller reactivity

of the Fe2+ grafted on amorphous support, the answer is that

the ions sink in the more flexible support and thus exhibit

much less pronounced coordinative unsaturation. In fact when

titrated with NO these centres do not form the usual popula-

tion of trinitrosylic groups.138

Concerning the detailed structure of Fe2+ centres many

authors have advanced hypothesis. For instance, Kachurovs-

kaya et al.87 suggest the structure represented in Scheme 2a,

while Yang et al.83 consider a larger number of structures and

Berlier et al.171 simply speak of Fe2+ species grafted to fully

siliceous or to silicoaluminate frameworks via SiOFe and

AlOFe bridges. We think that these structures are an over-

simplied representation of the ligands sphere of Fe2+ sites,

which is better and more realistically schematised as repre-

sented in Scheme 2b. In this scheme Al3+ can be statistically

distributed in the cationic coordination spheres I (green

sphere) or/and II (red sphere). This representation, which does

not pretend to describe the exact location of the Fe2+ on the

walls of the cavities and channels, is able to explain at the same

time the heterogeneity of sites and the effect of the presence

of Al. This conclusion is very similar to that advanced by

Li et al.52,53 for the Fe-SBA-15 systems.

6. UV-Vis-NIR spectroscopy of Fex+

in Fe-ZSM-5

and Fe-silicalite and of Fe2+

(NO)n complexes

The valence and coordination state of Fex+ centres can be in

principle studied by electronic spectroscopy in the UV-Visible

region.

In particular, accurate information about the oxidation and

coordination states of transition metal ions usually comes

from the so-called d–d transitions rationalized in terms of

the ligand field (LF) theory. It is a fact that metal ions with dn

configuration and in well defined environment are associated

with characteristic spectroscopic features in the visible-NIR

range which can be used as a fingerprint for the recognition of

the average coordination sphere and for the evaluation of the

metal–ligand binding strength.172 This is the case of as synthe-

sized Fe-silicalite samples prepared by isomorphous substitu-

tion (see Fig. 7a) where Fe3+ are isolated and in perfect

tetrahedral position. Indeed the very weak bands observed in

the visible region are characteristic of the d5 Fe3+ in tetra-

hedral coordination. On the basis of the known spectroscopy

of Fe3+ in solution,172 the extinction coefficient e should be

o1. For this reason the d–d bands of Fe3+ are hardly

detectable by reflectance spectroscopy even when all Fe sites

have the same well defined coordination. When Fe3+ ions are

dislodged from their original position and migrate into less

defined and more heterogeneous situations, the chances to

detect the corresponding d–d transitions become negligible or

null because the bands are broadened. This is the reason why

on samples treated at high temperature, the d–d bands are

totally missing (Fig. 7a).

Fig. 6 Relative fraction of the observed high frequency (part a) and

low frequency (part b) components of the Fe2+(NO)3 and Fe2+(NO)2adducts (taken at 15 Torr and 10�3 Torr, respectively) in a series of Fe-

silicalite and Fe-ZSM-5 samples. Upper and lower sections refer to Al-

free Fe-silicalite and Al-containing Fe-ZSM-5 samples, respectively.

For some samples the experiment was repeated more than once,

resulting in 20 IR experiments for Fe-silicalites and 17 for Fe-ZSM-

5. Reprinted from J. Catal., vol. 215, G. Berlier et al., pp. 264–270.

Copyright (2003), with permission from Elsevier.

Scheme 2


Substitutional Fe3+ ions in the silicalite matrix are sur-

rounded by a sphere of 4 SiOd� ligands and hence LMCT

transition are also expected in the 40 000–50 000 cm�1 range

characterized by an extinction coefficient which is about two

or three order of magnitude larger than that of the d–d

bands.151,152

The Fe-silicalite spectra do indeed show (Fig. 7a), two

intense bands in the UV region which were assigned to LMCT

transitions in the [FeO4] tetrahedral group.151,152 These bands,

although very intense, cannot be safely considered as an

evidence of tetrahedral coordination. In fact, also octahedral

complexes of Fe3+ are characterized by two strong bands in

the same energy range.173 The frequency of LMCT bands is

affected by clustering and in Fe2O3 they are shifted down to

the visible region. Consequently they can overlap the d–d

bands. The consequence of this is that when we are in the

presence of both isolated, clustered species and Fe2O3 parti-

cles, the spectrum in the visible becomes very complex and

broad and the interpretation is troublesome or impossi-

ble.126,130,151 The formation of clusters and of Fe2O3 particles

by removal of isolated Fe3+ from substitutional positions

induced by high temperature treatments is, at least partially,

responsible of the shift of the LMCT band towards the lower

frequency of Fe-silicalite (Fig. 7a).

As we have discussed in the previous paragraphs, the

treatment at high temperature in vacuo or in inert gas should

cause the formation of Fe2+ species either isolated or clus-

tered. So the question arises whether it can be possible to

detect them by UV-Vis-NIR spectroscopy. We can so start by

considering first the d–d bands of isolated Fe2+ ions in low

coordination. On the basis of the smaller ligand field and of

the lower charge of the metal ion we expect weak d–d transi-

tions shifted to the lower wavenumbers (in the NIR region)38

where they should not overlap with any other ligand field

transition. So far we were not able to single out d–d bands

surely assigned to Fe2+ ions: this is likely due to the low

intensity. We think that more accurate measurements in the

NIR region are needed. As for the LMCT associated with

Fe2+ they are expected to fall at high energy, so that it is hard

to observe them in the UV range especially in the presence of

Fe3+ species.

In conclusion, the information which can be extracted from

the reflectance spectra in the UV-Vis- NIS range of the

Fe-ZSM-5 and Fe-silicalite samples activated at high T is very

modest, especially because of the low intensity of the d–d

bands and the heterogeneity of sites.

From the IR experiments of the titration of the Fe2+ sites

we have learnt that Fe2+(NO)n complexes are formed by

interaction with NO. Homogeneous nitrosylic complexes

of Fe2+ are known for their intense absorptions in the visible

range. For instance, the ‘‘brown ring’’ [Fe(H2O)5NO]2+

complex is characterized by three strong absorptions (A, B,

C) in the 10 000–30 000 cm�1 range with extinction coefficients

two or three order of magnitude larger that those of the d–d

bands of the parent [Fe(H2O)6]2+ complex.

We have thus investigated by reflectance spectroscopy the

formation of surface nitrosyls on Fe-silicalite and Fe-ZSM-5

by NO adsorption from the gas phase and compared the

obtained spectra with those obtained by dosing NO in a

solution containing [Fe(H2O)6]2+.165 The results are shown

Fig. 7 (a) UV-Vis spectra of Fe-silicalite: (1) as synthesized (broken line), (2) calcined at 773 K and (3) at 973 K. In the inset an exploded view of

the d–d transitions of (1) and (2) is reported. Reprinted from J. Catal., vol. 158, S Bordiga et al., pp. 486–501. Copyright (1996), with permission

from Elsevier. (b) UV-Vis spectra of NO adsorbed on Fe-silicalite previously activated in vacuum at 773 K (background subtracted): (1) spectrum

after dosage of 10 Torr NO at room temperature, (2) and (3): effect of progressive PNO lowering. Unpublished. (c) Absorption spectral changes

recorded for the reaction of [Fe2+(H2O)6]2+ complex with NO. Trace (a) [Fe2+(H2O)6]

2+ solution saturated with NO; (b), (c) and (d) effect of

flushing the solution with Ar. Reprinted with permission from ref. 165. Copyright 2002 American Chemical Society.


in Fig. 7b and c for the nitrosyl complexes formed on

Fe-silicalite and for [Fe(H2O)6]2+, respectively.

The similarity of the UV-Vis-NIR spectra is stringent. It is

thus inferred that UV-Vis-NIR experiments shown in Fig. 7b

are the analogues of the IR experiments shown in Fig. 3b, 4, 5.

However, a detailed assignment of the UV-Vis bands in terms

of mono-, di- and trinitrosyl was not possible. What is clear

(and this is the main message coming from this comparison) is

that the Fe site interacting with NO molecules, responsible for

the intense UV-Vis bands (Fig. 7b and c), is very similar to the

Fe2+ ion of the Fentom complex, so that the environment

formed by the water molecules in the latter must be similar to

that formed by the silicate matrix.

Regarding the nature of the UV-Vis nitrosyl bands, on the

basis of their position we propose that they are related to d–d

transitions with mixed p character. The high intensity of the

bands indicates an important mixing with the empty ligand

p-orbitals. In the case when there is an extensive mixing of the

metal and ligand wave functions in the molecular orbitals of

the complex, the distinction between ligand field and charge

transfer cannot be made sharply.172

From this we conclude that UV-Vis-NIR spectroscopy of

the NO/Fe2+ interaction can be used to titrate the isolated

Fe2+ species on Fe-ZSM-5 and Fe-silicalite, exactly as it is

done in solution. Furthermore it is a very sensitive and

diagnostic method.

7. The sites structure and distribution emerging

from XAS investigations

Usual structural investigation methods such as X-ray diffrac-

tion (XRD) are limited in determining the structure of iron

active sites inside MFI-zeolite because of the low concentra-

tion of heteroatoms and because of the lack of long-range

order after the substitution process. Few XRD studies just

report on the iron locations in the framework, which are not

the active sites in catalysts.25 XAS absorption spectroscopies

can in principle overcome these problems. In particular,

extended X-ray absorption fine structure (EXAFS) is emer-

ging as one of the preferred techniques for probing active site

structures in catalysis.54,122,131,140,151,152,174–181 While the

EXAFS part of the spectrum allows to gain information on

the first and second coordination sphere of the metal species,

X-ray absorption near edge structure (XANES) displays its

great utility in clarifying the average valence state of the

absorbing metal centre.

Looking at the XANES results that have appeared in

the literature on Fe-ZSM-5 and Fe-silicalite it can be con-

cluded that they confirm that after high temperature activation

a part of the iron is in a divalent state.62,152,153,174,176 As far

as the EXAFS results are concerned, a summary of the

diverse Fe–O, Fe–Fe and Fe–Si/Al distances reported in the

literature are plotted as a function of iron content in

Fig. 8.54,122,131,140,151,152,174–181

As clearly displayed in Fig. 8, the results are heavily

scattered and at low iron concentration (from 0.2 to 1.0

wt%), the Fe–O distances are found in the 1.80–2.25 A range.

Concerning other distances measured in XAS experiments,

most of the Fe–Fe distances in iron containing MFI appear

centred on those of a-Fe2O3 (2.96 and 3.49 A). Moreover the

Fe–Fe distances are superimposed to those of Fe–Si/Al in the

2.80 to 3.20 A range.

This impressive scattering of the 1st shell distances can be

explained assuming two main causes. The first cause concerns

the fact that, in most of the cases, different groups investigate

significantly different samples, as preparation and post synth-

esis procedures strongly affect the final form of iron species

(vide supra sections 3 and 4). Secondly, notwithstanding the

fact that the accuracy of a first shell distance determination

may be as good as �0.01 or �0.02 A, these error bars are

statistical and systematic errors are not accounted for. In the

specific case of Fe-zeolites systematic errors may have a double

origin. Usually phase-shifts and amplitude functions, crucial

in determining bond distances and coordination numbers, are

theoretically generated from a given guessed cluster. As the

actual geometry of the active Fe species is still unknown,

phases and amplitudes generated in that way can be question-

able. The second source of possible systematic errors concerns

the assumption of Gaussian distance distribution done in the

standard EXAFS formula, usually used in most of the cited

papers. It is known that in systems characterized by a high

degree of heterogeneity, like liquids or amorphous systems,

this assumption is no longer valid. In such cases, EXAFS data

should be analyzed according to the cumulant approach, as

elegantly shown recently in the field of catalysis by the ETH

group of Prof. van Bokhoven.182

These observations suggest that the low concentration of

iron and, more importantly, the heterogeneity of the sites are

at the basis of the scattering of data. This result does once

more confirm that after calcination and activation a broad

distribution of iron centres is present on Fe-ZSM-5 and

Fe-silicalite systems.

A few results in the figure merit a specific comment. In

particular, we think that the distances centred at about 2.5 A

are likely due to Fe–Cl groups on samples prepared from

Fig. 8 Summary of the diverse Fe–O (open square), Fe–Fe (open

circle) and Fe–Si/Al (open triangle) distances calculated in the litera-

ture on the basis of EXAFS data. Distances are reported as a function

of iron content. Full and dashed vertical lines indicate average Fe–O

and Fe–Fe distances, respectively, calculated from XRD of a-Fe2O3.

Data have been collected from ref. 54, 122, 131, 140, 151, 152,

174–181.


FeCl3 exchanged systems. The distance at about 1.4 A found

by Choi et al. in oxidized samples could be consistent with the

formation of Fe4+QO species.177,178

Coming to the important problem of nuclearity of the

extraframework iron species EXAFS in principle could be

the technique of choice to face this problem. By fitting the

EXAFS contribution in the 2–4 A interval with a Fe–Fe model

one could obtain the average Fe–Fe coordination number

(NFe�Fe).174,179,180 Unfortunately, the relationship between

such NFe�Fe value and the average Fe nuclearity if far from

straightforward, because complexity is introduced by the huge

heterogeneity of extraframework species. As a consequence,

an average Fe–Fe coordination number of e.g. NFe�Fe = 1.0

could be interpreted as 100% of dimers, as 50% of isolated

monomers and 50% of trimers (having two Fe neighbours)

etc. The situation is even more complex, because the 2–4 A

interval is the region where also the backscattering of frame-

work atoms (Al or Si) is potentially expected. In this regard,

the group of Bell178 showed that the peak at 2.5 A, previously

ascribed to Fe–Fe scattering and used to argue for the

presence of di-iron-oxo species was actually due to Fe–Al

contributions.

8. What is known about the ‘‘active’’ oxygen

Active oxygen on Fe-ZSM-5 and Fe-silicalite is formed upon

decomposition of N2O at T Z 523 K. It is widely accepted

that active oxygen is formed predominantly on Fe 2+ sites.

The fraction of these sites has recently been evaluated with

several methods by Yuranov et al.61 and it has been found to

vary from 50–70% for isomorphously substituted samples

with less than 0.035 wt% Fe and treated at high T (1323 K

in He) to less than 10–15% for post-synthesis exchanged

samples. On the basis of catalytic experiments, the same

authors have found an indication of the presence of at least

two types of Fe2+ sites characterized by different reactivity. It

has been hypothesized that these sites are isolated and paired,

respectively. In the first case the structure of the oxidized

centre is Fe4+QO (or Fe3+–O�), while in the second case an

oxo-bridged Fe3+O2�Fe3+ structure is more likely. All struc-

tures are EPR silent.183

On the basis of an elegant resonant inelastic X-ray scatter-

ing (RIXS) experiment on a post-synthesis exchanged sample

containing 0.36% and treated at 1218 K, Pirngruber et al.184

have concluded that the Fe4+QO is not present after oxida-

tion with N2O. Although they are favouring the dinuclear

structure, they do not exclude the formation of Fe3+–O�. The

interpretation of the spectra has been given by assuming that

the concentration of the active sites is about 0.01 mmol g�1 (i.e.

6.1018 atoms g�1). However the data from Yuranov et al.61

regarding post-synthesis samples with similar iron content

(0.55% corresponding to about 60.1018 atoms g�1) give smal-

ler values (about 3.1018 atoms g�1), the remaining iron frac-

tion (more than 90%) being presumably in the form of iron

oxide clusters and particles: thus the question arises whether

the method is sensitive enough to allow clear-cut conclusions

especially in presence of a large fraction of spectator species.

A dimeric structure containing two active Fe2+ sites is

favoured by Dubkov et al.156 on the basis of Mossbauer

spectroscopy. However as the authors have found that each

atom in the complex is able to adsorb one active oxygen, the

Fe2+ sites behaving as monoatomic entities in a paired

arrangement are registered by Mossbauer spectroscopy as

dinuclear complexes. This in turn suggests that active oxygen

is not an oxo-bridged species and that two adjacent Fe4+QO

are formed upon N2O decomposition.

Concerning the nature of adsorbed oxygen it has to be

pointed out that the first proposed structure was the dimeric

one.185 This hypothesis was substantially based on the

observed reactivity of oxygen species formed by N2O dissocia-

tion towards methane, this reactivity being similar to that of

the enzyme methane oxygenase MMO (which indeed contains

two metal centres).99 Today we know that many other mono-

nuclear non-heme iron complexes are active in the same

reaction.40 For this reason other structures have to be

considered.

From the above discussion it can be seen that nuclearity of

sites and structure of adsorbed oxygen are closely connected.

From our IR results concerning the titration of Fe2+ sites on

very diluted samples, we have concluded that a relevant

fraction is associated with isolated sites characterized by high

coordinative unsaturation (and hence highly reactive). This

means that isolated (FeO)2+ structure is emerging as our

preferred candidate for active adsorbed oxygen in these sam-

ples. This does not exclude the presence of a minor fraction of

paired Fe2+–Fe2+ active species, in particular located on the

surface of small, partially reduced FexOy clusters entrapped

into the framework cavities, where the formation of oxo-

bridged species is more likely. However as we have no direct

information on the vibrational properties of adsorbed oxygen

(like on the contrary obtained by resonant Raman spectro-

scopy on homogeneous counterparts and for oxygen adsorbed

on Fe-SBA-15)53 we cannot reach a completely safe conclu-

sion. This is not unexpected because we have seen in the

previous chapters that the Fe-ZSM-5 and Fe-silicalite catalysts

are not the simple model systems imagined initially.

9. Are the structure and reactivity of active sites in

Fe-zeolites and in homogeneous compounds really

comparable?

Before starting the discussion of this point, let us first empha-

sise that Fe2+ in Fe-ZSM-5 and Fe-silicalite formed by

activation in vacuo or in helium flow at high temperature are

coordinatively unsaturated species, i.e. their coordination site

is not complete. The formation of surface species by interac-

tion with gases is, to a first approximation, well represented by

a ligand insertion into a vacant sphere as shown below:

LnFe2 þ þ nNO! LnFe

2 þðNOÞn ðn ¼ 1; 2; 3Þ ð5Þ

LnFe2 þ þOðex-N2OÞ ! LnðFeOÞ2 þðL ¼ SiO� or AlO�Þ ð6Þ

This situation is never found in homogeneous conditions

because the ions are always surrounded by a full coordination

sphere of ligands.

For instance, the formation of the simple homogeneous

[Fe(H2O)5NO]2+ nitrosyl (the ‘‘brown ring complex’’ of Fe2+)


in solution is a ligand exchange reaction:186

½FeðH2OÞ6�2 þ þNO! ½FeðH2OÞ5NO�2 þ þH2O ð7Þ

For this reason in the following we will compare only the

structure and properties of (coordinatively saturated) Fe2+-

(NO)n (n = 1, 2, 3) and (FeO)2+ groups anchored, on the one

hand, to the internal surfaces of ZSM-5 and silicalite or

located in the ligands sphere of homogeneous complexes, on

the other hand. This comparison together with the evaluation

of the catalytic properties of the systems will form the basis of

our considerations.

The first comparison to be made is between the spectro-

scopic properties of nitrosylic complexes formed on Fe2+ ions

anchored on Fe-ZSM-5 and Fe-silicalite and the [Fe(H2O)5-NO]2+ and [LFe(NO)]2+ (L = polyamine carboxylate) cat-

ionic complexes.165 In a second step the spectroscopic properties

of heme and non-heme nitrosyls167,187–189 will be considered.

From the first comparison it is emerging that the n(NO) of

[Fe(H2O)5NO]2+ and of [LFe(NO)]2+ complexes are in the

1780–1810 cm�1 range. This range coincides with the range

where the n(NO) of Fe2+(NO)n complexes in Fe-ZSM-5 and

Fe-silicalite are observed: it is indicative of a strong similarity

between the two situations.

When the n(NO) of heme and non-heme nitrosyls are

considered, it is noticed that the frequency is consistently

downward-shifted with respect to the complexes discussed

before.

This means that the electron donating ligand sphere of heme

and non-heme complexes greatly enhances the d–p backdona-

tion ability of the metal centre with respect to the ligand sphere

present in [Fe(H2O)5NO]2+ and [LFe(NO)]2+ and in Fe2+-

zeolites.

The consequence is that NO bonded to heme and non-heme

compounds is better described as a bent NO� while the charge

back-donated to NO on Fe2+ ions on Fe-ZSM-5 and Fe-

silicalite is definitely smaller (so that the NO ligand is more

likely to be linear, or only slightly bent). In other words the

electron donating character of the ligand sphere modulates

the electron density at the Fe2+: a fact which of course induces

different reactivities. The different reactivity of Fe2+ in

Fe-zeolites, on the one hand, and in heme and non-heme

compounds, on the other hand, is testified by the different

affinity towards CO and O2. In fact the interaction of CO with

Fe-ZSM-5 and Fe-silicalite is very weak and O2 is not ad-

sorbed at all at RT (while it readily forms Fe3+O2� groups

with Fe2+ in heme compounds). From this it is concluded that

a simple parallelism between Fe2+ ions on Fe-ZSM-5 and

Fe-silicalite, on the one hand, and heme and non-heme (en-

zymatic) complexes, on the other hand, is not straightforward

and that the most correct and simple homogeneous counter-

part is the [(H2O)5FeO]2+ complex.

On the basis of these considerations we can now move to the

next step by comparing the electronic and vibrational proper-

ties of ferryl (FeO)2+ groups grafted on the internal surfaces

of Fe-ZSM-5 and Fe-silicalite with ferryl groups in the sim-

plest homogeneous complex [(H2O)5FeO]2+. Then we will

continue considering ferryl groups in heme (cytochrome

P450) and non-heme complexes, which are very active in CH

hydroxylation.41,190

The simple [(H2O)5FeO]2+ complex recently characterized

by Mossbauer and XAS spectroscopy48 (Scheme 3) is thought

to be the active species in the Fenton reaction158 and is

characterized by a quite covalent FeQO bond (d = 0.162

nm) which reacts with benzene to produce phenol but does not

react with methane at RT.44 The first reaction is the same

catalyzed by Fe-ZSM-5 and Fe-silicalite, so the analogy

between this compound and the species formed by the inter-

action with N2O is relevant. The XANES spectrum of this

compound48 shows an edge at 7126 eV, i.e. 3 eV downwards

shifted with respect to that of the [(H2O)6Fe]3+ structure. It is

interesting to note that a similar shift has been observed on

passing from Fe-silicalite contacted with N2O at 523 K (7123.1

eV) to the same sample outgassed at 973 K in vacuo.62,152,153

Unfortunately the samples prepared by activation at high

temperature always contain a large fraction of Fe2O3 particles

and FexOy clusters, so the comparison between the two cases is

not fully satisfactory.

As for the n(FeQO) of the ferryl group in Fe-ZSM-5 and

Fe-silicalite nothing is known.

Scheme 4 presents a ferryl group formed on isolated grafted

Fe2+ sites in Fe-zeolites after contact with N2O at 523 K, as

proposed by Berlier et al. on the basis of XANES and EXAFS

measurements.152 This structure is indeed similar to that of ‘‘a-oxygen’’ suggested by Kachurovskaya et al.87 formed by the

addiction of a ‘‘QO’’ ligand to the Fe complex presented in

Scheme 2a. We think that resonant Raman experiments which

have demonstrated their utility in determining the vibrational

properties of ferryl groups in heme and non-heme

Scheme 3

Scheme 4


complexes,38,40 could be of high utility. About the ferryl

groups in heme and non-heme complexes we can say that they

are more polar and that the FeQO bond is slightly longer with

respect to the same groups in [(H2O)5FeO]2+. The increased

polarity is associated with the high electron donating ability of

the ligands present in heme and non-heme complexes. This

increased polarity is making the ferryl group of these com-

pounds more exposed to an electrophilic attack and this fact is

at the basis of the reactivity of these species with C–H bonds of

aliphatic and aromatic hydrocarbons even under very mild

conditions.

In conclusion, the ferryl groups in Fe-ZSM-5 and

Fe-silicalite, should show reactivity similar to that of the

Fenton complex.191 However, as they are associated with a

ligand sphere with donating character smaller than that of the

enzymatic complexes, they are also expected to be definitely

less active. This expectation is in agreement with the observa-

tion that the two types of catalysts operate in entirely different

temperature regimes. These considerations also suggest that

the search of analogies between heterogeneous and homoge-

neous complexes although representing an important area of

investigation,192 must be made with caution.

All the conclusions reached before are valid only for the

isolated Fe2+, Fe2+(NO)n and (FeO)2+ groups. As we know

that these species are predominant only in very diluted samples

activated at high temperature, these considerations cannot be

taken for all Fe loadings. In particular we cannot exclude that

dimeric and other type of aggregated species present in the

channels and cavities can contribute to the catalytic properties

of more concentrated samples.

10. Conclusions

In this review we have examined the abundant literature on

Fe-ZSM-5 and Fe-silicalite 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 with the results

derived from catalytic experiments, it is concluded that

Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived

before and that many type 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 Fe loading.

Only at the 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 NO, we

conclude that the active species on extremely diluted samples

are isolated and highly coordinatively unsaturated Fe2+

grafted to the crystalline matrix. We have also found the proof

of the presence of a minor fraction of Fe2+ ions characterized

by a more complete coordination sphere, likely located on

small clusters entrapped in the framework. The nitrosylic

species have been analyzed in detail and the similarities and

differences with the cationic, heme and non-heme homoge-

neous counterparts have been put into evidence. The same has

been done for the oxygen species formed on very diluted

samples by N2O decomposition, whose properties are more

similar to those of the (FeO)2+ in cationic complexes (includ-

ing the [(H2O)5FeO]2+ ‘‘brown ring’’ complex considered

active in the Fenton reaction) than to those of ferryl groups

in heme and non-heme counterparts. The formation of bridged

oxospecies on Fe2+ pairs located on small clusters is not

excluded.

Acknowledgements

Progetto Regionale NANOMAT DOCUP 2000-2006 Ob. 2

Reg. (CE) 1260/99 and EC NoE IDECAT are acknowledged

for financial support.

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

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