Spectroscopic characterization of iron-containing MCM-58

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Microporous and Mesoporous Materials 89 (2006) 47–57

Spectroscopic characterization of iron-containing MCM-58

V. Umamaheswari a,b, Winfried Bohlmann b, Andreas Poppl b,A. Vinu c, Martin Hartmann a,*

a Department of Chemistry, Chemical Technology, TU Kaiserslautern, D-67653 Kaiserslautern, Germanyb Fakultat fur Physik und Geowissenschaften, Universitat Leipzig, Linnestr. 5, D-04103 Leipzig, Germany

c International Center for Young Scientists, National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan

Received 9 March 2005; received in revised form 27 May 2005; accepted 31 May 2005Available online 23 November 2005

Abstract

Fe(III) species were introduced into the zeolite MCM-58 by two different methods, namely by the addition of ferric nitrate to thesynthesis gel (FeMCM-58) and by ion exchange of the calcined (Al)MCM-58 material with ferric chloride (Fe-MCM-58). The mate-rials were characterized by XRD, SEM, AAS, UV–Vis, EPR and 27Al and 29Si MAS NMR spectroscopy. The studies indicate thepresence of framework substituted Fe(III), isolated Fe(III) and iron oxide clusters in the FeMCM-58 materials. Further NO adsorp-tion studies were carried out with dehydrated FeMCM-58 and Fe-MCM-58 and studied by EPR at two different frequencies (X- andQ-band at 77 K and 6 K, respectively) to investigate the possible reduction of the introduced Fe(III) ions. The studies show that thedehydration process resulted in the formation of Fe(II) species by autoreduction of isolated Fe(III) species in both materials, whichin turn were involved in the formation of a paramagnetic high spin Fe(II)–NO complex as the major species. Hence our studiesconfirm the presence of isolated Fe(III) species not only in the Fe-MCM-58 material, but also in the calcined FeMCM-58 materials.� 2005 Elsevier Inc. All rights reserved.

Keywords: Iron-containing MCM-58; Characterization; NO adsorption; EPR studies

1. Introduction

Zeolite MCM-58 (IZA structure code IFR) [1–3], isisostructural with SSZ-42 [4–6] and ITQ-4 [7,8]. TheIFR structure has an one-dimensional arrangement ofundulating 12-membered ring channels. The basic build-ing unit is a polyhedron formed by 16 T atoms, whichform five 4-membered rings, four 5-membered ringsand two 6-membered rings. Each unit cell consists oftwo polyhedra and, therefore, contains 32 T atoms.The pore diameter at the narrowest point is 0.64 nmand amounts to 1 nm at the widest point. ZeoliteMCM-58 with its unique structure has been shown to

1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.micromeso.2005.05.045

* Corresponding author. Tel.: +49 6312053559; fax: +496312054193.

E-mail address: [email protected] (M. Hartmann).

exhibit superior catalytic activity in the conversion ofvarious organic compounds such as short chain n-alk-anes and C7–C10 normal paraffins [9]. It has beenreported recently that MCM-58 catalysts favor the for-mation of n-propyl toluene in the alkylation of toluenewith propylene. The formation of 1,2-ditolylpropane isfavored in the pores of zeolite MCM-58, presumablybecause the reaction intermediate has a similar shapeas the N-benzyl-1,4-diazabicyclo[2.2.2]octane template,which is used for the synthesis of the IFR topology[10]. Despite the very attractive structure of MCM-58only few studies of its modification using heteroatomsother than aluminum and boron have been reported[11]. The introduction of heteroatoms into the alumino-silicate lattice may cause significant changes in the acidicand hydrothermal properties of the materials [12–15].Using different heteroelements (viz. B, Ga, Fe, Co, Ti

48 V. Umamaheswari et al. / Microporous and Mesoporous Materials 89 (2006) 47–57

etc.) different acidic and redox functions can beintroduced.

Especially iron-containing zeolites are receivingincreasing attention because of their high potential tocatalyze various reactions. It has been shown that bothframework and extraframework iron species in zeolitesare active in oxidation reactions [16–18]. Fe-ZSM-5plays a vital role in environmental catalysis in particularfor the reduction of NOx and N2O with hydrocarbons orammonia [19–26] and for direct decomposition of NOinto N2 and O2 [27]. It has been reported thatFe-ZSM-5 [28] and ferrisilicate analogues of ZSM-5[29] exhibit an unique catalytic performance in the directhydroxylation of benzene to phenol (with nitrous oxideas an oxidant). The catalytic performance in this reac-tion is mainly attributed to the presence of particularsites involving iron ions. Upon N2O decomposition theso-called a sites in Fe-ZSM-5 are efficient to generatea new active form of O2 (a oxygen) which exhibits anunusual reactivity [30] in the oxidation of benzene tophenol and methane to methanol. It is also interestingto note that the extraframework iron species in theFe-silicalite materials with low nSi/nFe ratio have beenshown to reduce the reaction temperature for the reduc-tion of NO by C3H6 [31]. Stockenhuber et al. [32] havesuggested that aluminum-containing Fe-AlMCM-41materials show better catalytic activity for the oxidationof large unsaturated hydrocarbons to oxygenates thantheir aluminum free analogue [33]. The increased effi-ciency of zeolites containing both Al and Fe was attrib-uted to the increased dispersion of the extraframeworkFe(III) species, which are present in the system becauseof Fe(III) ion exchange of the calcined material or areformed by heat treatment of the as-synthesized material[34]. Zhang et al. [35] reported that tetrahedrally coordi-nated and atomically isolated Fe sites in FeMCM-41catalyze the epoxidation of styrene with H2O2. Sincethe properties of zeolitic materials are inextricablyrelated to the characteristics of framework and extra-framework species depending on the reaction to be cat-alyzed, it is important to study the location of ironspecies in the material by suitable techniques.

Recently, the synthesis of iron-containing MCM-58zeolites has been reported and the acidic properties ofthe FeMCM-58 materials were studied [36]. However,the spectroscopic properties of Fe(III) introduced intoMCM-58 zeolites by either isomorphous substitutionor post synthesis modification such as ion exchange havenot been investigated in detail. Hence in the presentwork an attempt has been made to probe the natureof the Fe(III) species in both framework incorporatedand ion exchanged zeolite MCM-58. Fe(III) ions wereintroduced during the hydrothermal synthesis of theMCM-58 zeolite substituting Al by Fe step by step inthe framework. In this approach we are taking advan-tage of recent results [34] where the presence of alumi-

num in the framework was shown to stabilize isolatediron species in high siliceous zeolites. In order to com-pare the state of the iron in isomorphous substitutedFeMCM-58 materials with a sample prepared by postsynthesis modification, iron was also introduced byliquid state ion exchange of the calcined (Al)MCM-58zeolites.

The materials were characterized by varioustechniques such as X-ray diffraction (XRD), scanningelectron microscopy (SEM), atomic absorptionspectroscopy (AAS), ultraviolet–visible (UV–Vis) spec-troscopy, electron paramagnetic resonance (EPR) spec-troscopy, and 29Si and 27Al magic angle spinningnuclear magnetic resonance (MAS NMR) spectroscopy.In order to probe the autoreduction of Fe(III) to Fe(II)and the accessibility of potentially ferrous iron species,both iron ion exchanged and isomorphous frameworksubstituted MCM-58 materials were dehydrated undervacuum and subsequently NO was adsorbed. The for-mation of paramagnetic Fe(II)–NO adsorption com-plexes was monitored by EPR spectroscopy at X- andQ-band frequencies.

2. Experimental

2.1. Synthesis

The template N-benzylquinuclinidium bromide(BQBr) was prepared by refluxing an equimolar mixtureof benzylbromide and quinuclidine in ethanolic solutionfor 16 h. The solid was crystallized by cooling thereaction mixture in an ice bath. Elemental analysisconfirmed the formation of N-benzylquinuclinidium: C59.43% (theor. 59.57%), H 7.17% (theor. 7.09%), N4.83% (theor. 4.96%). The synthesis procedure is analo-gous to the hydrothermal synthetic route reported ear-lier for MCM-58 [3,11,36]. In a typical synthesis ofiron-substituted MCM-58, appropriate amounts of alu-minum sulphate and ferric nitrate were dissolved in 88 gof distilled water. To this solution, 3.37 g of KOH and8.46 g of N-benzylquinuclinidium bromide were addedunder continuous stirring. Subsequently, 30 g of colloi-dal silica sol (30 wt.% SiO2 in water, levasil VP 4038)was slowly added under stirring. A low viscosity liquidresulted, which was further homogenized for 2 minand then transferred to a teflon-lined stainless steel auto-clave. The following molar gel composition was used:1SiO2:xAl2O3:yFe2O3:0.2BQBr:0.25K2O:40H2O (wherex and y vary from 0 to 0.045). The syntheses were car-ried out at 150 �C under rotation for 5–7 days dependingon the nSi/nFe ratio. After synthesis, the solid was recov-ered by filtration, washed and dried. The organic tem-plate was removed by calcination in nitrogen andfinally in air at 540 �C. Throughout the paper, materialsprepared in that way are designated as FeMCM-58(90),

10 20 30 40 50 60

d

c

b

a

2 Theta

Fig. 1. XRD patterns of as-synthesized (a) FeMCM-58(90), (b)FeMCM-58(44), (c) FeMCM-58(29) and (d) FeMCM-58(22).

10 20 30 40 50 60

d

c

b

a

2 Theta

Fig. 2. XRD patterns of calcined (a) FeMCM-58(90), (b) FeMCM-58(44), (c) FeMCM-58(29) and (d) FeMCM-58(22).

V. Umamaheswari et al. / Microporous and Mesoporous Materials 89 (2006) 47–57 49

FeMCM-58(44), FeMCM-58(29), and FeMCM-58(22)where the nSi/nFe ratios of the synthesis gel are givenin brackets. The nSi/nAl ratios of the FeMCM-58(90),FeMCM-58(44), FeMCM-58(29) materials are 15, 22,and 44, respectively, whereas aluminum is totallyreplaced by iron in FeMCM-58(22). The ion-exchangedFe-MCM-58 material was prepared as follows: Aboutone gram of calcined AlMCM-58 (nSi/nAl = 22) materialwas combined with 0.0468 g of ferric chloride in waterand the solution was stirred at 80 �C for 8 h. The result-ing solid was filtered off and dried. The procedureensured that the amounts of iron in Fe-MCM-58 andFeMCM-58(90) are comparable.

2.2. Formation of NO adsorption complexes

FeMCM-58(90) and Fe-MCM-58 materials wereloaded into quartz tubes (3 mm o.d. for X-band and1.8 mm o.d. for Q-band) attached to a steel valve andthen heated under vacuum at 673 K for 18 h. Thereafter,the materials were cooled down to room temperature,10 mbar NO were introduced and the quartz tube wassealed at 77 K.

2.3. Characterization

The powder XRD patterns of the FeMCM-58 sam-ples were collected on a Siemens D5005 diffractometerusing CuKa (k = 0.154 nm) radiation. The diffracto-grams were recorded in the 2h range of 5–60� with a stepsize of 0.01� and a step time of 10 s. The SEM pictureswere taken using a Hitachi S-4800 instrument. Elemen-tary analysis was done by AAS using a Perkin ElmerAAnalyst 300 spectrometer. UV–Vis spectra were mea-sured with a Perkin Elmer Lambda 18 spectrometerequipped with praying-mantis diffuse reflectance acces-sory using BaSO4 as a reference. The EPR spectra wererecorded at X-band (microwave frequency mmw =9.36 GHz) and Q-band (mmw = 33.03 GHz) frequenciesat 77 K (X-band) and 6 K (Q-band), respectively. TheX- and Q-band measurements were carried out onBruker ESP 380 and EMX spectrometers, respectively.The field modulation amplitudes used were 0.75 mT(X-band) and 0.41 mT (Q-band) and the modulationfrequencies were adjusted to mmod = 100 kHz (X-band)and mmod = 70 kHz (Q-band). The microwave powerused was low enough to prevent saturation of the spinsystems. 27Al and 29Si MAS NMR spectra wererecorded on a Bruker MSL 500 instrument. The 27AlMAS NMR spectra were acquired at a frequency of130.32 MHz, employing 4 mm zirconia rotors with aspinning rate of 10 kHz, a pulse length of 2.1 ls, delaytime of 100 ms and spectral width of 33 kHz with12,500 scans. The 29Si MAS NMR spectra wererecorded at a frequency of 99.361 MHz, employing a7 mm zirconia rotor with a spinning rate of 3.9 kHz,

pulse length of 3 ls, delay time of 20 s and a spectralwidth of 40 MHz with 1300 scans. A 0.1 M aluminumnitrate solution and tetramethylsilane (TMS) were usedas external references for 27Al and 29Si MAS NMR,respectively.

3. Results and discussion

3.1. XRD

The XRD powder patterns of as-synthesized andcalcined FeMCM-58 with different nSi/nFe ratios (90,44, 29, 22) are shown in Figs. 1 and 2, respectively.The XRD patterns resemble those previously reportedfor MCM-58 [3].

The linewidth of the XRD reflections of both as-syn-thesized and calcined Fe-MCM-58 materials increaseswith decreasing nSi/nFe ratio. This could be interpretedin terms of increasing strain in the crystallites due to

Table 1Gel and product composition determined by AAS for calcinedFeMCM-58 materials with different nSi/nFe ratios

Sample nSi/nAl

(gel)nSi/nFe(gel)

nSi/nAl

(product)nSi/nFe(product)

nAl/nFe(product)

FeMCM-58(90) 15 90 21 132 6.2FeMCM-58(44) 22 44 26 40 1.5FeMCM-58(29) 44 29 45 30 0.67FeMCM-58(22) – 22 – 23 –

50 V. Umamaheswari et al. / Microporous and Mesoporous Materials 89 (2006) 47–57

the incorporation of Fe(III) ions and, hence, anincreased structural distortion. Broadening of XRDreflections due the small size of the crystallites can beruled out, because the mean size of the particles is above3 lm (see below).

3.2. SEM

The influence of isomorphous substitution on themorphology of the synthesized FeMCM-58 materialswas studied by SEM. Incorporation of heteroelementsoften results in a decrease of the crystallite size [28,37]which is reported to be accompanied by an increase inthe aspect ratio in some cases [37]. The SEM picturesof the as-synthesized FeMCM-58(90), FeMCM-58(44),FeMCM-58(29), and FeMCM-58(22) materials arepresented in Fig. 3. All pictures show nonhomogeneousparticle size distributions. The presence of elongatedcrystallites, with sizes varying from 3 to 25 lm couldclearly be discerned for FeMCM-58(90) which possessesthe lowest iron content (Fig. 3a). In general, the size ofthe crystallites decreases with increasing iron content ofthe zeolites and a minimum average size of about 3 lmwas obtained for FeMCM-58(22) (Fig. 3d). Further-more, the aspect ratio of the crystallites seems to dimin-ish with decreasing nSi/nFe ratio and increasing nSi/nAl

Fig. 3. SEM pictures of as-synthesized (a) FeMCM-58(90), (b) F

ratio. An opposite trend has been reported for theisostructural zeolite ITQ-4 [37], where an increase inthe aspect ratio was observed with decreasing nSi/nAl

ratios. Hence, the substitution of Al by Fe in MCM-58 zeolites leads to a further reduction of the crystallitesize whereas the aspect ratio seems to be controlled bythe nSi/nAl ratio of the synthesis gel.

3.3. Atomic absorption spectroscopy

The nSi/nFe and nSi/nAl ratios of the synthesis gel arecompared with the corresponding ratios of FeMCM-58obtained from atomic absorption spectroscopy (AAS)(Table 1). The nSi/nFe and nSi/nAl gel ratios found forthe final products are close to the gel compositions forall materials studied except for FeMCM-58(90). For this

eMCM-58(44), (c) FeMCM-58(29) and (d) FeMCM-58(22).

225 300 375 4500

2

4

6

8

10

d

c

b

a

Inte

nsity

λ (nm)

Fig. 5. UV–Vis spectra of calcined (a) FeMCM-58(90), (b) FeMCM-58(44), (c) FeMCM-58(29) and (d) FeMCM-58(22).

V. Umamaheswari et al. / Microporous and Mesoporous Materials 89 (2006) 47–57 51

material a nSi/nFe ratio of 132 was obtained from AASwhich is higher when compared to the gel ratio of 90,indicating a reduced iron incorporation.

3.4. UV–Vis spectroscopy

The UV–Vis spectra of as-synthesized FeMCM-58samples with different nSi/nFe ratios are shown inFig. 4. The spectra show a characteristic absorptionband around 245 nm; the intensity of this band increaseswith increasing iron content of the material. This band isassociated with ligand to metal charge-transfer (CT)that involves isolated four co-ordinated Fe(III) andhas been assigned to the t1 ! e transition involvingFe(III) in [FeO4] tetrahedra [38]. The concentration of[FeO4] species increases with the iron content of thematerials as indicated by the rising intensity of the bandat 245 nm with decreasing nSi/nFe ratio. No significantabsorption is observed above 300 nm, which indicatesthat the as-synthesized materials are free from ferricoxide [39]. Fig. 5 shows the UV–Vis spectra of calcinedFeMCM-58 materials with different nSi/nFe ratios. Aftercalcination, the UV–Vis spectra still show the transitionat 245 nm of the tetrahedral ferric [FeO4] species, whichseems to be broadened in comparison with the as-syn-thesized materials. The broadening might be due to adistribution of the Fe–O bond lengths and Fe–O–Sibond angles as a consequence of the extended structuraldistortion after calcination of the as-synthesized materi-als [40]. Again, the intensity of the [FeO4] band increaseswith rising iron doping. It is worth to note that thespectrum of calcined FeMCM-58(90) shows a broadCT absorption band around 275 nm, which is assignedto Fe(III) species in octahedral coordination [41]. Theappearance of this species indicates leaching of Fe(III)from the framework during calcination of the as-synthe-sized material.

225 300 375 4500

2

4

6

8

10

12

14

ab

c

d

Inte

nsity

λ (nm)

Fig. 4. UV–Vis spectra of as-synthesized (a) FeMCM-58(90), (b)FeMCM-58(44), (c) FeMCM-58(29) and (d) FeMCM-58(22).

For FeMCM-58(90), FeMCM-58(44) and FeMCM-58(29) a broad background signal extending up to350 nm and for FeMCM-58(22), even to 440 nm isobserved, which indicates the formation of minute ironoxide particles formed during calcination of the materi-als [41]. In summary, the UV–Vis spectra of the calcinedmaterials obtained in this study evidence the presence oftetrahedrally coordinated Fe(III) moieties, such as[FeO4], in the MCM-58 framework and a proceedingformation of minute iron oxide particles with increasingiron concentration.

3.5. 27Al and 29Si MAS NMR spectroscopy

27Al MAS NMR spectroscopy has been employed toverify the aluminum incorporation into tetrahedralframework sites. The 27Al MAS NMR spectra of thecalcined FeMCM-58 materials with different nSi/nFeratios are shown in Fig. 6 and exhibit an intense signal

150 100 50 0 -50

c

b

**

* *

**a

ppm

ig. 6. 27Al MAS NMR spectra of calcined (a) FeMCM-58(90), (b)eMCM-58(44) and (c) FeMCM-58(29). (The asterisks indicatepinning side bands.)

FFs

52 V. Umamaheswari et al. / Microporous and Mesoporous Materials 89 (2006) 47–57

at a chemical shift of about 58 ppm with a low fieldshoulder at 60 ppm. Spinning sidebands are marked byasterisks. The 58 ppm signal is indicative of tetrahe-drally coordinated aluminum in aluminoslicate frame-works [3,11]. Naturally, its amplitude decreases withrising nSi/nFe ratio as Al is more and more replaced byFe. The origin of the shoulder at 60 ppm has been dis-cussed before [3] and was ascribed to the existence ofcrystallographically nonequivalent aluminum frame-work sites. For FeMCM-58(90) and FeMCM-58(44)low-intensity peaks at �5 ppm and 9 ppm, respectively,were observed and ascribed to the presence of extra-framework aluminum in octahedral coordination intwo different chemical environments [42], which mighthave been formed by leaching during calcination.

The 29Si MAS NMR spectra of the calcinedFeMCM-58 materials with different nSi/nFe ratios(Fig. 7) exhibit a broad signal (from �105 to �115ppm) centered at �111 ppm. The spinning side bandsare usually not observed for pure aluminosilicates andare indicative of the presence of paramagnetic iron spe-cies. The broad signal is assigned Si(OSi)4 (Q4) struc-tural units. The observable splittings of the Q4 signalresembles the presence of four crystallographically non-equivalent silicon sites with slightly different chemicalshifts as inferred from 29Si MAS NMR spectrum ofthe pure siliceous isostructural ITQ-4 material [7,11].The linewidth of the Q4 signal increases with decreasingnSi/nFe ratios of the FeMCM-58 materials and the struc-tural information is lost. Nevertheless, the observed lossin the structural homogeneity of the silicon sites with ris-ing iron content supports the increasing incorporationof iron into the MCM-58 framework [43]. The broad,less intense signal ranging from �98 to �105 ppm isassigned to a superposition of the signals due toSi((OSi)3, OAl) or Si((OSi)3, OFe), namely the silicontetrahedrally coordinated to one aluminum or iron

-40 -60 -80 -100 -120 -140 -160 -180

d

c

a

**

**

**

**

b

ppm

Fig. 7. 29Si MAS NMR spectra of calcined (a) FeMCM-58(90), (b)FeMCM-58(44), (c) FeMCM-58(29) and (d) FeMCM-58(22). (Theasterisks indicate spinning side bands.)

and three silicon atoms, with those of Q3 Si(OSi)3OHgroups [44]. There is no peak centered at about�100 ppm indicating the absence of Si coordinated totwo metal atoms (either aluminum or iron), hence, indi-cating the uniform distribution of the metal atoms.

In FeMCM-58(22), the material with the highest ironconcentration, an additional sharp signal at �107 ppmis clearly observed, but also seems to be present withreduced intensity in the FeMCM-58(29) sample. Thesignal is very sharp compared to the other resonancesand is consistent with the presence of an impurity suchas cristoballite or quartz as also reported for MCM-65[45,46]. However, the powder diffraction patterns donot clearly indicate such an impurity although quartzis a commonly formed impurity in the synthesis ofMCM-58 [3]. We have to note that this silicon signalfrom quartz impurity phases gives likewise rise to theconspicuous spinning sidebands whose origin will be dis-cussed in the following. In general, the chemical shiftanisotropy of 29Si is small so that sidebands are notobserved in diamagnetic materials in contrast to 27AlNMR spectra. Therefore, the presence of sidebands inthe 29Si MAS NMR spectra indicates the existence ofadditional internal inhomogeneous magnetic fieldswithin the FeMCM-58 materials. Similar silicon side-band spectra have been reported for FeZSM-5 [47]and were attributed to the presence of ferromagneticor superparamagnetic iron oxide, Fe3O4, particles inthese materials [48] giving rise to large magnetic suscep-tibility broadenings. So it can be inferred that in the cal-cined FeMCM-58 materials, the total incorporation ofthe Fe(III) into the framework was not achieved andpart of the iron is existing as iron oxide particles onthe outer surface of the material.

In order to verify that the sidebands can be reallyattributed to iron oxide particles, the 29Si MAS NMRspectrum of calcined, iron-free containing parent(Al)MCM-58 is compared with those of iron-exchangedFe-MCM-58 and as-synthesized and calcined FeMCM-58(90) in Fig. 8. As expected, the parent AlMCM-58does not show any side bands, whereas the spectrumof the Fe(III) ion exchanged AlMCM-58, Fe-MCM-58, reveals intense side bands due to the presence of fer-romagnetic iron oxide clusters in this material, which areformed as a consequence of the liquid state ion exchangeprocess. The as-synthesized and calcined FeMCM-58(90) samples also display side bands, but theirintensity is small compared to that of Fe-MCM-58. Thisindicates smaller clusters or a smaller amount of ferro-magnetic iron oxide clusters and, thus, a better disper-sion of Fe(III) in FeMCM-58(90) when compared toFe-MCM-58. It is also interesting to note that the inten-sity of the side bands in the calcined FeMCM-58(90)material is somewhat higher when compared to that ofthe as-synthesized material. This is reasonable, becauseduring calcination extraframework Fe(III) is formed,

0 200 400 600 800

e

d

c

b

a

g = 2.00

g = 2.30g = 4.29g = 9.05

B(G)

Fig. 9. X-band spectra at 77 K of calcined (a) FeMCM-58(90), (b)FeMCM-58(44), (c) FeMCM-58(29), (d) FeMCM-58(22) and (e) Fe-MCM-58.

3000 6000 9000 12000 15000

e

d

c

b

a

g = 2.00

B (G)

Fig. 10. Q-band spectra at 6 K of calcined (a) FeMCM-58(90), (b)FeMCM-58(44), (c) FeMCM-58(29), (d) FeMCM-58(22) and (e) Fe-MCM-58.

-40 -60 -80 -100 -120 -140 -160

d

c

b

**

**

* *

a

ppm

Fig. 8. 29Si MAS NMR spectra of calcined (a) (Al)MCM-58, (b) Fe-MCM-58, (c) FeMCM-58(90) as-synthesized and (d) FeMCM-58(90).(The asterisks indicate spinning side bands.)

V. Umamaheswari et al. / Microporous and Mesoporous Materials 89 (2006) 47–57 53

which can exist as either isolated species or as iron oxideclusters. Comparing the 29Si MAS NMR spectra of cal-cined FeMCM-58 materials with different nSi/nAl ratios(Fig. 7), it can be discerned that the intensity of the sidebands versus the main band increases with rising con-centration of iron in the material. This suggests anincrease of the concentration of the ferromagnetic ironoxide clusters with increasing iron concentration of thematerial as already evidenced by UV–Vis spectroscopy.This again indicates that the incorporation of Fe(III)into the framework is facilitated with increasing amountof aluminum in the zeolite. However, we like to stressthat the concentration of ferromagnetic iron oxide clus-ter is smaller and the Fe(III) seems to be better dispersedin all the isomorphous substituted calcined FeMCM-58materials when compared to ion exchanged Fe-MCM-58.

3.6. EPR spectroscopy

In order to elucidate the isomorphous substitution ofiron into MCM-58, the paramagnetic Fe(III) ions hav-ing an electron spin S = 5/2 are studied by EPR spec-troscopy at X- and Q-band frequency. X- and Q-bandEPR spectra of the calcined FeMCM-58 zeolites withdifferent nSi/nFe ratios are compared with the spectrumof the fresh ion-exchanged Fe-MCM-58 in Figs. 9 and10, respectively. As-synthesized FeMCM-58 samplesgave comparable EPR spectra (not shown). As com-monly observed for Fe(III) in zeolites, the X-bandspectra consist of an intense signal at geff = 4.29 accom-panied by shoulders at geff = 9.05 and geff = 2.30 andanother intense broad signal at geff = 2.00. The relativeintensity of the signal at geff = 4.29 observed forFeMCM-58 decreases with decreasing nSi/nFe ratio. AllQ-band spectra show only a single signal at geff = 2.00.Signals at geff = 4.29 and geff = 2.3 are not observed.

In both frequency bands, the intensity and linewidthof the geff = 2.00 signal increases with increasing Fe(III)content of the material. The broadening may be due toan increase in the dipole-dipole interaction among theparamagnetic Fe(III) species existing in the materials.

The interpretation of the Fe(III) EPR signals in zeo-lites and their assignment to various iron species havebeen controversially discussed in numerous papers andare still a matter of debate. Though many reports havebeen published regarding the state of Fe(III) ions in zeo-lites studied by EPR spectroscopy, none of them couldclearly distinguish between the Fe(III) ions in frameworksites and ion exchange sites [14,18,39,41,49,50,52–55,60].In general, the occurrence of a signal at geff = 4.29 in theX-band spectrum is attributed to Fe(III) species withlarge zero field splitting (ZFS) parameters D � g Æ b ÆB0 � 9 GHz and significant asymmetry parametersE/D � 1/3, but can already be observed for D valuesin the order of 6–8 GHz as evidenced by spectral

0 200 400 600 800

6.29 4.272.00

4.014.27

2.05 2.00

2.00

4.299.06

c

b

a

B(mT)

Fig. 11. X-band spectra of (a) fresh, (b) dehydrated and (c) NO-loadedFe-MCM-58 materials.

54 V. Umamaheswari et al. / Microporous and Mesoporous Materials 89 (2006) 47–57

simulations. The absence of the geff = 4.29 signal at Q-band frequency puts an upper limit of D < 20 GHz onthe possible maximum ZFS parameters. Otherwise theobservation of a signal at geff = 2.0 at Q-band but alsoat X-band frequency suggests the presence of Fe(III)species with much smaller ZFS parameters ofD < 6 GHz and D < 4 GHz, respectively. A naturalexplanation of these contradicting results is the assump-tion of a broad distribution of ZFS parameters withupper limits of D < 20 GHz and E/D 6 1/3, whereX-band and Q-band experiments are especially sensitivewith respect to the large and low values of the D param-eter distribution.

In a recent paper, Ferretti et al. [56] demonstratedthat all the discussed typical spectral features observedin the X- and Q-band Fe(III) EPR spectra can be nicelyreproduced by large distributions in the ZFS parameterswith 0 6 E/D 6 1/3, D values centered at 2.4–3.2 GHzand ranging from about 0.4 GHz to more than 8 GHzdepending on the preparation of the high siliceous ironcontaining MFI type zeolites. It is worth to mention thatthese D values are comparable to those reported forFe(III) species incorporated at tetrahedral lattice sitesin ABO4 type single crystals [51]. This again supportsthe presence of tetrahedrally coordinated Fe(III) species[FeO4] in FeMCM-58. The large distribution in the ZFSparameters of the framework substituted Fe(III) foundfor the high siliceous zeolites is explained by inhomoge-neities present in the zeolite matrices such as differentlattice sites accessible for Fe(III) ions, lattice defects,and a random distribution of the Al atoms over theframework sites, or induced by the isomorphous substi-tution of the iron itself. In the calcined materials, extra-framework Fe(III) ions in clusters of various nuclearityare even expected to increase the inhomogeneity to a lar-ger extent. The fact that the EPR spectra of ionexchanged and framework incorporated Fe(III) are verysimilar (Figs. 9 and 10) could very well prove the pres-ence of similar tetrahedral coordination for the Fe(III)species in the framework as well as in the extra latticesites, as evident by recent XAFS results for a Fe(III)modified ZSM-5 zeolite [34]. In ion exchanged materialsferric iron cations are suggested to exist as eitherZ�[Fe(O)2]

+ or Z�[Fe(OH)2]+ moieties, where Z� repre-

sent the ion exchange sites in the zeolite [34,58] in agree-ment with our EPR results.

3.7. EPR spectroscopy of NO adsorption complexes

over dehydrated ion-exchanged Fe-MCM-58 and

framework-substituted FeMCM-58 samples

Ion exchange of Fe(III) ions into aluminosilicatematerials can result in iron existing inside the pores ofthe zeolite or on the surface, in the form of nanoclusterswith the typical composition Fe3O4 [57] and as isolatedcations species of type Z�[Fe(O)2]

+ or Z�[Fe(OH)2]+

[34,58]. Such isolated Fe(III) species can undergo facileautoreduction to Fe(II) under inert atmosphere at hightemperatures [59]. Fig. 11 presents the X-band spectraof fresh, dehydrated and NO-loaded Fe-MCM-58.Upon dehydration of Fe-MCM-58, the intensity ofthe characteristic signal features of Fe(III) in freshion-exchanged materials decreases drastically exceptthose of the signal at geff = 4.29, whose g value slightlyshifts to 4.27. Furthermore, a new weak signal can beobserved at geff = 6.29. The disappearance of the geff =2.00 signal in the thermally treated sample is presum-ably caused by sever structural distortions of the localsymmetry of the previously tetrahedrally coordinatedferric ion species such as for instance the loss of coordi-nated water in case of Z�[Fe(OH)2]

+, which likelyresults in drastic increase in the ZFS parameters. Thisspecies then either contribute to the signal featured atgeff = 4.29 or even become EPR silent because of extre-mely low symmetry [60,61]. Berlier et al. [60] haveobserved the same effect for the pretreated Fe-silicalitematerials. In their work they showed by IR and EPRspectroscopy that the geff = 2.00 signal is caused by iso-lated Fe(III) sites solvated by water and its disappear-ance upon thermal treatment is related to waterdepletion of the Fe(III) species. During this pretreat-ment process autoreduction of the Fe(III) species mayalso take place resulting in the formation of EPR silentFe(II) ions [62]. The spectrum of the subsequently NO-loaded Fe-MCM-58 material shows additional signals atgeff = 4.01, 2.05 and 2.00, when compared to the dehy-drated material. The low field signal at geff = 4.01 is typ-ical for a S = 3/2 spin system with large ZFS parametersD � tmw and only small deviation from axial symmetryE/D � 1/3 [63]. At Q-band frequency the signal isobservable at the same g value (Fig. 10a) which providesa lower limit of D > 70 GHz for the ZFS parameter.Similar results have been observed for the dehydratedand NO-loaded ion-exchanged zeolites Fe-ZSM-5 and

300 600 900 1200 1500

1.994.01

1.994.01

b

a

B(mT)

Fig. 12. Q-band spectra of NO-loaded (a) Fe-MCM-58 and (b)FeMCM-58(90).

150 300 450 600 750

c

b

a

9.064.29

2.00

4.274.00 2.05

2.00

2.004.275.46

6.38

B(mT)

Fig. 13. X-band spectra of (a) fresh, (b) dehydrated and (c) NO-loadedFeMCM-58(90).

V. Umamaheswari et al. / Microporous and Mesoporous Materials 89 (2006) 47–57 55

Fe-Y [52,63], where the geff = 4.01 signal could beassigned to high spin NO adsorption complexes,Fe(II)–NO, formed due to the reaction between Fe(II)and NO. The process of high spin Fe(II)–NO complexformation involves the transfer of one electron fromFe(II) to NO resulting in an electronic structure[Fe3+–NO�]2+ of the adsorption complex. The strongantiferromagnetic interaction of the high-spin Fe(III)(S = 5/2) ion with NO� (S = 1) results in the formationof a species with total spin S = 3/2. Actually NO mole-cules have been successfully used as a probe for thedetection of Fe(II) ions in many biological systems suchas iron containing proteins and enzymes [64–67], wherelikewise Fe(II)–NO complexes with electronic structure[Fe3+–NO�]2+ and a total electron spin of S = 3/2 areformed that give rise to ERP signals at geff = 4. In addi-tion to high spin Fe(II)–NO complexes, the formation ofminor low spin [Fe+–NO+]2+ complexes with S = 1/2has also been reported after NO adsorption on ZSM-5[62], zeolite Y, and mordenite ion-exchanged with fer-rous ions [63]. The [Fe+–NO+]2+ complexes give riseto an orthorhombic g tensor with gxx = 2.015,gyy = 2.055 and gzz = 2.089. Hence in the present casethe observed signal at geff = 2.05 is assigned to the lowspin form [Fe+–NO+]2+ of the Fe(II)–NO adsorptioncomplex.

In addition to the signals of the Fe(II)–NO adsorp-tion complexes and isolated Fe(III) ions a sharp signalat geff = 1.99 was observed in the X- and Q-band EPRspectra of Fe-MCM-58 materials upon NO adsorptionon the dehydrated samples. Such a signal is commonfor a number of thermally treated zeolites [68,69], butits exact nature, viz. paramagnetic oxygen species or lat-tice defects, has not been clarified yet.

In order to probe the autoreduction of ferric to fer-rous iron in the isomorphous iron substituted zeolitesFeMCM-58(90), this materials was also dehydratedand subsequently exposed to nitric oxide. The X- andQ-band spectra of pretreated and NO adsorbedFeMCM-58(90) material are shown in Figs. 12b and13c respectively. The X-band spectrum of deydratedFeMCM-58(90) exhibits the signals of the typical spec-trum of Fe(III) ions at geff = 4.27 with ZFS parametersD > 6 GHz and E/D � 1/3 in addition to the narrow sig-nal at g = 1.99 which have already been observed anddiscussed for the ion-exchanged Fe-MCM-58. Againthe broad signal component at geff = 4.27 indicativefor Fe(III) ions with smaller D values cannot beobserved after dehydration. But in contrast to Fe-MCM-58, the low field feature at geff = 9.06 of theFe(III) spectrum seems to be less affected by the thermaltreatment and two new spectral features at geff = 6.38and 5.46 are developed. According to previous results[52], they are assigned to the effective gx and gy compo-nents of Fe(III) high-spin (S = 5/2) species having astrongly tetragonally distorted octahedral symmetry.

The formation of such distorted octahedral Fe(III) com-plexes is assisted by the presence of hydroxyl groups inthe coordination sphere of iron ions at higher activationtemperature [52]. The existence of octahedral Fe(III)moieties in FeMCM-58 zeolites was likewise evidencedby UV–Vis spectra of fresh FeMCM-58(90) materials(Fig. 4a).

After pretreatment and NO adsorption, FeMCM-58(90) behaves similar to the ion exchanged Fe-MCM-58 material. NO adsorption introduces two new signalsat geff = 4.01 and 2.05 in addition to the Fe(III) signal atgeff = 4.27–4.29, which is observed in the fresh and dehy-drated material at X-band frequencies (Fig. 13). Notethat the geff = 4.01 signal is likewise observed at Q-bandfrequencies (Fig. 12b) but with lower intensity. Againthe two signals at geff = 4.01 and 2.05 are attributedFe(II)–NO adsorption complexes with high spin,

[Fe3+–NO�]2+ (S = 3/2), and low spin, [Fe+–NO+]2+

(S = 1/2), configuration. Theses results evidence the for-mation of Fe(II) ions in FeMCM-58(90) due to autore-duction of Fe(III) species during the dehydration of the

56 V. Umamaheswari et al. / Microporous and Mesoporous Materials 89 (2006) 47–57

framework-substituted materials. However, earlierreports claim that Fe(III) present in the framework ofthe zeolite is hard to reduce by vacuum treatment evenat very high temperatures [67]. The reduction of frame-work Fe(III) ions by electron transfer to a neighbored[AlO4] tetrahedron was only observed at strong dehy-dration conditions for FeZSM-5 zeolites having an unu-sual low nSi/nFe ratio of about 10 [70]. But in the presentcase with nSi/nFe = 90, reduction of framework Fe(III)to form Fe(II) seems very unlikely. Thus the Fe(II),which is responsible for the formation of Fe(II)–NOcomplexes, is probably created by autoreduction of iso-lated Fe(III) species, which must have before beenextracted from the framework during dehydration orcalcination [66].

The presence of nonframework Fe(III) ions has alsobeen observed for isomorphously substituted Fe-silica-lite, where about 25% of the framework iron wasextracted by thermal treatment (773 K) of the as-synthe-sized material in air [41]. Hence, NO adsorption studiescan be used as a powerful tool for the detection of Fe(II)ions in these material and also for the indirect measureof the presence of isolated extraframework Fe(III) ionsin zeolitic materials. This study also suggests that iron-containing FeMCM-58 zeolites could be interesting cat-alysts for NO decomposition.

4. Conclusions

FeMCM-58 zeolites with different nSi/nFe and/or nSi/nAl ratios were synthesized hydrothermally and com-pared with Fe(III) ion-exchanged Fe-MCM-58 materialsobtained from (Al)MCM-58. The incorporation ofFe(III) ions into FeMCM-58 materials has been evi-denced by various spectroscopic methods. UV–Vis and29Si MAS NMR spectroscopy revealed the partialextraction of the iron from the framework and the sub-sequent formation of minute iron oxide particles duringcalcination of FeMCM-58. However, better dispersionof iron was detected in these samples as compared toFe-MCM-58 prepared by liquid state ion-exchange. asindicated by the smaller amount of ferromagneticFe2O3 clusters in FeMCM-58. EPR experimentsrevealed the presence of distorted tetrahedrally coordi-nated Fe(III) species with substantial structural inhomo-geneity for both materials, calcined FeMCM-58 and Fe-MCM-58. This does not seem surprising as Fe(III)framework incorporation and ion exchange are expectedto lead to iron species [FeO4] and Z�[Fe(O)2]

+ orZ�[Fe(OH)2]

+ with comparable symmetry. The EPRstudies of NO adsorption over FeMCM-58 and Fe-MCM-58 reveal the formation of high spin, (Fe3+–NO�)2+ (S = 3/2) and low spin (Fe+–NO+)2+ (S =1/2) NO adsorption complexes, with the former one asthe most predominant species. Thus it is inferred that

Fe(II) species are formed by autoreduction of isolatedFe(III) species during the dehydration process in bothion-exchanged and framework-substituted materials.This indicates the presence of isolated Fe(III) speciesin both systems irrespective of the method of Fe(III)introduction. This study also suggests that iron-contain-ing MCM-58 materials are promising catalysts for NOdecomposition, which is of high interest in environmen-tal catalysis.

Acknowledgements

Financial support of this work by Deutsche Fors-chungsgemeinschaft (DFG) and by Fonds der Chemis-chen Industrie is gratefully acknowledged. We alsothank Prof. D. Freude, Prof. D. Michel (Leipzig) andProf. S. Ernst (Kaiserslautern) for their valuablesuggestions.

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