incorporation of iron in sodalite structures and their

7/27/2019 Incorporation of Iron in Sodalite Structures and Their

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Applied Catalysis A: General 223 (2002) 147160

Incorporation of iron in sodalite structures and theirtransformation into other iron containing zeolites

Synthesis of Fe-NaA (LTA)

Pl Fejes a,, Imre Kiricsi a, Kristf Kovcs b, Kroly Lzr c, Istvn Marsi d,Albert Oszk e, Antal Rockenbauer f, Zoltn Schay c

a Department of Applied and Environmental Chemistry, University of Szeged, Rerrich ter 1, 6720 Szeged, Hungaryb Department of Silicate and Materials Engineering, University of Veszprm, Veszprm, Hungary

c Institute of Isotope and Surface Chemistry, Chemical Research Center of the Hungarian Academy of Science, Szeged, Hungaryd Juhsz Gyula Teachers Training College, Institute of Chemistry, University of Szeged, Rerrich ter 1, 6720 Szeged, Hungary

e Institute of Solid State and Radiochemistry, University of Szeged, Rerrich ter 1, 6720 Szeged, HungaryfInstitute of Experimental Physics, Technical University, Budapest, Hungary

Received 15 May 2001; received in revised form 11 July 2001; accepted 11 July 2001

Abstract

In the low temperature digestion of gibbsitic aluminium ores an iron content impurity phase, desilication product (DSP),will be separated from the sodium aluminate liquor. Due to its main component: (hydroxy- or carbonate-) sodalite (SOD) andamount, reaching 710% of the feedstock, the DSP is an important raw material for synthesising various zeolites. Applyingthe solgel technique the authors developed synthesis methods to prepare iron content (chloride-) sodalites, in order to modelhow they can be transformed to commercially interesting types of zeolites (e.g. to NaA (LTA)).

Using diluted mineral acids (e.g. 5 wt.% sulfuric acid) in amounts equivalent (or rather in 10% excess) to the ion exchangecapacity, a heat-treatment at 100C, lasting 3 h leads to an amorphous product which can easily be recrystallised to variouszeolites. Outgoing from iron containing SOD, a new kind of NaA (LTA) could be prepared possessing various amountsof iron. Mssbauer, XP, ESR and UVVIS spectroscopic investigations revealed that up to 3 wt.% total iron content, as anaverage, about 6062% iron is sited in framework (FW) positions, in Th co-ordination. The other part is located in the voids,in extra-framework (EFW) positions, in Oh co-ordination, as highly dispersed iron oxide. This component exhibits strongmagnetic interaction which manifests itself in large linewidths when taking the ESRspectra. Mssbauer spectroscopy revealedthat the FW/EFW iron ratio decreased with the total iron content.

Due to the chemistry of solgel technique, in alkaline environments complete iron incorporation into zeolitic frameworks

can never be attained. Applying complexing agents or other methods EFW iron can be dissolved during the acid treatmentpermitting the synthesis of low module zeolites with uniform Fe(III) siting. The various kinds of iron content LTA zeolitesmay catalyse (biomimetic) selective oxidation reactions between reactants capable to enter the pore structure. 2002 ElsevierScience B.V. All rights reserved.

Keywords: Fe (and 57Fe) content NaA (LTA) zeolites; Instrumental characterisation by SEM; XRD; Mssbauer; DR UVVIS; XP and ESRspectroscopy

Corresponding author. Tel.: +36-62-544-316.E-mail address: [email protected] (P. Fejes).

0926-860X/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0926-86 0X(01 )0075 4-2


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148 P. Fejes et al. / Applied Catalysis A: General 223 (2002) 147160

1. Introduction

An alternative extraction process for Bayers uni-

versally used aluminium manufacture is low tem-perature (140160C) digestion of aluminium orescontaining predominantly oxides with composi-tions close to the trihydrate, like gibbsite and hy-drargillite (see, e.g. [1]). The removal of silica fromthe sodium aluminate pregnant liquor (desilication)produces an important by-product, the desilicationproduct (DSP), in amounts reaching 710 wt.% ofthe gibbsitic type feedstock whose composition ap-proximates that of (hydroxy- or carbonate-) sodalite:3[Na2OAl2O32SiO2(2 + k)H2O]Na2X (where 0 k 2, and X: 2OH, CO32, SO42, 2[Al(OH)4]

or 2Cl). As impurity phase the DSP contains sulfatesodalite (noseane) and cancrinite as well, in lowerquantities. By virtue of its composition DSP can beprocessed further to higher added value products suchas various commercialised zeolites (like NaA (LTA),NaPc (GIS) (e.g. maximum aluminium P, or simplyMAP), NaX (FAU) (e.g. low silica X, or LSX), etc.)provided the thermodynamically stable sodalite phasecan somehow be transformed into the metastable,thermodynamically not-preferred zeolite phases men-tioned previously.

Fig. 1. XRD of a desilication product from Ziar nad Hronom.

Sodalites are unbelievably stable in alkaline en-vironments (even in caustic melts!), but cannotwithstand acids: in diluted mineral acids (sulfuric acid

preferred) H3O+

ions (4mol dibasic acid is needed performula-weight sodalite if it contains 2 mol interca-late NaOH) exchange for Na+ ions and at the boilingtemperature the sodalite structure turns amorphouswithin a few hours without complete disintegration ofthe solid. In the amorphised sodalite the structural alu-minium is left practically intact and the sodium contentappears dissolved as the relevant sodium salt. The acidtreated solid sample can be washed out or even driedif necessary and used as such for synthesising variouszeolites. These experimental observations were the keyissues to a Hungarian invention providing possibilityfor the utilisation of amorphised alkali (earth alkali)aluminium hydrosilicates in zeolite syntheses [2].

At the onset of this research work we have got twoDSP samples from Ziar nad Hronom (Slovakia). TheXRD of this sample (see Fig. 1) reveals clearly that themost intense reflexions are due to (hydroxy-) sodalite,indeed, thus what we want to tell about the modifi-cation of sodalites is mutatis mutandis valid for DSPsamples as well.

This paper tries to find the answers to a few unre-solved questions concerning acid treatment (theoreti-


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P. Fejes et al. / Applied Catalysis A: General 223 (2002) 147160 149

cal and practical amounts of acid, experimental pro-cedures for reliable removal of traces of the sodalitephase, possibility of deferration during the treatment,

etc.) and attempts to open up new experimental waysto crystallise amorphised (iron containing) sodalitesinto NaA (LTA) or other zeolites which contain in-corporated Fe(III) ions in the framework structure. Itis worth appreciating that the known siting of ironin LTA structures might be advantageous in heteroge-neous catalysis as well.

Because the Ziar DSP samples, as far as theiramounts and iron impurity levels are concerned, werenot adept to the task, first of all it was necessary tosynthesise SOD (and thereafter NaA (LTA)) sam-ples with Fe(III) contents between 03 wt.%. Theirstudy by Mssbauer, ESR, XP and diffuse reflectanceUVVIS absorption spectroscopies was also an im-portant task of this paper.

2. Experimental

2.1. Synthesis methods to produce iron containing

sodalites and NaA zeolites

According to Szostak the synthesis of Fe(III) con-

tent zeolites has to be carried out at low pH of themedium in order to avoid precipitation of insolubleFe hydroxide [3] which, if formed, cannot be incor-porated in the zeolitic framework.

Pursuing this principle the (Fe/Al) silicate gels forthe planned sodalite and NaA (LTA) syntheses were al-ways prepared in strongly acidic solutions (hydrochlo-ric acid, sulfuric acid; Hammett constants around 2to 2). Using the solidified gel as such or after carefulwashing the composition of the synthesis slurry wasadjusted by adding the necessary amounts of sodium

aluminate (standard composition in mol/100 g solu-tion: 0.124Al2O30.307Na2O3.398H2O), NaOH,distilled water and a crystallisation promoter (ei-ther NaCl or NaClO4, or both). Characteristic slurrycompositions (in mol) are as follows.

For sodalites:

2SiO21.0(Al2O3 + Fe2O3)4.19Na2O98.0H2O

For NaA (LTA):

1.73SiO21.0(Al2O3 + Fe2O3)3.58Na2O92.0H2O

Typical Si/Fe ratios in the synthesis slurries were:200, 100, 50 and 25.

The crystallisation of slurries was preceded on all

occasions by milling (non-abrasive Teflon balls!) last-ing at least 5h.The synthesis containers were made of Teflon

placed in stainless steel casing. For syntheses inthe microwave oven pressure-tight autoclaves wereapplied, made fully of Teflon.

The fairly lengthy sodalite syntheses could be short-ened to 45 min (without ageing and mixing) in the mi-crowave oven. When air thermostat was applied themilled slurry was aged overnight (1415 h) using rock-ing agitation followed by 5 h crystallisation at 140C,maintaining the mixing.

Iron containing NaA (LTA) specimen were syn-thesised both from iron silicate gels and acid treatedsodalites (containing iron). The approximate compo-sition of this raw material was:

2SiO21.0(Al2O3 + Fe2O3)1.2H2O.

Outgoing from gels, part of the high content of wa-ter was removed (down to


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prepared by the method A from a slurry where theSi/Fe ratio was 50. A short description of the variousmethods can be found below.

2.1.1. SOD-A syntheses

The Fe (and Al) containing gels were preparedfrom Fe(NO3)39H2O and AlCl36H2O in hy-drochloric acid solution and Aldrich type water-glass (standard composition in mol/100g solution:0.450SiO20.175Na2O3.453H2O).

2.1.2. SOD-B syntheses

In the preparation of these sodalite samples care-fully washed (and partly by methanol dehydrated) ironsilicate gels (Si/Fe = 200, 100, 50 and 25) were used.

In order to increase resonance-absorption in Mss-bauer spectroscopy the Si/Fe = 200 samples wereprepared with 57Fe tracer.

2.1.3. SOD-C, acid treatments

Iron containing sodalites could be made fullyamorphous by treating them with 5 wt.% sulfuric acidat 100C for 3h. Application of a slight (10%) ex-cess (4.4 mol sulfuric acid per 3 mol SOD) is advan-tageous.

2.1.3.1. Description of a typical acid treatment. To10.0g of sodalite stored previously under ambientconditions add 85.4g of 5 wt.% sulfuric acid prefer-ably in a crystallisation container with Teflon lining.Close the container, place it in preheated air thermo-stat and let the acid react at 100C for 3h, applyingrocking agitation. After cooling filter off the amor-phised sodalite, wash it with warm water (and save thefirst 200 ml of filtrate for later ICP analysis (Al andFe)). Dry the solid at 110C for 45 h. Determine theyield.

This amorphised dry sodalite can be used as rawmaterial for various zeolite syntheses. For its compo-sition see the description of NaA-G syntheses.

The sodium content of sodalite appears in the aque-ous solution as sodium sulfate. Only 1.53% of thealuminium content can be found dissolved as an av-erage value of several treatments. However, it can beconjectured that even this small amount stems not fromthe lattice, rather from rests of the initial amorphousgel (i.e. from detrital material). The fate of iron iscompletely different (vide infra).

2.1.4. NaA-F syntheses using gels

As described previously, the sol/gel technique isvery effective to prepare iron silicate gels with differ-

ent Si/Fe ratios. After careful washing and (partial)dehydration with absolute methanol these gels are ex-cellent raw materials for NaA syntheses.

2.1.5. NaA-G syntheses outgoing from amorphised

sodalites

The acid treatment results in an excellent SiO2 and(Al2O3 + Fe2O3) source of the approximate composi-tion: 2SiO21.0(Al2O3 + Fe2O3)1.2H2O. Compar-ing this and the optimum slurry composition for NaA(LTA) it is felt that only the missing 3.58 mol Na2Oand (92.01.2) mol H2O should be supplied. Actually,in order to have smooth crystallisation it is highly rec-ommended to introduce 0.1 mol Al2O3 in excess inform of sodium aluminate.

2.1.5.1. A representative synthesis description for

recrystallisation of amorphised sodalites into NaA

(LTA). Pound in a mortar 5g dry, amorphised so-dalite. Put it into a (200 ml) Teflon-lined crystallisa-tion container, wet the sample with 12 ml distilledwater and make it slightly alkaline by adding a fewdrops of 50wt.% NaOH solution (check the pH).

Add 43.2 g 14.5 wt.% NaOH and 1.24 g standard alu-minate solution (vide supra). To ensure good mixingplace 23 Teflon balls in the vessel and age the slurrywhile mixing at 65C overnight (1214 h). The nextmorning rise the temperature to 88C and crystallisethe slurry at this temperature for an additional 2.5 hmaintaining the agitation. Wash and dry the NaAproduct as usual.

2.2. Transmission and scanning electron microscopy

The microstructure of tenderly powdered sampleswas investigated by transmission electron microscopy(TEM) and scanning electron microscopy (SEM) in aTESLA BS540 TEM at 120kV and a Philips XL30SEM using 20 kV accelerating voltage. Both TEM andSEM specimens were first ultrasonically dispersed inisopropyl alcohol for 10 min. Drops of the fluid weredried onto the Formvar coated TEM grids and polishedSEM specimen holders. TEM and SEM specimenswere carbon and gold coated, respectively. All TEMmicrographs confirmed SEM observations, therefore


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the paper contains the visually more descriptive SEMmicrographs only.

2.3. X-ray diffraction

X-ray powder diffraction spectra were registered inthe 2 = 343 interval using filtered Cu K ra-diation of the DRON-3 type X-ray diffractometer ofthe Department for Applied and Environmental Chem-istry, Szeged University (Hungary).

Both SOD and NaA (LTA) zeolites have prim-itive cubic crystal structure (P43n and Fm3c, re-spectively). For determining the u.c. parameters theDebyeScherrer powder spectra were registered withreduced scan rate (0.5min1) and increased sensi-tivity. A modified version of the POWDER code [4]allowed the determination of u.c. parameters within0.1% cumulative error (instrumental and XRD evalu-ation error).

2.4. Mssbauer spectroscopy

The zeolitic samples were pressed into wafers of12 mm thickness and submitted to various treatments(heat-treatment, evacuation, reduction by carbonmonoxide, etc.) before taking the Mssbauer spectra.

The reduction aimed at establishing the emplace-ment of Fe(III) ions in various (framework, FW,or extra-framework, EFW) positions. EFW Fe(III)ions can be reduced by CO more easily than thosesited in (oxygen-) shielded FW positions, thereby,as isomer shifts (IS) and quadrupole splittings (QS)differ markedly for (reduced) Fe(II) and (unreduced)Fe(III) ions, their presence and approximate amountscan at least in principle be assessed by computer de-convolution of the superimposed Mssbauer signals.

IS data are always referred to-iron as standard. Lo-

cation of the Mssbauer signals could be determinedwith an accuracy of 0.03 mm/s. Description of themeasuring cell and additional experimental details canbe found in [5].

2.5. ESR measurements

ESR spectra were taken in quartz tubes of 4mminner diameter at room temperature on an up-graded JEOL JES-FE3X spectrometer with 100 kHzfield modulation, using Mn(II)-doped MgO for the

calibration of g-measurements. A Lake Shore Model647 Magnet Power supply and a Stanford ModelSR830 DSP Lock-in Amplifier were applied. The field

was measured by temperature-stabilised Hall-probewith an accuracy of 105 T. The spectrometer wascontrolled by a Pentium 100 computer.

The spectra were fitted by a simulation programcapable to approximate each parameter by the com-bination of various iterative techniques [6]. Theexperimental spectra were considered as an S =(1/2) (1/2) transition with an effective g-tensorpossessing axial symmetry. Powder averaging wascarried out in 90 points using anisotropic linewidths.

2.6. XP spectroscopy

X-ray photoelectron spectra were registered on aKRATOS XSAM 800 equipment using Al K source,50meV step size, 300ms dwell time, 40eV pass en-ergy in FAT mode and 10 sweeps for the Fe(2p) range.The VISION software of KRATOS was used for dataevaluation.

The samples were pressed into wafers of 2mmthickness and measured in the as received state (in-cluding the heat-treated samples as well). The bindingenergies were referenced to Si(2p) at 103.3 eV.

2.7. Diffuse reflectance UVVIS absorption

spectroscopy (DRS)

Diffuse UVVIS reflectance absorption spectrawere registered by Perkin-Elmer Lambda 15 typespectrometer working under computer guidance in the200900 nm wave length domain, using MgO as ref-erence. The spectra were taken at room temperatureand ambient conditions without special precautions.

2.8. Chemical analysis

After acid treatments the supernatant liquors ofsamples were analysed for Al and Fe contents usingICP spectrometer at the Department of Inorganic andAnalytical Chemistry, Szeged University. The impor-tance of these investigations cannot be overestimated:eventually in the possession of these analytical datacould be assessed how much aluminium (or, if at all,iron) will be dissolved from the samples. Had thisamount exceeded an acceptable level, this would have


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inevitably contributed to the relinquishment of thispromising zeolite by-product utilisation technology.

3. Discussion of results

The synthesis of sodalites containing both alu-minium and iron is dealt with in the papers [79];there are a few describing the preparation and charac-teristics of sodalites with iron [10,11] or aluminium[12,13] content only. All silica sodalite analogueswere synthesised first by Bibby and Dale [14].

The usual way to carry out these syntheses (see, e.g.[7,8]) is to prepare a slurry similar to those seen pre-viously and to admix the aqueous solution of an ironsalt in the last step. This is absolutely incompatiblewith the claims of Szostak and Thomas [3] becauseof the inadvertent formation of Fe(III) hydroxidein the alkaline environment. Under common syn-thesis conditions (NaOH concentration 13wt.%,temperature < 150C) the iron hydroxide is insolublein the slurry and so it cannot be incorporated intothe lattice. In light of these considerations it is unbe-lievable that the materials thus obtained may appearwhite as we can read in the respective papers (see,e.g. [11]). It remains to be seen whether under ardu-

ous conditions (temperature 180C, and strongly

Fig. 2. SEM micrograph of a Fe-content SOD-B sample (produced from Si/Fe = 200 gel).

alkaline medium) some iron can be dissolved as ferratcapable to enter the zeolite lattice.

The alternative synthesis methods (from A to G and

still further) have been developed in order to com-ply with the principles stipulated by Szostak, alwayskeeping in mind to let orthosilicic acid react with pos-sibly mononuclear Fe(III) ions in strong acids beforethe slurry turns alkaline during the addition of sodiumaluminate and NaOH.

In Fig. 2a, Fe-SOD sample (crystallised by methodSOD-B from a Si/Fe = 200 washed iron silicate gel)appears in 30,000 magnification as conglomerateof irregular polyhedra, each of about 0.4 m size,stuck closely together. According to our experiencethe shape of product zeolites depends both on the ironcontent and the method of preparation. On TEM pic-tures (not shown) the sonicated samples are very oftencubes or parallelepipeds of 0.10.17m size.

It is a renowned view that isomorphic substitutionof silicon in the framework by another element ex-hibiting larger ionic radius in tetrahedral co-ordination(Th) gives rise to an increase of the u.c. constant(s)and therewith volume, and if it actually happens, thisattests that isomorphic substitution occurred.

Table 1 shows the most important data of sodalitesamples synthesised according to methods A and B.

EDX analysis proved that even when iron contents in


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Table 1Compositions and u.c. constants of Fe-content SOD-A and SOD-Bsamples

Sample Product Si/Fe ratio in slurry25 50 100 200

SOD-A Si/Fe 10.100 20.200 23.700 35.100Fe (wt.%) 3.360 1.700 1.450 0.980a ()a 8.922 8.906 8.905 8.918

SOD-B Si/Fe 13.400 21.500 41.600 79.300Fe (wt.%) 2.550 1.590 0.830 0.430a ()b 8.812 8.904 8.895 8.907

a a = 8.913 0.009.b a = 8.880 0.045; a = 8.896 0.035.

the slurries and solid phases changed in parallel, theprimitive cubic u.c. constants (designated by a) ofthe SOD-A and SOD-B products remained uninflu-enced within a standard deviation of 0.4% (this means0.035).

Including each sample the mean value of a:a = 8.896 0.035 is in good agreement witha = 8.893 0.002 published by Vaughan et al.

Fig. 3. XRDs of Fe-content NaA (LTA)-G samples prepared from acid treated SODs ((a) Si/Fe = 200 and (b) Si/Fe = 25, in the SODslurries).

[7] for sodalites with Fe contents in the interval:19.6 Si/Fe 50; only the standard deviationsdiffer slightly.

Just here we find a hint about the possible forma-tion of NaA (LTA) stating that at


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When turned to methods F (washed iron silicate gel)and G (syntheses outgoing from amorphised sodalites)the products were NaA (LTA) zeolites with iron con-

tent, as expected.Fig. 3 shows two XR diffractograms: a belongs toa NaA sample prepared from SOD-B200 (with an ironcontent of 0.36 wt.%) after amorphisation, whereas thestarting material to b was a SOD-B25 sample (con-taining 2.55 wt.% iron) and the method of preparationwas the same. The diffractograms imply that the pres-ence of iron hinders crystallisation and by reducingparticle size contributes to the increase of linewidths.The same sample a can be seen in Fig. 4 in 30,000magnification. The little glistening crystals underthe optical microscope are actually crystal balls of0.30.6m size, consisting of hundreds of elemen-tary crystals whose size is about 0.05 m. Here too,the appearance of the Fe-NaA product zeolites de-pends in a sensitive way on the raw material, the ironcontent and preparation method. TEM pictures (notshown) revealed very often Fe-NaA crystals as cubeswith rounded off edges (i.e. they expose [1 1 1] typefaces as well).

The compositions and u.c. constants of Fe-NaA(LTA) samples are compiled in Table 2. Vaughan et al.have been absolutely right in that the structure of

NaA was rigid, too. Whilst EDX analyses revealed a

Fig. 4. SEM micrograph of a Fe-content NaA (LTA) sample (from SOD-B200).

Table 2Compositions and u.c. constants of Fe-content NaA-F and NaA-Gsamples

Sample Product Si/Fe ratio in slurry25 50 100 200

LTA-F Si/Fe 17.200 36.200 54.100 71.000Fe (wt.%) 2.190 1.110 0.730 0.560a ()a 24.644 24.616 24.638 24.630

LTA-G Si/Fe 12.100 20.500 41.200 71.400Fe (wt.%) 3.020 1.860 0.930 0.550a ()b 24.611 24.636 24.657 24.596

a a = 24.632 0.012.b a = 24.625 0.027; a = 24.629 0.020.

monotonous increase of iron content in the solid phasein conjunction with the slurry composition, this had noprofound effect on the u.c. constants; they remainedpractically unchanged (for the whole set of samples)within a standard deviation of 0.08%, correspondingto the cumulative error of evaluation.

The virtual independence of u.c. constants, on theother hand the gradual shift of sample colour fromoff-white over buff to rust-red with the increase ofiron content gave rise to the suspicion that either noiron incorporation has taken place at all, or if had, it

could have been only partial: an unknown percentage


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was present as FW constituent, in Th co-ordination(and we know that in the absence of magnetic inter-actions this iron imparts a nearly invisible green hue

to the crystals [15]) and another percentage was sitedas EFW component in the voids and on the externalsurface of crystals in octahedral, Oh, co-ordination asiron oxide (-hydroxide).

It is worth appreciating that only one questionremained to be answered, notably the pertainingamounts of FW and EFW iron!

In contrast to isomorphously substituted high silicazeolites this question cannot be answered by measur-ing the zeolites ion exchange capacity. The zeoliticspecimen we are dealing with here are low moduletypes where part of the aluminium was substituted byiron, thus, for the FW iron Si/(Al + Fe(FW)) 1 andis independent of the FW/EFW iron ratio and this isvalid for the ion exchange capacity as well. Therefore,for estimating the FW/EFW iron ratio other experi-mental techniques should be used.

In order to provide reliable answer, in situ Mss-bauer spectra were obtained on SODB25 andSODB200 samples in calcined and CO-reducedstates. Then, having the samples recrystallised to NaAform, spectra were recorded again and were comparedto those of the SOD form.

Fig. 5 displays the 77 K in situ spectra of Si/Fe = 25sample. The calcined SOD sample exhibits a typicalFe(III) doublet (Fig. 5a; the respective data are com-piled in Table 3). The following CO treatment providesa means to estimate the portion of iron ions located inclose vicinity to each other: removal of oxygen atomsfrom Fe(III)OFe(III) pairs results in reduction toFe(II) state. Thus, presence of associated EFW ironions and Fe(FW)OFe(EFW) pairs can be confirmed.In contrast, single framework substituted Fe(III) ionscannot be reduced this way. Upon treating the SOD

sample with CO at 370

C ca. 2/3 of iron remains inferric form whereas ca. 1/3 is reduced to ferrous state(Fig. 5b). Thus, the greater portion of iron is probablylocated in FW position in the SOD-25 sample.

The recrystallisation of SOD-25 to NaA results in asignificant increase in the amount of EFW iron avail-able to CO molecules. The Fe(II) state is prevailing inthe spectrum recorded after the respective CO treat-ment (Fig. 5d and Table 3), and even evidences forpresence of ferromagnetic -iron carbide can be foundin separate particles (with characteristic size exceed-

Fig. 5. Mssbauer spectra of Si/Fe = 25 SOD (a, b) and NaA(ce) samples recorded at 77K and at ambient temperature (e);after calcination (a, c) and reduction by CO at 350C (b, d, e).

ing 3 nm, e.g. the peak at 1.8 mm/s). The presence ofFe5C2 is confirmed also in a 77 K spectrum recordedin a larger velocity range, not shown. For compari-son, the corresponding 300K spectrum is also pre-sented (Fig. 5e): having the CO desorbed, EFW Fe(III)in octahedral co-ordination appears in a large propor-tion (50% relative intensity) together with carbidicand ferrous iron species located mostly in octahedralco-ordination.


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Table 3Mssbauer parameters extracted from spectra of Figs. 5 and 6 a

Treatment Comp. IS QS FWHM RI

Fe SOD Si/Fe = 25Calculated (77 K) Fe3+ 0.42 0.74 0.62 100

620 K/CO (77 K) Fe3+ 0.39 0.87 0.90 62Fe2+ 0.81 1.86 0.95 15Fe2+ 1.18 2.21 0.69 23

Fe-NaA Si/Fe = 25Calculated (77 K) Fe3+ 0.44 0.54 0.54 64

Fe3+ 0.45 1.05 0.49 36

620 K/CO (77 K) Fecarbideb 0.08 3.33 0.45 29Fe3+ 0.52 0.88 0.60 17Fe2+tetr 0.95 2.19 0.57 25Fe2+ 1.36 2.19 0.69 29

620 K/CO (300 K) Fecarbideb 0.33 3.90 0.26 10Fe3+ 0.34 0.91 0.56 50Fe2+tetr 0.81 0.52 0.29 15Fe2+ 1.00 1.81 0.60 25

57Fe SOD Si/Fe = 200Calculated (77 K) Fe3+ 0.36 0.81 0.80 100

640 K/CO (77 K) Fe3+ 0.33 0.87 0.75 91Fe2+ 1.13 1.89 0.76 9

57Fe-NaA Si/Fe = 200Calculated (77 K) Fe3+ 0.47 0.64 0.36 35

Fe3+ 0.45 1.05 0.59 65

640 K/CO (77 K) Fe2+tetr 0.97 0.54 0.36 20Fe2+ 1.10 1.62 0.56 26Fe2+ 1.13 2.06 0.33 40Fe2+ 1.22 2.79 0.39 13

640 K/CO (300 K) Fe3+ 0.45 1.08 0.57 15Fe2+tetr 0.85 0.53 0.25 57Fe2+ 1.15 1.71 0.76 28

a Spectra were recorded at temperatures shown in brackets in the left column. IS: isomer shift, related to -iron, mm/s; QS: quadrupolesplitting, mm/s; FWHM: full line width at half maximum, mm/s; RI: relative spectral area, %.

b The pair of lines at 1.7 and +1.6 mm/s is the central part of a magnetic sextet of-Fe5C2.

The proportion of the FW iron is expected to in-crease by decreasing considerably the amount of ironin the sample (i.e. by increasing the Si/Fe ratio to200). Indeed, the majority of ferric iron cannot be re-duced, the proportion of EFW Fe(II) is only ca. 1/10in the spectrum of the CO treated SOD-B200 sam-ple (Fig. 6b). The acid treatment and recrystallisationof the sample exert a dramatic effect on the distri-bution of ions between FW and EFW emplacements.The CO treatment at 370C results again in a fullFe(III) Fe(II) reduction as the 77 K spectrum at-

tests (Fig. 6d). However, in contrast to the NaA-25sample, a planar co-ordination is dominating for Fe(II)ions (IS = 0.85mm/s, QS = 0.53 mm/s) in the spec-trum recorded at room temperature (see Fig. 6e). Thisprevailing component may most probably be attributedto single EFW ions (cf. [16]). A minor (about 15%)portion of iron is immediately reoxidised by raising thetemperature of measurement from 77 to 300 K whilemaintaining the CO atmosphere. This Fe(III) compo-nent is indicative of Fe(FW)OFe(EFW) pairs. In thiscomponent a slight increase in the IS value of Fe(III)


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Fig. 6. Mssbauer spectra of Si/57

Fe = 200 SOD (a, b) and NaA(ce) samples recorded at 77 K (ad) and at ambient temperature(e); after treatments of calcination (a, c) and reduction by CO at350C (b, d, e).

and a simultaneous decrease of the corresponding ISof Fe(II) can be explained by Fe(II) Fe(III) iner-valence charge transfer as suggested by Burns [17].

The studies by XP spectroscopy bear out the con-clusions drawn from the Mssbauer spectra in everyaspect. The sample studied here was a recrystallised

Fig. 7. XP Fe(2p) doublet spectra of a Fe-NaA-G specimen (fromSOD-B25) ((a) untreated and (b) heat-treated at 500C for 5h).

Fe-NaA-G specimen obtained from acid treatedFe-SOD-B25. The Fe(2p) doublet spectra are seenin Fig. 7 for the untreated a and treated b samplewhich was exposed to a 500C heat-treatment for 5 h.In the last case, the binding energies of the respectivecore levels are shifted by 0.45 eV towards lower ener-gies, but the 13.4 eV distance between the 2p1/2 and2p3/2 doublets did not change.

Not the slightest indication could be detected thatthe binding energies for FW and EFW Fe(III) ionswere perceptively different, contrary to Fe-ZSM-5

samples previously studied where a slight (1.9eV)shift in the respective peaks energies had been ob-served [18]. What can be seen is not more than adecrease to 2/3 value of the original line intensi-ties caused by the heat-treatment plus an increase oflinewidths revealing a change in the co-ordinationof iron. The change is obviously an increase of Ohpercentage manifested by deepening of the samplesrusty colour as well.

The diffuse reflectance UVVIS absorption spec-trum of a similar Fe-NaA-G sample (the raw material


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Fig. 8. Diffuse reflectance UVVIS spectrum of a Fe-NaA-Gsample (from Fe-SOD-B50).

for that was acid treated Fe-SOD-B50 with an ironcontent of 1.1 wt.%) is shown in Fig. 8. Although thespectrum is barely structured, it lends itself to furtherverify the simultaneous presence of FW and EFW ironco-ordination.

The first band centred at 246nm is a strongligand-to-metal charge transfer (CT) transition prov-ing the presence of Fe(III) ions in tetrahedralco-ordination by electronegative (in this case O2)ions [19]. The four to six possible electronic tran-sitions between states of different spin multiplicityin the 350570 nm region are actually forbidden,therefore, of low intensity. The band located at about370nm is of this type and characteristic of Fe(III)in tetrahedral co-ordination, too. The other possibletransitions of similar origin are superimposed hereby a wide diffuse band in the visible 470570 nm re-gion (bluish-green/green) due to iron oxide/hydroxidedebris in the zeolitic voids, whose complementary

colour we perceive as rust-red. This colour manifestsitself by strong magnetic interactions between theoxide particles which eventually leads to widening ofthe ESR signals (see later).

In a previous publication [20] we extensively dealtwith the partial incorporation of Fe(III) ions into ze-olitic frameworks, attributing the failure to achievecomplete substitution to various causes.

In relatively dilute (20 wt.%) solutions of sulfuric(and other common) acid(s) (pH 0) the Fe(III)

ions are present as binuclear (FeOFe)4+ twinsof various structures [21]. The same ions can beidentified in the iron silicate gels, too. However,

when these gels crystallise only one Fe(III) ion iscapable to enter the lattice, the other remains outsideand thereby develops an interesting heterolinkage(ThOh link).

In concentrated solutions of hard acids (like per-chloric acid; Hammett constant is less than 2) theconcentration of Fe(III) twins is considerably re-duced. But, in the slurry ready to be crystallised intoSOD, NaA and similar zeolites there is aluminatein excess and the pH is high, thus, from the iron sil-icate gel made at very low pH part of Fe(III) ionswill inevitably be set free to form insoluble iron hy-droxide increasing the rusty impurity in the voids.This is a serious obstacle which can be side-steppedonly by a masterly trick we would like to deal within the next publication.

The ligand fields for FW and EFW Fe(III) ions in thevery narrow and symmetric structure of sodalites andNaA (LTA) zeolites are similar, thus, the difference inthe binding energies of core level electrons might beso small that it cannot be resolved by todays ESCAor XP spectrometers. This can be the reason, too, that

experimental X-band ESR spectra of low (0.3 wt.%)and higher (3.0wt.%) iron contents in SOD and NaA(LTA) zeolites do not reveal any zero-field structure.The spectra can be interpreted as transitions between1/2 and 1/2 magnetic levels. Both the g-value andthe linewidth reveal significant anisotropy that can beascribed to the indirect effect of zero-field interactions,but the impact of locally disturbed ligand field due tothe statistically distributed paramagnetic Fe(III) ionscan also be important. Each spectrum can successfullybe modelled by one or two centres, both exhibiting

axial symmetry (i.e. in the effective spin Hamilto-nian g is anisotropic and D = 0, E = 0), while theincreasing spinspin interactions, as the iron contentis becoming larger, produce the linewidth anisotropy.Powder averaging of the sub-spectra was carried outin 90 points taking into account the g-factor andlinewidth dependence on the orientation of crystallites.

It is an obvious choice to assign one of thesesites to FW and the other to EFW iron types, butthe strong magnetic interactions currently hinder at-tempts to quantify the model. Nevertheless, the gzz


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and gxx = gyy values remain for both types near to 2.0uninfluenced by the iron content, as expected, onlythe respective (anisotropic) linewidths experience an

enormous increase (e.g. at 0.3% iron content thelinewidth Wzz equals to 50 Gauss for FW sites whichincreases to 2200 Gauss when the iron content reaches3.0 wt.%). Further details of the ESR spectroscopy ofthe samples will be published elsewhere.

It is worth noting that in the incompressible latticethe solid surface tension cannot develop fully rhombicenvironments (see [20]), so the characteristic signal atg = 4.29 is very weak or missing.

Following the acid treatment, the amorphised so-dalites can be recrystallised into various other types.Having practised this technique as a routine it wasrecognised soon that by adding various complex-ing agents or applying special techniques EFW ironcan be removed while that sited in the frameworkremains almost unimpaired. Among other indirectproofs this follows from the simple fact that while thedissolved amount of aluminium varies between 1.74and 2.97%, that for iron reaches the fairly high EFWlevels observed by spectroscopic methods, e.g. 11.1 gof a Fe-SOD-B200 sample (iron content 0.36 wt.%)contained 48 mg Fe. After treatment with 5 wt.%sulfuric acid and a reducing agent at 100C and last-

ing 3h, 15.4mg, i.e. 38% could be identified in thesupernatant liquor.

This part of the research work seems to be promis-ing: provided the low temperature digestion of gibb-sitic aluminium ores supply ample amounts of the so-dalite by-product, even the colour of the zeolites syn-thesised from this raw material can be advantageouslyinfluenced. This technique is capable to produce ze-olites (within a well defined family) with Fe(III) oreven 57Fe(III) tracer ions located exclusively in FWpositions permitting unequivocal matching of the re-

spective Mssbauer and ESR signals. It is thought thatit is of great theoretical importance. Not at last, thistype of Fe-content LTA zeolites (e.g. CaA) permit thepreparation of tailor-made (biomimetic) selective ox-idation catalysts for reactants not excluded from thepore structure by sieving effects.

From the experimental results and their variousevaluations the following conclusions can be drawn.

Syntheses of isomorphously substituted zeolites inalkaline media (irrespective of the module, cf. [22])

result inevitably in partial incorporation whicheversynthesis method is chosen. The co-ordination ofthe substituent is always mixed (FW and EFW); an

increasing part of the substituting ion (in our caseFe(III)) appears in EFW position at the expense ofFW substituent, as its amount in the synthesis slurryincreases.

Mssbauer spectroscopy revealed that at low (about0.4 wt.%) Fe loading the incorporation reachedabout 90% and it diminished approximately to 60%at 3 wt.% iron in the slurry.

Contrary to high module types where substitutingions larger than silicon always cause an increaseof u.c. volume, the rigidity and high symmetry ofSOD and LTA structures hampers lattice expansion.Their u.c. volumes remain strictly constant irrespec-tive of the degree of substitution. In full agreementwith this the X-ray photoelectron energies for FWand EFW species are inseparable. The difference ing-values of the respective ESR spectra for the sameions sited in ligand fields of (commencing) axialsymmetry is very little; the observed linewidthanisotropy is caused by spinspin interaction ofhigh dispersity EFW iron oxide, located in thevoids.

Treatment of low module natural or artificial zeo-

lites (like SOD, LTA, etc.) by diluted mineral acidsin amounts corresponding to the ion exchangecapacity, or 10% in excess, leads within 3h at100C to complete amorphisation, while the solidhabit of material so obtained will be retained. Af-ter washing (and drying, if necessary) this siliconand aluminium source may serve as raw mate-rial to synthesise other commercially interestingzeolites.

The acid treatment combined with addition of com-plexing and/or reducing agents (a theme for the sec-

ond publication) allows complete removal of EFWiron. The products exhibit white colour and are char-acterised by Fe(III) ions emplaced exclusively inframework position and tetrahedral co-ordination.The advantages of uniform Fe(III) ions siting fromthe point of view of Mssbauer and ESR spec-troscopy is obvious.

The possibility of influencing Fe(III) siting in LTAstructures may have importance in the preparationof selective oxidation catalysts for reactants of smallkinetic diameter.


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Acknowledgements

P. Fejes, I. Kiricsi, K. Lzr and A. Rockenbauer

acknowledge herewith the Grant no. T 029463 fromthe OTKA Commission. The authors express thanks toprofessor Lovat V.C. Rees (University of Edinburgh)for his valuable suggestions concerning Mssbauerspectroscopy measurements. Thanks are due to I. Saj(Central Research Institute for Chemistry of Hungar-ian Academy of Science, Budapest) for the diffrac-tograms, Mrs. F. Barna for the sometimes complicatedsynthesis work and to Mrs. Cs. Csikkel for the ICPspectral analyses. The contribution of P. Szab (Bu-dapest Technical University) in preparing the SEMmicrographs is also highly appreciated.

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incorporation of iron in sodalite structures and their

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