J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1329-1334 Solid-state Ion Exchange in Zeolites Part 53-N H,-Y-Iron( 11) Chloride Karoly Lazar Institute of Isotopes, Hungarian Academy of Sciences, Budapest, Hungary Gabriella Pal-Borbely and Hermann K. Beyer Central Research Institute of Chemistry, Hungarian Academy of Sciences, Budapest, Hungary Hellmut G. Karge* Fritz Haber Institute of the Max Planck Society, Berlin, Germany Solid-state ion-exchange reactions proceeding upon grinding of hydrated NH,-Y zeolite and FeCI, 4H,O in air and subsequent heating in vacuum have been followed by X-ray diffraction (XRD) and temperature-programmed evolution of volatile reaction products (H,O, NH, and HCI) monitored by mass spectrometry (MS). Changes in valence state and coordination environment of the exchanged iron ions upon heat treatment up to 720 K in vacuum and air, as well as in hydrogen after treatment in air, were studied by Mossbauer spectroscopy.Partial ion exchange and oxidation to cationic hydroxy-iron(iii) species proceed upon mere grinding. Heat treatment in vacuum results in partial autoreduction and characteristic environmental changes of di- and tri-valent Fe species, especially in the temperature range between 620 and 720 K. In an oxidizing atmosphere, autoreduction is suppressed; however, at temperatures >520 K, iron(ii1) oxide is formed in increasing amounts. Reduction by hydrogen subsequent to heat treatment in air at 720 K results in the formation of a magnetite-like oxide phase and some metallic a-iron.The phenomenon of solid-state ion exchange in zeolites was first observed by Rabo et ~1.'~~and Clearfield et aL3 In recent years, insertion of cations into zeolites via solid-state ion exchange has attracted increasing attention. Recently, the results of systematic investigations in this field obtained by several research groups were reviewed., Generally, solid-state ion exchange occurs at relatively high temperatures (550-1000 K) in intensively ground mix- tures consisting of (1) a hydrogen zeolite and (2) a metal halide or oxide. The process results in the formation of the respective metal zeolite and evolution of hydrogen halide and water. In contrast to conventional ion exchange proceeding in aqueous media, any desired ion-exchange degree can be achieved simply by applying the corresponding amount of salt, and, in many cases, even an ion-exchange degree of 100% can be reached easily in the solid state.Furthermore, this method does not require the handling of large volumes of salt solutions and long exchange times. Recently, the phenomenon of 'contact-induced' ion exchange between hydrated Na-Y zeolite and chlorides of Li, Na, K, Ca or and between NH,-Y zeolites and LaCl, has been reported. This reaction proceeds upon mere grind- ing of zeolite-salt mixtures at ambient temperature. Evidence of the exchange of cations under these experimental condi- tions included the formation of a separate crystalline phase of NaCl and NH,Cl, detected by XRD.In contrast to conven-tional ion exchange and similar to the solid-state variant, this type of ion exchange proceeds in solid mixtures of zeolites and salts, although intracrystalline water molecules adsorbed in the zeolite are obviously involved in the mechanism. Nevertheless, this process also provides a simple and, in many respects, advantageous method for cation insertion into zeolites and, after dehydration by subsequent heat treatment, it can be combined with 'true' solid-state ion-exchange reac- tions. For numerous chemical reactions, efficient mono- and bi- functional catalysts can be obtained by incorporation of -f Part 4: ref. 5. transition-metal ions into zeolites. The catalytically active species may be the zeolitic cation itself and, after reduction, highly dispersed transition-metal particles.Solid-state ion exchange has been shown to provide excellent possibilities for insertion of transition-metal ions (Pt, Pd, Cu, Fe, V, Cr etc.) into zeolites., The incorporation of easily oxidizable cations (e.g. Fe2+) in their lower valence state requires the exclusion of oxygen during the whole ion-exchange procedure. This leads to experimental complications, especially in the initial step, i.e. in the preparation of the ground zeolite-salt mix-tures. On the other hand, it is well known that, for example, Fe'" species formed during ion exchange of Fe2+ ions into zeolites in the presence of oxygen undergo, upon evacuation at higher temperatures, 'autoreduction' under evolution of molecular oxygen.8-' O The present study deals with the insertion of iron ions into Y zeolite by contact-induced ion exchange in mixtures of NH,-Y with iron@) chloride. In particular, changes of oxida- tion and coordination states of the iron cations during grind- ing of the mixtures in the presence of oxygen and upon successive heat treatments are investigated.Experimental Materials The parent Na-Y zeolite was provided by Union Carbide (Tarrytown, USA) and converted to NH,-Y by repeated ion exchange (15 times) with 1 mol dm-, NH,Cl solution. The (idealized) unit-cell composition of the exchanged zeolite was 10.3(~~4)54.iNa0.5 [Ai64.9Si 127.1O3841. Mechanical mixtures of this zeolite with crystalline iron(@ chloride tetrahydrate were prepared by intensive grinding in an agate mortar.The iron@) salt was applied in amounts equivalent to the aluminium content of the zeolite, i.e. the atom ratio of Fe to zeolitic framework aluminium was 0.5. No precautions were taken to exclude atmospheric moisture and oxygen during the grinding procedure. Methods X-Ray diffraction patterns of the mixture of NH,-Y zeolite and FeCl, before and after heat treatment at different tem- peratures were obtained with a Philips PW lo00 diffractome- ter using a graphite monochromator and Cu-Ka radiation. Temperature-programmed evolution (TPE) of volatile pro- ducts (HCl, H,O, NH, and 0,)during heating of the samples in high vacuum (< 5 x 10-Pa) up to lo00 K was measured by mass spectrometry.The method and apparatus are described in detail elsewhere." In situ Mossbauer measurements were carried out in a cell the description of which may be seen in ref. 12. The two series of 300 K spectra were recorded after various treatments (evacuation at 3 x lop2 Pa and calcination followed by reduction) at different temperatures (420, 520, 620 and 720 K) for 4 h. The calcination was carried out in air; reduction was performed in a flow of purified hydrogen. The isomer shift values are related to the centre of the a-iron spectrum mea- sured at room temperature. The lines are fitted with a Lor- entzian shape without constrained positional parameters. The accuracy of the positional data is kO.03mm s-'. Results X-Ray Diflkactometry Schematic XRD patterns of the parent NH,-Y zeolite and of its mixture with FeC1,.4H2O as prepared by grinding at ambient temperature and after heat treatment at 730 K are presented in Fig.1. Upon introduction of iron cations, the intensity of most of the XRD reflections considerably decreases. However, this effect does not indicate lattice damage. It is due partly to changes of the respective structure factors upon incorporation of iron cations, and partly to the higher X-ray absorption factor of the Fe component. The intensity re-increase of several reflections upon heat treat- ment and the lack of the broad peak at 20 = 20-30°, indica- tive of amorphous silica, show that the lattice cannot be seriously affected.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Contact-induced ion exchange proceeds upon grinding the NH,-Y-FeCl, mixture as evidenced by the appearance of the 110 reflection of crystalline NH,Cl at 28 = 32.68". However, the reflections indicative of FeCl, .4H20 do not completely disappear [Fig. l(b)]. Therefore, only partial ion exchange occurs. Upon heat treatment, the incorporation of iron cations gradually proceeds and at 520 K no separate phase of iron chloride can be detected. This ion exchange is accompa- nied by a steady decrease of the cubic unit-cell parameter and typical intensity changes of some reflections [c$ Fig. l(b) and (41.Using the relationship between the lattice parameter and the framework Si :A1 ratio of faujasite-type zeolites given by Breck and Flanigen,', the thus obtained Si : A1 ratio of the parent NH,-Y zeolite (1.8 :1) agrees well with the value determined by wet chemical analysis (2.0 :1).A small decrease of the lattice constant occurs upon grinding with FeCl, * 4H,O and a considerable shrinkage of the unit cell is observed after heating the mixture up to 720 K [see the cell parameters indicated in Fig. l(u)-(c)]. This unit-cell contrac- tion is due to the incorporation of iron lattice cations rather than to aluminium release from the framework and, hence, to an increase of the Si : A1 ratio of the lattice. TPEExperiments The intensities of the MS signals of the main products, HCl (mass number 36), NH, (mass number 17 minus the contribu- tion of H,O) and H,O (mass number 18), evolved from the FeC1, -4H2O-NH,-Y mixture during heating to lo00 K with a rate of 10 K min-were measured. The lower-temperature region of these evolution curves is shown in Fig.2(u). For comparison, the decomposition curves of crystalline ammon- ium chloride are presented in the same figure [Fig. 2(b)]. The maximum rate of release of ammonia and HCl is observed at 't n 10 20 30 28/degrees 400 500 600 Fig. 1 Schematic presentation of XRD patterns of (a) the parent temperature/K NH,-Y zeolite, a = 24.785 A; (b) the mixture of NH,-Fig. 2 TPE curves of hydrogen chloride (m/e = 36) (-), ammonia Y-FeCl, 1 4H,O ground in air at ambient temperature, a = 24.742 A; (m/e = 17) (---) and water (m/e = 18) (---*) evolved (a) from a (c) material (b) heated in air up to 720 K (heating rate: 10 K min-'), ground mixture of NH4-Y-FeC1,.4H,O (Fe : A1 = 0.5 :1); (b) from a = 24.542 .$; * FeCl, .4H,O; ** NH,Cl crystalline ammonium chloride J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 about 510-520 K and coincides with the maximum decompo- sition rate of ammonium chloride suggesting a preceding contact-induced ion exchange between the zeolitic com-ponent and the salt. A characteristic feature of the TPE pattern of the FeC1,-containing system is the small but sig- nificant and well reproducible delay of HCl evolution shifting the respective TPE curve to temperatures about 10 K higher than that for the ammonia evolution. In TPE patterns of mixtures containing other metal chlorides (e.g.CaCl, ,Fig. 3), ammonia and HCl evolution completely coincide, at least up to 570 K. As expected, the same is observed for the decompo- sition of ammonium chloride (Fig. 2). The water TPE curves of all NH,-Y-salt mixtures exhibit a maximum at about 400 K due to the desorption of water coordinatively bound to the metal cations. Among the systems investigated to date, only the mixture prepared with FeCI, * 4H,O evolves a second water species, revealed by an additional maximum at about 510 K coinciding with the ammonia release. This water probably originates from hydroxy groups which may have formed by hydrolytic pro- cesses involving iron ions and intracrystalline zeolitic water during the grinding procedure and then, at higher tem-peratures, were eliminated by dehydroxylation.At higher temperatures (>570 K for CaC1,-NH,-Y and 670 K for FeC1,-NH,-Y) the ammonia evolution declines faster than the HCl release. Therefore, part of the ammonium ions must be thermally decomposed and the resulting bridged hydroxy groups are eliminated only at higher temperatures by true solid-state ion exchange with the remaining CaC1, and FeCl, ,respectively, resulting in lattice metal cations and HC1. In summary, the main features of the TPE pattern of FeC1,-NH,-Y clearly reveal that, in this system, a complex reaction process is going on with a maximum rate at about Miissbauer Spectroscopy Mossbauer spectra (measured at 300 K) of the FeC1,-NH,-Y mixture heat-treated in vacuum at different temperatures are presented in Fig.4. Based on recently reported assign- ment~,'~the oxidation and coordination states of iron species were deduced from the Mossbauer parameters (isomer shift and quadrupole splitting) of the signals obtained by spectrum deconvolution (see Fig. 5). Depending on the pretreatment temperature, two Fe"' species differing in their coordination state (octahedral and trigonal, respectively) and six different Fe" species [viz. FeCl, xH,O, two types of iron(rr) ions in tetrahedral and trigonal coordination, respectively, and three Fe" species differing somehow in their octahedral environ- ment] could be distinguished. The presumption of the exis- tence of various iron components is proven by the low 2' values of the fits (x2 < 1.8).Fitting with smaller peak numbers resulted in a significant increase of x2. The results are summarized in Table 1; Fig. 6 illustrates the dependence of the spectral area of the individual Fe species on the pretreatment temperature of the zeolite-salt mixture. After grinding of hydrated NH,-Y and FeCl, .4H20 (Fe : Al,,,, = 0.5) at ambient temperature, the greater part (57%) of the iron applied is oxidized to the trivalent state. The Mossbauer parameters of this Fe"' species point to a filled (close to octahedral) coordination. The remaining Fe" is present in the form of three distinct species, one of which, amounting to about 21%, has been identified by its Moss-bauer parameters as partially dehydrated iron(1r) chloride.Another one (13%) is characterized by parameters similar to those of partially hydrated, tetrahedrally coordinated Fe" ions located in sodalite cages and large ~avities,'~ probably with one molecule water in its coordination sphere (Fei:tr). The third Fe" species (Fe:ct-l) has been shown to be in an octahedral environment probably provided by water and OH ligands [Fe(OH)+].' 400 500 600 -5 -3 -1 1 3 5 temperature/K velocity/mm s-' Fig. 3 TPE curves of hydrogen chloride (-), water (-.--) evolved from a groundY-CaCl, .2H20 ammonia (---) and mixture of NH,- Fig. 4 Mossbauer spectra of (a) NH4-Y-FeCl, * 4H20 ground at ambient temperature in air and material (a) after heat treatment in vacuum at (b)420 K, (c) 520 K, (d) 620 K and (e)720 K ,$ip'II I I -b I-5 I I veiocity/mm s-' Fig. 5 Mossbauer signals of individual iron species giving the best fit to the spectrum Fig.4(e)of ground NH,-Y-FeCl, heat-treated at 720 K. (e)Fe;;, ;(a)Fe:::, ;( El) Fe::,, ;(V) Fe:,-, ;(@I) Fe:t-3 Heating to 520 K results in a gradual autoreduction of the Fe:Et species as indicated by the decrease of its relative area to 44% at 420 K and to 38% at 520 K, accompanied by the formation of an Fe"' species (11Yo)with Mossbauer param- eters similar to those found for oxidized Fe-A zeolite and Table 1 Mossbauer parameters of iron species present in NH,-Y-FeCl, vacuum species parametef as prepared 0.37 0.63 57 ---0.69 0.35 13 ---0.83 1.95 9 ------1.13 1.83 21 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -60 --40 U--20 -300 400 500 600 700 temperature/K Fig. 6 Relative intensity (I,) of iron species found by Mossbauer spectroscopy in NH,-FeC1, ground at ambient temperature in air us. heat-treatment temperature. (e)Fe;, ;(W) Fe::, ;(0)Fe:it, ;( El) Fe:,-,F&,; (0) ;(V)Fe!c,-2 ;(0)Fe;c,-3 ;(+) FeC1, * xH,O attributed to trigonally coordinated Fe3 residing in the six- + membered window site SI." Furthermore, it has been shown by X-ray analysis16 that in oxidized samples of FeY zeolites iron is preferentially located in site SI'. Thus, the Fen' species appearing at 520 K (Fe:::J is also assigned to Fe3+ three-fold coordinated to three oxygen atoms of the D6R units.However, only part of the decrease of Fe!:t is due to conver- sion to Fei5,. A new Fen species (Fe&,J appears, in part due to autoreduction of Fe:;,, but also to full dehydration of the Fe::tr. Based on the interpretation of the Mossbauer spectra of Fe"'-A zeolite^,'^^'^ this species is attributed to Fe2+ ions coordinatively bound to three oxygen atoms of six-membered rings. Dramatic changes in the oxidation and coordination state of the iron species occur in the temperature range from 520 to 620 K. The concentration of Fe:Et abruptly decreases to 5% and new Fetct species appear. The processes are obvi- .4H,O mixtures after grinding and subsequent heat treatment in high after high-vacuum treatment 420 K 520 K 620 K 720 K 0.35 0.31 0.30 0.33 0.72 0.75 0.72 0.58 44 38 5 2 - 0.25 0.23 0.22 - 1.53 1.71 1.73 - 11 11 2 0.72 - - - 0.36 - - - 11 - - - - 1.08 0.88 0.92 - 0.78 0.62 0.68 - 21 21 30 0.79 - - - 2.00 - - - 22 - - - - - 0.95 0.99 - - 2.20 2.13 - - 37 45 - 1.06 1.23 1.27 - 2.60 2.19 2.16 - 9 26 20 1.04 1.06 1.79 1.90 23 21 a Parameter: 6, isomer shift; A, quadrupole splitting, I,, relative intensity.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ously accompanied by the evolution of ammonia, hydrogen chloride and water observed at about 520 K, when the tem- perature of the zeolite-salt mixture is continuously increased at a rate of 10 K min-' (see Fig.2). The parameters of the new FeLL,-, component do not exactly correspond either to those observed for iron@) ions in zeolite sites or to those for iron in crystalline aluminates (FeAl,O,).' Nevertheless, this species characterized by the parameters given in Table 1 may be related to iron(I1) ions interacting with oxidic extra-framework A1 species released from the framework during autoredu~tion.'~In line with this assumption, the amount of this species increases with increasing treatment temperature. The data for the Fefc,-, species are characteristic of ions located in the hexagonal prisms of the faujasite ~tructure.~.'~ The Mossbauer signal with parameters typical of FeCl, .xH,O suddenly disappears at temperatures above 520 K indicating that the part of FeCl, not incorporated in the zeolite lattice by contact-induced ion exchange during grind- ing (about 20%) is now involved in a 'true' solid-state ion- exchange reaction.The previous observations clearly demonstrate that signifi- cant autoreduction of Fe"' ions has taken place during treat- ments in vacuum. Therefore, the presumption that the autoreduction processes are probably suppressed under treat- ments in air seems plausible. However, a series of 300 K Mossbauer spectra has also been recorded after calcination in air and reduction at 520, 620 and 720 K. The spectra are displayed in Fig. 7. They exhibit characteristic features. As it is seen, reduction at 520 K has no severe consequences; the overwhelming part of the iron remains in the Fef;, state, only 12% of the spectral area belongs to a reduced iron(@ (Fefc,) component.The 620 K treatments have more severe effects ;after calcination even Fe203 lines can be detected (only the inner two lines of the sextet fall into the velocity range of the measurement; note the shoulders on the central doublet of spectrum (b). Reduction by hydrogen at the same temperature results in the appearance mainly of Fete, components. Fe" ions in tetra- hedral or trigonal coordination are hardly detected, indicat- ing low ion-exchange degrees. Calcination at 720 K results in the pronounced formation of a separate iron oxide phase; 45% of the spectral area belongs to the characteristic sextet.After the reduction performed at the same temperature, 19% of the spectral area can be assigned to a magnetite-like phase2' (with characteristic magnetic field values of 48.9 and 45.9 T for A and B sites, respectively, and isomer shift values of 0.32 and 0.67 mm s-'), and 11.5%of the area belongs to metallic a-iron (H,,, =33.0, 6 =0.0 mm s-'). Furthermore, this spectrum contains two doublets of Fefct (with relative -' ' ' ' ' ' ' ' ' :""""'-""""'""-3 -1 1 3 -3 -1 1 3 -6 0 6 velocity/mm s-' Fig. 7 In situ Mossbauer spectra of NH,-Y-FeCI, mixtures record- ed at 300 K after calcination [(a),(b)and (c)] and reduction in hydro- gen [(a'),(b')and (c')]at temperatures of (a)520, (b)620 and (c) 720 K areas of 15.5 and 40%)and one doublet of Fe:fi, (8%).Note here that for the appearance of magnetic sextets in the spectra a certain threshold particle size must be exceeded. In our case, depending on the particular compound, a critical size of about 4-8 nm can be estimated at room tem-perature.'l This value is definitely larger than any of the pore diameters characteristic of Y zeolite. The formation of a magnetite-like oxide phase upon reduction may indicate that the aluminium ions penetrate into the oxide phase and mixed (Fe, A1),0, is formed. (On the silica support, the reduction of Fe,O, results in the formation only of metallic iron, while on the alumina support, a mixed oxide is easily formed since A13+ and Fe3 + ions have similar sizes.,,) Discussion The NH,-Y-FeCl, mixture releases the greater part of the ammonia and HC1 in the temperature range typical of the decomposition of crystalline NH,C1 (Fig.2). Thus, ion exchange resulting in the formation of ammonium chloride probably proceeded to a high degree during the grinding process or during the successive heating operations in the temperature range in which decomposition of NH,C1 does not yet occur. NH,-Y-FeCl, differs from other NH,-Y-metal chloride systems in the release of water with increasing temperature. The peak at about 400 K associated with water coordi-natively bound to cations appears more or less pronounced in the TPE pattern of all zeolite-salt mixtures. The system containing iron chloride, however, shows a distinct second water desorption peak with a maximum at about 510 K coin-ciding with the release of ammonia.A further peculiarity of this system is the small but significant delay of the HC1 evol- ution compared with the ammonia release (Fig. 2). This unique TPE behaviour of NH,-Y-FeCl, mixtures points to the hydrolytic formation of hydroxy-iron cations during the grinding process or successive heat treatments : Fe2+ +H,O +Fe(OH)+ +H+ (1) As evidenced by Mossbauer spectroscopy, most of the iron ions applied are oxidized during grinding to Fe"' species and may be also present as hydroxy-cations, e.g. Fe(OH),+ and/or Fe(OH),+. The water release at 520 K, which coin- cides in the TPE experiments with the decomposition of ammonium chloride formed during grinding of the mixture by contact-induced ion exchange, may be due to the reaction: where n is the valence state of the cationic iron species.FeC1("-1)+ undergoes (at somewhat higher temperatures) solid-state ion exchange according to : FeCl("-')+ +H+ +Fen++HCl (3) The consumption of at least some of the HCl [reaction (2)] formed by decomposition of the ion-exchange product NH,Cl and its release at higher temperatures [reaction (3)] consistently explain the shift of the HCl evolution curve to higher temperatures. It is striking that the decomposition of the ammonium chloride observed in TPE experiments around 520 K and the abrupt change in the oxidation state of iron revealed by Mossbauer spectroscopy somewhere between 520 and 620 K proceed in nearly the same temperature range.The slight dif- ference may be due to the much higher vacuum in the TPE apparatus favouring the decomposition of NH,CI. The auto- reduction of Fe"' spontaneously proceeding in this tem-perature interval is associated with the appearance of an equivalent amount of a new octahedrally coordinated Fe" species (Fe$t-2), with Mossbauer parameters not typical of lattice iron cations in faujasite. This new species has been tentatively assigned to Fe" cations interacting with alu-minium oxide. The coincidence of these processes is probably not an acci- dental one. A plausible explanation consistent with all experi- mental findings can be given if it is assumed that stable Fe'" forms of zeolites contain hydroxy-cations [Fe(OH)2 or+ Fe(OH), '1 or, after dehydroxylation, dimeric cations con- nected via an oxygen bridge (Fe-00-Fe)~' (i.e.cationic FeIn species with less than three charges per Fe atom). Auto- reduction of these Fe'" zeolites results in the formation of the respective Fe" form and oxidation of the oxygen bound to Fe"' species (in hydroxy groups or oxygen bridges) to molec- ular oxygen. The formation of Fe3+ cations is believed to result in unstable or less stable structures, especially in zeo- lites with higher framework Si :A1 ratios and, hence, larger distances between the negatively charged tetrahedral struc- ture units with aluminium as the central atom. Therefore, it may be assumed that, at least at 520 K in Y zeolite, Fe3 cations are unstable and change their oxidation + state by autoreduction immediately after their formation according to eqn.(3). In this case, however, autoreduction must involve the oxidation of framework oxygen atoms to molecular oxygen associated with the release of oxidic alu- minium species from the framework: (A104/2}3-Fe3f-,(A104i2}2-Fe2++ A1,0, + 1/20, (4) where fA104,,)- denotes the part of the zeolite structure containing one tetrahedron with A1 as the central atom. Interactions between the simultaneously formed Fe" cations and aluminium oxide species are reflected by the Mossbauer parameters assigned to Fe:ct-2. Thus, the Fe" form of faujasite containing only very low amounts of two Fe"' species (2% each) can be obtained by contact-induced ion exchange and subsequent heat treatment at 720 K.However, the complex process involves reactions resulting in a partial release of framework aluminium in the form of oxidic species and, hence, affects the structure. It can be deduced that, although the evacuation results in the disturbance of the framework to a limited extent, the vacuum is essential for the solid-state ion-exchange processes, providing a fast removal of the small molecules formed (NH,, HC1 and H,O). The autoreduction can be suppressed with the treatment in air at 520 K, but at higher temperatures separate iron oxide phases form and the extent of the solid- state ion-exchange processes is, even at 720 K, rather limited.The size of the oxide particles is large and exceeds the size of the pores. Thus, lattice rupture also proceeds with treatments in air. The chlorine ions (and the various FeCl, species) have a distinguished role; their presence probably causes an inter- mediate step to be inserted among the ion-exchange pro- cesses [as is proposed in eqn. (2)]. Further studies of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 solid-state ion exchange of iron salts containing other anions are in progress. Partial funding provided by the Hungarian National Foun- dation for Scientific Research (OTKA grant T7364), as well as financial support by the Ministry of Technology and Research of the Federal Republic of Germany (BMFT, Project No. 03C 252 A7), is gratefully acknowledged.References 1 J. A. Rabo, M. L. Poutsma and G. W. Skeels, in Proc. 5th Znt. Congr. Catal., 1972, ed. J. W. Hightower, North-Holland, Amsterdam, 1973, p. 1353. 2 J. A. Rabo, in Zeolite Chemistry and Catalysis, ed. J. A. Rabo, ACS Monograph 17 1, American Chemical Society, Washington, DC, 1976, p. 322. 3 A. Clearfield, C. H. Saldariagga and R. C. 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