Solid-state exchange of palladium in zeolite NaX

i

A PA LE IY D CP AT L SS I A: GENERAL

Applied Catalysis A: General 167 (1998) 113-121

ELSEVIER

Solid-state exchange of palladium in zeolite NaX Cfcile

S t o l z a, A r n a u d

S a u v a g e b, P a s c a l e M a s s i a n i b, R e i n h a r d

K r a m e r a'*

alnstitut fiir Physikalische Chemie, Universitiit Innsbruck, A-6020 Innsbruck, Austria bLaboratoire de Rdactivitd de Surg[ace, Universitd P. et M. Curie, Paris Cddex 05, France

Received 9 June 1997; received in revised form 10 October 1997; accepted 13 October 1997

Abstract The migration of metal ions during solid-state exchange was studied by investigating the effect of grinding force and calcination conditions on the incorporation of Pd in zeolite NaX using various Pd salts as precursors. At temperatures lower than 600°C, it was found that PdC12 and Pd(NH3)4C12 lead to a significant Pd exchange whereas no Pd incorporation takes place with Pd(NO3) 2 as precursor. The grinding force influences the Pd 2+ exchange occurring in the subsequent calcination procedure. In the case of Pd(NH3)4Clz/NaX, the grinding also causes a decomposition of the precursor metal salt to Pd(NH3)2CI2 and a change in the crystal structure as shown by X-ray diffraction. During the calcination of PdClz/NaX samples the precursor salt is transformed according to the following scheme: /3-PdCI2 4 ~ c o~-PdC12500"c,02 PdO 75--+ °°c Pd° The Pd species are incorporated in the zeolite in two stages, one occurring at about 400°C via gas phase transport of Pd6Cl~2 molecules into the supercages followed by a transfer of fragments to the smaller zeolite cages and a second one occurring between 600 and 700°C via surface migration starting from PdO species. © 1998 Elsevier Science B.V. Keywords: Solid-state exchange; Palladium; Zeolite; NaX; Temperature programmed reduction

1. Introduction In the last decades the common amorphous supports (silica, alumina etc.) have been increasingly replaced by zeolites. For industrial applications the preparation of metal catalysts should be as simple and economical as possible. A widely used method to introduce metal cations into zeolites is conventional ion exchange in solution. One limitation of this method is the solubility of the metal precursor. Moreover, steric constraints

*Corresponding author. 0926-860X/98/$19.00 ~) 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00305-0

may result from the bulky hydration spheres of the metal ions in solution. An additional disadvantage for industrial applications is the handling of large volumes of solutions during the preparation process. Thus, an alternative method, the so called 'solid-state exchange', first described by Rabo [1] and Clearfield [2], has attracted increasing attention in the recent years [3-10]. In this process, the metal cations (usually alkali, rare earth or transition metal ions) are introduced into the zeolite by calcining a mechanical mixture of the zeolite (usually provided in its protonic form) and the metal precursor (usually in the form of a metal halide or oxide).

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Bifunctional Pd-containing zeolite catalysts, prepared by ion exchange in solution, have been studied for a long time [11-16]. Recently, Karge et al. [17,18] and Weitkamp et al. [19] have shown that the solidstate exchange procedure could also be used for the preparation of such catalysts. These studies were mostly devoted to the incorporation of palladium in silicon-rich (Si/Al=35-100) proton-containing (acidic) zeolites. However, the possibility of incorporating palladium by solid-state reaction in aluminumrich, alkali-containing zeolites would be also of great interest due to the specific catalytic behaviors of metal particles in such basic supports [20]. Solid-state exchange of LaC13 [21] and of CuC1 [22] on such zeolites has been reported recently. For zeolites with high aluminum content (and therefore high ion exchange capacity) one advantage of the alkali form compared to the protonic form is their higher thermal and hydrothermal stability [23-25]. So the high calcination temperatures necessary for ion exchange can be applied without loss of crystallinity. Moreover, the alkali forms of aluminum rich zeolites are those which are obtained by direct synthesis whereas the protonic forms require additional pretreatments. As a consequence, the study of solid-state exchange of Pd in basic high aluminum zeolites is of great interest. In this work, we investigated the solid-state exchange of palladium in the high-aluminum zeolite X (Si/AI= 1.2) in its sodium form (NaX). In particular, it was tried to find answers to the questions how the Pd incorporation can be influenced by preparation parameters (like grinding force or calcination conditions) and during which preparation step the migration of the metal ions takes place. Such data are essential for optimizing the preparation process and therefore important for industrial applications.

Thermal treatment of the catalyst was performed in a flow reactor. The gas flow was controlled by mass flow controllers (MKS). During TPO (Temperature Programmed Oxidation) the sample was heated in a flow of 5% 02 in He (25 ml/min) from room temperature to the maximum temperature (between 300 and 800°C, 700°C for 'standard'-TPO) with a heating rate of 1°C/min. The maximum temperature was held for 2 h before allowing the sample to cool and switching the flow to pure argon in order to sweep away O2. After TPO, TPR (Temperature-Programmed Reduction) experiments were performed in situ by heating the sample in 5% H2 in Ar (25 ml/min) from room temperature to 500°C with a heating rate of 7.5°C/rain. Changes in the gas composition during oxidation and reduction were detected by a thermal conductivity detector. All gases used were high grade purity and the Hz/Ar mixture was further purified by an oxygen removing unit.

2. Experimental

2.3. Methods of characterisation

2.1. Grinding

The crystallinity of the samples before and after different preparation steps was monitored by XRD (Siemens D500, CuK~ radiation). XRD allowed further to prove the presence of large particles of Pd species and to identify them by comparison with reference data. The micropore volumes of the samples were obtained from measurements on a BET apparatus (Quantasorb Jr., Quanta Chrome corporation) [26]. For observation of the homogeneity and size of the

PdCI2/NaX, Pd(NH3)4ClffNaX and Pd(NO3)2/ NaX with a Pd/NaX content of 3 wt.%, corresponding to 5 Pd atoms per unit cell, were prepared by mixing NaX (Union Carbide, unit cell composition Nas3(AIO2)s3(SiOz)lo9.(H20)235), with the [3-modification of PdCI2 (Merck), Pd(NH3)4CI2 (Johnson Matthey Chemicals) and Pd(NO3)2-(H20)× (Aldrich-

Sigma). In a first grinding step the sample was mixed gently (i.e. slowly and not powerfully) by hand in a mortar for 15 rain ('gentle grinding'). For 'strong grinding' the same grinding procedure was repeated but this time the sample was mixed powerfully. 'Ultrastrongly ground' samples were prepared by mixing the gently ground sample in a planetary micro mill for 15 min. A PdCIJNH4X sample was also prepared by 'gentle grinding' of PdCI2 with a NH4X zeolite of unit cell composition (NHa)y3Na m(A102)83(SiO2) rag. (H20)223. This NH4X zeolite has been obtained by exchanging NaX 5 times in a 1 M NH4NO3 solution at room temperature.

2.2. Temperature programmed oxidation and reduction

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Pd ° particles in the zeolite, a TEM (Jeol 100 CXII) was used. 3. Results

3.1. Analysis of the TPR-curves Typical TPR-profiles are monitored in Fig. 1. First, a negative peak at 60-70°C is observed. This hydrogen excess peak is caused by the release of hydrogen from large Pd particles already present outside the zeolite before heating in H2 [27]. These large particles, also confirmed by TEM, are formed during the first contact of the calcined samples with hydrogen at room temperature, as shown by the hydrogen consumption when H2 is introduced into the reactor (data not reported). The source for this reduction are mostly large PdO or PdC12 particles, which are easily reducible in hydrogen [28,29]. The resulting large Pd ° particles dissolve a high quantity of H 2 at room temperature (formation of a palladium hydride) and release it at 60-70°C. Secondly, positive peaks due to H2 consumption are found in the tern-

90°C

o t~

~

~

b _

60°C 0

perature range of 80-250°C (Fig. 1). These peaks can be assigned to the reduction of Pd 2+ species located in the zeolitic framework [14,28]. The various temperatures observed reveal the presence of Pd z+ ions with different reducibilities. The reducibility depends on the nature of the species involved and on its interaction with the support. In particular, it is affected by the location in the different zeolite cavities, higher temperatures corresponding to location in smaller cavities where the higher negative charge density improves the stabilization of the Pd 2+ ions. Referring to the literature [14,16,28] the broad peak at 170°C (Fig. 1, curve (b)) may be attributed to the reduction of bare Pd 2+ ions located in smaller cavities like sodalite cages and/or hexagonal prisms. The peak at 90°C (Fig. 1, curve (a)) most probably corresponds to the reduction of Pd species located in the supercages. The position of the various Pd 2+ species incorporated into the zeolite is only one information available from the TPR profiles. Additionally, the amount of the Pd 2+ ions exchanged in the different zeolitic cages can be calculated from the area of the corresponding peaks.

3.2. Effect of the precursor materials and of grinding

170°C

=

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h

I

I

I

100

200

300

400

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temperature (°C) Fig. 1. TPR-profilesof PdCIz/NaX samples after calcinationup to (a) 400°C (b) 700°C (TPO rate l°C/min, 2h at maximum temperature).

PdC12, Pd(NH3)4C12 and Pd(NO3)2 were tested as Pd precursors for solid-state exchange in NaX. For comparison, experiments were also conducted using NH4 X as the parent zeolite. First, all TPO-TPR and TEM results obtained with the Pd(NO3)2 precursor indicate that no significant Pd incorporation takes place in NaX after calcination at temperatures as high as 600°C. At that temperature the decomposition of Pd(NO3)2 to PdO is only partial as revealed by mass spectrometry (not shown). On the other hand, calcination of PdC12/ NH4X mechanical mixtures at temperatures between 300 to 600°C always resulted in a strong amorphization of the zeolitic structure, evidenced by both XRD and pore volume measurements. Therefore, the solid-state exchange using NH4X as the parent zeolite or Pd(NO3)2 as the Pd precursor was not further investigated. The TPR-curves of PdC1JNaX samples, prepared by 'gentle', 'strong' and 'ultrastrong' grinding and calcined under 'standard'-TPO conditions

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170°C

170°C

180°C

c

Fig. 3. Electron micrograph of PdC12/NaX alter standard-calcination (up to 700'C, TPO rate 1' C/rain, 2 h at 700°C) and subsequent TPR, exhibiting small incorporated Pd particles and one large Pd particle outside the zeolite. Table 1 Influence of grinding force on the amount of Pd 2~ exchange, calculated from the TPR-curves in Fig. 2

0

i

i

i

i

100

200

300

400

Grinding force

% Incorporated Pd

500

t e m p e r a t u r e (°C) Fig. 2. TPR-profiles of (a) gently, (b) strongly and (c) ultrastrongly ground PdCI2/NaX samples after calcination up to 700°C (TPO rate 1:'C/rain, 2 h at 700°C).

(Tmax=700°C), are reported in Fig. 2. The intense negative peak present at about 60°C on all curves indicate that after reduction all samples contain a large amount of big Pd ° particles outside the zeolite. TEM inspection also confirmed the presence of large Pd ° particles in the reduced samples in addition to smaller particles, 2--4 nm in diameter, incorporated into the zeolite (Fig. 3). These particles are comparable to those formed in H-ZSM-5 upon solid-state exchange treatments using PdC12 as precursor [17,18]. The different area of the positive TPR-peaks at about 170°C show that the amount of the Pd 2+ cations exchanged in small cages (sodalite cages and/or hexagonal prisms) is significantly influenced by the grinding force. The values for Pd incorporation, calculated from the H2 consumption peaks, are given in Table 1. After 'standard'-calcination, the highest quantity of exchanged Pd 2+ is found in the 'strongly' ground sample (31%). The low Pd incorporation in the 'ultrastrongly' ground and calcined sample (7%) is due to a loss of the zeolitic crystallinity during the treatment in

Gentle Strong Ultrastrong

PdClffNaX

Pd(NH3)4CIffNaX

20% 31% 7%

25% 33% -

the planetary mill as observed by XRD. A similar effect of amorphisation by such a treatment has been reported by Kaliaguine et al. [30]. Essentially similar TPR-curves are obtained with 'gently' and 'strongly' ground samples of Pd(NH3)4C12/NaX and thus similar extents of incorporation are calculated for these samples (Table 1). The X-ray diffraction powder patterns (Fig. 4) of these samples show, moreover, that grinding at room temperature causes the formation of new crystalline Pd species, evidenced by additional peaks when compared with the diffractograms of pure NaX and pure Pd(NH3)4CI> From literature data [31] the additional peaks in the diffractograms of the 'gently' ground sample (marked with @) can be attributed to transPd(NH3)2C12. The sample prepared by 'strong' grinding contains additional to these Pd(NH3)2C12 peaks new peaks (marked with ~), assigned to the [3-modification of trans-Pd(NH3)2Cl> as reported by Smirnov [31]. So it appears that Pd(NH3)4C12 loses two NH3 molecules during the grinding process.

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5

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25 30 20 (degree)

35

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45

511

Fig. 4. X-ray diffractograms of (a) Pd(NH3)4C12, (b) NaX, (c) gently ground Pd(NH3)4C12/NaX and (d) strongly ground Pd(NH3)4CI2/NaX. Peaks are marked for trans-Pd(NH3)2Cl2 (0) and for 13-trans-Pd(NH3)2Cl2(~). Furthermore the grinding force seems to influence the structure of the Pd(NH3)2CI2 crystals. 3.3. Effect o f calcination temperature

Different steps of Pd incorporation in the 'strongly' ground PdC12/NaX sample were followed by perform-

ing TPO up to different maximum temperatures (300800°C). X-ray diffractograms of the calcined samples are given in Fig. 5. As indicated by the marked XRD peaks different crystalline Pd species, located outside the zeolite could be observed after various calcination temperatures. After low temperature calcination (300°C) the structure of the initial PdCI2 modification (I3-PdC12) [32] was retained (peaks * in Fig. 5(a)). On the other hand, upon calcination up to 400°C the initial ~3-PdCI2 particles are completely transformed into another modification, called ct-PdCl2 [32-34], as indicated by a new XRD peak of low intensity in Fig. 5(b) (marked with ,L)- Calcination at temperatures above 500°C leads to the formation of PdO as extra-zeolitic Pd-species (peaks (~) in Fig. 5(c) and (d)) and calcination up to 800°C causes partial decomposition of the big PdO crystallites into the elements, as is proved by the appearance of new peaks in the XRD (peaks • in Fig. 5(e)), corresponding to crystalline Pd °, and by the formation of 02 excess peaks at about 780°C during TPO. The samples calcined up to different temperatures were also investigated by TPR (Fig. 6). At calcination temperatures lower than 400°C, no hydrogen consumption peaks were found between 80 and 250°C, showing that no Pd incorporation into the zeolite took

¢o . N

5

I0

15

20

25 30 20 (degree)

35

40

45

50

Fig. 5. X-ray diffractograms of PdCl2/NaXaffer calcination up to different maximumtemperatures (TPO rate l°C/min, 2 h at Tmax): T.... 300°C (b) 400°C (c) 500°C (d) 700°C (e) 800°C. Peaks are marked for ~-PdC12(*), ~-PdC12 (~), PdO ((~) and Pd° (O).

(a)

C. Stolz et al./Applied Catalysis A: General 167 (1998) 113 121

118

35

,, iiiiii+iii,lii~iiil;ii ¸¸'

300°C 30

350°C

,= 25 1 ea

20

i

15

°

400°C 10 e~ 0

500°C

em

=

200

700°C

300

400

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600

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900

TPO-temperature (°C)

600°C

Fig. 7. Extent of Pd incorporation during calcination (TPO rate 1C / m i n , 2 h at maximum temperature). The Pd precursor particles located outside the zeolite are monitored by differently shaded areas.

800°C

0

100

200

300

400

500

600

temperature (°C)

Fig. 6. TPR-proflles of PdC1jNaX after calcination up to different maximum temperatures (TPO rate l'C/min, 2h at Tnl,,x): (a) Tma×-300:C, (b) 350:'C, (c) 400~C, (d) 500°C, (e) 600C, (f) 700~C, (g) 800'C. The negative peaks at 60-70':C are suppressed.

place (Fig. 6(a) and (b)). After calcination to 400°C part of the Pd species were incorporated into supercages and small cavities as shown by the positive peaks centered at 90 and 170°C in Fig. 6(c). Upon calcination up to 500cC nearly no further Pd incorporation was found, but the location of the intrazeolitic Pd species has changed to the small cavities (Fig. 6(d)). A further increase of Pd incorporation in the small cavities was observed after calcination at 600 and 700°C. However, calcination up to 800°C resulted in no additional Pd incorporation. Regardless the calcination temperature, the pore volume of the sample after reduction was 0.27+0.02 ml/g confirming that the zeolitic porosity was preserved during TPO-TPR treatments. Fig. 7 shows the amount of incorporated Pd, calculated from the positive TPR-peaks in Fig. 6, as a

function of the final calcination temperature. The Pd species present outside the zeolite after the corresponding calcination are represented by differently shaded areas in Fig. 7. These Pd species, monitored by XRD measurements, can act as the source for the Pd migration at the corresponding calcination temperature. From these data Pd 2~' exchange seems to occur in two different temperature ranges, the first around 400°C and the second around 600~C.

4. D i s c u s s i o n

The present study demonstrates that the solid-state exchange of Pd in zeolites is not limited to acidic zeolites but can also be applied to Na-containing zeolites. Under the given conditions, the maximum amount of Pd incorporation into NaX is around 31% as evaluated from TPR analysis. This is about half of the value (55-60%) obtained by solid-state exchange of Pd in H-ZSM-5 as found by IR analysis [17,18]. The lower incorporation in the present study may result not only from the nature of the initial counter cations (Na + rather than H +) but also from a higher Pd content (about double) in the investigated samples. This work also confirms that the aluminum-rich X zeolite exhibits a high thermal stability when used in its Na-form making possible the high-temperature

c. Stolz et al./Applied Catalysis A: General 167 (1998) 113-121

treatments applied during solid-state exchange procedures. In contrast, NH4X (or HX upon heat treatment) is amorphized at temperatures as low as 300°C inhibiting any metal incorporation by simple thermal treatment. This amorphisation is in line with the known unstability of the acidic forms of faujasites. With respect to the effect of grinding, it is found that in all systems investigated calcination of 'strongly' ground samples leads to the highest Pd incorporation. 'Strong' grinding crushes the crystals of the metal salt into smaller particles. As the contact area between the smaller Pd salt particles and the zeolite is increased the smaller distance of transport facilitates the Pd incorporation. In a separate experiment it could be verified that smaller Pd salt particles, produced by separate grinding in a mortar prior to the mixing procedure with the zeolite, lead to higher Pd 2+ exchange. In the case of Pd(NH3)4CIz/NaX the grinding procedure not only affects the amount of exchangeable Pd 2+ but also the composition and the structure of the initial metal salt. During grinding Pd(NH3)4C12 loses two NH3 ligands forming Pd(NH3)2Cl2 crystals. These crystals change their symmetry group with stronger grinding resulting in a different modification. However, it can be summarized that for optimal Pd 2+ exchange the grinding force should be as strong as possible but not exceed the limit where the zeolite loses its crystallinity, e.g. by milling. It is worth noting that the optimal grinding force will depend on the zeolite type used. Considering the effect of grinding it should be pointed out that on the one side the grinding procedure itself does not lead to any Pd incorporation, since no Pd 2+ exchange was observed when the final calcination temperature was as low as 300°C. On the other side, the grinding procedure improves the Pd 2+ exchange in the subsequent calcination step. With respect to the mechanisms involved during calcination, it is found by XRD that the starting material I3-PdCI2 (the low temperature modification of PdC12) transforms at a calcination temperature of about 400°C into a-PdC12, the intermediate temperature form. This agrees with experiments of Soulen et al. [321] who observed the transition from the [3- to the a-modification at 401°C. The ~t-PdCl2 crystals outside the zeolite are transformed to PdO during calcination up to 500°C. PdO itself decomposes into the elements at about 780°C. Actually, at an oxygen

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pressure of 50 mbar, as applied in this study, the decomposition of PdO becomes thermodynamically favoured at temperatures above 720°C [35]. The transformation of the initial [3-PdC12 particles during calcination can be summarized by the following reaction: 400~c 500~, o2 75~c pd 0 d-PdC12 ~ c~-PdC12 PdO Moreover, the migration process was found to occur in two different temperature ranges, around 400 and 600°C. Based on these results it should be possible to determine the Pd species which is the source for the migration: in the low temperature range (_<400°C), the incorporation of Pd coincides with the transition of [3PdC12 to a-PdC12. The vapour pressure of PdC12 is given in the literature [35] for the temperature range between 610 and 680°C. For a rough estimation the partial pressure of PdC12 was extrapolated to 400°C resulting in a value of about 1 Pa. In view of this high vapour pressure the onset of Pd incorporation occurs most likely by gas phase transport. This assumption was further confirmed by an experiment stimulated by one of the referees of this paper. Two small connected glass tubes were filled with PdC12 and with zeolite, respectively. Thereafter, the apparatus was evacuated and the temperature was slowly raised simulating calcination to 500°C. It was observed that during this treatment PdC12 moved to the zeolite chamber causing a colour change in the upper part of the NaX powder (about 1 mm thick). This result demonstrates on the one hand that gas phase transport of PdC12 occurs over large distances but on the other hand that its migration across the NaX powder is limited. In the literature the gaseous molecules were first proposed to be PdsCllo [36] but more recent studies [34] proved that their stoichiometry is rather Pd6Cll2. These molecules form also the solid [3-PdCI2 phase. From the density of ~-PdC12 the volume of the Pd6Cl12 molecules was calculated and thereby the size of these molecules could be estimated to be roughly 7.6 A. Since the aperture of the dehydrated supercages and small cavities is about 8 and 2.8 A [25], respectively, the Pd6Cll2 molecule is just able to migrate into the supercages. In summary, at calcination temperatures of 400°C, the incorporation of Pd species involves the sublimation of the PdCI2 precursor and the gas phase

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transport of Pd6C112 molecules into the zeolite framework, followed by the decomposition of these molecules due to interaction with the zeolite. Such a gas phase process was also proposed in order to explain the redispersion of palladium by chlorine in Y zeolite [29]. The TPR experiments strongly suggest that the corresponding Pd ions are mainly located in the supercages where they are reduced at temperatures around 90cC. The fact that Pd(NO3)2 is not incorporated in NaX in this temperature range supports the importance of a chlorine species for migration, as already concluded in the literature [29]. In Pd(NH3)4C12/NaX samples the incorporation may occur by transformation of Pd(NH3)2CI2 to PdCI2 5~)llowed by gas phase transport o f Pd6Cl12 as described above. Upon calcination at 500°C the total amount of incorporated Pd does not further increase, but the distribution of the Pd location changes towards the small cavities. At these temperatures the precursor substance outside the zeolite is transformed to PdO, which exhibits a much lower volatility than PdC12. However, the Pd 2+ ions formed by decomposition of Pd6C112 molecules inside the supercages can migrate either into the sodalite cages or into the hexagonal prisms. There they exchange with Na + cations or with protons still present in the zeolite [37]. During TPR they are reduced at temperatures around 170°C. In the high temperature range of Pd incorporation the precursor species outside the zeolite is PdO. The thermodynamic data for PdO [38] predict that its vapour pressure at 600°C is lower than 1.10 -j3 Pa. This pressure is insufficient to account for gas phase transport of PdO. So in this temperature range the migration of palladium occurs most likely by surface diffusion of a molecular PdO species or by surface spreading of a two-dimensional PdO-layer [27,39]. This diffusion process is interrupted as soon as PdO is decomposed during calcination to Pd and O2. Above this decomposition temperature no further incorporation of Pd is observed (Fig. 7). Based on these mechanistic studies of the migration process, the pretreatment conditions for optimizing the Pd incorporation can be described as follows: (i) Strong grinding of a chlorine containing precursor is recommended in order to bring precursor and zeolite in intimate contact and to initiate the formation of Pd6Cl12 molecules during calcination.

(ii) For a sufficient time, the calcination temperature should be held just below the temperature at which PdC12 is transformed to PdO in order to enable the gas phase transport into the supercages. (iii) At higher calcination temperatures PdO migrates into the zeolitic framework either by surface diffusion of Pd 2+ or by spreading of PdO. The optimum temperature for this process is just below the decomposition temperature of PdO, that is at about 700°C. Experiments are in progress where the temperature program and the composition of the calcination gas are changed systematically in order to further optimize the solid-state exchange of palladium in zeolites.

5. Conclusion In the solid-state exchange of Pd salts in zeolite NaX it was found that the extent of Pd incorporation is increased by using PdCI2 or Pd(NH3)aCI2 as a precursor salt instead of Pd(NO3)2, and by applying strong grinding during the solid-state mixing procedure. However, incorporation occurs only during the subsequent calcination procedure in two different temperature regions: (i) At about 400°C F'd6C112 molecules are transferred to the supercages of the zeolite by gas phase transport followed by surface transport of molecular fragments into small cavities. This gas phase transport is terminated as soon as the PdC12 precursor is transformed to PdO. (ii) Between 600 and 700°C molecular PdO species can migrate into the zeolite by surface diffusion. This process is again interrupted as soon as the calcination temperature reaches the temperature of PdO decomposition.

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