H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V. All rights reserved.
1901
P o s t - S y n t h e s i s M o d i f i c a t i o n of M i c r o p o r o u s M a t e r i a l s b y Solid-State Reactions Hellmut G. Karge Fritz Haber Institute of the Max Planck Society, Berlin, Faradayweg 4-6, 14195 Berlin, Germany
This study reviews the development and application of a novel method of ion exchange with microporous materials. The experimental procedures to carry out and techniques to monitor such solid-state reactions are presented and are illustrated by a number of selected examples including the incorporation of alkaline, alkaline earth, rare earth and transition metal cations. Particular attention is paid to more recent achievements such as the introduction of noble metals into narrow-pore zeolites, reductive solid-state ion exchange and to problems such as the mechanism and kinetics of solid-state ion exchange.
1. INTRODUCTION Early in the '70s a few observations of the solid-state reaction between hydrogen forms of zeolites and salts were reported [1-4]. Rabo et al. [1-3] used the solid-state reaction between Y-type zeolites and sodium chloride to remove acidic OH groups and to eliminate Br~nsted acid-based catalytic properties of these materials. Almost at the same time, Clearfield and co-workers [4] provided evidence for the solid-state ion exchange in systems such as NH4,Na-A or NH4~a-X mixed with halides. In particular they used chlorides of copper, cobalt and iron and monitored the reaction via ESR spectroscopy. Surprisingly, for about more than one decade there was no further interest in the p h e n o m e n o n of solid-state ion exchange in zeolites. An exception were the experiments carried out by Fyfe et al. [5] who contacted, e.g., Li-A and Na-A and tracked the interchange of Li§ and Na § by solid-state NMR or XRD until a homogeneous distribution of both kinds of cations over the physical mixture of Li-A/Na-A was reached. It was only by the mid '80s that systematic studies of the potential of solid-state ion exchange commenced. Two research groups started independent
1902 investigations of a broad variety of systems of microporous solids and compounds of ions to be incorporated into these stuctures. Slinkin and colleagues [6-10] focused on the exchange of transition metal ions into zeolites, particularly into high-silica structures such as ZSM-5 and mordenite. They mainly used ESR spectroscopy as a powerful tool for obtaining evidence of incorporation of transition metal ions. In that respect the studies of these authors appeared closely related to the seminal early work of Clearfield et al. [4]. A topic of particular interest in the studies of Slinkin and his group was, however, the reaction of compounds of polyvalent transition metal cations, e.g., V5+, Mo 5+, Cr 6+ etc.. The group of the present author started systematic investigations on solidstate ion exchange in systems similar to those employed in the previously mentioned experiments by Rabo et al. [1-3] even though initially without any knowledge of these. Thus, Beyer et al. [11] and Karge et al. [12] carried out a series of quantitative investigations on solid-state reaction between the alkaline halides and hydrogen or ammonium forms of Y-type or ZSM-5 zeolites. Since their original main interest was, however, not to eliminate but to create active acidic sites in zeolite catalysts, the studies were soon extended to alkaline earth and rare earth systems [13-16]. Also, compounds of copper, manganese and iron were included because of the great importance of these cations in oxidation catalysis [17-19]. Bifunctional catalysts, e.g., Pd, H-ZSM-5, were shown to be attainable via solid-state ion exchange as well [20-21]. In the above-mentioned studies the starting form of the microporous materials was either a hydrogen or an ammonium zeolite. In view of the potential of the preparation of acidic catalysts it seemed worthwhile to attempt reacting an as-synthesized sodium form with a salt of a bi- or trivalent cation. It could be shown that this is, in fact, a promising route to obtain active, acidic zeolite catalysts [15,22]. It seems that solid-state reactions between microporous solids such as zeolites and compounds of cations, which are desired to enter the porous structure, have now become a frequently used method. Advantages over the conventional ion exchange are seen in the fact that solid-state ion exchange (i) does not require handling large volumes of salt solutions, (ii) avoids the problem of discarding waste salt solutions and (iii) provides an opportunity to introduce metal cations into narrow pore cavities in cases where the ion exchange in aqueous solutions would be impeded by the large solvation shell of the cations. In the current contribution, most relevant earlier results will be reviewed, including pertinent examples of the previously mentioned aspects of solid-state reaction with microporous materials. More recent developments will be treated as well, viz. (i) succesful incorporation of noble metals into narrow pore zeolites [23-25]; (ii) modification of the method, called reductive solid-state ion exchange [26-29]; (iii) elucidation of the role of traces of water, i.e. solid-state reaction v e r s u s contact-induced ion exchange [5,30-31] and (iv) questions of the mechanism and kinetics of solid-state ion exchange [12,32-33].
1903 2. BASIC CONCEPTS OF ION EXCHANGE In this paragraph the main features of conventional and solid-state ion exchange will be contrasted. Basically, conventional ion exchange in aqueous solution is described by eq. (1), where, for the sake of simplicity, a chloride is used as the dissolved salt of the in-going cation M. a [ M ( H 2 0 ) w ] n§ + na CI" + bNaZ r
(1)
xMZ n + (a-x)[M(H20)w] n§ + naCl" + nxNa + + (b-nx)NaZ M: n-valent cation, Z: monovalent zeolite fragment. Usually the equilibrium does not lie heavily on the right hand side of eq. (1). Thus, the (partially exchanged) solid and the solution have to be separated. In many cases, one has to repeat the exchange procedure rather frequently in order to obtain a high degree of exchange. The degree of exchange is nevertheless in several cases limited due to the relative dimensions of the in-going cation (eventually increased by the solvation shell) and the "windows" which must be penetrated. Thus the highest degree of exchange, which can be obtained by merely suspending Na-Y in an aqueous solution of lanthanum salts, is usually about 70%. Because of the fundamental importance of ion exchange in zeolite chemistry, there exists a large body of literature on thermodynamics and kinetics of ion exchange in solution (see, e.g. [34-38] and References therein). Solid-state ion exchange may be described by eqs. (2a) or (2b), depending on whether the starting material is a cation-containing form, such as a sodium form of a zeolite, or the hydrogen (ammonium) form. Again, for the sake of clarity it is assumed that the solid-state reaction takes place between a chloride of the cation M and a Na-zeolite: aMCl n +bNaZ r
xMZ n + (a-x)MCl n + (b-nx)NaZ + nxNaCl
aMCl n +bHZ --~ xMZ n + (a-x)MCl n + (b-nx)HZ + nxHC1 $
(2a)
(2b)
In a stoichiometric mixture one may reach a complete exchange without residual salt: aMCl n + naHZ ~ aMZ n + naHCl 1"
(3)
In the case of (2a), an equilibrium will be obtained, similar to conventional ion exchange (vide infra, e.g., Section 6.1). In order to obtain a higher degree of exchange here, one has to remove the solid chloride, e.g. NaCI, which forms during the solid-state ion exchange, via brief washing with a small amount of solvent. If one starts with the hydrogen (or ammonium) form as in case (2b) and
1904 continuously removes the hydrogen halide (HCI) in an inert gas stream or high vacuum, the equilibrium may completely shift to the right hand side resulting in a 100% exchange (compare, e.g., Sections 5.1 and 5.2). To date, only qualitative observations are reported related to the thermodynamics of solid state ion exchange [17-18]. A few results have been obtained which seem to shed some light on the possible mechanism of solid-state ion exchange (see Section 7). Systematic investigations of the kinetics of solid-state reactions between salts and zeolites have been started only recently (Section 8). 3. EXPERIMENTAL PROCEDURE FOR SOLID-STATE ION EXCHANGE The main prerequisite for an efficient solid-state ion exchange of microporous materials is to prepare an intimate mixture of the two components, i.e. the microporous solids and the compound which contains the in-going cation. This can be achieved, for instance, by grinding or milling a mixture of both components. In cases where a too intense grinding or milling may affect the integrity of the porous framework [39], it is preferable to prepare a suspension of the powdered components in an inert solvent such as hexane. After thorough mixing of the solids in the suspension, the solvent may be easily removed by evaporation [11]. The intimate mixture is subsequently heated in an inert gas stream or high vacuum to remove volatile products such as hydrogen halides, ammonia, water, etc.. Usually reaction temperatures of 525 - 625 K and a reaction time of a few hours are required to obtain a maximum degree of exchange. However, in some cases solid-state reactions occur already at temperatures as low as 400 K, in others significantly higher heating is necessary. In any case, the crystallinity of the product obtained via solid-state reaction should be checked, even though in general the evolution of, e.g., HCI or HF does not affect the structure in the absence of water vapor. This can be achieved if the mixture of solids was carefully pretreated at about 400 K in a stream of dry inert gas or in high v a c u u m prior to the reaction in order to desorb physically adsorbed water. The reaction between the solids may in some cases be advantageously carried out in the presence of an oxidizing (e.g., chlorine) or reductive gas. In the former case the exchange may be in fact mediated through the gas phase after formation of volatile reactants such as PdCI 2 (see Section 6.2). 4. EXPERIMENTAL TECHNIQUES TO MONITOR SOLID-STATE ION EX-
CHANGE 4.1. IR
spectroscopy
According to the two types of processes described by eqs. (2a) and (2b), there are basically two ways to employ IR spectroscopy in order to detect the occurrence and determine the degree of solid-state ion exchange. First, one could use a probe such as pyridine or CO which provides, upon adsorption, bands typical of the in-
1905 going cation, e.g., Be2+, La3+, Cu § Mn 2§ etc. or the replaced cation of the starting material, e.g. Na § Secondly, if the compound of the in-going cation M n§ such as a halide reacts with the OH groups of the hydrogen form of the parent zeolite, the solid-state reaction will result in a decrease of the typical OH bands. As an indispensable cross-check one has to carry out an analogous experiment with the pure hydrogen zeolite, i.e. without the compound (halide, oxide) of the in-going cation M n+. Comparison of the spectra obtained in the presence and the absence of the M n+ compound should ascertain that a decrease in the intensity of the OH band under inspection is not simply caused by dehydroxylation. Of course, the incorporation of the cation M n+ may be additionally confirmed by observing the IR bands typical of suitable Mn+-probe complexes.
4.2. ESR spectroscopy Many transition metal cations, if introduced into a zeolite framework structure, will give rise to typical ESR spectra. Usually treatment of the respective mixtures of salts or oxides and zeolites is carried out ex-situ. However, employing a particularly designed high-temperature ESR reactor [40-41], in-situ ESR experiments on solid-state ion exchange should also be feasible. This would possibly enable us also to follow the kinetics of the transition metal incorporation. ESR spectroscopy is not only able to detect and monitor solid-state reactions between solids and zeolites but also to provide insight into the particular type of coordination which the incorporated transition metal cation has obtained. 4.3. Temperature-programmed evolution (TPE) of volatile gases When the compound of the cation, M n+ , desired to go into the zeolite structure, reacts with the parent zeolite under the formation of one or more volatile products such as HCI, NH3, H20, etc. these products can be detected and/or quantitatively determined. As analytical tools (continuous) titration of the evolved volatiles (HC1, NH 3 or H20, the latter via Karl Fischer's method), gas chromatography or mass spectrometry may be employed. Temperatureprogrammed heating of the salt-zeolite mixture is advantageous because the profiles obtained enable one to discriminate between different types and paths of solid-state reaction. Moreover, different maximum temperatures in the TPE profiles of a homologous series of systems, e.g. alkaline chlorides/H-ZSM-5, provide a measure of the ease of the respective solid-state reactions. 4.4. Thermogravimetric analysis Solid-state reactions between salts or oxides and zeolites can be studied by thermal gravimetric analysis if they are accompanied by a change in the weight of the mixtures. For instance, the loss of weight, which occurs when a chloride reacts with a hydrogen form of a zeolite, provides a measure of the degree of exchange. Monitoring the weight loss by a microbalance during temperature-
1906 programmed heating of the chloride/H-zeolite mixture may be combined with one of the techniques described in Section 4.3, e.g. with the titration of the evolved HCI. Thus it is possible to distinguish between changes in weight, which are caused by mere desorption of volatiles such as physisorbed water and those which are due to solid-state reaction generating a volatile product. 4.5. Solid-state NMR spectroscopy There are various possibilities to employ solid-state NMR spectroscopy for the
investigation of solid-state ion exchange. Similar to IR, one could use 1H MAS NMR to determine the extent of reaction between OH groups and compounds of cations which one would like to introduce into the structure. Moreover, since a number of cations, e.g., Li+, Na +, AI 3+, are directly observable via MAS NMR, their introduction into microporous solids can be studied even without the help of probe molecules. Also, replacement of the cations such as Na + of the parent sodium form of the zeolite and subsequent formation of, e.g., small crystallites of NaCI according to eq. (2a) may be detected by 23Na MAS NMR. In favorable cases MAS NMR furthermore provides information about the location and chemical surrounding of the introduced cation. Recently, techniques were developed which allow heating the solid samples inside the spinning rotor [42-44]. This will render possible in-situ Magic Angle Spinning NMR studies on solid-state reactions in addition to the conventional experiments, where the reacting salt/zeolite mixtures were pretreated and reacted ex-situ. 4.6. X-ray diffraction To date, XRD was used in the field of solid-state ion exchange (i) to check as to whether the crystallinity of the exchanged materials were affected by the solidstate reaction and (ii) to demonstrate that solid-state ion exchange has in fact occurred. The latter can be achieved through monitoring the decrease of the reflections of the compound containing the in-going cation M n+. In favorable cases, the appearance of the reflections originating from the corresponding compound, e.g. NaCI, may be observed. Furthermore, the reflections typical of the respective zeolite framework are affected by the in-going cation, and the resulting XRD pattern may be compared with that of a conventionally exchanged zeolite sample, if available. Similarity or coincidence of the XRD patterns of the products of solid-state and conventional ion exchange would suggest that the solid-state reaction has indeed led to an incorporation of Mn+'[22]. In a heatable XRD chamber in-situ measurements of solid-state ion exchange are possible. To date reported in-situ XRD investigations merely qualitatively confirmed solid-state ion exchange and provided information about the required reaction temperature. However, such measurements should enable one to carry out relatively precise kinetics investigations as well and moreover, should allow
1907 determining the population of the various cation sites in the zeolite structure as a function of time.
4.7. M6ssbauer spectroscopy M6ssbauer spectroscopy may be employed for the study of solid-state ion exchange as well. Unfortunately, there is only a very limited number of nuclei suitable for M6ssbauer experiments. The most prominent one is Fe; Eu and S n are also promising candidates. Experiments carried out with Fe-salt/zeolite mixtures gave, however, a wealth of information about the effect of the reaction temperature on the fraction of Fe 2+ and Fe3§ spedes incorporated into the structure, the degree of exchange, the chemical surrounding of the Fe-containing cationic species, and the coordinative configuration [19]. 4.8. Extended X-ray absorption fine structure (EXAFS) It was only recently that EXAFS measurements were carried out in order to study solid-state ion exchange. This method not only provides evidence whether solid-state ion exchange occurred, it also makes available data on the coordination of, e.g., introduced metal cations and, after their reduction, on the size of the particles formed. 5. SYSTEMS INVESTIGATED FOR SOLID-STATE ION EXCHANGE Even though systematic studies on solid-state ion exchange were carried out only during approximately the last decade, there has nevertheless been a relatively large number of reports in this field. Thus no exhaustive review can be given, and in what follows, selected examples of solid-state ion exchange of cations of alkaline, alkaline earth, rare earth, transition metals and noble metals into a small selection of zeolite structures will be discussed. 5.1. Incorporation of alkaline, alkaline earth and rare earth cations into zeolite struchLres via solid-state ion exchange Hydrogen forms and ammonium forms of Y-type and ZSM-5-type zeolites were reacted with halides of alkaline, alkaline earth and rare earth metals. It turned out that chlorides reacted more easily than fluorides and bromides. Iodides of alkaline metals usually decomposed in the presence of zeolites. The mixtures turned yellow due to free iodine already upon merely grinding. Also carbonates, nitrates and sulfates seemed to be less suitable. In the systems MIcI/NH4-Y, NH4-ZSM-5, or H-ZSM-5 (M I = Li+, K+, Rb +, Cs+), II II 2+ 2+ M Cl2/H-mordenite (M = Ca , Mg ) and MIII CI3/NH4-Y or H-ZSM-5 (Me III= La3+) IR spectroscopy, TPE combined with mass spectrometry (MS) or thermogravimetric analysis (TGA) to prove and quantitatively determine the effect of solid-state ion exchange [11,13-14] were employed. As an example Figure 1 displays the decrease in absorbances of both the high-frequency (3640 cm q) and
1908
the low-frequency (3550 cm "1) bands upon reaction with CsCl in high vacuum as a function of temperature and reaction time [45]. It is worth noting that the bulky Cs § cation enters the large cavities (decrease of the HF band) as well as the small cavities (decrease of the LF band) resulting in a relatively high degree of exchange, viz. almost 100% compared to a degree of only 60% obtained by conventional exchange in aqueous solution.
3.5 (a) NH4-Y(95) 723 K, 7 h (b) NH4-Y(95)/CsCI, AI/Cs - 1, 723 K, 12 h 3.0 uJ
o
LF band
Z
" I I
I !"
H F band
nO o0 rn
<
2.0
(a) 1.5
1.0 3000
(b)
3100
3200
3300
3400 3500 3600 3700 WAVENUMBER [CM -1]
3800
3900 4000
Figure 1. Heat-treatment at 723 K in high vacuum (10-5 Pa) of (a) NH4-Y (95) for 7 h and (b) CsCI/NH4-Y (95) with Cs/AI = 1 for 12 h; degree of exchange in NH4-Y: 95%. Figure 2 shows TPE/MS profiles recorded upon heating the same CsCI/NH4-Y mixture. Dehydration is seen between 400 and 500 K; the H20 peaks are, however, not significantly above the continuous background signal m / e = 18. As it is generally the case with the alkaline metal chloride/NH4-Y series one can distinguish between a low-temperature (LT) and a high-temperature (HT) reaction regime at around 500 and 860 K, respectively. In the range of the LT reaction, there is an overlap of the deammo~iation (evolution of NH 3) and the reaction of the OH groups, which formed thereby, with CsCI (evolution of HCI). At high temperatures (around 860 K) the reaction between less accessible OH groups, which have survived, and CsCI takes place.
1909
10
~' 8 ~.
m/o = 16 (NH3)
z
,
//
9
/
9
..
,s, r
~--- m/e = 36 (HCI) I~
,
"~
m/o = 36 (HCI) m/o = 18 (H20)
J'
..-'"-"
2
0 300
400
500
600 700 TEMPERATURE [K]
800
900
1000
Figure 2. Temperature-programmed evolution of NH 3, HCI and H20 from a mixture of CsCI/NH4-Y (95) with Cs/AI=I monitored by MS; degree of exchange in NH4-Y:95%. Reaction of MeICl with H-ZSM-5 or NH4-ZSM-5 gave similar results [11]. Mixtures with MeI/AI = 1 yielded an exchange degree of almost 100% as derived from the elimination of the OH band intensity. In this context it is worth noting that also the seemingly non-acidic so-called silanol groups (indicated by an IR band around 3740 cm "~) reacted to some extent. However, these OH groups immediately reappeared after brief washing of the exchanged zeolite due to rapid hydrolysis confirming their low strength of acidity. Since the solid-state reaction between MeIcI (especially LiCI) and acidic OH groups proceeds so easily, it provides a convenient means to eliminate residual Brensted acidity. The latter is sometimes undesired. An example is the preparation of a selective hydrogenation/dehydrogenation catalyst via H2-reduction of a noble metal-containing zeolite where, due to the formation of protons, acidic OH groups are unavoidably generated. These may be subsequently eliminated v/a solid-state reaction with LiCI or NaCl. As examples for reactions between salts of bivalent alkaline earth cations with hydrogen or a m m o n i u m forms of zeolites; the systems CaCI2.2H20/ NH 4mordenite, CaCl2.2H20/H-mordenite, MgCl2.6H20/H-mordenite, and
1910 CaC12-2H20/H-ZSM-5 were investigated. Also, MgF 2 was employed as a salt of the in-going cation, Mg 2+.
/, t~
./
/i\
,S"
s
18 .......
LLI 0
...--"
; 3610
Y
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61
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3500
I
3000
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-" 1 8 0 0
WAVENUMBER
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1600
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1400
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1200
[CM -1]
Figure 3. Solid-state ion exchange in the system CaCI2/H-MOR (Ca/AI=0,5); spectrum la, lb: pure H-MOR activated at 775 K in high-vacuum; spectrum 2a: CaCI2/H-MOR heat-treated at 770 K in high vacuum; spectrum 2b: CaCI2/HMOR heat-treated at 770 K in high vacuum followed by pyridine adsorption and removal of excess pyridine at 475 K; spectnml 3a: sample of 2a rehydroxylated by brief contact with H20 vapor at 400 K; spectrum 3b: sample of 3a after pyridine adsorption and subsequent degassing at 475 K. After Ref. [13], with permission. Figure 3 illustrates the changes in the IR spectra of H-mordenite (spectra la, lb), after solid-state reaction (spectrum 2a) and subsequent pyridine adsorption (spectrum 2b). Spectrum 2a shows the complete removal of the acidic OH groups and partial reaction of the silanol groups. Spectrum 2b reveals the incorporation of the Ca 2+ residing on cation sites of the mordenite structure as indicated by the band at 1446 cm d typical of pyridine coordinatively bound to Ca 2+ cations in zeolites. The small band at 1455 cm "1 is due to pyridine adsorbed onto residual true Lewis sites (extra-framework Al-containing species [13]). Another sample of a CaCl2/H-mordenite was heat-treated at 775 K and characterized by a spectrum like spectrum 2a in Figure 3. After rehydroxylation
1911 (brief contact with 1.3 kPa H20 vapor at 400 K) and removal of excess water, spectrum 3a in Figure 3 exhibited again a strong OH band at 3618 cm d due to acidic OH groups. Subsequent pyridine adsorption and deg~ssing gave rise to spectrum 3b. It exhibits a pyridinium ion band at 1540 cm originating from Bmnsted acid OH groups newly formed according to the Plank-Hirschler mechanism [46-47] Ca 2+ (OZ)~ + H20 ---) Ca(OH) + + (OZ)- + HOZ (4) OZ: monovalent negatively charged framework fragment of the structure. In agreement with these findings, a sample prepared via solid-state ion exchange and characterized by spectrtun 2 was almost inactive in acid-catalyzed reactions such as the disproportionation of ethylbenzene [4849] but became a much more active catalyst after the treatment with small amounts of water vapor. This observation confirms that the bare Ca 2§ cations were unable to act as carboniogenic centres in hydrocarbon reactions. Rather, the presence of acidic OH groups is an indispensable prerequisite for such a catalysis to occur. Table 1 illustrates, as an example, the stoichiometry of the solid-state reactions of the mordenite samples. There are interesting differences between the results obtained with H-mordenite and NH4-mordenite. The H-mordenite parent sample contains an appreciable amount of true Lewis sites as determined from the difference between AltotaI (AAS) and Altetrahedr" (27A1 MAS NMR). Nevertheless, a total of 1.09 mmol/g Ca 2§ were, after solid-state reaction, irreversibly held in the structure. This corresponded exactly to the total AI content (2.18 mmol Al/g). It was concluded that Ca 2§ was not only tightly bound to the framework AI (1.68 mmol/g) but has also reacted with extra-framework A1containing species according to, e.g. 2 AI OOH + CaCI 2 --r Ca(AIOO)2 + 2HCI 1' (5) Another fraction, i.e. 0.17 mmol CaCl2/g zeolite, has reacted with the weakly acidic part of the silanol groups which are indicated by the relatively strong OH -1 9 band around 3750 cm m spectrum la of Figure 3. This fraction was extractable as Ca(OH)2. By contrast, NH4-mordenite did not contain measurable amounts of true Lewis sites. The content of tetrahedraUy coordinated AI and the total AI content coincided (see Table 1). Thus, the amount of Ca 2§ irreversibly held on cation sites equaled the total amount of Altetrahedr. or acidic Bransted sites. However, a thorough evaluation of the analytical data revealed that a certain amount of CaCl 2 was occluded as salt molecules, viz. 0.66 and 0.27 mmol.CaCl2/g zeolite non-extractable at 300 K and 375 K, respectively. According to the findings of Rabo et al. [3] such occluded salt molecules enhance the thermal stability of the structure.
1912 Table 1 Mass Balance for ion exchange in the systems CaCI2.2H20/NH4-MOR and CaCI~.2H20/NH4-MOR (1) Zeolite
(2) AI total
(3) AI tetrah,
H-MOR NH4_MOR
2.18 2.50
1.68 2.50
(8) Zeolite
(9) (10) extracted extracted CaCI 2 Ca(OH) 2 1/2x(6)
H-MOR NH4-MOR
(4) (5) (6) CaCI 2 HCI; NH~CI extracted employed evolved CI"
0.72 0.94
(7)-(9) 0.17 0.01
1.98 2.48
2.52 2.54
(11) reacted CaCI 2
(12) irrev, held Ca 2§
1/2x(5)
(11)-(10)
1.26 1.27
1.09 1.26
1.44 1.88
(7) extracted Ca 2§ 0.89 0.95
(13) (14) occluded occluded CaCI 2 CaCI 2 extr. 300K extr. 375K (4)-(7)-(12)
0.66
0.27
t
* All data in millimoles per gram water-free zeolite Basically similar results were obtained with the systems CaCI2.~H20/H-ZSM5 and MgCl2.6H20/H-mordenite. However, compared to the experiments with the M I CI series (Me I = Li, Na, K, Rb, Cs), solid-state reaction was not as rapid with MIIcI2 . Particularly with MgCl 2 the reaction did not easily proceed and was frequently not completed. With MgF 2 the solid-state ion exchange was even more difficult. Only about 40% of the protons of the OH groups of the parent H2+ mordenite were replaced by Mg . This is certainly not due to the fact that MgF 2 does not possess crystal water, since in the case of the mixtures with CaCI2.~H20 or MgCI2-6H20 the H20 partial pressure during the high-temperature solid-state reaction was also below 10"s Pa as monitored by MS. Rather, the higher lattice energy of MgF 2, viz. 659 kcal.mol "1 vs. 595 kcal.mol d for MgCI 2 and 525 kcal-mo1-1 for CaCI 2, (see, e.g. Ref. [50]) provides a higher barrier for separation of MgF 2 entities from the MgF 2 cry stallites admitted to the zeolite powder. W h e n different structure types are compared, e.g., Y-type, mordenite-type and ZSM-5type zeolites, the ratio l~si/nAl, i.e. the distance between tetrahedrally-coordinated framework AI may also play an important role for the feasibility of solid-state ion exchange (vide infra, cf. system LaCI3/H-ZSM-5). Because rare earth cation-containing faujasite-type zeolites as components of acidic catalysts are important for various hydrocarbon reactions, in particular for
1913 hydrocarbon cracking, the system LaCI3-7H20/NH4-Y was also investigated with respect to solid-state ion exchange [14-16]. In fact, an exchange degree of almost 100% could be achieved in a one-step process. Such a high degree of exchange is usually achieved via conventional exchange only by repeated suspending the solid in aqueous La-salt solutions and intermittent separation of the suspension and.calcination of the (partially) exchanged material.
100 /~
. 8O
i \\ R ~ , , NH4-Y; m/e = 16 ,,/'lf'~/[,,,,l~ (deammoniation)
~ 6o " Z iii I--. Z -- 40 . -J 7
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,
' I I
Or) 20
-
0 300
LaCI3/NH4-Y;m/e=16
+
+
II II !! !! II
500
INH4-Y; m/e=18 I(dehydroxylation) i/!J,,,
\.~LaCI3/NH4-Y; mle=36 / \
\
'.: '("\ \ \ " . :
,
\ \ ~
~ \
", "
' ' I I
-~_ ~
\
~
700
~
TEMPERATURE [K]
~
+
I
900
',I I
t I l
I
11O0
Figure 4. Temperature-programmed evolution of NH 3, HCI and H20 from parent NH4-Y(98) and a mixture of LaCI3/NH4-Y(98) with La/AI=0.33, monitored by MS; degree of exchange in NH4-Y(98) 998%. After Ref. [14], with permission. In Figure 4, the TPD profiles are shown for the parent NH4-Y (m/e=16, deammoniation; m/e=18, dehydroxylation) as well as for LaCI3.7H20/NH4-Y (m/e=16, deammoniation; m/e=36, evolution of HCI). Again, one distinguishes a LT and a HT solid-state reaction. Moreover, the TPE experiments confirmed that in the system LaCI3/NH4-Y no dehydroxylation occurred as with NH4-Y (or H-Y) at about 950 K because all the OH groups were consumed via solid-state ion exchange at lower temperatures.
1914
I
3518
3616_i/A \
____ 29721
-
A = 0.1
LU
16Z
Total
2936
LU 0
B
LU N
-"
Z
F 2887
Z
<: rn nO or) rn .,r
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/ 3800
3400
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WAVENUMBER [CM -1]
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A
Total "
,,P ~_~_,.x_x-X-X, x 2600 0 1 2
0
z
O I-2 0 I--
TIME ON STREAM [H]
Figure 5. Solid-state ion exchange in a mixture LaCI3/NH4-Y(98) with La/CI=0.33 and catalytic activity in ethylbenzene disproportionation. Spectra a, b and c after heat-treatment at 455, 575 and 675 K, respectively; A: total conversion on the wafer characterized by spectrum c. Spectrum d: after rehydroxylation by short contact with H20 vapor (0.1 kPa); spectrum e: after contact with the feed stream (1.3 vol.-% EB in dry He, 5 ml 9min "z; mass of catalyst: 0.03 g; 455 K); B: total conversion on the wafer characterized by spectra d and e. After Ref. [14], with permission. Figure 5 displays IR spectra obtained in a similar way as those of Figure 3. One realizes that upon heating a (stoichiometric) mixture of LaCI3-7H20 and NH4-Y to 675 K for 20 h in high vacuum all the bands due to OH groups, which intermittently formed upon deammoniation, have disappeared. Since these experiments were carried out in a IR micro-flow reactor cell, the catalytic activity could be tested. The lower part of Figure 5 proves that the material obtained via solid-state exchange under exclusion of any H20 (Pactiv. < 10"5 Pa), was inactive in catalyzing the disproportionation of ethylbenzene. However, after a 2 minutes contact with 0.1 kPa H20 vapor at675 K OH bands typical of La-Y developed (see spectrum d in Figure 5, upper par0. Upon passing a stream of ethylbenzene in N 2 through the wafer (characterized by spectrum e in the upper part of Figure 5), the feed interacted with the acidic OH groups and a significant conversion of ethylbenzene was obtained. The catalytic performance of La-Y obtained via solidstate reaction of LaCI3-7H20 with NH4-Y was, after subsequent hydroxylation,
1915
even superior to that of a conventionally exchanged La-Y catalyst with the same degree of exchange.
I
': La-Y (98, S.E.) 9 9 9 9 9 La-Y (96; C.E.) : o o o o
a r" 3630-~A = 0.2
Tpretreat" Thvdrat
= 725 K = 525 K
Tr~act."
= 725 K
- 15.0
.i 10.0 Z
O ....., u)
O,
LU i ~5.0
Z
0 0
I
3500
I
3000
WAVENUMBER
0
[ C M "1]
I
I
I
I
I
50
100
150
200
250
0.0
T I M E O N S T R E A M [MIN]
Figure 6. Solid-state ion exchange in a mixture LaC13/NH4-Y(98) , resulting in LaY (98; SE), conversion in n-decane cracking and comparison with conventionally exchanged La-Y (96; C.E.). Spectra a and b: La-Y (98; S.E.) and La-Y (96; C.E.) after pretreatment at 725 K in high vacuum in an IR flow reactor cell (rehydroxylation step not shown). A and B: total conversion on wafers characterized by spectra a and b and subsequently rehydroxylated (feed: 0.1 kPa n-decane in dry helium; total pressure: 101.3 kPa, ~, = 10 m l . m i n d ; mass of catalyst: 0.03 g, Treact: 725 K). After Ref. [16], with permission. Similar results were obtained using the cracking of n-decane as a test reaction. Figure 6 displays the in-situ IR spectrum of highly-exchanged La-Y (98%), obtained via solid-state reaction, i.e. La-Y (98; SE) as described before and, for comparison, the spectrum of a La-Y prepared by conventional exchange with almost the same high degree of exchange (96%), i.e. La-Y (96; CE). After equal pretreatment both samples exhibited the same steady-state activity in n-decane cracking (see Figure 6). Main differences in the product distribution were due to a higher content of C3 through C 5 over La-Y (98; SE)and formation of C1, 1hexene and a higher yield of hexane over La-Y (96; CE). The results from solid-state reaction between LaCI3.7H20 and NH4-Y described so far were obtained using stoichiometric mixtures, i.e. La/AI = 0.33. It is worthwhile to note, however, that an excess of LaCI3.7H20 (La/A1 = 0.66) did
1916 not cause any difference in the results of the solid-state ion exchange. Brief washing of the heat-treated mixture removed exactly the excess of LaCI 3 (see Ref. [15]).
5.2. Incorporation of transition metal cations into zeolite structures via solidstate ion exchange Investigation of solid-state ion exchange of transition metal cations in zeolites is the particular domain of ESR spectroscopy, even though, of course, other techniques as discussed in section 4 are applicable as well. After the pioneering work by Clearfield et al. [4], especially Slinkin and his co-workers contributed significantly to elucidating solid-state ion exchange by ESR spectroscopy. These authors not only studied solid-state reactions in such important systems as Cucompounds/H-ZSM-5 and Cu-compounds/Y-type zeolite but they also investigated the incorporation of polyvalent, complex cations such as VO(OH) +, 5+ 3+ . . MoCI , Cr etc. using hahdes, oxides, sulfides as the compounds of transition metals. Because of the ever growing interest in the incorporation of transition metal cations into microporous solids in view of the potential for oxidation catalysis, several authors reported on these or similar systems. Focusing on the application of ESR, Kucherov and Slinkin [51] recently reviewed a great number of such studies. In the following subsections a selection of pertinent investigations will be summarized.
5.2.1 Solid-state ion exchange with compounds of copper, zinc, cadmium and mercury Introduction of copper, in particular into high silica zeolites such as mordenite and ZSM-5, was and still is intensely studied in view of denox catalysis [52-53]. A considerable effect of the reaction temperature was found in that calcination at 1073 K markedly increased the intensities of the ESR lines in comparison with the result of a 793 K pretreatment. Figure 7 shows, as an example, the result of the reaction between CuO and H-ZSM-5 at 1073 K and subsequent evacuation at 293 K (spectrum b) [6]. The ESR spectrum b is in complete agreement with spectrum a which was obtained by the same treatment of a conventionally exchanged Cu, H-ZSM-5 [10]. A full coincidence was observed regarding the g-values and hyperfine splitting (hfs) constants. Therefore the authors arrived at the conclusion that also after introduction of Cu 2+ into HZSM-5 via solid-state reaction, the isolated cations resided at two differently coordinated even though similar sites. One was characterized by gu=2.29, g• A,=15.6 roT, A• mT and is typical of a square planar environment. The second site corresponded to a fivefold coordinated state, giving rise to gn=2.31, g• An=15.3 mT and A• roT. The first site is assumed to be close to the wall of the straight elliptical channel where the Cu 2+ is linked with three oxygen atoms of the framework and one extra-framework ligand. The second site is
1917 surrounded by six oxygen atoms of the framework and somewhat apart from the wall of the channel. Essentially, the Cu 2§ is in a square pyramidal position (see Figure 8). These positions are supposed to be identical with those of Ni 2+ in HZSM-5 which could be confirmed by quantum-chemical calculations. The computed g-values were in satisfactory agreement with the experimentally obtained data of g• and g..
I l
I
n
gnl
I
9
x8
H dr
!
20 mT
x6
H
qll
I
20 rnT
Figure 7. ESR spectra of a mixture CuO/H-ZSM-5 and comparison with conventionally exchanged Cu,H-ZSM-5; spectrum a: Cu, H-ZSM-5, conventionally exchanged, calcined for 4h at 1073 K in air, evacuated at 293 K; spectrum b: Cu,HZSM-5 obtained by solid-state ion exchange from CuO/H-ZSM-5 upon calcination for lh at 1073 K in air and evacuation at 293 K; spectrum c: after contact of the sample of spectrum b with air. After Refs. [10] and [6], with permission.
1918
Z
Cu
Figure 8. Cluster model of an isolated Cu 2+ cation inside the H-ZSM-5 structure. After Ref. [54], with permission. Cu 2+ cations introduced by solid-state ion exchange into the zeolite structure were accessible for adsorbates such as 02. The interaction with the adsorbate was indicated by a significant change in the ESR spectrum (see spectrum c in Figure 7). The hyperfine splitting (hfs) entirely disappeared. However, this change was fully reversible. Coordinatively unsaturated isolated Cu 2§ cations were formed only in the reaction between completely exchanged H-ZSM-5 and CuO but not with H,Na2+ . ZSM-5. In the latter case, the Cu cations were located m an octahedral environment. When H-ZSM-5 samples with various nsi/nAl ratios were reacted, a linear relationship between the ESR signal intensity and the AI content of the framework (corresponding to the number of Br~nsted acid sites) was obtained (Figure 9). This confirms the important role of acidic OH groups as active centres for the solid-state reactions with CuO. No ESR signals due to isolated Cu 2§ cations were obtained after heat-treating a mixture of CuO and Na-ZSM-5. Results similar to those obtained upon calcining the system CuO/H-ZSM-5 were reported for the solid-state reactions of CuCI 2, CuF 2, Cu-hydroxy-carbonate, Cu3(PO4) 2 and Cu ~ In fact, the rates of the appearance of the ESR signals differed: the reaction of Cu ~was slowest, whereas CuCI 2 reacted most easily. Comparison with the results of ESR measurements on conventionally exchanged Cu-zeolites provided evidence that isolated Cu 2* cations were not only coordinated to
1919 oxygen anions of the framework but also coordinated to one extra-framework anion such as (OH)', CI', F', PO43- .
m
LU 0
0.5
1.0
1.5
AI203 [MOL %] Figure 9. Linear relationship between the maximum ESR intensity of C u 2+ i n Cu,H-ZSM-5 (obtained by solid-state reaction in the mixture CuO/H-ZSM-5) and the AI content of the ZSM-5 framework. After Ref. [6], with permission. Furthermore, it was shown that also monovalent Cu § migrated into the structure, for instance when H-ZSM-5 was reacted with Cu2S. Mild oxidation under conditions which could not cause migration of CuO, produced the typical ESR spectrum of isolated Cu 2§ Incorporation of Cu § and Cu 2§ was also investigated by Wichterlova et al. [55] and Karge et al. [18] who employed ESR, IR and TPE-MS to study the incorporation of the cations into H-ZSM-5. They found that the solid-state reaction proceeded most easily with the chlorides and was less feasible with sulfates, acetates and carbonates. Based on their quantitative measurements they were able to show that the degree of exchange increased with the amount of applied copper compound.
1920 More recently, Jiang and Karge [32-33] used the reaction of CuCl with mordenites and Y-type zeolites to study the kinetics of the solid-state exchange
(vide infra). Hartmann et al. [56] found that the solid-state reaction of CuCI with H-Y was the most reliable and successful way to incorporate Cu § Roessner et al. [57-58] and Salzer [59] investigated the solid-state reaction of ZnO with H-ZSM-5 using transmission and diffuse reflectance spectroscopy, respectively. They could show that Zn,H-ZSM-5 obtained via solid-state reaction exhibited catalytic activities in n-hexane isomerization comparable to those observed with Zn,H-ZSM-5 samples which were prepared by conventional ion exchange or by the incipient wetness method. The latter is somehow between exchange in aqueous solution and solid state [60-61]. In a study by Onyesty~ik et al. [62], Zn and Cd cations were also exchanged into faujasite-type zeolites such as NH4-X and NH4-Y as well as NH4-mordenite. Chlorides and, in the case of Cd 2§ oxides, sulfides and nitrates were reacted. The authors found that a greater number of Cd 2§ was introduced by solid-state reaction than by conventional exchange. The former route led to materials which were more efficient in dissociative adsorption of H2S. The reaction of mercury compounds such as Hg2CI 2 [12] with hydrogen forms of zeolites was studied in order to show that solubility in water of the compound of the in-going cation is by no means a prerequisite of solid-state ion exchange to occur (vide infra).
5.2.2 Solid-state ion exchange with compounds manganese
of iron, cobalt, nickel and
Halides of these metals have already been used in the early work of Clearfield et al. [4] who reacted them with A and X type zeolites to show qualitatively (with the help of ESR spectroscopy) the phenomenon of solid-state ion exchange. More recently, the incorporation of Fe3§ into H-ZSM-5 [8] and Fe 2§ into NH4-Y [19,63] were carried out. Kucherov and colleagues [8] showed that upon solid-state reaction of FeCI 3 at 793 K, Fe3§ migrated into cationic positions of H-ZSM-5 where they resided as isolated cations in a strong crystal field of low symmetry and were accessible to gas phase molecules. Their properties were completely different from those of Fe located in the framework, even though the ESR spectra resembled each other. The main signal appeared at g--4.27, as was also observed by Wichterlova et al. [55] compared with g--4.25 for H-[Fe]-ZSM-5. However, upon admission of 0 2 or NH 3, the ESR spectrum of Fe,H-ZSM-5 obtained by solid-state ion exchange drastically changed, whereas that of ferrisilicalite H-[Fe]ZSM-5 remained essentially unchanged. Furthermore, Fe3§ on cation sites could be replaced by reaction of Fe,H-ZSM-5 with CuO (the ESR spectrum of Cu-ZSM-5 appeared, vide supra) which did not occur upon heat treatment of CuO/H-[Fe]-ZSM-5.
1921
Finally, it is very typical of Fe3§ on cation sites that their ESR spectrum shows an anomalous temperature effect in that, upon cooling to 77 K, the intensity of the line at g~4.27 strongly increases (see Fig. 10). Thus, it is possible to distinguish between Fe in extra-framework and (tetrahedrally coordinated) framework sites via the different behavior of the 4.27 line upon various treatments, even though Fe3§ may occur in an tetrahedral coordination in extra-framework positions as well [55].
H r
Figure 10. ESR spec~ of Fe,H-ZSM-5 obtained by solid-state reaction between FeCl 3 and H-ZSM-5. Spectrum a: uncalcined mixture; spectrum b: after calcination at 573 K for lh; spectrum c: after subsequent cooling to 77 K. After Ref. [8], with permission.
1922 Investigation of mixtures between FeCl 3 and dealuminated H-Y led to results similar to those obtained with FeCI3/H-ZSM-5. Table 2 Data obtained from 300 K M6ssbauer spectra after sequential 4 h evacuations of mechanical mixture of FeCIT4H~O+NH~-Y Temp. (K)
Component
IS
QS
RI
300
Fe(III)oct Fe(II)tetr Fe(II)oct_l FeCI.xH20 Fe(III)oct Fe(II)tet r Fe(II)octd FeCI.xH20 Fe(III)oa Fe(III)tri8 Fe(II)trig Fe(II)oct_3 FeCI.xH20 Fe(III)oct Fe(III)~$ Fe(II)tri8 Fe(II)oct.2 Fe(II)oct_3 Fe(III)oct Fe(III)trig Fe(II)trig Fe(II)oct_2 Fe(II)oct.3
0.37 0.69 0.83 1.13 0.35 0.72 0.79 1.04 0.31 0.25 1.08 1.06 1.06 0.30 0.23 0.88 0.95 1.23 0.33 0.22 0.92 0.99 1.27
0.63 0.35 1.95 1.83 0.72 0.36 2.00 1.79 0.75 1.53 0.78 2.60 1.90 0.72 1.71 0.62 2.20 2.19 0.58 1.73 0.68 2.13 2.16
57 13 9 21 44 11 22 23 38 11 21 9 21 5 11 21 37 26 2 2 30 45 20
420
520
620
720
(IS: isomer shift, relative to (z-iron, m m / s , QS: quadrupole splitting, m m / s , RI relative spectral area, %) Introduction of Fe 2+ and Fe 3§ via solid-state reaction was extensively studied by Lazar and colleagues [19,63] who mainly used M6ssbauer spectroscopy complemented by TPD and XRD. Figure 11 displays a set of M6ssbauer spectra obtained upon reaction between FeCI 2 and NH4-Y at various reaction tempera-
1923 tures. Table 2 provides the results of the analysis of those spectra. One recognizes . . . . 3+ z+ . that the sohd-state reaction results m various Fe and Fe speaes as a function of temperature. The chemistry may be described by eqs. (6-8). Fe 2+ + H20 ---> Fe(OH) + + H + (6) Fe(OH) (n'z)+ + (n-1)HCI --) FeCI (n'l)+ + (n-1)H20 , (n=2 or 3) (7) FeCI (n'l)+ + H + --) Fe n+ + HCI (8)
•
~mbient
^
,_.,
0K
<:~
Fe
Z
Z
O
20 K
I--Q. n" O 03 nn <:
20 K
m
~
Fe(ll)tetr'2
nO
Fe(ll)~
CO m < ,,
20 K
oct"4
I,,- QS -,I Its I
b - e" treatment in high vacuum I
-5
I
I
-3
I
'
-1
i
'
1
'
'
3
I
I
5
VELOCITY [MM/S]
Fe(lll)trig -~-
~
I"
I
I
-3
'
'
-1
I
'
1
'
'
'
5
VELOCITY [MM/S]
Figure 11. (A) M6ssbauer spectra of (a) FeCI2.4H20/NH4-Y ground at ambient temperature in air and of (a) heat-treated in vacuum at (b) 420 K; (c) 520 K; (d) 620 K and (e) 720 K. (B) M6ssbauer signals of individual iron species giving the best fit to spectrum e of Figure 11 A. After Ref. [19], with permission. This reaction scheme was supported by the results of TPD/MS. Moreover, the disappearance of the FeCI2.4H20 and appearance of NH4C1 reflections (at low temperatures) proved the occurrence of a solid-state reaction in the mixture of FeCI2-4H20 and NH4-Y. After heating the mixture to 720 K in high v a c u u m only about 4% of Fe 3+ species (FelocgtandFc~g)remained, i.e. the material was almost exclusively an Fe2+-Y zeolite. The results were explained by assuming that the intermittently formed fraction of Fe 3+ species was almost completely reduced through autoreduction which proceeds according to eq. (9).
1924 2 {AIO4/2} 3 Fe3+ ---) 2 {AIO4/2}2 Fe2+ + A1203+ 89 2
,
(9)
where {A104/2}-denotes the part of the zeolite structure containing one tetrahedron with AI as the central atom. As frequently observed (compare Section 5.1)the reaction proceeded most easily with Fe-chloride, whereas the solid-state exchange with Fe(COO) 2 or Fe(CH3COO) 2 occurred to a much lesser extent exhibiting an even more complex chemistry. Onyesty/ik et al. [62] in their comparative study on the incorporation of bivalent metal cations into faujasite-type zeolites also reacted COC12 with NH4-Y. They found deep ion-exchange in all cases as measured by the disappearance of the OH stretching bands. For the HF band (around 3640 cm d) the sequence was Co < Mn < Cd ~ Zn < Ca. However, since the LF band (around 3550 cm d) was preferentially eliminated, the authors concluded that M 2+ cations were to a high degree selectively incorporated by reaction with the OH groups in the small cavities. Solid-state reaction of manganese with H-ZSM-5 was studied by Wichterlova et a1.[55] and Beran et al. [17]. ESR, XPS, TPD of ammonia, TPE of HC1 and test reactions (conversion of methanol or toluene) were employed to study the solidstate exchange and characterize the exchanged zeolites. XIX3 showed the same metal concentration as bulk analysis and, therefore, the authors concluded that the metal cations incorporated via solid-state reaction were homogeneously distributed. Neither surface enrichment nor depletion was observed. As an example, a set of ESR spectra obtained after introduction of Mn 2+ is shown in Figure 12. The parent mixture of MnSO4 and H-ZSM-5 did not show any hyperfine splitting of the signal at g = 0 due to isolated Mn 2+ cations on exchange sites in the zeolite structure (compare spectra a and d). However, upon heattreatment of the mixture at progressively higher temperatures, the hyperfine splitting became more and more pronounced until the spectrum appeared to be identical with that of conventionally exchanged Mn,H-ZSM-5 (spectrum d). Conversion of toluene and formation of aromatics from methanol was similar on conventionally exchanged Mn,H-ZSM-5 and Mn-containing ZSM-5 obtained via solid-state reaction but higher than for zeolite-supported MnO. However, introduction of the metal resulted in a decrease of the overall catalytic activity when compared to H-ZSM-5 with a similar nsi/nnl ratio. Moreover, in the methanol reaction the yield of aromatics was lower and that of olefins higher over Mn-loaded H-ZSM-5 than in the parent zeolite H-ZSM-5 [55]. Beran et al. [17] studied not only the solid-state reaction between MnSO4, Mn(CH3COO) 2 and Mn30 4 and H-ZSM-5. Rather they employed MnCI 2, which yielded by far the highest degrees of exchange (60-85%). They were able to show that the degree of exchange is strongly affected by the reaction temperature (see Figure 13). The occurrence of solid-state reaction in the mixtures of H-ZSM-5 and the above M n
1925 compounds was confirmed by IR, EPR and TPE of HCI, and the degree of exchange determined v/a the consumption of acidic OH groups or the intensities of the IR bands due to pyridine attached to Mn 2§ on cation sites.
Figure 12. ESR spectra (X band) of Mn,H-ZSM-5 obtained by solid-state reaction. Spectrum a: physical mixture MnSO 4 + H-ZSM-5, untreated (sample a); spectrum b and c: sample a heat-treated at 770 and 870 K, respectively; spectrum d: conventionally exchanged Mn,H-ZSM-5, for comparison, g--4.27 9 Mn 2§ in distorted tetrahedral coordination; g=2.00 isolated Mn 2§ cations in octahedral coordination; splitting into six hyperfine lines with A=9.8 mT. After Ref. [55], with permission.
1926
770 K .o-. or) 50 t3.
670 K
0 rr' (..9 1" 0 25 C3 LU
570 K
Z 0 0
I Heat treatment in vacuum ] I
I
I
5 10 15 R E A C T I O N TIME [H]
I
2O
Figure 13. Effect of the temperature on the solid-state reaction between MnCI 2 and H-ZSM-5, monitored by the IR measurement of the consumption of acidic OH groups. After Ref. [17], with permission.
5.23 Solid-state ion exchange with compounds of vanadium, chromium and molybdenum
niobium,
Incorporation of vanadium, chromium and molybdenum cations into highsilica zeolites was first studied in detail by Kucherov and Slinkin using ESR spectroscopy. H-ZSM-5 was reacted with V20 5 [7,51,64] at 873-1073 K, and an ESR spectrum typical of isolated vanadyl cations was obtained (Figure 14). The parameters were: g,=1.93; g• A,=19.8 mT; A• mT). In case of an interaction of VO(OH) + with a framework AI, where the negative charge should be concentrated, one would expect a super-hyperfine splitting. This was indeed observed (see Figure 14, b and c) with a splitting of 0.7 mT. Thus, the vanadyl cation is supposed to be rather close to the aluminum. Its dxy orbital, which carries the unpaired spin, is assumed to lie in the plane drawn perpendicular to the plane of Figure 15 and the V=O bond directs away from the O-AI-O fragment. The vanadyl cations were readily accessible by adsorbates such as H20, 02, ammonia, pyridine, p-xylene and nitrobenzene which was concluded from the changes in the ESR spectra upon adsorption. The oxidation state of V(IV), however, was very stable and remained unchanged by adsorption and treatment in a reductive or oxidizing atmosphere.
1927
I
I
I
I
I
I
I
I
I
-i
"
H
I!
m
,
20 mT
|
:
:
a
I I I I
I
I
Im
I
L
i
I
'
i I
I
I
m
I
gml .-
-l b
c
L.
i
.J
.5 mT.
0.7 mT .J I_ ~1 r"
C
2mT
Figure 14. ESR spectra of isolated vanadyl cations (5IV(IV)) introduced into HZSM-5 (nsi/nAl = 35) by solid-state reaction with V20 5 at 1025 K. After Ref. [64], with permission. Kucherov and Slinkin [9] also investigated the co-introduction of pairs of cations such as V/Cu and Cr/Cu. The reaction of V20 s with H-ZSM-5 at 1073 produced a maximum number of isolated V(IV) cations, e.g. VO(OH) § Sub-
1928 sequent interaction of the obtained VO(OH),H-ZSM-5 with Cu compounds, however, led to a decrease of the VO(OH) + concentration and a corresponding increase in the Cu 2§ concentration. Obviously, Cu 2§ is more strongly held and is able to readily replace VO(OH) §
O
,,
M-/-,-"Ill,'"
:
-" ,'" C , ~ . . . . _-:.~r~,. . . . . . -7"-
7
,,f
sSI
\ J
J
Si
Si Sw i/
AI
7"
i SJ
Si
\
Figure 15. Close proximity of the vanadyl cation to framework A1 as the origin of the super-hyperfine splitting of the ESR signal of Figure 14. After Ref. [64], with permission. The solid-state reaction in a mixture of V2Os and Na-Y was studied by Marchal et al. [65] who employed XRD, 29Si MAS NMR, 129Xe NMR and EPR. They observed solid-state reaction at 973 K, indicated by the disappearance of the V20 5 reflections. At low loadings, the zeolite structure was not damaged; this was confirmed by 129Xe N1VIR. Some pores, however, seemed to be blocked by Vcontaining species which had formed. Also, no significant change in the nsi/nal ratio of the framework was seen by 29Si MAS NMR. Only at higher loadings the structure collapsed. Weakly loaded samples gave spectra similar but not identical to those observed by Kucherov and Slinkin; V5§ and V 4+ species were observed. The results were interpreted as being due to the formation of sodium vanadates and vanadium bronze phases; NaV~VIVoI5 and NasVVvIVo32 were identified. Introduction of vanadium into H-ZSM-5 and H-mordenite was further investigated by Shen et al. [66-67]. The authors suggested that VO 3§ formed upon
1929 solid-state reaction of V20 5 with these zeolites. Characterization was carried out with the help of XRD, XPS, surface area determination and FTIR using pyridine as a probe. The catalytic behavior of the V-containing zeolites prepared through solid-state ion exchange in toluene oxidation to benzaldehyde seemed to depend on their acidic properties. As reported by Huang et al. [68], incorporation of vanadium-containing cationic species may be facilitated by the presence of water vapor. This proved to be particularly valuable in the case of high aluminum zeolites which cannot stand the high temperatures used in the solid-state exchange between V20 5 and high-silica zeolites (vide infra). Such findings will be presented in Section 6.2 Solid-state reaction between niobium oxide and Y-type zeolites was studied by Ziolek et al. [69]. This reaction seems to be the only suitable way of introducing Nb into zeolites. An exchange between Na § in Na-Y for niobium-containing cationic species was not observed when a mixture of Nb205 and Na-Y was heated to 975 K whereas such an exchange did occur with NH4-Y and dealuminated forms of zeolites (D-NH4-Y, nsi/n~a=4,25). XRD showed a disappearance of the Nb205 reflections upon heating the mixtures of Nb2Os/NH4-Y or Nb205/DNH4-Y. Moreover, XRD confirmed, in contrast to the report in Ref. [70], that the crystallinity of the zeolites was maintained after calcination of Nb205/Y-zeolite mixtures. The solid-state reaction led to a decrease in the density of Bronsted acid sites and an increase in the density of Lewis acid centres. This was evidenced by TPE of water, TPD of ammonia and IR using pyridine as a probe. The observed changes in acidity were accompanied by an enhancement of redox properties, in particular in the case of Nb-modified D-NH4-Y where isopropanol was partly converted to acetone. The introduction of chromium into zeolites was investigated by several authors [7,71-76]. Both Cr203 and CrO 3 were used as reacting oxides. Calcination in air of their mixtures with H-ZSM-5 produced identical spectra, which revealed that chromium was introduced as Cr(V). Since the average distance between the framework A1 atoms in the ZSM-5 structure is rather large, it was assumed that, similar to the cases of other polyvalent cations such as v a n a d i u m or molybdenum, not bare Cr5+ cations but complex cations, e.g., (CrO2) + were introduced. However, the maximum amount of Cr(V) cations, which could be introduced via solid-state reaction, did not exceed 20% of the framework AI. Excess chromium compounds remained at the external surface of the zeolite crystallites. Adsorption of gases such as 0 2, NH 3 or pyridine gave rise to significant changes in the ESR spectra showing that the Cr(V) species introduced into the interior of the structure were accessible. The results obtained by Kucherov et al. [77] upon reaction of CrO 3 with H-[Ga]ZSM-5 very much resembled those found with H-[AI]ZSM-5 (vide supra). However, Cr, H-[Ga]ZSM-5 proved to be thermally less stable than H-[AI]ZSM-5.
1930 Finally, as was found with the system (VO2),H-ZSM-5/CuO, co-introduction of Cr and Cu was also possible [9,78]. The ESR spectrum of a mixture of CuCrO 4 and H-ZSM-5, for instance, was a superimposition of the two ESR signals due to Cu (II) and Cr (V) ions. The hfs parameters indicated that Cu (II) as well as Cr (V) ions resided as isolated cations inside the zeolite structure and the dipole-dipole interaction was negligible. Similar to vanadium, the introduction of Cr(V), which possesses a nuclear spin I--5/2, generates a super-hyperfine splitting with sextets of g• (splitting constant: 0.6 mT) and g. (splitting constant: 0.75 mT). This shows a close proximity of introduced Cr(V) to tetrahedrally coordinated A1 in the framework.
I
I
x a
3 400 ft. v
I
LL
o5
I
I-.
Z :::::)
z
b
200 I
I
_J UJ nn
:D v 0
I
40000
I
20000 WAVENUMBER [CM 1 ]
Figure 16. Diffuse reflectance spectra of Cr-containing Y-type zeolite. (a), Cr-X, conventionally exchanged; (b), Cr-Y, obtained via impregnation and calcination; (c), Cr-Y, obtained via solid-state reaction. After Ref. [75], with permission. In their systematic studies of preparation and properties of Cr-containing zeolites, Weckhuysen and Schoonheydt [75-76] employed diffuse reflectance spectroscopy (DRS) and ESR. They compared various zeolite structures (X, Y, mordenite) loaded via different techniques (conventional ion exchange; impregnation-incipient wetness method; and solid-state ion exchange). In any event, CrCI 3 9 6 H20 was used as the Cr-compound. After calcination at 823 K differently prepared samples gave rise to very similar DRS spectra (see Figure
1931 -1 16). They exhibited two pronounced bands at 28,000 and 38,000 to 39,000 cm which are typical of chromate-like species. The spectrum of the Cr-Y sample, which was prepared via solid-state ion exchange, showed an additional weak band around 10,000 cm d. Detailed analysis of the ESR spectra and decomposition of the DRS bands revealed the presence of various oxidation states of Cr, viz. Cr (III), Cr(V) and Cr(VI). The ESR parameters of Cr, H-ZSM-5 (gj=1.99; g.=1.93, Cr(V) in supercages; g• gn=1.94, Cr(V) in small cavities) coincided with those reported by other authors [73,79]. Weckhuysen and Schoonheydt [75-76] also assumed that Cr(V) resides inside the structure as a chromyl species, CrO~. An interesting sequence in the propensity for reduction by CO treatment was found, viz. Cr-mordenite > CrY (impregnated) > CrY (conventionally ion exchanged) > Cr-X > Cr-Ga-Y > CrY (solid-state ion exchanged). The authors suggested that there exists a relationship between hardness of the Cr(V)- or Cr(VI)-containing zeolite and its reducibility: the hardness increases from Crmordenite over Cr-Y and Cr-X to Cr-Ga-Y. Thus, the harder the zeolite, the less reducible it is. The low reducibility of CrY prepared via solid-ion exchange, however, was explained by the fact that this material showed residual acidity. Incorporating Mo-containing cationic species into zeolite structures is of great interest because of the redox properties the resulting materials are expected to have. Molybdenum occurs in various oxidation states and the incorporation of Mo into zeolites might provide us with valuable shape-selective oxidation catalysts. Early work on solid-state ion exchange between H-Y and MoOCI 4 was reported by Dai and Lunsford [80]. These authors heated MoOCI4/H-Y mixtures to 673 K and characterized the products by IR, ESR and XRD. As reacting Mocompounds, frequently MoCI 5, MoO 3 or MoC13 were also used [7,23,81-82]. 6+ 5+ Again, one has to assume that not Mo or Mo cations as such, but complex entities, e.g., MoCI4 § enter the zeolite structure. Solid-state incorporation of Mo into H-ZSM-5 upon heating a mixture of MoCI 5 and the zeolite at a relatively low temperature (423 K ) i n vacuum produced a strong ESR signal typical of Mo(V) ions (6 components with a splitting of 8 mT due to the presence of 95M o and 97Mo) indicating the uptake of Mo species [7]. A similar result was obtained when H-mordenite was used instead of H-ZSM-5, but no reaction occurred with Na-mordenite. The Mo(V) cations introduced were, however, not stable against oxidation in air at 573 K. Such a treatment caused irreversible elimination of the ESR signal. In his Thesis, Bock [23] described systematic work on the solid-state ion exchange of MoCI 3 with narrow or medium pore zeolites such as H-ZSM-5 (10membered rings, 3-dimensional pore system), zeolite H-EU-1 (10-membered rings, large side-pockets, 1-dimensional pore system), H-ZSM-35 (ferrierite structure, crossing 10-and 8-membered ring channels) and zeolite L (narrowed 12-membered rings, 1-dimensional pore system). The effect of acidity, the nsi/nA1 ratio, crystallite size and zeolite structure on the solid-state ion-exchange were
1932 studied. The solid-state reaction was monitored by TPE of HCI. An interesting result was that only two thirds of the chlorine reacted and was detected as HCI. It was concluded that species like [Mo2C12]4+ (or MoC12+) remain in the structure and populate the cation sites there. Moreover, from the effects of particle size and pore size it was derived that solid-state ion exchange occurs via the migration of ion pairs rather than independent transport of cations and anions. In many cases, the solid-state ion exchange appears to be diffusion-controlled (see Section 8). Attempts to introduce Mo-containing cations into zeolites by reacting with MoO3 were unsuccessful [7]. Wang et al. [81] and Yuan et al. [82] reported that the reaction between MoO 3 and H-Y was significantly facilitated by the presence of water vapor. This modification of solid-state ion exchange will be dealt with in Section 6. 5.2.4
Noble metals
40 30 20 10
-~ Io~ 40-
o
o ""
30-
r Z
10 0.
I pd: 1.0 Wt-%; Ca(O.2)/H-ZSM-5 ]L B
i Pd: 1.0 wt-%; Ca(O.2)/H-ZSM-51
I Pd: 1.0 wt-%; Ca(0.4)/H-ZSM-5 I t
[ Pd: 1.0 wt-%; Ca(0.4)/H-ZSM-5 !
[...,Pd:1.0 wt-%; Ca(0.6)/H-ZSM-5 I
I Pd: 1.0 wt-%; Cal0.6)/H-ZSM-5 I
20
40 30 20 lO
00.0 1.0
2.0 3.0 4.0
5.0
6.0 7.0 0.0 1.0 2.0 D I A M E T E R [NM]
3.0
4.0
5.0
6.0
7.0
Figure 17. Size distribution of Pd-particles from electron microscopic images of PdCI~-CaCI 2 /H-ZSM-5 after simultaneous (A) and successive (B) introduction of Ca z+ and Pd 2+ via solid-state exchange followed by reduction; details see text. After Ref. [21], with permission. It could be shown that solid-state ion exchange is also a suitable route to noble metal-containing zeolites. Bifunctional catalysts, containing both acidic and hydrogenation-dehydrogenation centres were prepared by Karge et al. [20-21] via incorporation of Pd or Pt in H-ZSM-5. The solid-state reaction was carried out
1933 with PdCl2, PdO or PtCI 2. The materials were, after reduction, tested by hydrogenation of ethylbenzene. Subsequent or simultaneous introduction of the noble metal component and alkaline earth cations or La 3+ improved the performance of the reduced catalysts. The presence of Ca 2* or La 3+ decreased the acidity. Moreover, these cations seem to play the role of anchors for the metal particles which form upon reduction and thus improve the particle size distribution (Figure 17).
60
i
0.54 Pd /
Rho
0.54 Pd
/ ZK-5
1.0 Pd / S A P O - 4 2 mm
i
50
B
m
i
Yn-Hexane
0
m Y2,2,4-Tdmelhylpentane
m Y2r2,4-Tdm~~n~e
40
Yn-Hexe.ne
i
"1o >, -r-
>a ..J
30
ul >.
20
=,=
A v
w
m
=l
1
Yn-Hexane im
m Y2,2,4-Trimelhylpen~e
10 m -
0(~
m
m
m
m
m
m
,--I _._,_. 60 120
mm
_ 180
m
0
m
,_,_ 60
- -
._'
_-~_ 120 180
9 0
--"
m 60
,
m
m 120
m
i
m
m 180
TIME ON STREAM [MINI
Figure 18. Time-on-stream behavior of three narrow-pore catalysts in the competitive hydrogenation of an equimolar hexene-(1)/2,4,4-trimethyl pentene(1) mixture (Treact=343 K, W/Falkenes=10.g.h/mol). After Ref. [25], with permission. The study of Weitkamp et al. [24-25] provides an excellent example of the advantages of solid-state ion exchange. These authors successfully introduced 2+ 3+ noble metal cations (Pt2§ Pd , Rh ) into small pore zeolites (ZSM-58, zeolite Rho, zeolite ZK5 and SAPO-42) via reacting the hydrogen forms of these zeolites with PdCI 2, PtCI 2 or RhCI 3. The conventional ion exchange with noble metal complexes is not applicable in the case of small pore zeolites because of the bulkiness of the salt complexes, e.g. [Pt (NH3)4]CI 2. Weitkamp et al. [25] were able to prove that solid-state ion exchange of noble metal cations into small pore zeolites resulted, after reduction, in highly shape-selective hydrogenation
1934 catalysts. This is illustrated by Figure 18, which shows the results of competitive hydrogenating slim n-hexene-(1) (inside the structure) and bulky 2,2,4trimethylpentene-(1) (at the external surface) of Pd/Rho, Pd/ZK-5 and Pd/SAPO42.
(111) (220)
I/I
PtCI2 (400)
(311)
7500 5000 2500
6
8
10
12
14
16
18
20
22
24
BRAGG ANGLE, 2 0 [DEGREE]
Figure 19. In situ XRD patterns of a mixture PtCI2/NH4-Y obtained during solidstate ion exchange in air at the temperatures indicated. After Ref. [83], with permission. Introduction of platinum into Y-type zeolite was also studied by Hatje et al. [83] who monitored the solid-state reaction between PtO 2 or PtC12 and NH4-Y by temperature-programmed reduction (TPR), XRD and dispersive extended x-ray absorption fine structure (DEXAFS) measurements. In the absence of air, reduction to zero valent metal (Pt~ was observed. When, however, the solidstate ion exchange was conducted in air, no reduction and metal aggregation was observed. Rather, it was found that starting at 555 K the Pt-C1 shell at 0.175 n m was replaced by a Pt-O shell at somewhat lower distances (0.150 nm). The in-situ XRD measurements proved the migration of platinum to cationic sites of the zeolite structure in that the disappearance of the PtCI 2 reflections (20 =12.85 ~ was observed. Concomitantly the (111) and (220) reflections, mainly due to cations on the respective positions of the structure, increased (Figure 19).
1935 Recently, Schlegel et al. [84] reported on the solid-state ion exchange between Rh chloride and dealuminated Y-type zeolite studied by FTIR. A significant effect of the presence of CO in the gas phase was detected (vide infra). 6. MODIFICATIONS OF SOLID-STATE ION EXCHANGE There are a few modifications of the process of solid-state ion exchange such as contact-induced ion exchange, vapor phase-mediated cation exchange and reductive solid-state ion exchange which should be briefly discussed.
6.1. Contact-induced solid-state ion exchange
1400 -
35OO 3000
1200
A
25OO
1000
B
20OO 1500
8O0
1000 500
600
0
5
10
2o
15
4OO 2OO I
10
I
t
I
20 30 BRAGG ANGLE, 2 0 [DEGREE]
40
Figure 20. Contact-induced solid-state ion exchange between Li-A and Na-A (A) under exclusion of even traces of water and (B) under admission of ambient moisture; details see text. In a very interesting experiment, Fyfe et al. [5] were able to show that a cation exchange occurred between, e.g., Li-A and Na-A merely due to intimate contact of the zeolite crystallites. These authors used 29Si MAS NMR and XRD to demonstrate this phenomenon. Initially, the mixture of Li-A and Na-A exhibited, as expected, two sharp 29Si MAS NMR signals, i.e. at 8 = - 85.1 and 8 =-88.9 p p m (referenced to TMS) according to the different local environments in Li-A and Na-A. After some time needed for equilibration, however, only one sharp line
1936 was observed between the original doublet, indicating that now a single phase with a homogeneous cation distribution had formed. The result was confirmed by XRD where the corresponding splitting of the reflections disappeared upon equilibration. Similar results were obtained with the pairs Li-A/Na-Y and LiA/Na-MOR. Koy and Karge [31] succeeded in proving that this type of ion exchange requires residual adsorbed water in the pores of the zeolite crystallites. When completely dehydrated Li-A and Na-A samples were used and all experimental steps (mixing, filling the XRD capillary, sealing the capillary, etc.) was carried out in an efficiently working glove box (PH20 < 10" Pa), no collapse of the doublets of XRD reflections was observed (Figure 20A). When, however, the capillaries were opened and contacted with ambient moisture, the splitting of the reflections vanished (Figure 20B). '
I
'
I
'
I
'
I
'
I
'
I
NaCI,cryst. Na-Y
b
i/\~
J~l-
~.. CaCI2/Na-Y
c
d a 10
~, i
I 0
I
I -10
I
I -20
LiCI/Na-Y 9
I -30
i
I -40
CHEMICALSHIFT,~NaCl,cryst.[ppm] Figure 21. Contact-induced solid-state ion exchange between Na-Y (a: parent zeolite) with (b) CaCI2.2H20, (c) BeCI2 and (d) LiCI; details see text. After Ref. [30], with permission.
1937 Also, 23Na MAS NMR proved to be a suitable tool to detect and determine cation migration upon contact-induced ion exchange. Figure 21 displays the results obtained upon grinding mixtures of CaCI2/Na-Y, BeC12/Na-Y and LiC1/Na-Y [30]. The reference was NaC1. Thus, appearance of the strong signal at zero ppm indicates solid-state ion exchange upon mere contact of the chloride powders and zeolite grains. Obviously, sodium was replaced by the in-going Be2+, Ca 2+ or Li § and formed tiny NaCI crystallites. The broad signal at -12.5 ppm is due to Na § located in the truncated octahedra of the Y-type structure [85]. As was proven by IR spectroscopy [30], the process is to some extent reversed at higher temperatures, i.e. the equilibrium is shifted to the left of eq. (2a).
0 -8.2
-9.1
"'7,
23
u
~
03 Z W I--z
-13.1
_.1
Z
0
-20 0 C H E M I C A L SHIFTS,
-20
~iNaCi' cryst.
0
-20
[PPM]
Figure 22. Contact-induced solid-state ion exchange between Na-Y (a: parent zeolite) and crystalline LaCI3 (b: ground at ambient; c: heat-treated at 850 K); details see text. After Ref. [22], with permission. Similar observations were made with the system LaCI3/Na-Y [15,22]. Again, contact-induced solid-state exchange was revealed by 23Na MAS NMR (see Figure 22). A sharp signal at about-9 ppm (referenced to NaCI) is indicative of Na + in the large cavities; Na + inside the truncated octahedra gives rise to a signal at about -13 ppm (vide supra, Ref. [85]). In hydrated NaY, both signals overlap, and the resulting broad band appears at-8.2 ppm (Figure 22a). When a mixture
1938
of LaC13 and Na-Y was intimately ground one obtained the spectrum b of Figure 22, where the strong signal at zero ppm indicated replacement of Na t by La3§ La(OH) 3+ or La(OH)+2 and the corresponding formation of NaCI. The second signal at-13.1 ppm showed that all the remaining sodium cations resided in the truncated octahedra (sodalite cages). However, when the ground mixture was heat-treated at 850 K, the exchange was partially reversed, the signal at zero p p m decreased and the signal around -9 ppm reappeared. The appearance of N aC1 upon grinding the mixture LaCI3/Na-Y and re-migration of Na § at higher temperatures was confirmed by XRD [22]. Also, the changes in the intensities of reflections other than those of NaCI were in agreement with the 23Na MAS NMR measurements of contact-induced solid-state exchange in the system LaCI3/Na-Y, as caused by grinding and heat-treatment. Contact-induced solidstate ion exchange in the system LaCI3/Na-Y yielded acidic catalysts active in acid-catalyzed reaction such as disproportionation of ethylbenzene (see Figure 23).
Z iii Z I.U N Z ILl rn ,_1
5
0 0000
0
A
_
~9 L9 (J
2 0
10
20 TIME ON STREAM [H]
3O
40
Figure 23. Total conversion of ethylbenzene in selective disproportionation to benzene and diethylbenzenes over La,Na-Y obtained via contact-induced solidstate exchange between LaC13 and Na-Y; details see text. After Ref. [22], with permission.
1939
6.2. Vapor phase-mediated cation exchange A few observations of cation exchange between solid phases are reported, where obviously the presence of a gas phase rendered possible or facilitated the process. It was already mentioned that attempts to react MoO 3 and H-ZSM-5 were unsuccessful (see Section 5.2.3). However, Wang et al. [81] observed that heating a mechanical mixture of MoO 3 and H,Na-Y in the presence of water vapor led to cation exchange. The authors assumed that due to the interaction with water vapor, molybdenyl cations formed from MoO 3. These reacted with the protons of H,Na-Y. Incorporation of Mo-containing species into the zeolite was confirmed by XRD, FTIR, ESR and back-exchange of the Mo-containing cationic species. A similarly gas-phase mediated cation exchange was reported by Sachtler et al. [86]. They studied the incorporation of Pd 2§ into Y-type zeolites upon reaction of Pd ~ particles supported by the zeolite in the presence of chlorine. They assumed that PdCI 2 formed and that this compound diffused into the pore structure to react and replace the protons. In earlier studies Beyer et al. [87-89] had observed remigration of Ag ~ and Cu ~ in proton-containing Y-type zeolite upon calcination in oxygen. In the course of their studies of incorporation of Rh into dealuminated Y-type zeolite, Schlegel et al. [84] found the degree of solid-state ion exchange in vacuum or oxygen limited to about 25% even at temperatures as high as 675 875 K. The ion exchange occurred at much lower temperatures (375 - 425 K) if CO was present in the gas phase. The degree of exchange thus obtained was significantly higher than in vacuum, viz. more than 50%. From their results the authors derived a model which includes the formation of dicarbonyl species at the outer surface of the zeolite crystallites. Subsequently, these specie s are assumed to migrate into the pores and react with acidic protons and finally form stable Rh(CO)~ cations on cation positions. 6.3. Reductive solid-state ion exchange An interesting extension of the method of solid-state ion exchange was reported by Kanazirev et al. [26-28]. These authors conducted solid-state ion exchange under reductive conditions, i.e. in the presence of hydrogen. Systems studied were, for instance, Ga203/H-ZSM-5, In203/H-ZSM-5 or CuO/H-ZSM-5. The solid-state ion exchange occurred according to eqs. (10-11). M20 3 + 2H 2 + 2H-Z ~
2M-Z + 3H20
(10)
or MO + 1/2 H 2 + H-Z --~ M-Z + H20 (11) where Z represents a monovalent, negatively charged zeolite fragment and M (prior to reduction) a tri- or bivalent metal. The solid-state ion exchange was mainly monitored by TPR or thermogravimetric measurements. Very recently,
1940
Richter et al. [29] successfully conducted similar experiments with In203/Hmordenite. 7. MECHANISM OF SOLID-STATE ION EXCHANGE !
B
LU O Z <
I= m
Z < n-' I-
4000
3500
WAVENUMBER[CM1]
3000
Figure 24. Solid-state reaction between dehydrated NaCl and hydrogen mordenite under complete exclusion of even traces of water as monitored by the consumption of the (acidic and non-acidic) OH groups in an ultra-high v a c u u m cell; (A): hydrogen mordenite, activated at 675 K and 10-4 Pa; for comparison; (B) mixture NaCI/H-MOR after heat-treatment at 675 K. After Ref. [12], with permission.
1941 The mechanism of solid-state ion exchange is still not clarified. Most likely, different mechanisms are operative, depending on the system under investigation. First of all, one should realize that the presence of traces of water (crystal water such as in CaCI 2 92H20 or adsorbed water) may facilitate the solid-state ion exchange. However, the presence of water is certainly not an indispensible prerequisite for all cases of solid-state ion exchange in zeolites. This follows from the observation that even under complete exclusion of water, solid-state ion exchange between NaCI and H-mordenite proceeded. In the experiment illustrated by Figure 24, each experimental step (mixing the pre-dehydrated components, preparation of an IR-transmittant wafer, placing it into an ultrahigh-vacuum tight IR cell) was carried out in a glove-box where the water partial presssure was always below 10-4 Pa. Nevertheless, the band of acidic OH-groups completely disappeared upon heating. Moreover, the fact that solid-state ionexchange occurs with many insoluble cation compounds, such as Hg2C12, AgC1 [12] and oxides (vide supra), confirms that the presence of water is, in many cases, not necessary. Another question is as to whether the cations and anions of the salts exchanging with zeolites in the absence of water migrate independently or as pairs: (i) a possible mechanism would be that the cations migrate into the zeolite structure, replace, e.g., the protons which counter-diffuse to the external surface where they combine with the anions; (ii) another mechanism would involve the migration of salt molecules. Even though a general decision between these two possibilities cannot yet be made, there are several observations which favour the mechanism of migrating molecules. Comparison between the reaction in the systems CsC1/H-ZSM-5 and Cs4 [PW12040]/H-ZSM-5 showed a significantly lower degree of exchange in the case 4~ of the big Keggin ions (PW12040) which are unable to penetrate into the zeolite channels. This seems to indicate that for the solid-state reaction with alkaline metal salts the diffusion of the salt molecules into the pores is a prerequisite [90]. In a sense, one could similarly interpret the lacking solid-state ion exchange between absolutely dry Li-A and Na-A (see Section 6.1): here the role of the bulky anion is played by the zeolitic framework itself. Another mechanism might be operative in the case of volatile compounds. After vaporazation of the compound containing the in-going cation in systems such as Pd~ or MoO3-H20/zeolite the transport may proceed through the gas phase, followed by sorption of the volatile species, diffusion inside the structure and, finally, replacement of the protons or other cations of the parent zeolite.
1942 8. KINETICS STUDIES
time/min a. 0.25 b. 10 c. 20 d. 40 e. 60 f. 120 g. 240
1604
A
1451
Io.,o 1592
1489
UJ 0 Z .< rn rv
0
O3 rn
1700
1650
1600
1550
1500
1450
1400
W A V E N U M B E R [CM -1]
Figure 25. IR spectra of pyridine adsorbed on CuC1/Na-mordenite at 535 K as a function of the time of the solid-state reaction; details see text. After Ref. [32], with permission. Kinetics studies on solid-state ion exchange were conducted with the systems CuC1/Na-Y and CuC1/Na-mordenite. Pyridine was used as a probe. The basic idea of these experiments was to utilize the reaction-time dependent intensities of the IR bands characterizing pyridine attached to either Na § or Cu § T h r o u g h this time-dependence the exchange process could be monitored. Figure 25 displays a set of spectra obtained from an IR transmittant wafer of a CuCI/Namordenite mixture at 535 K and various reaction times. Prior to reaction, the sample had been degassed for 15 h in high vacuum at 400-425 K, a temperature
1943 too low for measurable solid-state reaction. Subsequently, the wafer was contacted with pyridine in a lower part of the IR cell which had been preheated to the reaction temperature, e.g. 535 K. At zero time only those bands were visible which are characteristic of pyridine attached to sodium ions (1442 and 1592 cm'l). As the reaction proceeded, bands developed that are typical of pyridine adsorbed on Cu t on cation sites (1451 and 1604 cm-1). Even though there was a considerable overlap of the Na + ~ py and Cu (-- py bands, decomposition was possible using mixed Gaussian and Lorentzian functions. Upon appropriate correction for the temperature effect on adsorption well-reproduced normalised integrated absorbances of pyridine adsorbed on CuCl/mordenite as a function of reaction time were obtained. Jiang et al. [33] were able to show that the kinetics results are well described by a solution of Fick's second law (see Figure 26). The expression In (Dt/R2), where D stands for the "diffusivity", t for the (selected) reaction time and R for the average particle diameter, was plotted as a function of the reciprocal reaction temperature. From such Arrhenius plots an activation energy for the solid-state reaction between CuCI and Na-mordenite (or Na-Y) was derived, amounting to 70 kJ.mol "1.
1.0 ,o.6"
~ ....
.6 0.8
8
...,~ ....
. ..... "6"""
..4" ,."" ......'" ....R"
/r , ./
0.6
: 0.4 "
0.2
i
y"
/
9
9 v,," ; ,V.,'i 9 j , q' ,7 /
513K v 493 K 9 473 K ......... fitting results
i iM.
r 0.0
._:::...::,
-
9 5
10
15
20 25 tl/2 [MIN 1/2 ]
30
35
40
45
Figure 26. Fitting results for CuCl/Na-mordenite from the integrated absorbances of Cu+(-- py bands based upon a diffusion model (solution of Ficks's second law); details see text. After Ref. [33], with permission.
1944 9. CONCLUDING REMARKS Solid-state reaction has proven to be a powerful technique for post-synthesis modification of microporous solids. A great number of efficient and reliable methods has been developed to provide evidence and quantitatively determine solid-state ion exchange. Solid-state ion exchange was successfully conducted with alkaline, alkaline earth, rare earth, transition and noble metal cations. Questions of mechanisms, thermodynamics and kinetics of solid-state ion exchange in microporous materials are still not satisfactorily clarified and require further work. However, it appears that these challenging problems should be successfully tackled with modern techniques such as in situ infrared spectroscopy, nuclear magnetic resonance, electron spin resonance and x-ray diffraction. REFERENCES
1 2 3 4
5 6 7 8 9 10 11 12
13 14 15
J.A. Rabo, M.L. Poutsma and G.W. Skeels, Proc. 5th Int. Congress on Catalysis, Miami Beach, Flo., USA, August 20-26, 1972, (J.W. Hightower, Ed.), North-Holland Publishing Co., New York, 1973, pp. 1353-1361. J.A. Rabo and P.H. Kasai, Progress in Solid State Chemistry 9 (1975) 1-19. J.A. Rabo, "Salt Occulusion in Zeolite Crystals", in "Zeolite Chemistry and Catalysis", (J.A. Rabo, Ed.), ACS Monograph 171, Am. Chem. Soc., Washington, D.C., USA, 1976, pp. 332-349. A. Clearfield, C.H. Saldarriaga and R.C. Buckley, Proc. 3rd Int. Conference on Molecular Sieves; Recent Progress Reports, Zfirich, Switzerland, Sept. 37, 1973; (J.B. Uytterhoeven Ed.), University of Leuwen Press, 1973, Leuwen, Belgium, Paper No. 130, pp. 241-245. C.A. Fyfe, G.T. Kokotailo, J.D. Graham, C. Browning, G.C. Gobbi, M. Hyland, G.J. Kennedy and C.T. DeSchutter, J. Am. Chem. Soc. 108 (1986) 522-523. A.V. Kucherov and A.A. Slinkin, Zeolites 6 (1986) 175-180. A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38-42. A.V. Kucherov and A.A. Slinkin, Zeolites 8 (1988) 110-116. A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 43-46. A.V. Kucherov, A.A. Slinkin, D.A. Kondrat'ev, T.N. Bondarenko, A.M. Rubinstein and Kh.M. Minachev, Zeolites 5 (1985) 320-324. H.K. Beyer, H.G. Karge and G. Borb~ly, Zeolites 8 (1988) 79-82. H.G. Karge, V. Mavrodinova, Z. Zheng and H.K. Beyer, "Guidelines for Mastering the Properties of Molecular Sieves", (D. Barthomeuf, E.G. Derouane and W. H61derich, Eds.), NATO ASI Series B, Physics Vol. 221, Plenum Press, New York, 1990, pp. 157-168. H.G. Karge, H.K. Beyer and G. Borb~ly, Catalysis Today 3 (1988) 41-52. H.G. Karge, G. Borb~ly, H.K. Beyer and G. Onyesty~ik, Proc. 9th Int. Congress on Catalysis, Calgary, Ottawa, Canada, June 26-July 1, 1988 (M.J. Philips and M. Ternan, Eds.), Chemical Institute of Canada, Ottawa, 1988, pp. 396-403. H.G. Karge and H.K. Beyer, in DGMK-Berichte-Tagungsbericht 9101, DGMK-Fachbereichstagung "C1-Chemie- Angewandte Heterogene Katalyse
1945
16 17 18
19 20 21
22 23 24
25
26 27 28
29 30 31 32 33
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