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Introduction of Cations into Zeolites by Solid-State Reaction
P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam
43
IN'I'KODUCTION OF CATIONS INTO ZEOLITES BY SOLID-STATE KEACTION
Hellmut G. Karge and Hermann K. Beyer Fritz-Haber-Institut der Max-Planck-Gesellschaft Berlin, 1000 Berlin 33, FRG
ABSTRACT A brief review is given of early observations of cation incorporation into zeolites through solid-state reaction. The general procedure and techniques suitable for investigation of solid-state ion exchange in zeolites are described. Results of systematic studies on introduction of alkaline, alkaline earth, rare earth, transition metal and noble metal cations into hydrogen, ammonium and sodium forms of zeolites are reported. In these studies, halides or oxides of the in-going cations are preferentially employed. Particular attention is paid to the stoichiometry of the solid-state ion exchange. It is shown that in several cases a one-step solid-state reaction leads to a 100% cation incorporation, whereas such a high degree of exchange is difficult to obtain by conventional methods. As is illustrated by a few selected examples, solid-state ion exchange might be an interesting way to prepare active acid and bifunctional catalysts. INTKODUCTION In the early seventies some interesting results were reported which showed th a t ion exchange in zeolites might take place not only by interaction of the solid zeolite phase (containing the cation I to be replaced) and a solution (containing the cation II to migrate into the zeolite structure and replace cation I). Rather, it turned out that a reaction of two solid phases, for instance of a zeolite with cation I and a salt of cation It,may also result in the zeolite containing cation II. Thus, Rabo et al. [l-31 found th a t proton-containing samples of zeolite Y reacted with sodium chloride under evolution of hydrochloric acid. By means of IR spectroscopy i t was demonstrated that this reaction eliminated acidic OH groups and thus, removed residual Brprnsted centres from the zeolite structure which are active in butene isomerisation [ll. This observation was in line with the early finding that cracking catalysts were deactivated by interaction of the zeolite component with the matrix. Obviously, in these cases cations from the matrix (clays, alumina) migrated into the zeolite structure and eliminated acidic hydroxyl groups. Clearfield and co-workers [4] were the first to apply ESR spectroscopy in order to confirm solid-state ion exchange. They introduced transition metal cations such as Cu2+, Zn2+, Ni2+ and Cr3+ into zeolites A, X and Y via reaction of salts of those cations and deammoniated ammonium forms of the zeolites. Occupation of the cation
44
sites in the zeolite structure by transition metal cations was indicated by the appearance of the typical ESR spectra. These spectra were identical to those obtained from conventionally exchanged zeolites. Surprisingly, for a relatively long time the studies on solid-state ion exchange were not continued. Only in the mid-eighties did three research groups start systematic work in this area, viz. the groups of Kucherov and Slinkin, Karge and Beyer, and Wichterlova and Beran. Since then considerable activity in this field has emerged. A special case of solid-state ion exchange was found by Fyfe et al. [5] who investigated the cation transfer between two zeolite pairs, for instance L i - m a - A , L i - m a - Y or Li-AINa-MORby 29Si MAS NMR and XRD. Obviously, there is a steadily increasing interest in the phenomenon of solid-state ion exchange. This is partly due to possible advantages of solid-state ion exchange with respect to several applications. In contrast to conventional ion exchange, solid-state ion exchange does not require handling and disposal of large volumes of salt solutions. Moreover, in many cases solid-state ion exchange proceeds more easily than the conventional exchange in suspension. This is especially true when, in aqueous solution, the in-going cation is strongly hydrated and prevented by its hydration shell from penetrating into small cavities and narrow channels of the zeolite structure. Examples are the incorporation of lanthanum, nickel or platinum via solid-state reaction. In view of the many interesting aspects of solid-state ion exchange, recent developments in this field will be reviewed. General Procedure a n d Experimental Techniques The usual procedure for carrying out solid-state ion exchange is simple. Finely dispersed powders of the zeolite and a salt or oxide of the in-going cation are carefully mixed and heated, either in a stream of inert gas or in high vacuum. In fact, the temperature required for achieving a certain degree of exchange frequently depends on the nature of the cations and anions involved. In some cases a fraction of the cations is exchanged even during grinding of the salt-zeolite mixtures a t ambient temperatures. The effect of the duration of heat treatment is less pronounced. In most cases the exchange is fast in the initial period of the solid-state reaction and then levels off. Examples will be given below. Solid-state ion exchange may be carried out in stoichiometric mixtures of salts (or oxides) and zeolites, related to the A1 content (in case of aluminosilicates) of the framework. However, i t also works with under-stoichiometric mixtures or mixtures containing a n excess of the in-going cation. In the latter case, the excess may be removed by brief extraction of the solid-state reaction product with water; also, salt occlusion may occur during heat treatment [2-31. Thus, with many important salt(oxide)-zeolite
45
systems a desired degree of exchange, even a 100% exchange, can be achieved in a onestep procedure. There are quite a number of experimental techniques to prove qualitatively, and determine quantitatively, the extent of solid-state ion exchange. In the first instance, electron spin resonance (ESR) and infrared (IR) spectroscopy should be mentioned. The experiments employing these techniques are carried out in essentially the same way a s frequently described in zeolite research [6-lo]. Due to its high sensitivity, especially in the case of many transition metals, ESR is appropriate for deciding whether or not solid-state ion exchange has taken place. Because of calibration problems, however, i t is frequently less suitable for determining the degree of exchange. With IR both qualitative and quantitative analysis is feasible. If the solid-state ion exchange is conducted with hydrogen or ammonium forms of zeolites, the change in absorbance of the OH stretching bands caused by the consumption of OH groups upon ion exchange is measured. To indicate the cations themselves, appropriate probe molecules such as pyridine, CO, CH,CN etc. may be used. Furthermore, the reaction of hydrogen or ammonium forms of zeolites with halides is easily monitored via titration of the hydrohalic acid evolved. This works particularly well with chlorides. Similarly, if gases such as hydrogen chloride or water are evolved due to solid-state reaction of hydrogen forms of zeolites with chlorides or oxides, mass spectrometry and gas chromatography are frequently employed. These techniques enable u s not only to detect the gases and thus confirm the fact of solid-state reaction but also provide means to evaluate the rate of exchange and the amount of exchanged cations. The loss of weight, which originates from the reaction of a halide or oxide with a hydrogen form of a zeolite, i.e. from evolution of hydrohalic acid or water, can be measured by a balance. This thermogravimetric analysis may be combined with GC or MS measurements or with continuous titration. In most instances, however, zeolites and zeolite-like materials are synthesized a s sodium forms. Thus, when sodium forms are the starting materials for solid-state ion exchange, magic angle spinning-nuclear magnetic resonance of 23Na (23Na MAS NMR) is advantageously employed to monitor that process. Examples are the exchange of sodium for other alkaline metal cations and the incorporation of lanthanum cations (vide infra). Solid-state Ion Exchange with Alkaline Metals Systematic studies on solid-state ion exchange with alkaline metals were carried out in the systems M'Cl/H-ZSM-5 and M'Cl/NH,-Y, where MI represents Li, Na, K,
46
Rb and Cs [ll-121.Also, the solid-state replacement of Na+ by Li+ or K + in Na-Y was investigated [ 131. As the first example, Figure 1 shows a sequence of IR spectra obtained with NaCVHZSM-5, where NaCl was admixed in excess (NaCVAl= 1.9). These spectra provide evidence for the following points. (i) Solid-state ion exchange took place upon heating the NaCl/H-ZSM-5 mixture in high vacuum (HV) a t 723 K (change of the absorbance of the OH bands). (ii) Not only the strong acidic OH groups (band a t 3605 cm-') but also the weak ones (band a t 3740 cm-l of so-called silanol groups) were involved (the latter band was weakened). (iii) The Na+ 0'-Si f groups formed by reaction of NaCl with silanol OH'S were easily hydrolyzed, since upon treatment with H,O the OH band a t 3740 cm-l was fully restored whereas the band at 3605 did not reappear; i.e. Na+ cations, which had replaced protons of the bridging, strongly acidic OH groups, were irreversibly held. (iv) No lattice destruction due to HCl evolution (vide infra) had occurred, since conventional re-exchange with NH,Cl solution and subsequent deammoniation resulted ina complete recovery of the original OH band a t 3605 cm-'. In this context it should be stressed that the structural integrity of the product of the solid-state ion exchange (NaZSM-5) was also confirmed via comparison of its X-ray diffraction pattern with th a t of the starting material (H-ZSM-5). No change in the intensities of the zeolite lattice reflections was detected. However, a significant decrease in the NaCl reflections was observed which is a further proof for the occurrence of a solid-state reaction. The same result holds for all the other examples of solid-state ion exchange reported here. The IR results of the NaCVH-ZSM-5 reaction were in excellent agreement with the stoichiometric data obtained from AAS analysis and titration. These data are listed in Table 1.
3605 I
3800
I
3600
W A V E N U M B E R [crn-']
Fig. 1. IR spectra of the OH stretching region after evacuation at 723 K (2h, 10" pa). a, HZSM-5; b, NaCl/H-ZSM-5 mixture (NaCl/Al= 1.89) calcined at 900 K (1 h); c, (b) washed with water; d, (b) twice exchanged with 1 N NH, C1 solution (Ref. [I11, with permission).
47
Table 1 Mass balance* for solid-state ion exchange in the system NaCl/H-ZSM-5
Zeolite H-ZSM-5 (1) H-ZSM-5 (2)
(1) SiiAl
(2) A1
155 23
0.107 0.691
0.01
0.808 1.306
(5)
(6) C1' extracted
(7) Na+ extracted
(8) Na+ non-extracted
mixture
HC1 evolved
NaCl/H-ZSM-5 (1) NaCVH-ZSM-5 (2)
0.549 0.828 =(4) - (6)
*
0.260 0.478
(3) Na+ -_
0.707 0.670
(4)
NaCl
0.101 0.636 (4) - (7)
all data in m m o l per gram zeolite fired a t 1273 K
(1) to (3) data of the starting zeolites; (4) added to the zeolite; (5) evolved during calcination a t 900 K for 1 hour; (6) and (7) extracted from the heat-treated mixture by
washing with water; (8) not extractable with water; compare with Al, column (2).
One recognises from Table 1 that the stoichiometry is excellent: (i) the amount HC1 evolved agrees well with the difference between employed and extracted Cl-, see columns (4) and (6); (ii) the amount of non-extractable N a + , i.e. the difference between N a + employed and extracted, see columns (4) and (71, coincides with the A1 content. Since these H-ZSM-5 samples did not contain essential amounts of extra-framework Al, each A1 corresponded to one bridging OH group. Therefore, the data of column (8) in comparison with column (2) show that a 100%or almost 100%exchange of H + for N a + occurred. With the same system, i.e. NaCUH-ZSM-5,it is illustrated in Figure 2 how the solidstate reaction upon temperature-programmed heating is monitored by thermal gravimetric analysis TGA and simultaneous continuous titration of HCl evolved (TPE). Curve (a) indicates the weight-loss of the NaCl/H-ZSM-5 mixture, curve (b) the amount of HC1 released. Both curves exhibit three steep steps. The first one (at about 400 K) is mainly caused by dehydration and only to a minor extent by evolution of HCl (see small step in curve (b)). The second step in curve (a) centered around 830 K coincides with the strongest evolution of HCI (curve (b)). Finally, at very high temperatures decomposition of the excess NaCl occurred and gave rise to steps a t ca. 1200 K i n both curves.
48 1.10
-
2
c
I
E
w W
' w
Y
n W
1.05
> A
-I
a.
0
Q
-
I
> w
VI
U
I
1.oo
300
500
700
900
1100
T E M P E R A T U R E [K]
Fig. 2. a, TGA and b, continuous titration of evolved HC1 upon temperature-programmed heating of a NaCV HZSM-5 mixture (Na/Cl= 1.89)i heating rate 2.5 K min(Ref, [ll], with permission).
-
Investigation of solid-state ion exchange with the help of mass-spectrometrically monitored evolution of HCl and NH, was carried out with the system M'CVNH,ZSM-5. Results are presented in Figure 3. Two regions, viz. a low-temperature (LT) and a high-temperature (HT) regime of solid-state reaction, can be distinguished. With LiCl
r
1
c
5
TEMPERATURE
[K]
Fig. 3. Temperature-programmed evolution (TPE) of HCl upon heating M'CY NHA-ZSM-5mixtures; MI, Li(--); Na. (---); K (-*-a); Rb (-*--*=); c s (-**--*--); Li refers t o right scale (Ref. [Ill,with permission).
49
the LT reaction is eminently prevailing. The exceptional ease of solid-state ion exchange with LiCl was observed with other zeolites as well (vide infra). In the case of the other M'C1 salts, the contribution of the HT reaction to the overall process is significantly higher than that of the LT reaction. The temperature maxima both of the LT and the HT peaks decrease in the sequence T(Na) > T(K) > T(Rb) > T(Cs). Very similar results, including the exceptional behaviour of LiCl and the sequence of the temperature maxima, were obtained with mixtures of M'Cl/NH,-Y. The sequence of the peak temperatures corresponded to the decrease in the lattice energies of the alkaline chlorides. Thus, i t seems t h a t the solid-state ion exchange between alkaline chlorides and hydrogen forms of zeolites proceeds more easily the lower the lattice energy of M'C1. This suggests t h a t a low lattice energy would facilitate the separation of M'C1 entities from the M'C1 structure which then can migrate to and into the zeolite structure in order to react there with the OH groups. Note, however, t h a t such a relationship between the lattice energy of the compound of the in-going ion and the ease of solid-state ion exchange was not established with, e.g., Mn compounds and H-ZSM-5 (see below and Ref. [14]). Thus, parameters other than lattice energies may be operative as well. Cation exchange with alkaline metal cations can also be achieved by solid-state reaction between M'C1 and sodium forms of zeolites [13]. This was proven, inter alia, via 23Na MAS NMR. For the sake of comparison i t seems useful first to look at the 23Na MAS NMR signals of sodium in various surroundings (Figure 4). In the 23Na MAS NMR spectra, sodium chloride is used as a reference. Thus, the signal of crystalline NaCl appears a t 0 ppm. Sodium cations in aqueous solutions give rise to a sharp line at - 12.5 ppm whereas the sodium cations in hydrated Na-Y are indicated by a broad signal around - 8 ppm. When the solid-state reaction is carried out with LiCl and Na-Y a dramatic change i n the 23Na MAS NMR spectrum occurs even when the reaction temperature is as low as ambient. As is demonstrated i n Figure 5, the signal typical of Na in Na-Y disappears. Instead, the line typical of crystalline NaCl is developed and, furthermore, a broad signal appears a t about - 12.5 ppm where sodium cations in aqueous solution are indicated (compare Figure 4, line b). However, the linewidth of the 12.5 ppm band in Figure 5 is very large compared to that of Figure 4. The most likely explanation for these observations is t h a t a fraction of t h e sodium cations, which were expelled from the cation sites by Li cations, form tiny NaCl crystallites at the outer surface of the zeolite particles and give rise to the 0 pprn signal (simultaneously, i n the X-ray pattern of this sample the reflections of crystalline NaCl were detected). Another fraction of the sodium cations remains in the intracrystalline water of Na-Y. But in the cavities of Na-Y the mobility of the water molecules is restricted, and a large variety of coordination states are available for the sodium cations. These
50
factors may give rise to the observed broadening of the - 12.5 ppm signal, via a homogeneous (relaxational) and/or inhomogeneous (chemical shift) effect on the 23Na resonance.
I
I
l
l
I
1
I
I b, Nain
NaCl soh.
b
0
I
5
I
0
I
-5
-10
C H E M I C A L SHIFT,
-15
I
-20
6Nacl.cryrt.
-25
[ppml
Fig. 4. 23Na MAS NMR spectra. a, crystalline NaC1; b, saturated NaCl solution; c, Na-Y, without pretreatment (Ref. 11, with permission).
I
I0
I
I
0
I
I
-10
I
I
-20
C H E M I C A L 5 H I FT,
I
I
-30
6NaCI.cryrt.
I
I
-40
[ppm]
Fig. 5. 23Na MAS NMR spectra of Na-Y, without pretratment and LiCU Na-Y mixture, ground and intimately mixed (Ref. 14, with permission).
Again, LiCl reacts much more easily than other alkaline chlorides. Most probably, this is due to a particularly large decrease in the free energy when Li+ is transferred from crystalline LiCl or aqueous LiCl solution to the zeolite where the small Li+ cations are strongly "solvated" by the oxygens of the framework [ 151. The ease of exchange with LiCl is of great interest with respect to the important dealumination method using SiCl, [16-171, which yields silicon-rich, hydrophobic zeolites, i.e. valuable adsorbents. Sulikowski et al. 1181 have found that, when Li-Y is dealuminated instead of Na-Y, no self-inhibition of the dealumination by SiC1, occurs. This is due to the fact that, in contrast to NaA1C14, the product LiAlC1, decomposes a t the reaction temperature, and thus plugging of the zeolite pores is avoided.
51
Solid-state Ion E x c h a n g e with Alkaline E a r t h Metals Results were reported on the systems MgC12/H-MOR,CaC12/H-MOR or NH,-MOR [19] a s well a s on CaCl,/Na-FAU and BeC12/Na-FAU [ 131.
The stoichiometry of solid-state ion exchange was checked in detail with CaC12/ €1 MOR and CaCl,/NH,-MOR. Again, when an excess of CaC1, was used, part of the weak silanol-type OH groups also reacted. But they were easily hydrolysed. Excellent agreement between the aluminium content (bridging OH groups) and the amount of irreversibly held calcium cations was found.
1.40
3
-
m
Y
+
1.30
I
W W
3
1.20
1
1.10
1.oo
,+, LUV
*++.
, ,
+
%+-
wuu
TEMPERATURE [“C]
I
,+-+-+-+-+-+-
-
isothermal heating at 55OoC
Fig. 6 a, TGA and b, continuous titration of evolved HCl upon heating of a CaCl / H-MOR mixture (Ref. r191. with permisssion)
However, the exchange with CaCl, proceeded less easily than with alkaline metal chlorides. Temperature-programmed heating of the CaC12/H-MOR mixture (Ca/Al= 0.51, similar to the case of alkaline metal chlorides, provided two steps of HC1 evolution corresponding to a low-temperature and a high-temperature process (Figure 6). While the LT exchange gave rise to a sharp step between 400 and 575 K, the HT exchange resulted in a less pronounced evolution of HCl. Only after repeated heating and a second isothermal treatment a t 875 K was i t completed. Then the reaction ceased and a total of 2.5 mmol HCl per gram zeolite was evolved. This compared satisfactorily with the A1 content (2.2 mmol per gram zeolite). The agreement between HC1 or NH,Cl evolved and A1 content was perfect in the case of CaCl,/NH,-MOR (Ca/A1=0.5), viz. 2.54 and 2.52 mmol per gram.
52
Thermogravimetric analysis showed only one steep decrease in weight of the CaC12/ H-MOR mixture due to release of H 2 0 and HCl. The system initially contained significant amounts of water because the hydrated chloride, CaC12 * 2H20, was employed. Thus, the LT exchange was not, in a strict sense, a solid-state ion exchange and was probably facilitated by the presence of water molecules in the zeolite pores. However, as will be pointed out later, the presence of water is not a n indispensable condition for solid-state ion exchange to occur. Rather, the solid-state exchange can be conducted even in ultrahigh vacuum, i.e. in the absence of any traces of water. Solid-state ion exchange with CaC12/H-MORand MgCl,/H-MOR was also monitored by IR. Figure 7 demonstrates, as a n example, the removal of acidic OH groups (spectrum 2a) in H-MOR due to replacement of the protons by Ca2+. Subsequent pyridine adsorption gave rise to a sharp band a t 1446 cm-*typical of pyridine coordinatively bonded to calcium cations and a smaller band a t 1455 cm-l which is due to pyridine attached to "true" Lewis sites (spectrum 2b). Most likely, both types of pyridine coordination contributed to the small signal a t 1610 cm-l. Note, however, that only a very weak pyridinium ion band at 1540 cm-' was observed. Another sample, which had been prepared via solid-state ion exchange (spectrum 2a), was contacted with small amounts I
I
I
/
iI
I
I
Y
U
z
a
II-
r
v)
z a CT
c
3av I
4000
I
3500
I
3000
W A V E N U M B E R [cm-'I
Fig. 7. IR spectra of mordenite samples. l a , lb, spectra of H-MOR after activation (775 Pa); 2b, same treatment a s Pa); 2a, CaCl /H MOR after heating at 775 K, (2a), subsequent pyrikne adsorption and removal of excess pyridine (475 K, loe5Pa); 3a, same treatment as (2a) and brief contact with H,O, followed by degassing at 775 K ( Pa); 3b, same treatment as (3a), subsequent pyridine adsorption and removal of excess pyridine (475K, Pa).
53
of water vapour and again degassed. This resulted in spectrum 3a showing a strong OH stretching band around 3618 cm-l. Obviously, interaction of H 2 0 molecules with Ca2+ introduced by solid-state ion exchange generated acidic OH groups according to the Hirschler-Plank mechanism [20-211.In fact, subsequent pyridine adsorption produced a strong band a t 1540 cm-' indicating pyridinium ions. While the ion exchange with alkaline metal cations does not result in catalysts active in acid-catalysed hydrocarbon reactions and, in contrast, may be carried out to remove any residual activity [l],the incorporation of alkaline earth cations by solidstate reaction should lead to active catalysts. I t was shown, however, that the solidstate ion exchange had to be followed by contact with water vapour in order to obtain calcium or magnesium mordenites which are sufficiently active in, for instance, disproportionation of ethylbenzene. This is in full agreement with the IR spectroscopic results which indeed showed that only upon interaction of the heat-treated CaC12/H-MOR mixture with water vapour were acidic OH groups generated. Also, in the case of alkaline earth chlorides some investigations were undertaken using sodium forms of zeolites instead of hydrogen forms as starting materials for solidstate ion exchange. These experiments, however, were conducted a t ambient temperature and moisture. 23Na MAS NMR was used to evaluate the degree of exchange when, for instance, BeC1, or CaC12was contacted with Na-Y. The results were similar to what was found in the case of LiC1. The 23Na MAS NMR spectra suggested complete exchange. The line indicating N a + in Na-Y completely disappeared. Instead, the signal of Na+ in crystalline NaCl and the broad band a t -12.5ppm due to N a + in intracrystalline water developed. The exchange between BeC1, and Na-Y was also studied by IR, using pyridine a s a probe. Before solid-state reaction, pyridine indicated Na+ on cationic sites through a band a t 1444 cm-I.After solid-state reaction N a + was completely replaced by Be2+ and the corresponding band of pyridine coordinatively bonded to Be2+ was observed a t 1453 cm-I. Interestingly, i t was important to avoid heating of the BeC12/Na-Y mixture higher than 400 K, because a t temperatures higher than this, partial re-exchange of N a + for Be2+ occurred. Solid-state Ion Exchange with Hare-earth Metals In view of catalyst preparation, the solid-state ion exchange with lanthanum or rare-earth cations is of particular interest. Rare-earth-containing zeolites are widely used a s cracking catalysts. However, special measures are required [22-231to obtain a degree of exchange higher than about 75% of, e.g., La3+ or Ce3+ when the exchange is carried out with the conventional technique, i.e. by suspending the zeolite powder in a solution of the in-going trivalent ion. The reason is that in solution the highly solvated
54
M3+ cation cannot enter the sodalite cages. To achieve higher degrees of exchange, usually the conventional exchange procedure with suspensions is repeated several times and the exchange product intermittently heated to push the, a t least partially, desolvated La3+ cations into the sodalite cages. It turns out, however, that solid-state ion exchange may lead to an almost 100% exchanged La-Y zeolite in a one-step procedure and provide highly active catalysts [24-251. Similar to the results already described, also with LaC13/NH4-Y the stoichiometric measurements were very instructive (Table 2).The data of Table 2 show the results of asolid-state reaction between LaCl, and NH4,Na-Y. The zeolite had a degree of exchange of 89% NH4+ for Na+. The LaC13/NHq,Na-Ymixture contained 1 La3+ per 3 A1 of the zeolite structure. The experimental results were obtained by titration of the evolved gases and chemical analysis of both the water-extracted product and the extract solution. From these data it is evident that (within the limits of error) the amount of La3+ introduced by solid-state reaction (4.95meq./g) corresponded exactly to the amount of framework aluminium (4.83meq./g) or maximum of bridging OH groups. However, even the main fraction of Na+ of the starting material (1.61 meq./g) was removed from the zeolite. Only about 0.7 meq. Na+ and 0.8 meq. Cl- remained in the structure, corresponding to roughly one NaCl molecule per P-cage. It is assumed th a t this NaCl is occluded in the structure. According to Rabo's study [2-31this would enhance the thermal stability of the exchange product. Tabl e 2.
Solid-state Ion Exchange*, LaCl,/NH,,Na-Y (LafAl =0.33); thermal treatment at 850 K La3
c1-
Na+
NH4+
A1
_-
--
1.61
4.83
1.61
4.83
3.29
+
Starting zeolite Salt admixed Evolved gas (as NH,Cl)
*
3.29
3.29
Extracted with K,O
0.06
0.72
0.94
irreversibly held
1.60
0.82
0.67
4.80
__ __
4.83 4.83
data in mmol per gram zeolite, with the exception of the last line (meq./g)
The results were exactly the same when an excess of LaC13, e.g., a ratio of 2 La3+ per 3 A1 of the zeolite structure, was employed. The only difference was that, after completion of the reaction, a higher amount ofLaC1, was extracted [251.
55
In the same way, a 100%La-Y sample was obtained by stoichiometric solid-state ion exchange when the starting zeolite was a 100% exchanged NH,-Y and a ratio La/A1= 0.33 employed. In contrast to the conventional procedure, the solid-state reaction yielded the 100% ion-exchanged La-Y by one exchange step [25]. Solid-state incorporation of La3+ into Y-type zeolite was also studied by IR. Figure 8 exhibits a set of spectra in the OH and NH stretching region obtained upon thermal treatment of a LaCl3/NH,,Na-Y(89) mixture a t successively higher temperatures (spectra a-c) and after contact of the exchange product with water vapour followed by degassing (spectrum d). At 475 K deammoniation was still incomplete; however, the prominent high frequency and low frequency OH bands of hydrogen Y a t 3640 and 3542 crn-l, respectively, were already partly developed. Upon heating to higher temperatures, solid-state reaction of LaCl, with the OH groups eliminated the OH bands almost completely. Subsequent treatment with H,O vapour and degassing produced the OH bands typical of La-Y (spectrum d). 3738 3560
I
ll I l l
CII
I 1
3800
l
h
I
Y
3738
3738 3530
c
d'
I
I
I
.u
I
725 K.30 min, HV
I If1 I /I I I /I I 3400 3000"38003400"3800 3400 " 3800 W A V E N U M B E R [cm'l
I
3400
I
3000
Fig. 8. IR spectra of the OH stretching region of a LaCl /NH -Y mixture. a, b and c, after heating a t 475, 625 and 725 K, res ectively h a , 2%);d,gfter (c) and brief contact with H,O (0.6 kPa, 3 min) followefby evacuation (725 K, 10- Pa, 30 min). Adsorption of pyridine subsequent to spectrum c gave rise to a band a t 1452 cm-l typical of pyridine coordinatively bonded to lanthanum cations; only a tiny pyridinium ion band a t 1542 cm-l was observed. When, however, generation of spectrum d (i.e. after H,O contact of the sample) was followed by pyridine adsorption, the band of acidic OHgroups at 3630 cm-' was completely removed and a strong band a t 1542 cm-l (pyridium ions) appeared.
56
The intensities of the bands were similar to those of La-Y samples obtained by conventional ion exchange. TPD of ammonia showed that also the strength of the acidic sites were essentially the same as found with conventionally exchanged La-Y. Thus, the nature, density and strength of acidity of La-Y prepared via solid-state ion exchange are comparable to those of the conventionally obtained products and, therefore, similar catalytic behaviour in acid-catalysed reactions was expected. This was, indeed, found when La-Y was employed as a catalyst for ethylbenzene disproportionation.
I
I
w
z w
N
'
l
~
l
'
1
~
1
-
~
1
1
1
~
1
-
16-
0
v
U
T I M E O N S T R E A M [hl
Fig. 9. Selective disproportionation of ethylbenzene at 425 K (1.3 vol % EB in He, 5 ml min-', m[catl=O.25 g). A, over a La-Y (98) catalyst obtained by solid-state ion exchange; B, over a conventionally prepared La-Y (96)catalyst. Finally, La-Y zeolites were obtained by solid-state ion exchange between LaCl, and the sodium form of Y-type zeolites as well [25]. This was proven by chemical analysis, IR, X-ray diffraction (XRD) and a test reaction. Details of the preparation and characterisation are outlined in Ref. [25]. Chemical analysis gave evidence for a partial replacement of sodium by lanthanum cations. IR showed the formation of acidic OH groups. Finally, XRD demonstrated, via the appearance of reflections of crystalline NaC1, that obviously Na + cations were expelled from the interior of the zeolite crystals by in-going La3+. Outside the zeolite particles they had formed small NaCl crystallites.
~
l
51
Even though a 100% exchange in the system LaC13/Na-Y was not yet achieved by solid-state ion exchange, the product obtained showed catalytic activity i n acid-catalysed ethylbenzene disproportionation similar to that of conventionally obtained La-Y with a n exchange degree of 74%. Solid-state Ion E x c h a n g e with Transition Metals As mentioned in the Introduction, Clearfield et al. [4] were the first to incorporate transition metal cations into zeolites via solid-solid reaction. These authors reacted partially (i.e. to 36 - 58%) NH4-exchanged and subsequently deammoniated forms of Na-A, Na-X or Na-Y with transition metal compounds such a s CuC12, ZnCl,, NiCl,, CoCl2, CrC1, or MnC1,. The extent of solid-solid reaction was monitored by titration of HCl evolved. Depending on the reaction conditions, up to 100%exchange of the zeolite protons by metal cations could be achieved. Investigation of transition metal cations and their reactions fall in the realm of ESR spectroscopy. Such studies benefit by the high sensitivity of that spectroscopic technique. Moreover, ESR spectroscopy of cations in zeolites provides in many cases deeper insight into their coordination state. Thus, Clearfield et al. [41 were also the first to successfully apply ESR spectroscopy to prove the incorporation of transition metal cations into a zeolite structure by solid-solid interaction. In Figure 10, ESR spectra of Cu,Na-Y
Fig. 10. ESR spectra of heat-treated Na-Y and Cu(II)-containing Y-type zeolite s m ples. A, 15% of original N a + replaced through solid-state reaction between CuCl, and H,Na-Y; B, 15% of original N a + replaced by conventional exchange in aqueous suspension; C, original Na-Y (after Ref. [51, with permission).
58
obtained via solid-state exchange (line A), conventionally exchanged Cu,Na-Y (line B) and the starting zeolite Na,H-Y (line C) are compared. In both Cu,Na-Y samples about 1 5 6 of the original Na+ cations were replaced by Cu2+. It is evident from comparison of spectra (A) and (B) that very similar Cu2+-containing zeolite Y samples were obtained from solid-state and conventional aqueous exchange. Both spectra (A) and (B) exhibit 8 lines indicative of two different environments for the Cu2+ cations. The first set of signals is characterised by g,,=2,35 and g,=2,06 whereas the g-values of the second set are g,,= 2,30 and gl= 2,06.
ESR spectroscopy was extensively used by the group of Kucherov and Slinkin in their systematic work on solid-state reaction between high-silica zeolites and transition metal compounds. As zeolites they preferentially employed hydrogen mordenites, hydrogen forms of ZSM-5 or highly dealuminated H-Y. In their experiments, the transition metal compounds reacted with the zeolites are mostly oxides such a s CuO [261 or CrO,, Cr20,, MOO, or V,O, [27]. However, salts were also applied, e.g. MoCl, [271, CuCl,, CuF2, Cu3 (PO,),, Cu2S [261 or FeCl3[281. Finally, the studies were extended to simultaneous or successive reaction of two oxides (CuO + CrO,, CuO + V20& with HZSM-5 [29]. In particular, the system CuO/H-ZSM-5 was studied in great detail. An interesting result was that Cu2+ introduced by solid-solid reaction of CuO and H-ZSM-5 at 825 K o r 1075 K exhibited ESR spectra which were completely identical with those obtained from Cu,H-ZSM-5 prepared via conventional ion exchange in aqueous solution (Ref. [301, Figure 11).The g-values and hyperfine splitting constants for samples prepared via either route showed full agreement, hence the authors concluded that also the
I
Fig. 11. ESR spectra of Cu(II)-containing MFI-type zeolite samples. A, CuCl,/ H-ZSM-5 mixture heat-treated in vacuum a t 10 75 K; B, Cu,H-ZSM-5 obtained by conventional exchange, calcined in air at 1075 K and evacuated a t 300 K (after Refs. [261 and 1301, with permission).
59
isolated Cu2+ ions introduced by solid-state reaction were present in two different coordination states, viz. in a square planar environment (g,,=2.32, g,=2.045, A,,= 17 mT, Al= 2.9 mT) and a fivefold coordinated state (g,,=2.32, gL= 2.06-2.07, A,,= 14 - 14.2 mT, Al= 1.8 mT). The latter coordinatively unsaturated state, however, was completely absent when CuO was reacted with H,Na-ZSM-5 (40% Na+ exchanged for H + ) instead of H-ZSM-5 (95% N a + exchanged for H f ) . The H,Na-ZSM-5 starting zeolite was prepared by burning-off of the organic template without any further exchange of the remaining N a t cations. Therefore, on the basis of the above ESR observation i t was suggested that, upon synthesis, N a + and organic cations have been located in the ZSM-5 structure not randomly but, rather, in a somehow ordered manner. The Cu2+ cations introduced by solid-solid reaction were easily accessible for adsorbates such as 0, a s was evidenced by the dramatic but reversible change, i.e. loss of intensity, broadening and removal of hyperfine splitting of the ESR signals upon oxygen admission. The amount of Cu2+i which could be introduced into H-ZSM-5 by solid-state reaction, increased linearly with the A1 content of the framework. Similar results were obtained with H-MOR. However, when Na-MOR was calcined in a mixture with CuO no appearance of the Cu2+ ESR signal was observed. Solid-state reaction also occurred between H-ZSM-5 and CuC12, CuF, or Cu3(P0,),. In these cases a n effect of the anions was realised. The Cu2+ cations introduced were coordinated not only to the framework but also to F' or a s a ligand. With Cu,S i t is supposed that also Cut cations migrate. The Cu2+ ESR spectrum appeared upon oxidation of the thus introduced Cut cations residing on cation sites. It is stressed that reaction of CuF, with H-ZSM-5 caused no destruction of the lattice. X-ray diffraction patterns obtained after solid-state reaction gave evidence for the integrity of the structure. Thermal treatment of mixtures of hydrogen zeolites with CrO,, MoCl, and V205 gave rise to ESR spectra typical of isolated Cr(V), MOW)and V(W) cationic species [271, respectively. The authors do not comment on the reduction of Cr(VT) or V(V). Most probably this occurred due to the presence of organic contaminants such a s residual template. Because of the large distance between the framework A1 atoms (or protonic sites) in the highly siliceous hydrogen zeolites used, the authors assume that isolated complex cations such as CrO,+, MoC14+, VO(OH)+ were introduced by solid-state reaction, rather than Cr5+,Mo5+ or V4+. Interestingly, Kucherov and Slinkin observed a "superhyperfine splitting" in the case of ESR spectra of Cr(V) or V ( N ) obtained after solid-state reaction [27,31]. The additional splitting (see Figure 12) is ascribed to a n electronic interaction of 53Cr (nuclear spin I=3/2) or (I=7/2) with adjacent framework A1 (I=3/2) [27,31]. Yang et al. [32], who also investigated by ESR the introduction of Cr or Mo into H-ZSM-5, arrived a t similar results.
60
There is also one publication by Kucherov et al. [33]concerning the introduction of Cu, Fe, Cr or V cations into the gallium analog of H ZSM-5. In this work it was found that, in line with the well-known lower acidity strength in H-[Gal ZSM-5, the cations introduced are less stabilized in the Ga form than in the A1 form of ZSM-5 type zeolite.
Fig. 12. ESR spe tra of isolated vanadyl cations tS1V (IV)) introduced into H-ZSM-5 (Si/Al = 35) by solid-state reaction with V 0, a t 1025 K (after Ref. 1311, wizh permission). Polyvalent cations could be co-introduced, e.g. by reacting H-ZSM-5 with CuCrO,. This provided ESR spectra identical to those of isolated Cu(II) and Cr(V) randomly distributed over cation positions. When CuO was reacted with the product of solid-state exchange between CrO, and H-ZSM-5, the signal intensity of Cr(V) considerably decreased, indicating exchange of Cr(W cations for Cu(II) species. Hence the Cu(lI) cations are more strongly bound than Cr(V) cations. Similarly, V(IV) introduced by solid-state reaction could be replaced by successive introduction of Cu(II). No solid-state exchange was observed upon thermal treatment of mixtures of FeO or Fe304 with H-ZSM-5 (but see Ref. [341 later in the text). However, Fe(lII) cations were successfully introduced into H-ZSM-5 or H-Y (Si/Al= 25) through reaction with FeC1, [281. Typical ESR signals (gl =4.27; g2= 5.65; g3 = 6.25) were most intense after stepwise oxidation of the reaction product in air at 575 and 825 K. The signals indicated that Fe(lTI) species were located in strong crystal fields of low symmetry. Again, the authors assume that FeC12+ or FeO+ is incorporated into the zeolite rather than Fe3+. The ESR spectra were compared with those of ferrisilicate which also exhibit a prominent line a t g, =4.25 (with additional signals a t g2=5.2 and g3 =7.9). However, the extra-framework Fe(III) species were distinguished from framework Fe: In contrast to framework Fe, (i) the Fe(lTI) cations (introduced by solid-state reaction or by aqueous
61
ion exchange) exhibit a n anomalous temperature effect, i.e. the intensity of the signal with g=4.27 was a t least 200 fold increased by cooling the sample to 78 K; (ii) the Fe(III) species were accessible for sorbates such as O,, NH, and pyridine which caused a dramatic change of the ESR spectrum (disappearance of the low field lines a t g2 = 5.25 and g3 = 6.25); (iii) the Fe(III) species on cationic sites could be replaced by subsequent reaction with CuO (vide supra). Solid-state ion exchange of Fe3+ into H-ZSM-5 was also investigated by Wichterlova et al. [34]. These authors used, besides ESR, temperature-programmed desorption of ammonia (TPDA) to monitor solid-solid reaction. Their results obtained with mixtures of Fe203 and H-ZSM-5 showed that significant amounts of Fe3+ were incorporated. Fe3 ions introduced in this way gave rise to a n ESR signal with g=4.27 and were easily reduced. In the reduced form the samples were tested for methanol conversion and toluene disproportionation. Due to a reduced density of acidic sites, the activity was lower than that of the starting zeolite H-ZSM-5. The yield of aromatics in methanol conversion, however, was about the same as in the case of Fe,H-ZSM-5 obtained by conventional exchange. Wichterlova e t al. [35] and Beran et al. [14] also studied the solid-state ion exchange of nickel and manganese compounds (chlorides, sulphates, acetates and oxides) with KZSM-5. Again, IR, ESR, TPDA, TPE, but also X-ray photoelectron spectroscopy (XPS)were employed a s techniques for investigation. With NiC1, the exchange reaction was optimum; 100% of the Ni2+ applied could be introduced a s long as no excess of NiCl, was used. In a stoichiometric mixture all of the acidic OH groups were consumed, i.e. one Ni2+ replaced two bridging hydroxyls upon 6 h calcination a t 770 K in a stream of dry oxygen. Such a high degree of exchange with Ni2+ is difficult to achieve by the conventional aqueous solution method. Reduction in H, a t 720 K or re-exchange with NH,N03 solution and subsequent deammoniation restored the original density of OH groups. Under identical conditions reaction with NiSO, was less efficient. No ion exchange was obtained with Ni(CH3 COO), or NiO. After reduction, zeolites obtained through solid-state ion exchange in the system NiCl,/H-ZSM-5 exhibited an activity in hydrogenation of ethylene comparable to that of samples prepared by standard Ni2+ ion exchange in aqueous medium and subsequent reduction. When mixtures of Mn(NO,), and H-ZSM-5 were thermally treated for 2 h a t 870 K, XPS revealed a significant decrease in the surface concentration of manganese [351. Further evidence for introduction of Mn2+ into H-ZSM-5 was provided by the ESR spectra of MnO/H-ZSM-5 and MnCl,/H-ZSM-5 mixtures after heat-treatment a t 870 K and 770 K followed by rehydration a t ambient temperature [14,351. The spectra showed a signal with six hyperfine lines typical of Mn2+ in cation sites with 0, symmetry. Also, TPDA, TPE and IR (consumption of acidic OH groups, decreased pyridinium ion formation upon pyridine admission to the heat-treated mixture, increased density of
62
pyridine attached to Mn2+ cations) proved solid-state ion exchange between Mn compounds and H-ZSM-5. Under identical conditions the degree of exchange obtainable decreased in the sequence MnC12>Mn304>MnS04. In contrast to the case of NiCl,, however, even with MnC1, no complete elimination of the acidic OH'S was achieved. Moreover, a fraction of the manganese compound admixed always remained unreacted. In the case of MnC12/H-ZSM-5the solid-state ion exchange was studied as a function of reaction time and temperature. Figure 13 clearly demonstrates the effect of temperature. Raising the reaction temperature from 570 K to 770 K resulted in a significant increase in the degree of exchange. Most of the manganese ions were introduced during the first stage of reaction (within 1 h), and then the further reaction proceeded very slowly.
-
1
&?
Y
cn 3
-
I 0 U
I
770 K
670K
W
z
I
I
0-
P / O - O
50-
0 W e* 0 P 25-
3
\;I.-- I 1
-
0-
I
o-
Heat treatment in vacuum
I
0
I
I
I
5 10 15 R E A C T I O N T I M E [hl
I
20
I
Fig. 13. Solid-state ion exchange with manganese chloride. Number of bridgin OH groups consume via solid-state exchange in a mixture of MnCl, and H-Z8M-5 (SiiA1=13.5, M$+iOH=0.33) as a function of reaction time a t 570, 670 and 770 K (after Ref. [141,with permission). Modification of H-ZSM-5 zeolites through solid-state reaction with ZnO was described by Yang et al. [32]. On the basis of XPS results they reported that, upon heattreatment of a ZnO/II-ZSM-5 mixture, Zn ions migrated from the outer surface into the channels of the zeolite. This finding was supported by TPDA, IR (decrease of acidic Br#nsted sites upon solid-state reaction between ZnO and H-ZSM-5) and temperatureprogrammed reduction (TPR).The latter showed increased uptake and reducibility after thermal treatment of ZnO/H-ZSM-5 compared to ZnO. Zeolites Zn,H-ZSM-5 exhibited, after reduction in H,, pronounced selectivity in propane aromatization. More recently, Karge et al. [361 have shown that also noble metals can be easily introduced into zeolites via solid-state reaction. Various zeolites, such a s NH4-Y, US-Y,
63
H-MOR and H-ZSM-5, and noble metal compounds (PdCl,, Pd(NO,),, PdO, PtCl,, orPtC1,) were used. It was demonstrated with the help of several techniques (IR, TPDA, TPE etc.) that the noble metal cations upon solid-state reaction occupy cation sites inside the zeolite structure. After reduction in H, the thus-obtained materials possessed hydrogenation properties. Provided a suitable balance between the acid function (residual acidic OH groups) and the hydrogenation function (noble metal aggregates) was established, these catalysts were efficient in hydroisomerisation of, for instance, ethylbenzene. Role of Water i n Solid-state Ion Exchange In most cases solid-state ion exchange in zeolites was conducted in the presence of ambient moisture or residual water vapour. However, it was shown that this type of exchange also occurs whenever traces of water are carefully excluded [ 121. Moreover, solid-state ion exchange into zeolites was also achieved with compounds insoluble in water, e.g. with AgCl o r Hg,Cl,. This suggests that the presence of residual water is not necessarily a prerequisite for the solid-state ion exchange in zeolites to occur, even though small amounts of water such as the crystal water might facilitate the low-temperature solid-state reaction (vide supra). However, more subtle details of solid-state ion exchange in zeolites as, for instance, the particular mechanism of ion migration remain a mystery, and their clarification needs further experimental work. Acknowledgment Financial support by the Bundesminister fur Forschung und Technologie (BMFT, Project No. 03C 257 A7) is gratefully acknowledged.
K K F E KENCES 1 J.A. Rabo, M.L. Poutsma and G.W. Skeels, in J.W. Hightower (Editor), Proc. 5th Int. Congress on Catalysis, Miami Beach, Flo., USA, August 20-26, 1972, NorthHolland Publishing Co., New York, 1973, pp. 1353-1361. 2 J.A. Rabo and P.H. Kasai, Progress in Solid State Chemistry 9 (1975) 1-19. 3 J.A. Rabo, "Salt Occlusion in Zeolite Crystals", in J.A. Rabo (Editor), "Zeolite Chemistry and Catalysis", ACS Monograph 171, Am. Chem. SOC.,Washington, D.C., USA, 1976, pp. 332-349. 4 A. Clearfield, C.H. Saldarriaga and R.C. Buckley, in J.B. Uytterhoeven (Editor), Proc. 3rd Int. Conference on Molecular Sieves; Recent Progress Reports, Zurich, Switzerland, Sept. 3-7, 1973; University of Leuwen Press, 1973, Leuwen, Belgium, Paper No. 130, pp. 241-245. 5
6 7 8
9 10
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. J.H. Lunsford, Adv. Catal. 22 (1972) 265-344. H.G. Karge, S. Trevizan de Suarez and I.G. Dalla Lana, J. Phys. Chem. 88 (1984) 1782-1784. J.B. Uytterhoeven, L.G. Christner and W.K. Hall, J. Phys. Chem. 69 (1965) 21172126. M.L. Hair, "Infrared Spectroscopy on Surface Chemistry", Marcel Dekker Inc., New York, 1967. H.G, Karge, Z. Phys. Chem. [NF] 122 (1980) 103-116.
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11 H.K. Beyer, H.G. Karge and G. Borbely, Zeolites 8 (1988) 79-82. 12 H.G. Karge, V. Mavrodinova, 2. Zheng and H.K. Beyer, in D. Barthomeuf, E.G. Derouane and W. Holderich (Editors), "Guidelines for Mastering the Properties of Molecular Sieves", NATO AS1 Series, Series B, Physics Vol. 221, Plenum Press, New York, 1990, pp. 157-168. 13 G. Borbely, H.K. Beyer, L. Radics, P. SBndor and H.G. Karge, Zeolites 9 (1989) 428-431. 14 S. Beran, B. Wichterlovh and H.G. Karge, J. Chem. SOC.Faraday Trans. I 86 (1990)3033-3037. 15 R. Schollner, P. Nobel, H. Herden and G. Korner, in P. Fejes (Editor), Proc. Symp. on Zeolites, Szeged, Hungary, Sept. 11-14,1978, Acta Universitatis Szegediensis, Acta Physica e t Chemica, Nova Series 24 (1978) 293-298. 16 H.K. Beyer and I. Belenykaja, in B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud (Editors), Proc. Int. Symp."Catalysis by Zeolites", Ecull (Lyon), France, Sept. 9-11, 1980; Elsevier, Amsterdam, 1980; Stud. Surf. Sci. 5 (1980)203-210. 17 H.K. Beyer, I.M. Belenykaja, F. Hange, M.Tielen, P.J. Grobet and P.A. Jacobs, J. Chem. SOC. Faraday Trans. I81 (1985) 2889-2901. 18 B. Sulikowski, G. Borbely, H.K. Beyer, H.G. Karge and I.W. Mishin, J. Phys. Chem. 93 (1989)3240-3243. 19 H.G. Karge, H.K. Beyer and G. Borbely, Catalysis Today 3 (1988)41-52. 20 A.E. Hirschler, J. Catal. 2 (1963) 428-439. 21 C.J. Plank, in W.M. Sachtler, G.C.A. Schuit and P. Zwietering (Editors), Proc. 3rd Congress on Catalysis, Amsterdam, The Netherlands, July 20-25, 1964, NorthHolland Publ. Comp., Amsterdam, 1965, pp. 727-728. 22 D. Keir, E.F.T. Lee and L.V.C. Rees, Zeolites 8 (1988) 228-231. 23 S. HoEevar and B. Dr&aj,in L.V.C. Rees (Editor), Proc. 5th Int. Conf. Zeolites, Na les, Italy, June 2-6,1980,Heyden, London, 1980, pp. 301-310. 24 H.8. Karge, G. Borbely, H.K. Beyer and G. Onyestyhk, in M.J. Philips and M. Ternan (Editors), Proc. 9th Int. Congress on Catalysis,Calgary, Ottawa, Canada, June26-July 1,1988, Chemical Institute of Canada, Ottawa, 1988, pp. 396-403. 25 H.G. Karge and H.K. Beyer, in DGMK-Berichte-Tagungsbericht 9101, DGMKFachbereichstagung "Cl-Chemie - Angewandte Heterogene Katalyse - C4Chemie", Leipzig, FRG, Febr. 20-22, 1991, ISBN No. 3-928164-07-4, ISSN No. 0938-068 X, pp. 191-206. (English Version to be published in Erdol & Kohle, Erdgas, Petrochemie). 26 A.V. Kucherov and A.A. Slinkin, Zeolites 6 (1986) 175-180. 27 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38- 42. 28 A.V. Kucherov and A.A. Slinkin, Zeolites 8 (1988) 110-116. 29 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987)43-46. 30 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. 31 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 583-584. 32 Y. Yang, X. Guo, M. Deng, L. Wang and Z. Fu, in H.G. Karge and J. Weitkamp (Editors), Proc. Int. Symp. "Zeolites as Catalysts, Sorbents and Detergent Builders-Applications and Innovations",Wiirzburg, FRG,Sept. 4-8,1988;Elsevier, Amsterdam, 1989; Studies Surface Sci. Catalysis 46 (1989) 849-858. Faraday 33 A.V. Kucherov, A.A. Slinkin, H.K. Beyer and G. Borbely, J. Chem. SOC. Trans. I, 85 (1989) 2737-2747. 34 B. Wichterlovh, S. Beran, S. BednaFovh, K. NedomovB, L. Dudikovh and P. J iru in P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff (Editors), Proc. Int. Symp. "Innovation in Zeolite Materials Science", Nieuwpoort, Belgium, Sept. 13-17,1987, Elsevier, Amsterdam; Studies Surf. Sci. Catalysis 37 (1988) 199-206. 35 8. Wichterlovh, S. Beran, L. Kubelkovh, J, NovhkovB, A. SmiegkovA and R. Sebik, in H.G. Karge and J. Weitkamp (Editors), Proc. Int. Symp. "Zeolites as Catalysts, Sorbents and Detergent Builders - Applications and Innovations", Wurzburg, FRG, Sept. 4-8, 1988, Elsevier, Amsterdam, 1989; Studies Surf. Sci. Catalysis 46 (1989)347-353. H.G. Kame. Y. Zhanp and H.K. Bever. Dublication in DreDaration.
catalysis
43
IN'I'KODUCTION OF CATIONS INTO ZEOLITES BY SOLID-STATE KEACTION
Hellmut G. Karge and Hermann K. Beyer Fritz-Haber-Institut der Max-Planck-Gesellschaft Berlin, 1000 Berlin 33, FRG
ABSTRACT A brief review is given of early observations of cation incorporation into zeolites through solid-state reaction. The general procedure and techniques suitable for investigation of solid-state ion exchange in zeolites are described. Results of systematic studies on introduction of alkaline, alkaline earth, rare earth, transition metal and noble metal cations into hydrogen, ammonium and sodium forms of zeolites are reported. In these studies, halides or oxides of the in-going cations are preferentially employed. Particular attention is paid to the stoichiometry of the solid-state ion exchange. It is shown that in several cases a one-step solid-state reaction leads to a 100% cation incorporation, whereas such a high degree of exchange is difficult to obtain by conventional methods. As is illustrated by a few selected examples, solid-state ion exchange might be an interesting way to prepare active acid and bifunctional catalysts. INTKODUCTION In the early seventies some interesting results were reported which showed th a t ion exchange in zeolites might take place not only by interaction of the solid zeolite phase (containing the cation I to be replaced) and a solution (containing the cation II to migrate into the zeolite structure and replace cation I). Rather, it turned out that a reaction of two solid phases, for instance of a zeolite with cation I and a salt of cation It,may also result in the zeolite containing cation II. Thus, Rabo et al. [l-31 found th a t proton-containing samples of zeolite Y reacted with sodium chloride under evolution of hydrochloric acid. By means of IR spectroscopy i t was demonstrated that this reaction eliminated acidic OH groups and thus, removed residual Brprnsted centres from the zeolite structure which are active in butene isomerisation [ll. This observation was in line with the early finding that cracking catalysts were deactivated by interaction of the zeolite component with the matrix. Obviously, in these cases cations from the matrix (clays, alumina) migrated into the zeolite structure and eliminated acidic hydroxyl groups. Clearfield and co-workers [4] were the first to apply ESR spectroscopy in order to confirm solid-state ion exchange. They introduced transition metal cations such as Cu2+, Zn2+, Ni2+ and Cr3+ into zeolites A, X and Y via reaction of salts of those cations and deammoniated ammonium forms of the zeolites. Occupation of the cation
44
sites in the zeolite structure by transition metal cations was indicated by the appearance of the typical ESR spectra. These spectra were identical to those obtained from conventionally exchanged zeolites. Surprisingly, for a relatively long time the studies on solid-state ion exchange were not continued. Only in the mid-eighties did three research groups start systematic work in this area, viz. the groups of Kucherov and Slinkin, Karge and Beyer, and Wichterlova and Beran. Since then considerable activity in this field has emerged. A special case of solid-state ion exchange was found by Fyfe et al. [5] who investigated the cation transfer between two zeolite pairs, for instance L i - m a - A , L i - m a - Y or Li-AINa-MORby 29Si MAS NMR and XRD. Obviously, there is a steadily increasing interest in the phenomenon of solid-state ion exchange. This is partly due to possible advantages of solid-state ion exchange with respect to several applications. In contrast to conventional ion exchange, solid-state ion exchange does not require handling and disposal of large volumes of salt solutions. Moreover, in many cases solid-state ion exchange proceeds more easily than the conventional exchange in suspension. This is especially true when, in aqueous solution, the in-going cation is strongly hydrated and prevented by its hydration shell from penetrating into small cavities and narrow channels of the zeolite structure. Examples are the incorporation of lanthanum, nickel or platinum via solid-state reaction. In view of the many interesting aspects of solid-state ion exchange, recent developments in this field will be reviewed. General Procedure a n d Experimental Techniques The usual procedure for carrying out solid-state ion exchange is simple. Finely dispersed powders of the zeolite and a salt or oxide of the in-going cation are carefully mixed and heated, either in a stream of inert gas or in high vacuum. In fact, the temperature required for achieving a certain degree of exchange frequently depends on the nature of the cations and anions involved. In some cases a fraction of the cations is exchanged even during grinding of the salt-zeolite mixtures a t ambient temperatures. The effect of the duration of heat treatment is less pronounced. In most cases the exchange is fast in the initial period of the solid-state reaction and then levels off. Examples will be given below. Solid-state ion exchange may be carried out in stoichiometric mixtures of salts (or oxides) and zeolites, related to the A1 content (in case of aluminosilicates) of the framework. However, i t also works with under-stoichiometric mixtures or mixtures containing a n excess of the in-going cation. In the latter case, the excess may be removed by brief extraction of the solid-state reaction product with water; also, salt occlusion may occur during heat treatment [2-31. Thus, with many important salt(oxide)-zeolite
45
systems a desired degree of exchange, even a 100% exchange, can be achieved in a onestep procedure. There are quite a number of experimental techniques to prove qualitatively, and determine quantitatively, the extent of solid-state ion exchange. In the first instance, electron spin resonance (ESR) and infrared (IR) spectroscopy should be mentioned. The experiments employing these techniques are carried out in essentially the same way a s frequently described in zeolite research [6-lo]. Due to its high sensitivity, especially in the case of many transition metals, ESR is appropriate for deciding whether or not solid-state ion exchange has taken place. Because of calibration problems, however, i t is frequently less suitable for determining the degree of exchange. With IR both qualitative and quantitative analysis is feasible. If the solid-state ion exchange is conducted with hydrogen or ammonium forms of zeolites, the change in absorbance of the OH stretching bands caused by the consumption of OH groups upon ion exchange is measured. To indicate the cations themselves, appropriate probe molecules such as pyridine, CO, CH,CN etc. may be used. Furthermore, the reaction of hydrogen or ammonium forms of zeolites with halides is easily monitored via titration of the hydrohalic acid evolved. This works particularly well with chlorides. Similarly, if gases such as hydrogen chloride or water are evolved due to solid-state reaction of hydrogen forms of zeolites with chlorides or oxides, mass spectrometry and gas chromatography are frequently employed. These techniques enable u s not only to detect the gases and thus confirm the fact of solid-state reaction but also provide means to evaluate the rate of exchange and the amount of exchanged cations. The loss of weight, which originates from the reaction of a halide or oxide with a hydrogen form of a zeolite, i.e. from evolution of hydrohalic acid or water, can be measured by a balance. This thermogravimetric analysis may be combined with GC or MS measurements or with continuous titration. In most instances, however, zeolites and zeolite-like materials are synthesized a s sodium forms. Thus, when sodium forms are the starting materials for solid-state ion exchange, magic angle spinning-nuclear magnetic resonance of 23Na (23Na MAS NMR) is advantageously employed to monitor that process. Examples are the exchange of sodium for other alkaline metal cations and the incorporation of lanthanum cations (vide infra). Solid-state Ion Exchange with Alkaline Metals Systematic studies on solid-state ion exchange with alkaline metals were carried out in the systems M'Cl/H-ZSM-5 and M'Cl/NH,-Y, where MI represents Li, Na, K,
46
Rb and Cs [ll-121.Also, the solid-state replacement of Na+ by Li+ or K + in Na-Y was investigated [ 131. As the first example, Figure 1 shows a sequence of IR spectra obtained with NaCVHZSM-5, where NaCl was admixed in excess (NaCVAl= 1.9). These spectra provide evidence for the following points. (i) Solid-state ion exchange took place upon heating the NaCl/H-ZSM-5 mixture in high vacuum (HV) a t 723 K (change of the absorbance of the OH bands). (ii) Not only the strong acidic OH groups (band a t 3605 cm-') but also the weak ones (band a t 3740 cm-l of so-called silanol groups) were involved (the latter band was weakened). (iii) The Na+ 0'-Si f groups formed by reaction of NaCl with silanol OH'S were easily hydrolyzed, since upon treatment with H,O the OH band a t 3740 cm-l was fully restored whereas the band at 3605 did not reappear; i.e. Na+ cations, which had replaced protons of the bridging, strongly acidic OH groups, were irreversibly held. (iv) No lattice destruction due to HCl evolution (vide infra) had occurred, since conventional re-exchange with NH,Cl solution and subsequent deammoniation resulted ina complete recovery of the original OH band a t 3605 cm-'. In this context it should be stressed that the structural integrity of the product of the solid-state ion exchange (NaZSM-5) was also confirmed via comparison of its X-ray diffraction pattern with th a t of the starting material (H-ZSM-5). No change in the intensities of the zeolite lattice reflections was detected. However, a significant decrease in the NaCl reflections was observed which is a further proof for the occurrence of a solid-state reaction. The same result holds for all the other examples of solid-state ion exchange reported here. The IR results of the NaCVH-ZSM-5 reaction were in excellent agreement with the stoichiometric data obtained from AAS analysis and titration. These data are listed in Table 1.
3605 I
3800
I
3600
W A V E N U M B E R [crn-']
Fig. 1. IR spectra of the OH stretching region after evacuation at 723 K (2h, 10" pa). a, HZSM-5; b, NaCl/H-ZSM-5 mixture (NaCl/Al= 1.89) calcined at 900 K (1 h); c, (b) washed with water; d, (b) twice exchanged with 1 N NH, C1 solution (Ref. [I11, with permission).
47
Table 1 Mass balance* for solid-state ion exchange in the system NaCl/H-ZSM-5
Zeolite H-ZSM-5 (1) H-ZSM-5 (2)
(1) SiiAl
(2) A1
155 23
0.107 0.691
0.01
0.808 1.306
(5)
(6) C1' extracted
(7) Na+ extracted
(8) Na+ non-extracted
mixture
HC1 evolved
NaCl/H-ZSM-5 (1) NaCVH-ZSM-5 (2)
0.549 0.828 =(4) - (6)
*
0.260 0.478
(3) Na+ -_
0.707 0.670
(4)
NaCl
0.101 0.636 (4) - (7)
all data in m m o l per gram zeolite fired a t 1273 K
(1) to (3) data of the starting zeolites; (4) added to the zeolite; (5) evolved during calcination a t 900 K for 1 hour; (6) and (7) extracted from the heat-treated mixture by
washing with water; (8) not extractable with water; compare with Al, column (2).
One recognises from Table 1 that the stoichiometry is excellent: (i) the amount HC1 evolved agrees well with the difference between employed and extracted Cl-, see columns (4) and (6); (ii) the amount of non-extractable N a + , i.e. the difference between N a + employed and extracted, see columns (4) and (71, coincides with the A1 content. Since these H-ZSM-5 samples did not contain essential amounts of extra-framework Al, each A1 corresponded to one bridging OH group. Therefore, the data of column (8) in comparison with column (2) show that a 100%or almost 100%exchange of H + for N a + occurred. With the same system, i.e. NaCUH-ZSM-5,it is illustrated in Figure 2 how the solidstate reaction upon temperature-programmed heating is monitored by thermal gravimetric analysis TGA and simultaneous continuous titration of HCl evolved (TPE). Curve (a) indicates the weight-loss of the NaCl/H-ZSM-5 mixture, curve (b) the amount of HC1 released. Both curves exhibit three steep steps. The first one (at about 400 K) is mainly caused by dehydration and only to a minor extent by evolution of HCl (see small step in curve (b)). The second step in curve (a) centered around 830 K coincides with the strongest evolution of HCI (curve (b)). Finally, at very high temperatures decomposition of the excess NaCl occurred and gave rise to steps a t ca. 1200 K i n both curves.
48 1.10
-
2
c
I
E
w W
' w
Y
n W
1.05
> A
-I
a.
0
Q
-
I
> w
VI
U
I
1.oo
300
500
700
900
1100
T E M P E R A T U R E [K]
Fig. 2. a, TGA and b, continuous titration of evolved HC1 upon temperature-programmed heating of a NaCV HZSM-5 mixture (Na/Cl= 1.89)i heating rate 2.5 K min(Ref, [ll], with permission).
-
Investigation of solid-state ion exchange with the help of mass-spectrometrically monitored evolution of HCl and NH, was carried out with the system M'CVNH,ZSM-5. Results are presented in Figure 3. Two regions, viz. a low-temperature (LT) and a high-temperature (HT) regime of solid-state reaction, can be distinguished. With LiCl
r
1
c
5
TEMPERATURE
[K]
Fig. 3. Temperature-programmed evolution (TPE) of HCl upon heating M'CY NHA-ZSM-5mixtures; MI, Li(--); Na. (---); K (-*-a); Rb (-*--*=); c s (-**--*--); Li refers t o right scale (Ref. [Ill,with permission).
49
the LT reaction is eminently prevailing. The exceptional ease of solid-state ion exchange with LiCl was observed with other zeolites as well (vide infra). In the case of the other M'C1 salts, the contribution of the HT reaction to the overall process is significantly higher than that of the LT reaction. The temperature maxima both of the LT and the HT peaks decrease in the sequence T(Na) > T(K) > T(Rb) > T(Cs). Very similar results, including the exceptional behaviour of LiCl and the sequence of the temperature maxima, were obtained with mixtures of M'Cl/NH,-Y. The sequence of the peak temperatures corresponded to the decrease in the lattice energies of the alkaline chlorides. Thus, i t seems t h a t the solid-state ion exchange between alkaline chlorides and hydrogen forms of zeolites proceeds more easily the lower the lattice energy of M'C1. This suggests t h a t a low lattice energy would facilitate the separation of M'C1 entities from the M'C1 structure which then can migrate to and into the zeolite structure in order to react there with the OH groups. Note, however, t h a t such a relationship between the lattice energy of the compound of the in-going ion and the ease of solid-state ion exchange was not established with, e.g., Mn compounds and H-ZSM-5 (see below and Ref. [14]). Thus, parameters other than lattice energies may be operative as well. Cation exchange with alkaline metal cations can also be achieved by solid-state reaction between M'C1 and sodium forms of zeolites [13]. This was proven, inter alia, via 23Na MAS NMR. For the sake of comparison i t seems useful first to look at the 23Na MAS NMR signals of sodium in various surroundings (Figure 4). In the 23Na MAS NMR spectra, sodium chloride is used as a reference. Thus, the signal of crystalline NaCl appears a t 0 ppm. Sodium cations in aqueous solutions give rise to a sharp line at - 12.5 ppm whereas the sodium cations in hydrated Na-Y are indicated by a broad signal around - 8 ppm. When the solid-state reaction is carried out with LiCl and Na-Y a dramatic change i n the 23Na MAS NMR spectrum occurs even when the reaction temperature is as low as ambient. As is demonstrated i n Figure 5, the signal typical of Na in Na-Y disappears. Instead, the line typical of crystalline NaCl is developed and, furthermore, a broad signal appears a t about - 12.5 ppm where sodium cations in aqueous solution are indicated (compare Figure 4, line b). However, the linewidth of the 12.5 ppm band in Figure 5 is very large compared to that of Figure 4. The most likely explanation for these observations is t h a t a fraction of t h e sodium cations, which were expelled from the cation sites by Li cations, form tiny NaCl crystallites at the outer surface of the zeolite particles and give rise to the 0 pprn signal (simultaneously, i n the X-ray pattern of this sample the reflections of crystalline NaCl were detected). Another fraction of the sodium cations remains in the intracrystalline water of Na-Y. But in the cavities of Na-Y the mobility of the water molecules is restricted, and a large variety of coordination states are available for the sodium cations. These
50
factors may give rise to the observed broadening of the - 12.5 ppm signal, via a homogeneous (relaxational) and/or inhomogeneous (chemical shift) effect on the 23Na resonance.
I
I
l
l
I
1
I
I b, Nain
NaCl soh.
b
0
I
5
I
0
I
-5
-10
C H E M I C A L SHIFT,
-15
I
-20
6Nacl.cryrt.
-25
[ppml
Fig. 4. 23Na MAS NMR spectra. a, crystalline NaC1; b, saturated NaCl solution; c, Na-Y, without pretreatment (Ref. 11, with permission).
I
I0
I
I
0
I
I
-10
I
I
-20
C H E M I C A L 5 H I FT,
I
I
-30
6NaCI.cryrt.
I
I
-40
[ppm]
Fig. 5. 23Na MAS NMR spectra of Na-Y, without pretratment and LiCU Na-Y mixture, ground and intimately mixed (Ref. 14, with permission).
Again, LiCl reacts much more easily than other alkaline chlorides. Most probably, this is due to a particularly large decrease in the free energy when Li+ is transferred from crystalline LiCl or aqueous LiCl solution to the zeolite where the small Li+ cations are strongly "solvated" by the oxygens of the framework [ 151. The ease of exchange with LiCl is of great interest with respect to the important dealumination method using SiCl, [16-171, which yields silicon-rich, hydrophobic zeolites, i.e. valuable adsorbents. Sulikowski et al. 1181 have found that, when Li-Y is dealuminated instead of Na-Y, no self-inhibition of the dealumination by SiC1, occurs. This is due to the fact that, in contrast to NaA1C14, the product LiAlC1, decomposes a t the reaction temperature, and thus plugging of the zeolite pores is avoided.
51
Solid-state Ion E x c h a n g e with Alkaline E a r t h Metals Results were reported on the systems MgC12/H-MOR,CaC12/H-MOR or NH,-MOR [19] a s well a s on CaCl,/Na-FAU and BeC12/Na-FAU [ 131.
The stoichiometry of solid-state ion exchange was checked in detail with CaC12/ €1 MOR and CaCl,/NH,-MOR. Again, when an excess of CaC1, was used, part of the weak silanol-type OH groups also reacted. But they were easily hydrolysed. Excellent agreement between the aluminium content (bridging OH groups) and the amount of irreversibly held calcium cations was found.
1.40
3
-
m
Y
+
1.30
I
W W
3
1.20
1
1.10
1.oo
,+, LUV
*++.
, ,
+
%+-
wuu
TEMPERATURE [“C]
I
,+-+-+-+-+-+-
-
isothermal heating at 55OoC
Fig. 6 a, TGA and b, continuous titration of evolved HCl upon heating of a CaCl / H-MOR mixture (Ref. r191. with permisssion)
However, the exchange with CaCl, proceeded less easily than with alkaline metal chlorides. Temperature-programmed heating of the CaC12/H-MOR mixture (Ca/Al= 0.51, similar to the case of alkaline metal chlorides, provided two steps of HC1 evolution corresponding to a low-temperature and a high-temperature process (Figure 6). While the LT exchange gave rise to a sharp step between 400 and 575 K, the HT exchange resulted in a less pronounced evolution of HCl. Only after repeated heating and a second isothermal treatment a t 875 K was i t completed. Then the reaction ceased and a total of 2.5 mmol HCl per gram zeolite was evolved. This compared satisfactorily with the A1 content (2.2 mmol per gram zeolite). The agreement between HC1 or NH,Cl evolved and A1 content was perfect in the case of CaCl,/NH,-MOR (Ca/A1=0.5), viz. 2.54 and 2.52 mmol per gram.
52
Thermogravimetric analysis showed only one steep decrease in weight of the CaC12/ H-MOR mixture due to release of H 2 0 and HCl. The system initially contained significant amounts of water because the hydrated chloride, CaC12 * 2H20, was employed. Thus, the LT exchange was not, in a strict sense, a solid-state ion exchange and was probably facilitated by the presence of water molecules in the zeolite pores. However, as will be pointed out later, the presence of water is not a n indispensable condition for solid-state ion exchange to occur. Rather, the solid-state exchange can be conducted even in ultrahigh vacuum, i.e. in the absence of any traces of water. Solid-state ion exchange with CaC12/H-MORand MgCl,/H-MOR was also monitored by IR. Figure 7 demonstrates, as a n example, the removal of acidic OH groups (spectrum 2a) in H-MOR due to replacement of the protons by Ca2+. Subsequent pyridine adsorption gave rise to a sharp band a t 1446 cm-*typical of pyridine coordinatively bonded to calcium cations and a smaller band a t 1455 cm-l which is due to pyridine attached to "true" Lewis sites (spectrum 2b). Most likely, both types of pyridine coordination contributed to the small signal a t 1610 cm-l. Note, however, that only a very weak pyridinium ion band at 1540 cm-' was observed. Another sample, which had been prepared via solid-state ion exchange (spectrum 2a), was contacted with small amounts I
I
I
/
iI
I
I
Y
U
z
a
II-
r
v)
z a CT
c
3av I
4000
I
3500
I
3000
W A V E N U M B E R [cm-'I
Fig. 7. IR spectra of mordenite samples. l a , lb, spectra of H-MOR after activation (775 Pa); 2b, same treatment a s Pa); 2a, CaCl /H MOR after heating at 775 K, (2a), subsequent pyrikne adsorption and removal of excess pyridine (475 K, loe5Pa); 3a, same treatment as (2a) and brief contact with H,O, followed by degassing at 775 K ( Pa); 3b, same treatment as (3a), subsequent pyridine adsorption and removal of excess pyridine (475K, Pa).
53
of water vapour and again degassed. This resulted in spectrum 3a showing a strong OH stretching band around 3618 cm-l. Obviously, interaction of H 2 0 molecules with Ca2+ introduced by solid-state ion exchange generated acidic OH groups according to the Hirschler-Plank mechanism [20-211.In fact, subsequent pyridine adsorption produced a strong band a t 1540 cm-' indicating pyridinium ions. While the ion exchange with alkaline metal cations does not result in catalysts active in acid-catalysed hydrocarbon reactions and, in contrast, may be carried out to remove any residual activity [l],the incorporation of alkaline earth cations by solidstate reaction should lead to active catalysts. I t was shown, however, that the solidstate ion exchange had to be followed by contact with water vapour in order to obtain calcium or magnesium mordenites which are sufficiently active in, for instance, disproportionation of ethylbenzene. This is in full agreement with the IR spectroscopic results which indeed showed that only upon interaction of the heat-treated CaC12/H-MOR mixture with water vapour were acidic OH groups generated. Also, in the case of alkaline earth chlorides some investigations were undertaken using sodium forms of zeolites instead of hydrogen forms as starting materials for solidstate ion exchange. These experiments, however, were conducted a t ambient temperature and moisture. 23Na MAS NMR was used to evaluate the degree of exchange when, for instance, BeC1, or CaC12was contacted with Na-Y. The results were similar to what was found in the case of LiC1. The 23Na MAS NMR spectra suggested complete exchange. The line indicating N a + in Na-Y completely disappeared. Instead, the signal of Na+ in crystalline NaCl and the broad band a t -12.5ppm due to N a + in intracrystalline water developed. The exchange between BeC1, and Na-Y was also studied by IR, using pyridine a s a probe. Before solid-state reaction, pyridine indicated Na+ on cationic sites through a band a t 1444 cm-I.After solid-state reaction N a + was completely replaced by Be2+ and the corresponding band of pyridine coordinatively bonded to Be2+ was observed a t 1453 cm-I. Interestingly, i t was important to avoid heating of the BeC12/Na-Y mixture higher than 400 K, because a t temperatures higher than this, partial re-exchange of N a + for Be2+ occurred. Solid-state Ion Exchange with Hare-earth Metals In view of catalyst preparation, the solid-state ion exchange with lanthanum or rare-earth cations is of particular interest. Rare-earth-containing zeolites are widely used a s cracking catalysts. However, special measures are required [22-231to obtain a degree of exchange higher than about 75% of, e.g., La3+ or Ce3+ when the exchange is carried out with the conventional technique, i.e. by suspending the zeolite powder in a solution of the in-going trivalent ion. The reason is that in solution the highly solvated
54
M3+ cation cannot enter the sodalite cages. To achieve higher degrees of exchange, usually the conventional exchange procedure with suspensions is repeated several times and the exchange product intermittently heated to push the, a t least partially, desolvated La3+ cations into the sodalite cages. It turns out, however, that solid-state ion exchange may lead to an almost 100% exchanged La-Y zeolite in a one-step procedure and provide highly active catalysts [24-251. Similar to the results already described, also with LaC13/NH4-Y the stoichiometric measurements were very instructive (Table 2).The data of Table 2 show the results of asolid-state reaction between LaCl, and NH4,Na-Y. The zeolite had a degree of exchange of 89% NH4+ for Na+. The LaC13/NHq,Na-Ymixture contained 1 La3+ per 3 A1 of the zeolite structure. The experimental results were obtained by titration of the evolved gases and chemical analysis of both the water-extracted product and the extract solution. From these data it is evident that (within the limits of error) the amount of La3+ introduced by solid-state reaction (4.95meq./g) corresponded exactly to the amount of framework aluminium (4.83meq./g) or maximum of bridging OH groups. However, even the main fraction of Na+ of the starting material (1.61 meq./g) was removed from the zeolite. Only about 0.7 meq. Na+ and 0.8 meq. Cl- remained in the structure, corresponding to roughly one NaCl molecule per P-cage. It is assumed th a t this NaCl is occluded in the structure. According to Rabo's study [2-31this would enhance the thermal stability of the exchange product. Tabl e 2.
Solid-state Ion Exchange*, LaCl,/NH,,Na-Y (LafAl =0.33); thermal treatment at 850 K La3
c1-
Na+
NH4+
A1
_-
--
1.61
4.83
1.61
4.83
3.29
+
Starting zeolite Salt admixed Evolved gas (as NH,Cl)
*
3.29
3.29
Extracted with K,O
0.06
0.72
0.94
irreversibly held
1.60
0.82
0.67
4.80
__ __
4.83 4.83
data in mmol per gram zeolite, with the exception of the last line (meq./g)
The results were exactly the same when an excess of LaC13, e.g., a ratio of 2 La3+ per 3 A1 of the zeolite structure, was employed. The only difference was that, after completion of the reaction, a higher amount ofLaC1, was extracted [251.
55
In the same way, a 100%La-Y sample was obtained by stoichiometric solid-state ion exchange when the starting zeolite was a 100% exchanged NH,-Y and a ratio La/A1= 0.33 employed. In contrast to the conventional procedure, the solid-state reaction yielded the 100% ion-exchanged La-Y by one exchange step [25]. Solid-state incorporation of La3+ into Y-type zeolite was also studied by IR. Figure 8 exhibits a set of spectra in the OH and NH stretching region obtained upon thermal treatment of a LaCl3/NH,,Na-Y(89) mixture a t successively higher temperatures (spectra a-c) and after contact of the exchange product with water vapour followed by degassing (spectrum d). At 475 K deammoniation was still incomplete; however, the prominent high frequency and low frequency OH bands of hydrogen Y a t 3640 and 3542 crn-l, respectively, were already partly developed. Upon heating to higher temperatures, solid-state reaction of LaCl, with the OH groups eliminated the OH bands almost completely. Subsequent treatment with H,O vapour and degassing produced the OH bands typical of La-Y (spectrum d). 3738 3560
I
ll I l l
CII
I 1
3800
l
h
I
Y
3738
3738 3530
c
d'
I
I
I
.u
I
725 K.30 min, HV
I If1 I /I I I /I I 3400 3000"38003400"3800 3400 " 3800 W A V E N U M B E R [cm'l
I
3400
I
3000
Fig. 8. IR spectra of the OH stretching region of a LaCl /NH -Y mixture. a, b and c, after heating a t 475, 625 and 725 K, res ectively h a , 2%);d,gfter (c) and brief contact with H,O (0.6 kPa, 3 min) followefby evacuation (725 K, 10- Pa, 30 min). Adsorption of pyridine subsequent to spectrum c gave rise to a band a t 1452 cm-l typical of pyridine coordinatively bonded to lanthanum cations; only a tiny pyridinium ion band a t 1542 cm-l was observed. When, however, generation of spectrum d (i.e. after H,O contact of the sample) was followed by pyridine adsorption, the band of acidic OHgroups at 3630 cm-' was completely removed and a strong band a t 1542 cm-l (pyridium ions) appeared.
56
The intensities of the bands were similar to those of La-Y samples obtained by conventional ion exchange. TPD of ammonia showed that also the strength of the acidic sites were essentially the same as found with conventionally exchanged La-Y. Thus, the nature, density and strength of acidity of La-Y prepared via solid-state ion exchange are comparable to those of the conventionally obtained products and, therefore, similar catalytic behaviour in acid-catalysed reactions was expected. This was, indeed, found when La-Y was employed as a catalyst for ethylbenzene disproportionation.
I
I
w
z w
N
'
l
~
l
'
1
~
1
-
~
1
1
1
~
1
-
16-
0
v
U
T I M E O N S T R E A M [hl
Fig. 9. Selective disproportionation of ethylbenzene at 425 K (1.3 vol % EB in He, 5 ml min-', m[catl=O.25 g). A, over a La-Y (98) catalyst obtained by solid-state ion exchange; B, over a conventionally prepared La-Y (96)catalyst. Finally, La-Y zeolites were obtained by solid-state ion exchange between LaCl, and the sodium form of Y-type zeolites as well [25]. This was proven by chemical analysis, IR, X-ray diffraction (XRD) and a test reaction. Details of the preparation and characterisation are outlined in Ref. [25]. Chemical analysis gave evidence for a partial replacement of sodium by lanthanum cations. IR showed the formation of acidic OH groups. Finally, XRD demonstrated, via the appearance of reflections of crystalline NaC1, that obviously Na + cations were expelled from the interior of the zeolite crystals by in-going La3+. Outside the zeolite particles they had formed small NaCl crystallites.
~
l
51
Even though a 100% exchange in the system LaC13/Na-Y was not yet achieved by solid-state ion exchange, the product obtained showed catalytic activity i n acid-catalysed ethylbenzene disproportionation similar to that of conventionally obtained La-Y with a n exchange degree of 74%. Solid-state Ion E x c h a n g e with Transition Metals As mentioned in the Introduction, Clearfield et al. [4] were the first to incorporate transition metal cations into zeolites via solid-solid reaction. These authors reacted partially (i.e. to 36 - 58%) NH4-exchanged and subsequently deammoniated forms of Na-A, Na-X or Na-Y with transition metal compounds such a s CuC12, ZnCl,, NiCl,, CoCl2, CrC1, or MnC1,. The extent of solid-solid reaction was monitored by titration of HCl evolved. Depending on the reaction conditions, up to 100%exchange of the zeolite protons by metal cations could be achieved. Investigation of transition metal cations and their reactions fall in the realm of ESR spectroscopy. Such studies benefit by the high sensitivity of that spectroscopic technique. Moreover, ESR spectroscopy of cations in zeolites provides in many cases deeper insight into their coordination state. Thus, Clearfield et al. [41 were also the first to successfully apply ESR spectroscopy to prove the incorporation of transition metal cations into a zeolite structure by solid-solid interaction. In Figure 10, ESR spectra of Cu,Na-Y
Fig. 10. ESR spectra of heat-treated Na-Y and Cu(II)-containing Y-type zeolite s m ples. A, 15% of original N a + replaced through solid-state reaction between CuCl, and H,Na-Y; B, 15% of original N a + replaced by conventional exchange in aqueous suspension; C, original Na-Y (after Ref. [51, with permission).
58
obtained via solid-state exchange (line A), conventionally exchanged Cu,Na-Y (line B) and the starting zeolite Na,H-Y (line C) are compared. In both Cu,Na-Y samples about 1 5 6 of the original Na+ cations were replaced by Cu2+. It is evident from comparison of spectra (A) and (B) that very similar Cu2+-containing zeolite Y samples were obtained from solid-state and conventional aqueous exchange. Both spectra (A) and (B) exhibit 8 lines indicative of two different environments for the Cu2+ cations. The first set of signals is characterised by g,,=2,35 and g,=2,06 whereas the g-values of the second set are g,,= 2,30 and gl= 2,06.
ESR spectroscopy was extensively used by the group of Kucherov and Slinkin in their systematic work on solid-state reaction between high-silica zeolites and transition metal compounds. As zeolites they preferentially employed hydrogen mordenites, hydrogen forms of ZSM-5 or highly dealuminated H-Y. In their experiments, the transition metal compounds reacted with the zeolites are mostly oxides such a s CuO [261 or CrO,, Cr20,, MOO, or V,O, [27]. However, salts were also applied, e.g. MoCl, [271, CuCl,, CuF2, Cu3 (PO,),, Cu2S [261 or FeCl3[281. Finally, the studies were extended to simultaneous or successive reaction of two oxides (CuO + CrO,, CuO + V20& with HZSM-5 [29]. In particular, the system CuO/H-ZSM-5 was studied in great detail. An interesting result was that Cu2+ introduced by solid-solid reaction of CuO and H-ZSM-5 at 825 K o r 1075 K exhibited ESR spectra which were completely identical with those obtained from Cu,H-ZSM-5 prepared via conventional ion exchange in aqueous solution (Ref. [301, Figure 11).The g-values and hyperfine splitting constants for samples prepared via either route showed full agreement, hence the authors concluded that also the
I
Fig. 11. ESR spectra of Cu(II)-containing MFI-type zeolite samples. A, CuCl,/ H-ZSM-5 mixture heat-treated in vacuum a t 10 75 K; B, Cu,H-ZSM-5 obtained by conventional exchange, calcined in air at 1075 K and evacuated a t 300 K (after Refs. [261 and 1301, with permission).
59
isolated Cu2+ ions introduced by solid-state reaction were present in two different coordination states, viz. in a square planar environment (g,,=2.32, g,=2.045, A,,= 17 mT, Al= 2.9 mT) and a fivefold coordinated state (g,,=2.32, gL= 2.06-2.07, A,,= 14 - 14.2 mT, Al= 1.8 mT). The latter coordinatively unsaturated state, however, was completely absent when CuO was reacted with H,Na-ZSM-5 (40% Na+ exchanged for H + ) instead of H-ZSM-5 (95% N a + exchanged for H f ) . The H,Na-ZSM-5 starting zeolite was prepared by burning-off of the organic template without any further exchange of the remaining N a t cations. Therefore, on the basis of the above ESR observation i t was suggested that, upon synthesis, N a + and organic cations have been located in the ZSM-5 structure not randomly but, rather, in a somehow ordered manner. The Cu2+ cations introduced by solid-solid reaction were easily accessible for adsorbates such as 0, a s was evidenced by the dramatic but reversible change, i.e. loss of intensity, broadening and removal of hyperfine splitting of the ESR signals upon oxygen admission. The amount of Cu2+i which could be introduced into H-ZSM-5 by solid-state reaction, increased linearly with the A1 content of the framework. Similar results were obtained with H-MOR. However, when Na-MOR was calcined in a mixture with CuO no appearance of the Cu2+ ESR signal was observed. Solid-state reaction also occurred between H-ZSM-5 and CuC12, CuF, or Cu3(P0,),. In these cases a n effect of the anions was realised. The Cu2+ cations introduced were coordinated not only to the framework but also to F' or a s a ligand. With Cu,S i t is supposed that also Cut cations migrate. The Cu2+ ESR spectrum appeared upon oxidation of the thus introduced Cut cations residing on cation sites. It is stressed that reaction of CuF, with H-ZSM-5 caused no destruction of the lattice. X-ray diffraction patterns obtained after solid-state reaction gave evidence for the integrity of the structure. Thermal treatment of mixtures of hydrogen zeolites with CrO,, MoCl, and V205 gave rise to ESR spectra typical of isolated Cr(V), MOW)and V(W) cationic species [271, respectively. The authors do not comment on the reduction of Cr(VT) or V(V). Most probably this occurred due to the presence of organic contaminants such a s residual template. Because of the large distance between the framework A1 atoms (or protonic sites) in the highly siliceous hydrogen zeolites used, the authors assume that isolated complex cations such as CrO,+, MoC14+, VO(OH)+ were introduced by solid-state reaction, rather than Cr5+,Mo5+ or V4+. Interestingly, Kucherov and Slinkin observed a "superhyperfine splitting" in the case of ESR spectra of Cr(V) or V ( N ) obtained after solid-state reaction [27,31]. The additional splitting (see Figure 12) is ascribed to a n electronic interaction of 53Cr (nuclear spin I=3/2) or (I=7/2) with adjacent framework A1 (I=3/2) [27,31]. Yang et al. [32], who also investigated by ESR the introduction of Cr or Mo into H-ZSM-5, arrived a t similar results.
60
There is also one publication by Kucherov et al. [33]concerning the introduction of Cu, Fe, Cr or V cations into the gallium analog of H ZSM-5. In this work it was found that, in line with the well-known lower acidity strength in H-[Gal ZSM-5, the cations introduced are less stabilized in the Ga form than in the A1 form of ZSM-5 type zeolite.
Fig. 12. ESR spe tra of isolated vanadyl cations tS1V (IV)) introduced into H-ZSM-5 (Si/Al = 35) by solid-state reaction with V 0, a t 1025 K (after Ref. 1311, wizh permission). Polyvalent cations could be co-introduced, e.g. by reacting H-ZSM-5 with CuCrO,. This provided ESR spectra identical to those of isolated Cu(II) and Cr(V) randomly distributed over cation positions. When CuO was reacted with the product of solid-state exchange between CrO, and H-ZSM-5, the signal intensity of Cr(V) considerably decreased, indicating exchange of Cr(W cations for Cu(II) species. Hence the Cu(lI) cations are more strongly bound than Cr(V) cations. Similarly, V(IV) introduced by solid-state reaction could be replaced by successive introduction of Cu(II). No solid-state exchange was observed upon thermal treatment of mixtures of FeO or Fe304 with H-ZSM-5 (but see Ref. [341 later in the text). However, Fe(lII) cations were successfully introduced into H-ZSM-5 or H-Y (Si/Al= 25) through reaction with FeC1, [281. Typical ESR signals (gl =4.27; g2= 5.65; g3 = 6.25) were most intense after stepwise oxidation of the reaction product in air at 575 and 825 K. The signals indicated that Fe(lTI) species were located in strong crystal fields of low symmetry. Again, the authors assume that FeC12+ or FeO+ is incorporated into the zeolite rather than Fe3+. The ESR spectra were compared with those of ferrisilicate which also exhibit a prominent line a t g, =4.25 (with additional signals a t g2=5.2 and g3 =7.9). However, the extra-framework Fe(III) species were distinguished from framework Fe: In contrast to framework Fe, (i) the Fe(lTI) cations (introduced by solid-state reaction or by aqueous
61
ion exchange) exhibit a n anomalous temperature effect, i.e. the intensity of the signal with g=4.27 was a t least 200 fold increased by cooling the sample to 78 K; (ii) the Fe(III) species were accessible for sorbates such as O,, NH, and pyridine which caused a dramatic change of the ESR spectrum (disappearance of the low field lines a t g2 = 5.25 and g3 = 6.25); (iii) the Fe(III) species on cationic sites could be replaced by subsequent reaction with CuO (vide supra). Solid-state ion exchange of Fe3+ into H-ZSM-5 was also investigated by Wichterlova et al. [34]. These authors used, besides ESR, temperature-programmed desorption of ammonia (TPDA) to monitor solid-solid reaction. Their results obtained with mixtures of Fe203 and H-ZSM-5 showed that significant amounts of Fe3+ were incorporated. Fe3 ions introduced in this way gave rise to a n ESR signal with g=4.27 and were easily reduced. In the reduced form the samples were tested for methanol conversion and toluene disproportionation. Due to a reduced density of acidic sites, the activity was lower than that of the starting zeolite H-ZSM-5. The yield of aromatics in methanol conversion, however, was about the same as in the case of Fe,H-ZSM-5 obtained by conventional exchange. Wichterlova e t al. [35] and Beran et al. [14] also studied the solid-state ion exchange of nickel and manganese compounds (chlorides, sulphates, acetates and oxides) with KZSM-5. Again, IR, ESR, TPDA, TPE, but also X-ray photoelectron spectroscopy (XPS)were employed a s techniques for investigation. With NiC1, the exchange reaction was optimum; 100% of the Ni2+ applied could be introduced a s long as no excess of NiCl, was used. In a stoichiometric mixture all of the acidic OH groups were consumed, i.e. one Ni2+ replaced two bridging hydroxyls upon 6 h calcination a t 770 K in a stream of dry oxygen. Such a high degree of exchange with Ni2+ is difficult to achieve by the conventional aqueous solution method. Reduction in H, a t 720 K or re-exchange with NH,N03 solution and subsequent deammoniation restored the original density of OH groups. Under identical conditions reaction with NiSO, was less efficient. No ion exchange was obtained with Ni(CH3 COO), or NiO. After reduction, zeolites obtained through solid-state ion exchange in the system NiCl,/H-ZSM-5 exhibited an activity in hydrogenation of ethylene comparable to that of samples prepared by standard Ni2+ ion exchange in aqueous medium and subsequent reduction. When mixtures of Mn(NO,), and H-ZSM-5 were thermally treated for 2 h a t 870 K, XPS revealed a significant decrease in the surface concentration of manganese [351. Further evidence for introduction of Mn2+ into H-ZSM-5 was provided by the ESR spectra of MnO/H-ZSM-5 and MnCl,/H-ZSM-5 mixtures after heat-treatment a t 870 K and 770 K followed by rehydration a t ambient temperature [14,351. The spectra showed a signal with six hyperfine lines typical of Mn2+ in cation sites with 0, symmetry. Also, TPDA, TPE and IR (consumption of acidic OH groups, decreased pyridinium ion formation upon pyridine admission to the heat-treated mixture, increased density of
62
pyridine attached to Mn2+ cations) proved solid-state ion exchange between Mn compounds and H-ZSM-5. Under identical conditions the degree of exchange obtainable decreased in the sequence MnC12>Mn304>MnS04. In contrast to the case of NiCl,, however, even with MnC1, no complete elimination of the acidic OH'S was achieved. Moreover, a fraction of the manganese compound admixed always remained unreacted. In the case of MnC12/H-ZSM-5the solid-state ion exchange was studied as a function of reaction time and temperature. Figure 13 clearly demonstrates the effect of temperature. Raising the reaction temperature from 570 K to 770 K resulted in a significant increase in the degree of exchange. Most of the manganese ions were introduced during the first stage of reaction (within 1 h), and then the further reaction proceeded very slowly.
-
1
&?
Y
cn 3
-
I 0 U
I
770 K
670K
W
z
I
I
0-
P / O - O
50-
0 W e* 0 P 25-
3
\;I.-- I 1
-
0-
I
o-
Heat treatment in vacuum
I
0
I
I
I
5 10 15 R E A C T I O N T I M E [hl
I
20
I
Fig. 13. Solid-state ion exchange with manganese chloride. Number of bridgin OH groups consume via solid-state exchange in a mixture of MnCl, and H-Z8M-5 (SiiA1=13.5, M$+iOH=0.33) as a function of reaction time a t 570, 670 and 770 K (after Ref. [141,with permission). Modification of H-ZSM-5 zeolites through solid-state reaction with ZnO was described by Yang et al. [32]. On the basis of XPS results they reported that, upon heattreatment of a ZnO/II-ZSM-5 mixture, Zn ions migrated from the outer surface into the channels of the zeolite. This finding was supported by TPDA, IR (decrease of acidic Br#nsted sites upon solid-state reaction between ZnO and H-ZSM-5) and temperatureprogrammed reduction (TPR).The latter showed increased uptake and reducibility after thermal treatment of ZnO/H-ZSM-5 compared to ZnO. Zeolites Zn,H-ZSM-5 exhibited, after reduction in H,, pronounced selectivity in propane aromatization. More recently, Karge et al. [361 have shown that also noble metals can be easily introduced into zeolites via solid-state reaction. Various zeolites, such a s NH4-Y, US-Y,
63
H-MOR and H-ZSM-5, and noble metal compounds (PdCl,, Pd(NO,),, PdO, PtCl,, orPtC1,) were used. It was demonstrated with the help of several techniques (IR, TPDA, TPE etc.) that the noble metal cations upon solid-state reaction occupy cation sites inside the zeolite structure. After reduction in H, the thus-obtained materials possessed hydrogenation properties. Provided a suitable balance between the acid function (residual acidic OH groups) and the hydrogenation function (noble metal aggregates) was established, these catalysts were efficient in hydroisomerisation of, for instance, ethylbenzene. Role of Water i n Solid-state Ion Exchange In most cases solid-state ion exchange in zeolites was conducted in the presence of ambient moisture or residual water vapour. However, it was shown that this type of exchange also occurs whenever traces of water are carefully excluded [ 121. Moreover, solid-state ion exchange into zeolites was also achieved with compounds insoluble in water, e.g. with AgCl o r Hg,Cl,. This suggests that the presence of residual water is not necessarily a prerequisite for the solid-state ion exchange in zeolites to occur, even though small amounts of water such as the crystal water might facilitate the low-temperature solid-state reaction (vide supra). However, more subtle details of solid-state ion exchange in zeolites as, for instance, the particular mechanism of ion migration remain a mystery, and their clarification needs further experimental work. Acknowledgment Financial support by the Bundesminister fur Forschung und Technologie (BMFT, Project No. 03C 257 A7) is gratefully acknowledged.
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11 H.K. Beyer, H.G. Karge and G. Borbely, Zeolites 8 (1988) 79-82. 12 H.G. Karge, V. Mavrodinova, 2. Zheng and H.K. Beyer, in D. Barthomeuf, E.G. Derouane and W. Holderich (Editors), "Guidelines for Mastering the Properties of Molecular Sieves", NATO AS1 Series, Series B, Physics Vol. 221, Plenum Press, New York, 1990, pp. 157-168. 13 G. Borbely, H.K. Beyer, L. Radics, P. SBndor and H.G. Karge, Zeolites 9 (1989) 428-431. 14 S. Beran, B. Wichterlovh and H.G. Karge, J. Chem. SOC.Faraday Trans. I 86 (1990)3033-3037. 15 R. Schollner, P. Nobel, H. Herden and G. Korner, in P. Fejes (Editor), Proc. Symp. on Zeolites, Szeged, Hungary, Sept. 11-14,1978, Acta Universitatis Szegediensis, Acta Physica e t Chemica, Nova Series 24 (1978) 293-298. 16 H.K. Beyer and I. Belenykaja, in B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud (Editors), Proc. Int. Symp."Catalysis by Zeolites", Ecull (Lyon), France, Sept. 9-11, 1980; Elsevier, Amsterdam, 1980; Stud. Surf. Sci. 5 (1980)203-210. 17 H.K. Beyer, I.M. Belenykaja, F. Hange, M.Tielen, P.J. Grobet and P.A. Jacobs, J. Chem. SOC. Faraday Trans. I81 (1985) 2889-2901. 18 B. Sulikowski, G. Borbely, H.K. Beyer, H.G. Karge and I.W. Mishin, J. Phys. Chem. 93 (1989)3240-3243. 19 H.G. Karge, H.K. Beyer and G. Borbely, Catalysis Today 3 (1988)41-52. 20 A.E. Hirschler, J. Catal. 2 (1963) 428-439. 21 C.J. Plank, in W.M. Sachtler, G.C.A. Schuit and P. Zwietering (Editors), Proc. 3rd Congress on Catalysis, Amsterdam, The Netherlands, July 20-25, 1964, NorthHolland Publ. Comp., Amsterdam, 1965, pp. 727-728. 22 D. Keir, E.F.T. Lee and L.V.C. Rees, Zeolites 8 (1988) 228-231. 23 S. HoEevar and B. Dr&aj,in L.V.C. Rees (Editor), Proc. 5th Int. Conf. Zeolites, Na les, Italy, June 2-6,1980,Heyden, London, 1980, pp. 301-310. 24 H.8. Karge, G. Borbely, H.K. Beyer and G. Onyestyhk, in M.J. Philips and M. Ternan (Editors), Proc. 9th Int. Congress on Catalysis,Calgary, Ottawa, Canada, June26-July 1,1988, Chemical Institute of Canada, Ottawa, 1988, pp. 396-403. 25 H.G. Karge and H.K. Beyer, in DGMK-Berichte-Tagungsbericht 9101, DGMKFachbereichstagung "Cl-Chemie - Angewandte Heterogene Katalyse - C4Chemie", Leipzig, FRG, Febr. 20-22, 1991, ISBN No. 3-928164-07-4, ISSN No. 0938-068 X, pp. 191-206. (English Version to be published in Erdol & Kohle, Erdgas, Petrochemie). 26 A.V. Kucherov and A.A. Slinkin, Zeolites 6 (1986) 175-180. 27 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38- 42. 28 A.V. Kucherov and A.A. Slinkin, Zeolites 8 (1988) 110-116. 29 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987)43-46. 30 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. 31 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 583-584. 32 Y. Yang, X. Guo, M. Deng, L. Wang and Z. Fu, in H.G. Karge and J. Weitkamp (Editors), Proc. Int. Symp. "Zeolites as Catalysts, Sorbents and Detergent Builders-Applications and Innovations",Wiirzburg, FRG,Sept. 4-8,1988;Elsevier, Amsterdam, 1989; Studies Surface Sci. Catalysis 46 (1989) 849-858. Faraday 33 A.V. Kucherov, A.A. Slinkin, H.K. Beyer and G. Borbely, J. Chem. SOC. Trans. I, 85 (1989) 2737-2747. 34 B. Wichterlovh, S. Beran, S. BednaFovh, K. NedomovB, L. Dudikovh and P. J iru in P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff (Editors), Proc. Int. Symp. "Innovation in Zeolite Materials Science", Nieuwpoort, Belgium, Sept. 13-17,1987, Elsevier, Amsterdam; Studies Surf. Sci. Catalysis 37 (1988) 199-206. 35 8. Wichterlovh, S. Beran, L. Kubelkovh, J, NovhkovB, A. SmiegkovA and R. Sebik, in H.G. Karge and J. Weitkamp (Editors), Proc. Int. Symp. "Zeolites as Catalysts, Sorbents and Detergent Builders - Applications and Innovations", Wurzburg, FRG, Sept. 4-8, 1988, Elsevier, Amsterdam, 1989; Studies Surf. Sci. Catalysis 46 (1989)347-353. H.G. Kame. Y. Zhanp and H.K. Bever. Dublication in DreDaration.
catalysis