- Email: [email protected]
Infrared spectroscopic study of adsorption and reactions of methyl chloride on acidic, neutral and basic zeolites
I APPL’E CATALYSIS
6 ENVlRONMEKlAL
Applied Catalysis B: Environmental
8 ( 1996) 391-404
Infrared spectroscopic study of adsorption and reactions of methyl chloride on acidic, neutral and basic zeolites Z. Khya, Applied Chemisty Received
I. Hannus, I. Kiricsi
*
Department, Jbzsef Attila Uniuersity, H-6720 Szeged, Rerrich B&I te’r I., Hungay 16 August 1995; revised 30 November
1995; accepted
1 December
1995
Abstract Infrared (IR) spectroscopic investigations were performed of the adsorption and surface reactions of CH,Cl over acidic, basic and neutral zeolite catalysts. In each case, the spectral changes reflected the appearance of the Fermi resonance phenomenon. The resonance parameters determined pointed to a decreasing extent of resonance in consequence of the adsorption. No influence of the acidity of the adsorbents was observed. The formation and consumption of HCl and OH groups were established at elevated temperatures. The reactions assumed to have occurred are described and explained. Keyvords:
Infrared spectrometry,
Fourier transform;
Methyl chloride; Fermi resonance;
Y-FAU zeolite
1. Introduction Studies of the adsorption and reactions of halogen-containing hydrocarbons over solid materials as potential decomposition catalysts are significant from the aspect of hazardous waste treatment. A knowledge of the adsorption and transformation of CH,Cl, the simplest of such chloro compounds, is of basic importance. It is not surprising, therefore, that an increasing number of observations has been published during the past ten years. Certain papers deal with the adsorption of CH,Cl from a theoretical point of view, since a peculiar change occurs in the Fermi resonance interaction between v,, the C-H symmetric stretching vibration, and 2 v~, the overtone of the
* Corresponding
author. Tel./fax.
( + 36-62) 321523, e-mail [email protected]
0926.3373/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0926-3373(95100083-6
392
Z. Kbnya et al./Applied
Catalysis B: Environmental 8 (1996) 391-404
asymmetric deformation of CH, upon adsorption. Crowell et al. [I] described the spectral changes taking place during the adsorption of CH,Cl on alumina. In a recent paper, McGee et al. [2] presented the resonance parameters for methyl halides adsorbed on silica. Other authors investigated the adsorption of CH,Cl in order to acquire an insight into the formation of methoxy species on different oxides [3]. These species are assumed to be the intermediates in the reaction of methanol to dimethyl ether and eventually to alkenes [4]. It has also been reported that alkenes such as ethene and propene, and traces of aromatic compounds, mainly benzene, were formed from CH,Cl over zeolite catalysts above 470 K [5]. Furthermore, a process is known in which CH, is converted over a supported catalyst containing a complex mixture of CuCl, KC1 and LaCl,, first to CH,Cl, followed by the conversion of this to gasoline over a zeolite catalyst [6]. A search of the relevant literature did not yield any detailed investigation on the adsorption and transformation of CH,Cl over zeolites. The aim of our work, therefore, was to study the adsorption and decomposition of CH,Cl over zeolites of different acid-base characters. This paper reports the behaviour of methyl chloride over basic, neutral and acidic zeolite catalysts.
2. Experimental
2. I. Materials The parent zeolite was Nay-FAU with the composition Na58A158Si1340384, obtained from Union Carbide. This sample is regarded as a neutral material, since only traces of Lewis acidity were detected by pyridine adsorption [7]. HY-FAU, the acidic form, was prepared from Nay-FAU by ion-exchange in 0.1 M NH&l solution. After repeating the ion-exchange procedure three times, the solid material was separated by filtration and was washed to become Cl--free, followed by drying in air. The unit cell composition of this zeolite was HY-FAU was obtained from NH,Y-FAU by deam(NH,),,Na,,Al**Si,,,0,,,. monization at 723 K under vacuum, carried out in situ in an infrared (IR) cell. This sample exhibited both Brijnsted and Lewis acidity, as shown by pyridine adsorption measurements. The ratio of Brijnsted to Lewis acid sites, measured as the ratio of the integrated areas of the respective pyridine bands, was 1.5, demonstrating that the sample contained more Briinsted than Lewis acid sites. For preparation of a sample of basic character, Nay-FAU was mixed with a methanolic solution of NaN,. After evaporation of the solvent, the air-dried sample contained 3.1 mmol NaN, per g zeolite. Sodium clusters as strong basic sites [8] were produced by thermal decomposition of NaN, in the zeolite at 723 K under vacuum in the IR cell [9].
Z. Kbnya et al. /Applied Catalysis B: Enuironmental8 (1996) 391-404
Methyl chloride (Aldrich, without further purification.
99.5 + %) and HCl (Aldrich,
393
99 + %) were used
2.2. Spectroscopy For the in situ IR spectroscopic measurements, self-supported wafers with a thickness of 10 mg/cm* were pressed and outgassed at 723 K in the IR cell. After cooling to the desired (generally ambient) temperature, CH,Cl or HCl, the product of its surface transformation, was adsorbed and the adsorption was monitored by IR spectroscopy. In each experiment, 10 Torr of adsorbate was used. The spectra were run on a Matson Genesis Fourier transform (ET)-IR spectrophotometer. Samples were treated in the presence of adsorbate (or in vacuum) at different temperatures. In some cases, the displayed spectra were corrected for the basic spectrum of the zeolite, and are referred to as difference spectra.
3. Results 3.1. Adsorption
on Nay-FAU
Fig. 1 shows the spectra of adsorbed CH,Cl at room temperature for increasing contact time and at increasing temperatures. Band characteristics of physisorbed CH,Cl are seen at 3050, 2966, 2862, 1443 and 1351 cm-‘. These bands are assigned as follows: v,, asymmetric C-H stretching; vi, symmetric C-H stretching; 2v,, overtone; v5, asymmetric C-H deformation; and v2, symmetric C-H deformation vibration. The C-Cl band cannot be detected in the adsorbed phase because of the very intense absorption of the zeolite framework below 1200 cm- ’. The intensity of the bands increased slightly with time at room temperature. Evacuation at room temperature resulted in the disappearance of all bands (not shown), indicating that only physisorption took place. Heat treatment of the wafer in the presence of CH,Cl vapour induced irreversible changes. With increasing temperature, the intensities of the bands decreased. Furthermore, the band at 1443 cm-’ broadened on the high-frequency side and shifted somewhat to higher wavenumbers ( Av = 10 cm-‘). Meanwhile, a new band appeared at 1647 cm- ’, as can be seen in Fig. 1B. Surprisingly, a weak absorption indicating the formation of OH groups developed at 3650 cm-’ in the spectrum registered after heat treatment at 573 K for 30 min (not shown). 3.2. Adsorption
on basic sample, Na/NaY-FAU
The adsorption bands developing
of CH,Cl at 3055,
on the sample containing sodium clusters led to 2963, 2864, 1636, 1447 and 1349 cm-‘. The
394
T r a n S m i t t a ” c e
2. Khya et al/Applied
Catalysis B: Environmental 8 (1996) 391-404
-:/--\/I-
T r a ” s m i t t a ” c e
m
---+--Ta
B
A
3100
3000
2900
Wavenumbers
1600
1500
400
> 1300
Wavenumbers
Fig. 1. Difference spectra of CH,Cl adsorbed on NaY-FAU zeolite at different temperatures; (a) immediately after adsorption at room temperature, (b) after 30 min, (c) at 373 K, (d) at 473 K, (e) at 573 K.
intensities of the two latter absorptions increased slightly with increasing contact time at room temperature. The same trend was established for the bands in the C-H stretching region, but the shapes of these bands were not similar to those observed for the neutral sample. As the inset in Fig. 2B shows, the integrated area of the absorption at 1636 cm-’ first increased rapidly and then levelled off. With increasing temperature, the intensities of the bands in general decreased. The only exception was the band at 1636 cm- *, which exhibited an increasing intensity (Fig. 2B). Here again, a small shift to higher wavenumbers was found for the band appearing at 1447 cm-’ at room temperature. 3.3. Adsorption
on acidic sample, HY-FAU
No substantial changes can be seen in the spectra registered at room temperature after prolonged contact times (see spectra a and b in Fig. 3). However, the
Z. Khya et al. /Applied Catalysis B: Enuironmental8 (1996) 391-404
395
T T a n s m
T r
a n
s In i
i t t a ” c e
t t
a n
c e
___m_7
3100
3000
2900
2800
_
1700
1600
1500
__.-,J
1400
1300
W%tXXlUlbXS
Fig. 2. Difference spectra of CH,Cl adsorbed on Na/NaY-FAU at different temperatures; (a) at room temperature, (b) at 373 K, (c) at 573 K. The inset shows the development of the band at 1636 cm-’ at room temperature with time (min).
general features of the spectra were essentially different from those obtained for the neutral and the basic samples. The band at 1349 cm-’ is sharp, but the shape of the absorption at 1447 cm- ’ is much broader than for the neutral or the basic sample. These spectra are a combination of the spectra of the adsorbed and the gas phase (with a rotational band structure). On this zeolite (as compared with the neutral and basic samples), the third band, near the C-H deformation region, is situated at the lowest wavenumber, appearing at 1623 cm- ’. The bands in the region of C-H deformation vibrations did not change much with time at room temperature. A unique feature of the adsorption on the acidic sample is the involvement of various OH groups in the adsorption and reaction of CH,Cl. In the spectrum of the acidic zeolite pretreated at 723 K in vacua, three absorptions were obtained,
3800
3600
t”. , , 3200
3000
,., , .,*
3400
109
1700
1500 W~Venumbers
1600
1400
1300
Fig. 3. (A) Spectra of CH,Cl adsorbed on HY-FAU zeolite at different temperatures; (a,) activated zeolite before adsorption, (a) immediately temperature, (b) 1 h later, (c) at 400 K, (d) at 473 K, (e) at 573 K, (f) at 673 K. (B) Difference spectra under identical conditions.
a n c e
i t t a n c e
In
a n s
I
T
after adsorption
at room
Z. Kbnya et al. /Applied Catalysis B: Enuironmental8 (19961391-404
397
at 3736, 3641 and 3545 cm-’ (spectrum a, in Fig. 3A). These are assigned, respectively, as a non-acidic terminal SiOH, and two acidic, bridging OH groups sited in super- and sodalite cages. Upon CH,Cl adsorption on the wafer, the intensities of the OH bands gradually decreased and they shifted to lower wavenumbers (Fig. 3A). The frequency shift measured from the acidic OH groups was 300-400 cm-‘, indicating the involvement of a very strong hydrogen bond or even probably a weak chemical interaction between the OH groups and CH,Cl. The decrease in the intensity of the SiOH band at 3736 cm-’ is evidence of the involvement of the neutral silanol groups in a similar interaction. Evacuation after the adsorption of CH,Cl at room temperature for 1 h did not lead to the disappearance of the absorption bands (not shown in the figure). Adsorption at increasing temperatures resulted in essential changes in the spectra, as shown in Fig. 3. First, the band at 1349 cm-’ disappeared after treatment at 400 K, accompanied by partial restoration of the spectral feature in
3500
3000
2500
2000
1500
Wavenumbers Fig. 4. Spectra of HCl adsorbed on NaY-FAU zeolite at different temperatures; (a) spectrum of activated zeolite, (b) HCl adsorbed at room temperature, (c) after treatment at 373 K, (d) at 473 K, (e) at 573 K, (f) at 673 K.
398
Z. Kbnya et al/Applied
Catalysis B: Environmental 8 (1996) 391-404
the OH range. Secondly, the spectra became similar to the gas-phase spectrum of CH,Cl, particularly in the C-H deformation region. However, the spectrum after treatment at 673 K (see spectrum f in Fig. 3B) was identical to that generally observed upon alkene adsorption on acidic zeolites [lo]. Thirdly, the OH vibrations were no longer as intense as for the original sample after treatment of the wafer in the presence of CH,Cl vapour above room temperature. For this acidic sample, the formation of HCl was detected by analysing the gas phase by means of IR spectroscopy. 3.4. HCl adsorption In order to ensure the proper assignment of the IR bands observed during CH,Cl adsorption, control experiments were performed with HCl as adsorbate.
T I
a n
s m
i t t a n c e
Fig. 5. Spectra of HCl adsorbed on HY-FAU zeolite at different temperatures; (a) spectrum of activated zeolite, (b) HCl adsorbed at room temperature, (c) at 423 K, (d) at 573 K, (e) at 723 K. (Inset shows the development of the band around 910 cm -’ for identical conditions.)
Z. K6nya er al. /Applied Catalysis B: Environmental 8 (1996) 391-404
399
Upon the adsorption of HCl on Nay-FAU zeolite at room temperature, the bridging OH bands developed and the terminal SiOH bands disappeared, as can be seen in Fig. 4. At high temperature, the broad band centred at around 2700 cm-‘, characteristic of adsorbed HCl [ 111, became flatter, and simultaneously the OH bands became clearly visible. Furthermore, weak bands developed in the range 3 100-2700 cm- ‘, as an indication of the presence of HCl in the gas phase. The adsorption of HCl on the basic sample resulted in the development of a band at around 2700 cm- ‘, characteristic of adsorbed HCl. After evacuation at room temperature, the spectrum did not regain its original shape. On HY-FAU, the adsorption of HCl led to the disappearance of the band of terminal SiOH groups at 3736 cm- ‘. The intensity of the bridging OH absorptions simultaneously decreased, and they finally disappeared above 573 K (Fig. 5).
4. Discussion 4.1. Interaction
at room temperature
The adsorption of CH,Cl at room temperature results in the formation of hydrogen-bonded surface complexes, particularly over materials possessing OH groups on their surfaces. In these cases, the bands due to the OH groups generally shift to lower wavenumbers as a consequence of adsorption interactions. This was observed for the HY-FAU sample. Since this sample contains three different types of OH groups, the assignment of the shifted bands is rather difficult. If we choose the band at 3641 cm- ’ (vibration of OH groups sited in the super cage) as reference and measure the shifts relative to the position of this band, the estimated shift of 400 cm-‘, caused by the adsorption of CH,Cl, indicates a strong interaction. Basila [12] assumed the formation of a hydrogenbonded complex upon the adsorption of CH,Cl on silica, since the observed shift in the frequency of the OH groups was as low as 106 cm-‘. Hertl and Hair [13] reported an even smaller shift ( Av = 25 cm-‘). McGee et al. [2] showed that CH,Cl adsorption causes a shift of 124 cm -’ in the band position of OH on silica. Crowell et al. [l] measured a 136 cm-’ shift in the OH vibration frequency of alumina. Since the HY-FAU zeolite is the most acidic of the listed adsorbents, the highest shift we observed seems reasonable. As far as the type of bonding is concerned, the adsorption of CH,Cl at room temperature proved reversible only for Nay-FAU, since the surface species formed on both the basic and the acidic sample could not be desorbed completely by evacuation even above room temperature. It follows from this that the adsorption of CH,Cl on HY-FAU and Na/NaY-FAU generates not only
400
Z. Kbnya et al/Applied
hydrogen bonds; real chemical adsorbent may also take place.
Catalysis B: Emironmental
interactions
8 (1996) 391-404
between
the adsorbate
and the
4.2. Fermi resonance In the spectrum of the CH,Cl molecule, an unusual spectral feature, the Fermi resonance phenomenon, is generally observed. Fermi resonance occurs between vl, the C-H symmetric stretching, and 2 v5, the first overtone of the asymmetric C-H deformation levels [14]. This phenomenon is observed for other molecules too [15]. The question arises of whether this resonance is preserved after adsorption. Crowell et al. [I] first investigated this problem in the course of the adsorption of CH,Cl on an alumina surface. They concluded that Fermi resonance can be observed even in the adsorbed phase, but the interaction between the energy levels is weaker than in the gas phase. McGee et al. [2] showed that, in addition to the positions of the bands, their intensities can also be used to characterize the extent of Fermi resonance in methyl halides. These two teams studied strongly acidic (alumina) and slightly acidic (silica) adsorbents. We investigated the adsorption of CH,Cl on acidic, neutral and basic adsorbents. Spectra recorded under identical experimental conditions are depicted in Fig. 6. It is seen that the shapes of the spectra are different. For Nay-FAU, it is clear that the ratio of the intensities of v1 to 2 us increases upon adsorption. The same phenomenon is less pronounced for HY- and Na/NaYFAU. The calculated Fermi resonance parameters are listed in Table 1. Values calculated from the data of Herzberg [14] and those published by Crowell et al. [I] and McGee et al. [2] are also included. Since the A, W, and Wp values for the different adsorbates are very close to each other, and fairly independent of the acidic character of the adsorbents, the resonance interactions can be assumed to be similar. This follows from the studies of CH,Cl adsorption on alumina and silica and from comparisons of the calculated Fermi resonance parameters [ 1,2]. The second feature is that the extent of the resonance is markedly decreased relative to that for the gas-phase molecules. Table 1 shows that A increased from 12 to 28 as a consequence of adsorption on Nay-FAU. The smaller value of A means more complete resonance. This rule is somewhat confusingly described in [l]. 4.3. Interactions
occurring above ambient temperature
The results showed that the adsorption of CH,Cl on zeolites led to its decomposition at (for HY and Na/NaY) and above (for NaY) ambient temperature. In the first step, the formation of surface methoxy groups is to be expected, as described by the following reactions: -OH + Cl-CH, + -0-CH, + HCl
401
Z. Khya et al. /Applied Catalysis B: Enuironmental8 (1996) 391-404
1 r a n s m
i t
t a n c e
3100
3000
2900
1700
2800
WaVtXlUmb05
1600
1500
1400
1300
Wavenumbers
Fig. 6. Spectra of CH,CI in gas phase and in adsorbed phase, used to estimate Fermi resonance parameters; (a) gas-phase spectrum, (b) adsorbed on HY-FAU, (c) on Nay-FAU, (d) on Na/NaY-FAU at room temperature after 30 min (difference spectra).
and or -0Na
+ Cl-CH,
+ -0-CH,
+ NaCl
These equations reveal two important problems. First, the formation of methoxy species is accompanied by the consumption of OH groups. Several spectroscopic studies have been carried out on the formation of surface methoxy species [ 161. As a general conclusion, it can be stated that the formation of surface methoxy was unequivocally proved over materials possessing OH groups serving as reaction partners for CH,Cl. Beebe et al. [3] assigned the 2960 cm-’ band to the asymmetric C-H, and the 2849 cm-’ band to the symmetric C-H stretching vibration of the methoxy species. These values are very close to the respective vibration of adsorbed CH,Cl. This can be readily understood, since in both physisorbed CH,Cl and the methoxy species, the bonds in the CH, group are not strongly influenced by the substitution of the 0 for Cl.
402 Table 1 Characteristic
Z. K&ya et al./Applied
absorption
bands of CH,Cl
Catalysis B: Enuironmental8
and calculated
Gas phase
Fermi resonance parameters A b= w, % E, Et3
parameters
Adsorbed on zeolites
Adsorbed on
This work
HY
NaY
Na/NaY
SiO, [2]
Al,O,
1355 1455 2879 2966 3042
2966 3046
1349 1447 2865 2965 3054
1351 1443 2862 2966 3050
1349 1447 2864 2963 3055
1351 1445 2865 2965 3039
1352 1446 2861 2965 3044
12.5 3706 56 -31 2965 2879
12 3128 62 -28 2976 2886
21 4749 71 -29 2965 2865
28 5016 80 -24 2966 2862
19.5 4710 69 -30 2963 2864
25 4688 75 -31 2965 2865
21 5292 73 -31 2965 2861
I141 Vibrational mode va, sym. CH, def. v~, asym. CH, def. 2 vg, overtone v,, sym. CH stretch. v4, asym. CH stretch.
Fermi resonance
(1996) 391-404
[l]
a
a The energy levels of the two vibrational bands (labelled LYand /3) were calculated via the following Eq. [2]: E, =(2X v,)+ W,; Ea =(2X v,)+ Wp where W, = A2 +[A2/4+ b2/2]‘12; Wa = A2 -[A2/4+ b2/2]‘12 and A=[(~v, + v,)/2-(2X v,)]; b2 =[(v, -2~s)~ - A2]/2.
Secondly, the formation of HCl is obvious experimental evidence of the generation of methoxy groups on the surface. HCl is formed most easily on HY-FAU, whereas its formation is most difficult on the sodium-cluster-containing Na/NaY. For Nay-FAU, only the reaction between the terminal SiOH groups and CH,Cl leads to the formation of HCl. In this case, NaCl formation accompanies the generation of methoxy groups. HCl formation takes place above 400 K on alumina upon reaction with CH,Cl, as reported by Beebe et al. [3]. Since HY-FAU zeolite is more acidic than alumina or silica [17], readier formation of surface methoxy groups and concomitant HCl generation were expected, and this was found in our experiments. The formation of HCl in the adsorbed phase involves the risk of framework destruction, since HCl is able to break both Si-0 and Al-O bonds. This is particularly true in the presence of water. For HY-FAU, this was actually the case, as evidenced by the formation of water and =Si-0-Si= through the reaction of HCl and OH groups. The experimental evidence that this reaction occurs is seen in Fig. 5, where the band at 3735 cm-’ due to SiOH groups disappears upon HCl 5dsorption while a new band develops around 910 cm- ’ due to the partial dealumination of the framework (see the inset in Fig. 5). In fact, the neutral SiOH group acts as a basic site towards such a strongly acidic molecule as HCl. As far as the situation over Nay-FAU is concerned, the development of OH bands was found (see Fig. 4). This feature can be explained by the reaction of HCl with Naf, resulting in the formation of NaCl and OH groups.
Z. Kbnya et al/Applied
Catalysis B: Environmental 8 (19961 391-404
403
In summary, the adsorption of CH,Cl over acidic zeolite leads to the formation of surface methoxy groups and HCl. Because of its stronger adsorption, HCl remains on the surface, hindering further CH,Cl adsorption by occupying the adsorption centres. Therefore, much less CH,Cl can adsorb and more remains in the gas phase, the gas-phase spectrum overlapping the spectrum of the adsorbed molecules. This feature was seen in the spectra of CH,Cl over HY-FAU (compare the spectra in Fig. 6). The role of water as a product also seems to be important, since it likewise adsorbs very strongly on zeolites, resulting in decreasing adsorption capacities. At higher temperatures, water could bring about the collapse of the framework of Y-type zeolite. However, the highly acidic medium locally formed from H,O and HCl in the surroundings of the adsorption centre is even more destructive. On the basis of these spectroscopic results, a catalytic reactor system has been designed and kinetic experiments are in progress.
5. Conclusions Upon the adsorption of CH,Cl on each of the samples investigated, the Fermi resonance phenomenon that appears in the gas-phase spectrum is preserved in the adsorbed phase, but the extent of the resonance was decreased. This decrease proved to be independent of the acid-base character of the adsorbent. Good agreement was found between the Fermi resonance parameters published in the literature and those calculated from our spectra. The generation of surface methoxy species accompanied by HCl or NaCl formation is assumed for the interaction observed in the spectral changes. At elevated temperatures, reactions leading to the collapse of the zeolite framework may also take place.
Acknowledgements Financial support from the National Science Foundation of Hungary (OTKA T 07577/93) and stimulating discussions with Prof. A. Molnar are gratefully acknowledged.
References [l] [2] [3] [4]
J.E. Crowell, T.P. Beebe, Jr. K.C. McGee, A.T. Capitano T.P. Beebe, Jr., J.E. Crowell T.R. Forester and R.F. Howe, Trans., 70 (1974) 1527.
and J.T. Yates, Jr., and V.H. Grassian, and J.T. Yates, Jr., J. Am. Chem. Sot.,
J. Chem. Phys., 87 (1987) 3668. Langmuir, 10 (1994) 632. J. Phys. Chem., 92 (1988) 1296. 109 (1987) 5076; B.A. Morrow, J. Chem. Sot. Faraday
404
Z. K6nya et al/Applied
Catalysis B: Environmental 8 (1996) 391-404
[5] V.N. Romannikov and K.G. Ione, Kinet. Katal., 25 (1984) 92. 161 C.E. Taylor and R.P. Noceti, in Proceedings of the 9th International Congress on Catalysis, Calgary, 1988, Vol. 4., p. 990. [7] I. Kiricsi, Gy. Tasi, H. Fdrster and P. Fejes, Acta Phys. Chem. Szeged, 33 (1987) 69. [8] L.R.M. Martens, P.J. Grobet and P.A. Jacobs, Nature, 315 (1985) 568. [9] I. Hannus, Gy. Tasi, I. Kiricsi, J.B. Nagy, H. Forster and P. Fejes, Thennochim. Acta, 249 (1995) 285. [IO] L. Kubelkova, J. Novakova, B. Wichterlova and P. Jiru, Coll. Czeh. Chem. Commun., 45 (1980) 2290; L. Kubelkova, J. Novakova, Z. Dolejsek and P. Jiru, Coll. Czeh. Chem. Commun., 45 (1980) 3101. [ll] G.A. Ozin, S. Gzkar and G.D. Stucky, J. Phys. Chem., 94 (1990) 7562. [12] M.R. Basila, J. Chem. Phys., 35 (1961) 1151. [13] W. Hertl and M.L. Hair, J. Phys. Chem., 72 (1968) 4676. [14] G. Herzberg, Molekula-Szinkepek II, Akademia Kiad6, Budapest, 1959. [15] I. Hannus, I. Kiricsi, Gy. Tasi and P. Fejes, Appl. Catal., 66 (1990) L7. [16] V. Bosacek, J. Phys. Chem., 97 (1993) 10732. [17] B. Umansky, J. Engelhardt and W.K. Hall, J. Catal., 127 (1991) 128.
6 ENVlRONMEKlAL
Applied Catalysis B: Environmental
8 ( 1996) 391-404
Infrared spectroscopic study of adsorption and reactions of methyl chloride on acidic, neutral and basic zeolites Z. Khya, Applied Chemisty Received
I. Hannus, I. Kiricsi
*
Department, Jbzsef Attila Uniuersity, H-6720 Szeged, Rerrich B&I te’r I., Hungay 16 August 1995; revised 30 November
1995; accepted
1 December
1995
Abstract Infrared (IR) spectroscopic investigations were performed of the adsorption and surface reactions of CH,Cl over acidic, basic and neutral zeolite catalysts. In each case, the spectral changes reflected the appearance of the Fermi resonance phenomenon. The resonance parameters determined pointed to a decreasing extent of resonance in consequence of the adsorption. No influence of the acidity of the adsorbents was observed. The formation and consumption of HCl and OH groups were established at elevated temperatures. The reactions assumed to have occurred are described and explained. Keyvords:
Infrared spectrometry,
Fourier transform;
Methyl chloride; Fermi resonance;
Y-FAU zeolite
1. Introduction Studies of the adsorption and reactions of halogen-containing hydrocarbons over solid materials as potential decomposition catalysts are significant from the aspect of hazardous waste treatment. A knowledge of the adsorption and transformation of CH,Cl, the simplest of such chloro compounds, is of basic importance. It is not surprising, therefore, that an increasing number of observations has been published during the past ten years. Certain papers deal with the adsorption of CH,Cl from a theoretical point of view, since a peculiar change occurs in the Fermi resonance interaction between v,, the C-H symmetric stretching vibration, and 2 v~, the overtone of the
* Corresponding
author. Tel./fax.
( + 36-62) 321523, e-mail [email protected]
0926.3373/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0926-3373(95100083-6
392
Z. Kbnya et al./Applied
Catalysis B: Environmental 8 (1996) 391-404
asymmetric deformation of CH, upon adsorption. Crowell et al. [I] described the spectral changes taking place during the adsorption of CH,Cl on alumina. In a recent paper, McGee et al. [2] presented the resonance parameters for methyl halides adsorbed on silica. Other authors investigated the adsorption of CH,Cl in order to acquire an insight into the formation of methoxy species on different oxides [3]. These species are assumed to be the intermediates in the reaction of methanol to dimethyl ether and eventually to alkenes [4]. It has also been reported that alkenes such as ethene and propene, and traces of aromatic compounds, mainly benzene, were formed from CH,Cl over zeolite catalysts above 470 K [5]. Furthermore, a process is known in which CH, is converted over a supported catalyst containing a complex mixture of CuCl, KC1 and LaCl,, first to CH,Cl, followed by the conversion of this to gasoline over a zeolite catalyst [6]. A search of the relevant literature did not yield any detailed investigation on the adsorption and transformation of CH,Cl over zeolites. The aim of our work, therefore, was to study the adsorption and decomposition of CH,Cl over zeolites of different acid-base characters. This paper reports the behaviour of methyl chloride over basic, neutral and acidic zeolite catalysts.
2. Experimental
2. I. Materials The parent zeolite was Nay-FAU with the composition Na58A158Si1340384, obtained from Union Carbide. This sample is regarded as a neutral material, since only traces of Lewis acidity were detected by pyridine adsorption [7]. HY-FAU, the acidic form, was prepared from Nay-FAU by ion-exchange in 0.1 M NH&l solution. After repeating the ion-exchange procedure three times, the solid material was separated by filtration and was washed to become Cl--free, followed by drying in air. The unit cell composition of this zeolite was HY-FAU was obtained from NH,Y-FAU by deam(NH,),,Na,,Al**Si,,,0,,,. monization at 723 K under vacuum, carried out in situ in an infrared (IR) cell. This sample exhibited both Brijnsted and Lewis acidity, as shown by pyridine adsorption measurements. The ratio of Brijnsted to Lewis acid sites, measured as the ratio of the integrated areas of the respective pyridine bands, was 1.5, demonstrating that the sample contained more Briinsted than Lewis acid sites. For preparation of a sample of basic character, Nay-FAU was mixed with a methanolic solution of NaN,. After evaporation of the solvent, the air-dried sample contained 3.1 mmol NaN, per g zeolite. Sodium clusters as strong basic sites [8] were produced by thermal decomposition of NaN, in the zeolite at 723 K under vacuum in the IR cell [9].
Z. Kbnya et al. /Applied Catalysis B: Enuironmental8 (1996) 391-404
Methyl chloride (Aldrich, without further purification.
99.5 + %) and HCl (Aldrich,
393
99 + %) were used
2.2. Spectroscopy For the in situ IR spectroscopic measurements, self-supported wafers with a thickness of 10 mg/cm* were pressed and outgassed at 723 K in the IR cell. After cooling to the desired (generally ambient) temperature, CH,Cl or HCl, the product of its surface transformation, was adsorbed and the adsorption was monitored by IR spectroscopy. In each experiment, 10 Torr of adsorbate was used. The spectra were run on a Matson Genesis Fourier transform (ET)-IR spectrophotometer. Samples were treated in the presence of adsorbate (or in vacuum) at different temperatures. In some cases, the displayed spectra were corrected for the basic spectrum of the zeolite, and are referred to as difference spectra.
3. Results 3.1. Adsorption
on Nay-FAU
Fig. 1 shows the spectra of adsorbed CH,Cl at room temperature for increasing contact time and at increasing temperatures. Band characteristics of physisorbed CH,Cl are seen at 3050, 2966, 2862, 1443 and 1351 cm-‘. These bands are assigned as follows: v,, asymmetric C-H stretching; vi, symmetric C-H stretching; 2v,, overtone; v5, asymmetric C-H deformation; and v2, symmetric C-H deformation vibration. The C-Cl band cannot be detected in the adsorbed phase because of the very intense absorption of the zeolite framework below 1200 cm- ’. The intensity of the bands increased slightly with time at room temperature. Evacuation at room temperature resulted in the disappearance of all bands (not shown), indicating that only physisorption took place. Heat treatment of the wafer in the presence of CH,Cl vapour induced irreversible changes. With increasing temperature, the intensities of the bands decreased. Furthermore, the band at 1443 cm-’ broadened on the high-frequency side and shifted somewhat to higher wavenumbers ( Av = 10 cm-‘). Meanwhile, a new band appeared at 1647 cm- ’, as can be seen in Fig. 1B. Surprisingly, a weak absorption indicating the formation of OH groups developed at 3650 cm-’ in the spectrum registered after heat treatment at 573 K for 30 min (not shown). 3.2. Adsorption
on basic sample, Na/NaY-FAU
The adsorption bands developing
of CH,Cl at 3055,
on the sample containing sodium clusters led to 2963, 2864, 1636, 1447 and 1349 cm-‘. The
394
T r a n S m i t t a ” c e
2. Khya et al/Applied
Catalysis B: Environmental 8 (1996) 391-404
-:/--\/I-
T r a ” s m i t t a ” c e
m
---+--Ta
B
A
3100
3000
2900
Wavenumbers
1600
1500
400
> 1300
Wavenumbers
Fig. 1. Difference spectra of CH,Cl adsorbed on NaY-FAU zeolite at different temperatures; (a) immediately after adsorption at room temperature, (b) after 30 min, (c) at 373 K, (d) at 473 K, (e) at 573 K.
intensities of the two latter absorptions increased slightly with increasing contact time at room temperature. The same trend was established for the bands in the C-H stretching region, but the shapes of these bands were not similar to those observed for the neutral sample. As the inset in Fig. 2B shows, the integrated area of the absorption at 1636 cm-’ first increased rapidly and then levelled off. With increasing temperature, the intensities of the bands in general decreased. The only exception was the band at 1636 cm- *, which exhibited an increasing intensity (Fig. 2B). Here again, a small shift to higher wavenumbers was found for the band appearing at 1447 cm-’ at room temperature. 3.3. Adsorption
on acidic sample, HY-FAU
No substantial changes can be seen in the spectra registered at room temperature after prolonged contact times (see spectra a and b in Fig. 3). However, the
Z. Khya et al. /Applied Catalysis B: Enuironmental8 (1996) 391-404
395
T T a n s m
T r
a n
s In i
i t t a ” c e
t t
a n
c e
___m_7
3100
3000
2900
2800
_
1700
1600
1500
__.-,J
1400
1300
W%tXXlUlbXS
Fig. 2. Difference spectra of CH,Cl adsorbed on Na/NaY-FAU at different temperatures; (a) at room temperature, (b) at 373 K, (c) at 573 K. The inset shows the development of the band at 1636 cm-’ at room temperature with time (min).
general features of the spectra were essentially different from those obtained for the neutral and the basic samples. The band at 1349 cm-’ is sharp, but the shape of the absorption at 1447 cm- ’ is much broader than for the neutral or the basic sample. These spectra are a combination of the spectra of the adsorbed and the gas phase (with a rotational band structure). On this zeolite (as compared with the neutral and basic samples), the third band, near the C-H deformation region, is situated at the lowest wavenumber, appearing at 1623 cm- ’. The bands in the region of C-H deformation vibrations did not change much with time at room temperature. A unique feature of the adsorption on the acidic sample is the involvement of various OH groups in the adsorption and reaction of CH,Cl. In the spectrum of the acidic zeolite pretreated at 723 K in vacua, three absorptions were obtained,
3800
3600
t”. , , 3200
3000
,., , .,*
3400
109
1700
1500 W~Venumbers
1600
1400
1300
Fig. 3. (A) Spectra of CH,Cl adsorbed on HY-FAU zeolite at different temperatures; (a,) activated zeolite before adsorption, (a) immediately temperature, (b) 1 h later, (c) at 400 K, (d) at 473 K, (e) at 573 K, (f) at 673 K. (B) Difference spectra under identical conditions.
a n c e
i t t a n c e
In
a n s
I
T
after adsorption
at room
Z. Kbnya et al. /Applied Catalysis B: Enuironmental8 (19961391-404
397
at 3736, 3641 and 3545 cm-’ (spectrum a, in Fig. 3A). These are assigned, respectively, as a non-acidic terminal SiOH, and two acidic, bridging OH groups sited in super- and sodalite cages. Upon CH,Cl adsorption on the wafer, the intensities of the OH bands gradually decreased and they shifted to lower wavenumbers (Fig. 3A). The frequency shift measured from the acidic OH groups was 300-400 cm-‘, indicating the involvement of a very strong hydrogen bond or even probably a weak chemical interaction between the OH groups and CH,Cl. The decrease in the intensity of the SiOH band at 3736 cm-’ is evidence of the involvement of the neutral silanol groups in a similar interaction. Evacuation after the adsorption of CH,Cl at room temperature for 1 h did not lead to the disappearance of the absorption bands (not shown in the figure). Adsorption at increasing temperatures resulted in essential changes in the spectra, as shown in Fig. 3. First, the band at 1349 cm-’ disappeared after treatment at 400 K, accompanied by partial restoration of the spectral feature in
3500
3000
2500
2000
1500
Wavenumbers Fig. 4. Spectra of HCl adsorbed on NaY-FAU zeolite at different temperatures; (a) spectrum of activated zeolite, (b) HCl adsorbed at room temperature, (c) after treatment at 373 K, (d) at 473 K, (e) at 573 K, (f) at 673 K.
398
Z. Kbnya et al/Applied
Catalysis B: Environmental 8 (1996) 391-404
the OH range. Secondly, the spectra became similar to the gas-phase spectrum of CH,Cl, particularly in the C-H deformation region. However, the spectrum after treatment at 673 K (see spectrum f in Fig. 3B) was identical to that generally observed upon alkene adsorption on acidic zeolites [lo]. Thirdly, the OH vibrations were no longer as intense as for the original sample after treatment of the wafer in the presence of CH,Cl vapour above room temperature. For this acidic sample, the formation of HCl was detected by analysing the gas phase by means of IR spectroscopy. 3.4. HCl adsorption In order to ensure the proper assignment of the IR bands observed during CH,Cl adsorption, control experiments were performed with HCl as adsorbate.
T I
a n
s m
i t t a n c e
Fig. 5. Spectra of HCl adsorbed on HY-FAU zeolite at different temperatures; (a) spectrum of activated zeolite, (b) HCl adsorbed at room temperature, (c) at 423 K, (d) at 573 K, (e) at 723 K. (Inset shows the development of the band around 910 cm -’ for identical conditions.)
Z. K6nya er al. /Applied Catalysis B: Environmental 8 (1996) 391-404
399
Upon the adsorption of HCl on Nay-FAU zeolite at room temperature, the bridging OH bands developed and the terminal SiOH bands disappeared, as can be seen in Fig. 4. At high temperature, the broad band centred at around 2700 cm-‘, characteristic of adsorbed HCl [ 111, became flatter, and simultaneously the OH bands became clearly visible. Furthermore, weak bands developed in the range 3 100-2700 cm- ‘, as an indication of the presence of HCl in the gas phase. The adsorption of HCl on the basic sample resulted in the development of a band at around 2700 cm- ‘, characteristic of adsorbed HCl. After evacuation at room temperature, the spectrum did not regain its original shape. On HY-FAU, the adsorption of HCl led to the disappearance of the band of terminal SiOH groups at 3736 cm- ‘. The intensity of the bridging OH absorptions simultaneously decreased, and they finally disappeared above 573 K (Fig. 5).
4. Discussion 4.1. Interaction
at room temperature
The adsorption of CH,Cl at room temperature results in the formation of hydrogen-bonded surface complexes, particularly over materials possessing OH groups on their surfaces. In these cases, the bands due to the OH groups generally shift to lower wavenumbers as a consequence of adsorption interactions. This was observed for the HY-FAU sample. Since this sample contains three different types of OH groups, the assignment of the shifted bands is rather difficult. If we choose the band at 3641 cm- ’ (vibration of OH groups sited in the super cage) as reference and measure the shifts relative to the position of this band, the estimated shift of 400 cm-‘, caused by the adsorption of CH,Cl, indicates a strong interaction. Basila [12] assumed the formation of a hydrogenbonded complex upon the adsorption of CH,Cl on silica, since the observed shift in the frequency of the OH groups was as low as 106 cm-‘. Hertl and Hair [13] reported an even smaller shift ( Av = 25 cm-‘). McGee et al. [2] showed that CH,Cl adsorption causes a shift of 124 cm -’ in the band position of OH on silica. Crowell et al. [l] measured a 136 cm-’ shift in the OH vibration frequency of alumina. Since the HY-FAU zeolite is the most acidic of the listed adsorbents, the highest shift we observed seems reasonable. As far as the type of bonding is concerned, the adsorption of CH,Cl at room temperature proved reversible only for Nay-FAU, since the surface species formed on both the basic and the acidic sample could not be desorbed completely by evacuation even above room temperature. It follows from this that the adsorption of CH,Cl on HY-FAU and Na/NaY-FAU generates not only
400
Z. Kbnya et al/Applied
hydrogen bonds; real chemical adsorbent may also take place.
Catalysis B: Emironmental
interactions
8 (1996) 391-404
between
the adsorbate
and the
4.2. Fermi resonance In the spectrum of the CH,Cl molecule, an unusual spectral feature, the Fermi resonance phenomenon, is generally observed. Fermi resonance occurs between vl, the C-H symmetric stretching, and 2 v5, the first overtone of the asymmetric C-H deformation levels [14]. This phenomenon is observed for other molecules too [15]. The question arises of whether this resonance is preserved after adsorption. Crowell et al. [I] first investigated this problem in the course of the adsorption of CH,Cl on an alumina surface. They concluded that Fermi resonance can be observed even in the adsorbed phase, but the interaction between the energy levels is weaker than in the gas phase. McGee et al. [2] showed that, in addition to the positions of the bands, their intensities can also be used to characterize the extent of Fermi resonance in methyl halides. These two teams studied strongly acidic (alumina) and slightly acidic (silica) adsorbents. We investigated the adsorption of CH,Cl on acidic, neutral and basic adsorbents. Spectra recorded under identical experimental conditions are depicted in Fig. 6. It is seen that the shapes of the spectra are different. For Nay-FAU, it is clear that the ratio of the intensities of v1 to 2 us increases upon adsorption. The same phenomenon is less pronounced for HY- and Na/NaYFAU. The calculated Fermi resonance parameters are listed in Table 1. Values calculated from the data of Herzberg [14] and those published by Crowell et al. [I] and McGee et al. [2] are also included. Since the A, W, and Wp values for the different adsorbates are very close to each other, and fairly independent of the acidic character of the adsorbents, the resonance interactions can be assumed to be similar. This follows from the studies of CH,Cl adsorption on alumina and silica and from comparisons of the calculated Fermi resonance parameters [ 1,2]. The second feature is that the extent of the resonance is markedly decreased relative to that for the gas-phase molecules. Table 1 shows that A increased from 12 to 28 as a consequence of adsorption on Nay-FAU. The smaller value of A means more complete resonance. This rule is somewhat confusingly described in [l]. 4.3. Interactions
occurring above ambient temperature
The results showed that the adsorption of CH,Cl on zeolites led to its decomposition at (for HY and Na/NaY) and above (for NaY) ambient temperature. In the first step, the formation of surface methoxy groups is to be expected, as described by the following reactions: -OH + Cl-CH, + -0-CH, + HCl
401
Z. Khya et al. /Applied Catalysis B: Enuironmental8 (1996) 391-404
1 r a n s m
i t
t a n c e
3100
3000
2900
1700
2800
WaVtXlUmb05
1600
1500
1400
1300
Wavenumbers
Fig. 6. Spectra of CH,CI in gas phase and in adsorbed phase, used to estimate Fermi resonance parameters; (a) gas-phase spectrum, (b) adsorbed on HY-FAU, (c) on Nay-FAU, (d) on Na/NaY-FAU at room temperature after 30 min (difference spectra).
and or -0Na
+ Cl-CH,
+ -0-CH,
+ NaCl
These equations reveal two important problems. First, the formation of methoxy species is accompanied by the consumption of OH groups. Several spectroscopic studies have been carried out on the formation of surface methoxy species [ 161. As a general conclusion, it can be stated that the formation of surface methoxy was unequivocally proved over materials possessing OH groups serving as reaction partners for CH,Cl. Beebe et al. [3] assigned the 2960 cm-’ band to the asymmetric C-H, and the 2849 cm-’ band to the symmetric C-H stretching vibration of the methoxy species. These values are very close to the respective vibration of adsorbed CH,Cl. This can be readily understood, since in both physisorbed CH,Cl and the methoxy species, the bonds in the CH, group are not strongly influenced by the substitution of the 0 for Cl.
402 Table 1 Characteristic
Z. K&ya et al./Applied
absorption
bands of CH,Cl
Catalysis B: Enuironmental8
and calculated
Gas phase
Fermi resonance parameters A b= w, % E, Et3
parameters
Adsorbed on zeolites
Adsorbed on
This work
HY
NaY
Na/NaY
SiO, [2]
Al,O,
1355 1455 2879 2966 3042
2966 3046
1349 1447 2865 2965 3054
1351 1443 2862 2966 3050
1349 1447 2864 2963 3055
1351 1445 2865 2965 3039
1352 1446 2861 2965 3044
12.5 3706 56 -31 2965 2879
12 3128 62 -28 2976 2886
21 4749 71 -29 2965 2865
28 5016 80 -24 2966 2862
19.5 4710 69 -30 2963 2864
25 4688 75 -31 2965 2865
21 5292 73 -31 2965 2861
I141 Vibrational mode va, sym. CH, def. v~, asym. CH, def. 2 vg, overtone v,, sym. CH stretch. v4, asym. CH stretch.
Fermi resonance
(1996) 391-404
[l]
a
a The energy levels of the two vibrational bands (labelled LYand /3) were calculated via the following Eq. [2]: E, =(2X v,)+ W,; Ea =(2X v,)+ Wp where W, = A2 +[A2/4+ b2/2]‘12; Wa = A2 -[A2/4+ b2/2]‘12 and A=[(~v, + v,)/2-(2X v,)]; b2 =[(v, -2~s)~ - A2]/2.
Secondly, the formation of HCl is obvious experimental evidence of the generation of methoxy groups on the surface. HCl is formed most easily on HY-FAU, whereas its formation is most difficult on the sodium-cluster-containing Na/NaY. For Nay-FAU, only the reaction between the terminal SiOH groups and CH,Cl leads to the formation of HCl. In this case, NaCl formation accompanies the generation of methoxy groups. HCl formation takes place above 400 K on alumina upon reaction with CH,Cl, as reported by Beebe et al. [3]. Since HY-FAU zeolite is more acidic than alumina or silica [17], readier formation of surface methoxy groups and concomitant HCl generation were expected, and this was found in our experiments. The formation of HCl in the adsorbed phase involves the risk of framework destruction, since HCl is able to break both Si-0 and Al-O bonds. This is particularly true in the presence of water. For HY-FAU, this was actually the case, as evidenced by the formation of water and =Si-0-Si= through the reaction of HCl and OH groups. The experimental evidence that this reaction occurs is seen in Fig. 5, where the band at 3735 cm-’ due to SiOH groups disappears upon HCl 5dsorption while a new band develops around 910 cm- ’ due to the partial dealumination of the framework (see the inset in Fig. 5). In fact, the neutral SiOH group acts as a basic site towards such a strongly acidic molecule as HCl. As far as the situation over Nay-FAU is concerned, the development of OH bands was found (see Fig. 4). This feature can be explained by the reaction of HCl with Naf, resulting in the formation of NaCl and OH groups.
Z. Kbnya et al/Applied
Catalysis B: Environmental 8 (19961 391-404
403
In summary, the adsorption of CH,Cl over acidic zeolite leads to the formation of surface methoxy groups and HCl. Because of its stronger adsorption, HCl remains on the surface, hindering further CH,Cl adsorption by occupying the adsorption centres. Therefore, much less CH,Cl can adsorb and more remains in the gas phase, the gas-phase spectrum overlapping the spectrum of the adsorbed molecules. This feature was seen in the spectra of CH,Cl over HY-FAU (compare the spectra in Fig. 6). The role of water as a product also seems to be important, since it likewise adsorbs very strongly on zeolites, resulting in decreasing adsorption capacities. At higher temperatures, water could bring about the collapse of the framework of Y-type zeolite. However, the highly acidic medium locally formed from H,O and HCl in the surroundings of the adsorption centre is even more destructive. On the basis of these spectroscopic results, a catalytic reactor system has been designed and kinetic experiments are in progress.
5. Conclusions Upon the adsorption of CH,Cl on each of the samples investigated, the Fermi resonance phenomenon that appears in the gas-phase spectrum is preserved in the adsorbed phase, but the extent of the resonance was decreased. This decrease proved to be independent of the acid-base character of the adsorbent. Good agreement was found between the Fermi resonance parameters published in the literature and those calculated from our spectra. The generation of surface methoxy species accompanied by HCl or NaCl formation is assumed for the interaction observed in the spectral changes. At elevated temperatures, reactions leading to the collapse of the zeolite framework may also take place.
Acknowledgements Financial support from the National Science Foundation of Hungary (OTKA T 07577/93) and stimulating discussions with Prof. A. Molnar are gratefully acknowledged.
References [l] [2] [3] [4]
J.E. Crowell, T.P. Beebe, Jr. K.C. McGee, A.T. Capitano T.P. Beebe, Jr., J.E. Crowell T.R. Forester and R.F. Howe, Trans., 70 (1974) 1527.
and J.T. Yates, Jr., and V.H. Grassian, and J.T. Yates, Jr., J. Am. Chem. Sot.,
J. Chem. Phys., 87 (1987) 3668. Langmuir, 10 (1994) 632. J. Phys. Chem., 92 (1988) 1296. 109 (1987) 5076; B.A. Morrow, J. Chem. Sot. Faraday
404
Z. K6nya et al/Applied
Catalysis B: Environmental 8 (1996) 391-404
[5] V.N. Romannikov and K.G. Ione, Kinet. Katal., 25 (1984) 92. 161 C.E. Taylor and R.P. Noceti, in Proceedings of the 9th International Congress on Catalysis, Calgary, 1988, Vol. 4., p. 990. [7] I. Kiricsi, Gy. Tasi, H. Fdrster and P. Fejes, Acta Phys. Chem. Szeged, 33 (1987) 69. [8] L.R.M. Martens, P.J. Grobet and P.A. Jacobs, Nature, 315 (1985) 568. [9] I. Hannus, Gy. Tasi, I. Kiricsi, J.B. Nagy, H. Forster and P. Fejes, Thennochim. Acta, 249 (1995) 285. [IO] L. Kubelkova, J. Novakova, B. Wichterlova and P. Jiru, Coll. Czeh. Chem. Commun., 45 (1980) 2290; L. Kubelkova, J. Novakova, Z. Dolejsek and P. Jiru, Coll. Czeh. Chem. Commun., 45 (1980) 3101. [ll] G.A. Ozin, S. Gzkar and G.D. Stucky, J. Phys. Chem., 94 (1990) 7562. [12] M.R. Basila, J. Chem. Phys., 35 (1961) 1151. [13] W. Hertl and M.L. Hair, J. Phys. Chem., 72 (1968) 4676. [14] G. Herzberg, Molekula-Szinkepek II, Akademia Kiad6, Budapest, 1959. [15] I. Hannus, I. Kiricsi, Gy. Tasi and P. Fejes, Appl. Catal., 66 (1990) L7. [16] V. Bosacek, J. Phys. Chem., 97 (1993) 10732. [17] B. Umansky, J. Engelhardt and W.K. Hall, J. Catal., 127 (1991) 128.