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Adsorption sites for benzene in KL zeolite: An infrared study of molecular recognition
r'
T - RWO R T .
~]1= I N E M A N N
Adsorption sites for benzene in KL zeolite: An infrared study of molecular recognition Bao Lian Su and Denise Barthomeuf Laboratoire de R~activit~ de Surface, URA I 106, CNRS, Universit~ Pierre et Marie Curie, Paris, France The adsorption of benzene is followed by infrared spectroscopy. A detailed study in the range
P/Po from 0 to 10 -4 is extended further to P/Po = 0.5. It shows that about two molecules of benzene per unit cell strongly interact with cations for P/Po = 10-4. At higher benzene Ioadings the excess of aromatic is simply filling the pores. No significant interaction is detected with the 12R windows. The comparison with KY, NaY, NaEMT, and NaBeta suggests that in KL a molecular recognition effect directs the adsorption on the cations and keeps it from the 12R aperture. Keyword$: Benzene absorption; KL; NaY; NaEMT; NaBeta; IR; molecular recognition
INTRODUCTION KL zeolite is often used as a support for highly dispersed Pt. The Pt/KL was reported early as a very selective catalyst for the aromatization of hexane to benzene. 1'2 It may be of interest to understand the mode of interaction of the aromatic with the zeolite since this step may participate in the overall catalytic process of aromatization reaction. The adsorption of benzene in KL was studied at low loading (one molecule/unit cell) by neutron diffraction at 78 Ks and by n.m.r, of deuterium 4--6 between 100 and 350 K. It showed that benzene interacts with the cation but not with oxygen atoms of the 12R windows. 3'4'6 The question then arises as to the location of benzene at high loadings and at room temperature. I n f r a r e d spectroscopy was used successfully to study the location o f benzene in a large number of zeolites (X and Y exchanged with alkaline cations, 7- 12 NaEMT, 13 Na- and CsBeta 14). Comparative studies of NaY or NaX revealed similar locations by i.r. 7'9-I 1or neutron diffraction, x~'16 neutron inelastic scattering,17 or n.m.r.18 The i.r. C - H out-of-planebands of benzene (1,960 (v5 + P17) and 1,815 c m - 1 (vl0 + v,7 ) for the liquid phase) are shifted to high wavenumbers by a r o u n d 2 0 - 3 0 c m - 1 w h e n b e n z e n e is a d s o r b e d on the cations a n d by a r o u n d 50-60 cm-1 if the aromatic molecule is in the 12R windows, the C - H interacting with the framework oxygen. 7'9-11"13'14 The two possible adsorption sites are first, the accessible cations acting as Lewis acids, which interact with the ~r electrons of benzene; and Address reprint requests to Dr. Barthomeuf at the Laboratoire de R6activit6 de Surface, Universit6 Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris cedex 05 France. Received 29 November 1994; accepted 16 January 1995 Zeolites 15:470-474, 1995 © Elsevier Science Inc. 1995 655 Avenue of the Americas, New York, NY 10010
second, the oxygen atoms of the large aperture behaving as basic sites. Their average basic strength may be expressed as their mean charge 8 calculated from the Sanderson principle of equalization of electronegativities. ~9'2° It was shown that the amount of benzene adsorbed in the 12R window of faujasites X and Y exchanged with various alkaline cations increases in parallel with the oxygen basicity.7'9--11'2° KL zeolite also has 12R windows, and one might expect that considering its average oxygen charge close to that of faujasites (Table I) benzene could be adsorbed in this aperture at high loadings. To shed some light on these points the present paper describes the behavior of benzene in KL as studied by i.r. spectroscopy.
EXPERIMENTAL KL zeolite provided by Union Carbide and NaEMT synthesized as in Ref. 21 have the formula Ks.z5 Na0.15 (AIO2)8. 4 (8i02)27 and Na21 (A10~)2I (8i02)75 , respectively. Self-supported wafers (around 15 mg of hydrated zeolite, 18 mm in diameter) were calcined in situ in a pyrex i.r. cell with CaF z windows at 773 K in a flow of dry oxygen for 6 h. T h e n the samples were evacuated for 6-8 h at the same temperature. The adsorption of known and increasing amounts of benzene was carried out at room temperature as described previously7'9-ILIa'22 on the already pretreated samples. After 1 h equilibration, the i.r. spectra were recorded using a Perkin Elmer 1750 Infrared Fourier Transform Spectrometer coupled with a Data Station 3600. Desorption experiments were conducted by 1 h evacuation at the desired temperature. The relative absorbance, Ar, was obtained by using the expression as in Refs. 9-11, 22.
0144-2449/95/$10.00 SSDI 0144-2449(95)00003-O
Adsorption sites for benzene in KL zeolite: B.L. Su and D. Barthomeuf Table I
Characteristics of benzene adsorption and desorption on zeolites Adsorption on
Zeolite
Average oxygen charge a
Cations
12R window
Desorption temperature from cations (K)
NaBeta NaEMT
- 0.240 -0.318
Yes Yes
Yes No
473 423
KL NaY KY
- 0.350 - 0.352 - 0.382
Yes Yes Yes
No Yes Yes
>443 383 n&
Ref. 14 13 and this work This work 7, 11 11
a Calculated from Refs. 19 and 20.
b NO data.
(1)
Ar= km fbandA(V)dv
where M is the weight of 1 unit cell of zeolite; m, the weight of the wafer; v, the wavenumber in c m - 1; and k, a coefficient arbitrarily chosen equal to 2.95 X 10 -7.
RESULTS AND DISCUSSION It was reported that the nitrogen adsorption capacity of KL is around 0.12 g/g.23 This suggests a benzene adsorption capacity of around three molecules/unit cell (m/u.c.). Considering the size of the absorbent molecule, a nitrogen molecule, being much smaller than a benzene molecule in dimension, can enter not only the 12R channels but also the cancrinite cages and hexagonal prisms, whereas only the 12R channels are accessible for benzene. Therefore, the real adsorption capacity of benzene in KL might be less than the value estimated from the nitrogen adsorption capacity, that is, less than three m/u.c. Figure 1 reports the transmission spectra of KL zeolite before (a) and after (b) benzene loading at condition (6.23 m/u.c.) where there is condensation of benzene between the particles of the zeolite. Benzene adsorption gives some changes around 3,800-3,200 c m - I (hydroxyl region), 3,200-2,900 c m - t, 2,200-1,700 c m - i ( C - H out-of-plane vibrations region), and 1,5001,300 cm -1 ( C - C stretching range). T h e range
around 3,200-2,900 c m - x consists of different combinations of C - H stretching and C - C out-of-plane and stretching vibrations and is very difficult to study because of the overlapping of bands. It is not considered in this paper.
Interaction with hydroxyls Some O H groups are still observed in KL zeolite 24'25 after evacuation at 773 K (Figure2). The adsorption of benzene moves the silanol band vibrating at 3,746 to 3,633 cm-1 and does not change the two O H groups at 3,695 and 3,676 c m - ~ much. The shift of 113 c m - t for the silanol groups is slightly less than that observed for Na- or CsBeta (130 c m - ~),~4 dealuminated NaY (136 cm-1),~6 or LZY-82 (141 cm-x).22 This suggests that in KL the silanol groups are even weaker acids than in the other zeolites.
Type and amounts of benzene adsorbed The shift of the C - H out-of-plane i.r. bands provides information on the type of site interacting with benzene, that is, cation (metal ion or proton, 20-30 c m - t shift) or oxygen atoms of the 12R window (5060 c m - t shift). In addition, the changes in the intensity of the same bands with benzene loading give ac-
37(,~ ,l~ 0
4000
I
It
3000
2000
I
1600 cm-1
Figure 1 Infrared spectra in transmission (%) of KL pretreated at 773 K without (a) and with (b) adsorbed benzene (6.23 molecules/unit cell).
O I
4000
3750
3500
cm-1
Figure 2 Changes in the absorbance of the hydroxyl groups of KL upon the adsorption of increasing amounts of benzene (in molecules/unit cell) (a) 0.0, (b) 0.40, (c) 3.34, (d) 6.23.
Zeolites 1 5 : 4 7 0 - 4 7 4 , 1995
471
Adsorption sites for benzene in KL zeolite: B.L. Su and D. Barthomeuf
I
-1o.o13
,-AI
i
=
l
T,
g
o
2000
2200
1850
cm-1
Figure 3 Changes in the absorbance of C - H out-of-plane vibrations as a function of the amount of the benzene introduced in the i.r. cell (in molecules/unit cell) (a) 0.39, (b) 0.78, (c) 1.00, (d) 1.94, (e) 2.98, (f) 3.34, (g) 6.23.
cess to the amount of aromatic adsorbed on each type of site. 7'9-11'1B'14'22 This approach is applied to the C - H out-of-plane bands of benzene adsorbed in KL. Figure 3 shows the spectra in the 2,200-1,700 cm-1 range at increasing benzene loadings. Up to around 3.3 m/u.c. (corresponding to a relative pressure P/Po of 10- 4), one primary set of bands is observed vibrating at around 1,985 and 1,845 cm-a. They may be assigned to benzene interacting with cations K +, from the shift of about 25-30 c m - i compared to liquid benzene bands (1,960 and 1,815 cm-1). Very weak bands are seen at around 2,012-1,869 cm -1 (about 55 cm-1 shift). The results suggest that most of the benzene is interacting with the accessible K + cations in the main channel, and only few molecules are located in the 12R window. At overloading (curve g, Figure 3) even when the pores of the zeolite are filled bands in the range of liquid benzene are seen, very likely because of condensation in the zeolite. The amount of benzene adsorbed on cations may be obtained by plotting the absorbance of the two bands at 1,985 and 1,845 c m - i as a function of the amount of benzene contacting the zeolite and ex-
0,04
/ i I ............ j
_"
o,o i Ar
0,02
. . . .
pressed in m/u.c, of KL. Up to P/Po = 10- 4 it is considered that all of the molecules of benzene are adsorbed. Figure 4 indicates that after adsorption of around two molecules the benzene molecules in excess do not interact with the cations. Assuming that one molecule of benzene is adsorbed per cation this suggests that only two K + ions/u.c, may retain benzene on cations. It was checked that up to P/Po = 0.5, no more benzene is adsorbed on cations (Figure 5), and only the bands of liquid benzene are growing. This indicates that the benzene molecules continuously condensate between the particles of the zeolite. It is reported that 3.6 K + ions are located in the main channel of K L . 27 The present results suggest that only two of them interact with benzene at room temperature. This feature is confirmed by the changes in the wave number of the two bands reported in Figure 6. The wave number of the two bands decreases down to around two m/u.c. This indicates that there is no further influence of the cations above this value. The estimated value of the benzene adsorption capacity from nitrogen adsorption is confirmed by the present study. The almost negligible interaction of benzene with the oxygen of the 12R window (weak shoulders at 2,012-1,869 c m - 1) is surprising. Based on the charge on the oxygen (Table 1) one might have expected that, as in faujasite for instance, the C - H would interact with the framework oxygen. It was observed in this last zeolite structure that the extent of the interaction C - H / o x y g e n increases as the basicity rises. 7'9-11"2° The average oxygen charge (Table I) is close for KL and NaY, but only NaY adsorbs benzene in the 12R window. In addition, NaBeta with a low calculated oxygen charge, also adsorbs benzene in its 12R apertureq4; NaEMT does not. 13 The results suggest that within a given zeolite structure a trend is found between the amount of benzene adsorbed in the 12R window and the average oxygen basicity, v'9--ll but a comparison can not be made easily between different structures on this basis. Very likely, parameters other than the average basicity are important. The shape of
b .-=, ,T, .i
Figure 4 Variations of the relative absorbance of the (re + v~) (a) and (vl0 + vl>) (b) C - H out-of-plane bands of adsorbed benzene as a function of the number of benzene molecules introduced in the i.r. cell.
[]
0,01
0,00 0
472
Zeolites 1 5 : 4 7 0 - 4 7 4 , 1995
2
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Adsorption sites for benzene in KL zeolite: B.L. Su and D. Barthomeuf
2,5
n/u.c. 2,0, ~O
O
1,5' Figure 5 Adsorption isotherm of benzene on K + cations in KL zeolite at room temperature.
1,0
0,5
0,0 0,0
I
I
,
0,1
I
,
0,2
I
I
0,3
0,4
the 12R aperture is not much different in KL, ~s NaY, or Ky. 29 It cannot explain the KL behavior. It could be that the actual basicity of oxygen atoms in the 12R window in KL is very different from the average calculated value. Alternatively, in faujasites the adsorption on cations and 12R windows could be related to the formation of benzene clusters. ~7 The supercages in faujasites are connected in the three directions of space by the 12R windows. Benzene clusters very similar to the ones existing in the solid state may easily be formed in these supercages and extend in the whole pore volume of the zeolite. 17 By contrast, in L zeolites the pores look like cylinders parallel to the c axis of 1992
I)(crn-1) 1988
t
I
i
0,5
P/Po
the crystal and not interconnected. The packing of benzene in L zeolite could favor the adsorption mainly on cations located on the walls of the main channel but not in the window, which is perpendicular to the axis of the channel. The benzene clusters would not be formed. In any case, the results suggest a molecular recognition between the benzene molecule and the 12R aperture of the zeolites.
Strength o f benzene/cation interaction This strength may be evaluated from the temperature of the disappearance of the i.r. benzene bands upon evacuation. The C - C stretching vibration at 1,479 cm-1 is more sensitive than the C - H out of plane bands in the 1,800-2,000 cm - l range.8,22,30 Table I shows that both KL and NaEMT adsorb benzene only on cations. The two samples are then compared with regard to the strength of adsorption in Figure 7 for the C - C range. It is seen that KL still adsorbs benzene after evacuation for 1 h at a higher temper-
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i
i
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1846
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_
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Figure 8 Dependence of the wave number of the (vs + v17) (a) and (Vlo + v17) (b) bands on the amounts of benzene introduced in the i,r. cell (in molecules/unit cell) on KL pretreated at 773 K.
Figure 7 Changes in the absorbance of C - C stretching vibration of adsorbed benzene for NaEMT (A) and KL (B) as a function of the evacuation temperature (K) (1 h for each temperature) A:
(a) 298, (b) 333, (c) 353, (d) 368, (e) 383; B: (al) 298, (bl) 323, (cl) 353, (dl) 383, (el) 418, (fl) 443.
Zeolites 15:470-474, 1995
473
Adsorption sites for benzene in KL zeolite: B.L. Su and D. Barthomeuf
ature than NaEMT. Table 1 compares the two zeolites with NaY and Na Beta for the desorption temperatures from the cations. In the faujasite series it was observed that a high adsorption on cations (strong Lewis acidity) occurred simultaneously with a small adsorption in the 12R window (weak oxygen basicity) and reversibly. 7'9-11,20,26 The comparison of the various zeolite structures in Table 1 does not show such a simple relationship. This confirms that the structure of the zeolite exerts a strong influence on the adsorption on cations and in the 12R window in line with a molecular recognition effect. This effect seen in the present adsorption experiments is very likely to exist in catalysis also. In the aromatization reaction on Pt/ KL the benzene formed on Pt diffused through the zeolite from site to site, being adsorbed in a way probably related to the one described here. The high selectivity of L zeolite compared with faujasite, for instance, 1,2 for the production of benzene might also be related to the different modes of adsorption of the aromatic in the two zeolites. The formation of specific adsorbed phases such as the benzene cluster may well restrict the mobility of the benzene molecules in the pores of the zeolite and the departure of this reaction product toward the gas phase. The molecular recognition effect would then strongly influence the selectivity in this catalytic reaction. More work is still needed to understand better the energetics and dynamics of benzene adsorption in the various zeolites to quantify this effect. In conclusion, the adsorption of benzene in KL occurs only on cations even at full loading of the cages. No significant interaction with the 12R window is observed. This may arise from a chemical or geometric effect and may indicate a molecular recognition between the benzene and KL. This effect may be an important parameter governing the selectivity in catalytic reactions like aromatization in Pt/alkaline L zeolites.
ACKNOWLEDGMENTS We thank Isabelte Virlet for very helpful assistance.
REFERENCES 1 Besoukhanova C., Guidot, J., Barthomeuf, D., Breysse, M.
474
Zeolites 15:470-474, 1995
2 3 4 5 6 7
8 9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
and Bernard, J.R.J. Chem. Soc. Faraday Trans. I 1981, 77, 1595 Davis, R.J. Heterogeneous Chemistry Reviews 1994, 1, 41 Newsam, J. J. Phys. Chem. 1989, 93, 7989 Newsam, J., Silbernagel, B.G., Garcia, A.R. and Hulme, R. J. Chem. Soc. Chem. Commun. 1987, 664 Silbernagel, B.G., Garcia, A.R. and Newsam, J.M. Colloids Surf. A 1993, 72, 71 Silbernagel, B.G., Garcia, A.R., Newsam, J. and Hulme, R. J. Phys. Chem. 1989, 93, 6506 De Mallmann, A. and Barthomeuf, D. New developments in zeolite science and technology, in Studies in Surface Science and Catalysis, Vol. 28 Eds. Y. Murakami, A. Ijima and J.W. Ward) 1986, p. 609 Coughlan, B., Carroll, W., O'Malley, P. and Nunan, J. J. Chem. Soc. Faraday Trans. I 1981, 77, 3037 De Mallmann, A. and Barthomeuf, D. Zeolites 1988, 8, 292 De Mallmann, A. and Barthomeuf, D. J. Chem. Soc. Chem. Commun. 1986, 476 De Mallmann, A., Dzwigaj, S. and Barthomeuf, D. Zeolites: Facts, figures, future in Studies in Surface, Science and Catalysis, Vol. 49B (Eds. P.A. Jacobs and R.A. Van Santen) 1989, p. 935 Coughlan, B., and Keane, M.A.J. Chem. Soc. Faraday Trans. I 1990, 86, 3961 Su, B.L., Manoli, J.M., Potvin, C. and Barthomeuf, D. J, Chem. Soc. Faraday Trans, I 1993, 89, 857 Dzwigaj, S. De Mallmann, A. and Barthomeuf, D. J. Chem. Soc. Faraday Trans. I 1990, 86, 431 Fitch, A.N., Jobic, H. and Renouprez, A. J. Chem. Soc. Chem. Commun. 1985, 294 Fitch, A.N., Jobic, H. and Renouprez, A. J. Phys. Chem. 1986, 90, 1311 Jobic, H., Renouprez, A., Fitch, A.N. and Lauter, H.J.J. Chem. Soc. Faraday Trans. I 1987, 83, 3199 Lechert, H. and Wittern, K.P. Ber. Bunsenges Phys. Chem. 1978, 82, 1054 Sanderson, R.T. Chemical Bonds and Bond Energy, Academic Press, New York, 1976 Barthomeuf, D. Stud. Surf. Sci. Catal. 1991, 65, 157 Delprato, F. PhD Thesis, University of Haute Alsace, Mulhouse, France, 1990 Su, B.L. and Barthomeuf, D. Zeolites 1993, 13, 623 Parra, C.F., Ballivet, D. and Barthomeuf, D. J. Catal. 1975, 40, 52 Ballivet, D. and Barthomeuf, D. J. Chem. Soc. Faraday Trans. I 1977, 73, 1581 Tsitsishvili, G.V. Adv. Chem. Ser. 1973, 121,291 De Mallmann, A. Thesis, 3rd cycle, University of Paris, France, 1986 Barrer, R.M. and Villiger, H. Z. Kristallogr. 1969, 128, 352 Meier, W.M. and Olson, D.H. Zeolites, 1992, 12, 96 Meir, W.M. and Olson, D.H. Zeolites 1992, 12, 124 Su, B.L. and Barthomeuf, D. J. Catal. 1993, 139, 81
T - RWO R T .
~]1= I N E M A N N
Adsorption sites for benzene in KL zeolite: An infrared study of molecular recognition Bao Lian Su and Denise Barthomeuf Laboratoire de R~activit~ de Surface, URA I 106, CNRS, Universit~ Pierre et Marie Curie, Paris, France The adsorption of benzene is followed by infrared spectroscopy. A detailed study in the range
P/Po from 0 to 10 -4 is extended further to P/Po = 0.5. It shows that about two molecules of benzene per unit cell strongly interact with cations for P/Po = 10-4. At higher benzene Ioadings the excess of aromatic is simply filling the pores. No significant interaction is detected with the 12R windows. The comparison with KY, NaY, NaEMT, and NaBeta suggests that in KL a molecular recognition effect directs the adsorption on the cations and keeps it from the 12R aperture. Keyword$: Benzene absorption; KL; NaY; NaEMT; NaBeta; IR; molecular recognition
INTRODUCTION KL zeolite is often used as a support for highly dispersed Pt. The Pt/KL was reported early as a very selective catalyst for the aromatization of hexane to benzene. 1'2 It may be of interest to understand the mode of interaction of the aromatic with the zeolite since this step may participate in the overall catalytic process of aromatization reaction. The adsorption of benzene in KL was studied at low loading (one molecule/unit cell) by neutron diffraction at 78 Ks and by n.m.r, of deuterium 4--6 between 100 and 350 K. It showed that benzene interacts with the cation but not with oxygen atoms of the 12R windows. 3'4'6 The question then arises as to the location of benzene at high loadings and at room temperature. I n f r a r e d spectroscopy was used successfully to study the location o f benzene in a large number of zeolites (X and Y exchanged with alkaline cations, 7- 12 NaEMT, 13 Na- and CsBeta 14). Comparative studies of NaY or NaX revealed similar locations by i.r. 7'9-I 1or neutron diffraction, x~'16 neutron inelastic scattering,17 or n.m.r.18 The i.r. C - H out-of-planebands of benzene (1,960 (v5 + P17) and 1,815 c m - 1 (vl0 + v,7 ) for the liquid phase) are shifted to high wavenumbers by a r o u n d 2 0 - 3 0 c m - 1 w h e n b e n z e n e is a d s o r b e d on the cations a n d by a r o u n d 50-60 cm-1 if the aromatic molecule is in the 12R windows, the C - H interacting with the framework oxygen. 7'9-11"13'14 The two possible adsorption sites are first, the accessible cations acting as Lewis acids, which interact with the ~r electrons of benzene; and Address reprint requests to Dr. Barthomeuf at the Laboratoire de R6activit6 de Surface, Universit6 Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris cedex 05 France. Received 29 November 1994; accepted 16 January 1995 Zeolites 15:470-474, 1995 © Elsevier Science Inc. 1995 655 Avenue of the Americas, New York, NY 10010
second, the oxygen atoms of the large aperture behaving as basic sites. Their average basic strength may be expressed as their mean charge 8 calculated from the Sanderson principle of equalization of electronegativities. ~9'2° It was shown that the amount of benzene adsorbed in the 12R window of faujasites X and Y exchanged with various alkaline cations increases in parallel with the oxygen basicity.7'9--11'2° KL zeolite also has 12R windows, and one might expect that considering its average oxygen charge close to that of faujasites (Table I) benzene could be adsorbed in this aperture at high loadings. To shed some light on these points the present paper describes the behavior of benzene in KL as studied by i.r. spectroscopy.
EXPERIMENTAL KL zeolite provided by Union Carbide and NaEMT synthesized as in Ref. 21 have the formula Ks.z5 Na0.15 (AIO2)8. 4 (8i02)27 and Na21 (A10~)2I (8i02)75 , respectively. Self-supported wafers (around 15 mg of hydrated zeolite, 18 mm in diameter) were calcined in situ in a pyrex i.r. cell with CaF z windows at 773 K in a flow of dry oxygen for 6 h. T h e n the samples were evacuated for 6-8 h at the same temperature. The adsorption of known and increasing amounts of benzene was carried out at room temperature as described previously7'9-ILIa'22 on the already pretreated samples. After 1 h equilibration, the i.r. spectra were recorded using a Perkin Elmer 1750 Infrared Fourier Transform Spectrometer coupled with a Data Station 3600. Desorption experiments were conducted by 1 h evacuation at the desired temperature. The relative absorbance, Ar, was obtained by using the expression as in Refs. 9-11, 22.
0144-2449/95/$10.00 SSDI 0144-2449(95)00003-O
Adsorption sites for benzene in KL zeolite: B.L. Su and D. Barthomeuf Table I
Characteristics of benzene adsorption and desorption on zeolites Adsorption on
Zeolite
Average oxygen charge a
Cations
12R window
Desorption temperature from cations (K)
NaBeta NaEMT
- 0.240 -0.318
Yes Yes
Yes No
473 423
KL NaY KY
- 0.350 - 0.352 - 0.382
Yes Yes Yes
No Yes Yes
>443 383 n&
Ref. 14 13 and this work This work 7, 11 11
a Calculated from Refs. 19 and 20.
b NO data.
(1)
Ar= km fbandA(V)dv
where M is the weight of 1 unit cell of zeolite; m, the weight of the wafer; v, the wavenumber in c m - 1; and k, a coefficient arbitrarily chosen equal to 2.95 X 10 -7.
RESULTS AND DISCUSSION It was reported that the nitrogen adsorption capacity of KL is around 0.12 g/g.23 This suggests a benzene adsorption capacity of around three molecules/unit cell (m/u.c.). Considering the size of the absorbent molecule, a nitrogen molecule, being much smaller than a benzene molecule in dimension, can enter not only the 12R channels but also the cancrinite cages and hexagonal prisms, whereas only the 12R channels are accessible for benzene. Therefore, the real adsorption capacity of benzene in KL might be less than the value estimated from the nitrogen adsorption capacity, that is, less than three m/u.c. Figure 1 reports the transmission spectra of KL zeolite before (a) and after (b) benzene loading at condition (6.23 m/u.c.) where there is condensation of benzene between the particles of the zeolite. Benzene adsorption gives some changes around 3,800-3,200 c m - I (hydroxyl region), 3,200-2,900 c m - t, 2,200-1,700 c m - i ( C - H out-of-plane vibrations region), and 1,5001,300 cm -1 ( C - C stretching range). T h e range
around 3,200-2,900 c m - x consists of different combinations of C - H stretching and C - C out-of-plane and stretching vibrations and is very difficult to study because of the overlapping of bands. It is not considered in this paper.
Interaction with hydroxyls Some O H groups are still observed in KL zeolite 24'25 after evacuation at 773 K (Figure2). The adsorption of benzene moves the silanol band vibrating at 3,746 to 3,633 cm-1 and does not change the two O H groups at 3,695 and 3,676 c m - ~ much. The shift of 113 c m - t for the silanol groups is slightly less than that observed for Na- or CsBeta (130 c m - ~),~4 dealuminated NaY (136 cm-1),~6 or LZY-82 (141 cm-x).22 This suggests that in KL the silanol groups are even weaker acids than in the other zeolites.
Type and amounts of benzene adsorbed The shift of the C - H out-of-plane i.r. bands provides information on the type of site interacting with benzene, that is, cation (metal ion or proton, 20-30 c m - t shift) or oxygen atoms of the 12R window (5060 c m - t shift). In addition, the changes in the intensity of the same bands with benzene loading give ac-
37(,~ ,l~ 0
4000
I
It
3000
2000
I
1600 cm-1
Figure 1 Infrared spectra in transmission (%) of KL pretreated at 773 K without (a) and with (b) adsorbed benzene (6.23 molecules/unit cell).
O I
4000
3750
3500
cm-1
Figure 2 Changes in the absorbance of the hydroxyl groups of KL upon the adsorption of increasing amounts of benzene (in molecules/unit cell) (a) 0.0, (b) 0.40, (c) 3.34, (d) 6.23.
Zeolites 1 5 : 4 7 0 - 4 7 4 , 1995
471
Adsorption sites for benzene in KL zeolite: B.L. Su and D. Barthomeuf
I
-1o.o13
,-AI
i
=
l
T,
g
o
2000
2200
1850
cm-1
Figure 3 Changes in the absorbance of C - H out-of-plane vibrations as a function of the amount of the benzene introduced in the i.r. cell (in molecules/unit cell) (a) 0.39, (b) 0.78, (c) 1.00, (d) 1.94, (e) 2.98, (f) 3.34, (g) 6.23.
cess to the amount of aromatic adsorbed on each type of site. 7'9-11'1B'14'22 This approach is applied to the C - H out-of-plane bands of benzene adsorbed in KL. Figure 3 shows the spectra in the 2,200-1,700 cm-1 range at increasing benzene loadings. Up to around 3.3 m/u.c. (corresponding to a relative pressure P/Po of 10- 4), one primary set of bands is observed vibrating at around 1,985 and 1,845 cm-a. They may be assigned to benzene interacting with cations K +, from the shift of about 25-30 c m - i compared to liquid benzene bands (1,960 and 1,815 cm-1). Very weak bands are seen at around 2,012-1,869 cm -1 (about 55 cm-1 shift). The results suggest that most of the benzene is interacting with the accessible K + cations in the main channel, and only few molecules are located in the 12R window. At overloading (curve g, Figure 3) even when the pores of the zeolite are filled bands in the range of liquid benzene are seen, very likely because of condensation in the zeolite. The amount of benzene adsorbed on cations may be obtained by plotting the absorbance of the two bands at 1,985 and 1,845 c m - i as a function of the amount of benzene contacting the zeolite and ex-
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pressed in m/u.c, of KL. Up to P/Po = 10- 4 it is considered that all of the molecules of benzene are adsorbed. Figure 4 indicates that after adsorption of around two molecules the benzene molecules in excess do not interact with the cations. Assuming that one molecule of benzene is adsorbed per cation this suggests that only two K + ions/u.c, may retain benzene on cations. It was checked that up to P/Po = 0.5, no more benzene is adsorbed on cations (Figure 5), and only the bands of liquid benzene are growing. This indicates that the benzene molecules continuously condensate between the particles of the zeolite. It is reported that 3.6 K + ions are located in the main channel of K L . 27 The present results suggest that only two of them interact with benzene at room temperature. This feature is confirmed by the changes in the wave number of the two bands reported in Figure 6. The wave number of the two bands decreases down to around two m/u.c. This indicates that there is no further influence of the cations above this value. The estimated value of the benzene adsorption capacity from nitrogen adsorption is confirmed by the present study. The almost negligible interaction of benzene with the oxygen of the 12R window (weak shoulders at 2,012-1,869 c m - 1) is surprising. Based on the charge on the oxygen (Table 1) one might have expected that, as in faujasite for instance, the C - H would interact with the framework oxygen. It was observed in this last zeolite structure that the extent of the interaction C - H / o x y g e n increases as the basicity rises. 7'9-11"2° The average oxygen charge (Table I) is close for KL and NaY, but only NaY adsorbs benzene in the 12R window. In addition, NaBeta with a low calculated oxygen charge, also adsorbs benzene in its 12R apertureq4; NaEMT does not. 13 The results suggest that within a given zeolite structure a trend is found between the amount of benzene adsorbed in the 12R window and the average oxygen basicity, v'9--ll but a comparison can not be made easily between different structures on this basis. Very likely, parameters other than the average basicity are important. The shape of
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Figure 4 Variations of the relative absorbance of the (re + v~) (a) and (vl0 + vl>) (b) C - H out-of-plane bands of adsorbed benzene as a function of the number of benzene molecules introduced in the i.r. cell.
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Zeolites 1 5 : 4 7 0 - 4 7 4 , 1995
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Adsorption sites for benzene in KL zeolite: B.L. Su and D. Barthomeuf
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the 12R aperture is not much different in KL, ~s NaY, or Ky. 29 It cannot explain the KL behavior. It could be that the actual basicity of oxygen atoms in the 12R window in KL is very different from the average calculated value. Alternatively, in faujasites the adsorption on cations and 12R windows could be related to the formation of benzene clusters. ~7 The supercages in faujasites are connected in the three directions of space by the 12R windows. Benzene clusters very similar to the ones existing in the solid state may easily be formed in these supercages and extend in the whole pore volume of the zeolite. 17 By contrast, in L zeolites the pores look like cylinders parallel to the c axis of 1992
I)(crn-1) 1988
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i
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the crystal and not interconnected. The packing of benzene in L zeolite could favor the adsorption mainly on cations located on the walls of the main channel but not in the window, which is perpendicular to the axis of the channel. The benzene clusters would not be formed. In any case, the results suggest a molecular recognition between the benzene molecule and the 12R aperture of the zeolites.
Strength o f benzene/cation interaction This strength may be evaluated from the temperature of the disappearance of the i.r. benzene bands upon evacuation. The C - C stretching vibration at 1,479 cm-1 is more sensitive than the C - H out of plane bands in the 1,800-2,000 cm - l range.8,22,30 Table I shows that both KL and NaEMT adsorb benzene only on cations. The two samples are then compared with regard to the strength of adsorption in Figure 7 for the C - C range. It is seen that KL still adsorbs benzene after evacuation for 1 h at a higher temper-
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Figure 8 Dependence of the wave number of the (vs + v17) (a) and (Vlo + v17) (b) bands on the amounts of benzene introduced in the i,r. cell (in molecules/unit cell) on KL pretreated at 773 K.
Figure 7 Changes in the absorbance of C - C stretching vibration of adsorbed benzene for NaEMT (A) and KL (B) as a function of the evacuation temperature (K) (1 h for each temperature) A:
(a) 298, (b) 333, (c) 353, (d) 368, (e) 383; B: (al) 298, (bl) 323, (cl) 353, (dl) 383, (el) 418, (fl) 443.
Zeolites 15:470-474, 1995
473
Adsorption sites for benzene in KL zeolite: B.L. Su and D. Barthomeuf
ature than NaEMT. Table 1 compares the two zeolites with NaY and Na Beta for the desorption temperatures from the cations. In the faujasite series it was observed that a high adsorption on cations (strong Lewis acidity) occurred simultaneously with a small adsorption in the 12R window (weak oxygen basicity) and reversibly. 7'9-11,20,26 The comparison of the various zeolite structures in Table 1 does not show such a simple relationship. This confirms that the structure of the zeolite exerts a strong influence on the adsorption on cations and in the 12R window in line with a molecular recognition effect. This effect seen in the present adsorption experiments is very likely to exist in catalysis also. In the aromatization reaction on Pt/ KL the benzene formed on Pt diffused through the zeolite from site to site, being adsorbed in a way probably related to the one described here. The high selectivity of L zeolite compared with faujasite, for instance, 1,2 for the production of benzene might also be related to the different modes of adsorption of the aromatic in the two zeolites. The formation of specific adsorbed phases such as the benzene cluster may well restrict the mobility of the benzene molecules in the pores of the zeolite and the departure of this reaction product toward the gas phase. The molecular recognition effect would then strongly influence the selectivity in this catalytic reaction. More work is still needed to understand better the energetics and dynamics of benzene adsorption in the various zeolites to quantify this effect. In conclusion, the adsorption of benzene in KL occurs only on cations even at full loading of the cages. No significant interaction with the 12R window is observed. This may arise from a chemical or geometric effect and may indicate a molecular recognition between the benzene and KL. This effect may be an important parameter governing the selectivity in catalytic reactions like aromatization in Pt/alkaline L zeolites.
ACKNOWLEDGMENTS We thank Isabelte Virlet for very helpful assistance.
REFERENCES 1 Besoukhanova C., Guidot, J., Barthomeuf, D., Breysse, M.
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8 9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
and Bernard, J.R.J. Chem. Soc. Faraday Trans. I 1981, 77, 1595 Davis, R.J. Heterogeneous Chemistry Reviews 1994, 1, 41 Newsam, J. J. Phys. Chem. 1989, 93, 7989 Newsam, J., Silbernagel, B.G., Garcia, A.R. and Hulme, R. J. Chem. Soc. Chem. Commun. 1987, 664 Silbernagel, B.G., Garcia, A.R. and Newsam, J.M. Colloids Surf. A 1993, 72, 71 Silbernagel, B.G., Garcia, A.R., Newsam, J. and Hulme, R. J. Phys. Chem. 1989, 93, 6506 De Mallmann, A. and Barthomeuf, D. New developments in zeolite science and technology, in Studies in Surface Science and Catalysis, Vol. 28 Eds. Y. Murakami, A. Ijima and J.W. Ward) 1986, p. 609 Coughlan, B., Carroll, W., O'Malley, P. and Nunan, J. J. Chem. Soc. Faraday Trans. I 1981, 77, 3037 De Mallmann, A. and Barthomeuf, D. Zeolites 1988, 8, 292 De Mallmann, A. and Barthomeuf, D. J. Chem. Soc. Chem. Commun. 1986, 476 De Mallmann, A., Dzwigaj, S. and Barthomeuf, D. Zeolites: Facts, figures, future in Studies in Surface, Science and Catalysis, Vol. 49B (Eds. P.A. Jacobs and R.A. Van Santen) 1989, p. 935 Coughlan, B., and Keane, M.A.J. Chem. Soc. Faraday Trans. I 1990, 86, 3961 Su, B.L., Manoli, J.M., Potvin, C. and Barthomeuf, D. J, Chem. Soc. Faraday Trans, I 1993, 89, 857 Dzwigaj, S. De Mallmann, A. and Barthomeuf, D. J. Chem. Soc. Faraday Trans. I 1990, 86, 431 Fitch, A.N., Jobic, H. and Renouprez, A. J. Chem. Soc. Chem. Commun. 1985, 294 Fitch, A.N., Jobic, H. and Renouprez, A. J. Phys. Chem. 1986, 90, 1311 Jobic, H., Renouprez, A., Fitch, A.N. and Lauter, H.J.J. Chem. Soc. Faraday Trans. I 1987, 83, 3199 Lechert, H. and Wittern, K.P. Ber. Bunsenges Phys. Chem. 1978, 82, 1054 Sanderson, R.T. Chemical Bonds and Bond Energy, Academic Press, New York, 1976 Barthomeuf, D. Stud. Surf. Sci. Catal. 1991, 65, 157 Delprato, F. PhD Thesis, University of Haute Alsace, Mulhouse, France, 1990 Su, B.L. and Barthomeuf, D. Zeolites 1993, 13, 623 Parra, C.F., Ballivet, D. and Barthomeuf, D. J. Catal. 1975, 40, 52 Ballivet, D. and Barthomeuf, D. J. Chem. Soc. Faraday Trans. I 1977, 73, 1581 Tsitsishvili, G.V. Adv. Chem. Ser. 1973, 121,291 De Mallmann, A. Thesis, 3rd cycle, University of Paris, France, 1986 Barrer, R.M. and Villiger, H. Z. Kristallogr. 1969, 128, 352 Meier, W.M. and Olson, D.H. Zeolites, 1992, 12, 96 Meir, W.M. and Olson, D.H. Zeolites 1992, 12, 124 Su, B.L. and Barthomeuf, D. J. Catal. 1993, 139, 81