Langmuir 1991, 7, 3027-3033
3027
The Origin of Strong Acidity in H-ZSM-20 Zeolites Yao Sun, Po-Jen Chu, and Jack H. Lunsford' Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received March 7, 1991. In Final Form: August 6, 1991 A ZSM-20zeolite has been synthesized in which the ratio of faujasite to Breckstructure six is approximately 1.8. This material has a framework aluminum concentration of 41 Alf atoms/unit cell. At this Alf concentration, most of the 4-rings of the zeolite contain a single Alf atom, which is one of the requirements that has been proposed for strong acidity. But the normal H-ZSM-20 zeolite contained almost no strongly acidic sites as determined by its activity for the n-hexane cracking reaction. When the zeolite was mildly steamed to form a small amount of extraframework Al, the catalytic activity increased remarkably. It appears that the activity reached a maximum at -32 Alf atoms and 9 extraframework A1 atoms. Upon more extensive dealumination, the activity decreased in a linear mrlnner. Solid-state 27AlNMR revealed that the extraframework A1 was present in at least two states: an octahedral form having a chemical shift of 0 ppm and a tetrahedral form having a chemical shift of ca. 54 ppm. The latter form is believed to reside in the /3 cages as Al(OHP+ions, with the A1 being bonded to three framework oxygen atoms. Through inductive effects the structural hydroxyl groups that are in the region of the cation become strongly acidic. These hydroxyl groups are characterized by infrared bands at 3600 and 3526 cm-l. On the basis of the relative intensities of the hydroxyl bands and Na+poisoningexperiments,it is estimated that approximately half of the structural hydroxyl groups become strongly acidic. This fraction is considerably greater than was previously observed in steam-dealuminated H-Y zeolites and is consistent with the higher activity of the dealuminated H-ZSM-20 zeolite.
Introduction It is now recognized that mild steaming greatly enhances the catalytic activity of H-Y and H-ZSM-5 zeolites for demanding acid-catalyzed reactions, such as hexane cracking.1,2 There is a lack of agreement, however, concerning the origin of this improved activity. The steaming process removes A1 and Si from the framework, and all or part of this extraframework material may be responsible for the enhanced activity. Lago et al.3 concluded that in ZSM-5 the framework aluminum, Alf,was removed from a paired Alf region of the zeolite. The extraframework aluminum acts as a strong electron-withdrawing center for the remaining tetrahedral Alf, thus creating a stronger Bronsted acid site. Beyerlein et aL4suggested that, in dealuminated H-Y zeolites, there is a synergistic effect between framework Bronsted sites and Lewis sites associated with the extraframework aluminum. Zholobenko et al.5 have concluded that these Lewis acid sites are capable of polarizing light alkanes, and thus they enhance the catalytic activity for hexane cracking. They noted, however, that bridged OH groups are also needed for the cracking reaction. The model put forth by Lunsford and c o - w o r k e r ~for ~ *dealuminated ~~~ H-Y is similar to that of Lago et aL3for H-ZSM-5. In this model, two requirements are essential for strong acidity: (i) there must be only one Ab atom in a 4-ring of the zeolite, and (ii) the /3 cage must contain aluminum cations. Corma and co-workers8 attributed the strong acidity to the presence of an amorphous (1) DeCanio, S. J.; Sohn, J. R.; Fritz, P. 0.; Lunsford, J. H. J. Catal. 1986,101, 132. (2) Sohn, J. R.; DeCanio, S. J.; Fritz, P. 0.; Lunsford, J. H. J . Phys. Chem. 1986,90, 4847. (3) Lago, R. M.; Haag, W. 0.;Mikovsky, R. J.; Olson, D. H.; Hellring,
S. D.; Schmitt, K. D.; Kerr, G. T. In Proceedings 7th International Zeolite Conference;Murakami, T., Iifima, A., Ward, J. W., Eds.; Kodansha Ltd.: Tokyo, 1986; p 677. (4) Beyerlein, R. A.; McVicker, G. B.; Tacullo, L. N.; Ziemiak, J. J. J. Phys. Chem. 1988,92, 1967. ( 5 ) Zholobenko, V. L.; Kustov, L. M.; Kazansky, V. B.; Loeffer, E.; Lohser, V.; Peuker, Ch.; Oehlmann, G. Zeolites 1990,10, 304. (6) Fritz, P. 0.; Lunsford, J. H. J. Catal. 1989, 118, 85. (7) Lunsford, J. H. In Fluid Catalytic Cracking III; Occelli, M. L., Ed.; ACS Symposium Series 452; American Chemical Society: Washington, DC, 1991; p 1.
0143-1463f 91f 2407-3027$02.50f 0
silica-alumina phase in the large cavities of a Y-type zeolite, but more recently they have agreed that a cationic form of extraframework aluminum could enhance the acidity of framework hydroxyl groups.9 Early theoretical arguments of DempseylO and also of Mikovsky and Marshallll suggested that the acid strength was related to the A1 distribution in the framework and that only Alf atoms with no second-neighbor Alf atoms in the 4-rings are responsible for strong Br6nsted acidity. The number of aluminum atoms with 0, 1, 2, and 3 aluminum second neighbors in the 4-rings has been calculated by Beagley et al.,12 and the number with zero neighbors, N O ) , peaks at 32 Alf atoms/ unit cell for a faujasite-type zeolite. This calculation assumes a uniquestructures model which takes into account repulsive interactions between Alf atoms. Using cumene dealkylation and hexane cracking as tests of strong acidity in zeolite Y that had been dealuminated either with S i c 4 or by steaming, it was found that the activity results could be interpreted by the N(0) distribution of Alf.lp2 Subsequent experiments by Beyerlein et aL4and results from our laboratory13 showed that the N(0) distribution alone was inadequate to account for the acid-catalyzed activities of dealuminated H-Y zeolites. In particular, Beyerlein et aL4dealuminated a zeolite with ammonium hexafluorosilicate, which removed part of the Alf corn. pletely out of the zeolite so that very little extraframework A1 remained. The resulting material had 32 Alf/ unit cell, which, according to the N(0)distribution, should have resulted in a catalyst having nearly maximum activity, but in fact, the activity was considerably less than that of a zeolite which had been steamed to the same A4 concentration. We have confirmed these results for additional samples that had been dealuminated with (8) Garralon, G.; Corma, A.; Fornes, V. Zeolites 1989,9,84. Sanz, L.; Fornes, V.; Corma, A. J. Chem. SOC.,Faraday Trans. 1 1988,84, 3113. (9) Corma, Ad;Fornes, V.; Rey, F. Appl. Catal. 1990,59, 267. (IO) Dempsey, E. J. Catal. 1974, 33,497; 1975,39, 155. (11) Mikovsky, R. J.; Marshall, J. F. J. Catal. 1976, 44, 170. (12) Beagley, B.; Dwyer, J.; Fitch, F. R.; Mann, R.; Walters, J. J. J. Phys. Chem. 1984,88,1744. (13) Carvajal, R.; Chu, P.-J.;Lunsford, J. H. J. Catal. 1990,125, 123.
0 1991 American Chemical Society
Sun et al.
3028 Langmuir, Vol. 7, No. 12, 1991
(NH4)2SiF6.I3 After mild steaming, Beyerlein et aL4further showed that the activity of their sample was considerably greater than expected, and they suggested that the improved activity might result from a more favorable distribution of extraframework Al. It also is possible that fluorine, introduced during this treatment, may have acted in a synergistic manner with the extraframework A1 to enhance the acidity, and hence the activity.14 Although the previous experiments point to the need for both a proper Alf distribution and the presence of extraframework Al, they do not address the nature of the extraframework Al. Several studies have shown the extraframework A1 is present in different forms,15-18and we have proposed that cationic species of the type [A1 (::)All4+ or A1(OH)2+,present in the /3 cages of the zeolite, are responsible for the enhanced a ~ i d i t y . ~This ,~ proposal is supported by a study in which La3+ions were introduced into Y-type zeolites that had differing amounts of A l p It has been established by diffraction techniques that lanthanum ions enter the /3 cages, where they form speciesanalogousto those noted above for aluminum ions.lg The effect of La3+on catalytic activity was parallel to that observed with the steamed H-Y zeolites, although the polarizing or inductive effect of the lanthanum ions was not as great as that of the aluminum ions. Therefore, the absolute activities in the La3+-exchangedzeolites were less than in the zeolites that contained extraframework Al. To separate the effects of the Alf distribution and the presence of extraframework Al, ideally one would like to synthesize Y-type zeolites having from 0 to 54 Alf/unit cell. Zeolite Y with an Alf concentration less than -50 Alf/unit cell has not been synthesized, but it is possible to synthesize the closely related ZSM-20 zeolite with ca. 40 Alf/unit ell.^*^^ Newsam et have demonstrated that ZSM-20 is an intergrowth of the cubic faujasite (FAU) and Breck structure six (BSS). As shown in Figure 1, pairs of sodalite cages, when connected through a hexagonal prism, can be related by an inversion center (FAU) or by a mirror operation (BSS). In the FAU all supercages are identical, but in the BSS two types of supercages occur. In the larger supercages the 12-ring apertures provide lateral connections between channels. Recently, Davis and c o - ~ o r k e r have s ~ ~ synthesized pure BSS. In the present study it was of interest to determine the acidic properties of H-ZSM-20, both in its normal state and after steaming. The acidic properties were evaluated from the hexane cracking activity of the zeolite. In addition, 29Siand 27Alsolid-state NMR studies have been (14) Becker, K. A.; Kowalak, S. In Recent Advances in Zeolite Science; Klinowski, J., Barrie, P., Eds.; Elsevier: Amsterdam, 1989; p 123. (15) Ray, G. J.; Meters, B. L.; Marshall, C. L. Zeolites 1987, 7, 307. (16) Samoson, A.; Lippmaa, E.; Engelhardt, G.; Lohse, V.; Jerschkewitz, H.-G. Chem. Phys. Lett. 1987, 134, 589. (17) Grobert, P. J.; Beerts, H.; Tielen, M.; Martens, J. A.; Jacobs, P. A. In Zeolites as Catalysts, Sorbents and Detergent Builders; Karge, H. G., Weitkamp, J., Eds.; Elsevier: Amsterdam, 1989; p 721. (18) Man, P. P.; Klinowski, J. Chem. Phys. Lett. 1988, 147, 581. (19) Cheetham, A. K.; Eddy, M. M.; Thomas, J. M. J . Chem. SOC., Chem. Commun. 1984, 1337. (20) Fulop, V.; Borbely, G.; Beter, H. K.; Ernst, S.; Weitkamp, J. J . Chem. SOC.,Faraday Trans. 1 1989,85, 2127. (21) Dewaele, M.; Maistriau, L.; Nagy, J. B.; Gabelica, Z.; Derouane, E. G. Appl. Catal. 1988, 37, 273. (22) Dwyer, J.; Millward, D.; OMalley, P. J.; Araya, A,; Corma, A,; Fornes, V.; Martinez, A. J. Chem. Soc., Faraday Trans. 1990,86, 1001. (23) Newsam, J. M.; Treacy, M. M. J.; Vaughan, D. E. W.; Strohmaier, K. G.;Mortier, W. J. J.Chem. SOC.,Chem. Commun. 1989,493. Vaughan, D. E. W.; Treacy, M. M. J.; Newsam, J. M.; Strohmaier, K. G.; Mortier, W. J. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; ACS Symposium Series 398; American Chemical Society: Washington, DC, 1989; p 544. (24) Annen, M. J.; Young, D.; Arhancet, J. P.; Davis, M. E. Zeolites 1991, 1 1 , 98.
a
b
Figure 1. Line drawings of (a) faujasite and (b) Breck structure six zeolites.
carried out in order to determine the distribution of the framework and the extraframework A1 in the zeolite. Evidence will be presented for a second type of tetrahedral A1 in the zeolite that may be present in the 0 cages. Experimental Section Synthesis. Although attempts to synthesize ZSM-20 according to examples 1 and 5 of ref 25 were unsuccessful, the desired product was obtained by using a variation of the method described by Derouane and co-workers.21 In this synthesis, 90 mL of tetraethylammonium hydroxide solution (40 w t % , Aldrich) was added to a stirred solution of 5 g of NaAlOz (25.2 wt % A1203, 20 wt % NazO, Natal). The mixture was cooled in a water bath. After -10 min the solution became milky white, and 69 mL of tetraethyl orthosilicate was added, with stirring. The beaker containing the mixture was cooled with water for 1 h, after which the slurry was poured into a polypropylene bottle. The bottle was loosely capped and heated in an autoclave for 24 h a t 100 O C . During this period the autoclave was opened for 1 min three or four times. The bottle was then capped tightly and kept at 100 "C for 11 days. The resulting ZSM-20 was filtered from the slurry and washed with water. If left for longer periods in the highly alkaline mother liquor the ZSM-20was transformed, and after approximately 2 weeks, crystalline zeolite fl was detected. Preparation of the Catalysts. The normal H-ZSM-20 catalyst was prepared by treating the synthesized ZSM-20 with a 1 M NH4N03 solution three times at 70-80 OC for 12 h each. The residual sodium content was C0.4 atom/unit cell. The NH4ZSM-PO was transformed into the H-ZSM-20 as described below. Normally no attempt was made to burn out the template, although on one occasion the sample was heated to 400 O C in flowing air. This treatment had no affect on the catalytic activity of the zeolite. Steam-dealuminated ZSM-20 samples containing a range of framework aluminum atoms per unit cell were prepared according to the method of Ward.26 Different temperatures and flow rates of Nz were employed to achieve the desired extent of dealumination. The more extensively dealuminated samples were prepared from the ammonium form, and after steaming, the zeolites were exchanged an additional time in 1M NH4N03. The
-
(25) Ciric, J. S. U S . Patent 3,972,983, 1976. (26) Ward, J. W. U S . Patent 3,929,672, 1975.
Langmuir, Vol. 7, No. 12, 1991 3029
Strong Acidity in H-ZSM-20 Zeolites less extensively dealuminated samples were prepared from the sodium form. Intermediate samples were prepared from zeolites that contained both Na+ and NH4+ ions. The steamed samples that contained Na+ were exchanged three times in 1M NH4N03 to remove residual Na+ and other cations. Evaluation of Catalytic Activity. Catalytic activity measurements were made in a plug flow Pyrex reactor having an internal diameter of 5 mm. A stream of Nz (20 mL mi+) was passed through a n-hexane saturator at 0 "C and then into the catalytic reactor. The catalyst (20-45 mesh) in the amount of 42-44 mg was in the form of an undiluted bed, placed on quartz wool. The reactor above the catalyst was packed with quartz chips to preheat the reactant gases. An external thermocouple was placed adjacent to the catalyst bed. The samples were activated in situ by heating under flowing NZfor 1h at 100 OC, 40 min at 200 OC, 40 min at 300 OC, and 2 h at 400 OC. The activity was determined by GC product analysis (Cl-CS hydrocarbons) using a Carle I11 chromatograph equipped with a 3M porapak Q (80-100 mesh) column at 145 OC. Isohexane was includedas a product of the cracking reaction. The maximum n-hexane conversion was 8% at 350 OC and 21% at 400 "C; therefore, differential conditions existed at 350 "C. The first sample was taken after 5 min on stream to determine the initial activity, and a second sample was taken after 1h to determine the decay in activity. Activity measurements were carried out separately at 350 and 400 OC, with a fresh catalyst being used at each temperature. Sodium Poisoning. The effect of Na+ ions on catalytic activity was determined for a zeolite that had been dealuminated to a level of 20 Alf/unit cell. The steamed sample was ion exchanged three times in a 1 M NH4N03 solution to remove residual sodium. Sodium was then added back by exchanging alloquats of the zeolite in dilute solutions of NaN03. Characterization. The ZSM-20 zeolite was identified and its crystallinity was determined from its X-ray diffraction (XRD) pattern using a Seifert-Santag Pad I1 automated diffractometer. All samples were uniformly crystalline. The unit cell parameters were determined relative to Pb(N03)~(a0 = 7.8568 A) with the zeolite in the hydrated state. The SSi and z7AlNMR spectra were obtained using a Bruker MSL-300 spectrometer (7.05 T). Zirconium rotors were filled with -0.2 g of hydrated zeolite, and the samples were spun (3 kHz) at the magic angle to remove line broadening caused by homonuclear or heteronuclear interactions. Data were collected for ca. 3.5 h for each sample. The Si/Al ratios were calculated using the method of Engelhardt et alez7For the one-dimensional 27Alexperiment, the radiofrequency field strength was kept as low as 42 kHz (32 G, ?r/2 in 6 ps) so that a 2-ps pulse gave rise to the maximum intensity for the central transition in the strongly quadrupolar coupled species. In the two-dimensional nutation experiment the radiofrequency field strength was maximized at 93 kHz (84G, 7r/2 in 2.7 rs). The two-dimensional data array was composed of normal transient decays with progressive 1-ps increments of the excitation pulse. A total of 48-60 data sets were taken. These parameters yielded a spectral width of 1MHz in the rf dimension or 500 kHz in the nutation experiment. This width covered the total region of interest, which was about 3-4 times the rf field (279-472 kHz). The procedure for obtaining the phase-sensitive 2-D Fourier transform in the nutation experiment was the same as that described by Samoson et al.16 Inductively coupled plasma (ICP) and atomic absorption (AA) spectroscopy were used to determine the aluminum and sodium contents, respectively,by analyzinga known amount of dissolved zeolite. The infrared spectra of hydroxyl groups were obtained using a DigilabFTS-40 Fourier transform spectrometer equipped with a MCT detector. The zeolite samples were pressed into selfsupporting wafers (5-10 mg cm-z). The wafers were pretreated under the same conditions as the catalytic samples, except that they were degassed under vacuum rather than flowing Nz.
andfromthelinesat 28= 10.11°and10.90,itwasestimated that the amount of BSS intergrowth in the ZSM-20 synthesized in our laboratory was 36 f 4 % . The absence of a line at 28 = 22.4' confirms that the sample was essentially free of zeolite 0. In a particular sample the Si/Al ratios determined for the parent material by ICP and NMR spectroscopy were 3.91 and 3.76, respectively, which correspond to 39.1 and 40.3 Alf/unit cell. These values are within an experimental error of f l Alf/unit cell. The value obtained by NMR will be used here because the Alf concentrations in the dealuminated zeolites were determined by this method. As noted previously for dealuminated zeolite Y, the unit cell size decreases as Ab is substituted by Si.28 In the cubic FAU structure, the unit cell is defined by a single unit cell constant, ao; however, the hexagonal BSS unit cell is defined by two constants, a and c. For empirical purposes, the XRD pattern may be used to obtain a single unit cell parameter. T h e unit cell constants as a function of A 4 per unit cell are depicted in Figure 3 for the dealu-
(27) Engelhardt, G.; Lohse, V.; Lippmaa, E.; Tarnak, M.; Magi, M. Z. Anorg. A&. Chem. 1981, 482,49.
(28) S o h , J. R.; DeCanio, S. J.; Lunsford, J. H.; O'Donnell, D. J. Zeolites 1986,6, 255.
I
I
I
40
35
30
25
I
I
I
1
20
15
10
5
28
Figure 2. Powder X-ray diffraction pattern of ZSM-20 zeolite. Table I. X-ray Diffraction Powder Pattern of ZSM-20 20 value
this work 5.9 6.2 6.6 10.1 10.8
a
from ref 21 6.0 6.3 6.65 10.25 10.7
11.1
11.1
11.9 15.6 18.7 20.4 23.3 23.7 27.2 29.8 30.9 31.3 31.6 32.6
11.9,12.0 15.7 18.8 20.5 23.4 23.75 27.2 29.85 31.1 31.25 31.55 32.6
d,'
A
14.98 14.26 13.39 8.76 8.19 7.97 7.44 5.68 4.75 4.35 3.82 3.75 3.28 3.00 2.89 2.86 2.83 2.75
re1 intens
vs vs M
M
W W M S M S
M S M W M M M W
Based on 219 values from this study.
Results Composition of ZSM-20. The XRD powder pattern of ZSM-20 is shown in Figure 2 and compared in Table I with literature results.21 The BSS zeolite is readily distinguished from FAU by additional lines at 28 = 5.9O,
10.9', 18.7', 23.3', 25.7', and 31.3', which are present in
BSS,but not in FAU. On the basis of the simulated powder patterns of FAU and BSS reported by Vaughan et al.,23
Sun et al.
3030 Langmuir, Vol. 7,No. 12, 1991 24.7 I
I
h
'524.6 C
2 '
24.5
0
u
-aJ 24.4
0
C
24.3
3
24.2
0
10
20
30
40
50
Framework AI/U.C. I
Figure 3. Variation of unit cell size, aluminum content of H-ZSM-20 zeolite.
ao,
with framework
120
80
E '2-
40
g3
o
v
400 .-,X > .-e
2 300 200
100
0
-80 -100 -120
PPm
PPm
Figure 5. 'Wi NMR spectra of (a) H-ZSM-20 and (b) H-Y zeolites after both had been steam-dealuminatedto 26 Alf/unit cell.
160
7 .-
1
-80 -100 -120
0
10
20
30
40
50
60
Framework AI/U.C. Figure 4. Activity of normal and steam-dealuminatedzeolites: 0 ,activity measured after 5 min on stream; 0, activity measured after 1 h on stream. minated ZSM-20 samples. The values of a0 were determined from the (333) and (440) reflections. The equation that represents the best fit of the data in Figure 3 is Alf/unit cell = 131.1(a0- 24.277)
(1) By use of this equation it is, of course, possible to determine the Alf concentration from the XRD data. Hexane Cracking Activity. The catalytic activities of the normal H-ZSM-20 zeolite and the respective dealuminated samples are shown in Figure 4. The solid line represents the N ( 0 )distribution calculated by Beagley et al.,12 which has been adjusted in amplitude to match the activity data. Clearly, the normal H-ZSM-20 is relatively inactive, and is well below the line; however, the activity increased sharply with a very modest extent of dealumination. The maximum activity (extrapolated) occurred when only ca. 9 Alf/unit cell were removed, which corresponds to about one extraframework A1 atom per /3 cage. 27AlNMR experiments (see below) demonstrate that part of this extraframework aluminum is present in tetrahedral sites and part in octahedral sites. A second point of interest is the fact that the activities of the dealuminated H-ZSM-20 zeolites at a given A4 per unit cell were approximately twice as great as those of dealuminated H-Y zeolites prepared in our laboratory either by steaming or by treatment with SiC14.2,6J3The product distributions obtained over the dealuminated
H-ZSM-20 and the H-Y zeolites were essentially the same. The extent of deactivation by coking was considerably greater with the dealuminated H-ZSM-20 than was found previously with H-ZSM-20 or dealuminated H-Y zeolites, although the extent of deactivation also parallels the activities of the respective zeolites. DSi and 27AlNMR Spectra. Insight into the differences between the dealuminated H-ZSM-20 and H-Y zeolites can be obtained by comparing the *%i spectra of Figure 5. It is apparent that even though the two samples had similar concentrations of A4 atoms, the aluminum distributions were different. The ZSM-20 sample had a larger fraction of Si(1) atoms; whereas the Y-type zeolite had a larger fraction of Si(0) and Si(3) atoms. Here, the numbers in parentheses indicate the number of Alf atoms in adjacent tetrahedral sites. On the basis of spectrum 5a and those of other ZSM-20 samples, it appears that this material did not have any Si(3) atoms. If a Si atom has two or three neighboring Alf atoms, it follows that these Ab atoms also have next-nearest Alf neighbors. The presence of Si(2) atoms in samples containing 26 Alf/unit cell a t first seems inconsistent with the distributions for N(0) and N(1) obtained by Beagley et al.,12 assuming a significant-structures model. That is, at Alf 132/unit cell there should be no 4-rings that contain two Alf atoms. But the disagreement is not definitive because it is possible to have second-nearest Alf neighbors without them being in the same 4-ring. In fact, consideringrepulsive interactions, the former is more probable than the latter. For an Si(3) atom, however, the N(1) configuration must be present. Thus, in the case of zeolite Y, the significant structures model is not strictly obeyed. There is considerable evidence from 27AlNMR that part of the extraframework A1 is present in tetrahedral symmetry. Early work by Ray et al.15 demonstrated that, except for the initial stages of dealumination, the ratio of tetrahedral to octahedral A1remained essentially constant as H-Y zeolites were progressively steamed for longer periods. These authors suggested that the tetrahedral extraframework A1was trapped in the center of the /3 cages. In addition to the resonances of octahedral A1 at 0 ppm and tetrahedral Alf at 60 ppm, several groups have reported an additional resonance at 50-54 ppm in dealuminated Y-type zeolites.lGl8 This line becomes even more apparent in the 2-D NMR experiments described by Samoson et al.'6 I t is generally agreed that the species responsible for the resonance at 50-54 ppm is tetrahedral extraframework Al, but its chemical form and location in the zeolite is uncertain. A dealuminated ZSM-20 zeolite having 26 Alf/unit cell also exhibited 27Alresonances at 0, ca. 54, and 60 ppm, as
Langmuir, Vol. 7, No. 12,1991 3031
Strong Acidity in H-ZSM-20Zeolites
400
1
300 N
5
200
100
100
80
60
40
20
0
-20
PPm Figure 6. *'Al NMR spectraof steam-dealuminatedH-ZSM-20 that contained 26 Alf/unit cell. The inset is a conventional spectrum;the main figure is a 2-D spectrum. The ordinate is the rf dimension.
,
'
,
4000
.
3800
,
'
3600
I
3400
WAVENUMBERS
Figure 8. Infrared spectra of steam-dealuminatedH-ZSM-20 (20Alf/unitcell) after the addition of Na+ions: (a)0.20, (b)3.05, (c) 4.23, and (d) 8.15 Na+/unit cell. 400
0
I
2
4 6 8 1012 Na' IONS/U .C .
Figure 9. Poisoning of n-hexanecracking activity by the addition of Na+ions to H-ZSM-20 with 22 (A,A) and 20 Alf/unitcell (0, 0 ) . Data for solid symbols obtained at 350 O C ; data for open symbols obtained at 400 "C.
1
I
I
shown in Figure 6. The lines at 40 and 98 ppm are spinning side bands of the resonances at 0 and 60 ppm, respectively. The resonances at ca. 54 ppm provide convincing evidence for a tetrahedral form of extraframework A1 in the ZSM20 samples. Infrared Spectra and Sodium Poisoning. Infrared spectra of a normal H-ZSM-20 and a dealuminated H-ZSM-20 that contained 20 Alf/unit cell are shown in Figure 7. The spectrum of the normal H-ZSM-20 zeolite is characterized by three bands a t 3744, 3631, and 3546 cm-'. By analogy with the normal H-Y zeolites, the band at 3744 cm-l is attributed to terminal hydroxyl groups in the lattice and the bands at 3631 and 3546 cm-l are associated with bridging hydroxyl groups that function as relatively weak Bronsted acids. After dealumination, the zeolite exhibited bands a t 3667,3624,3600, and 3540 cm-l, in addition to the usual band at 3745 cm-'. A similar set of bands has been reported for steamed H-Y zeolites? Sodium ions are a strong poison for acidic sites in zeolites, and they also affect the more acidic hydroxyl groups
that are detected by infrared spectroscopy.6 Changes in the acidic hydroxyl bands observed in Figure 8 following the ion exchange of the dealuminated H-ZSM-20 zeolite are described above with Na+ ions. The band intensities for the several samples were normalized by assuming that Na+ exchange did not affect the band of terminal SiOH groups. It is evident that the other bands showed remarkable changes as a result of the exchange reaction. The addition of 3.0 Na+/unit cell appears to improve the resolution of all of the OH bands, without decreasing the amplitude of any. In spectrum b the broad band at 3540 cm-l is resolved into bands at 3526 and 3550 cm-l. At a level of 4.1 Na+/unit cell (spectrum c) the bands at 3600 and 3526 cm-' decreased and a new band appeared at 3693 cm-l. As more Na+ was added, the bands at 3667,3600, and 3526 cm-' decreased more significantly than the bands at 3624 and 3550 cm-I. The effect of sodium poisoning on catalytic activity is shown in Figure 9. The amount of Na+ required to completely destroy the activity was about 10-12 Na+/ unit cell, which is about half the value of Alf per unit cell in this sample. A Na+/Alf ratio of 0.5 is considerably greater than the value of 0.2 that was required to completelypoison the activity in several dealuminated H-Y samples.6
3032 Langmuir, Val. 7,No. 12, 1991 Discussion The fraction of FAU and BSS in a ZSM-20 zeolite appears to vary significantly with the method of preparation. In the samples of Vaughan et al.F3 the ratio of FAU to BSS was 0.33, whereas in the sample described here, the ratio was approximately 1.8. For the purposes of this study the larger ratio is an advantage because it was of interest to compare the effect of dealumination on twozeolites in which the local structures were similar, but which had different Alf concentrations in the starting material. It is, of course, true that ZMS-20 contains microscopicintergrowths and multiple twinning which may affect transport properties in certain reactions, but these factors would not be expected to alter the intrinsic acidity. Several characteristics of the normal H-Y and H-ZSM20 zeolites, such as the frequencies of hydroxyl bands and the chemical shifts of the 29Siresonances, are quite similar. Moreover, the two types of zeolites qualitatively exhibit the same catalytic properties with respect to the normal and the steam-dealuminated samples. That is, the zeolites in their normal hydrogen form are relatively inactive, but upon dealumination, the activity increases to a maximum value a t ca. 32 Alf/unit cell and then decreases linearly with respect to further dealumination. It is significant that lanthanum-exchanged Y-type zeolites exhibited a similar functional response. But as noted previously, the maximum (interpolated) activity of the dealuminated ZSM-20 for n-hexane cracking was about twice as great as that reported previously for steam-dealuminated H-Y zeolites. Moreover, the activities observed for zeolites in which [Allf S32/unit cell also were greater by a factor of 2. The catalytic behavior of the H-ZSM-20 zeolite is consistent with the model described in the Introduction, in which both a single Alf atom per 4-ring and the presence of cationic aluminum in the pertinent p cage are required for strong acidity. The linear relationship between activity and Alf per unit cell from 0 to ca. 32 Alf/unit cell, as shown in Figure 4, is predicted by the N(0) distribution, based on the significant-structures model. The presence of Si(2) atoms in some of the more A1 rich ZSM-20 samples does not invalidate this model. It is evident, however, from the inactivity of the normal H-ZSM-20 zeolite that an additional requirement must be met for strong acidity to develop. Clearly, a relatively small amount of extraframework (
