J. Phys. Chem. 1988, 92, 1967-1970
1967
Influence of Framework and Nonframework Aluminum on the Acidity of High-Silica, Proton-Exchanged FAU-Framework Zeolites R. A. Beyerlein,+ G . B. McVicker,* L. N. Yacullo, and J. J. Ziemiak Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801 (Received: August 6, 1987)
Measured selectivities and activities in isobutane conversion investigations on high-silica FAU materials prepared by different methods of dealumination indicate that the strong acidity exhibited by hydrothermally dealuminated (ultrastable) materials, while directly related to framework aluminum content, depends on a balance between aluminum in the framework and dislodged or nonframework aluminum that is entrained in the lattice during steam treatment. Isobutane conversions over ultrastable FAU materials containing low fractions of dislodged aluminum exhibit markedly enhanced activities, indicating that conventionally prepared ultrastable materials may contain a higher fraction of dislodged aluminum than is optimum.
Introduction The removal of aluminum from the framework of Y-type zeolites strongly influences the activity and selectivity patterns of a wide range of hydrocarbon conversion For ultrastable, high-silica FAU materials prepared by steam dealumination, interpretation of catalytic data is complicated by the presence of entrained, nonframework aluminum species. The individual and collective roles of framework and nonframework aluminum species on catalyst performance are not well understood. The influence of zeolite composition (acidity) on catalytic cracking activity can be established by using a variety of test reactions. Recent investigations by Bremer and co-workers’ of hexadecane cracking over a series of ultrastable, FAU-type materials revealed a linear dependence of total conversion rates on the quantity of aluminum retained in the zeolites following acid extraction. This finding supports the prediction of Beaumont and Barthomeufl that acid-site strength in dealuminated faujasite should be constant for Si/A1 2 6. Similar results, with hexane cracking over a series of H-ZMS-5 materials, 20 5 Si/A1 5 20000, were reported by Haag et aL6 Both the Bremer et aL5 and Haag et a1.6 papers conclude that the activity per acid site is essentially constant. In the hexadecane-cracking studies over dealuminated faujasites, no results pertaining to the fraction of aluminum residing in nonframework positions were presented, nor were the possible catalytic consequences of extra-framework aluminum species addressed. We have examined the catalytic properties of a series of ultrastable FAU materials prepared by steaming/acid extraction, using isobutane conversion to make direct correlations between “catalytic acidity” and framework composition. Isobutane conversion selectivities were determined as previously described’ in order to obtain the relative contribution of carbonium ion pathways for each framework composition. Two important findings have emerged. First, carbonium ion activity, as evidenced by skeletal isomerization and polymerization and back-cracking of isobutane, is directly proportional to framework aluminum content. Second, comparison with results on “clean framework” high-silica FAU materials prepared by low-temperature chemical dealumination with ammonium hexafluorosilicate (AHF) indicates that the number of acid sites capable of initiating carbonium ion pathways depends on a balance between aluminum in the framework and dislodged or nonframework aluminum. The latter statement is strengthened by results of isobutane conversions employing high-silica FAU materials produced by mild steaming of those initially dealuminated with AHF. These materials, which now contain a substantial fraction of dislodged aluminum and are themselves ultrastable materials, show a dramatic increase in carbonium ion activity with respect to their “clean framework” parent. *Author to whom correspondence should be addressed. ‘Present address: Amoco Oil Co., P.O. Box 400, Naperville, IL 60566.
0022-3654/88/2092-1967$01.50/0
Experimental Section Materials Preparation and Characterization. High-silica, FAU materials with framework compositions of 5 5 %/AI 5 60 were prepared in a conventional manner by multiple steamings of H-Y zeolite at 550-650 OC for 2 h with intermediate ammonium-exchange treatments and aluminum extractions using 0.01-0.1 N HC1. The starting material was an ultrastable faujasite, LZ-Y82, obtained from Union Carbide. For comparison, an ultrastable FAU material, USY-A, was prepared by a single 2-h steam treatment at 650 “C of an (NH4+,Na+)-Y zeolite (75% exchanged; Si/Al = 2.3) followed by two ammonium-exchange treatments. Plots of nonframework aluminum and n-hexane sorption capacity (Figure l ) , and lattice parameter (Figure 2), are given against framework A1 per unit cell (AI,) as determined by 29SiMASNMR.*-l0 High crystallinity, as evidence by adsorption isotherms for n-hexane, is maintained throughout the 5 5 Si/Al, 5 60 composition range (Figure 1b) even though these materials contain substantial quantities of nonframework aluminum Al, (Figure 1a). For increasingly dealuminated materials, the 29Si MASNMR spectra showed no increase in background for chemical shifts in the vicinity of -1 10 ppm,” providing additional support for crystallinity maintenance. The initial N a content of this dealuminated series of materials was low, 1 0 . 2 wt 56 N a 2 0 for both LZ-Y82 and USY-A, and was progressively lowered by subsequent steaming/acid extraction treatments. The linear correlation shown in Figure 2 between X-ray lattice pa(1) Beaumont, R.; Barthomeuf, D. J . Catal. 1972.26, 218. 1972,27, 45. Barthomeuf, D.; Beaumont, R. J . Catal. 1973, 30, 288. (2) Ward, J. W. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 118. (3) Poutsma, M. L. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 437. (4) Barthomeuf, D. In Catalysis by Zeolites; Imelik, B., Ed.; Elsevier: Amsterdam, Netheiland, 1980; p 55. (5) Bremer, H.; Wendlandt, K. P.; Chuong, Tran Khac; Lohse, U.; Stach, H.; Becker, K. In Proceedings of the 5th International Symposium on Heterogeneous Catalysis; Varma, 1983; Part 1; p 435. (6) Haag, W. 0.;Lago, R. M.; Weisz, P. B. Nature (London) 1984, 309, 589. (7) McVicker, G. B.; Kramer, G. M.; Ziemiak, J. J. J . Catal. 1983, 83, 286. (8) Engelhardt, G.; Lohse, U.; Lippmaa, E.; Tarmak, M.; Magi, M. Z . Anorg. Allg. Chem. 1981, 482, 49. (9) (a) Ramdas, S.; Thomas, J. M.; Fyfe, C. A.; Hartman, J. S. Nature (London) 1981, 292, 228. (b) Klinowski, J.; Ramdas, S.; Thomas, J. M.; Fyfe, C. A,; Hartman, J . S. J . Chem. Soc., Faraday Trans 2 1982, 78, 1025. (IO) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J. J . Am. Chem. Soc. 1982, 104, 4859. (1 1) Lattice destruction of thermochemically treated FAU materials, leading to formation of an amorphous silica phase, may be identified in the 29Si NMR spectra by a broadened peak or shoulder at about -110 ppm (TMS). See Engelhardt, G.; Lohse, U.; Magi, M.; Lippmaa, E. In Structure and Reactivity of Modified Zeolites; Jacobs, P. A,, et al., Eds.; Elsevier: Amsterdam, Netherlands, 1984; p 23.
0 1988 American Chemical Society
Beyerlein et al. I
a
I
b
I
,
1
h
0
i
0
10 20 30 AIF, Framework Alluc
0
20 30 Framework Alluc
10 AI,
0
L
Figure 1. (a) Nonframework aluminum content per unit cell, AIN,versus framework aluminum per unit cell, AIF, for steam-dealuminated, mild acid extracted FAU materials. (b) n-Hexane adsorption capacity versus AIF for dealuminated FAU materials.
0 0
-
/
/' /
24.40
e
2
24.20
r1 e
0
I
I
1
10 20 AI,, Framework A l h c
30
60 Pressure-(torr)
90
3
120
Figure 3. n-Hexane adsorption isotherms at 25 O C for a parent Na-Y material with Si/AI = 2.3, 0;ammonium hexafluorosilicate dealuminated FAU material prepared from an ammonium-exchanged, Na-Y parent (using the method of ref 13) which was further treated to lower its Na content as described in the text, 0.
24.50
z E
/I
Na-Y
o (NH,),SiF6 Treated FAU
1
30
Figure 2. Lattice parameter a,, for dealuminated FAU materials versus AIF. The dashed line gives the dependence of a. on A1F determined by Fichtner-Schmitteret al., ref 12.
rameter and framework aluminum content, as determined by 29Si MASNMR, is in good agreement with the correlation recently reported by Fichtner-Schmittler et a1.I2 The Fichtner-Schmittler correlation is reproduced as the dashed line. The two data points at the extreme right of Figure 2 denote L Z Y 8 2 and USY-A, respectively. Dealuminated FAU materials containing minimal amounts of dislodged aluminum were prepared by treating an (NH4+,Na+)-Y zeolite (75% exchanged; Si/A1 = 2.3) with ammonium hexafluorosilicate (AHF).13 The product material (sample A) showed Si/A1 = 4.6 by both elemental analysis and 29SiMASNMR.*-'O A portion of this sample was ammonium exchanged, calcined for 1 h at 350 OC, and then ammonium exchanged twice more to lower the sodium concentration from 1.2 to 0.06 wt % (sample B). Both samples A and B showed a lattice parameter of a. = 24.54 A, which is close to that measured for the ultrastable materials LZ-Y82 (ao = 24.56) and USY-A (ao = 24.58) with similar framework composition (Figure 2). As shown in Figure 3, the n-hexane adsorption isotherm of sample B measured at 25 O C is comparable to that of the parent Na-Y material. These results indicate that near-defect-free materials are produced by this low-temperature, chemical dealumination method in which silicon is believed to replace tetrahedral-site vacancies created by aluminum remova1.l3 Two additional ultrastable FAU materials were prepared by mild hydrothermal treatment of the low-sodium, AHF-treated (12) Fichtner-Schmittler, H.; Lohse, U.; Engelhardt, G.; Patzelovl, V. Crysr. Res. Technol. 1984, 19, K1. (1 3) Skeels, G. W.; Breck, D. W. In Proceedings of the Sixth Inkmational Zeolite Conuerence; Olson, D . , Bisio, A., Eds.; Butterworths: UK, 1984, p 87.
material, sample B. A portion of this sample was steamed at 570 "C (PH20= 1/4 atm; 2 h) and then ammonium exchanged. Half of the resulting material received a final ammonium exchange (sample C) while the second half was subjected to an acid extraction with 0.033 N HC1 (sample D). The refined lattice parameters for these materials were a. = 24.42 f 0.01 8, for sample C and a,, = 24.36 f 0.01 8,for sample D, showing that substantial dealumination had occurred. n-Hexane adsorption capacities for each material, 0.17 g/g, were only slightly lower than that measured for the parent material, sample B, 0.18 g/g. Chemical analyses were performed by using a Jarrell Ash 1100 (ICPES) Spectrometer. Powder X-ray diffraction measurements were performed by using an analog Philips diffractometer and an automated Siemens D500 system (both using Cu K a radiation). Capacities for adsorption of n-hexane were determined from full isotherms measured at 25 OC. 29SiMASNMR data were accumulated on a JEOL FX200 spectrometer operating at 4.7 T with 0.4 g of sample in the spinning probe. All MASNMR samples were examined in the fully hydrated state. Apparatus for Model Compound Studies. Isobutane conversions were carried out at 1.0 atm (101 kPa) total pressure in a 22-cm3, stainless steel, fixed-bed reactor operated once through. The reactor bed configuration consisted of 2-cm3pre- and postheat zones (mullite, 10/16 mesh) which preceded and followed a catalyst zone containing 1.Og of neat zeolite (previously compacted at 100 MPa and broken up to 10/20 mesh), diluted to 18 cm3 with mullite beads.' Kinetic measurements were performed at 500 OC with a stream of 0.25 atm of isobutane in helium (200 cm3/min) following pretreatment of the sample in dry helium at 500 OC for 1 h. Carbonium ion selectivities were estimated with the molar product ratio (n-butane propane + isopentane)/total conversion products. This ratio accounts for the major carbonium ion based products arising from isomerization and chain-cracking sequences. Methane and olefinic products were associated with a radicallike cracking mechanism as previously discussed.'
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+
Results and Discussion Conuentionally Prepared Ultrastable Materials. Carbonium ion rates for the conventionally prepared ultrastable FAU materials were determined from the measured isobutane conversion patterns at 500 OC, and the results are given with respect to framework aluminum content in Figure 4. For %/AIF I5, a h e a r dependence of carbonium ion rate on framework aluminum is observed. This finding indicates that framework aluminum is uniquely related to active sites and suggests that acid strength per active site is essentially constant for this series of materials. The USY-A material (Si/AIF = 4.5, A1F = 34.9) shows comparable carbonium ion activity to that of LZ-Y82 (Si/AIF = 5.1, AIF = 3 1,5), Thus, despite differences in preparation, carbonium
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 1969
Influence of Aluminum on the Acidity of Zeolites
TABLE I: Influence of Sample Size on Isobutane Conversion:' Results for Ultrastable FAU Materials
convn rates framework composn Si/AIFb ii: 5
Si/AIF'
== 12.4
g
convn, mol %
% carb ion act.
0.30 0.40 0.50 0.60 0.70 0.80 1.oo 1.oo 2.30
7.28 9.48 12.6 15.7 18.8 21.1 26.9 13.0 30.1
69 70 73 74 75 76 80 71 78
total, mol/(h.g)
X lo3
mol/(h.g)
32.5 31.7 33.7 34.9 35.9 35.3 36.0 17.4 17.5
carb ion l o 3 molecules/(min.AIF)
X
22.4 22.2 24.6 25.8 26.9 26.8 28.8 12.4 13.6
0.196 0.194 0.215 0.225 0.235 0.234 0.251 0.227 0.249
'500 OC, 1.0 atm, 0.25 atm of i-C4/He (200 cm3/min); pretreat 1 h, 500 OC, He. bLZ-Y82 obtained from Union Carbide. 'High-silica FAU materials prepared from LZ-Y82; 650 'C steam, 2 h, PHZO = 1/3 atm; NH4* exchange; 0.033 N HCI extraction.
TABLE 11: Ammonium Hexafluorosilicate" (AHF) Dealuminated FAU Materials: Comparison with Conventional Ultrastable FAU Materials and with Ultrastable FAU Materials Produced from AHF Dealuminated Materials
NMR
chem anal.
framework A1 (NMR), AI,
4.6 4.9 5.1 4.5 8.1 (12.4)r
4.5 5.1 2.7 2.7 5.7 8.6
34.3 32.5 31.5 34.9 21.1 14.3
Si/AI
sample A" Bb LZ-Y82'
USY-Ad Ce D'
lattice param, A
Na content, wt %
24.54 24.54 24.56 24.58 24.42 24.36
1.24 0.06 0.15 0.10 0.08 0.08
i-C4convn rates, mol/(h.g) X lo3 total carb ion 4.1 13.7 35.8 36.8 43.5 33.5
5% carbon ion act.
51 62 80 79 76 74
2.1 8.5 28.7 28.9 33.1 24.8
"Dealuminated by method of Skeels and Breck, ref 13. bSample A after Na extraction (NH4+exchange; 350 O C , 1 h; NH4* exchange (2)). 'Obtained from Union Carbide; rate values are averages from seven runs. dUltrastable FAU material prepared from a Na-Y zeolite; see text. e Ultrastable FAU materials produced from AHF-dealuminated materials; see text. JFramework aluminum content determined from X-ray lattice parameter correlation (ref 12); NMR analysis was compromised by poor resolution in the 29SiMASNMR spectrum. ion rates for the conventional ultrastable FAU materials prepared in this study appear to depend only on framework composition. Additional isobutane conversion investigations were carried out with varying amounts of conventional ultrastable FAU materials with compositions Si/AIF = 5 and 12.4. The results, given in Table I, show that, over a wide range of sample sizes, total rates and carbonium ion activity are little affected and that carbonium ion activity per framework aluminum atom, 0.23 molecules/(min.AIF), is essentially independent of both sample size and framework aluminum content. Such consistency strongly supports the contention that carbonium ion activity is directly dependent upon framework aluminum content. "Clean Framework", High-Silica FAU Materials. For comparison, isobutane conversions were carried out on high-silica FAU materials dealuminated with A H F (samples A and B) that contained minimal amounts of dislodged aluminum. Compositional analyses and isobutane conversion results for these materials are summarized in Table I1 and are compared with those for the two conventional ultrastable materials, USY-A and LZ-Y82, with similar framework compositions. The low-sodium material, sample B, displays a poor carbonium ion selectivity (62%) and a carbonium ion rate which is a factor 3 lower than that exhibited by the conventional ultrastable FAU materials, including USY-A which was prepared from the same starting Na-Y. The lowered activity and selectivity exhibited by this "clean framework" material (sample B) imply that dislodged aluminum species entrained in the lattice during hydrothermal treatment are playing an essential role in the enhanced performance displayed by conventional ultrastable FAU catalysts. This interpretation is supported by the recent investigations of Ward and Carlson,14 who found that high hydrocracking activity was observed only if the as-prepared, AHF-treated materials were subjected to a steam treatment, thereby producing nonframework aluminum species entrained in the zeolite lattice. Ultrastable FAU Materials Prepared from AHF- Treated Materials. In order to further investigate the effects of dislodged aluminum, isobutane conversions were carried out on two ul(14) Ward, J. W.; Carlson, T. L. U S . Patent 4 5 1 7 0 7 3 , May 14, 1985.
3
n V
SOO'C
0
I
0
1
I
20
30
AI,, Framework Alluc
Figure 4. Carbonium ion rates over low-sodium, dealuminated FAU materials versus framework aluminum per unit cell, AIF, from isobutane conversion measurements at 500 OC.
trastable catalysts, samples C and D, prepared from the low-sodium, AHF-treated material, sample B. A dramatic enhancement in carbonium ion activity is exhibited by each of these materials. Sample C, which is estimated to have 21 aluminum atoms per unit cell (Table 11) on the basis of both 29SiMASNMR and X-ray diffraction measurements, shows a carbonium ion rate, 33.1 mol/(h.g) X that is a factor of 1.65 greater than that for a conventionally prepared ultrastable FAU material (Figure 4) having the same framework aluminum content. The carbonium ion selectivities for these materials, 76% and 75% respectively, are essentially equivalent. As shown in Figure 5 , both conqentionally prepared and mildly steamed AHF-treated, ultrastable FAU catalysts exhibit a linear dependence of carbonium ion activity upon framework aluminum content. In the absence of ultrastabilization, low-sodium, AHF-
1970 The Journal of Physical Chemistry, Vol. 92, No. 7 , 1988
30
/
lsobutane
2s
2-
c x
-OE
20
'E f Of
e0
6 6 ,o
I
10
20
30
AI,, Framework Alluc
Figure 5. Carbonium ion rates over ultrastable materials: conventionally prepared, 0 ; prepared from materials initially dealuminated by using AHF, A. The single data point at the lower right represents the carbonium ion rate over the low-sodium, AHF-treated FAU material, sample B (see text).
treated materials exhibit inferior carbonium ion activity as noted by the single data point off the curves in the lower right corner of Figure 5. Even though X-ray diffraction measurements and n-hexane adsorption isotherms showed that both conventionally prepared and mildly steamed AHF-treated materials are highly crystalline FAU materials, the carbonium ion activity per framework aluminum atom for the latter materials is substantially greater. Only a fraction of this activity difference, however, can be attributed to the 15-20% higher n-hexane adsorption capacities exhibited by the mildly steamed, AHF-treated materials. From Table I1 and Figure l a , it is evident that the ultrastable FAU materials produced from those initially dealuminated with A H F have fewer nonframework aluminum species (AlN) than do those which are conventionally prepared. AlN/AlF = 0.42 and 0.44 for samples C and D produced from materials initially dealuminated with A H F as contrasted with the range 0.66 I AlN/AIF 5 2.2 for the conventionally prepared materials. As discussed in the following section, the lower fraction of nonframework aluminum may be a significant factor in the activity increase exhibited by the mildly steamed A H F materials.
Conclusions The present investigation provides compelling evidence that the presence of dislodged aluminum is essential to good catalytic performance in acidity-dependent reactions over high-silica FAU
Beyerlein et al. materials. The marked activity enhancement that occurs upon mild steaming of "clean framework", AHF-treated materials is consistent with previous work on H-ZSM-51S.16and also with recent investigations by Ward and Carlson14 on composite hydrocracking catalysts containing AHF-treated FAU materials. Increased catalytic activity for the ultrastable materials may involve a synergism between framework Bronsted sites and Lewis sites associated with dislodged aluminum as has been described in the previously proposed concept of superacidity.17J8 The correlations of activity with composition support a direct dependence of carbonium activity on framework aluminum in all of the ultrastable materials. For the series of conventionally prepared materials, the fact that no dependence is evident between activity and the amount of nonframework aluminum species (AlN) over a range 0.7 IAlN/AlF I2.2 (Figures l a and 4) suggests that these materials contain dislodged aluminum in excess of that required to maximize the formation of strong acid sites. The ultrastable FAU materials produced by mild steaming of those initially dealuminated with A H F contain a significantly lower fraction of nonframework aluminum, AlN/AlF = 0.4, and these materials exhibit higher carbonium ion activity than do conventionally prepared ultrastable FAU materials. While these studies do not indicate the preferred level of dislodged aluminum for highest catalytic acidity as was recently found for H-ZSM-5,16J9 it is clear that conventional preparations of ultrastable FAU materials contain a significantly higher fraction of dislodged aluminum than is optimum. Other factors, such as local aluminum environment and siting of dislodged aluminum atoms, could play a significant role. Extensive 29Siand 27AlMASNMR investigations and aluminum doping/insertion studies in concert with model compound conversions are in progress to address these issues.
Acknowledgment. We gratefully acknowledge W. L. Schuette and L. A. Pine for their continuing interest in and support of this work and H. Malone for performing the 29SiN M R measurements. Registry No. AI, 7429-90-5; isobutane, 75-28-5. (15) Ashton, A. G.; Batmanian, S.; Clark, D. M.; Dwyer, J.; Fitch, F. R.; Hinchcliffe, A.; Machado, F. J. In Catalysis by Acids and Bases; Imelik, B., Ed.; Elsevier: Amsterdam, Netherlands, 1985; p 101. (16) Lago, R. M.; Haag, W. 0.;Mikovsky, R. J.; Olson, D. H.; Hellring, S. D.; Schmitt, K. D.; Kerr, G. T. In "New Developments in Zeolite Science and Technology"; Proceedings of the 7th International Zeolite Conference, Murakami, Y . ,Iijima, A,, Ward, J. W., E&.; Kodansha: Tokyo, 1986; p 677. (17) (a) Jacobs, P. A. CarboniogenicActivity of Zeolites; Elsevier: Amsterdam, Netherlands, 1977. (b) Jacobs, P. A.; Leeman, H. E.; Uytterhoeven, J. B. J . Catal. 1974, 33, 17. (18) Mirodatos, C.; Bathomeuf, D. J . Chem. SOC.,Chem. Commun. 1981, 39. (19) Ashton, A. G.; Batmanian, S.; Dwyer, J.; Elliott, I. S.; Fitch, F. R. J . Mol. Catal. 1986, 34, 73.
