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J . Phys. Chem. 1995, 99, 1276-1280
Structure Identification of Intermediate Aluminum Species in USY Zeolites Using High-Resolution and Spin-Lattice Relaxation 27AlNMR Xiaolin Yang Engelhard Corporation, R&D, 101 Wood Avenue, Iselin, New Jersey 08830 Received: August 31, 1994; In Final Form: October 18, 1994@
Using high-resolution and spin-lattice relaxation MAS 27AlNMR methods, we obtained structural information about A1 species in steamed Y zeolites that are intermediate between NMR-detectable tetrahedral and octahedral A1 species. The A1 species, NMR shifted from aluminum nitrate aqueous solution at about 30 ppm, responded to various chemical and physical sample treatments dramatically differently than the tetrahedral framework and octahedral extraframework A1 sites. Spin-lattice relaxation resolves at least two new peaks, previously unreported, near the 30 ppm peak. The TI relaxation times of the 30 ppm A1 sites are also different from the tetrahedral and octahedral A1 sites. These experimental results lead to a conclusion that the 30 ppm peak reported previously is actually a mixture of extraframework A1 species. The results are of fundamental importance in understanding steamed zeolite structures and in quantifying catalytically active sites. This work illustrates how sample treatment and spin relaxation can provide additional "resolution" to NMR spectroscopy of a complex solid material. It also serves as a reminder that the structure of steamed zeolites is more diverse than previously thought.
Introduction Zeolitic aluminosilicates have important industrial uses, such as catalysts and adsorbents.' There is also a growing scientific interest for developing novel materials based on zeolites.2 Aluminum substituted for some of the Si(+4) zeolite framework atoms is directly related to catalytic active sites in many cases due to its trivalent oxidation state. For a freshly crystallized zeolite, almost all of the A1 atoms are in the framework (FW) lattice. After calcination and steaming, which are necessary for making ultrastable zeolites, some A1 is released from the framework structure, forming extraframework Al species (EFW).3 The structural nature of the EFW A1 species and their roles in zeolite properties, e.g., acidity, are not well understood and are being actively investigated. High-resolution solid-state 27Al NMR has been extensively used for studying Al-containing materials, particularly A1 sites in zeolites:-* For a typical fresh zeolite, 27AlNMR spectra show mainly one peak at about 60 ppm due to FW tetrahedral A1 sites. For a calcined and/or steamed zeolite, two additional peaks are located at about 30 and 0 ppm, using magic-angle-spinning (MAS) high-resolution NMR. The 0 ppm peak is conventionally assigned as octahedrally-bonded EFW A1 sites.'-8 Controversies exist over the structure and origin of the 30 ppm peak since its discovery a decade ago. Some believe it is due to five-coordinated A1 species; others consider it a tetrahedrally-bonded A1 species (either FW or EFW) that suffers greatly from second-order quadmpolar interactions. The controversy is not just conceptual or terminological: it affects our ability to understand and control zeolite structures and to make quantitative measurement of catalytic active sites. The present work provides new experimental data on this issue. In the mid-l980s, several research groups reported the observation of a five-coordinated A1 species at about 30 ppm in a high-resolution solid-state "AI NMR spectrum in barium aluminate gl~colate,~ andalusite and dehydrated kaolin, lo barium aluminum glycolate and andalusite," steamed faujasite, and ZSM-5.12 X-ray diffraction provided evidence for the existence Abstract published in Advance ACS Abstracts, December 15, 1994.
0022-3654/95/2099-1276$09.00/0
of the 5-coordinate A1 dimer species in barium aluminum glycolate and trigonal-bipyrimidal nonframework A1 species in andalusite." The assignment of 5-coordinate A1 species for steamed zeolites was based on (1) a comparison to barium aluminum glucolate and andalusite in terms of their chemical shifts and (2) an extrapolation from the chemical shifts of tetrahedral and octahedral A1 sites based on the fact that the chemical shift monotonically depends on the number of oxygen atoms that are connected to the A1 atom probed.12 Almost at the same time, another school of explanation of the intermediate A1 species was introduced. Using two-dimensional 27AlNMR followed by a quantitative estimation of the quadrupolar interactions, Samoson et al. were the first to propose that the 30 ppm peak was due to a 4-fold coordination extraframework A1 site.13 Using CP/MAS and ultrahigh-speed (20 kHz) 27Al NMR, Kellberg et al. showed that the nonframework 30 ppm peak of USY is an individual signal and not part of a second-order quadrupole line shape of tetrahedral sites.l4 Similar conclusions were also reached by Schmitt et al. using a spin-echo spectral editing technique.l5 Very recently, the structural origin of the 30 ppm peak of USY was again investigated by Ray and Samoson and by Haddix et al. using double-orientation-rotation (DOR) NMR probes.I6J7 DOR averages the anisotropy of the second-order quadrupolar interactions and thus, in principle, could reveal intrinsic resonances only due to isotropic chemical shifts. In contrast to all of the earlier observations, both publications revealed two, not one, small peaks between 60 and 0 ppm under DOR conditions. Ray and Samoson found that one of the two peaks, located at 47.5 ppm, is due to tetrahedral F W A1 sites. The EFW A1 species associated with the second peak located at about 30 ppm are different in samples that had different hydrothermal treatment. For a singly thermal-dealuminated sample, the 30 ppm peak is caused by a significant secondorder quadrupolar shift of a tetrahedrally-coordinated A1 species. No large second-order shift was found for the doubly thermaltreated sample, and the 30 ppm peak was attributed to either a tetrahedrally-bound species with an unusual chemical shift or 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 4, 1995 1277
Intermediate Aluminum Species in USY Zeolites a pentacoordinated A1 species. In contrast to the conclusions reached by Ray and Samoson, Haddix et al. showed that one of the two small peaks, located at about 40 ppm, is due to spin sidebands, while the other one at about 20 ppm could be either due to spin sidebands or a unique isotropic species. This background summary demonstrates that the structural origin of the 30 ppm peak is far from being resolved. In the present work, we used two approaches for more insight about the structural nature of the 30 ppm A1 species. First, we used physical and chemical treatments to disturb all the AI sites before probing their different responses. The sample treatments include steam dealumination, dehydration, acid impregnation, and NH3 adsorption. Second, we applied a spin-lattice relaxation method (saturation-comb) to disturb all of the spins as we probe their different responses. We report here the structural information indicated by these experiments and NMR analysis.
7 -
100
I
i
--=-
-I&
--
- 0 -.,w
100
(c) LZY82-650s. Asterisk indicates spin sidebands.
Parent Zeolites. The USY zeolites studied in the present work are commercially available Union Carbide Linde Y zeolite LZY62, LZY82, and steam-dealuminated Linde Y zeolite 82 (LZY82-650s). LZY62 is a N&+-exchanged fresh Y zeolite. Almost all the A1 sites are the tetrahedral framework type. LZY82 was produced by mild steam dealumination of LZY62 and followed by N&+ exchange, which transforms about 50% of the tetrahedral A1 sites into extraframework ones. The LZY82-650s sample was obtained by further steam dealumination of LZY82 at 650 "C and 50% water partial pressure in air for 4 h. This causes severe destruction of the zeolite lattice, leaving about 20% of the tetrahedral framework sites. The framework atomic SUA1 ratios for the three zeolites are about 2.5, 5.0, and 12.5, respectively, based on a 29SiNMR measurement.I8 All the zeolites were stored in a closed container to equilibrate with water vapor prior to the various treatments we conducted. Sample Treatment. Samples were generated for each of the three treatments: acid impregnation, NH3 adsorption, and dehydration. Acid treatment was carried out by equilibrating the water-saturated zeolites at room temperature with dilute HCl aqueous solution and then drying the sample in air at room temperature. The NH3-treated sample was made by equilibrating the HzO-saturated sample with ammonia gas. Dehydration was done at 150 "C in a vacuum of about Torr for about 15 h. NMR Measurement. All the 27AlNMR spectra were taken at room temperature using a Varian Unity-400 spectrometer which is operated at 104.2 MHz for the 27Alnucleus. A Doty two-channel probe is used which has a 5 mm sample rotor spinning at about 10 kHz. A single pulse was applied for the excitation. We used a zI12 pulse and a pulse which produces a maximal peak intensity. Both give very similar results. A total of 1000 scans were collected for each spectrum. Spin-lattice ( T I )relaxation measurement was carried out using a saturation-comb pulse s e q u e n ~ e . ~The ~ - pulse ~ ~ train consists of ten short pulses with a varying space between 0.2 and 0.9 ms. The recovery is sampled after a time of z. A total of 4000 scans were collected for each spectrum.
TABLE 1: AI Intensity Measurement Results
Effect of Steam Dealumination. Figure 1 displays the 27Al NMR spectra of LZY62, LZY82, and LZY82-650s saturated with water vapor. Three peaks are clearly seen for the steamed samples: tetrahedral F W at about 60 ppm, octahedral EFW at about 2-4 ppm, and the intermediate species at about 30 ppm.
v
-
-
-100
6 (ppm, u(o%7 Figure 1. *'A1 NMR of HZO-saturated (a) LZY62, (b) LZY82, and
Experimental Section
Results
r
0
relative intensity (%) 60 PPm
30 PPm
0 PPm
0.85
100 66 48
11 21
23 31
0.90 0.97 0.91 0.98
39 41 75 89
7 3 4 1
54 56 21 10
0.50
25
44
31
1m9 samples untreated LZY62 LZY82 LZY82-650s treated LZY82-650s-HC1-24 LZY82-605s-HC1-45 LZY82-650s-NH3-24 LZY82-650s-NH3-45 dehydrated LZY82-650s-dried
W0(Vf) 1.00 0.97 h days h days
"Total A1 intensity per gram of sample on a volatile-free basis, assuming 1.00 for LZY62. The volatile material was measured using TGA at 700 "C.
Deconvolution was performed to make a quantitative estimation of each peak as a function of steam dealumination. The deconvolution results, together with the NMR measured total A1 intensity per gram of a dried sample, are listed in Table 1. We found in the deconvolution that it is difficult to get a stable line width value because it depends on the number of components and the GaussinLorentzian fraction used. However, the relative intensity of each peak is relatively stable regardless of the deconvolution function used, and the result is accurate enough for the purpose of pointing out the general trend of the data. Several trends are seen regarding the data in Table 1 for the untreated samples. First, the total A1 intensity decreases as the sample gets more dealuminated. LZY62 is a freshly crystallized zeolite. Almost all of the A1 is in a symmetric, tetrahedral bonding environment. The total A1 intensity measured should reflect all the A1 species present. The A1 contents in the three untreated samples are nearly the same: 22.3, 22.8, and 23.2 wt %, respectively. The difference between the elemental analysis and the NMR measurement indicates that steam dealumination makes some A1 sites NMR invisible even when the samples are water-saturated, and a short pulse is used. Second, as the steam condition becomes more severe, the relative intensities of 30 and 0 ppm peaks increase while the 60 ppm tetrahedral peak intensity decreases, indicating that the former two peaks are probably the products of steam dealumination. Acid Treatment. Figure 2a-c compares the 27Al NMR spectra of untreated and HC1-treated LZY82-650s for 24 h and 45 days. The deconvolution data are listed in Table 1. The
Yang
1278 J. Phys. Chem., Vol. 99, No. 4, 1995
I ',
I
100
I
\
' ' I
\
/
00
\
.a
.W
20 1 (ppn, *I(OH),')
Figure 2. 27AlNMR of LZY82-650s (a) saturated with H20, (b) HC1treated for 24 h, and (c) HCI-treated for 45 days. The spectra are scaled to a same dried weight for a quantitative comparison.
-
,
rm
,
eo
-
~
.
-
~
20
-
-10
,
Jo
6 @Pm, 4W.S
Figure 3. 27AlNMR of LZY82-650s (a) saturated with H20, (b) NH3treated for 24 h, and (c) NH3-treated for 45 days. The spectra are scaled to a same dned weight for a quantitative comparison.
most prominent features after the acid treatment are (1) the 30 ppm peak intensity drops significantly from 21% to 3% 45 days after the HC1 treatment, (2) the 0 ppm peak intensity is increased while the 60 ppm tetrahedral peak remains relatively constant, and (3) the total NMR-observable intensity is increased and the trend becomes more pronounced as the acid impregnation time increases. 29SiNMR shows that the framework A1 sites remain intact after the HC1 treatment.18 It is known that an acid leaching can dissolve and remove extraframework A1 species from steamed zeolites.21s22The present work demonstrates the kinetic effect of acid treatment on the total A1 intensity and site distribution as a function of time. It also clearly indicates that the 30 ppm is mobilized by the acid treatment. In other words, the present data indicate that the 30 ppm peak is not likely attributed to framework A1 species. NH3 Treatment. In contrast to the acid treatment, after the sample is saturated with NH3, the 60 ppm peak intensity increases while both the 30 ppm and octahedral peaks decrease dramatically; see Figure 3a-c and Table 1. The 30 ppm peak intensity drops from 21% in the untreated sample to 1% 45 days after the NH3 treatment. Similarly to the HCl treatment, the total A1 intensity is increased after the NH3 treatment. 29Si NMR shows that the NH3 adsorption does not alter the framework structure.18 Why do the 60 ppm peak and total A1
intensity increase? What causes the 30 and 0 ppm peaks decrease? We do not know but are investigating these issues. However, it is clear that the 30 ppm species behaves much like the 0 ppm extraframework species in responding to the NH3 treatment. In other words, the N H 3 treatment experiment results offer another piece of evidence that the 30 ppm peaks is in the extraframework. Dehydration. After dehydration, shown in Table 1, both 60 and 0 ppm peak intensities are dramatically decreased while the 30 ppm peak remains almost constant. The total A1 intensity is also significantly decreased, as expected, to about half of its original value. It was reported that the decrease of the tetrahedral intensity upon dehydration is due to the significant increase of quadrupolar inter at ion^.^^ The same reason may be also responsible for the decrease of the octahedral peak. The indifference of the 30 ppm peak toward dehydration indicates that this species may not possess dehydratable hydroxyl ligands. Spin-Lattice Relaxation Measurement. Spin-lattice ( T I ) relaxation is one of the most commonly used methods for distinguishing different structures and species. However, this method has not been used much in solid-state 27AlNMR. This is partly due to the strong quadrupolar interactions and inhomogeneity of the steamed zeolite samples which present different tip angles for different sites. Traditional inverserecovery (18Oo-t-9O0) and saturation-recovery (90°-t-900) pulse sequences provide incomplete information. Instead, we used a saturation-comb method in the present work. With this method, a pulse train with a short space between adjacent pulses forces the magnetization of all the spins to zero, and the recovery is sampled after a time of z. This allows, in principle, saturation of the entire spin system and detection of the relaxation behavior of all the A1 sites. Figure 4 displays saturation-comb relaxation spectra of dried LZY82-650s as a function of relaxation time t. At long t value, only the three peaks described earlier can be discerned. However, as t decreases, several new peaks become observable. More than two peaks are associated with the 30 and 0 ppm peaks that are observed previously. To examine the new peaks in more detail, Figure 5 shows the spin-lattice spectra of dried samples as a function of steam condition at the shortest t value. Clearly, the new peaks near the 30 and 0 ppm are observed again in the singly steamed sample but not in the unsteamed sample. Most relevant to the present work, as the relaxation time becomes longer, the three peaks located at about 36, 27, and 21 ppm gradually merge into one peak located at 30 ppm. Thus, the 30 ppm peak reported previously is by no means a unique, single peak. It is probably a collection of at least three A1 sites of similar structural nature. The 0 ppm peak demonstrates similar spectral features: it consists of at least three peaks at about 2.2, -8.1, and -16 ppm. Looking more carefully in Figure 5, even the 60 ppm peak is not a single, unique peak. A shoulder component is located at about 48 ppm. This position is almost identical to the peak observed by Ray and Samoson using the DOR probe.I6 They assigned that peak as a framework tetrahedral site. To have a more quantitative comparison among the different Al species observed, the natural logarithms of the magnetization recoveries of the tetrahedral FW site (58 ppm), intermediate site (28 ppm), and octahedral EFW site (2 ppm) are plotted in Figure 6 as a function of t. The magnetization recovery is defined as
M(0) - M ( t ) = M ( 0 ) exp(-dT,) where M ( t ) and M(0) are the peak heights at time t and "infinity" when the magnetization reaches its maximum value.
Intermediate Aluminum Species in USY Zeolites
J. Phys. Chem., Vol. 99, No. 4, 1995 1279
6 (ppm, W H ) ; 3
Figure 4. Spin-lattice relaxation *’A1 NMR spectra of dried LZY82-650s as a function of relaxation time
t
the three sites by their clearly separated magnetization recovery curves.
a (PPm ww.3
Figure 5. Spin-lattice relaxation *’A1 NMR spectra of dried (a) LZY62, (b) LZY82, and (c) LZY82-650s at t = 1 0 0 , ~ .The spectra are not scaled to a same dried weight.
0
0.05
0. I
0.15
0.2
(8)
Figure 6. Natural logarithm of magnetization recovery, In[ 1 - M(t)/ M(O)], as a function of relaxation time
5.
T I is the spin-lattice relaxation time. Figure 6 shows that the magnetization recovery is not a simple, single-exponentialform, and the TI relaxation times for all the three A1 sites are not dramatically different. The nonexponentialbehavior is probably due to the presence of nonsaturated residue, which is about 30% of the total signal. The strong quadrupolar interaction associated with the A1 nucleus makes it difficult to completely saturate all the spins. Qualitatively, significant differences are found among
Discussion The experimental results provide answers to several key questions regarding the structure of the 30 ppm A1 species. The 30 ppm Peak Is Not Due to a Tetrahedral Framework A1 Site. If the 30 ppm species were part of the FW tetrahedral A1 sites, we would expect the 60 and 30 ppm peaks to respond similarly to the physical and chemical treatments. This is clearly not the case. Dramatically different responses to the HC1 treatment (no change vs significantly decrease), NH3 treatment (significant increase vs dramatic decrease), and dehydration (significant decrease vs constant) demonstrate that they are different species. The different T1 relaxation behaviors among the three peaks provide additional proof that the 30 ppm peak is not part of the framework Al. This conclusion is in good agreement with those reached by the CP/MAS results of Kellberg et al.,14spin-echo T2 relaxation results of Schmitt and co-workers,15and DOR results of Ray and Samoson.16 The 30 ppm Peak Is Attributed to EFW A1 Species. If the 30 ppm peak A1 species were in the framework structure, we would expect that HCl and NH3 treatments will have little effect on its peak intensity since 29Si NMR shows that the framework structure remains the same after these treatments. Instead, we would expect that the dehydration will decrease its intensity dramatically as happened to the FW tetrahedral sites. All these expectations are opposite to the expenmental observations. Most people believe the 30 ppm peak is in EFW. We provide here direct experimental evidence to support this assumption. The 30 ppm Peak Is a Mixture of at Least Three AI Species. The spin-lattice relaxation data clearly show that the structural nature of the steamed zeolites is more complex than previously thought. The 30 ppm peak is not a single, “unique” peak. It is a mixture of at least three peaks. This is consistent with the DOR observations that two peaks are associated with the 30 ppm peak.l6-I7 We hope our new findings may serve as a reminder that a more diverse structure should be used in considering properties of steamed zeolites, such as their acidity. What Is the Structure of the 30 ppm “Peak”? The fact that dehydration has little affect on the 30 ppm peak intensity
1280 J. Phys. Chem., Vol. 99, No. 4, 1995 indicates that this species may not possess dehydratable hydroxyl groups. The pentacoordinated A1 site in barium aluminate glucolate and andalusite are also not connected to hydroxyl groups and are not in a framework structure.” Thus, a comparison of USY and the two minerals in terms of their 30 ppm peak may not be unrealistic. The dramatic intensity decrease after the acid and NH3 treatment indicates that the A1 species represented by the 30 ppm peaks have small geometric sizes such that they are relatively easily mobilized by HCl or NH3. In other words, even if this A1 species should be proved tetrahedrally-bonded, its structure is dramatically different from the framework tetrahedral A1 sites. In this sense, we are more inclined to the “pentacoordinate” hypothesis. However, the present work does not provide direct evidence to prove this. Other techniques, such as theoretical structural calculations, are needed to further clarify this issue.
Conclusion Structural identification of the 30 ppm intermediate Al species in a steamed zeolite is a controversial, but important, issue to be resolved. We studied perturbation of all A1 sites caused by various chemical and physical treatments. We also investigated the relaxation behaviors of all the A1 sites by applying saturation-comb spin-lattice relaxation method. The study show that the intermediate A1 site is not a unique, single species. Spin-lattice relaxation reveals at least two more species near 30 ppm. The intensity of the 30 ppm species decreases dramatically after HCl or NH3 treatment, indicating they are not in the framework structure and probably have much smaller geometric sizes.
Acknowledgment. The author thanks Mr. Cesar Tolentino for preparing the LZY82-650s sample. Helpful comments and full support by Dr. Ralph Truitt of Engelhard are gratefully acknowledged. The author also thanks two anonymous reviewers for their insightful comments.
Yang
References and Notes (1) See, for example: Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry, and Use; Krieger Publishing: Melboume, FL, 1984. (2) See, for example, a special topic on new frontiers in material science: Science 1994, 263, 1698- 1728. (3) Klinowski, J.; Fyfe, C. A,; Gobbi, G. C. J . Chem. SOC.,Faraday Trans. I 1985, 81, 3003-3019. (4) Klinowski, J. Prog. NMR Spectrosc. 1984, 16, 237-309. ( 5 ) Thomas, J. M.; Klinowski, J. Adv. Catal. 1985, 33, 199-374. (6) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; John Willy & Sons: New York, 1987. (7) Fyfe, C. A.; Mueller, K. T.; Kokotailo, G. T. In NMR Techniques in Catalysis; Bell, A. T., Pines, A., Eds.; Marcel Dekker: New York, 1994; Chapter 1, pp 11-67. (8) Heifer, H.; Emst, H. Annu. Rep. NMR Spectrosc. 1994, 28, 91187. (9) Cruickshank, M.; Dent Glasser, L. S.; Bani, S. A. I.; Poplett, I. J. F. J. Chem. Soc., Chem. Commun. 1986, 23-24. (10) Lippmaa, E.; Samoson, A,; Magi, M. J. Am. Chem. Soc. 1986,108, 1730-1735. (11) Alemany, L. B.; Kirker, G. W. J.Am. Chem. Soc. 1986,108,61586162. (12) Gilson, J.-P.; Edwards, G. C.; Peters, A. W.; Rajagopalan, K.; Wormsbecher, R. F.; Roberie, T. G.; Shatlock, M. P. J . Chem. SOC.,Chem. Commun. 1987, 91-92. (13) Samoson, A,; Lippmaa, E.; Engelhardt, G.; Lohse, U.; Jerschkewitz, H.-G. Chem. Phys. Lett. 1987, 134, 589-592. (14) Kellberg, L.; Linsten, M.; Jakobsen, H. J. Chem. Phys. Lett. 1991, 182, 120-126. (15) Schmitt, K. D.; Haase, J.; Oldfield, E. Zeolite 1994, 14, 89-100. (16) Ray, G.; Samoson, A. Zeolite 1993, 13, 410-413. (17) Haddix, G. W.; Narayana, M.; Gillespie, W.; Georgellis, M.; Wu, Y. J . Am. Chem. SOC. 1994, 116, 672-674. (18) Yang, X. Unpublished data. (19) Fukushima, E.; Roeder, S. B. W. Experimental Pulse NMR; Addison-Wesley: Reading, MA, 1981. (20) Yang, X.; Sibemagel, B. G.; Larsen, J. W. Energy Fuels 1994, 8, 266-275. (21) Klinowski, J.; Thomas, J. M.; Fyfe, C. A,; Gobbi, G. C. Nature 1982, 296, 533-536. (22) BosaEek, V.; Matikkin, V. M. J . Phys. Chem. 1987,91,260-262. (23) Kentgens, A. P. M.; Scholle, K. F. M. J.; Veeman, W. S. J. Phys. Chem. 1983, 87, 4357-4360. JP9423 5 5 9
