Zeolite Synthesis - American Chemical Society

Chapter 32

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Quadrupole Nutation NMR Studies of Second Generation Faujasitic Catalysts Halimaton Hamdan and Jacek Klinowski Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, England

27Al quadrupole nutation NMR reveals the presence of four kinds of aluminium in dealuminated zeolite Y: framework tetrahedral (F), non-framework tetrahedral (NFT), distorted framework tetrahedral (DFT) and non-framework octahedral (NFO). The DFT aluminium, which is reported for the first time here, is bonded to hydroxyl groups formed during dealumination. In samples realuminated by hydrothermal isomorphous substitution in aqueous KOH at elevated temp eratures both the NFO and NFT aluminium is reinserted into the framework and the intensity of the DFT signal is corresp ondingly reduced. The NFT aluminium resonates at a lower frequency than the F aluminium, and is subject to stronger quadrupolar interactions. Quantification of the various NMR signals is essential for the determination of the distribution of aluminium in zeolites. All NMR transitions of nuclei with I>l/2 are subject to first-order quad rupole interactions with the exception of the central (1/2 -1/2) transition which experiences only the second-order effects. Spectra of quadrupolar nuclei in polycrystalline samples give characteristic powder patterns for the central transition, whereas all other transitions are usually broadened beyond detection. While magic-angle spinning (MAS) reduces the linewidth of the central transition by a factor of ca. 4 in comparison with a static spectrum, second-order quadrupole interactions cannot be averaged to zero by MAS. Approximately 85% of all nuclei have I>l/2, and many quadrupolar components of common solids, such as N a and K , have in addition narrow chemical shift ranges, which greatly complicates the interpretation of their spectra. Quadrupole nutation NMR of nuclei with half-integer spin in pow dered samples (1-8) can distinguish between nuclei subjected to different quadrupole interactions, the signals from which overlap in ordinary NMR spectra. The technique can be used for the determination of the local 23

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0097-6156/89A)398-0465$06.00A) ο 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

466

ZEOLITE SYNTHESIS

environment of Al in zeolitic catalysts, which is essential for the under standing of their chemistry. One of the difficulties involved is that, as a result of strong quadrupole interactions, the amounts of framework and non-framework Al in thermally treated samples determined by the joint application of Si magic-angle-spinning (MAS) NMR (which monitors framework Al) and of chemical analysis (which gives the total Al content) are in striking disagreement with the results of A1 MAS NMR (9-11). The latter underestimates the amount of Al, and indirect methods, such as impregnation of the sample with ethanolic acetylacetone (12,13) prior to measurement have been used to observe the "invisible" aluminium. We have recently reported a series of advances towards quantitative determin ation of aluminium in zeolites by NMR (6-8), showing that all the Al can be detected by NMR in the solid state provided certain experimental conditions are met. We shall now demonstrate that: 1. During the course of hydrothermal dealumination (ultrastabilisation) of zeolite Y quadrupole nutation NMR detects four kinds of aluminium sites. In addition to signals from the framework (F), non-framework tetrahedral (NFT) and non-framework octahedral (NFO) aluminium there is a signal which we attribute to distorted framework (DFT) aluminium bonded to hydroxyl nests formed during dealumination. 2. When dealuminated zeolite Y is treated with an aqueous solution of a strong base such as KOH hydrothermal isomorphous substitution of silicon by aluminium takes place. Reinsertion of non-framework tetrahedral (NFT) and non-framework octahedral (NFO) aluminium reduces the distortion of the aluminate tetrahedra in the framework. 3. Realumination can only occur if there are sufficient suitable Si sites in the framework. Treatment of amorphous faujasites containing various kinds of aluminium leads to the formation of NFT aluminium with characteristic chemical shift and quadrupolar interactions. A quadrupole nutation NMR experiment (see Figure 1) is performed as follows. During the preparation period the spin system reaches thermo dynamic equilibrium. During the evolution period the sample is irradiated with an rf field with a pulse of length ti. The detection period corresponds to the acquisition of the free induction decay (FID) over time period t2. By keeping t2 constant and increasing ti by equal increments at regular intervals, a series of FIDs is acquired. A double Fourier transformation in t2 and ti gives a two-dimensional NMR spectrum with the axes F2 (containing the chemical shift and the second order quadrupolar shift) and Fi (containing only the quadrupolar information). The projection of the spectrum onto F2 is equivalent to a normal powder spectrum showing the combined effect of the chemical shift and quadrupolar interactions. The projection onto Fi gives the precession frequencies around the rf field in the 29


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In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. 27

Detection

Figure 1. Schematic representation of the A1 quadrupole nutation experi ment. The rf pulse of length ti is followed by the detection of the free induc tion decay in the absence of rf fields. Two-dimensional Fourier transform ation of the series of FIDs gives the nutation spectrum.

Preparation


4t ON ^1

!

i

a

1

I

ZEOLITE SYNTHESIS

468

rotating frame which depend on the ratio of the quadrupolar parameter CUQ and the rf field strength ω \ When cûQ«œ f a strong peak appears at the nutation frequency cûi=co f, while for cûQ»cûrf the peak is at o>2 = (1+1/2) cof. More complicated line shapes result for stationary samples in the intermediate cases (1,5). The strength of the quadrupolar interaction, COQ, can be up to several MHz depending on the nucleus and the structure of the solid, while the strength of the rf pulse is normally insufficient to allow the quadrupolar interaction to be neglected during the irradiation of the spin system. ΓΓ

r


r

r

Experimental 27

The A1 nutation spectra were measured at 104.26 MHz using a Bruker MSL-400 multinuclear NMR spectrometer with a high-power static probehead and a 5 mm diameter horizontal solenoidal coil. Using an aluminium nitrate solution the amplitude of the rf pulse (ω ί/2π) was adjusted and kept at a constant value of 70±5 kHz unless otherwise stated. Nutation experiments were performed on the same amount of each sample and the same number of transients was accumulated for samples in the same series. The rf pulse length was increased in Ιμβ increments from 2 to 65 μβ. The spectral width was 125 kHz, the recycle delay 0.2 s and the number of transients accumulated in each measurement was between 2000 to 4000. The FIDs were doubly Fourier transformed in the magnitude mode. A sine bell digitizer filter and zero filling were used in the Fi dimension with no filter in the F2 dimension. Γ

Preparation of Samples. Dealuminated samples were prepared by hydrothermal treatment of 62% ammonium-exchanged zeolite NH4,Na-Y (sample 1). Samples D-l and D-2 were made by steaming 20 g portions of sample 1 in a tubular quartz furnace (14) at 525°C with water being injected by a peristaltic pump with a flow rate of 12 ml/hour for 5 and 18 hours, respectively. The amorphous sample A-5 was prepared by repeated washing of sample 1 with 1M H N O 3 and steaming at 525°C. Another amorphous sample (A-6) was made by treating sample 1 with excess mixed aqueous solution of C H 3 C O O N H 4 and (NH4) SiF . Realuminated samples R-3 and R-4 were prepared by hydrothermal isomorphous substitution (6,14) by stirring 1 g of sample D-2 in 50 ml of 0.5M and 2M KOH at 80°C for 24 hours. The treatment of amorphous alumino-silicates A-5 and A-6 in 0.5M KOH under the same conditions gave samples AR-7 and AR-8, respectively. The conditions of preparation of all samples are summarised in Table I. 2

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32. HAMDAN AND KLINOWSKI

Second Generation Faujasitic Catalysts

469

TABLE I. Conditions of preparation of samples. (S) denotes hydrothermal treatment (steaming); (R) denotes treatment with aqueous KOH; (Ch) denotes chemical treatment as described. Sample no.

Prepared from sample no.

Treatment

1

parent Na-Y (Si/Al = 2.56)

62% N H 4 exchanged in 2 M NH4NO3

D-l

1

(S) 525°C, 5 hr.

D-2

1

(S) 5 2 5 0 Q 18 hr.

R-3

D-2

(R) 0 . 5 M KOH, 80°C, 24 hr.

R-4

D-2

(R) 2 M KOH, 80°C, 24 hr.

A-5

1

(S) 525°C, 18 hr, 1M H N O 3

(twice) A-6

1

(Ch) Aqueous C H 3 C O O N H 4 + (NH4)2SiF , 6

80°C, lhr. AR-7

A-5

(R) 0.5M KOH, 80°C, 24 hr.

AR-8

A-6

(R) 0 . 5 M KOH, 80°C, 24 hr.


ZEOLITE SYNTHESIS

470 Results and Discussion

29

All samples were characterized by x-ray diffraction (XRD) and by Si and A1 MAS NMR. The unit cell parameters, framework Si/Al ratios and the numbers of framework Si and Al atoms per unit cell calculated from deconvolution using Gaussian peak shapes are given in Table Π. All samples were fully hydrated over saturated N H 4 C I for 24 hours prior to NMR experiments. XRD patterns of dealuminated samples D-l and D-2 agree well with previous work (15). The samples are highly crystalline and the (cubic) unit cell parameter is reduced (Table Π) by the dealumination of the framework. The Si/Al ratios of samples D-l and D-2 were 3.10 and 4.91, respectively, which confirms that the degree of dealumination increases with the duration of hydrothermal treatment. A1 MAS NMR spectra of samples 1, D-2 and R-3 in the absolute intensity mode are given in Figure 2. As a result of dealumination, the intensity of the framework aluminium (F) signal at ca. 60 ppm in sample D-2 is much lower than in sample 1. The absence of spectral features at 0 ppm shows that aluminium extracted from the framework is in tetra hedral coordination, but the broadening of the A1 signal indicates that there is a wider range of quadrupole interactions than before thermal treatment. The nutation spectrum of sample 1 (Figure 3) consists of two signals [at (60 ppm, 78 kHz) and (60 ppm, 195 kHz)], both with the same linewidth of 855 Hz and both corresponding to framework (F) aluminium. The presence of two signals is due to the fact that the quadrupole interaction characteristic of framework A1 is of the same order of magnitude as the strength of the rf pulse. Since the latter is insufficient to overcome the quadrupole interaction entirely, the excitation is not fully non-selective (8). We have confirmed this by examining the nutation spectrum (not shown) of the same sample irradiated with a stronger rf pulse (96 kHz, as compared to 70 kHz for the spectra shown in Figure 3). The spectrum still contained two signals, at (60,101) and (60, 230), but the intensity of the first signal increased and that of the second decreased. This proves that both signals correspond to the same type of aluminium. In general, dealumination of sample 1 for various periods leads to spectra composed of up to four signals:


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1. 2. 3. 4.

60 ppm, 78 kHz; -2 ppm, 78 kHz; 56 ppm, 195 kHz; 74 ppm, 195 kHz;

(F) (NFO) (NFT) (DFT, see below).

Signals 1-3 have been assigned earlier (4,16). The highly crystalline dealuminated samples D-l and D-2 apparently do not contain NFO alum inium (signal 3) but do contain the three remaining types: the projection of



471



(A-5)

(D-2)

(A-6)

\

(AR - 8)

(R-3)

"1 -200

200

Γ"

—ι

200

-200

—I 200

-200

3

ppm from Al( H2O )g *

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Figure 2. A1 MAS NMR spectra. The spectra of samples 1, D-2 and R-3, but not those of the remaining samples, are given in the absolute intensity mode. TABLE Π.

Unit cell parameters and the composition of the samples. Sip and Alp denote numbers of Si and Al atoms per unit cell of 192 tetrahedral atoms.

Sample

ao(À)

1

24.69 24.58 24.52 24.72 24.98

D-l D-2 R-3 R-4

(Si/ A1)NMR 2.56 3.10 4.91 2.59 1.54

SIF

A1

138 145 160 139 116

54 47 33 54 76


F

472


ZEOLITE SYNTHESIS

F

2

( ppm)

F ( ppm ) 2

27

Figure 3. A1 quadrupole nutation spectra of crystalline samples.





473

the nutation spectra in Figure 3 onto F2 gives a very broad signal at 60 ppm, while the projection onto Fi clearly reveals three component signals (1,4,5). Signal 4 does not coincide with any of the previously assigned peaks. In order to assign it correctly, we must consider several arguments. Figure 3 shows that, as dealumination progresses, there is a change in relative intensities of the F and NFT signals and of the signal we wish to assign. The last two peaks first appear in sample D-l and become more intense in comparison to the F signal in the more highly dealuminated sample D-2. Signal 4 cannot be part of either the F or the NFT peak since it occurs at a lower field: its projection onto F2 is at 74 ppm and has a larger linewidth than signal F. The first step in the thermal treatment of zeolite NH4,NaY is the formation of the H-form (17,18), which generates tetrahedral framework aluminium associated with the hydroxyl nests. These nests can be "healed" by the rearrangement of the framework during which the vacancies left by the expulsion of Al are reoccupied by Si from other regions of the sample. However, there is evidence that some hydroxyl nests do remain (19,20). The proton is more electronegative than silicon and deshields the A1 nucleus so that it resonates at lower field. The presence of the proton makes the environment of A1 asymmetric, i.e. increases quadrupole effects. A simple calculation based on the Si MAS NMR spectra (see Table Π) and the obvious fact that the total amount of Al in the sample remains constant upon heat treatment shows that 61% of the total aluminium (33 out of 54 atoms) in sample D-2 is part of the framework. Deconvolution of the A1 spectrum in Figure 2 based on the (incorrect) assumption that the linewidth of the A1 signal remains constant upon hydrothermal treatment, always gives much lower amounts of framework Al than calculated from Si MAS NMR and XRD (9,10,18). Since it has been demonstrated that hydrated zeolites contain no "invisible" aluminium (6-8), one of the two signals at 195 kHz in Fi in Figure 3 must represent framework Al in sites distorted as a 27

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result of the hydrothermal treatment.

Signal 4 (at 74, 195) is therefore

assigned to tetrahedral framework aluminium in a distorted environment (DFT). Since it occurs at a higher nutation frequency than the "ordinary" (F) aluminium, such Al is subject to stronger quadrupole effects. Earlier work (13) has shown that almost all non-framework aluminium dissolves in ethanolic acetylacetone but that no framework aluminium does so unless the treatment is continued for extended periods. The nutation spectrum of sample D-2 treated with acetylacetone indicates (not shown) that a consider able amount of NFT aluminium has been removed, but that the F and DFT signals remain virtually unaffected. This further confirms that framework aluminium is not complexed by acetylacetone, and that signal 4 comes from Al in the framework, but subjected to stronger quadrupole interactions than in untreated samples. The intensity of the A1 MAS NMR spectrum of the realuminated sample R-3 (Figure 2) is greater than that of the dealuminated sample D-2. The F2 projection indicates that aluminium in the latter sample is in the tetrahedral coordination. The nutation spectra (Figure 3) clearly show that 27



474

ZEOLITE SYNTHESIS

the aluminium does go back into the framework. As realumination pro gresses, the F signal at (60, 78) increases. By comparing nutation spectra of many realuminated samples containing different amounts of framework aluminium as a result of treatment with KOH solutions of different concen trations we have found that both the NFT and (whenever present in the ultrastable precursors) the NFO species are involved. Upon treatment with 0.5M KOH (sample R-3) the NFT signal disappears faster than the DFT signal, indicating that the initial stage of the reaction involves primarily a conversion of NFT aluminium into F aluminium. The intensity of the DFT signal falls significantly only after most NFT aluminium has gone back into the framework (as in sample R-4 which has been treated with a more concentrated base). Also, the F signal in the realuminated sample R-4 is much narrower than in sample D-2 from which it was prepared, which indicates that the distribution of F aluminium is more ordered and symmetrical. However, the effect is not sufficiently large for the chemical shifts and/or quadrupole interactions to be manifested as a change in the Fi frequency. Aluminium reinsertion is evidently accompanied by a "relaxation" of the zeolitic framework. A treatment of the realuminated sample R-4 with acetylacetone does not remove aluminium from either the F or the remaining DFT sites. Although there is ample proof by NMR in one and two dimensions that aluminium is indeed reinserted into the framework, it could be argued that part of the tetrahedral aluminium observed in hydrothermally treated samples is attributable to amorphous parts of the sample which cannot be distinguished from the crystalline part by Si MAS NMR or the conven tional A1 MAS NMR. Si(nAl) units in the amorphous phase generally give rise to very broad Si NMR signals which overlap with signals from the crystalline regions of the sample (19). In order to demonstrate that our samples contain only crystalline aluminosilicate and therefore that this possibility may be dismissed, we have acquired nutation spectra of zeolite samples which have been deliberately made amorphous by chemical treat ment (see Table I). We did not use thermal methods of amorphization so as to avoid recrystallization of compact aluminosilicates. The XRD patterns of the amorphous samples A-5 and A-6 are featureless with a broad hump at 29

27

29

29

20=22°. The Si MAS NMR spectra of amorphous samples A-5 and A-6 (Figure 5) both feature a large broad peak at -112 ppm, typical of amorphous silica (18). The A1 MAS NMR spectrum of sample A-5 (Figure 2) is broad and contains both octahedral and tetrahedral signals, indicating that many kinds of non-equivalent environments for aluminium are simultaneously present. This is further confirmed by the nutation spectrum of sample A-5 (Figure 4), composed of numerous weak features obscured by the noise. By contrast, the A1 MAS NMR spectrum of the amorphous sample A-6 features a narrow peak at -2 ppm (Figure 2) indicating that the aluminium is mobile and in octahedral coordination. There is an additional low intensity signal at 60 ppm indicating that a small amount of tetrahedral aluminium 27

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Figure 4. A1 quadrupole nutation spectra of amorphous samples.



ZEOLITE SYNTHESIS

29

Figure 5. Si MAS NMR spectra.




477

is also present. The nutation spectrum, however, shows only one signal, at (-2, 78), corresponding to NFO aluminium. The treatment of samples A-5 and A-6 with aqueous KOH provides much information. The XRD pattern of sample AR-7 indicates that a new amorphous phase has been formed responsible for the appearance of the new broad hump at 26=25°. The Si MAS NMR spectrum features a large peak at -113 ppm and a shoulder at -115 ppm (Figure 5). Both are narrower than in the starting amorphous samples A-5 and A-6. The A1 MAS NMR spectrum of sample AR-7 (Figure 2) contains one narrow signal at 56 ppm, i.e. in the tetrahedral region. The nutation spectrum (Figure 4) gives two signals [at (56, 78) and (56,195)], for instrumental reasons both corresponding to the same kind of Al (see above). The position of these peaks in F2 and Fi is as reported by Samoson et al. (4) and assigned to nonframework tetrahedral (NFT) aluminium. We agree with their conclusion, since the sample is completely amorphous and cannot contain tetrahedral framework (F) aluminium. The A1 MAS NMR spectrum of KOH-treated sample AR-8 (Figure 2) is composed of two signals (at -2 ppm and at 56 ppm), which indicates that some of the NFO aluminium has been converted to NFT aluminium. The nutation spectrum (Figure 4) shows three signals: (-2, 78), (56, 78) and (56,195), which proves that all the forms of Al in the amorphous phase are converted to tetrahedral Al upon treatment with the base. However, the increased linewidth of the resulting NMR signals and the fact that they are located at higher field (56 ppm) along the F2 axis compared with 60 ppm for the F signal indicates that they are due to NFT aluminium. Furthermore, large quadrupolar effects and low concentration of the latter species make the small difference in peak position unobservable by conventional A1 MAS NMR. It appears that the increase in A1 linewidth observed upon dealumination of zeolites is due to a superposition of the F and NFT signals. 29


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Since the Si MAS spectra of both base-treated amorphous samples (AR-7 and AR-8) contain peaks at more negative chemical shifts than in the crystalline materials (for example samples R-3 and R-4 in Figure 3) it is clear that NMR signals from the amorphous phase cannot interfere with the determination of Si/Al ratios in realuminated crystalline zeolites. Our assignment of signal 4 as due to DFT aluminium is therefore vindicated, and the reinsertion of Al into the zeolitic framework demonstrated quan titatively. Acknowledgment We are grateful to Shell Research, Amsterdam, and to Universiti Teknologi Malaysia, for support, and to Dr. P.P. Man for discussions.


ZEOLITE SYNTHESIS

478

Literature Cited


1. 2. 3. 4.

Samoson, Α.; Lippmaa, E. Phys. Rev., 1983, B28, 6567. Samoson, Α.; Lippmaa, E. Chem. Phys. Lett. 1983, 100, 205. Samoson, Α.; Lippmaa, Ε. T. Magn. Res., 1988, 79, 255. Samoson, Α.; Lippmaa, E.; Engelhardt, G.; Lohse, U.; Jerschkewitz, H.-G. Chem. Phys. Lett., 1987, 134, 589. 5. Kentgens, A.P.M.; Lemmens, J.J.M.; Geurts, F.M.M.; Veeman, W.S. J. Magn. Res., 1987, 71, 62. 6. Man, P.P.; Klinowski, J. Chem. Phys. Lett., 1988, 147, 581. 7. Man, P.P.; Klinowski, J.J.Chem.Soc.,Chem. Comm., 1988, 1291. 8. Man, P.P.; Klinowski, J.; Trokiner, Α.; Zanni, H.; Papon, P. Chem. Phys. Lett., 1988,151,143. 9. Klinowski, J. Prog. Nucl. Magn. Reson. Spectrosc., 1984, 16, 237. 10. Klinowski, J.; Fyfe, C.A.; Gobbi, G.C.J.Chem.Soc.,Faraday Trans. I, 1985, 81, 3003. 11. Sanz, J.; Serratosa, J.M.J.Am. Chem.Soc.,1984, 106, 4790. 12. Bosáček, V.; Freude, D.; Fröhlich, T.; Pfeifer, H.; Schmiedel, H.J.Colloid Interface Sci., 1982, 85, 502. 13. Grobet, P.J.; Geerts, H.; Martens, J.A.; Jacobs, P.A.J.Chem.Soc.,Chem. Comm., 1987, 1688. 14. Hamdan, H; Sulikowski, B.; Klinowski, J.J.Phys. Chem. 1989,93,350. 15. Breck, D.W. "Zeolite Molecular Sieves: Structure, Chemistry and Use" Wiley, New York (1974). 16. Geurts, F.M.M.; Kentgens, A.P.M.; Veeman, W.S. Chem. Phys. Lett., 1985, 120, 206. 17. Kerr, G.T.J.Phys. Chem., 1967, 71, 4155. 18. Engelhardt, G.; Michel, D. "High-resolution Solid-State NMR of Silicates and Zeolites", Wiley, Chichester (1987). 19. Engelhardt, G.; Lohse, U.; Samoson, Α.; Mägi, M.; Tarmak, M.; Lippmaa, Ε. Zeolites, 1982, 2, 59. 20. Engelhardt, G.; Lohse, U.; Patzelová, V.; Mägi, M.; Lippmaa, Ε. Zeolites, 1983, 3, 233. RECEIVED December 22, 1988


Zeolite Synthesis - American Chemical Society

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