27Al quadrupole nutation NMR studies of amorphous aluminosilicates

Volume

158,number 5

CHEMICAL

27AI QUADRUPOLE Halimaton

NUTATION

HAMDAN

PHYSICS LETTERS

NMR STUDIES

16 June 1989

OF AMORPHOUS

ALUMINOSILICATES

and Jacek KLINOWSKl

Department of Chemistry, University of Cumbrrdge, Lensfield Road, Cambridge CB2 IEW UK Recerved 30 January

1989

“Al quadrupole nutation NMR provides valuable information on the structure of amorphous aluminosilicates derived from zeolite Y by chemical treatment. The local Al environment, ofwhich several kinds can be resolved in the two spectral dimensions, depends vitally on the mode of preparation of the sample. Treatment of the samples with aqueous KOH converts six-coordinated Al to four-coordination.

The results arc of great assistance

for the interpretation

1. Introduction Amorphous alurninosilicates are of considerable interest to the solid state chemist who wishes to know the coordination numbers of Si and Al as well as the interatomic distances and the radial distribution functions. These questions are especially important in the case of Al, which can be four-, five- and sixcoordinated, while Si is almost always present in tetrahedral coordination. Such considerations are essential for the study of ceramics and of materials prepared from crystalline aluminosilicates. The dividing line between crystalline and amorphous aluminosilicates is often unclear. Jacobs et al. [ 1 ] described an “X-ray amorphous” material which nonetheless exhibits infrared spectra and catalytic properties typical of zeolite ZSM-5. The sample was crystalline on a scale insufficient for X-ray detection, with crystals less than 8 8, in diameter. Such materials can be studied using infrared spectroscopy [ 2 1, 13C MAS NMR [ 31 and high-resolution electron microscopy [ 4-6 1. Zeolite-derived amorphous matrices are useful for trapping environmental poisons as well as the radioactive products of nuclear fission [ 710]. As an essentially “local” probe (in the sense that it does not require long-range order) NMR is well equipped for the study of these materials. “Si magicangle-spinning (MAS) NMR provides information on the coordination of Si and the distribution of Si0-Si and Si-O-Al angles, but the experiments are 0 009-2614/89/$ ( North-Holland

03.50 0 Elsevier Science Publishers Physics Publishing Division )

of *‘Al spectra of crystalline

aluminosilicates.

difficult and time-consuming because of the very long spin-lattice relaxation times of “Si in amorphous solids. “Al MAS NMR can, in principle, provide similar information, but is often of limited value in the study of poorly crystalline materials, the spectra of which normally contain only one or two very broad lines. It can, however, be used as an analytical tool: we have recently reported an advance towards quantitative determination of aluminium in zeolites by NMR [ 1l-1 3 1, showing that all of the Al can be detected in the solid state. The central (l/2*l/2) NMR transition of quadrupolar nuclei (those with I> l/2) is subject only to first-order quadrupole interactions with the electric field gradients in the solid. Only the central transition is usually observed in such spectra, all other transitions being broadened beyond detection by the first- and second-order effects. While MAS reduces the linewidth of the central transition, it does not average second-order quadrupole interactions. The recently introduced technique known as quadrupole nutation NMR [ 1 l-l 91 can, however, distinguish between nuclei with half-integer spin subjected to different quadrupole interactions, and resolve signals which overlap in ordinary NMR spectra. We demonstrate that the method provides valuable information on the local environment of Al in amorphous aluminosilicates. In quadrupole nutation NMR, a series of free induction decays (FIDs) is acquired during the interval tz using powerful resonant radiofrequency pulses B.V.

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while monotonically increasing the length, t,, of the pulse. A double Fourier transformation in tr and tl gives a two-dimensional NMR spectrum with the axes F2 (containing the chemical shift and the second-order quadrupolar shift) and F, (containing only the quadrupolar information). The projection of the spectrum onto r;, is equivalent to a normal powder spectrum showing the combined effect of the chemical shift and quadrupolar interactions; the projection onto F, gives the precession frequencies around the rf field in the rotating frame which depend on the quadrupolar parameters e’qQ/h and q and on the rf field strength.

2. Experimental The starting material (sample 1) was 62% ammonium-exchanged zeolite NH4, Na-Y. Sample 2A was prepared by repeated washing of sample 1 with 1 M HNO, and steaming in a tubular quartz furnace [ 201 at 525 ‘C for 18 h with water being injected by a peristaltic pump with a flow rate of 12 ml/h. Sample 3A was made by treating sample 1 with excess mixed aqueous solution of CH,COONH, and ( NH4)$iF6 at 80°C for 1 h. We have not used thermal treatment as a means of preparing amorphous samples, since zeolites tend to recrystallize at high temperatures. KOH-treated samples 2B and 3B were prepared [20] by stirring 1 g of samples 2A and 3A, respectively, in 50 ml of 0.5 M KOH at 80” C for 24 h. The preparation conditions of the samples are summarized in table 1. All samples were characterized by powder X-ray diffraction (XRD ) and by 29Si and “Al MAS NMR. They were fully hydrated over Table 1 Conditions of preparation of the samples Sample No.

Prepared from sample No.

Treatment

1 2A

parent Na-Y (Si/Al=2.56) 1

3A

1

2B 3B

2A 3A

62% NH,-exchanged in 2M NH,N03 525”C, 18 h, 1 M HNO, (twice) aqueous CH,COONH, t (NH4)$iFh, 8O”C, I h 0.5 M KOH, SO”C, 24 h 0.5 M KOH, 8O”C, 24 h

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16 June 1989

saturated NH&l for 24 h prior to NMR experiments. XRD patterns were acquired on a Philips P W 17 10 vertical goniometer using Cu Ka radiation selected by a graphite monochromator in the diffracted beam. 29Si MAS NMR spectra were measured at 79.5 MHz using a Bruker MSL-400 multinuclear spectrometer. Samples were spun in Andrew-Beams rotors at 2.6 kHz using air as the spinning gas. Radiofrequency pulses of 4 ps duration were applied with 20 s recycle delay, and 1000 transients were acquired for each spectrum. 29Si chemical shifts are quoted in ppm from external tetramethylsilane (TMS). 27A1NMR spectra were measured at 104.26 MHz using powerful 0.6 p’s radiofrequency pulses with a 0.2 s recycle delay and 10000 transients for each spectrum. Samples were spun at 3.4 kHz in an aluminium-free probehead and Vespel rotors. Chemical shifts are quoted in ppm from external Al(H,O)z+. Nutation spectra were measured with a high-power static probehead and a 5 mm diameter horizontal solenoidal coil. The amplitude of the rf pulse (w,,/2n) was adjusted using an aluminium nitrate solution and kept at a constant value of 80-t 5 kHz. The rf pulse length was increased in 1 p’s increments from 2 to 65 ps. The spectral width was 125 kHz, the recycle delay 0.2 s and between 2000 and 4000 transients were accumulated in each measurement. The FIDs were doubly Fourier transformed in the magnitude mode. A sine bell digitizer filter and zero filling were used only in the F, dimension and no filter in the F2 dimension.

3. Results and discussion XRD patterns (fig. 1) show that samples 2A and 3A are almost completely amorphous with a broad hump at 20=22”, although some diffraction peaks are present, particularly in sample 3A. However, with the exception of the peak with d~3.18 A, they do not correspond to the parent zeolite (sample 1 )_ The “Si MAS NMR spectra of these samples (fig. 2 ) both feature a large broad resonance at - 112 ppm, typical of amorphous silica [ 2 11. The much smaller signal at = - 100 ppm in both samples is due to the residual crystalline material. The 27Al MAS NMR spectrum of sample 2A (fig. 3 ) contains a number

Volume IS8, number 5

16 June 1989

CHEMICAL PHYSICS LETTERS

A (‘1

-80

-90

-100

-110

/

\

GW

-80

-100

-80

-120

ppm

-100

-120

fromTMS

Fig. 2. %i MAS NMR spectra.

3

13

23

33

43

53 2 e

(degrees)

Fig. 1. XKD patterns of samples I, ZA, 2B, 3A and 3B

of signals, both octahedral and tetrahedral, indicating that many kinds of non-equivalent environments for aluminium are simultaneously present. In contrast, the spectrum of sample 3A features a single narrow peak at -2 ppm, due to relatively mobile non-framework octahedral (NFO) aluminium. The nutation spectrum of sample 1 (fig. 4) con-

sists of two signals (at 60 ppm, 78 kHz) and (60, 195)), both with the same linewidth of 355 Hz and both corresponding to framework (F) aluminium. The presence of two signals is due to the fact that w,/ 21~is not sufficiently strong to overcome the quadrupole interaction entirely, the excitation being not totally non-selective [ 131. We have confirmed this by performing the experiment with a stronger rfpulse, whereupon the relative intensity of the two signals changed. The nutation spectrum of sample 2A (fig. 4) is composed of numerous broad features obscured by noise. This confirms the presence of a number of different environments for aluminium, already deduced from the spectrum in fig. 3. On the 449

Volume 158. number

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:!_:

-200

,

200

0

/

200

PHYSICS LETTERS

\

.

, 0

200

200

0

-200

Fig. 3. “‘Al MAS NMR spectra.

other hand, the nutation spectrum of sample 3A shows only one sharp signal, at (-2, 78), corresponding to NFO aluminium. The chemical environments of Al in samples 2A and 3A are clearly very different. In other words, the term “amorphous zeolite” isnot unequivocal. Both amorphous samples were treated with a KOH solution, capable of causing profound structural transformations in dealuminated crystalline zeolites [ 201. In these zeolites, KOH treatment results in reinsertion of non-framework Al into the framework. The XRD pattern of sample 2B (fig. 1) indicates that a structural change has taken place re450

16June

1989

sponsible for the shift of the broad hump to 28=25” and the appearance of two low-intensity peaks. The 29Si MAS NMR spectrum (fig. 2) shows a peak at - 113 ppm and a shoulder at - 115 ppm. Bath are considerably narrower than in the starting amorphous samples. There is also a sharp low-intensity signal at - 111 ppm. The nutation spectrum (fig. 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 F, is as reported by Samoson et al. [ 161 and corresponds to non-framework tetrahedral (NFT) aluminium. The XRD pattern (fig. 1) and the 29Si MAS NMR spectrum (fig. 2) of sample 3B are very similar to those from sample 3A. On the other hand, the “Al MAS NMR spectrum (fig. 3) features a new signal at 56 ppm, which indicates that some of the NFO aluminium has been converted to tetrahedral (NFT) coordination, The nutation spectrum of this sample (fig. 4) contains three signals, (- 2, 78), (56, 78) and (56, 195), which shows that the NFO aluminium in the amorphous phase is converted to tetrahedral coordination following treatment with the base. In sample 3B this conversion is only partial, which is probably caused by the fact that only the outer surface of the amorphous particle is accessible to the base. The considerable linewidth of the tetrahedral signal in samples 2B and 3B, its high field position ( 56 ppm ) along the F2 axis (compare with 60 ppm for the F signal) and the fact that the samples obviously do not contain any aluminosilicate “framework”, allow the NFT signal to be observed in isolation. The large quadrupolar effects, small difference in chemical shift and low concentration of the NFT species make the F and the NFT signals inseparable by conventional (one-dimensional ) “Al MAS NMR. The increased 27A1linewidth found upon dealumination of zeolites, something which greatly complicates quantitative interpretation of their conventional spectra, is thus caused by a superposition of F and NFT signals. The study of amorphous materials is therefore of assistance in the reliable assignment of NMR signals in crystalline aluminosilicates.

CHEMICAL

Volume 158, number 5

16June

PHYSICS LETTERS

1989

NFO

1 I-2.w

(56 ,781

to ,781

NFO

1

1

400

0 Fz

1

-4cm

I

1

I

400

0

- 400

I

F2 ( ppml

(pm

Fig. 4. “Al quadrupole

nutation

NMR spectra.

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I6 June 1989

PHYSICS LETTERS

Acknowledgement

We are grateful to Shell Research, Amsterdam, and to Universiti Teknologi Malaysia, for support.

[9] E.F. Vansant, A. Thijs, G. Peeters, P. De Bicvre, R.D. Penzhorn, A. Dorea and P. Schuster, Zeolites 4 ( 1984) 35.

[ lo] M. Miyake, T. Konishi, T. Suzuki, S. Osada and H. Kojima, Zeolites 4 ( 1984) 291.

[ 111 P.P. Man and J. Klinowski, Chem. Phys. Letters 147 ( 1988) 581.

[ 121 P.P. Man and J. Klinowski, Chem. Sot. Chem. Commun. References

(1988)

1291.

[ I3 ] P.P. Man, J. Klinowski, A. Trokiner, H. Zanni and P. Papon, 1 ] P.A. Jacobs, E.G. Derouane and J. Weitkamp, J. Chem. Sot. Chem. Commun. ( 198 I ) 59 1. 21 G. Coudurier, C. Naccache and J.C. Vedrine, J. Chem. Sot. Chem. Commun. (1982) 1413. 31 Z. Gabelica, J.B. Nagy and G. Debras, J. Catal. 84 (1983) 256. .4] J.M. Thomas and L.A. Bursill, Angew. Chem. Intern. Ed. Engl. 19 (I 980) 745. : 51 L.A. Bursill, J.M. Thomas and K.J. Rao, Nature 289 (1981) 157. 161 L.A. Bursill and J.M. Thomas, J. Phys. Chem. 85 (1981) 3007. 71 J. Klinowski, X. Lm, R.-D. Penzhorn, P. Schuster, N.J. Clayden and C.M. Dobson, J. Chem. Sot. Faraday Trans. I 81 (1985) 1435. .8] R.-D. Penzhorn and W. Mertin, J. Solid State Chem. 54 (1984) 235.

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[ 141 A. Samoson and E. Lippmaa, Phys. Rev. B 28 ( 1983) 6567. [ 15]A. Samoson and E. Lippmaa, Chem. Phys. Letters 100 (1983)

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[ 161A. Samoson, E. Lippmaa,

G. Engelhardt, U. Lohse and H.-G. Jerschkewitz, Chem. Phys. Letters I34 ( 1987) 589. [ 171 A. Samoson and E. Lippmaa, J. Magn. Reson. 79 ( 1988) 255. [ 181 A.P.M. Kentgens, J.J.M. Lcmmens,F.M.M. Gcurtsand W.S. Veeman, J. Magn. Reson. 71 ( 1987) 62.

[ 191F.M.M. Geurts, A.P.M. Kentgens and W.S. Veeman, Chem. Phys. Letters 120 (1985) 206. [20] II. Hamdan, B. Sulikowski and J. Klinowski, J. Phys. Chem. 93 (1989) 350.

[ 2I I G. Engelhardt NMR ofsilicates

and D. Michel, High-resolution solid-state and zeolites (Wiley, New York, 1987).