E L S E V I E R Microporous Materials 5 (1995) 29-37

MICROPOROUS MATERIALS

Structural investigations of SAPO-37 molecular sieve by coherence-transfer and dipolar-dephasing solid-state nuclear

magnetic resonance experiments

C.A. Fyfe *, K.C. Wong-Moon, Y. Huang, H. Grondey Department of Chemistry, University of British Columbia, 2036 Main Mall. Vancouver, B.C. V6T IZ1. Canada

Received 13 March 1995; accepted 14 April 1995

Abstract

In this work, dipolar-based coherence-transfer and dipolar-dephasing solid-state nuclear magnetic resonance (NMR) experiments were performed to investigate the structure of the silicoaluminophosphate molecular sieve SAPO-37. The 27A1~29Si transferred-echo double-resonance (TEDOR) experiment directly proves, for the first time, that silicon substitutes for phosphorus atoms in the framework via the SM2 mechanism. The 27A1 (31p) and 2~A1 (29Si) dipolar- dephasing difference experiments and the two-dimensional 27Al~31P TEDOR experiment indicate the presence of three types of aluminum environments: tetrahedral A1 surrounded by symmetrical [4P] or [4Si] environments which give rise to a sharp resonance, tetrahedral A1 surrounded by asymmetrical [3P, Si], [2P, 2Si] or [P, 3Si] environments which give rise to a broad resonance, and also extra-framework amorphous octahedral A1.

Keywor&v SAPO-37 molecular sieve; Coherence-transfer; Dipolar-dephasing; Transferred-echo double-resonance

1. Introduction

Zeolites are an important class of aluminosilicate open-framework materials that are widely used as molecular sieves because of the selectivity of their channel systems towards the adsorption of organic molecules [ 1 ]. Combination with their acidic prop- erties in the acid form gives them unique catalytic properties. Zeolites have been established indus- trial materials for thirty years, but recent years have shown the growth of a second class of related molecular sieves, namely the aluminophosphates (A1PO4s) [2]. Unlike most zeolites, the A1PO4 structures are ordered, with an exact alternation of A10 4 and P O 4 tetrahedra. The A1/P ratio,

* Corresponding author.

0927-6513/95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-6513(95)00032-1

therefore, is always 1, and the system is neutral with no charge-balancing extra-framework ions present. As a result, the A1PO4s cannot act as acid catalysts, although they have similar sieving prop- erties as zeolites.

With the introduction of silicon into the A1PO4 framework, however, the charge balance is altered, and acidity becomes possible. These silicoalumino- phosphate molecular sieves (SAPOs) exhibit some properties characteristic of zeolites and some unique properties that reflect their own chemical compositions [3]. SAPO-37, which has the faujas- ite framework, has been the subject of numerous investigations [4-11 ], particularly to establish that the Si atoms are in the framework and to determine the mechanism of silicon substitution. There are three possible mechanisms that can be envisaged for silicon substitution into an AIPO4 framework:

30 CA. Fyfe et al./Microporous Materials 5 (1995) 29-37

replacement of a single A1 atom by Si (SM1 mechanism), replacement of single P atom by Si (SM2 mechanism), or replacement of one P together with one A1 atom by two Si atoms (SM3 mechanism). It is postulated that silicon substitu- tion into the SAPO-37 framework proceeds via the SM2 and SM3 mechanisms, with the latter mechanism becoming more important for condi- tions of high silicon content (greater than 10% of the tetrahedral sites).

Over the last decade, high-resolution solid-state NMR spectroscopy [12] has emerged as a powerful technique for structural investigations of molecular sieves. In previous work, we have introduced dipo- lar-based NMR connectivity experiments involving quadrupolar nuclei, and applied them to alumino- phosphate (27A1/31P) [ 13-15] and zeolite molecular sieves (27A1/298i) [16]. Specifically, we have used the cross-polarization (CP), transferred- echo double-resonance (TEDOR), rotational- echo double-resonance (REDOR) and dipolar- dephasing (DD) difference experiments to select 27A1-O-31p and 27A1-O-298i connectivities in these molecular sieve systems via the heteronuclear dipolar interactions between the pairs of nuclei. In this work, we apply these experiments to select 27A1-O-3~P and 27A1-O 29Si connectivities in the silicoaluminophosphate molecular sieve SAPO-37.

2. Experimental

A sample of SAPO-37 was provided by W. R. Grace & Co. The sample was analysed by X-ray fluorescence (XRF) and found to have the composition (Sio.azvAlo.525Po.3490 2. A 1H/13C CP experiment confirmed that the sample contained tetramethylammonium (TMA) and tetrapropyl- ammonium (TPA) ions as templates, and the sample was studied in the as-synthesized form. Powder X-ray diffraction (XRD) measurements were performed using a Rigaku rotating anode powder diffractometer (CuKc~ radiation, 2= 1.5418 A).

All NMR experiments were performed on a Bruker MSL 400 spectrometer modified to include a third radiofrequency channel. Resonance fre- quencies were 79.495 MHz for 298i, 104.264 MHz for 27A1 and 161.977 MHz for 31p, and chemical

shifts are reported with respect to tetramethylsilane (TMS), aqueous AI(NO3)3 and H3PO4, respec- tively. In all experiments, the magic angle was set using the 79Br resonance of KBr. Due to the individual requirements of each experiment, three different NMR probes were used.

(1) The 27AI/29Si connectivity experiments were performed using a home-built double-tuned probe incorporating a 14-mm "pencil spinner" supplied by Chemagnetics, with an internal volume of 2.8 ml. This large sample volume was necessary due to the low chemical content of silicon in our sample of SAPO-37 coupled to the low natural abundance of 298i (4.7%). Samples were spun at 2.2-2.5 kHz with the spinning rate monitored by an optical sensor unit. This low spinning rate leads to more efficient CP. The 298i 90 ° pulse and the 27A1 90 ° pulse (central transition only) were 14.5-18.0 gs.

(2) The 2VA1/alP connectivity experiments were performed using a home-built double-tuned probe incorporating a 10-ram Supersonic spinner sup- plied by Doty Scientific with an internal volume of 1.0 ml. Samples were spun at 5.3-5.5 kHz with the spinning rate monitored by the spinning side- bands of the V9Br MAS spectrum of KBr. The alp 90 ° pulse and the 27A1 90 ° pulse (central transition only) were 22.0 gs.

(3) The 27A1 fast-spinning MAS experiments were performed using a home-built single-tuned probe incorporating a 5-mm high-speed spinner supplied by Dory Scientific with an internal volume of 100 gl. Samples were spun at 4-12 kHz with the spinning rate monitored by the spinning side- bands in the 79Br MAS spectrum of KBr. The 27AI 90 ° pulse (central transition only) was 1.5 las.

The pulse sequences used for the CP, TEDOR, DD and two-dimensional CP and TEDOR experi- ments are described in detail in Refs. [15] and [16]. For the one- and two-dimensional TEDOR experiments, the number of rotor cycles before and after the coherence transfer (n and m, respec- tively) are optimized experimentally and indicated in each figure caption.

3. Results and discussion

The powder XRD spectrum (Fig. 1) confirms the identity of the sample as SAPO-37 and shows

CA. Fyfe et aL/Microporous Materials 5 1995) 29 37 31

5 10 15 20 25 30 :35 40 28

Fig. I. Powder X-ray diffraction spectrum of SAPO-37.

that both the crystallinity and purity of the sample are high. XRF analysis shows that the composition of the sample is (Sio.127A10.52sPo.349)O 2. The com- bined Si and P content of 47.6% is lower than the A1 content of 52.5%. The framework A1 content, however, must be equal to (SM2 mechanism) or less than (SM3 mechanism) the combined Si and P content. Therefore, the difference must be due to extralattice octahedral A1. In addition, this means that the contents of Si and P in the frame- work must be slightly higher than 12.7% and 34.9%, respectively.

The 29Si NMR spectrum (Fig. 2a) agrees with those published previously [4-11]. The spectrum consists mainly of a sharp peak at -89 .4ppm (together with spinning sidebands as indicated in the figure), consistent with a single environment for Si. The nature of this environment, however, cannot be unambiguously assigned without further experiments, although the value of the chemical shift suggests that the environment could be Si[4A1]. The spectrum also shows a broad peak underneath the main sharp peak at around - 9 0 ppm which may be due to aluminosilicate environments (e.g. Si[3A1, Si]) and a small sharp peak at -107.1 ppm which has been previously assigned as a Si[4Si] environment arising from small siliceous domains perhaps formed by the SM3 mechanism.

(a)

-40 -60 -80 -100 -120 -I40 Frequency (ppm from TMS)

t I , I j I , I ~ I t

20 0 -20 -40 -60

Frequency (ppm from 85% H3PO 4)

I

-80

Fig. 2. (a) 298i MAS spectrum of SAPO-37. Spinning sidebands are indicated with an asterisk. (b) 3~PMAS spectrum of SAPO-37. Spinning sidebands are indicated with an asterisk.

The 31PMAS spectrum (Fig. 2b) also agrees with those published previously [4-8]. The spectrum consists of a sharp peak at -26.1 ppm,

32 C.A. Fyfe et al./Microporous Materials 5 (1995) 29-37

which can be assigned as P[4A1] from the expected P/A1 alternation.

The nature of the silicon environment can be unambiguously assigned, however, through the 27A1---~29Si TEDOR spectrum (Fig. 3). The TEDOR spectrum shows the same sharp peak as in the 298i MAS spectrum, while the corresponding null experiment (not shown) shows no signal. Since 27A1 is the only source of the coherence for the 298i signal, 2 7 A 1 - O - 2 9 8 i connectivities must exist, proving the Si[4A1] environment. This is consistent with Si substitution for P atoms via the SM2 mechanism.

The local environment of phosphorus can also be probed through the 27Al~31P TEDOR spectrum (Fig. 4). As expected, the TEDOR spectrum shows the same single peak as in the 31p MAS spectrum, verifying the P[4A1] environ-

I ~ I ~ I ~ I

-60 -80 -100 -120 Frequency (ppm from TMS)

Fig. 3. 27Al~Zgsi TEDOR experiment for SAPO-37, with n = 2 and m = 1. A total of 879 897 scans were acquired with a recycle delay of 0.1 s, resulting in a total experimental time of 24.4 h.

ment. Again, a corresponding null experiment was performed (not shown) and showed no signal.

However, the situation for aluminum is not as straightforward as for silicon and phosphorus. The 27A1 MAS spectra obtained at different spinning rates (Fig. 5) all show two sharp lines at 37.4 and 5.5ppm, with evidence of a third very broad resonance lying underneath these two sharp peaks. The peak at 37.4 ppm can be assigned to tetra- hedral A1. The peak at 5.5 ppm is typical of octa- hedral A1, and has previously been assigned to residual extralattice amorphous alumina [5,6 ]. The octahedral A1 resonance has a much shorter T2 value than the tetrahedral A1 (approximately 1.0 and 9.0 ms, respectively). The spinning sidebands of the two sharp lines disappear when spinning faster than 6.4 kHz, while those of the third broad resonance are still seen when spinning at 12.1 kHz. This third broad resonance, therefore, has a much

spinning rate/kHz

12.1

9.6

8.5

7.6

, I ~ I , I , I ~ I ~ I 20 0 -20 -40 -60 -80

Frequency (ppm from 85% H3PO4)

Fig. 4. 27A1--*31p TEDOR experiment for SAPO-37, with n = 4 and m = 1. A total of 6400 scans were acquired with a recycle delay of 0.1 s, resulting in a total experimental time of only 11 min.

6.4

5.3

~ J V ~ . ~ 4.1

200 0 -200 Frequency (ppm from AI(NO3)3)

Fig. 5. 27A1 MAS spectra of SAPO-37 acquired at the different spinning rates indicated.

C.A. Fyfe et al./Microporous Materials 5 (1995) 29 37 33

larger anisotropy (over 300 ppm) and a lower symmetry compared to the two sharp peaks. The chemical shift of the third broad resonance is difficult to measure directly since it is broad and lies underneath the two sharp signals. Fig. 5 shows that as the spinning speed increases, the relative intensity of the octahedral A1 peak seems to increase. Since the sidebands from the octahedral A1 resonance disappear when spinning faster than 6.4 kHz, the increase in intensity of the octahedral A1 resonance at higher spinning speeds must arise from another source. This increase is actually due to the third broad resonance, since its sidebands are still observable at high spinning rates. In addition, the resolution between the two sharp peaks decreases as the spinning speed increases. This implies that the isotropic chemical shift of the broad resonance is centred slightly downfield from the octahedral A1 resonance, at around 10 ppm.

It was previously proposed [17] that evidence for asymmetrical environments such as AI[3P, Si] comes from the large anisotropies of both the tetrahedral and octahedral A1 sites. However, when spinning faster than 6.4 kHz, the spinning side- bands of the two sharp lines disappear, and only spinning sidebands from the third broad peak remain (Fig. 5). Therefore, the large anisotropy observed must arise from the third broad peak and not from the sharp tetrahedral or octahedral peaks. In addition, in the two-dimensional (2D) 27Al~3aP TEDOR experiment (see Fig. 6 below) there is a large 27A1 anisotropy observed, but no intense correlation with the sharp octahedral A1 resonance. This provides additional support ruling out the octahedral A1 peak as the source of the large anisotropy.

Further information regarding the nature of the three A1 resonances can be obtained from the two-dimensional 27AI---~3tpTEDOR experiment (Fig. 6), which shows the individual 2VAI-O-3XP connectivities in SAPO-37. The sharp tetrahedral 27A1 site at 37.4 ppm shows a strong correlation to the single 3~p site, as expected. There is also a set of lower-intensity correlations over a range of about 300 ppm from the 27A1 spectrum to the 31p site. The source of these correlations can be made clearer by comparing the 27A1 2D TEDOR pro-

31p

20

o o o

@ 0

o

L I , I , I , I

0 -20 -40 -60

Frequency (ppm f rom H3PO4)

1

' 6 ~ z "7 "<

-' E o

.4

Fig. 6. Two-dimensional 27Al-,3~PTEDOR experiment for SAPO-37, with n =4, m = 1, and a spinning rate of 5.30 kHz. The spinning sidebands from the sharp tetrahedral peak at 37.4ppm are indicated ('1 (see Fig. 7). For each of the 128 experiments in tl, 3584 scans were acquired. The recycle delay of 0.1 s resulted in a total experimental time of 12.7 h.

jection (Fig. 7a) with the corresponding one- dimensional 27A1 MAS spectrum (Fig. 7b). It can be seen that half of the set of smaller peaks in the 27A1 2D TEDOR projection (indicated by aster- isks) are the sidebands of the sharp tetrahedral 27A1 peak since they are separated by the rotational frequency around the isotropic shift of this reso- nance. However, the other half of the set of smaller peaks is due to the third broad resonance in the 27A1 spectrum (isotropic peak and sidebands). (It should be noted that the contribution from spin- ning sidebands is enhanced in a TEDOR experi- ment.) A repeat of the 2D 27Al-*31pTEDOR experiment at a lower spinning speed (not shown) confirms that the isotropic shift of this third broad peak is centred at 7.4 ppm. This broad resonance has a large anisotropy (over 300ppm) and is therefore likely due to an asymmetrical environ- ment. Although it is possible that it could be due to an amorphous A1 O -P environment, the 3tp MAS spectrum is sharp, thereby making this possibility unlikely.

A 2D 27A1--+31P CP experiment (not shown) was

34 CA. Fyfe et al./Microporous Materials 5 (1995) 29-37

27A1 2D T E D O R pro jec t ion

27A1 1D M A S

, I , I I , I , I 200 100 0 -100 -200 Frequency (ppm from Al(NO3)3)

Fig. 7. /VA1 experiments on SAPO-37. In both experiments, the spinning rate was 5.30 kHz. (a) aVAl 2D TEDOR projection from Fig. 6, with spinning sidebands from the sharp tetrahedral peak at 37.4ppm indicated (*). (b) 2~AI one-dimensional MAS spectrum.

The difference between these two signals represents the 27A1 sites which are connected to the 31p site. As can be seen from the pulse sequence in Fig. 8, since there is no frequency-encoding period in the DD experiment and the echo time can be made very short, the decay due to T2 is minimized compared to the 2D TEDOR and CP experiments. In addition, since the 3~p spectrum consists of only a single line, there is no loss of information in the DD expertment compared to possible 2D experiments.

The 27A1 spin echo signal and the 27A1 (3~p) DD signal are compared in Fig. 9. In each experiment, the total echo time was made as short as possible (two rotor cycles) in order to minimize the effects of T2 relaxation. The 27A1 spin echo signal (Fig. 9a) shows sharp resonances from tetrahedral and octahedral A1, and the third broad resonance underneath. In the ZTA1 (3ap) DD experiment (Fig. 9b), the octahedral A1 peak is removed and

also performed. The experiment gives the same correlation of the sharp tetrahedral Z:A1 site at 37.4ppm to the single 31p site, as in the 2D T EDOR experiment. However, the sideband inten- sities are lower in the 2D CP experiment (as expected, since the contribution from spinning sidebands is enhanced in a TEDOR experiment), so the correlation to the third broad peak is not quite as clear.

The 2D 27Al~3Xp TEDOR and CP experiments do not show any correlations to the octahedral A1 site, consistent with the assignment of this site to extraframework amorphous alumina. However, care must be taken in coming to this conclusion based on such a negative result. It is possible that the octahedral A1 is in the framework, and correla- tions to this site may be missed as it has a very short T2 value (approximately 1.0 ms), making it possible that the octahedral A1 coherence might decay completely during the frequency encoding and echo periods of the 2D experiments.

To eliminate this possible ambiguity, an 2VA1 (31p) DD difference experiment was performed in which the 27A1 spin echo was recorded first with 3~p irradiation, and then without 3ap irradiation.

(a) 27A1 (90 (180)

[ a c q ] +

~ A ^ - _ .

iliV : -- vvil! I i v ' - - : lil |,, ,

. . . . . . . . . . . . . . . . . . . . i

S ~ t ! ~ .... : . . . . . ~" " i i i

~r-~ ,.

n'Cr •

(90) (180)

(b) 27A1 h / l^ . . . . I I l l [ a c q ] -

i

S ~'~iiiii .......... ~ZI..." '

• n ' C r ,

Fig. 8, Pulse sequence for 27A1/(S), with S=31P or 29Si, DD difference experiment under MAS conditions with a rotational period of zr. An 27A1 spin echo signal is recorded first ([acq] +) without S spin decoupling (due to 10MHz offset), The experiment is immediately repeated with subtraction ([acq]-) of the 27A1 spin echo signal obtained with on-resonance decoupling of the S spins during the first half of the experiment.

CA. F3fe et al./Microporous Materials 5 (1995) 29-37 35

•• 27A1 spin echo

• J ~/~ 27A1/31pD D

200 0 -200 Frequency (ppm from Al(NO3)3)

Fig. 9, 27A1 experiments on SAPO-37. In each experiment, the spinning rate was 5.65 kHz, and the total echo time was two rotor cycles (n=2). (a) 27A1 spin echo signal (no 31p dephasing) with the octahedral signal indicated (*). (b) 2VAl (31p) DD difference experiment, with one rotor cycle of 31p dephasing. A total of 116248 scans was acquired with a recycle delay of 0.5 s, resulting in a total experimental time of 16.1 h.

a) / ~ ~ i n echo

, J , I I

200 0 -200 Frequency (ppm from AI(NO3 }3)

Fig. 10. 27A1 experiments on SAPO-37. In each experiment, the spinning rate was 2.36 kHz, and the total echo time was two rotor cycles (n=2). (a) 27A1 spin echo signal (no ~'~Si dephasing). (b) 2-A1 (29Si) DD difference experiment, with one rotor cycle of 29Si dephasing. A total of 275268 scans was acquired with a recycle delay of 0.5 s, resulting in a total experimental time of 38.2 h.

is thus due to A1 which is not connected to P. Therefore, the octahedral AI atoms are not in the SAPO-37 framework. In addition, the presence of the third broad A1 resonance in the DD experiment verifies that this A1 site is connected to P in the lattice. A corresponding null DD experiment was performed (not shown), and this confirmed that no 27A1 signals arose from imperfect subtraction in the difference experiment.

In an analogous manner, a 27A1 (29Si) DD difference experiment was performed to detect only those A1 atoms with attached Si atoms. The 27A1 spin echo signal (Fig. 10a) shows a sharp reso- nance from tetrahedral AI and a broad resonance underneath. The 27A1 (298i) DD signal (Fig. 10b) also shows a sharp tetrahedral AI peak and a broad resonance underneath. The presence of the sharp tetrahedral A1 resonance in the 27A1 (298i)

DD experiment suggests that this resonance con- tains contributions from A1 nuclei with one or more Si nuclei as first-nearest neighbours. It is known that the Si sites in SAPO-37 tend to cluster together, rather than evenly distribute in the A1PO4 framework [4-11 ]. Therefore, at 12.7% Si substitu- tion for P, there should be detectable amounts of

AI[4Si] environments. The chemical shift of the tetrahedral AI[4Si] environment in zeolite Y, which has the same topology as SAPO-37, is significantly higher than the tetrahedral A1 peak observed in SAPO-37 (60 versus 37 ppm at 9.4 T). However, it is felt that zeolite Y is not an appro- priate chemical shift reference since at 10% Si incorporation, 90% of the SAPO-37 lattice is com- posed of AI and P tetrahedral sites, and the local geometries they impose may well determine the exact chemical shift.

Therefore, in the 27A1 (298i) DD experiment (Fig. 10b), it is suggested that the environment of the nuclei which contribute to the sharp tetrahedral AI resonance at 37 ppm is AI[4Si], and that the deciding factor determining the appearance of the 27A1 spectrum is the symmetry of the local AI environments. Thus, in the 2VAl MAS spectra (Fig. 5), the relatively narrow tetrahedral AI signal contains contributions from AI[4P] and AI[4Si], both of which are in symmetrical local environ- ments. Furthermore, in a study [5] of SAPO-37 samples with 12, 16, and 21% Si incorporation, all of which contain AI[4Si] environments, a sharp tetrahedral resonance was seen near 37 ppm, but

36 C.A. Fyfe et al./Microporous Materials 5 (1995) 29-37

no other sharp resonance was observed around 60 ppm (as in zeolite Y). This provides additional evidence that the AI[4Si] environments are contributing to the observed tetrahedral signal at 37ppm. This resonance is relatively broad (Avl /2~2OOOHz), and could accommodate two narrow resonances with somewhat different chemi- cal shifts.

Although there is no A1PO4-37 aluminophos- phate analog of SAPO-37 available, a comparison of A1PO4-5 [18] with SAPO-5 [19] and AIPO4-11 [20] with SAPO-11 [21] shows that there is no noticeable change in the chemical shift of the sharp tetrahedral A1 resonance when Si is incorporated into the lattice. This provides additional support for the argument that the chemical shift of the tetrahedral A1 resonance does not change greatly on going from an AI[4P] to an AI[4Si] environ- ment in SAPO molecular sieves.

The observation of the third broad A1 resonance in the 2TA1 (29Si) DD experiment (Fig. 10b) shows that this A1 site is connected to Si atoms in the lattice. This A1 site is also connected to P, as indicated by the 27A1 (31p) DD experiment (Fig. 9b). The broad resonance, therefore, is assigned to tetrahedral A1 surrounded by either [3P, Si], [2P, 2Si], or [P, 3Si]. These three A1 environments are all asymmetrical since they con- tain a mixture of P and Si atoms, which gives rise to the large anisotropy observed. This is consistent with the known effect of symmetry on quadrupolar interactions. The investigation of other SAPO-37 materials with lower and higher degrees of Si incorporation would perhaps confirm these conclu- sions, and such experiments are planned for the future.

4. Conclusions

Although the value of the chemical shift suggests the same, the 27A1~29Si TEDOR experiment unambiguously shows that the nature of the main Si environment in SAPO-37 is Si[4A1]. The 27Al-*3xP TEDOR experiment shows that there is only a single P[4A1] environment. These experi- ments are consistent with Si substitution for P atoms via the SM2 mechanism.

The 27A1 variable-speed MAS, 2D 27AI--*31P TEDOR and 27A1 (31p) and 27A1 (295i) DD difference experiments indicate the presence of three types of A1 environments: (a) tetrahedral A1 surrounded by symmetrical [4P] or [4Si] envi- ronments which give rise to a relatively sharp resonance at 37.4 ppm; (b) tetrahedral Al sur- rounded by asymmetrical [3P, Si], [2P, 2Si] or [P, 3Si] environments which give rise to a broad resonance with a large anisotropy (over 300 ppm), centred at 7.4 ppm; and (c) extraframework octa- hedral A1 which is not connected to either P or Si, which gives rise to a sharp resonance at 5.5 ppm.

Acknowledgements

The authors acknowledge the financial assis- tance of the Natural Sciences and Engineering Research Council (NSERC) of Canada in the form of operating and equipment grants (C.A.F.), a Postgraduate Scholarship (K.C.W.-M.) and a Postdoctoral Fellowship (Y.H.). The authors thank W.R. Grace & Co. for kindly providing the sample of SAPO-37, Dr. A.W. Peters of W.R. Grace & Co. for helpful discussions and Mr. T. Markus for assistance with probe electronics.

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