J. Phys. Chem. 1996, 100, 17889-17892
17889
Two-Dimensional Triple-Quantum 27Al MAS NMR Spectroscopic Study of the High-Temperature Phase Transformation of Microporous VPI-5 Joa˜ o Rocha* and Ana P. Esculcas Department of Chemistry, UniVersity of AVeiro, 3810 AVeiro, Portugal
Christian Fernandez and Jean-Paul Amoureux Laboratoire de Dynamique et Structure des Mate´ riaux Mole´ culaires, CNRS URA 801, UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France ReceiVed: May 10, 1996; In Final Form: August 7, 1996X
Two-dimensional (2D) triple-quantum 27Al magic-angle spinning (3Q MAS) NMR spectra of VPI-5 have been recorded in situ at room temperature and 85 °C. The results confirm that at 70-80 °C VPI-5 undergoes a reversible phase transformation from space group P63 to P63cm. This is the first study of a phase transformation by 2D MQ MAS NMR spectroscopy.
Introduction VPI-5 is a crystalline aluminophosphate molecular sieve containing 18-membered rings of Al and P atoms and having the chemical formula AlPO4. The large channel diameter at 11.7 Å is one of the largest found so far among crystalline molecular sieves. Several attempts have been made to determine the details of the structure of VPI-5, the most successful of which is the refinement of McCusker et al.1 According to these authors, as-prepared VPI-5 crystallizes in space group P63 with a ) 18.975 Å and c ) 8.104 Å. The water molecules have been located inside the VPI-5 channels: two molecules complete an octahedral coordination sphere around the framework Al(1) atom between the fused four-membered rings (Figure 1a); four of the remaining water positions form a hydrogen-bonded chain linking the six-coordinated Al atoms, thus forming a triple helix of water molecules. The last, ill-defined, water position links the helixes to one another. Double-rotation (DOR) NMR of room-temperature VPI-5 detects two four- and one sixcoordinated Al resonances in a 1:1:1 intensity ratio.2,3 On the other hand, three 31P MAS NMR resonances (at ca. -23, -27, and -33 ppm) are also observed with similar populations.4,5 Hence, the McCusker et al. refinement is in agreement with the NMR evidence. Variable-temperature 31P MAS NMR studies have shown that above room temperature VPI-5 undergoes a phase transformation, probably due to a breakdown of the triple-helix structure of water inside the pores, leading to increased molecular mobility and to the P63cm space group.4,5 Changes are observed already at 60 °C and become prominent at 70 °C. The resonance at -27 ppm grows at the expense of that at -23 ppm. The transition is essentially complete at 80 °C when only the peaks at ca. -27 and -33 ppm are seen in a 2:1 intensity ratio. Rocha et al. also studied this transformation by 27Al quadrupole nutation MAS NMR and found some evidence that only one four-coordinated Al site is present above 80 °C.5 In this paper we shall show that the new 3Q MAS NMR technique, recently introduced by Frydman and Harwood,6 allows the hightemperature VPI-5 phase transformation to be studied in situ. Our results confirm that this phase transition is fully reversible.
the general ideas. Consider the symmetric multiquantum transitions (m S -m) of a nucleus with a 1/2 integer spin (e.g., 27Al, I ) 5/ ). The transition frequency for a single crystallite 2 in a fast spinning powdered sample can be written as10
νp )
Theory The theoretical framework of MQ MAS NMR spectroscopy is discussed in detail in refs 6-9. Here we shall only highlight X
Figure 1. (a) Schematic drawing of part of the room-temperature VPI-5 structure showing the labeling of the Al and P atoms. P(2) and P(3) sites and Al(2) and Al(3) sites are inequivalent as a result of distortion. All Al and P atoms are linked, via oxygens (not shown), to atoms in another layer in the structure, which makes them four-coordinated, except for the Al(1) site which is six-coordinated as a result of bonding to four bridging oxygens and two “framework” water molecules.1 (b) Single-quantum room-temperature 27Al MAS NMR spectrum of VPI5.
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01350-0 CCC: $12.00
CQ2 [A (I, p) B0(η) + A2(I, p) B2(η, R, β) P2(cos θ) + ν0 0 A4(I, p) B4(η, R, β) P4(cos θ)] (1)
where p ) ∆m is is the order of the pQ multiquantum coherence, © 1996 American Chemical Society
17890 J. Phys. Chem., Vol. 100, No. 45, 1996
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Figure 2. Contour plots of the four-coordinated Al region of the 3Q MAS NMR spectra recorded in situ at (a) 20 °C and (b) 85 °C.
TABLE 1: Summary of Isotropic Chemical Shifts and SOQE Parameters for the Different Al Species Extracted from the 2D 3Q 27Al MAS NMR Spectra Recorded in Situ at 20 and 85 °C δiso (ppm)a
a
SOQE (MHz)b
S1 S2 S3
20 °C -9,9 44.5 42.1
4.1 2.8 1.3
S1 S3
85 °C -9.7 42.4
4.3 1.8
Estimated error: (0.5 ppm. b Estimated error: (0.1 MHz.
θ is the spinning angle, and CQ and η are the quadrupole coupling constant and asymmetry parameter, respectively. P2 and P4 are second- and forth-order Legendre polynomials. The orientation of the rotor axis with respect to the quadrupole tensor is given by angles R and β. The terms An(I, p) and Bn(η, R, β) can be found elsewhere.10 It is clear from eq 1 that p acts as an independent variable. Hence, the pQ coherence can be manipulated in a 2D experiment in order to refocus the anisotropies during observation. Using a two-pulse sequence, the pQ coherence evolves during a time t1 and is subsequently transferred selectively into a single-quantum (-1)Q coherence which is observed during the acquisition time t2. When spinning at the magic angle P2(θ) ) 0 isotropic echoes are expected at times
t2 ) -
A4(I, p)
t ) R(I, p)t1 A4(I, -1) 1
(2)
For a triple (p ) +3)-quantum transition of a I ) 5/2 nucleus, the anisotropic ratio R takes the value 19/12. After a 2D Fourier transform in t2 and t1, the various species appear along the anisotropic axis A with a direction
ν1 ) R(I, p) ν2
(3)
where ν2 and ν1 are the frequencies in the (-1)Q and pQ multiquantum dimensions, respectively. The lines of the different species are p times more separated by their isotropic chemical shifts in the multiple-quantum
dimension than in the classical MAS single-quantum dimension. The multiple-quantum spectrum can be described to a first approximation as a spectrum recorded at a virtual magnetic field p times greater than the actual field of the spectrometer. On the other hand, the lines do not appear at their isotropic chemical shifts; rather they are displaced (in both dimensions) by the quadrupole shifts. The induced quadrupole shift for p ) +3 and I ) 5/2 relative to the single-quantum quadrupole shift is ξ ) 3/4. This means that it is possible to separate species with similar isotropic chemical shifts but different quadrupole coupling constants. From the centers of gravity δ1 and δ2 (F1 and F2 dimensions, respectively) of the 2D spectrum, it is possible to estimate the isotropic chemical shift δiso and the SOQE parameter of the lines. For p ) +3 and I ) 5/2
δiso (ppm) ) 4/3δ1 - 1/3δ2
(
SOQE2 (MHz) ) CQ2 1 +
)
(δ2 - δiso)ν0 η2 ) 3 6000
(4) 2
(5)
Experimental Section VPI-5 was prepared according to Davis et al.11 using di-npropylamine (DPA) as a template, hydrated for 12 h (ca. 25 wt % water) over a saturated solution of NH4Cl, and characterized by powder X-ray diffraction (XRD) and scanning electron microscopy prior to NMR measurements. Elemental analysis indicates that the DPA content in the samples corresponds to about one molecule per three unit cells. Powder XRD shows that the sample is highly crystalline and contains less than 3% AlPO4-11 impurity. Single- and triple-quantum 27Al MAS NMR spectra were recorded at νo ) 104.3 MHz on a Bruker MSL-400P spectrometer. The 4-mm rotors were spun at 15 kHz. The recycle delay was 0.5 s. The radio frequency magnetic field amplitude was ca. 135 kHz. A total of 512 data points were acquired in the t1 dimension in increments of 12 µs. To produce pure-absorption line shapes in the 3Q MAS spectra, the optimum conditions for excitation and transfer of the ((3Q) coherences using a simple two-pulse sequence were used.7 The phase cycling was composed of six phases for the selection of triple-quantum coherences. This phase cycling was combined with a classic overall four-phase cycle in order to minimize phase and
Phase Transformation of Microporous VPI-5
J. Phys. Chem., Vol. 100, No. 45, 1996 17891
Figure 3. Stacked plots of the four-coordinated Al region of the 3Q MAS NMR spectra recorded in situ at (a) 20 °C and (b) 85 °C.
Figure 4. Contour plots of the six-coordinated Al region of the 3Q MAS NMR spectra recorded in situ at (a) 20 °C and (b) 85 °C.
amplitude mis-settings of the receiver. In our experiments we have recorded 192 transients per spectrum, that is eight full cycles. The ppm scale was referenced to ν0 frequency in the ν2 domain and to 3ν0 in the ν1 domain [reference (Al(H2O)63+)]. We have recorded 3Q rather than 5Q spectra because (i) the resolution of the former is already quite good and (ii) a 3Q spectrum can be acquired in a few hours and this is convenient when performing high-temperature in situ NMR studies. We have started by measuring a spectrum at 20 °C. Then the temperature was slowly raised (5 °C/min) up to 85 °C; after 30 min of equilibration a 3Q spectrum was recorded. Finally, the sample was cooled to 20 °C and a third spectrum measured. Results and Discussion Figure 1b shows the single-quantum 27Al MAS NMR spectra of VPI-5 (20 °C). The four-coordinated Al region of the 3Q MAS 27Al NMR spectrum recorded at 20 °C is shown in Figure 2a. No differences were found between the spectra recorded before heating in situ at 85 °C and after cooling from this temperature to 20 °C (the actual spectrum shown is the latter).
Isotropic chemical shifts (δiso) and SOQE parameters are collected in Table 1. At 20 °C the spectrum displays two tetrahedral Al peaks at δiso ) 42.1 and 44.5 ppm with very different SOQE parameters (1.3 and 2.8 MHz, respectively). S2 displays a characteristic powder pattern (see stacked plot in Figure 3a) which (after shearing of the spectrum) can be simulated to yield δiso, CQ, and η. These results are in accord with previous DOR and quadrupole nutation studies.3,5 Notice that the site with larger SOQE (S2) lies along the A axis while site S3 is not in line with either the A or the QIS axes. Moreover, S3 also does not lie along the axis δiso, perpendicular (slope -1/ξ) to QIS.7 These facts indicate for S3 a small distribution of both quadrupole coupling constants and chemical shifts.7 After heating to 85 °C (contour and stacked plots in Figures 2b and 3b, respectively), only one tetrahedral Al resonance is observed at δiso ) 42.4 ppm. This site is rather similar to site S3 seen at 20 °C, displaying only a slightly larger (1.8 MHz) SOQE. The six-coordinated Al region of the 3Q 27Al NMR spectra measured at 20 and 85 °C are shown in Figures 4 and 5. The small differences in the δiso and SOQE
17892 J. Phys. Chem., Vol. 100, No. 45, 1996
Rocha et al.
Figure 5. Stacked plots of the six-coordinated Al region of the 3Q MAS NMR spectra recorded in situ at (a) 20 °C and (b) 85 °C.
parameters (see Table 1) are consistent with simulations of the single-quantum MAS quadrupole powder pattern.5 It is interesting to note that the resonance recorded at 20 °C is slightly broader than the 85 °C one (see contour plots in Figure 4). Since the spectra were acquired without 1H high-power decoupling, this could indicate that 1H-27Al dipolar interactions are stronger at 20 °C than at 85 °C. This is not altogether unexpected since Rocha et al.5 reported a strong decrease in the efficiency of the 1H-27Al cross-polarization (CP) with increasing temperature. In addition, Houckgeest et al. observed that the 1H-31P CP signal is almost unobservable above 60 °C.4 We cannot discard the possibility that other mechanisms contribute to the broadening of the resonance measured at 20 °C, namely 27Al-31P dipolar interactions or a small dispersion of chemical shifts. There has been much discussion concerning the detailed assignment of the 27Al and 31P resonances (see, for example, ref 12). 27Al-31P correlation NMR spectroscopy clearly shows that the 31P line at -33.2 ppm is attributed to P(1).12 In addition, the peak at -23.3 ppm is strongly coupled with the 27Al site with the larger quadrupole coupling (S2). Our results support these findings. Indeed, at 85 °C, the peak S2 is not seen in the 3Q MAS NMR spectrum and simultaneously the -23.3 ppm resonance also disappears from the 31P MAS NMR spectrum. Grobet et al. assigned line S2 to Al(2) and if this is correct P(2) resonates at -23.3 ppm and P(3) at -27.5 ppm.3 In contrast, Engelhardt (quoted in ref 12) has suggested that S2 is attributed to Al(3) and if so the assignment of the 31P resonances should be reversed.
In conclusion we have shown that 2D MQ NMR spectroscopy is a powerful technique to study in situ the structure and phase transformations of crystalline microporous materials. Acknowledgment. J.R. and A.P.E. thank Junta Nacional de Investigac¸ a˜o Cientı´fica e Tecnolo´gica and PRAXIS XXI for funding. References and Notes (1) McCusker, L. B.; Baerlocher, Ch.; Jahn, E.; Bu¨low, M. Zeolites 1991, 11, 308. (2) Wu, Y.; Chmelka, B. F.; Pines, A.; Davis, M. E.; Grobet, P. J.; Jacobs, P. A. Nature 1990, 346, 550. (3) Grobet, P. J.; Samoson, A.; Geerts, H.; Martens, J. A.; Jabobs, P. A. J. Phys. Chem. 1991, 95, 9620. (4) van Braam Houckgeets, J. P.; Krushaar-Czarnetzki, B.; Dogterom, R. J.; de Groot, A. J. J. Chem. Soc., Chem. Commun. 1991, 666. (5) Rocha, J.; Kolodziejski, W.; He, H.; Klinowski, J. J. Am. Chem. Soc. 1992, 114, 4884. (6) Frydman, L.; Harwood, J. S. J. Am. Chem. Soc. 1995, 117, 5367. (7) Fernandez, C.; Amoureux, J. P. Chem. Phys. Lett. 1995, 242, 449. (8) Medek, A.; Harwood, J. S.; Frydman, L. J. Am. Chem. Soc. 1995, 117, 12779. (9) Amoureux, J. P.; Fernandez, C.; Frydman, L. Chem. Phys. Lett. 1996, 259, 347. (10) Amoureux, J. P. Solid-State NMR 1993, 2, 83. (11) Davis, M. E.; Montes, C.; Hathaway, P. E.; Garces, J. M. In Zeolites: Facts, Figures, Future; Jacobs, P. A., van Santen, R. A., Eds.; Elsevier: Amsterdam, 1989; p 189. (12) van Eck, E. R. H.; Veeman, W. S. J. Am. Chem. Soc. 1993, 115, 1168.
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