812
J. Phys. Chem. B 1999, 103, 812-817
27Al
Multiple-Quantum Magic Angle Spinning NMR Study of the Thermal Transformation between the Microporous Aluminum Methylphosphonates AlMePO-β and AlMePO-r Steven P. Brown,† Sharon E. Ashbrook, and Stephen Wimperis* Physical Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 3QZ, U.K. ReceiVed: June 4, 1998; In Final Form: NoVember 20, 1998
The 27Al multiple-quantum magic angle spinning (MQMAS) NMR technique is used to investigate the thermal transformation between AlMePO-β and AlMePO-R. The required spectral resolution is achieved using a novel split-t1 version of the basic five-quantum MAS experiment. The five-quantum split-t1 27Al MAS NMR spectrum of a sample where the thermal transformation has been interrupted before complete conversion is compared with that of a simple physical mixture of the R- and β-polymorphs. The two spectra are shown to be different, a result that does not appear to support the proposition by Carter et al. (J. Mater. Chem. 1997, 7, 2287) of a topotactic reconstructive transformation. On the basis of our 27Al MQMAS results, we outline a possible alternative (and rather more commonplace) mechanism for the thermal transformation.
Introduction The ability to obtain structural information using 27Al NMR spectroscopy of solids is often hindered by the presence of a significant second-order quadrupolar broadening of the 27Al resonances. As a result, much attention has been focused on the fact that, by use of the newly developed multiple-quantum magic angle spinning (MQMAS) technique,1,2 remarkable enhancements in spectral resolution have recently been demonstrated in a range of 27Al NMR applications, most notably in the study of microporous materials and amorphous solids.1,3-16 In this paper, the 27Al MQMAS NMR technique is applied to the investigation of the thermal transformation between two recently characterized microporous aluminum methylphosphonates.17-20 A novel version of the basic five-quantum MAS experiment is also demonstrated. The wide utility of molecular sieves as ion exchange materials, adsorbents, and catalysts has generated much interest in the development of novel structural forms. In 1994, the synthesis and characterization of the first member of a new class of microporous materials, the aluminum methylphosphonates (AlMePOs), were described.17 In contrast to the aluminophosphates (AlPOs), the aluminum methylphosphonates are hydrophobic as a result of the presence of the methyl groups attached to the phosphorus atoms. The first two forms to be described, AlMePO-R19 and AlMePO-β,17 are structurally very similar, both containing unidimensional channels, with the three oxygen atoms in each CH3PO3 unit being bonded to one six-coordinate (octahedral) and two four-coordinate (tetrahedral) aluminum atoms. There are, therefore, three tetrahedral aluminum sites to each octahedral site, although the higher symmetry of AlMePO-R means that there is only one crystallographically distinct site of each type, compared with three such tetrahedral sites and one octahedral site in AlMePO-β. Carter et al. have reported recently that the AlMePO-β polymorph undergoes a transformation into the R-polymorph when heated to 500 °C under water vapor partial pressures of * To whom correspondence should be addressed. Fax: +44-1865275410. Email:
[email protected]. † Present address: Max-Planck-Institut fu ¨ r Polymerforschung, Postfach 3148, D-55021 Mainz, Germany.
25 Torr.20 This type of conversion is not common in framework solids, where elevated temperatures usually result in the loss of crystallinity and the development of a denser phase, and Carter et al. have proposed that it is a topotactic reconstructive transformation.20 To test this hypothesis further, an AlMePO sample was prepared (by Mr. V. J. Carter of the University of St. Andrews, Scotland) where the thermal transformation was interrupted before the conversion from the β- to the R-polymorph was complete. In this paper, the five-quantum 27Al MAS NMR spectrum of this sample is compared with that of a physical mixture of the R- and β-polymorphs. The results do not appear to support the proposition of a topotactic reconstructive transformation. Figure 1 shows conventional 27Al MAS NMR spectra of AlMePO-R, AlMePO-β, and the intermediate R/β form recorded on a Bruker MSL 400 spectrometer (ω0(27Al)/(2π) ) 104.3 MHz). The peak in the spectrum of AlMePO-R at δ ) -22 ppm and that in the spectrum of AlMePO-β at δ ) -18 ppm correspond to the octahedral sites and, as expected, the same two peaks appear to occur in the spectrum of the R/β form. The apparent ratio of the R- to β-polymorphs in the intermediate form can be determined from the relative areas of the two octahedral peaks in Figure 1c; i.e., in this sample, the apparent R:β ratio equals approximately 1:2. The presence of secondorder quadrupolar broadening (and possibly other broadening mechanisms) prevents any of the tetrahedral sites at δ ≈ 40 ppm from being resolved in parts b and c of Figure 1; only a slight shoulder on the high-frequency side of the line shape hints at the presence of more than one site. It is the failure of conventional 27Al MAS NMR to resolve these tetrahedral sites that in this case provides the motivation for using twodimensional 27Al MQMAS NMR. Three-Quantum
27Al
MAS NMR of AlMePOs
Figure 2 presents two-dimensional three-quantum 27Al MAS spectra of the intermediate R/β form obtained on a Bruker MSL 500 spectrometer (ω0(27Al)/(2π) ) 130.3 MHz). Parts a and b of Figure 2 show the tetrahedral and octahedral regions, respectively, of a three-quantum MAS spectrum recorded with the amplitude-modulated z-filtered experiment described by Amoureux et al.9 In Figure 2a, two “ridge” line shapes can be
10.1021/jp9824858 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/15/1999
27Al
MQMAS NMR of AlMePO-β and AlMePO-R
J. Phys. Chem. B, Vol. 103, No. 5, 1999 813
Figure 2. Tetrahedral (a) and octahedral (b) regions of a twodimensional three-quantum 27Al MAS NMR spectrum of the intermediate R/β form of AlMePO recorded on a Bruker MSL 500 spectrometer (ω0(27Al)/(2π) ) 130.3 MHz) using an amplitude-modulated z-filtered MQMAS experiment.9 In (b), dashed lines indicate the direction of the second-order quadrupolar broadening (Q) and isotropic chemical shift (CS) axes, with gradients 19/12 and 3, respectively. The displayed F1 and F2 spectral widths equal 4.2 and 3 kHz (cut down from 50 and 31.3 kHz), respectively. Contours are drawn at 4, 8, 16, 32, and 64% of the maximum peak height. The MAS rate was 6.7 kHz, 192 transients (consisting of 512 points) were averaged for each of 256 increments of t1, and the relaxation interval was 500 ms. For the final p ) 0 to -1 pulse, the radio frequency field strength, ω1/(2π), was reduced to ∼7 kHz, while the pulse duration was 18 µs. Sign discrimination was restored using the TPPI method of incrementing the phase of the first pulse by 30° for each increment of t1.
observed a ridge line shape for the single octahedral site in pure AlMePO-β that similarly appears to have a gradient close to 3.10 Figure 1. 27Al MAS NMR spectra of (a) AlMePO-R, (b) AlMePO-β, and (c) the intermediate R/β form of AlMePO recorded on a Bruker MSL 400 spectrometer (ω0(27Al)/(2π) ) 104.3 MHz). The displayed spectral widths equal 15 kHz (cut down from 25 kHz). The MAS rate was 9.0 kHz in (a) and 7.4 kHz in (b) and (c). The positions of spinning sidebands are indicated by *. The ppm scale is referenced with respect to a 1 M Al(NO3)3 solution. Additional broad peaks in (b) and (c) are believed to correspond to amorphous impurities.
distinguished, indicating that three-quantum MAS has enabled partial resolution of the tetrahedral sites. The gradients of the two ridges are both close to 19/12, i.e., that predicted by theory for a spin I ) 5/2 three-quantum MAS spectrum where the second-order quadrupolar interaction is the dominant linebroadening mechanism. In contrast, the ridge line shapes of the two octahedral sites in Figure 2b have gradients that are clearly not equal to 19/12 (for reference, a line with gradient 19/12 is labeled Q in the figure). This is not surprising, since the 27Al quadrupolar coupling constants for these octahedral sites are very small10 and, as a result, there is little inhomogeneous second-order broadening. Instead, the two ridges in Figure 2b are observed to have gradients close to 3, i.e., that predicted by theory for a distribution of isotropic chemical shifts (a reference line with gradient 3 is labeled CS in the figure). Although such distributions are typical of amorphous solids, in a nominally crystalline sample, as in the present case, their presence may be attributed to the occurrence of aluminum atoms at, or close to, defect sites, domain boundaries, and surfaces. In view of this, it is important to note that the small chemical shift broadenings observed in the 27Al line shapes of the octahedral sites in Figure 2b could also be present in the line shapes in Figure 2a, except that in this latter case, they would be masked by the significantly larger inhomogeneous second-order broadening. Rocha et al. have
Five-Quantum
27Al
5/ 2
MAS NMR of AlMePOs
For spin I g nuclei, in addition to the three-quantum MAS experiment, it is also possible to perform a five-quantum MAS experiment. Although the excitation and detection of fivequantum coherence are less efficient than that of three-quantum coherence, Amoureux and co-workers have shown that significant enhancements in resolution can be achieved in 27Al NMR using this experiment.3,4,10,11,15 The improvements in resolution with respect to three-quantum coherence are due to two factors: first, the five-quantum isotropic chemical shift is a factor of 5/3 larger; second, for spin I ) 5/2 nuclei, the fivequantum isotropic second-order quadrupolar shift is larger by a factor of 25/3. A shearing transformation is commonly applied to twodimensional MQMAS spectra so that second-order broadened ridge line shapes, such as those shown in Figure 2a, appear parallel to the F2 frequency axis. In our recent work, however, we have developed alternative MQMAS experiments in which the t1 evolution period is split between multiple- and singlequantum precession such that the second-order quadrupolar broadening is refocused at the end of t1, thereby avoiding the need for shearing.21-23 As discussed in ref 23, these “split-t1” MQMAS experiments, particularly those that yield phasemodulated t1 data, are simple to implement and relatively very sensitive. The pulse sequence and coherence transfer pathway diagram24 for a phase-modulated split-t1 five-quantum MAS experiment are presented in Figure 3. It can be seen that the five- and singlequantum evolution periods are partitioned in the ratio 12:25. Of the various split-t1 five-quantum experiments possible, both amplitude- and phase-modulated, this one was selected for the present work because it readily yields pure absorption-mode
814 J. Phys. Chem. B, Vol. 103, No. 5, 1999
Brown et al.
Figure 3. Pulse sequence and coherence transfer pathway diagram for the optimum phase-modulated split-t1 five-quantum MAS NMR experiment for spin I ) 5/2 nuclei. The evolution period, t1, is split between five- and single-quantum evolution as indicated. A whole echo always forms at the center of the acquisition period, t2, for all values of t1. A phase-cycling scheme for the pulse phases φ1, φ2, and φ3 and for the receiver Rx is given in Table 1.
Figure 4. Two-dimensional 27Al MQMAS NMR spectra of the intermediate R/β form of AlMePO obtained on a Bruker MSL 400 spectrometer. The tetrahedral regions of (a) three-quantum and (b) fivequantum MAS spectra are shown, both recorded using phase-modulated “split-t1” experiments that yield second-order broadened ridges parallel to the F2 axis, thereby avoiding the need for shearing.22,23 In both spectra, the displayed F1 and F2 spectral widths equal 3 kHz. In both experiments, the full F2 spectral width was 25 kHz, the relaxation interval was 500 ms, the MAS rate was 7.4 kHz, and the duration of the central transition inversion pulse was 60 µs, with the radio frequency field strength, ω1/(2π), reduced to ∼4 kHz. For the three- and fivequantum experiments, respectively, the spin echo interval, τ, was 5.3 and 2.7 ms, 96 and 160 transients (consisting of 512 and 256 points) were averaged for each of 192 increments of t1, and the full F1 spectral widths equalled 19.4 and 30.8 kHz. Contours are drawn at 8, 16, 32, and 64% of the maximum peak height.
line shapes, because a short τ interval can be used, and because the coherence transfer pathway is optimum (i.e., p ) -5 to p ) +1 and p ) +5 to p ) -1 coherence transfer steps, both |∆p| ) 6, are avoided in favor of a |∆p| ) 4 step). The phase cycling we used for this experiment is given in Table 1. Parts a and b of Figure 4 compare the 27Al MQMAS spectrum of the intermediate R/β form of AlMePO obtained on the MSL 400 spectrometer using a phase-modulated split-t1 three-quantum MAS experiment (as described in Figure 14b of ref 23) with that obtained using the five-quantum MAS experiment in Figure 3. In both spectra, the displayed spectral widths in the F1 and F2 frequency dimensions are equal to 3 kHz (only the tetrahedral region is shown) and it is very clear that the frequency dispersion of the individual peaks in the F1 dimension is greatly increased in the five-quantum spectrum in Figure 4b. In particular, compared with the two ridge line shapes apparent in Figure 4a, three ridge line shapes can be distinguished in Figure 4b. According to a simple theoretical calculation, the increase in the F1 frequency dispersion is by a factor of 155/37 ≈ 4.2. Owing to a significant increase in the line width in F1, however, the impressive increase in F1 frequency dispersion afforded by the five-quantum MAS experiment is not fully translated into such
Figure 5. Two-dimensional 27Al five-quantum MAS NMR spectra of (a) AlMePO-R, (b) AlMePO-β, (c) a physical mixture of AlMePO-R and AlMePO-β in the mass ratio 1:2, and (d) intermediate R/β form of AlMePO, all obtained on a Bruker MSL 400 spectrometer with the phase-modulated split-t1 experiment in Figure 3. The last spectrum is merely a repeat of Figure 4b for ease of comparison. Only the tetrahedral regions are shown. In all spectra, the displayed F1 and F2 spectral widths equal 3 kHz (cut down from 25 and 30.8 kHz, respectively). In all experiments, the relaxation interval was 500 ms and the spin echo interval, τ, was 2.7 ms. In (a), (b), and (c), respectively, 320, 160, and 480 transients (consisting of 256 points) were averaged for each of 192 increments of t1, the MAS rate was 9.0, 7.4, and 6.5 kHz, and the duration of the central transition inversion pulse was 50, 60, and 50 µs, with the radio frequency field strength, ω1/(2π), reduced to ∼5, ∼4, and ∼5 kHz. Contours are drawn at 8, 16, 32, and 64% of the maximum peak height.
an impressive increase in resolution. Although some of this increased line width may be due to the fact that 1H decoupling was not used in our 27Al MQMAS spectra (five-quantum 27Al coherence is more sensitive to residual heteronuclear couplings than is three-quantum coherence), much of it seems to be an inherent feature of five-quantum coherence, as can be seen in many of the 27Al spectra presented by Amoureux and coworkers.3,4,10,11 The tetrahedral regions of five-quantum 27Al MAS spectra of samples of pure AlMePO-R and pure AlMePO-β (again prepared by Mr. V. J. Carter20) are presented in parts a and b of Figure 5, respectively. As expected, although three distinct ridge line shapes are observed for the β-polymorph, only one such ridge line shape is observed for the R-polymorph. One distinguishing feature of this latter line shape (Figure 5a), however, is that it is considerably broader and more asymmetric in the F1 dimension than those in Figure 5b, and this is probably indicative of the presence of a range of 27Al local environments. The confirmation, in Figure 5b, of the existence of three tetrahedral aluminum sites in AlMePO-β is in agreement with spectra previously published by Rocha et al.10 who have performed simple amplitude-modulated three- and five-quantum MAS experiments on this sample. The 27Al MAS NMR spectrum of the intermediate R/β form of AlMePO in Figure 1c appeared to indicate that the R- and β-polymorphs were present in the ratio 1:2. Therefore, a physical mixture of the R- and β-polymorphs was made up in this ratio and the resulting five-quantum 27Al MAS spectrum is presented in Figure 5c. It is the comparison of this spectrum with that of
27Al
MQMAS NMR of AlMePO-β and AlMePO-R
J. Phys. Chem. B, Vol. 103, No. 5, 1999 815
TABLE 1: Phase Cycle for Phase-Modulated Split-t1 Five-Quantum MAS Experiment in Figure 3 φ1 φ2 φ3 Rx
0° 18° 36° 54° 72° 90° 108° 126° 144° 162° 180° 198° 216° 234° 252° 270° 288° 306° 324° 342° 0° 20 (0°) 20 (45°) 20 (90°) 20 (135°) 20 (180°) 20 (225°) 20 (270°) 20 (315°) 5 (0° 270° 180° 90°) 5 (90° 0° 270° 180°) 5 (180° 90° 0° 270°) 5 (270° 180° 90° 0°)
the intermediate R/β form of AlMePO in Figure 5d which we will return to in the Discussion.
the experiment in Figure 3, Ω1 and Ω2, are then given by
85 160 Ω + Ω 37 CS 111 Q
Ω1 )
Extraction of the Isotropic Shifts The relatiVe position of two ridge line shapes obtained with the same MQMAS NMR experiment varies according to the magnetic field strength, B0, of the spectrometer used. This is because the F1 and F2 frequencies of a ridge line shape depend on both the isotropic chemical shift and the isotropic secondorder quadrupolar shift, and these are, respectively, directly and inversely proportional to the Larmor frequency, ω0. By use of both the F1 and F2 frequencies of a MQMAS ridge line shape, it is possible to determine the two components of the overall isotropic shift without resorting to performing an experiment at a second magnetic field strength.25 In this section, we show how this approach can be applied to our phase-modulated splitt1 five-quantum 27Al MAS spectra. It is first necessary to determine the dependence of the F1 and F2 frequencies on the two isotropic shifts for the phasemodulated split-t1 five-quantum MAS experiment in Figure 3. This can be calculated by considering the time-domain signal modulation, s(t1,t2), arising as a result solely of, first, the isotropic chemical shift, here represented by the rotating-frame angular frequency ΩCS,
sCS(t1,t2) )
{
} { }
}
12t1 25t1 exp -iΩCS exp{+iΩCSt2} exp -i5ΩCS 37 37
{
85ΩCS t exp{+iΩCSt2} 37 1
) exp -i
(1)
and, second, the isotropic second-order quadrupolar shift
sQ(t1,t2) )
{
exp -iA5ΩQ
} {
}
12t1 25t1 exp -iA1ΩQ exp{+iA1ΩQt2} (2) 37 37
The isotropic shift parameter ΩQ and the I ) 5/2 coefficients A|p| have been calculated from second-order perturbation theory to be26
16 A1 ) 15 A5 ) ΩQ )
(3a)
20 3
(3b)
( )(
e2qQ 9 16I2(2I - 1)2ω0 p
2
1+
)
η2 3
(3c)
where e2qQ/p is the quadrupolar coupling constant. Thus, eq 2 reduces to
{
sQ(t1,t2) ) exp -i
} {
}
160ΩQ 16ΩQ t exp -i t 111 1 15 2
(4)
The F1 and F2 frequencies of a ridge line shape recorded using
Ω2 ) ΩCS -
16 Ω 15 Q
(5a) (5b)
The frequencies ΩCS, ΩQ, Ω1, and Ω2 have all been defined above in angular frequency units (s-1), but the expressions in eq 5 are equally valid for frequency units of s-1, Hz, or ppm. A rearrangement of eqs 5a and 5b yields
ΩCS
37 85 Ω - Ω 144 1 37 2
( ) 37 50 ) Ω + Ω) 135( 37
ΩQ )
1
2
(6a) (6b)
Therefore, measurement of the frequencies Ω1 and Ω2 is all that is required to calculate the two isotropic shifts. The Ω1 and Ω2 values for the single peak in the AlMePO-R spectrum in Figure 5a, the three peaks in the AlMePO-β spectrum in Figure 5b, and the middle peak (in terms of F1 frequency) in Figure 5d are given in Hz and ppm in Table 2. The Hz and ppm scales are referenced with respect to the transmitter frequency and a 1 M Al(NO3)3 solution, respectively; 0 ppm corresponds to an offset of +254 Hz in the F2 dimension and (85/37) × 254 ) +584 Hz in the F1 dimension of Figure 5. Since the second-order quadrupolar broadening is small, estimation of the frequency Ω2 is straightforward in the present example. In other cases it may be necessary to determine, using computer simulations, which feature of the second-order quadrupolarbroadened line shape corresponds to the isotropic shift. The isotropic shifts ΩCS and ΩQ can then be calculated using eqs 6a and 6b, and their values are given in Table 2. A comparison with ref 10 reveals that the results presented in Table 2 for AlMePO-β are in close agreement with those obtained by Rocha et al. From the isotropic second-order quadrupolar shift alone, the quadrupolar coupling constant e2qQ/h and the asymmetry parameter η cannot be determined independently. Instead, using eq 3c, a quantity referred to as the “second-order quadrupolar effect” (SOQE) parameter4,6,8 can be defined by
(
)
η2 e2qQ 1+ h 3
1/2
)
2I(2I - 1)ω0xΩQ 3π × 103
(7)
where e2qQ/h and ω0/(2π) are in units of Hz and where ΩQ is in (dimensionless) units of ppm. The SOQE parameters calculated from our measured values of ΩQ are given in Table 2. In addition, we have attempted to determine the individual quadrupolar coupling parameters e2qQ/h and η by performing a computer fitting of F2 cross sections through the ridge line shapes. In the present case, however, although the overall results show good agreement with the SOQE parameters in Table 2, the asymmetry parameter η remains undetermined owing to insufficient resolution of any distinctive line shape features in the F2 frequency dimension. Discussion As mentioned previously, Carter et al. have proposed that the conversion of AlMePO-β to AlMePO-R that is observed at
816 J. Phys. Chem. B, Vol. 103, No. 5, 1999
Brown et al.
TABLE 2: F1 and F2 Frequencies (Ω1 and Ω2), Isotropic Chemical Shifts (ΩCS), Second-Order Quadrupolar Shifts (ΩQ), and Second-order Quadrupolar Effect Parameters (SOQE) for AlMePO-r, AlMePO-β, and the Middle Peak in the Intermediate r/β Form site
Ω 1/ Hz
Ω1/ ppm
Ω2/ Hz
Ω2/ ppm
ΩCS/ ppm
ΩQ/ ppm
SOQE/ MHz
R β(1) β(2) β(3) ?
11 085 11 750 11 149 10 895 11 054
100.7 107.1 101.3 98.9 100.4
4517 4590 4541 4443 4492
40.9 41.6 41.1 40.2 40.7
42.7 44.8 43.0 42.0 42.6
1.7 3.0 1.8 1.7 1.8
1.8 2.4 1.9 1.8 1.9
high temperatures in the presence of water vapor is a topotactic reconstructive transformation.20 These authors suggest that (a) there is no stable intermediate and (b) the rate of growth of the AlMePO-R domains is much faster than the rate of nucleation of these domains. The AlMePO sample prepared by interrupting the thermal transformation should, therefore, be essentially identical to a physical mixture of the R- and β-polymorphs. The main evidence presented for this in ref 20 is that the X-ray diffraction peaks of the product, AlMePO-R, are narrow, even in the early stages of the transformation, apparently indicating large crystal domains (>1000 Å) only. If the intermediate R/β form of AlMePO is virtually identical to a physical mixture of the R- and β-polymorphs, then the fivequantum 27Al MAS spectra in parts c and d of Figure 5 should also be very similar. This cannot be judged easily from twodimensional contour plots, however, and the additional resolution provided by split-t1 MQMAS NMR spectra is best exploited by viewing projections onto the F1 frequency axis. Figure 6 shows such F1 projections for all four two-dimensional spectra in Figure 5. Parts a and b of Figure 6 reveal clearly the one and three tetrahedral aluminum sites in AlMePO-R and AlMePOβ, respectively, while parts c and d of Figure 6 are F1 projections of the spectra of the physical mixture in Figure 5c and the intermediate R/β form in Figure 5d. These last two spectra (Figures 6c and 6d) are shown expanded and overlaid in Figure 6e. The results presented in Figure 6, in particular the two spectra superimposed in Figure 6e, do not appear to support the mechanism proposed by Carter et al. for the thermal transformation between the β- and R-polymorphs. Instead of being very similar, as would be expected for a topotactic reconstructive transformation, the two spectra in Figure 6e are quite different. The intensities of peaks in MQMAS spectra are known to have little quantitative significance, since they depend largely on the efficiency with which multiple-quantum coherences are excited. However, the distinctive difference between the two spectra in Figure 6e is one of frequency, not intensity. Furthermore, even allowing for a relative frequency shift between the two (that might arise, for example, from a bulk susceptibility effect), we have been unable to find any combination of the spectra in parts a and b of Figure 6 that matches closely the spectrum in Figure 6d. The spectrum of the physical mixture (Figure 6c and dashed line in Figure 6e) shows three peaks, and these must correspond to the single tetrahedral site in AlMePO-R and to the three tetrahedral sites β(1), β(2), and β(3) in AlMePO-β. Therefore, it appears that the R and β(2) sites are both contributing to the intensity of the middle (in terms of F1 frequency) of the three peaks, since, from parts a and b of Figure 6 and Table 2, it is these two sites that are most nearly isochronous in the F1 dimension. In the spectrum of the intermediate R/β form (Figure 6d and solid line in Figure 6e), however, although three peaks
Figure 6. In (a), (b), (c), and (d), projections onto the F1 axis of the corresponding two-dimensional 27Al NMR spectra in Figure 5 are shown. The projections in (c), for the physical mixture of AlMePO-R and AlMePO-β, and (d), for the intermediate R/β form of AlMePO, are overlaid in (e), with the former being shown as a dashed line. The assignment of the peak marked ? in (d) and (e) is debated in the text.
can still be identified clearly, the F1 frequency of the middle peak (marked as ? in parts d and e of Figure 6) seems to have shifted “upfield” by ∼0.9 ppm. Furthermore, there is some evidence of there being a broad spectral component underlying the three sharp peaks (as there is also in the tetrahedral region of the 27Al MAS spectrum in Figure 1c). The assignment of the peak marked ? in the spectrum of the intermediate R/β form is uncertain. One possibility is to assign the peak to AlMePO-R (which it most resembles in terms of the parameters Ω1, Ω2, ΩCS, and ΩQ in Table 2) and to explain the frequency shift as a microscopic susceptibility effect arising as a result of the nascent AlMePO-R being dispersed in small domains within an AlMePO-β matrix. The second possibility is to assign it to the β(2) site of AlMePO-β and to assign the underlying broad component to AlMePO-R; this would imply that the nascent AlMePO-R is largely amorphous and is dispersed in very small domains with a range of local environments. The observation of the broadness in the F1 dimension of the pure AlMePO-R peak in Figures 5a and 6a, noted above and indicating residual amorphous structure, can be used to lend
27Al
MQMAS NMR of AlMePO-β and AlMePO-R
support to either of these alternative assignments. However, both assignments suggest a rather more commonplace mechanism for the thermal transformation between AlMePO-β and AlMePO-R than that proposed by Carter et al., namely, that the rate of nucleation of new AlMePO-R domains is much greater than the rate of domain growth. Conclusions Unlike X-ray diffraction, which is preferentially sensitive to domains exhibiting long-range order, 27Al NMR spectroscopy is, in principle, sensitive to all aluminum atoms within a sample, regardless of whether they occur in crystalline or amorphous domains. In this paper, we have used five-quantum 27Al MAS NMR to investigate the thermal transformation between AlMePO-β and AlMePO-R and have shown that the results do not appear to support the proposition by Carter et al.,20 based largely on X-ray diffraction data, of a topotactic reconstructive transformation. Further work is necessary to resolve this apparent contradiction. The present work demonstrates, however, that the five-quantum 27Al MAS NMR technique yields enhancements in spectral resolution and, even more importantly, that the isotropic shift parameters that can be obtained in this way are extraordinarily sensitive indicators of the local environment. Acknowledgment. We are grateful to the Royal Society for support. S.P.B. and S.E.A. thank EPSRC for the award of studentships. S.P.B. also thanks Courtaulds Research for a CASE award. The MSL 400 spectrometer was purchased with the aid of a grant from SERC. We are very grateful to the following: Mr. V. J. Carter of the University of St. Andrews for introducing S.P.B. to AlMePOs and for providing us with the samples used in this work; Dr. Stephen Heyes and Dr. Ann Chippindale for informative discussions; Dr. Frank Riddell for giving S.P.B. access to the MSL 500 spectrometer at St. Andrews; Dr Paul Hodgkinson for writing the two-dimensional Fourier transform program.
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