TPAF Microporous Aluminophosphate - American Chemical Society

J. Phys. Chem. B 2001, 105, 12249-12256

12249

Solid-State NMR Studies of the As-Synthesized AlPO4-5/TPAF Microporous Aluminophosphate Re´ gis D. Gougeon,*,†,| Eric B. Brouwer,† Philippe R. Bodart,†,§ Luc Delmotte,‡ Claire Marichal,‡ Jean-Michel Che´ zeau,‡ and Robin K. Harris† Department of Chemistry, UniVersity of Durham, South Road, Durham DH1 3LE, U.K., LMM, UPRES-A-CNRS 7016, UniVersite´ de Haute Alsace, 3 rue A. Werner, 68093 Mulhouse Cedex, France, and LDSMM, URA 8024, UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France ReceiVed: March 27, 2001; In Final Form: June 16, 2001

As-synthesized AlPO4-5 aluminophosphate molecular sieve prepared in a fluoride medium, using the quaternary tetrapropylammonium (TPA+) cation template, is characterized by a variety of solid-state NMR techniques. 19F MAS spectra show two distinct populations, with the same isotropic chemical shift. While 31P MAS experiments distinguish four different crystalline environments, both 19F-31P CP-MAS and HETCOR experiments reveal that only two signals belong to the AlPO4-5 framework, whereas the remaining two are attributed to crystalline byproducts. 27Al MAS and 3Q-MAS spectra indicate, in addition to the tetrahedral environment, the presence of pentacoordinated Al species in close proximity to fluorine atoms, as is additionally shown by the 19F-27Al HETCOR experiment. An 27Al-{19F} REDOR experiment strongly supports the existence of an Al-F bond (rAl-F < 2 Å which forms the fifth coordination position of one of the twelve aluminum sites of the unit cell.

Introduction Aluminophosphate (AlPO4) molecular sieves represent a class of microporous materials whose structures are similar to those of zeolites. The AlPO4-5 phase displays a hexagonal structure of type AFI containing the organic template within the pores in the as-synthesized form.1,2 Interest in the AFI-type materials arises from their microporous architecture, which is composed exclusively of one-dimensional linear 12-ring channels characterized by polar micropore walls.1 This channel system runs in the c direction (Figure 1) through the (Al,P)O4 framework built up by alternating AlO4 and PO4 tetrahedra.3 The structure provides interesting potential applications such as optical devices for second harmonic generation4 and molecular sieve membranes.5 Several studies6-10 have been devoted to the synthesis of the AlPO4-5 phase, which can be prepared with different templates.6-11 Large crystals can be obtained either by microwave heating,12 or by hydrothermal heating of a gel2,13 or of a clear solution,14 in the presence of HF in all cases. The concomitant use of amines or quaternary ammonium cations as templates, and of a fluoride medium, appears to be a prerequisite for the formation of both large and well-crystallized single crystals of AlPO4-5,15 whereas the conventional hydrothermal methods often lead to byproducts.16 As with many industrially relevant materials such as zeolites and other micro- and meso-porous materials, the characterization of both the intermediate and final synthetic products poses significant challenges. Efforts to produce pure phases are often * Author to whom correspondence should be addressed at Universite´ de Haute Alsace, Mulhouse, France. Phone: +33-389-336886. Fax: +33389-336885. E-mail: [email protected]. † University of Durham. ‡ Universite ´ de Haute Alsace. § Universite ´ des Sciences et Technologies de Lille. | Current address: Universite ´ de Haute Alsace.

confounded by unwanted reaction pathways, which frequently remain unknown. Even the knowledge of desired reaction pathways, and the corresponding synthetic intermediates and formation mechanisms, are often elusive. As a result, there has been an ongoing interest to develop characterization techniques to guide and refine the synthetic efforts. To the best of our knowledge, only one structural study has been carried out on the framework structure of the AlPO4-5 phase obtained via a fluoride medium.2 From a single-crystal X-ray diffraction (XRD) structure refinement, it was shown that the as-synthesized [AlPO4]12TPAF phase crystallizes in space group P6cc.2 The tetrapropylammonium (TPA+) cations were located in the large 12-ring channels, whereas F anions were located between two (Al,P)O4 4-rings of the framework, adjacent to the channels (Figure 1). The 31P MAS NMR spectrum of the as-synthesized sample showed two distinct peaks (∼2:10 intensity ratio) for the single type of tetrahedral phosphorus, which was thought to arise from the shorter distance between F- anions and two of the twelve phosphorus sites of the unit cell. In this paper, we apply advanced solid-state NMR spectroscopic techniques to probe structural features of the AlPO4-5 framework, synthesized in a fluoride medium, which are either obscured, or at best, ambiguous from the diffraction studies. The interaction of fluoride anions with the aluminum and phosphorus framework elements are described. Furthermore, we demonstrate that NMR techniques can effectively deal with crude as-synthesized materials, which sometimes contain impure phases in addition to the one desired. These mixed phases can shed light on the reaction pathways followed by the active species, such as fluorinated entities, during the synthesis, and thus assist understanding of the mechanisms of synthesis for microporous materials. As-synthesized AlPO4-5 contains a rich variety of NMRactive nuclei (1H, 13C, 17O, 19F, 27Al, and 31P) whose chemical

10.1021/jp0111214 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/13/2001

12250 J. Phys. Chem. B, Vol. 105, No. 49, 2001

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Figure 2. XRD powder pattern of A recorded with Cu KR radiation (λ ) 1.5418 Å) on a Philips PW 1800 diffractometer. The data were collected between 0 and 50° (2Θ) with a step size of 0.02° and a counting time of 60 s. Asterisks indicate unidentified byproducts and dots indicate peaks which could be attributed to the crystalline phase of the augelite type Al2PO4(OH)3.

Figure 1. Framework topology of the as-synthesized AlPO4-5/TPAF material with F atoms located in a particular 6(c) position, with an occupancy factor of 1/6, according to ref 2; (top) view perpendicular to the c-axis of the unit cell, and (bottom) detail of the different Al-F and P-F distances in the 4-ring channels.

shifts and heteronuclear dipolar interactions give rise to specific NMR spectral features which in turn provide valuable information on the local nuclear environments in the AlPO4-5 structure. We hope to illustrate, in this paper, the utility of these increasingly accessible NMR experiments, as a valuable complement to the more widely used diffraction techniques, to address a number of structural issues that are typically encountered in the characterization of such framework materials. Experimental Section 1. Synthesis and Characterization. Sample A. This AlPO4-5 sample was hydrothermally synthesized at 170 °C in a Teflonlined autoclave from 85% H3PO4 (Prolabo, Normapur), 98% aluminum isopropoxide (Aldrich), tetrapropylammonium hydroxide (20% aqueous solution, Fluka), hydrofluoric acid (40% aqueous solution, Fluka), and doubly distilled water, with a molar composition of 2:2:1:1:75, respectively. Aluminum isopropoxide (4.1 g) was slowly added to a solution of phosphoric acid (2.3 g) in H2O (4.6 mL), with stirring until a homogeneous gel was obtained. Tetrapropylammonium hydroxide, and then HF, were added and the mixture was aged under stirring for 17 h. The gel was transferred into a Teflon-lined stainless steel autoclave and heated with stirring for 6 days at 170 °C. After cooling, the solid was recovered by filtration, washed with distilled water, and dried overnight at 90 °C. Scanning electron microscopy of the as-synthesized product revealed large crystals with traces of amorphous materials (which were partially

removed by sonication), and also the presence of a few additional crystals of a type other than the AFI structure. Powder XRD (Figure 2) confirmed the AFI-type structure and also indicated the existence of a small amount of crystalline byproducts. The 13C MAS NMR spectrum (not shown) of the as-synthesized sample confirmed the occlusion of the template within the pores, in agreement with the literature.1 Sample B. Comparative measurements were performed on a previously studied sample (therein labeled sample 2), synthesized under conditions similar to sample A, in which the AlPO4-5 phase was the sole crystalline phase detected after thorough sonication.2 2. Solid-State NMR Spectroscopy. The NMR experiments were carried out using three different spectrometers. A Chemagnetics CMX 200 spectrometer operating at frequencies of 200.1, 188.3, 81.0, and 52.1 MHz for 1H, 19F, 31P, and 27Al, respectively, was used with (i) A double-resonance 1H/19F 4-mm probe to record the 19F MAS spectra; (ii) A triple-resonance 1H/19F/X 7-mm probe to record the 1H-31P and 19F-31P CPMAS spectra; (iii) A triple-resonance 1H/X/Y 5-mm probe to perform the 31P-{27Al} TRAPDOR experiment 17,18 (Figure 3a). A Bruker ASX 400 spectrometer operating at 104.3 MHz (27Al) was used to record the 27Al triple-quantum (3Q) MAS spectrum 19 with the three-pulse z-filtered sequence 20 (Figure 3b) and a double-channel high-speed 2.5-mm probe. A Bruker DSX 400 spectrometer operating at frequencies of 376.5, 162.0, and 104.3 MHz for 19F, 31P, and 27Al, respectively, was used with (i) A Bruker double-channel 4-mm probe fitted with an HFX box accessory to record the 27Al, 31P MAS, 19F-27Al CPMAS, and the 19F-27Al and 19F-31P HETCOR experiments 21 (Figure 3c); (ii) A Bruker double-channel high-speed 2.5mm probe to record 19F MAS spectra and the 27Al-{19F}REDOR 22 experiment (Figure 3d). Chemical shifts of fluorine, phosphorus, and aluminum spectra are referenced to the signals for liquid C6F6 (δF ) -166.4 ppm), brushite (δP ) -1.2 ppm), and 1 M Al(NO3)3 solution (0 ppm), respectively. Additional experimental details are given in the appropriate figure captions. A was heated at 90 °C overnight under vacuum to effect dehydration, and then transferred to the 4-mm rotor in a glovebox under an argon atmosphere. We have checked that although the rotor was spun by air, it was sufficiently tight to prevent any water uptake, even after several days.

As-Synthesized AlPO4-5/TPAF Microporous Aluminophosphate

J. Phys. Chem. B, Vol. 105, No. 49, 2001 12251 TABLE 1: Fluorine-19 Chemical Shift Tensor Parameters for A, B, and AlPO4-CHAa sample A B AlPO4-CHA

proportion δ11 δ22 δ33 δiso Ω % (ppm) (ppm) (ppm) (ppm) (ppm) 13 87 14 86 100

-68 -71 -73

-120 -114 -177 -120 -120 -108 -179 -120 -128 -182 -128

κ

109

0.16

107 108

0.33 0

a The corresponding experimental spectra were recorded at a magnetic field B0 ) 9.4 T. Errors in δIi: (3 ppm. The shielding span is defined as Ω ) |δ33 - δ11| and the shielding skew as κ ) -3(δiso - δ22)/Ω.27,28

Figure 3. Pulse sequences used: (a) TRAPDOR, (b) Multiple-Quantum MAS, HP are optimized hard pulses, (c) HETCOR, (d) REDOR, shown for four rotor cycles.

Figure 4. 19F MAS spectra of (a) A and (b) B, recorded at spinning speeds between 11 and 12 kHz, in a magnetic field B0 ) 9.4 T (Spectrometer Frequency: SF ) 376.5 MHz), using a Hahn echo with 90° and 180° pulses of 1.6 and 3.2 µs, respectively; recycle delay of 150 s and 360 transients; (c) Simulated spectrum of B; (d) and (e) Individual components of the simulation. Spinning sidebands are marked by asterisks, and impurities indicated by arrows.

Results and Discussion 19F NMR. Figure 4a displays the 19F MAS spectrum of A, obtained at a spinning speed of 12 kHz. The spectrum exhibits a single intense peak centered at -120 ppm, which is associated with F- species balancing the charge of the TPA+ template within the pores.2,15 Although such a chemical shift has been attributed to isolated F- anions in AlPO4-11,15 it is suggested

that fluorine ions could bridge two aluminum atoms from two four-ring units of the AlPO4-5 framework,23 in view of the unusually short distance (2.19 Å) between F and two of the twelve Al sites in the unit cell.2 This is further supported by a recent NMR study of the AlPO4-CJ2 phase 24 which reports chemical shifts between -115 and -124 ppm for bridging and terminal framework Al-F species. Additional minor 19F signals also appear at -123 ppm and between -127 and -130 ppm. Such signals are often observed in 19F MAS spectra of as-synthesized microporous AlPO4 materials prepared via a fluoride medium.15,25 These signals, which remain after calcination, are attributed to framework or extraframework Al-F-type impurities, and are present in very small amount.15 The 19F MAS spectrum of B (Figure 4b) shows that these peaks disappear after thorough sonication, and confirms that, even if such impurities do belong to the framework, the corresponding amount of fluorine species is negligible. Due to the weak peak intensity, we were unable to further explore this hypothesis. No line-narrowing is observed in the 19F MAS spectrum with proton-decoupling (not shown), indicating that 19F-1H dipolar interactions with the protons from the TPA+ template are negligible in this sample. Furthermore, the 19F MAS spectrum recorded at a slow spinning speed (5 kHz) shows no significant line-broadening compared to that at 12 kHz, indicating that residual 19F-19F, 27Al-19F, and 31P-19F dipolar couplings already average out at slow spinning speeds, and are therefore weak. As shown recently for F- nuclei occluded in zeolite-type materials, the 19F chemical shift anisotropy (CSA) is an excellent tool to investigate dynamics of the F- species.26 Motional processes reduce the CSA, and consequently the full range of the spinning sideband pattern. Τwo distinct fluorine populations, with identical isotropic chemical shift, were required for the best simulation of the 19F MAS spectra of A and B. Figure 4c,d,e shows for instance the simulated spectrum and the two components, respectively, corresponding to the simulation of the 19F MAS spectrum of B. All the parameters derived from these simulationssthe principal chemical shift components, the isotropic chemical shift δiso, the shielding span Ω, and the shielding skew parameters κ27,28sare gathered in Table 1. For comparison, the corresponding values for a chabazite-like microporous aluminophosphate (AlPO4-CHA), in which two fluorine atoms bridge two aluminum atoms from the framework,23 are given in Table 1. The CSA values were determined by simulation of the spinning sideband patterns 29 using the QUASAR30 or WINFIT31 softwares. The proportions of the two populations required for the best fit of the 19F spinning sideband patterns of A and B are of the order of 87%/13% (Table 1), with a good reliability ensured by the comparison of the simulation at two different spinning speeds. Surprisingly, the major population, characterized by a


Gougeon et al.

Figure 5. 31P MAS NMR spectra of; (a) A: SF ) 162.0 MHz, νr ) 5 kHz, 3 µs (45°) pulse duration, 20 s recycle delay, 12 transients; (b) B: SF ) 162.0 MHz, νr ) 10 kHz, 1.5 µs (30°) pulse duration, 20 s recycle delay, 16 transients; (c) A, 1Hf31P CP-MAS: SF ) 81.0 MHz, νr ) 5 kHz; 2 ms contact time, 2 s recycle delay; (d,e) A, 19Ff31P CP-MAS: SF ) 81.0 MHz, νr ) 5 kHz, 30 s recycle delay, contact times of 3 ms (d) and 18 ms (e); Hartmann-Hahn conditions were achieved with radio frequency fields equivalent to 65 kHz for both 1Hf31P and 19Ff31P CP-MAS experiments. When used, 1H or 19F decoupling field strengths were set to 32 kHz.

tensor with significant CSA, is sufficient to satisfactorily fit the spinning sidebands (Figure 4c, d). The minor population only seems to be required for the correct fit of the isotropic signal (Figure 4e), even at low spinning speed (6 kHz, not shown). Furthermore, the consistent fit of the 19F spectrum of both samples recorded at two different magnetic fields (9.4 and 4.7 T), with the same two components, confirms that the 19F spinning sideband pattern is due to CSA. Clearly, there is an excellent agreement between the two different samples, indicating that the existence of the second minor fluorine population really has a physical significance in the as-synthesized AlPO45/TPAF material. The shielding parameters (and especially Ω) of the tensorial component are very similar to those of fluorine in the AlPO4CHA material, where F atoms are bound to framework aluminum (Table 1). These comparisons suggest strongly that the major fluorine population in the AlPO4-5 sample corresponds to fluorine atoms rigidly bound to the framework, whereas the other population corresponds to fluorine species that are either more mobile or in a more symmetrical environment. These features imply the formation during the synthesis of the AlPO4-5 sample of two kinds of charge compensation of the cationic template, either as F- mobile species (∼10%) or, in majority (∼90%) as (AlO4F)- centers in which F is rigidly bound to an aluminum site of the framework. 31P NMR. The 31P MAS spectrum of A (Figure 5a) has four distinct peaks, labeled P1 to P4 at ca. -17, -20, -24, and -31 ppm, respectively, with respective intensities of 1.9:0.8:1.7:10.3 derived from deconvolution. Only the two latter peaks were observed for the AlPO4-5 structure,2 as also shown by the 31P MAS spectrum of B (Figure 5b), where an intensity ratio close to 5:1 was observed.2 P1 and P2 correspond to byproducts formed during the synthesis, which are not removed after a first sonication. The attribution of these peaks, which are also observed in the 1Hf31P CP-MAS spectrum of A (Figure 5c), is discussed further below. The existence of the two 31P peaks P3 and P4 at -24 and -31 ppm has been attributed to the occurrence of a shorter distance between F and two of the twelve crystallographically equivalent P atoms of the AlPO4-5 unit-cell (Figure 1). However, the observation of two 31P peaks is a priori contradictory with the existence of only one phosphorus crystallographic site in

Figure 6. (a) 19F-31P HETCOR experiment on A: B0 ) 9.4 T, νr ) 5 kHz, 4 ms contact time, 54 s recycle delay, 67 experiments of 60 transients each were acquired in the second dimension. The onedimensional 31P and 19F MAS spectra were recorded separately, rather than as projections of 2D plots. (b) 19F-27Al HETCOR experiment on B: B0 ) 9.4 T, νr ) 8 kHz, 500 µs contact time, 54 s recycle delay, 72 experiments of 120 transients each were acquired in the second dimension. The one-dimensional 27Al and 19F MAS spectra were recorded separately, rather than as projections of 2D plots.

the AlPO4-5 asymmetric unit. A shorter P-F distance might explain a deshielding of these two phosphorus nuclei 2, but would consequently mean that the AlPO4-5 asymmetric unit contains at least two phosphorus sites. The 19Ff31P CP-MAS spectra (Figure 5d,e) and the two-dimensional 19F-31P HETCOR experiment (Figure 6a) clearly support this shorter distance. For a short contact time of 3 ms (Figure 5d), the P3 signal is more intense than the P4 one, due to a more efficient polarization transfer from F to the closer P3 nuclei. Furthermore, two different rates of cross-polarization can be distinguished within the P4 signal centered at -31 ppm (Figure 5d,e). For a short contact-time of 3 ms, the signal maximum is centered at -29.5 ppm, with a shoulder at -31 ppm; for the longer contact time of 18 ms, the peak maximum is at -31 ppm. To summarize: the phosphorus atoms associated with the P3 peak are closer to fluorine, and those associated with the major P4 peak can be distinguished by different phosphorus-fluorine distances. While the 19Ff31P CP-MAS spectra confirm the existence of different regimes of 19F to 31P polarization transfer, the 19F31P HETCOR experiment (Figure 6a) clearly confirms that the two 31P signals corresponding to the AlPO4-5 phase (and only them) are polarized from the framework F- anions resonating


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Figure 8. 27Al 3Q-MAS MAS sheared spectrum (B0 ) 9.4 T) of A; νr ) 15 kHz, 0.5 s recycle delay, 2.1 and 0.7 µs pulse durations for the two hard pulses (rf field of about 300 kHz) and 10 µs duration for the soft pulse (rf field of about 50 kHz), 99 experiments of 240 transients each were acquired in the isotropic dimension.

TABLE 2: Isotropic Chemical Shifts δiso and Composite Quadrupolar Parameters λ for Sample Aa Figure 7. 27Al MAS NMR spectra (B0 ) 9.4 T) of (a) A and (b) B: 0.7 µs (15°) pulse duration, 1 s recycle delay, 8 transients; (c) Dehydrated A: MAS, 0.7 µs (15°) pulse duration, 0.5 s recycle delay, 256 transients; (d) Dehydrated A, 1Hf27Al CP-MAS, 850 µs contact time, 3 s recycle delay, 256 transients; (e) Dehydrated A,19Ff27Al CPMAS, 500 µs contact time, 54 s recycle delay, 24 transients; νr ) 8-10 kHz.

at -120 ppm. Close examination reveals that the correlation peak between fluorine and the P4 signal is centered on the highfrequency side of the 31P signal. Since this experiment used a short contact-time of 4 ms, it confirms that this 31P peak is a composite band, with fluorine closer to some of the relevant phosphorus sites than others. To check whether the two abovementioned fluorine populations are involved in the 19F-31P correlation observed in Figure 6a, we have simulated the spinning sideband pattern (not shown) corresponding to the vertical slices at δ(31P) of -24 and -31 ppm (Figure 6a). Although the signal-to-noise ratio is not as good as in the 19F MAS spectra, both slices required two fluorine populations consistent with those reported from the 19F MAS study. On this basis, it is possible to confirm that the two fluorine populations belong to the AlPO4-5 crystalline structure. 27Al NMR. The 27Al MAS spectrum of A (Figure 7a) exhibits a major peak at 36 ppm, labeled Al1, corresponding to tetrahedral Al species of the framework,2,32 an additional peak at 10 ppm, labeled Al2, and a broader signal between 0 and -20 ppm centered at -7 ppm, labeled Al3. The spectrum of B (Figure 7b) is similar, except for a significant reduction of the broad Al3 signal, which is probably due to a byproduct in A. The nature of the latter is discussed in the text below. It is well established that aluminum pentacoordination exists in crystalline aluminophosphates, with characteristic chemical shifts ranging from ca. 10 to 30 ppm, and the Al2 signal clearly agrees with that range. However, pentacoordination has been shown to arise from tetrahedral aluminum framework sites that are additionally coordinated either by F- 24 or by OH-/H2O species.33-35 To characterize the fifth coordination of the Al2 site, we acquired 27Al MAS and 1Hf27Al CP-MAS spectra of dehydrated A. After dehydration, Al2 is still present (Figure 7c) indicating either that this Al site does not interact specifically

signal

δiso (ppm)

λ (MHz)

Al1 Al2 AlPO4-5-OHb

39 15 44

2.8 3.5 2.6c

a Errors are (1 ppm for the isotropic chemical shift and (0.2 MHz for the composite quadrupolar parameter. λ ) χ(1 + η2/3)1/2, χ is the quadrupolar coupling constant (e2qQ/h) and η the asymmetry parameter. The parameters for AlPO4-5 synthesized in an alkaline medium are also given b From ref 36. c Calculated from the χ value of 2.3 MHz and the η value of 0.95.

with water, or that the dehydration conditions were unable to remove strongly bound water molecules. The 1Hf27Al CP-MAS spectrum of dehydrated A (Figure 7d) precludes this second possibility, since both Al1 and Al2 signals from the AlPO4-5 framework are poorly cross-polarized, in contrast with the Al3 peak associated with byproducts. A specific interaction with F- anions to explain the Al2 signal is clearly established by the 19Ff27Al CP-MAS spectrum (Figure 7e) and the twodimensional 19F-27Al HETCOR experiment (Figure 6b). Both experiments confirm that only the Al2 signal is enhanced by polarization transfer from 19F atoms resonating at -120 ppm. Note that the Al1 signal is not observed even with long contact times (not shown); likely reasons for this are (a) a much greater distance between F- and the corresponding aluminum species and (b) a short 27Al (T1F) value, preventing any efficient transfer of polarization for long contact-times. The deconvolution of the 27Al MAS spectrum of A and B gives in both cases an Al1/Al2 intensity ratio of about 9.6, suggesting that only one of the twelve aluminum sites per unit cell exhibits a specific interaction with F- species. 27Al 3Q-MAS NMR. To identify and characterize the framework aluminum species, we ran an 27Al 3Q-MAS experiment on A. Three distinct resonances are observed (Figure 8, Table 2), in agreement with the 27Al MAS spectrum. The Al3 signal, associated with the byproducts, is not clearly resolved and suggests that the corresponding aluminum species may be characterized by distributions in both chemical-shift and quadrupolar parameters. In contrast, both the Al1 and Al2 signals are resolved, with signals nearly parallel to the horizontal axis,


Gougeon et al.

Figure 9. 27Al-{19F} REDOR (a) and difference (b) spectra of B for 6 rotor cycles; B0 ) 9.4 T, νr ) 15990 Hz, 1.4 s recycle delay, 1024 transients, 90° and 180° pulses of 15 and 30 µs, respectively, for 27Al, 180° pulses of 1.6 µs for 19F.

Figure 10. (O) Measured 27Al-{19F} REDOR data for signal Al2 (δAl ) 10 ppm) of B. Calculated curves correspond to: dotted line: Al-F dipolar coupling of 2800 Hz (rAlF ) 2.19 Å); plain line: Al-F dipolar coupling of 4150 Hz (rAlF ) 1.92 Å).

though a weak observable bending away at low-frequencies suggests some distribution of the quadrupolar coupling parameters. The tetrahedral Al1 signal has an isotropic chemical shift of 39 ppm and a composite quadrupolar parameter λ of 2.8 MHz (Table 2). This isotropic chemical shift is 5 ppm less than that calculated for the as-synthesized AlPO4-5 material obtained in an alkaline medium.36 According to the empirical relation between the P-O-Al angle and the 27Al chemical shift proposed by Mu¨ller et al.,37 the lower chemical shift value of 39 ppm corresponds to a mean P-O-Al bond angle of 151°. While this value is in good agreement with the mean angle of 150° (adjusted for oxygen displacement) in the alkaline phase,1 it is slightly less than the mean value of 155° expected from the structure of B.2 A reason for this discrepancy might be that no adjustment for oxygen displacement has been considered in the XRD study of B.2 The observed value of λ (2.8 MHz) is in reasonable agreement with the quadrupolar coupling constant χ of 2.3 MHz for the alkaline AlPO4-5 homologue (Table 2). The Al2 signal is characterized by δiso ) 15 ppm and λ ) 3.5 MHz (Table 2). This isotropic chemical shift is in excellent agreement with other values for pentacoordinated aluminum 24,35 and confirms that the Al2 signal of the AlPO4-5 framework results from the additional coordination of a tetrahedral site by a fluorine atom. The higher λ value than that of the Al1 site also agrees with some distortion of the Al2 site. 27Al-{19F} REDOR NMR. The existence of an Al-F bond in the as-synthesized AlPO4-5 material can be assessed from the measurement of the fluorine-aluminum distance associated with the Al2 aluminum species, using the REDOR NMR technique.22 Figure 9 clearly shows, through the 27Al echo spectrum and the 27A-{19F} REDOR difference spectrum for B, that only Al2 is dipolar coupled to fluorine. The experimental REDOR curve, along with REDOR simulations calculated for two different values of isolated Al-F dipolar couplings, are shown in Figure 10. The data represent the evolution of the intensity of the Al2 signal (obtained by deconvolution), as a function of the dephasing time NτR, where N is the number of rotor cycles and τR is the rotor period. Since the 27Al spectrum is already resolved by MAS, the MQ-REDOR experiment38,39 is unnecessary. The isolated spin-pair assumption is considered to be valid since there is only one fluorine atom per unit cell, and the next nearest fluorine neighbor is more than 8 Å from the aluminum.2 The two simulated REDOR curves correspond to 19F-27Al dipolar couplings of 4150 and 2800 Hz, i.e., internuclear

distances of 1.92 Å, as in other AlPO4 materials with Al-F bonds40,41 and 2.19 Å, as obtained from structure refinement,2 respectively. The dipolar coupling of 2800 Hz is clearly not large enough to fit the observed REDOR data. Since the Al2 signal is weak, and is further underlain by a broad impurity signal, the deconvolution may be subject to some errors, and a precise distance from the experimental points is thus not expected. However, the analytical curve calculated for a rAl-F distance of 1.92 Å (4150 Hz) is in rather good agreement with the experiment (Figure 10). Location of Fluorine Atoms. The combination of the NMR techniques reported above clearly establishes the existence of Al-F bonds in the as-synthesized AlPO4-5/TPAF microporous material and further suggests a possible location for the corresponding F atoms. As can be seen from Figure 1, an Al-F bond would result from the slight displacement of F from its 6(c) particular position, toward one of the two closest Al atoms, while keeping it in the same Al-O2-P - P-O2-Al plane. Such breaking of the symmetry would agree with all our NMR results: (i) one pentacoordinated and 11 tetrahedral Al sites leading to a 1:11 ratio, which is consistent with the 27Al MAS spectra; (ii) two P sites neighboring the pentacoordinated Al site, which are equally close to F, leading to the P3 signal of the 31P MAS spectra; (iii) the more remote P sites, associated with the P4 signal of the 31P MAS spectra, can now be distinguished by different P-F distances. Nevertheless, it must be born in mind that the 31P chemical shift is mainly a function of the Al-O-P angle.37 Therefore, the P3 peak deshielding with respect to P4 can also be seen as a result of F atoms pushing O1, O3, and O4 away from O2, hence leading to smaller AlO-P angles.2 The remaining minor fluorine species can be associated with unbound F atoms located in the particular position derived from the XRD study.2 In that context, the fact that the 19F vertical slices drawn from the 19F-31P HETCOR experiment (Figure 6a) show a faithful reproduction of the 19F MAS spectrum, can be explained by the similarity of the two F environments. Altogether, these results provide a detailed complementary picture in agreement with the initial XRD study.2 However, the difficulty of XRD to locate fluorine positions could be explained by the likely random distribution of the two F species in the material, with possible exchange between them. New single-crystal and powder-diffraction experiments are currently underway in order to verify these solid-state NMR observations. Byproducts. A survey of the literature indicates that a number of additional phases such as AlPO4, AlPO4-11, AlPO4-H3,


Figure 11. 31P-{27Al} TRAPDOR spectra of A for four rotor cycles, obtained by initial 1Hf31P cross-polarization; (a) echo spectrum, (b) TRAPDOR spectrum, (c) difference spectrum; νr ) 4 kHz, 3 s recycle delay, 16 transients, 27Al irradiation field equivalent to 25 kHz.

tridymite, or AlPO4•2H2O (metavariscite/variscite) 2,7,42,43 may form during the synthesis of the AlPO4-5 material, depending on the starting gel composition and the synthesis conditions. Besides, it has been shown that several octahedral aluminophosphate-fluoride complexes are formed in starting gels of SAPO-34 containing only the sources of aluminum, phosphorus, and fluorine.44 However, reasons such as small quantities, poor crystallinity, or short-range crystals may explain a weak efficiency of XRD to detect these additional phases, as suggested by Figure 2. The best match of some of the small additional peaks in Figure 2 would be with a crystalline form of the natural mineral augelite Al2PO4(OH)3, which has a single type of PO4 tetrahedra and two types of Al polyhedra, i.e., five-coordinated AlO2(OH)3 and six-coordinated AlO4(OH)2.45 As indicated by the 31P NMR spectra of A (Figure 5 a,c), the two additional peaks P1 and P2, at -16 and -20 ppm, respectively, exhibit line widths comparable to those of the AlPO4-5 material, supporting the suggestion that the corresponding species have some crystalline order. Such chemical shifts are still in the range of chemical shifts for phosphorus having three to four aluminum atoms in the second sphere of coordination, as confirmed by the 31P-{27Al} TRAPDOR experiment recorded for A (Figure 11). The TRAPDOR and difference spectra (Figure 11 b,c) show an overall line shape which is similar to that of the echo spectrum (Figure 11a) and indicate that the two additional signals are associated with phosphorus species experiencing a comparable aluminum environment to those of the P sites in the AlPO4-5 phase. However, a closer look at the difference spectrum (Figure 11c) shows that the relative intensities of the two signals P1 and P2 are almost equal, whereas P1 is more intense than P2 in the echo spectrum (Figure 11a). Since the Al-P distance is approximately the same in all aluminophosphates, this suggests that while the P2 phosphorus atoms have four O-Al bonds, P1 phosphorus atoms probably have only three. The comparison of the 31P MAS and the 1Hf31P CP-MAS spectra (Figure 5 a,c) shows that proton cross-polarization enhances the relative intensities of P1 and P2. Furthermore, the 1Hf31P variable contact-time study (not shown) indicates a faster rate of crosspolarization for P1 and P2, than for the AlPO4-5 framework P3 and P4 signals. Therefore, the P1 and P2 phosphorus species are either closer to, or have more nearby protons than P3 and P4 which only experience the proximity of protons of the TPA+ template. Finally, from the observation of a small shoulder centered at ca. -16 ppm in the 19F-31P CP-MAS spectra of A (Figure 5d,e) it must be concluded that the P1 byproduct also contains some fluorine species. All these observations agree with

J. Phys. Chem. B, Vol. 105, No. 49, 2001 12255 the presence of an aluminophosphate of the augelite type, i.e., having PO4 tetrahedra and nearby OH groups to efficiently polarize 31P (Figure 5c) and 27Al nuclei (Figure 6e). Besides, it is well established that in a fluoride medium, F atoms can partially replace OH groups in the final products.24,44 The formation of a phase of the augelite type with Al-F bonds replacing some of the Al-OH ones would explain the polarization of the P1 phosphorus by 19F nuclei. The two aluminum sites in augelite have 27Al chemical shifts centered at 20 and -10 ppm.46 Whereas the latter agrees with the Al3 signal observed for A (Figure 7a), the high quadrupolar coupling constant (5.7 MHz) of the former could explain why it is not observed. Unfortunately, no 31P chemical shifts of augelite are reported in the literature, which could be compared to P1 and P2 signals. Conclusion A wide variety of advanced solid-state NMR techniques have been used to investigate the crystalline as-synthesized microporous aluminophosphate AlPO4-5 obtained in a fluoride medium, with the tetrapropylammonium cationic template. Although the synthesis of AlPO4-5 in both fluoride and alkaline media has been reported,2 the NMR spectral characteristics have not been thoroughly studied hitherto. In particular, the NMR data indicate two 31P MAS signals (at -24 and -31 ppm), in contrast to the diffraction structure indicating crystallographically equivalent phosphorus atoms within the unit cell. The second, additional 31P NMR signal was proposed to arise from the closer proximity of two of the twelve phosphorus of the unit cell.2 In the present work and through 19Ff31P CP-MAS and correlation experiments on both the original 2 and a newly synthesized sample, different phosphorus-fluorine distances are distinguished. While the small additional 31P peak at -24 ppm corresponds to framework phosphorus atoms closest to fluorine, the more intense 31P signal at -31 ppm appears to be a composite signal representing at least two different phosphorusfluorine distances. The 19F CSA analysis suggests the existence of two populations of fluorine, having the same isotropic chemical shift. The main component is characterized by CSA parameters similar to those of the fluorine species in the microporous AlPO4-CHA material where two rigid F atoms bridge two framework Al atoms. The second minor component does not show any CSA, which is considered to reflect the absence of an Al-F bond and/or a dynamic process involving the corresponding fluorine. The 27Al MAS NMR spectra of the two samples exhibit an additional peak to the main tetrahedrally coordinated aluminum signal, which corresponds to weakly distorted five-coordinated aluminum species, the fifth coordination being with F, as confirmed by the 27Al 3Q-MAS and the 19F-27Al CP-MAS, HETCOR, and REDOR experiments. Finally, in agreement with the XRD analysis, 31P-{27Al} TRAPDOR and 19F-31P CP-MAS experiments confirm that some crystalline aluminophosphate of the augelite type, and containing fluorine, can form as byproducts during the synthesis of the AlPO4-5 material. We show in this paper that the combined use of several solidstate NMR techniques can bring a detailed analysis of the crystalline microporous AlPO4-5 framework, which is complementary to diffraction analyses. In this particular case, we show that a REDOR experiment analyzed in conjunction with other techniques such as MQ-MAS and HETCOR experiments can bring together sufficient evidence to propose an internuclear distance prior to any single-crystal XRD measurements.

12256 J. Phys. Chem. B, Vol. 105, No. 49, 2001 Acknowledgment. The authors thank Dr. H. Kessler for providing one of the samples previously studied in ref 2 of this paper, and Dr. J. Baron for helpful discussions. Dr. P. Hazendonk is warmly acknowledged for running some of the 19F MAS spectra. References and Notes (1) Bennet, J. M.; Cohen, J. P.; Flanigen, E. M.; Pluth, J. J.; Smith, J. V. Intrazeolite Chemistry; Stucky, G. D., Dwyer, F. G., Eds; Am. Chem. Soc. Symp. Ser. 1983, 218, 109. (2) Qiu, S.; Pang, W.; Kessler, H.; Guth, J. L. Zeolites 1989, 9, 440. (3) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types; Butterworth: London, 1987. (4) Werner, L.; Caro, J.; Finger, G.; Kornatowski, J. Zeolites 1992, 12, 658. (5) Caro, J.; Finger, G.; Kornatowski, J.; Richter-Mendau, J.; Werner, L.; Zibrowius, B. AdV. Mater. 1992, 4, 273. (6) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannon, T. A.; Flanigen, E. M. Intrazeolite Chemistry; Stucky, G. D., Dwyer, F. G., Eds; Am. Chem. Soc. Symp. Ser. 1983, 218, 79. (7) Jahn, E.; Mu¨ller, D.; Wieker, W.; Richter-Mendau, J. Zeolites 1989, 9, 177. (8) Weyda, H.; Lechert, H. Zeolites 1990, 10, 251. (9) Finger, G.; Richter-Mendau, J.; Bu¨lov, M.; Kornatowski, J. Zeolites 1991, 11, 443. (10) Demuth, D.; Stucky, G. D.; Unger, K. K.; Schu¨th, F. Microporous Mater. 1995, 3, 473. (11) Flanigen, E. M.; Patton, R. L.; Wilson, S. T. InnoVation in Zeolite Materials Science; Grobet, P. J., Eds; Stud. Surf. Sci. Catal.; Elsevier Science B. V.: Amsterdam, 1988; 37, p 13. (12) Girnus, I.; Jancke, K.; Vetter, R.; Richter-Mendau, J.; Caro, J. Zeolites 1995, 15, 33. (13) Rackoczy, R. A.; Ernst, S.; Hartmann, M.; Traa, Y.; Weitkamp, J. Catalysis Today 1999, 49, 261. (14) Pang, W.; Qiu, S.; Kan, Q.; Wu, Z.; Peng, S. Proceedings of the Eighth International Zeolite Conference; Elsevier: Amsterdam, 1989; p 281. (15) Guth, F. Ph.D. Thesis, Universite de Haute Alsace, Mulhouse, France, 1989. (16) Kodaira, T.; Miyazawa, K.; Ikeda, T.; Kiyozumi, Y. Microporous Mesoporous Mater. 1999, 29, 329. (17) Van EcK, E. R. H.; Janssen, R.; Maas, W. E. J. R.; Veeman, W. S. Chem. Phys. Lett. 1990, 174, 428. (18) Grey, C. P.; Veeman, W. S.; Vega, A. J. J. Chem. Phys. 1993, 98, 7711. (19) Medek, A.; Harwood, J. S.; Frydman, L. J. Am. Chem. Soc. 1995, 117, 12779. (20) Amoureux, J. P.; Fernandez, C.; Steuernagel, S. J. Magn. Reson. 1996, A123, 116.

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TPAF Microporous Aluminophosphate - American Chemical Society

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