10385
J. Phys. Chem. 1993,97, 10385-10388
Solid-state NMR Studies of the Aluminophosphate Molecular Sieve AIPO4-18 Heyong He* and Jacek Klinowski' Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1E W,U.K. Received: March 12, 1993; In Final Form: June 11, 1993'
Solid-state NMR reveals that AlP04-18 contains three crystallographic sites for both P and A1 in a 1:l:l population ratio. In the as-prepared material two A1 sites are 4-coordinated and one is 5-coordinated. The latter becomes 4-coordinated upon calcination. These results are consistent with the structural refinement. There is a reversible structural change from space group C2/cin the calcined form to a lower symmetry space group in the calcined-rehydrated form. As a result of this transition, three P and A1 sites in an occupancy 8 in four double six-membered rings in one unit cell give rise to at least 12 crystallographic sites for P and Al.
Introduction
The preparation of aluminophosphatemolecular sieves, known as AIP04-n, where n is an integer denoting the structure type, was first reported in 1982.1 Many of them offer useful potential applications, such as shape-selective sorption. Of the more than 20 AlP04-n prepared so far, some have structures analogous to those of aluminosilicatezeolites, but many have completely novel structures. Of special interest are those involving one-dimensional channel systems. Unlike in zeolites, which are built of A104- and Si04 tetrahedra, the AlP04-n frameworks are composed of alternating A104-and P04-units and are thus electrically neutral. Another difference is that aluminum in AlP04-n can be 4-, 5-, and 6-~oordinated.~-~ Because singlecrystals of sufficient size are difficult to prepare, most of the structures of AlP04-n have been solved by XRD Rietveld refinement. However, the results are not always consistent with the conclusions drawn from other techniques, particularly solid-state NMR. For example, the structure of VPI-5 was originally refined in space group P63cm in which two framework P and A1 crystallographic sites are in an occupancy 2:l and all A1 atoms are 4m0rdinated.~However, 31Pmagicangle-spinning (MAS) NMR shows that there are three P rmonances withintensityratio l:l:l, whi1ez7AlMASNMRshows that one-third of aluminum atoms are 6 ~oordinated.~ NMR does not therefore support the original XRD refinement. This disagreementwas removed by the subsequentrefinementin space group P63 which calls for three P and A1 crystallographic sites in an occupancy 1:1:1 and two A1 sites are 4-coordinated and one is 6-coordinated.2 NMR is thus a very powerful technique for structural determination,particularly as concerns the coordination states of aluminum. We report solid-state NMR studies of different states of AlP04-18, the structure of which was refined by Simmen et al.5 NMR and XRD refinement are in agreement as to the structure of as-prepared and calcined AlPO4-18, but NMR reveals a structural distortion in the calcined-rehydrated material.
Experimental Section AlP04-18 was prepared according to the ref 5 except that, in order to improve the purity and crystallinity of the product, the composition of the synthesis gel was 0.36HCL:0.67(TEA)zO: A1203:P205:35HzO. The dehydrated as-prepared sample was prepared by evacuation in a vacuum line for 12 h. Calcined samples were prepared by heating the as-prepared material at 500 OC for 24 h. Calcined-rehydrated AlPO4-18 was prepared by hydration of the calcined sample over a saturated aqueous solution of NaCl for 12 h. Abstract published in Aduance ACS Abstracts, September 1, 1993.
0022-365419312097- 10385$04.00/0
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Figure 1. XRD patterns of AlPO4-18. (a) as-prepared;(b) calcined; (c)
calcined and rehydrated. The XRD patterns were recorded using a Philips 1710 powder diffractometer with Cu Ka radiation (40 kV, 40 mA), 0.025O step size and 5-s step time. MAS NMR spectra were recorded at 9.4 T with 4-mm double-bearing zirconia rotors spun in nitrogen at 8-10 kHz. UAl spectra were measured at 104.3 MHz with very short, 0.6 I.LS (less than loo),radiofrequencypulses and 0.3-s recycle delays. lHd7Al cross-polarization (CP) MAS spectra were recorded with a singlecontact pulse sequence, 500-I.LS contact time, 3.5-~slH 90° pulse, and 4-s recycledelay. 31Pspectra were recorded at 162.0 MHz with 30° pulses and 30-s recycle delays. IH-31P CP/MAS NMR spectra used a 5-ms contact time, 5-s recycle delay, and 3.594s lH 90° pulse. Chemical shifts of 27Al and 31P are given in ppm from external AI(H2O)p and 85% 0 1993 American Chemical Society
He and Klinowski
10386 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 AlP04
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NMR spectra (left) and 'H-"P CP/MAS NMR spectra (right) of A1P04-18. (a) as-prepared;(b) calcined; (c) calcined and rehydrated. Figure 2.
3lP MAS
H3P04,respectively. The Hartmann-Hahn condition for 'H27Al and lH-3IP CP/MAS was established using a sample of kaolinites and NaHzP04, respectively. Dehydrated samples were transferred into an NMR rotor in a drybox. The 27Aldoublerotation NMR spectrum was recorded using a Chemagnetics CMX-300spectrometer at 78.1 MHz with theouter rotor spinning at 718 Hz and the inner rotor spinning at 3 kHz.
Results and Discussion As-Prepared AlPO4-18. The XRD pattern of as-prepared AlP04-18 shown in Figure l a is the same as that given in ref 5 , indicating high purity of the material. Figure 2a shows 31Pand 'H-31P CP/MAS NMR spectra of dehydrated as-prepared AlP04-18. The3'Pspectrumconsists ofthree peaks withchemical shifts of -12.6, -28.7, and -30.1 ppm in the intensity ration of ca. 1:1:1. The latter two peaks overlap. The Z7Al spectrum given in Figure 3a shows three resonances at ca. 45,38, and 6 ppm, in the intensity ratio of ca. 1:l:l. The first peak is very narrow and symmetric, the other two have typical quadrupolar powder patterns and are very broad. Their lH-3lP and 1H-t7Al CP/ MAS spectra are very similar to the MAS spectra. There are three peaks with chemical shifts of ca. -13, -29, and -31 ppm in the 1H-31P CP/MAS spectrum and three peaks with chemical shifts of ca. 45,38, and 6 ppm in 1HJAl CP/MAS spectrum. No intensity ratio changes are found upon cross-polarization because water is absent and the interaction between template molecules and the framework P and A1 atoms is evidently weak. The structure of dehydrated as-prepared AlP04-18 (see Figure 4a) contains three crystallographic P and A1 sites with the occupancy of l:l:l, of which one A1 site is 5-coordinated.5 This is supported by our NMR results. The three resonances in the intensity ratio of 1:l:l in the 31PMAS spectrum correspond to three different P sites. Using the P coordinates given in ref 5 , we arrive at the P U A l angles of 144.3O (Pl), 148.5O (P2), and
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Figure 3. *'AI MAS NMR spectra (left) and lHA7AICP/MAS NMR spectra (right) of AlPO4-18 (a) as prepared; (b) calcined, (c) calcined
and rehydrated. Asterisks denote spinning sidebands. 131.4O (P3). On the basis of the relationship between the chemical shift and the P U A 1 angle in aluminophosphates? we assign the resonances at -12.6, -28.7, and -30.1 ppm to P3, P1, and P2 sites, respectively. Since there are three A1crystallographic sites in the occupancy 1:l:l and the intensities of the three peaks in "Al MAS NMR spectrum are equal, each peak must belong to a distinct A1 site. We assign the peak at 6 ppm to 5-coordinated All. The peaks at 45 and 38 ppm belong therefore to two 4-coordinated A1 sites, A12andA13. Since thelineshapesofthese two peaksaredifferent, so must be the symmetry of A12 and A13 sites. We estimate the symmetry of AI environment as follows. Since we are dealing with A1 atoms in the same coordination (that of A104), the symmetry of A1 depends on the bond lengths of A 1 4 and the locations of four oxygen atoms. Consider an A 1 4 bond as a a at the aluminum atom and assume that vector ? ~originating the properties of all four bonds are the same. The quantity r = I C ~ ~ l i ~where a d , i~1-0, is the vector linking the central aluminum atom and the ith oxygen, then depends only on the coordinates of the A1 and 0 atoms and is a measure of the symmetry of the environment of the A1 atom. When the distribution of oxygensis symmetric and the A104unit is a regular tetrahedron, r is zero, increases when the tetrahedron is distorted, and reaches a maximum when all oxygens are aligned on the same side of the aluminum. In dehdyrated as-prepaed AlP04-18 the values of r for A12 and A13 are 0.1 86 and 0.348, respectively. We therefore assign the symmetric peak at 44 ppm in 27Al MAS spectrum to A12 and the asymmetric peak at 38 ppm to A13. This method of assignment is not valid when both Z7Alresonances are asymmetric. However, in our particular case this problem does not arise. Calcined AIPOd-18. The XRD pattern of calcined AlP04-18 shown in Figure 1b is the same as in ref 5 . The3'P MAS spectrum
The Molecular Sieve AlP04-18 (Figure 2b) is very simple, consisting of a single resonance at -30.7 ppm. The line width is much greater than that from the as-prepared sample. The 27Al MAS spectrum (Figure 3b) contains an asymmetric resonance at 32 ppm. Calcined AlPO4-18 also contains three P and A1 crystallographic sites in the occupancy l:l:l, with all A1 atoms in 4-coordinati0n.~Upon removal of the template, the environment of framework P and A1 in calcined AlP04-18 becomes more symmetric. The P-O-A1 angle corresponding to the three P sites is 147.9-149.6’. Since 31PNMR chemical shifts of the three signals are very similar and also similar to that for site P2 in the as-prepared sample, the corresponding P-O-A1 angles must also be very close. The similar angles of the three P therefore cause the chemical shifts to be very close. The large line width in the 31PMAS spectrum is due to the overlapping of three phosphorus resonances. Upon calcination, the 5-coordinated All in the asprepared sample loses its extra 0 atom and changes to 4-coordination. The values of r for all three A1 atoms in the calcined sample are 0.130,0.161, and 0.170, respectively. The symmetry of the three A1 sites in calcined AlP04-18 is higher and similar to the A12 site in the as-prepared form. Of course, the single asymmetric peak in the 27AlMAS spectrum consists of three overlapping resonances. It could be composed of three similar pseudosymmetric peaks. Calcined and Rehydrated AIPO4-18. The XRD pattern and the 27Aland MAS spectra of AlPO4-18 show interesting changes after calcination and rehydration. The XRD pattern in Figure IC is different from that of its as-prepared and calcined form. The 31PMAS spectrum could be deconvoluted using a minimum of six peaks of unequal intensity between -1 8 and -3 1 ppm. The peak at -18.5 ppm is enhanced in the lH-3IP CP/ MAS spectrum. The 27Al MAS spectrum contains three resonances at ca. 44,38, and -13 ppm, respectively, with the two peaks overlapping. The intensity ratio of first two peaks to the third one is ca. 1.2:l. The intensity of the peak at -13 ppm in the 1H-27Al CP/MAS spectrum is considerably increased in comparison with the MAS spectrum. 27Aldouble-rotation NMR spectrum shows three asymmetric peaks at ca. 43, 37, and -13 ppm, respectively. XRD and NMR show that the symmetry changes in calcined AlP04-18 upon rehydration are fully reversible. The precise structure of calcined and rehydrated AlP04-18 is unknown. As with other aluminophosphate molecular sieve,such as VPI-5, the changes in the XRD pattern and NMR spectra are due to structural distortions brought upon by the adsorption of water molecules and formation of 6-coordinated A1 in the framework. Adsorption of water on calcined and rehydrated AlPO4-18 is reflected in the appearance of 1H-31Pand lHJ7Al CP/MAS spectra. The peak at -13 ppm in the 27Al MAS spectrum is clearly due to the formation of 6-coordinated framework Al. In the AlPO4-18 structure each unit cell contains four double six-membered rings and 24 P and A1 atoms. Deconvolution of the 31PMAS spectrum indicates that there are at least 12 equally occupied crystallographic sites for phosphorus. In other words, calcination and rehydration significantly lower the symmetry of the framework. In calcined AlP04-18,24 P and A1 atoms in one unit cell belong to three P and A1 crystallographic sites in an occupancy 8, and the space group is CZ/C.Upon rehydration, at least 12 P and 12 A1 atoms become inequivalent. This symmetry change between the calcined and calcinedrehydrated forms is also evident in the Z7Al spectra. The *7Al MAS spectrum of the latter form contains one resonance from 6-coordinated A1 and at least two from 4-coordinated Al. The doublet at 44 ppm could be due to the quadrupolar powder pattern or to two separate resonances. The two possible explanations for the 4-coordinated/6-coordinatedA1 intensity ratio of 1.2: 1 are as follows: (i) unlike in the as-prepared and calcined forms, there are more than three framework crystallographic sites for Al; (ii)
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10387
Figure 4. Framework structure of AlPO4-18 (a) as-prepared, with the template omitted for clarity; (b) calcined. Adapted from Simmen et al.’ AIPO, *’AI
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27Aldoublarotation NMRspectrumof calcinedand rehydratedAlP0,- 18. Asterisks denote spinning sidebands. Figure 5.
if there are still only three A1 crystallographic sites, they are hydrated to different extents. We investigated both these possibilities further by 27Al doublerotation (DOR) NMR1”l5 (see Figure 5 ) . DOR can remove not only the first-order effects such as chemical shift anisotropy but also the second-order quadrupolar interaction, which affects all quadrupolar nuclei. The removal of second-order quadrupolar interactions requires that the Legendre polynomials
10388 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993
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be averaged to zero. This can be achieved when the sample is spun simultaneously about two different axes 81 = 54.74O (the conventional "magic angle") and 82 = 30.56O or 7 0 . 1 2 O . Since DOR removes both first- and second-order quadrupole interactions, it makes single resonances symmetric. With explanation ii, a maximum of three symmetric 4-coordinated and 6-coordinated A1 resonances would be observed. Possibility i would lead to more than three symmetric resonances. The *'A1 DOR NMR spectrum is rather complicated. Each peak is asymmetric as a result of the overlap of at least two symmetric peaks. A comparison with the lower symmetry information obtained from MAS NMR and the fact that the framework P/Al atomic ratio is 1 indicates that explanation i is much more likely. This means that the number of crystallographic sites for A1 in calcined and rehydrated AlPO4- 18 is not three as in the case of as-prepared and calcined samples. There are at least twelve inequivalent crystallographicsites for both P and A1 in a unit cell of calcined AlPO.4-18, and the space group is lower than CZ/C.
He and Klinowski
Acknowledgment. We are grateful to Dr. J. S. Frye of Chemagnetics, Fort Collins, for acquiring the DOR spectrum and to the Ernest Oppenheimer Fund for support. References and Notes (1) Wilson,S.T.;Lok,B.M.;Flanigen,E.M.U.S.PatentNo.4,310,440, 1982. (2) McCusker, L. B.; Birlocher, Ch.; Jahn, E.;Bfllow, M.Zeolires 1991, 11, 308. (3) Parise, J. B. Inorg. Chem. 1985,24, 4312. (4) P!uth, J. J.; Smith, J. V. Acru Crysrullogr. 1986, CIZ, 283. ( 5 ) Simmen, A.;McCusker, L. B.; Birlochcr, C.; Mcier, W. M.Zeolites 1991, 11, 654. (6) Rudolf, P. R.; Crowder, C. E.Zeolires 1990, 10, 163. (7) Grobet, P. J.; Martens, J. A,;Balakrishnan, I.; M t r t e ~M.; , Jacobs, P. A. Appl. Card 1989, 56, L21. (8) Rocha, J.; Klinowski, J. J. Mugn. Reson. 1990,90, 567. (9) Miiller, D.;Jahn, E.;Ladwig, G.; Haubenreisscr, U. Chem. Phys. h r r . 1984, 109, 332. (10) Llor, A.;Virlet, J. Chem. Phys. h i t . 1988, 152, 248. (11) Samoson, A.;Lippmaa, E.; Pines, A.Mol. Phys. 1988,65, 1013. (12) Samoson, A.;Pines,A. Rev. Sci. Insrrum. 1989,60,3239. (13) Wu,Y.;Chmelka,B.F.;Pines,A.;Davis,M.E.;Grobet,P.J.;Jacobs, P. A. Nurure 1990, 346, 550. (14) Jelinek, R.; Chmelka, B. F.; Wu,Y.;Grandinetti, P. J.; Pines, A.; Barrier, P. J.; Klinowski, J. J. Am. Chem. Soc. 1991, 113, 4097. (15) Bame, P. J., Smith, M.E.;Klinowski, J. Chem. Phys. L r r . 1991, 180, 6.
