Interaction of Dipropylamine Template Molecules with the Framework

Determination of distances between aluminum and spin-1/2 nuclei using cross polarization with very weak radio-frequency fields. Gregor Mali , Venčesl...
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J. Phys. Chem. B 2002, 106, 63-69

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Interaction of Dipropylamine Template Molecules with the Framework of as-Synthesized AlPO4-31 Gregor Mali,*,† Anton Meden,‡ Alenka Ristic´ ,† Natasˇa Novak Tusˇar,† and Vencˇ eslav Kaucˇ icˇ †,§ National Institute of Chemistry, HajdrihoVa 19, SI-1000 Ljubljana, SloVenia, Faculty of Chemistry and Chemical Technology, UniVersity of Ljubljana, Asˇkercˇ eVa 5, SI-1000 Ljubljana, SloVenia, and UniVersity of Ljubljana, SI-1000 Ljubljana, SloVenia ReceiVed: July 31, 2001; In Final Form: October 9, 2001

The symmetry transformation of aluminophosphate molecular sieve AlPO4-31 and the interaction of its framework with dipropylamine template molecules were studied by variable temperature XRD and multinuclear MAS, CPMAS, and MQMAS NMR measurements. The as-synthesized material exhibited triclinic symmetry with a slightly distorted rhombohedral-like unit cell. Deviation from a rhombohedral symmetry was due to a large amount of template within cavities of a molecular sieve, which caused short contacts with the aluminophosphate framework. The template within AlPO4-31 was in the form of dipropylammonium hydroxide ionic pairs, whose arrangement did not possess a long-range order. Hydroxyl groups, not being isolated by surrounding alkyl groups, interacted with the aluminophosphate framework and gave rise to 5-coordinated aluminum sites. Dipropylammonium and hydroxyl groups were also coupled to the framework through hydrogen bonds with the framework oxygen and affected the local environments of the 4-coordinated aluminum and phosphorus sites. After one-half of the template was expelled from the sample, the interaction with the framework was reduced and the degree of crystal symmetry increased. Irreversible transformation from triclinic to rhombohedral symmetry occurred at 210 °C.

1. Introduction Aluminophosphate molecular sieves have been the subject of extensive studies over the past two decades mostly due to their interesting catalytic properties, an ion-exchanging role and a molecular sieve effect, which enables the discrimination of molecules according to their size and shape. AlPO4-31 was one of the first aluminophosphate molecular sieves, which were prepared in 1982.1 About a decade later, the structure of its calcined form was determined by synchrotron powder diffraction2 and space group R3h was assigned to this molecular sieve with ATO3 framework topology. Recently Finger et al.4 noted that the as-synthesized form of AlPO4-31 posessed triclinic symmetry, but they were not able to determine the structure. They also observed that when a portion of the template molecules were removed from AlPO4-31, its symmetry changed from triclinic to rhombohedral. Also when a small amount of divalent metal, such as magnesium or zinc, was incorporated into molecular sieve, the symmetry of corresponding assynthesized samples was rhombohedral rather than triclinic. With detailed XRD and NMR analyses we tried to explain the change of symmetry in aluminophosphate molecular sieve AlPO4-31. For this reason, pure AlPO4-31 as well as metal substituted analogues CoAPO-31 and MnAPO-31 were prepared. Particular effort was also devoted to elucidating the shortrange interaction between the template and the aluminophosphate framework with several NMR measurements. 2. Experimental Section AlPO4-31, CoAPO-31, and MnAPO-31 were synthesized hydrothermally with dipropylamine as a template. Aluminum * Corresponding author. † National Institute of Chemistry. ‡ Faculty of Chemistry and Chemical Technology. § University of Ljubljana.

and phosphorus sources were pseudoboehmite and orthophosphoric acid, respectively. Metal acetate hydrates Co(CH3COO)2‚ 4H2O and Mn(CH3COO)2‚4H2O were used as sources of cobalt and manganese. Template-free samples were prepared by heating the corresponding as-synthesized products up to 550 °C and keeping them at that temperature in an oxygen flow for 6 h. More details of the synthesis and calcination procedures are described in ref 5. X-ray powder diffraction patterns of all products were recorded on a Siemens D-5000 diffractometer using CuKR radiation. Room-temperature data were collected in 2θ range from 5 to 35° in steps of 0.04° spending 1 s per step. Data which were used for indexing and refinement of AlPO4-31 unit cell parameters were recorded in a range from 2 to 52° in steps of 0.02° with 20 s per step. LCLSQ6 computer program was used for the determination of lattice constants. The R2 peaks were removed and peak positions were determined by profile fitting. Temperature-resolved patterns of AlPO4-31 sample were recorded on the same diffractometer equipped with HTK-16 high-temperature chamber. Data were collected at selected temperatures between room temperature and 1000 °C in 2θ range from 5 to 35° in steps of 0.03° spending 4 s per step. Between scans the sample was heated with a rate of 10 °C/ min. Solid-state NMR experiments were performed on a narrowbore Varian Unity Inova spectrometer operating at 150.87, 156.35, 242.89, and 600.03 MHz for 13C, 27Al, 31P, and 1H nuclei, respectively. For all MAS, CPMAS, and MQMAS measurements on the AlPO4-31 sample, a single Doty 5 mm supersonic triple-tuned CPMAS probehead was used. The sample was typically spun with a spinning speed of 11 kHz. Some details about multinuclear MAS and CPMAS measurements are collected in Table 1. Note that in cross-polarization

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Figure 1. XRD patterns of as-synthesised MnAPO-31, CoAPO-31 and AlPO4-31 molecular sieves. Vertical bars mark reflection positions calculated from the appropriate unit cells (rhombohedral in the first two cases and triclinic in the third one: see Table 2).

TABLE 1: Details about MAS and CPMAS NMR Measurements nuclei 1H 27Al 31P 1H-13C 27Al-1H 27Al-31P 1H-31P

νRF1 [kHz] 50 65 30 39 1 2 36

νRF2 [kHz]

pulse length/ contact time [µs]

delay [s]

no. of scans

50 8 17 25

5 2 7.2 8000 0-10000 2000 4000

10 1 100 10 0.1 0.1 10

40 200 400 8000 800 8000 800

experiments involving quadrupole aluminum nuclei very weak radio frequency fields were used. On one hand such fields enable efficient aluminum magnetization spin-locking7,8 and magnetization transfer,9 while on the other hand they introduce extremely high sensitivity to the radio frequency offsets. Although the latter property often represents a drawback, in our case it turned into an advantage, because it enabled selective polarization transfer from aluminum nuclei belonging to different local environments. In this way we were able to obtain more detailed information about proximity of aluminum nuclei on one hand and phosphorus nuclei or protons on the other hand than we could do in a case of robust nonselective crosspolarization. 3. Results and Discussion 3.1. Phase Transition. Results of X-ray powder diffraction shown in Figure 1 confirm that the as-synthesized form of AlPO4-31 possesses triclinic symmetry, while the as-synthesized forms of CoAPO-31 and MnAPO-31 are rhombohedral. The triclinic lattice of AlPO4-31 only slightly deviates from the rhombohedral one (Table 2) and transforms to the latter at 210 °C (Figure 2). The phase transition is irreversible (Figure 2) and is associated with a loss of approximately one-half of the total amount of template within the sample (Table 3). After the phase transition, when the sample is at 220 °C, the amount of the template within the rhombohedral AlPO4-31 is almost equal to the amounts of template within the as-synthesized rhombohedral samples of CoAPO-31 and MnAPO-31. This indicates that dipropylamine plays a leading role in the phase transition and that its amount determines the symmetry of ATO framework. While the structure of an aluminophosphate molecular

sieve can be influenced also by the presence of water molecules,10-14 phase transitions associated with adsorption/ desorption of water are generally reversible and observed for calcined samples in which channels are empty and thus easily accessible to water molecules. The influence of water could not be detected either in the as-synthesized or in the calcined form of AlPO4-31. Presented variable temperature X-ray powder diffraction and elemental analyses suggest that rhombohedral symmetry is possible when there is about one-third of a molecule of dipropylamine in one unit cell of an ATO framework. In the above representation of a rhombohedral lattice (Table 2), the straight channel runs along the short body diagonal of the unit cell so that one cell “covers” about 0.5 nm long part of the channel.2,15 Each template molecule with the length of about 1 nm thus has in the average 1.5 nm long space, where it can be placed randomly. The maximum content of dipropylamine in the ATO framework is probably about two-thirds of a molecule per rhombohedral-like unit cell as found in AlPO4-31. In such a case, a template molecule can occupy in the average only about 0.75 nm long part of the channel, which further implies that molecules probably have to at least partially overlap or bend or both to accommodate within the channel. This crowding of molecules can cause short contacts between the template and the framework and therefore deform channels and result in the triclinic symmetry. Results of the elemental analysis provide also an indication for incorporation of cobalt and manganese atoms into aluminum framework positions of CoAPO-31 and MnAPO-31. The number of cobalt or manganese atoms added to the number of aluminum atoms is, namely, within experimental error, equal to the number of phosphorus atoms and is thus in agreement with strict alternation of aluminum and phosphorus sites within aluminophosphate molecular sieves. Furthermore, the number of dipropylamine molecules per unit cell of CoAPO-31 or MnAPO-31 is approximately equal to the number of cobalt or manganese atoms, which explains also why the amount of template in CoAPO-31 and MnAPO-31 is smaller than the amount of template in AlPO4-31. Substitution of trivalent aluminum with divalent metal generates negatively charged framework sites. To compensate the lack of positive electric charge, each metal site probably attracts a protonated dipropylammonium cation. In this way cobalt and manganese atoms can lock the template and disable the packing, which is required for larger population of template molecules, to be established. More rigorous proof for substitution of aluminum atoms can be found in ref 5, where local environments of aluminum, phosphorus, cobalt and manganese atoms are analyzed by several spectroscopic techniques and catalytic properties of AlPO4-31, CoAPO-31, and MnAPO-31 are studied. 3.2. Interaction of the Template with the Framework. Whereas the above discussion was pertinent to long-range order and bulk properties of our samples, in this section we elucidate a complemental aspect of interaction between aluminophosphate framework and dipropylamine template molecules on a shortrange scale. We begin the analysis with MAS NMR of aluminum, phosphorus, carbon, and hydrogen nuclei in assynthesized, partially and completely detemplated samples of AlPO4-31. A partially detemplated sample was obtained from the as-synthesized one by keeping it at 220 °C for 1 h and a completely detemplated sample was obtained by calcination. All spectra, which are shown in Figure 3, were recorded at room temperature. In agreement with the low symmetry of the assynthesized AlPO4-31, spectra of aluminum and phosphorus

Interaction of Template with the Framework

J. Phys. Chem. B, Vol. 106, No. 1, 2002 65

TABLE 2: Space Groups and Unit Cell Parameters for as-Synthesized AlPO4-31, CoAPO-31, and MnAPO-31 and for Calcined AlPO4-31a sample

space group

as-synthesized AlPO4-31 as-synthesized CoAPO-31 as-synthesized MnAPO-31 calcined AlPO4-31

lattice constants a ) 1.2007(1) nm R ) 118.345(5)° a ) 1.215(1) nm R ) 118.135(5)° a ) 1.216(2) nm R ) 118.132(7)° a ) 1.2147(8) nm R ) 118.143(3)° a ) 2.0840 nm

P1 R3hrhomb R3hrhomb R3hrhomb R3hhex

b ) 1.2283(1) nm β ) 117.052(5)°

c ) 1.2073(1) nm γ ) 118.921(4)°

c ) 0.5008 nm

a

Superscriptshex andrhomb denote hexagonal setting and rhombohedral setting, respectively. Standard deviation on the last decimal number is given in parentheses.

Figure 2. Variable temperature XRD patterns of the as-synthesised AlPO4-31. Left: Above 210 °C the diffraction maxima between 22 and 23° merge into a single one due to transformation from the triclinic to the rhombohedral symmetry. Right: The described phase transition is irreversible.

TABLE 3: Chemical Composition of as-Synthesized AlPO4-31, CoAPO-31, and MnAPO-31 Molecular Sievesa sample

NMe

NAl

NP

Ntemplate

0.28 0.31

5.99 5.99 5.69 5.68

6.01 6.01 6.04 6.01

0.64 0.33 0.30 0.30

-31b

AlPO4 AlPO4-31c CoAPO-31 MnAPO-31

a Number of incorporated divalent metal, aluminum, and phosphorus atoms, as well as number of template molecules within one rhombohedral unit cell, are given. Elemental analysis was performed with an EDAX analytical system attached to scanning electron microscope. Amount of template was determined with CHN analysis. Experimental error is (0.04. b As-synthesized sample. c Partially detemplated sample, which has undergone an irreversible phase transition at 210 °C.

nuclei both show several resonances, which correspond to several magnetically inequivalent aluminum and phosphorus sites. After partial removal of the template, the number of resonances in both spectra reduces to one and confirms that above 210 °C AlPO4-31 transforms to a more symmetric phase with a single tetrahedral phosphorus and aluminum crystallographic site. 1H-13C CPMAS spectrum of the as-synthesized material shows four narrow resonances, which can be assigned to carbon nuclei in -CH2-N- groups (44 ppm), -CH2groups (12 ppm), and in two -CH3 groups (6.3 and 4.2 ppm). When one-half of the template is expelled from the sample, the above-described resonances become not only lower but also broader. A larger breadth can be explained by increased freedom of dipropylamine molecules to occupy random positions or orientations within partially detemplated sample, giving rise to

a spread of carbon chemical environments. The proton spectrum is not as well-resolved as the carbon one, but it still enables us to distinguish two contributions at 1.7 and 3.8 ppm, which belong to hydrogen in alkyl and amino groups, respectively. Actually, this is not the only possible assignment of proton resonances, because the above chemical shifts correspond also to protons in terminal and bridging hydroxyl groups.16,17 After partial and afterward complete removal of dipropylamine, these two contributions are first lowered and finally completely diminished. Instead, a peak at about 4.7 ppm appears, which is due to water molecules which enter emptied channels of detemplated samples. Surprisingly and contrary to situation in many aluminophosphate molecular sieves these molecules influence neither the symmetry of ATO framework nor the coordination of its constituents. In a similar way as the proton spectrum of the as-synthesized material, spectrum of aluminum nuclei is also composed of two main contributions. The first one between 43 and 26 ppm belongs to 4-coordinated aluminum nuclei and the second one between 17 and 2 ppm belongs to 5-coordinated nuclei. Analysis of centerband and sideband intensities shows that 5-coordinated sites represent (1(0.05)/9 of all aluminum sites. If we recall that in the as-synthesized AlPO4-31 there is approximately one template molecule per nine aluminum atoms (Table 3), we can conclude that dipropylamine is responsible for the distortion of the local aluminum environment. In fact, the presence of 5-coordinated aluminum sites suggests that rather than dipropylamine molecules there are probably dipropylammonium hydroxide ionic pairs contained within channels of AlPO4-31. A similar phenomenon was already observed for some other

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Figure 3. Multinuclear MAS NMR of as-synthesized (top), partially (middle) and completely (bottom) detemplated AlPO4-31. In the case of 1H f 13C CPMAS, only spectra of the as-synthesized and partially detemplated samples are shown.

aluminophosphate molecular sives.18-20 It was suggested then that during the synthesis, template molecules became protonated and finally trapped within microporous structure as organic hydroxides, or more precisely, as organic cations and hydroxide anions. If such hydroxyl groups were not isolated from the framework by alkyl groups, they could interact with aluminum atoms and produce apparent 5-coordinated aluminum sites. Because nitrogen atom of a nonprotonated dipropylamine molecule also possesses a free electronic pair, in a similar way as oxygen in a hydroxyl, 5-coordinated aluminum sites could in principle be due to the coordination with dipropylamine as well. The dilemma can be solved by looking at the IR spectrum of the as-synthesized AlPO4-31 in Figure 4, which shows a sharp signal at 3604 cm-1. The signal falls within a typical region of hydroxyl stretching vibrations and clearly confirms the presence of hydroxyl groups as well as, indirectly, the presence of dipropylammonium cations, which are needed to counterballance the electric charge of hydroxyls. Resolution of aluminum MAS spectrum is much higher than resolution of the proton MAS spectrum and can even further be enhanced with a 3QMAS measurement. In a two-dimensional 27Al 3QMAS spectrum of the as-synthesized AlPO -31 at least 4 eight 4-coordinated (shown in Figure 5) and three 5-coordinated sites (not shown) can be resolved. The number of magnetically inequivalent aluminum sites exceeds the number of crystallographically distinct aluminum sites, which is in a noncentrosymmetric triclinic lattice of AlPO4-31 equal to six. The number of

Figure 4. IR spectrum of the as-synthesized AlPO4-31. The spectrum was recorded on Perkin-Elmer System 2000 FTIR spectrometer at a resolution of 4 cm-1.

crystallographically inequivalent sites could be larger than six if the true unit cell was doubled or trippled due to long-range ordering of template molecules. As it was shown in the case of AlPO4-41,13 the scattering power of template was high enough to enable the observation of superstructure reflections due to crystallographically ordered template. In our case, however, the diffraction pattern of the as-synthesised AlPO4-31 shows no such reflections, which means that a large number of magnetically distinct aluminum sites must be a consequence of a short-range order, which cannot be seen by X-ray diffraction.

Interaction of Template with the Framework

J. Phys. Chem. B, Vol. 106, No. 1, 2002 67 shift δH of the proton MAS NMR signal was observed. It was given by17

νOH[cm-1] ) 3870 - 67.8δH[ppm]

Figure 5. Sheared 27Al 3QMAS spectrum of the as-synthesized AlPO431 (only the contribution of the 4-coordinated aluminum nuclei is shown).

Figure 6. 27Al f 1H two-dimensional CPMAS experiment in the assynthesized AlPO4-31.

Additional information about local interactions in the assynthesized AlPO4-31 was obtained with 27Al f 1H crosspolarization measurements. A two-dimensional spectrum in Figure 6 shows three cross-peaks, among which the most intense is the one between 5-coordinated aluminum nuclei and protons with resonance at 3.8 ppm. Although protons from either amino or hydroxyl groups can exhibit chemical shifts of about 4 ppm, the above presented results suggested that 5-coordinated aluminum nuclei were coordinated with hydroxyl rather than amino groups. Proton MAS NMR and IR spectroscopic techniques have been extensively applied to study hydroxyls in aluminosilicate and silicoaluminophospahte molecular sieves in the past. It was found that proton NMR signals with chemical shifts between 3.8 and 4.3 ppm could be ascribed to bridging SiOH-Al groups in large cavities or channels of dehydrated materials.21 Furthermore, a nearly linear interdependence between the wavenumber νOH of the IR band and the chemical

If we put the observed chemical shift of 3.8 ppm in the above expression, we obtain the wavenumber of 3612 cm-1, which agrees well with the observed value of 3604 cm-1. These results suggest that hydroxyl groups within AlPO4-31 could be of the bridging type Al-OH-Al. Such hydroxyl bridges are actually quite common in aluminophosphate molecular sieves and were detected with XRD for example in AlPO4-17, AlPO4-18, and AlPO4-40.20,22,23 An alternative suggestion, according to which the proton NMR signal at 3.8 ppm and hydroxyl stretching IR band at 3604 cm-1 might be ascribed to hydroxyl groups of the terminal type Al-OH involved in additional hydrogen bonds, is less probable, because corresponding IR bands then usually appear between 3700 and 3800 cm-1.16,24 Nevertheless, the interpretation of NMR and IR signals is not totally reliable, since suggestions are based on comparison of the as-synthesized aluminophosphate sample with dehydrated silicoaluminophosphate materials. The two-dimensional 27Al f 1H CPMAS spectrum in Figure 6 shows also a less intense cross-peak between 4-coordinated aluminum sites with resonance at about 39 ppm and protons with resonance at 3.8 ppm. Aluminum nuclei, which contribute to this cross-peak, are probably those, whose contributions in the 3QMAS spectrum were denoted as S1 and S4. If lower cross-peak intensity and threfore weaker polarization transfer are due to longer distances among protons and aluminum nuclei, then this cross-peak is probably not due to the polarization transfer between aluminum and covalently bonded hydroxyls but between aluminum and hydrogen-bonded groups. These groups can be hydroxyls as well as amines. If one of them interacts with a framework oxygen via the hydrogen-bonding interaction, neighboring aluminum and phosphorus atoms remain 4-coordinated whereas their tetrahedral environments may become noticably distorted. A distortion of aluminum environment from a tetrahedral one can lead to a large electric field gradient at aluminum postion and therefore induce strong quadrupolar interaction. Indeed, quadrupolar coupling constants of sites S1 and S4 of 3.3 ( 0.6 MHz and 2.3 ( 0.4 MHz, estimated from a 3QMAS measurement, are large and thus suggest that environments of sites S1 and S4 might be distorted due to hydrogen bonding. The above assumption that different distances among protons and either 4-coordinated or 5-coordinated aluminum nuclei are responsible for different intensities of corresponding cross-peaks in the two-dimensional spectrum can be verified with detailed 27Al f 1H variable contact time analysis. The description of polarization transfer dynamics between two sets of abundant nuclei is difficult.25 When the expression for the dynamics of cross-polarization based on a spin thermodynamic treatment26 was used, several sets of time constants TCP, T1FAl, and T1FH yielded equally good fits of experimental data. Here TCP is crosspolarization time constant, which contains distance dependence of r6, and T1FAl and T1FH are aluminum and proton spin-lattice relaxation times in corresponding rotating frames. Only when the following constraintss(i) polarization transfers from the same aluminum environment to different proton environments have to be described with equal T1FAl and (ii) polarization transfers from different aluminum environments to the same proton environment have to be described with equal T1FHswere applied (see Table 4), more reliable values for time constants were obtained. Still the obtained values allow only qualitative

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TABLE 4: Results of 27Al f 1H Variable Contact Time CPMAS Experimentsa IfS

T1FI [ms]

T1FS [ms]

TCP [ms]

AlV f Ha AlIV f Ha AlIV f Hb

0.15 0.25 0.25

7.5 7.5 6.5

2.3 13 40

a AlIV and AlV denote 4- and 5-coordinated aluminum sites and Ha and Hb denote proton contributions at 3.8 and 1.7 ppm, respectively. All values were obtained by fitting theoretical experession to experimental data and allow only qualitative analysis.

Figure 8. 27Al f 31P two-dimensional CPMAS experiment in the assynthesized AlPO4-31. Figure 7. 27Al f 1H variable contact time CPMAS experiment in the as-synthesized AlPO4-31. Selective transfers of polarization from the 5-coordinated aluminum nuclei to the protons, resonating at 3.8 ppm (]), from the 4-coordinated aluminum nuclei to the protons, resonating at 3.8 ppm (O), and from the 4-coordinated aluminum nuclei to the protons, resonating at 1.7 ppm (b), were studied. Solid lines show fitted functions.

explanation of experimentaly observed dynamics. Results in Figure 7 and Table 4 show that the fastest is the polarization transfer from 5-coordinated aluminum sites, slower still is the cross-polarization from 4-coordinated sites S1 and S4, and the slowest is polarization transfer from other 4-coordinated sites to protons in alkyl groups. Cross-polarization among the latter nuclei was not discussed so far. It corresponds to the crosspeak of the smallest intensity in the two-dimensional 27Al f 1H CPMAS spectrum and presents an evidence that the template is crowded within channels and that many short contacts with the framework are generated in the as-synthesized AlPO4-31. Observed values of TCP are consistent with the assumption that 5-coordinated aluminum nuclei “see” protons in attached hydroxyl groups, 4-coordinated aluminum nuclei on sites S1 and S4 “see” more distant protons in hydrogen-bonded amines or hydroxyls and the rest of aluminum nuclei “see” the most distant protons in alkyl groups. Phosphorus NMR also sheds some light on the interaction between the template and the aluminophosphate framework. The spectrum of phosphorus nuclei of the as-synthesized AlPO4-31 (Figure 3) consists of at least seven resonances so that the number of magnetically inequivalent phosphorus sites again exceeds the number of crystallographycally inequivalent sites. Two resonances at -23.5 and -25 ppm are completely resolved from the rest of the spectrum. Since in the IR spectrum in Figure 4 no signal can be observed at wavenumbers, which are characteristic for terminal P-OH groups (about 3680 cm-1), and since the 1H f 31P CPMAS spectrum reveals that the two resolved phosphorus signals are not enhanced with respect to the rest of the phosphorus spectrum (not shown), the resonances at -23.5 and -25 ppm cannot be assigned to phosphorus nuclei in P-OH groups. Rather the decreased chemical shielding of these nuclei can be explained as a consequence of hydrogenbonding interaction between neighboring oxygen atoms and

either amino or hydroxyl groups. The two-dimensional 27Al f 31P CPMAS spectrum in Figure 8 namely shows cross-peaks among the phosphorus nuclei with resonances at -23.5 and -25 ppm, and the 4-coordinated aluminum nuclei, which contribute to signals S1 and S4. This means that the 4-coordinted aluminum nuclei, which are relatively close to protons and experience strong quadrupolar interaction, share the framework oxygen with the phosphorus nuclei, which are exposed to decreased chemical shielding. A combination of NMR measurements thus gives an indication for the presence of hydrogen bonds between dipropylammonium hydroxide ionic pairs and the framework of AlPO4-31. 4. Conclusions The as-synthesized form of AlPO4-31 exhibits triclinic symmetry with a slightly distorted rhombohedral-like unit cell. Deviation from a rhombohedral symmetry is due to a large amount of template within the sample, which causes short contacts with the aluminophosphate framework. There are about two-thirds of dipropylammonium hydroxide ionic pair per one unit cell of AlPO4-31, showing that the arrangement of template does not possess a long-range order. Hydroxyl groups most probably form Al-OH-Al bridges between framework aluminum sites and thus give rise to the 5-coordinated aluminum. Amino and/or hydroxyl groups are, through hydrogen bonds, also connected to framework oxygen between 4-coordinated aluminum and phosphorus sites. This causes a deformation of aluminum and phosphorus tetrahedral environments, which can be detected through an increased electric quadrupole interaction of aluminum nuclei and decreased chemical shielding of phosphorus nuclei. When at 210 °C approximately one-half of the template is removed from the sample, the interaction with the framework is reduced and irreversible phase transition to rhombohedral phase occurs. The amount of template within partially detemplated AlPO4-31 is approximately equal to amounts of template within as-synthesized samples of CoAPO31 and MnAPO-31, which is why these modified samples possess rhombohedral rather than triclinic symmetry. A reason for the lower template content of CoAPO-31 and that of the MnAPO-31 molecular sieves might be the incorporation of

Interaction of Template with the Framework divalent cobalt or manganese onto aluminum framework sites. In this way negatively charged centers are generated, which can lock dipropylammonium cations and thus give rise to their random distribution within channels of CoAPO-31 and MnAPO31. Acknowledgment. The authors acknowledge the financial support of the Slovenian Ministry of Education, Science and Sport through research projects Z1-3277 and Z2-3457. References and Notes (1) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. (2) Bennett, J. M.; Kirchner, R. M. Zeolites 1992, 12, 338. (3) Baur, W. H.; Joswig, W.; Kassner, D.; Kornatowski, J. Acta Crystallogr. 1994, B50, 290. (4) Finger, G.; Kornatowski, J.; Jancke, K.; Matschat, R.; Baur, W. H. Microporous Mesoporous Mater. 1999, 33, 127. (5) Novak Tusˇar, N.; Mali, G.; Arcˇon, I.; Ghanbari-Siahkali, A.; Dwyer, J.; Kaucˇicˇ, V. Microporous Mesoporous Mater. 2002, accepted for publication. (6) Burnham, C. W. Least Squares Refinement of Crystallographic Lattice Parameters, Manual; Harvard University: Cambridge, 1993. (7) Vega, A. J. J. Magn. Reson. 1992, 96, 50. (8) Sun, W.; Stephen, J. T.; Potter, L. D.; Wu, Y. J. Magn. Reson. A 1995, 116, 181. (9) Vega, A. J. Solid State Nucl. Magn. Reson. 1992, 1, 17. (10) Kolodziejski, W.; He, H.; Klinowski, J. Chem. Phys. Lett. 1992, 191, 117.

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