NMR Characterization of Mesostructured Aluminophosphates - The

Oct 21, 2000 - ... of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany ... nuclei using cross polarization with very weak radio-frequency...
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J. Phys. Chem. B 2000, 104, 10473-10481

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NMR Characterization of Mesostructured Aluminophosphates M. Schulz,† M. Tiemann,‡ M. Fro1 ba,*,‡ and C. Ja1 ger*,† Institute of Optics and Quantum Electronics, Friedrich Schiller UniVersity, Max-Wien-Platz 1, D-07743 Jena, Germany, and Institute of Inorganic and Applied Chemistry, UniVersity of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany ReceiVed: January 27, 2000; In Final Form: August 8, 2000

The atomic structure of ordered mesostructured aluminophosphates with dodecyl phosphate as the structuredirecting template was investigated by multinuclear solid-state NMR. Two different types of materials were studied: one with the template headgroup as the only phosphate source and one where additionally phosphoric acid was used in the synthesis. All 31P and 27Al NMR resonances can be subdivided into three groups depending on the sample composition. Furthermore, 2D heteronuclear correlation NMR measurements allow a conclusive and unambiguous identification of the phosphate headgroup resonances of the template molecules and the determination of water and/or hydroxyl units. Most notably, there is an obvious correlation between the d001 values of the samples as measured by powder X-ray diffraction and the appearance of the various groups of 27Al and 31P resonances.

1. Introduction Since its discovery in 1992 the synthesis of ordered mesoporous or mesostructured materials using supramolecular surfactant assemblies as structure-directing templates1 has been extended from silica and aluminosilicate phases to a large number of other materials, including metal oxides2-10 and chalcogenides.11-14 These materials exhibit unique structural properties on a length scale of several nanometers and are of interest for a large variety of potential applications, e.g., as catalysts or catalyst supports, molecular sieves, conducting or semiconducting devices, photoactive materials, or host-guest systems. The synthesis of mesoporous aluminophosphates is particularly important, as these materials are expected to be of great significance for catalytic purposes. Numerous members of the family of zeolite-analogous crystalline microporous aluminophosphates15 and silicoaluminophosphates,16 such as AlPO4-5, VPI-5,17 and many others, often containing additional (transition) metals in their frameworks, have proven to be excellent catalysts for a vast number of chemical reactions.18 Accordingly, materials that combine these catalytic properties with considerably larger pore systems are required. Mesoporous aluminophosphates with hexagonal pore structures have been synthesized by various groups.19-23 Likewise, several papers have been published reporting on mesostructured phases with lamellar or hexagonal structures which are not structurally stable upon removal of the template.24-29 Nevertheless, these nonporous materials are of interest for both basic structural investigations and mechanistic studies, particularly when the walls of the network are X-ray amorphous. Recently, lamellar mesostructured aluminophosphates have been synthesized as model compounds by Fro¨ba and Tiemann from H3PO4 and Al(OiPr)3 using monododecyl phosphate surfactant as the template.28a The structural investigation of these materials by means of powder X-ray diffraction, thermal analysis, and Al K-edge X-ray absorption spectroscopy (XANES) leads to two * To whom correspondence should be addressed. † Friedrich Schiller University. ‡ University of Hamburg.

results: (i) In some cases the inorganic networks consist not only of aluminophosphate (where Al is supposed to be tetrahedrally coordinated), but also of aluminum oxide or oxyhydroxide species (with octahedral Al, presumably γ-AlO(OH), boehmite). The relative amount of the latter species depends systematically on the Al/P ratio as employed for the preparation or as found in the products, respectively. (ii) The phosphate headgroups of the surfactant may become incorporated into the inorganic layers when no phosphoric acid is available during the synthesis, which means that the surfactant can act as both the template and reactant. Since detailed structural information is very limited using the above-mentioned methods, the aim of our investigations is to improve the current understanding of the microstructure of these mesostructured aluminophosphates. Using standard magic angle sample spinning (MAS) NMR experiments, the number of the various Al and P sites in the structure can be determined. Specifically, it will be shown that the 31P and 27Al MAS NMR spectra correlate directly with the d spacings measured by X-ray diffraction. However, a number of questions concerning the real structure remains unanswered. For example, it is not clear which 31P lines are caused by the phosphoric acid used in the synthesis and which of the lines are the template headgroup resonances. However, this piece of information is very important for building a structural proposal. Also, some of the 1H signals may be caused by water molecules and/or hydroxyl units. Such problems can be elegantly solved using 2D methods,30,31 specifically 1H-31P heteronuclear correlation (HETCOR) NMR, where the various 1H signals are correlated to the phosphate structures in direct proximity if short contact times are used. 2. Experimental Section 2.1. Sample Preparation. The procedure for the synthesis of the mesostructured aluminophosphates from aluminum isopropoxide(Al[OiPr]3),monododecylphosphate(C12H25OPO[OH]2, in the following denoted by C12PO4), andsin some casess phosphoric acid (H3PO4) is reported elsewhere.28a The sample compositions are summarized in Table 1. There are two different

10.1021/jp000337n CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000

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TABLE 1: Synthesis Parameters, Product Compositions, and d001 Valuesa of the Lamellar Aluminophosphates28a molar amts used prod composition d001 (nm) (from elem anal.), surfact in synthesis, concn Al[OiPr]3/H3PO4/ Al/P (H3PO4)/ first second sample (% w/w) C12PO4 P (C12PO4) phase phase A8 A10 A13 B5 B7 B13 a

23 13 13 9 16 13

1/1/1 1/1/1 3/1/1 1/0/1 1/0/1 1.5/0/1

1.66/1.32/1 1.88/1.37/1 2.59/1.08/1 1.19/0/1 1.13/0/1 1.51/0/1

2.71 2.70 2.69 2.73

3.55 3.52 3.65 3.58 3.54

Dominating phase bold.

series: the A samples were prepared from mixtures of Al[OiPr]3, H3PO4, and C12PO4 (in varying relative amounts), whereas no H3PO4 was employed for the syntheses of the B samples. 2.2. NMR. All NMR measurements were carried out using a Bruker ASX 400 spectrometer operating at a magnetic field B0 ) 9.4 T. The NMR frequencies are 400, 100, 102, and 161 MHz for 1H, 13C, 27Al, and 31P NMR, respectively. Two doubleresonant Bruker MAS probes were used with a 4 mm rotor system for MAS frequencies up to 14 kHz and a 2.5 mm rotor system for very fast MAS up to 35 kHz. For 1H NMR, spinning frequencies of 10 and 28 kHz were used with typically 32 scans and a 1 s repetition time. All spectra are referenced against TMS as for 13C NMR. The 13C crosspolarization MAS (CPMAS) measurements were carried out using about 1K scans, a 1 ms cross-polarization time, and highpower proton decoupling during data acquisition. Sample rotation frequencies of 10 and 28 kHz were used. The repetition time for the 31P NMR measurements was 20 s with 16-32 scans. The MAS frequencies were varied between 10 and 28 kHz, and proton high-power dipolar decoupling was required. Chemical shifts are referenced against a solution of 85% phosphoric acid. Finally, the 27Al MAS NMR spectra were recorded using a spinning frequency of 10 kHz with proton decoupling, acquiring typically 512 scans. These spectra are referenced against YAG as a secondary standard using the narrow signal of the 6-foldcoordinated Al site at 0.6 ppm. To separate all Al sites, 27Al multiple quantum MAS (MQMAS)32-34 was employed using the z-filter sequence with three rf pulses.34 The rotation frequency in these experiments was 10 kHz with a sweep width of the f1 triple quantum dimension of 66.7 kHz. The repetition time was 0.1 s, and pulse widths of 3.5 and 1.6 µs for the first two pulses were used at the B1 field strength that corresponds to 100 kHz. The third weak pulse was 8 µs long using a field strength of 20 kHz. A total of 256 t1 increments (15 µs incrementation time, hypercomplex data acquisition) was measured with 1K scans for each increment. The total measuring time was approximately 4 h. Additionally, 1H-31P HETCOR experiments were carried out using both the 4 and 2.5 mm probes with MAS frequencies of 12.5 and 28 kHz, respectively. The slower spinning speed was used to measure the spinning sideband patterns of the 1H resonances, which are obtained as cross-sections through the 31P peaks in the 2D NMR spectrum. Since strong spinning sidebands are usually the result of strong dipolar coupling between the protons, the presence of water molecules or methylene groups can be identified, whereas weak spinning sidebands can be attributed to protons in hydroxyl groups. A short cross-polarization time of 128 µs was used to ensure that the cross-polarization from protons to phosphorus atoms is restricted to distances of about 2-3 Å. The repetition times were 2 s, and for the 4 mm probe (12.5 kHz MAS frequency),

Figure 1. Effect of increased sample temperatures in the 31P MAS NMR spectra of samples A10 and B13 caused by different MAS frequencies on the 2.5 mm rotor system. Particularly the lines at -20.4 and -22.7 ppm (A10) and -10.0 and -10.6 ppm (B13) show a strong temperature dependence. This coalescence is caused by motional effects of the template molecules. The temperature effect is completely reversible.

an f1 sweep width of 150 kHz was chosen. A total of 96 t1 increments were acquired using 128-256 scans per increment. At a 12.5 kHz sample rotation frequency the resolution in the 1H spectra (slices along the f dimension) is not sufficient to 1 discriminate among the signals of water molecules, methylene groups, or hydroxyl groups by their isotropic chemical shift. The HETCOR experiment was therefore repeated using the 2.5 mm probe and a spinning frequency of 28 kHz, to enhance the resolution to separate the isotropic 1H resonances. Rotorsynchronized t1 incrementation was used to acquire the 2D data set. All other experimental parameters remained the same. 3. Results and Discussion 3.1. General Remarks. It is usually assumed that the heteronuclear dipole couplings between the protons (in hydroxyl groups, methylene groups, or water molecules) and the phosphorus or aluminum nuclei are suppressed sufficiently by MAS if spinning frequencies of about 10 kHz and higher are used. However, this assumption does not apply for these samples. High-power proton decoupling is required during the detection period. Otherwise a line broadening of about 2 is observed. Similar observations were made for the 27Al NMR spectra and, most importantly, for the MQMAS experiments. Proton decoupling is particularly needed in the triple quantum dimension f1. Hence, dipolar decoupling was used in all experiments. In addition to the MAS measurements at 10 kHz, we also used sample rotation frequencies of up to 30 kHz, particularly for measuring the 1H line shape and for the HETCOR experiments. According to the specification of the 2.5 mm Bruker MAS probe and various papers in the literature,35,36 the sample temperature varies with the rotation frequency. At rotation frequencies of 10, 20, and 28 kHz, the typical sample temperatures are approximately 300, 315, and 335 K, respectively. This means that especially for high spinning speeds the sample temperatures differ considerably from room temperature. The effect of a change in the sample temperature on the 31P and 13C NMR lines of typical samples is shown in Figures 1 and 2.

NMR Characterization of Aluminophosphates

Figure 2. Temperature dependence of the 13C CPMAS NMR spectra of the template molecule in samples A10 and B13. The NMR spectra of sample A10 do not depend on the temperature, whereas in B13 conformation changes of the alkyl chain are observed by the changed line shape at about 30-35 ppm.

Several lines of the 31P NMR spectra of samples A10 and B13 (Figure 1) show distinct changes of the isotropic chemical shifts with temperature. For the B13 sample the two right resonances at -11.5 and -12.5 ppm remain almost constant except that the line at -11.5 ppm shifts by about 0.5 ppm toward lower field at 28 kHz or about 335 K, which is negligible. However, the two lines at -10 and -10.6 ppm (10 kHz or about 300 K sample temperature) change completely with temperature. They merge to a single line found at -10.4 ppm when acquired at 28 kHz or about 335 K. Such an observation is typical for a beginning motional narrowing or exchange. Similar observations are made for sample A10. Although the resonances at -15.5, -24.4, and -26.5 ppm remain almost constant irrespective of the rotation frequencies and thus of the sample temperatures, the lines at -20.4 and -22.7 ppm merge together as seen in Figure 1. To prove that the line shifts are indeed caused by a temperature increase, a spectrum of sample A10 was recorded at 333 K with a rotation frequency of 10 kHz. The line positions are identical with those found in the spectrum measured at 28 kHz. Therefore, all spectral changes observed are due to a temperature increase at high spinning speeds. Additionally, the changes of the isotropic chemical shifts are completely reversible and are also observed in the 27Al spectra. Similar temperature effects can also be observed in the 13C NMR spectra of the samples (cf. Figure 2). It is interesting that the 13C CPMAS spectrum of sample A10 does not change irrespective of temperatures up to about 350 K. A similar behavior is found for sample A8. Contrary to this, the spectra of all other samples exhibit a temperature dependence of the methylene signals, which is exemplarily shown in Figure 2 for sample B13. Near room temperature the peak is at 32 ppm for all samples. However, a second peak at 30.6 ppm is observed at elevated temperatures, suggesting changes in the transgauche conformations of the alkyl chain of the template molecule in the samples as a function of the temperature. 3.2. 1D NMR Measurements. 3.2.1. 1H and 13C MAS NMR Spectra. The 1H MAS NMR line shapes also show specific differences between A10/A8 and A13/all B samples. Two typical spectra are shown in Figure 3. The MAS frequency is 28 kHz in both cases. Generally, the spectrum of sample A10 (and of A8) is much less resolved compared to that of B13, which is chosen as representative of the other samples. This observation can be explained by assuming that the template molecule is more mobile in the B samples and A13 than it is in A10 and A8.

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Figure 3. 1H MAS NMR spectra of samples A10 and B13 acquired at a 28 kHz MAS frequency. For B13 the narrow and strong resonances at 0.8 and 1.2 ppm correspond to the protons in the methyl and methylene groups. The weak resonance at 3.8 ppm is the signal of the methylene unit next to the phosphate headgroup of the template molecules (-CH2-OP unit). The lines are narrow because of a high internal mobility of the template molecule. The same resonances are found in all other samples, but in A10 they are not so well resolved due to restricted mobility. For further details see the text.

This assumption correlates with the temperature dependence of the 13C signals (section 3.1), where it is found that the methylene group signals for A10 (and A8) do not change with temperature whereas the same carbon signals show a temperature dependence for all B samples and A13. The 1H NMR line assignment is as follows: The strong and narrow peaks at 0.8 and 1.2 ppm are caused by the protons of the methyl and methylene groups of the template molecules, respectively. The signal of the methylene unit at the phosphate headgroups (-CH2-O-P-) is found at 3.8 ppm. Additionally, a weak peak occurs at 7.7 ppm for A10 (and A8), which can be attributed to P-OH groups. Later it will be shown by 2D HETCOR NMR that this assignment is correct, but for the moment this assignment is more intuitive as other arguments are not available from the 1H MAS NMR spectra. For the other samples this peak at about 7.7 ppm is missing. The fundamental question of whether water molecules and/or hydroxyl groups exist in the structure and where they are located cannot be answered from the 1H MAS NMR spectra. This kind of problem can be solved using HETCOR experiments (cf. section 3.3). It will also be shown that these NMR correlation experiments give indications of where the differently bonded protons reside in the structure of the lamellar aluminophosphates. The template molecules can of course be investigated directly by 13C NMR, and the spectra of samples A10 and B13 have already been shown in Figure 2. The assignment of the lines (cf. Table 2) is unambiguous and suggests that the template molecules remain intact during synthesis, at least the alkyl chain. However, there are obviously changes in the trans-gauche conformations as a function of the temperatures in all B samples and A13 in contrast to A10 and A8. 3.2.2. 31P and 27Al MAS NMR. The 31P MAS NMR spectra (MAS frequency 10 kHz) for the samples of the B series (B5, B7, and B13) and of the A series (A8, A10, and A13) are plotted on the right and left in Figure 4, respectively. Comparing the NMR spectra of all samples, the 31P resonances can be subdivided into three different groups. The first group (in the following called P-1) is formed by the narrow lines at about -10 ppm. These lines occur in the spectra of samples A13, B13, B7, and B5, and the peaks are at isotropic chemical shifts of -10, -10.6, and -11.5 ppm with a shoulder at about -12.5 ppm which is notably present in all B samples. Furthermore, but not belonging to this group P-1, there is a broad component in sample A13, probably caused by a residual amorphous phase. By fitting the 31P NMR spectrum of A10 with Gaussian lines, it is found that the isotropic chemical shift of the broad line is at about -15 ppm and that

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TABLE 2: Structural Assignment of the 13C NMR Signals of the Template Molecule chem shift (ppm) structure unit

14.6 -CH2-CH3

ca. 24 -CH2-CH2-CH3

ca. 32 -CH2-CH2-CH2-

65.1 -CH2-CH2-O-PO(OH)2

Figure 4. 31P MAS NMR spectra of all A and B samples. For detailed discussion see the text.

the phosphorus is distributed in equal amounts (1:1) between the narrow resonances and that broad signal. The second group of lines, P-2, is formed by the NMR resonances at -15.1, -20.4, and -22.7 ppm, which follows immediately by comparing the spectra of samples B7 and B5, where the relative intensity ratio (or phosphorus distribution) of the lines of groups P-1 and P-2 changes as a function of the concentration of the template solution (cf. Table 1). The signals of group P-2 are also found in the spectra of samples A10 and A8. Finally, the third group, P-3, is formed by the peaks at -24.4 and -26.5 ppm, which are only present in the two latter samples. It is also evident from the NMR spectra of samples A10 and A8 that the phosphorus distribution over the lines of groups P-2 and P-3 depends on the template concentration. These two group P-3 signals are obviously an indicator of a special structure of these two samples. There is a clear correlation between the appearance of these 31P signals and the fact that the 13C CPMAS NMR spectrum does not change with temperature (Figure 2) and that the line narrowing due to fast MAS is less pronounced for samples A8 and A10 (Figure 3). The 27Al MAS NMR spectra of these samples are shown in Figure 5 in the same manner as for the 31P NMR spectra. The 27Al NMR spectra of samples A10 and A8 differ completely from those of the other samples. Nevertheless, all 27Al resonances, which will be discussed now, can again be divided into three groups, Al-1, Al-2, and Al-3. The lines of group Al-1 are only present in samples A10 and A8. At first glance, two quite narrow resonances with peaks at 40.4 and -16.7 ppm seem to be present, which correspond to aluminum in 4- and 6-fold coordination. The first signal, assigned as Al-1a (the first signal of group Al-1; cf. Figures 5 and 6), is at 40.4 ppm and consists of a single line. This 27Al NMR signal is caused by 4-fold-coordinated Al(OP)4. The second signal of group Al-1 can only be resolved by 27Al MQMAS,32-34 as the 27Al MAS NMR line shape with its peak at -16.7 ppm is in fact a superposition of two different lines: the second group Al-1 signal (Al-1b) and the single Al-2a resonance (cf. Figure 6). According to the MQMAS spectrum, the quadrupole interac-

Figure 5. 27Al MAS NMR spectra of the samples. The nomenclature of the lines (Al-1a-Al-3c) is explained in the text.

Figure 6. Sheared 27Al MQMAS spectrum of sample A10. The Al signal at about -17 to -20 ppm (horizontal scale) consists of two separate signals, assigned as Al-1b and Al-2a, whereas the signal at -41 ppm is a single resonance. Their structural origin is explained in the text in detail.

tion is almost negligible for sites Al-1a and Al-2a, whereas small second-order effects are observed for the resonance Al-1b. In summary, group Al-1 consists of two different Al species present in samples A10 and A8: Al-1a belonging to an Al(PO)4 structural motif and Al-1b obviously corresponding to 6-foldcoordinated aluminum with phosphorus in the second coordination shell. Exclusively on the basis of these Al NMR spectra, the structure of site Al-1b cannot be identified unambiguously,

NMR Characterization of Aluminophosphates but it is plausible to assume that phosphorus tetrahedra, hydroxyl units, and water molecules coordinate this aluminum site. The second group, Al-2, consists of a single resonance, Al2a, which again has to be assigned as 6-fold-coordinated aluminum with phosphorus in the second coordination shell. The third group, Al-3, however, consists of three lines. They can best be identified in the spectra of samples A13 and B13 (Figure 5, bottom). These resonances are rather broad and asymmetric with peaks at about 7, -9, and -20 ppm. Such asymmetric and broad line shape features are typical for secondorder quadrupolar broadening effects including distributions of the quadrupole and chemical shift parameters. This means that in these aluminum-oxygen polyhedra the Al-O distances and O-Al-O bonding angles have certain distribution widths. The MQMAS line shapes of samples A13 and B7 are shown in Figure 7. All three lines, Al-3a, Al-3b, and Al-3c, are well resolved, and besides in A13 they also occur in the B samples, but the relative intensity ratios vary with the sample composition and concentration of the template solution. There is an obvious correlation of the 31P signals with the 27Al signals, which will be discussed in the Conclusions. The structural assignment of the group Al-3 signals is difficult and not yet clear, but the line at 7 ppm (Al-3a) may possibly be assigned to boehmite.37 The Al coordination number for the other two signals, Al-3b and Al-3c, is also assumed to be 6. 3.3. 2D 1H-31P HETCOR NMR and 31P Template Resonances. As mentioned previously protons are definitely present in the material in the form of methylene and methyl groups, as P-OH units, and of course as water. Unfortunately the resolution in the ultrafast 1H MAS spectra even at 28 kHz (cf. section 3.2.1) is not sufficient to distinguish all peaks completely and particularly to verify the existence of water molecules. Another problem refers to the assignment of the 31P NMR resonances for the A samples. In these samples two different phosphorus sources have been used for the synthesis: phosphoric acid and the phosphate headgroup of the template. For this reason, one important question arises: Which of the 31P NMR lines are caused by the template molecules and which correspond to aluminophosphate-like structures? These problems can partially be solved by comparing the 1D MAS NMR spectra. For example, in all B samples no phosphoric acid has been used in the synthesis. Hence, all the 31P signals of groups P-1 and P-2 (cf. section 3.2.2, Figure 4) are associated with the template because the 13C NMR spectra confirm that the template molecules are intact in all samples. However, since this is only an indirect approach, an independent proof is required. To this end 2D 1H-31P HETCOR measurements were carried out using a short cross-polarization time of about 100 µs. Such a short contact time ensures that the protons cross-polarize only the adjacent phosphorus atoms at a distance of less than about 200-300 pm. Therefore, the proton line shape (vertical cross-section taken at the various P resonances) is specific for the bonding scenario next to those phosphorus sites. 1H spin diffusion can be neglected for these short crosspolarization times in these samples as will be shown further below. The 1H-31P HETCOR spectra for sample A10 are shown in Figures 8 and 9 using MAS frequencies of 12.5 and 28 kHz, respectively. At the medium spinning speed of 12.5 kHz, where the spectral resolution is rather moderate, the 1H line shapes consist of a series of MAS spinning sidebands for protons with strong homonuclear dipole interaction between the protons (e.g., immobile water and methylene groups). Therefore, the signals of immobile water molecules and methylene groups in the spatial

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Figure 7. Comparison of the 27Al MQMAS spectra of samples B7 and A13. In both samples all three lines of group Al-3 are present but in different ratios. Additionally, B7 also shows the Al-2 group signal Al-2a. For details see the text.

proximity of phosphate units can be distinguished quite easily from the signals of hydroxyl units where a strong MAS center band is observed with weak spinning sidebands. These features are in fact observed for the various 1H crosssections or slices shown in the middle of Figure 8 for sample A10 using the short contact time of 100 µs. The two 1H slices at the bottom in the middle section are taken through the P-3 resonances at -24.4 and -26.5 ppm (cf. section 3.2.2). The peak position of the center band is at about 8 ppm, and strong MAS sidebands are observed. This proton line shape is obviously caused by water molecules, because the methylene proton resonances are at either 1.2 or 3.8 ppm (cf. section 3.2.1). Furthermore, these water molecules are at different distances from the two phosphate units. This conclusion follows from the projection on the f2 axis (31P dimension) (Figure 8, top). The cross-polarized P signal intensity is larger for the phos-

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Figure 9. 1H-31P HETCOR NMR spectrum (top) with the 31P and 1H skyline projections for sample A10 and a MAS frequency of 28 kHz. The 1H NMR spectra (cross sections at the various 31P signals) have the maximum resolution available at a spinning frequency of 28 kHz and are shown at the bottom. These spectra are explained in detail in the text.

Figure 8. 1H-31P HETCOR NMR spectrum (top) with the 31P and 1H skyline projection for sample A10 and a MAS frequency of 12.5 kHz. The 1H NMR spectra at the various 31P signals taken as crosssections (slices) are shown in the middle part for a short crosspolarization time of 100 µs, bottom. The three 1H slices at the bottom belong to the template resonances for a long cross-polarization time of 2 ms. The comparison of the line shapes for short and long crosspolarization times verifies that 1H spin diffusion is negligible for the short contact time such that these 1H spectra represent the chemical structures around the various 31P sites.

phorus site at -24.4 ppm compared with the -26.5 ppm line.

Other 1H-31P HETCOR signals are not found for these group P-3 31P lines, which means that these sites have nothing to do with the template molecule. Due to the isotropic chemical shifts of -26.5 and -24.4 ppm, these signals are attributed to P(OAl)4 structural units, and the phosphorus bound in these sites stems from the phosphoric acid. Due to the overall weak CP efficiency, it is assumed that water molecules are not in immediate spatial proximity of these units, which means that they do not coordinate these phosphate tetrahedra. The three upper 1H slices in the middle of Figure 8 which are taken at -15.1, -20.4, and -22.7 ppm (second group of 31P lines) are completely different. First of all there is a single line without strong spinning sidebands again at 8 ppm. This line is caused by the P-OH units. Also, with decreasing 31P chemical shifts, the intensity of this 1H line decreases as well, which is interpreted as the result of different numbers of hydroxyl units per headgroup. It is assumed that there are two, one, and no hydroxyl groups at the three template resonances at -15.1, -20.4, and -22.7 ppm, respectively. Such a result

NMR Characterization of Aluminophosphates can only be obtained by this 2D HETCOR experiment, and it is most valuable for interpreting the bonding of the template molecule to the inorganic layer. Besides these OH- resonances at 8 ppm, a shoulder at about 4 ppm is observed which has intense spinning sidebands. This signal is caused by the methylene protons in the CH2-O-P group next to the phosphate headgroup and is an independent proof that all 31P MAS lines at -22.7, -20.4, and -15.1 ppm belong to the phosphate headgroup of the template molecule and that the phosphate group is chemically bonded to the layer. To verify that 1H spin diffusion is not important for the short contact time of 100 µs, the same HETCOR spectrum has been acquired for a contact time of 2 ms. Now the 1H cross-sections of the template resonances at -22.7, -20.4, and -15.1 ppm are in fact equal due to the spin diffusion as shown by the spectra at the bottom of Figure 8. Generally, the ultimate resolution in the 1H dimension is not achieved for the 12.5 kHz spinning frequency. For measuring the correct isotropic 1H chemical shifts, the 1H-31P HETCOR experiment was repeated at a MAS frequency of 28 kHz under otherwise identical conditions (Figure 9). All previous conclusions are supported by this spectrum, showing the improved resolution in the 1H dimension. Particularly, the 1H lines at about 8 and 4 ppm are now well resolved. Most notably though, the water signal (at the 31P slices at -24.4 and -26.3 ppm) and the P-OH- resonance (slices at -15.5 and around -21 ppm) have an identical isotropic 1H shift of about 8 ppm. A last comment refers to the 1H line intensities at about 4 ppm (-CH2-O-P methylene proton signal) and 1.2 ppm (protons of the methylene groups in the alkyl chain) at the 31P peaks at -15.5 and near -21.5 ppm as shown in Figure 9. The relative ratio of their peak heights taken at 4 and 1.2 ppm is always about 2:1. The 2D HETCOR spectrum of sample B7 acquired at a MAS frequency of 28 kHz is shown in Figure 10. In B7, all 31P lines of groups P-1 and P-2 are present and, therefore, all 1H correlation peaks occur in a single 2D spectrum. The correlation peaks of P-1 (31P shift around -10 to -12 ppm) are on the left-hand side in Figure 10, whereas the peaks correlating the 1H signals with the 31P resonances of group P-2 (-15.3, -20.8, and -22.3 ppm) are found in the center and on the right. The corresponding 1H cross-sections are shown in the bottom part of Figure 10. Due to the increased mobility of the template molecules, the resolution of the spectra is much better than for sample A10 for the same spinning frequency. First of all the 1H-31P cross-peaks of group P-2 shall be discussed (Figure 10, bottom). Compared with A10, the spectra are slightly better resolved, but in general this part of the 2D spectrum consists of the same cross-peaks. A new feature is that the P-OH peak at about 8 ppm (1H dimension) seems to consist of two signals with slightly different chemical shifts (ca. 7.5 and 8.2 ppm). A structural interpretation cannot be given yet. Contrary to this the correlation peaks for group P-1 are much narrower. The corresponding cross-sections in Figure 11 show very sharp 1H peaks at isotropic chemical shifts of 3.5 and 1.3 ppm for all 31P resonances of the template molecules at about -10 ppm. However, compared with sample A10 the relative intensity ratio between the resonances at 3.5 and 1.3 ppm is 1:1 instead of 2:1. Furthermore, three of the 31P template resonances of group P-1 (the two lines merged at -10.4 ppm, see section 3.1.2, and the single line at -11.9 ppm) possess another broad 1H correlation signal with a chemical shift of about 6.5-7 ppm. This correlation signal, however, is not present for the 31P cross-section at -12.5 ppm (cf. the third

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Figure 10. 1H-31P HETCOR NMR spectrum (top) with the 31P and 1 H skyline projection for sample B7 and a MAS frequency of 28 kHz. The 1H NMR spectra taken as cross-sections (slices) at the various 31P signals are shown underneath. For details see the text.

slice from the top in Figure 10). It is worth noting that in none of these 1H cross-sections is a signal found at 8 ppm, which would correspond to P-OH groups. To interpret the 1H line at 6.5 ppm, it is useful to check the 1H MAS NMR line shape (Figure 3). As the 6.5 ppm peak cannot unambiguously be identified in the 1H MAS line shape, the 1H-31P HETCOR experiment was repeated at 12.5 kHz for measuring the MAS spinning sidebands. Instead of the full 2D spectrum, only the 1H cross-sections taken at the 31P chemical shifts of -11.7 and -12.5 ppm are compared in Figure 11. These two 31P chemical shifts have been chosen for taking the cross-sections because at -11.7 ppm (31P) the 6.5 ppm 1H cross-peak is present, but it is not found at -12.5 ppm. From the calculated difference plot of these two slices, the 1H NMR spectrum for the 6.5 ppm resonance with its spinning sidebands remains. This spectrum is shown in Figure 11 at the bottom. As a result strong MAS spinning sidebands are found for the

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Schulz et al. TABLE 3: Summary of the Results of the X-ray and NMR Investigations of the Various A and B Samplesa sample

A8b

A10

A13b

B5

B7

B13

d spacing (nm) 3.55 3.52 2.71, 3.65 2.70, 3.58 2.69, 3.54 2.73 31P group P-2, P-3 P-2, P-3 P-1 P-1, P-2 P-1, P-2 P-1 27Al group Al-1, Al-2 Al-1, Al-2 Al-3 Al-3, Al-1 Al-3, Al-1 Al-3 a For details see the text. Bold numbers indicate the dominating phase. b In samples A8 and A13 a broad 31P signal is observed besides the groups P-2 and P-3 resonances (for A8) and besides the group P-1 resonances (A13). These extra signals are not considered further.

Figure 11. Comparison of the 1H MAS NMR spectra taken as crosssections at 31P chemical shifts of -11.7 and -12.5 ppm from the 1 H-31P HETCOR NMR spectrum of sample B7 at a MAS frequency of 12.5 kHz. The difference trace (bottom spectrum) between the two yields the 1H signal with an isotropic chemical shift of 6.5 ppm (see the text). 1H

line at 6.5 ppm, and it must be concluded that either immobile water molecules are present in spatial proximity to the phosphate headgroup or that “clustered” hydroxyl units may be present. 4. Conclusions

In summary, it has been shown that the MAS NMR spectra provide a number of significant results on the structure of both the template molecule and the inorganic layer. For example, the number of different phosphate units in the structure can be determined, and by the comparison of the 31P MAS NMR spectra for different sample compositions, all 31P resonances can be divided into three groups, P-1, P-2, and P-3. As for the B series of samples no phosphoric acid has been used in the synthesis, and since both 1H MAS NMR and 13C CPMAS spectra show that the template molecules remain intact in all samples, the lines of groups P-1 and P-2 must belong to the template headgroup, although they have very different chemical shifts. The resonances of group P-3, however, are very specific, and because of their chemical shifts, it is likely that they represent Al(PO)4-like structures. Furthermore, the number of different aluminum sites and their coordination numbers can be determined by 27Al MAS NMR and MQMAS, and as valid for the 31P spectra, the 27Al signals can again be divided in three groups, Al-1, Al-2, and Al-3. The corresponding 27Al signals are very different. Although the Al1a, Al-1b, and Al-2a lines are very narrow, all group Al-3 lines are broader and asymmetric, which is caused by distributions of the quadrupolar and chemical shift parameters. Although these 1D MAS NMR data yield sometimes only limited information of the exact chemical structures and, particularly, of the bonding scenario of all specified units, a comparison between the appearance of the 27Al and 31P resonance signals in the various samples and the d spacing of the samples is most interesting (cf. Table 3). From the 31P and 27Al MAS NMR spectra (Figures 5 and 6) it is immediately evident that the 27Al signals of group Al-3 occur only if simultaneously the 31P signals of group P-1 are present in the samples. Furthermore, the 31P signals of group P-2 correlate only with the single Al-2 group resonance, and finally, the Al-1 peaks are only found with the P-3 lines. And last, but not least, there is a correlation between the d spacings measured by X-ray diffraction and the various NMR resonances.

For example, short d spacings correlate with the P-1 and Al-3 signals, whereas the larger d spacings occur only with the 31P signals of P-2 and/or P-3 in combination with the corresponding Al-2 and Al-1 resonances. Regardless of these very important results, various questions remain unsolved on the basis of these investigations. For example, for a number of 27Al and 31P signals the atomic structure of the corresponding groups is not clear. Another point is that, although the correlation of certain 27Al and 31P signals is obvious, it is not clear which connectivities exist between these structural units and where and how water molecules and hydroxyl units are bonded in the structure. Also, the incorporation of the template molecule into the layer structure is still to be investigated. This is the aim of further NMR measurements using 2D HETCOR techniques such as 1H-27Al and 27Al-31P HETCOR NMR and 31P double quantum NMR. Acknowledgment. We thank the Deutsche Forschungsgemeinschaft (Grants Ja 552/9-1,2,3, Fr 1372/1-1, and Fr 1372/ 5-1) and the Fonds der Chemischen Industrie for generous support. M.T. thanks the Freie und Hansestadt Hamburg for a Ph.D. scholarship. References and Notes (1) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli J. C.; Beck, J. S. Nature 1992, 359, 710-712. (b) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843. (2) (a) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. Nature 1994, 368, 317-321. (b) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D.; Chem. Mater. 1994, 6, 1176-1191. (3) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014-2017. (4) Ciesla, U.; Schacht, S.; Stucky, G. D.; Unger, K. K.; Schu¨th, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 541-543. (5) (a) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 426-430. (b) Antonelli, D. M.; Nakahira, A.; Ying, J. Y. Inorg. Chem. 1996, 35, 3126-3136. (6) Fro¨ba, M., Muth, O.; Reller, A. Solid State Ionics 1997, 101-103, 249-253. (7) Tian, Z.-R.; Tong, W.; Wang, J.-Y.; Duan, N.-G.; Krihnan, V. V.; Suib, S. L. Science 1997, 276, 926-930. (8) Wong, M. S.; Ying, J. Y. Chem. Mater. 1998, 10, 2067-2077. (9) Fro¨ba, M., Muth, O. AdV. Mater. 1999, 11, 564-567. (10) Ciesla, U.; Fro¨ba, M.; Stucky, G. D.; Schu¨th, F. Chem. Mater. 1999, 11, 227-234. (11) Fro¨ba, M.; Oberender, N. Chem. Commun. 1997, 1729-1730. (12) Bonhomme, F.; Kanatzidis, M. G. Chem. Mater. 1998, 10, 11531159. (13) Neeraj; Rao, C. N. R. J. Mater. Chem. 1998, 8, 279-280. (14) MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 397, 681-684. (15) Wilson, S. T.; Lok, B. 1M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146-1147. (16) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1984, 106, 6092-6093. (17) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. Nature 1988, 331, 698-699. (18) Hartmann, M.; Kevan, L. Chem. ReV. 1999, 99, 635-663.

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