by Solid-State NMR - American Chemical Society

Article pubs.acs.org/JPCC

Accurate Structural Description of the Two Nanoporous Fluorinated Aluminophosphates ULM-3(Al) and ULM-4(Al) by Solid-State NMR Charlotte Martineau,‡,* Boris Bouchevreau,‡ Renée Siegel,† Jürgen Senker,† Alenka Ristić,§ and Francis Taulelle‡ ‡

Tectospin − Institut Lavoisier de Versailles, CNRS UMR 8180, Université de Versailles Saint-Quentin en Yvelines, 45 avenue des États-Unis, 78035 Versailles Cedex, France † Anorganische Chemie III, Universität Bayreuth, Universitätsstr. 30, 95447 Bayreuth, Germany § National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia S Supporting Information *

ABSTRACT: This work reports an investigation of the localization of F atoms and OH groups in the two fluorinated aluminophosphates ULM-3(Al) and ULM-4(Al) by multinuclear and multidimensional high-resolution solidstate magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy. First, in ULM-3(Al), we show that the compound is fully fluorinated. A whole set of state-of-the-art homonuclear and heteronuclear 2D NMR experiments (19F−31P, 19F−19F, 31P−31P, 27Al−31P, and 1H−31P) have been recorded, yielding a unique line assignment of all the 31P and 19F resonances to the corresponding crystallographic sites. In a second part, ULM-4(Al) is shown to be a quite remarkable sample because it contains both fully and partially fluorinated building units. The structural differences between ULM3(Al) and ULM-4(Al) are finally discussed in the light of the results in terms of potential templating role of water.

1. INTRODUCTION Nanoporous aluminophosphates (AlPO4)1,2 have been extensively studied, in particular, for applications in molecular sieving.3,4 These materials usually are synthesized in the presence of one or several structural directing agents (SDAs), which play a large role in the final architecture of the AlPO4. Fluoride ions are also often added in the synthesis media because they are believed to act as mineralizing agents, favoring the framework formation of AlPO4 solids. The fluoride ions sometimes are incorporated in the final AlPO4 network, leading to fluorinated aluminophosphates.5−36 The localization of the fluoride ion in porous solids, which can either be connected to the framework or be trapped in pores or cages, can be difficult. A combination of X-ray diffraction studies and nuclear magnetic resonance (NMR) spectroscopy is therefore most often required to determine accurately their position, as was shown in microporous fluoride-containing zeolites37−43 or fluoroaluminophosphates.9,21,36,44−47 Although it has been much less studied, water is also known to be a potential co-SDA agent, as was, for example, reported in the metal−organic-frameworks MIL-5048 or MIL-74.49 The two highly similar fluorinated aluminophosphates Al3(PO4)3F2•N2C4H1418 (ULM-3(Al), with ULM standing for Université Le Mans) and Al3(PO4)3F2•N2C4H14•0.5H2O32 (ULM-4(Al)) are case studies to better understand the potential templating role of water. Chemically, these two samples indeed differ only by the presence of water molecules © 2012 American Chemical Society

in the latter. Structurally, if the building units (BUs) are also similar, then their assembly differs quite significantly, leading to two distinct 3D periodic frameworks, as was shown by singlecrystal X-ray diffraction.13,32 The presence of hydroxyl groups in the BUs has not been mentioned in these samples, partially because the isoelectronic OH groups and F atoms can hardly be distinguished from X-ray diffraction (single-crystal or powder) data. However, they are a trace of the complex processes that occur during crystallization; therefore, their accurate localization in the final crystal structure is essential. For example, in the aluminophosphate AlPO4−CJ2,6,9 an F/OH distribution exists on one of the crystallographic positions. We recently have shown through a solid-state NMR study that the F/OH distribution is random (i.e., the F and OH groups are not found in separate domains),50 which was the key information to understand the crystal formation of this sample.44 To be able to address such situations in fluorinated AlPO4s, it is essential to decrease the observation scale to the atomic level. Because of both its chemical and atomic sensitivity, solid-state NMR is an ideal tool to complement diffraction techniques and investigate such fine local distributions. 31P nucleus is a probe of choice because the 31P chemical shifts in these samples are extremely sensitive to the presence of water molecules or OH Received: August 13, 2012 Revised: September 15, 2012 Published: September 17, 2012 21489

dx.doi.org/10.1021/jp308056j | J. Phys. Chem. C 2012, 116, 21489−21498

The Journal of Physical Chemistry C

Article

groups in the surroundings of the phosphorus atoms.50 However, two major problems for the NMR data analysis may be encountered: (i) the resolution of the 31P resonances is often strongly reduced by heteronuclear dipolar couplings to the highly abundant and sensitive 1H, 19F, or 27Al nuclides and (ii) the assignment of the 31P resonances can be difficult, in particular, when the number of resonances of identical intensity is large. To overcome these difficulties, we have developed a special NMR51 probe that can be simultaneously tuned to the four frequencies of 1H, 19F, 31P, and 27Al (at 11.7 T magnetic field). With this probe, 19F−31P magnetization transfers can be achieved under 1H and 27Al decoupling for optimum resolution. We chose to perform the line assignment by analysis of a series of 2D NMR correlation spectra. In this work, we have investigated the localization of F atoms and OH groups in the two fluorinated aluminophosphates ULM-3(Al)13 and ULM-4(Al)32 by multinuclear and multidimensional high-resolution solid-state magic-angle spinning (MAS) NMR. In ULM-3(Al), we show that the compound is fully fluorinated. We have recorded a whole set of state-of-theart homonuclear and heteronuclear 2D NMR experiments (19F−31P, 19F−19F, 31P−31P, 27Al−31P, and 1H−31P) to progressively reduce the possible number of line assignments to a unique possibility for the 31P and 19F resonances. We show that ULM-4(Al) is a quite remarkable sample because it contains both fully and partially fluorinated BUs. The structural differences between ULM-3(Al) and ULM-4(Al) and the potential templating role of water are finally discussed.

The 19F double-quantum single-quantum61 (DQ-SQ) NMR spectra were recorded using the dipolar homonuclear homogeneous Hamiltonian (DH3) experiment.62 The recycle delay was 15 s for ULM-3(Al) and 30 s with presaturation train pulses for ULM-4(Al). The 31P POST-C763 NMR spectra were recorded at MAS frequency of 10 kHz. 1H→31P CP transfer was set prior to the 31P DQ excitation to enhance the 31P signal and reduce the recycle delay to that of 1H (relaxation time T1(1H) below 1 s for both samples). All DQ-SQ NMR spectra were sheared64,65 so that they read like usual SQ-SQ NMR spectra. The 1H and 1H→31P CPMAS NMR spectra were run at MAS of 10 kHz on an Avance III Bruker 400 spectrometer (static magnetic field B0 = 9.4 T, Larmor frequencies of 400.1, 161.7, and 100.6 MHz for 1H, 31P, and 13C, respectively) using a 4 mm double-resonance probe. The 1H MAS NMR spectra were recorded using a XX̅ supercycled version of DUMBO66,67 decoupling shape, with RF field ∼100 kHz and pulse length of 30 μs. The phase-shifted recoupling effects a smooth transfer of order (PRESTO-II68,69 block) was used to generate the 2D 1 H→31P HETCOR NMR spectra. The R1825 recoupling form was used with RF field ∼90 kHz and recoupling duration optimized to 155 μs. 1H DUMBO decoupling was applied in the indirect t1 dimension using the parameters mentioned above. 1H SPINAL-64 decoupling was applied during the 31P signal acquisition (pulse length 4.8 μs, RF field ∼95 kHz). 210 t1 slices with 32 transients each were recorded. In the 2D NMR experiments, phase-sensitive detection in the indirect dimension was obtained using the States70 or StatesTPPI71 method. The 19F, 27Al and 31P chemical shifts were referenced to CFCl3, H3PO4 85%, and a 1 M solution of Al(NO3)3, respectively. The 1H and 13C chemical shifts were referenced to proton and carbon signals in TMS. The 15N chemical shifts were referenced using the nitrogen signal from 15 N-enriched glycine (−347.5 ppm from nitromethane).72 The NMR spectra were reconstructed using the Dmfit73 software. All of the missing experimental details are provided in the Supporting Information.

2. MATERIALS AND METHODS 2.1. Sample Preparation. ULM-3(Al) was synthesized by hydrothermal synthesis, as described in ref 13. ULM-4(Al) was also hydrothermally synthesized according to ref 32 but with slightly different conditions of reaction time and temperature (190 °C for 19 h). The purity of the samples was verified by powder X-ray diffraction. 2.2. NMR Experiments. Most experiments were performed on an Avance 500 Bruker spectrometer (static magnetic field B0 = 11.7 T, Larmor frequencies of 500.1, 470.6, 202.5, 130.3, 125.7, and 50.7 MHz for 1H, 19F, 31P, 27Al, 13C, and 15N, respectively) using a quadruply tuned51 (1H, 19F, 27Al, 31P) 2.5 mm probe, a 3.2 mm triple-resonance (1H, 27Al, 31P), or a 4 mm double-resonance probe for 13C and 15N experiments. The 27Al multiple-quantum MAS (MQMAS) NMR spectra were recorded using the three-pulse z-filter sequence.52 A 2-D Fourier transformation followed by a shearing53 transformation yielded pure absorption 2D NMR spectra. For the 19F→31P cross-polarization (CP) heteronuclear correlation (CP-HETCOR)54 NMR spectra, 1H 64-step small-phase incremental alteration (SPINAL-64)55 decoupling was applied during the whole experiment, and 27Al rotor-asynchronized multipulses (RA-MP) 56 decoupling was applied during the direct acquisition of the 31P signal. The 27Al→31P MQ-D-R-INEPT57 MAS (20 kHz) NMR spectra were recorded using the N = 1 rotary resonance recoupling (R3) sequence,58−60 for excitation and reconversion of the 27Al−31P heteronuclear coherences. R3 durations were optimized to τ = 600 μs. The MQMAS block used the same parameters, as mentioned above. In this experiment, no shearing transformation in the 27Al indirect dimension is necessary;57 however, the apparent Larmor frequency was changed to achieve the universal ppm scale in the 27Al isotropic dimension.

3. RESULTS AND DISCUSSION 3.1. ULM-3(Al). The structure of ULM-3(Al)13 can be described with two hexameric building units (hBU), each one being constructed from an alternation of three PO4 tetrahedra, two five-fold (AlV) and one six-fold (AlVI) coordinated aluminum atoms (Figure 1a). The aluminum polyhedra are connected to each other by a corner, which is occupied by a fluoride atom in ULM-3(Al). The hBU1 and, respectively, hBU2, are connected to each other, forming sheets in the (b,c) plane, as shown in Figure 1b. hBU1 and hBU2 sheets are alternately stacked along the a-axis, generating 3D pores in which the diaminopropane (DAP) molecules are inserted (Figure 1c). The 31P MAS NMR spectrum of ULM-3(Al) is show in Figure 2a. Because of 31P−27Al and 31P−1H dipolar couplings not removed by the MAS, the lines are broad (Figure 2a, i). The resolution can be greatly improved under 1H/27Al doubleresonance decoupling (Figure 2a, iii), and the six 31P resonances of equal intensity expected from the structure can be resolved (Table 1). It should be mentioned that additional 19 F decoupling did not improve the 31P spectral resolution. The narrow 31P line width observed strongly suggests that the two hBUs in ULM3-(Al) are fully fluorinated. The 19F MAS NMR 21490



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Table 1. 31P Line, Isotropic Chemical Shift δiso (±0.1 ppm), Relative Intensity (Eq. P), and the Two Possible Assignment Sets in ULM-3(Al) line

δiso (ppm)

Eq. P

assignment 1

assignment 2

Pa Pb Pc Pd Pe Pf

−7.2 −8.4 −14.1 −14.9 −17.6 −18.0

1 1 1 1 1 1

P3 P6 P1 P4 P5 P2

P6 P3 P4 P1 P2 P5

spectrum, recorded under 1H decoupling for optimum resolution (Figure 2b) contains the expected four resonances of equal intensity (Table 2). The 27Al NMR spectrum contains Table 2. 19F Line, Isotropic Chemical Shift δiso (± 0.1 ppm), Relative Intensity (Eq. F), and the Two Possible Assignment Sets in ULM-3(Al) line

δiso (ppm)

Eq. F

assignment 1

assignment 2

Fa Fb Fc Fd

−114.5 −116.9 −117.8 −120.7

1 1 1 1

F1 F4 F2 F3

F2 F3 F1 F4

signals centered on 10 and −25 ppm, corresponding to AlV and AlVI atoms, respectively. In the indirect dimension of the 27Al MQMAS NMR spectrum (Figure 3), two of the four AlV resonances can be distinguished (Table 3). However, the two AlVI sites are not resolved. One of the major issues in such compounds is the assignment of the NMR lines to the corresponding atoms in the structure. This step is all the more difficult in ULM-3(Al) as (i) most resonances have the same relative line intensity and ii) the structure is highly symmetrical, thus the correlation patterns are also similar. Considering the six P, four F and six Al inequivalent atoms, there are 6! x 4! x 6! = 12 × 106 possibilities to assign the 31P, 19F and 27Al NMR spectra. To progressively reduce the number of possible assignments, we have carried out a whole set of 2D NMR experiments (19F−31P, 19F−19F, 31 P−31P, 27Al−31P and 1H−31P), whose results are presented in the following section. In the 2D 19F−19F DQ-SQ NMR correlation spectrum of ULM-3(Al), two pairs of cross-peaks can be readily identified

Figure 1. Representation of the building units (a) hBU1 and (b) hBU2 in ULM-3(Al), on which atoms are labeled. (c) Projection of the structure in the (a,b) plane, showing the alternate stacking of the hBU1s (yellow) and hBU2s (light green). (d) Projection of the structure in the (b,c) plane, showing the pores filled with the DAP molecules.

Figure 2. (a) 31P MAS (10 kHz) NMR spectra of ULM-3(Al) recorded (i) without decoupling, (ii) with 1H decoupling, and (iii) with 1H/27Al double resonance decoupling. The reconstructed spectrum is show in (iv). Star indicates the presence of an unknown impurity. (b) Experimental (i) and reconstructed (ii) 19F MAS NMR spectrum of ULM-3(Al) recorded under 1H decoupling. 21491



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intensity between the lines corresponding to Pa, Pc, and Pe are observed, indicating that they belong to the same hBU. On the other hand, cross-peaks between Pb, Pd, and Pf are observed, showing that these P atoms belong to the second hBU. Additional peaks of lower intensity are observed between most 31 P resonances. They correspond to longer-range P−P distances and indicate the connectivities between the hBUs. The 19F→31P 2D correlation NMR spectrum of ULM-3(Al), shown in Figure 5, allows the distinction of the P and F atoms

Figure 3. 27Al MQMAS NMR spectrum of ULM-3(Al). Top spectrum is the full projection of the MAS 27Al dimension. Right spectrum is the full projection on the isotropic dimension.

Table 3. 27Al Line, Relative Intensity (Eq. Al), and the Two Possible Assignment Sets in ULM-3(Al) line

Eq. Al

Ala Alb Alc Ald Ale

1 1 1 1 2

assignment 1

assignment 2

Al2 Al5 Al6 Al3 Al1

Al5 Al2 Al3 Al6 Al1

or Al3 or Al6 or Al5 or Al2 and Al4

or Al6 or Al3 or Al2 or Al5 and Al4

Figure 5. 19F→31P CP-HETCOR NMR correlation spectra of ULM3(Al) recorded with 1H decoupling in the indirect 19F dimension and 1 H/27Al double resonance decoupling in the direct 31P dimension.

(Figure 4a); they indicate the spatial proximities (F−F distances below 4 Å) between the F atoms from a given hBU: the fluorine resonances Fa and Fc belong to one of the hBUs, whereas Fb and Fd belong to the second hBU. In the 2D 31 P−31P DQ-SQ NMR correlation spectrum of ULM-3(Al), displayed in Figure 4b, the phosphorus atoms forming the two hBU units can be distinguished: correlation peaks of strong

forming the two hBUs: one can observe the correlation of strong intensity peaks between Pa, Pc and Pf and Fa and Fc, which therefore belong to the same hBU, and strong correlation peaks among Pb, Pd, and Pe and Fb and Fd, which thus form the second hBU. Additional peaks of lower intensity can be observed, which corresponds to P−F proximities between neighboring hBUs. In particular, Pa and Pb have cross-

Figure 4. Sheared (a) 19F−19F DH3 and (b) 31P−31P POST-C7 2D NMR correlation spectra of ULM3-(Al). Thick lines correspond to the DQ diagonal of slope 1. Dashed lines indicate F−F or P−P cross-correlations. Top spectra, on which lines are labeled, are the full projections on the 19F and 31P SQ dimensions. 21492



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Figure 6. 31P−27Al 2D correlation NMR spectra of ULM-3(Al) recorded with the (a) 31P→27Al CP-HETCOR experiment and (b) 27Al→31P MQD-R-INEPT experiment. Right spectra are the full projections on the 27Al dimension.

Therefore, additional information was sought from the template DAP molecules. The 13C CPMAS NMR spectrum is given in the Supporting Information, in which the four inequivalent NH3-CH2 carbon resonances are resolved. More interesting is the 1H MAS NMR spectrum. Under homonuclear decoupling (Figure 7a), five lines can be resolved: the three lines in the 1−5 ppm region correspond to the CH2 protons, whereas the two lines in the 7−9 ppm range correspond to the protons from the NH3. The two NH3 lines have a 3:1 ratio, thus one of the four inequivalent amines has a significant different environment from the others. A 1H→31P 2D correlation NMR spectrum was recorded using the PRESTOII68 block to ensure that only the closest protons from the phosphorus atoms will transfer magnetization. On this spectrum, one can notice that the protons from the NaH3 groups have a cross peak with both Pa and Pb, whereas the NbH3 protons have a single cross peak with Pb. Pa and Pb correspond to P3 and P6 (see above) and are, according to the structural data, in the proximity of the N2 and N4 atoms. Now, N2 is close to both P6 and P3, whereas N4 is only close to P3 (Table 4). Therefore, the NbH3 resonance corresponds to N4 and the correlated P atom corresponds to P6. This in turn provides a unique assignment of all four 19F and six 31P resonances to the corresponding crystallographic sites in ULM3(Al). Only an ambiguity remains for the assignment of the AlV resonances. This analysis has allowed a drastic decrease in the number of possible assignments from 12 × 106 to a unique set for the 19F and 31P resonances and four possibilities for the 27Al lines (Tables 1−3). 3.2. ULM-4(Al). The structure of ULM-4(Al)32 is very close to that of ULM-3(Al) and can also be described with two inequivalent hBUs build up from alternated PO4 tetrahedra, AlV and (AlVI) aluminum polyhedra (Figure 8a). The aluminum polyhedra are connected by corners. This corner position which, in the initial structure was believed to be fully occupied by a fluoride atom.32 The hBU1 units, respectively, hBU2 units, are connected to each other, forming sheets in the (b,c) plane (Figure 8b); the connectivity of the hBUs that generates the 3D structure is, however, different from that in ULM-3(Al). The stacking of the hBUs forms pores in which DAP and water

correlation peaks of significant intensity with Fa and Fd, which belong to the other hBU. This indicates that these two P atoms ensure the connection between the inequivalent hBUs. In agreement with the crystal structure, Pa and Pb are thus assigned to P3 and P6 (Table 1). The distinction between Pc/ Pd and Pe/Pf can be achieved by noticing on the 31P−31P 2D NMR correlation spectrum that Pc and Pd have a crosscorrelation peak of strong intensity, whereas the correlation peak between Pe and Pf is of much weaker intensity; that is, the Pc−Pd distance is much shorter than the Pe−Pf distance. Therefore, on the basis of the P−P distances in ULM-3(Al) (see the Supporting Information), the resonances Pe and Pf correspond to P2 and P5 and Pc and Pd correspond to P1 and P4 (Table 1). This data analysis shows the presence of two correlated sets of 31P and 19F resonances; one corresponds to the atoms forming hBU1s, and the second corresponds to the atoms forming hBU2s. It is so far, however, not possible to assign the sets to a given hBU because the F−F, P−P, and F−P correlation patterns are identical for the two units. To reduce further the possible line assignment, we carried out a 27Al−31P 2D NMR experiment. The 31P→27Al CPHETCOR NMR spectrum of ULM-3(Al) is displayed in Figure 6a. Although the high-resolution was ensured in the 31P dimension under 1H/27Al decoupling, the resolution remains poor in the 27Al dimension due to the large 27Al second-order quadrupolar interaction that broadens the resonances. Therefore, we have used a recently developed method,57 the 27 Al→31P MQ-D-R-INEPT sequence, which allows the removal of the quadrupolar anisotropic interaction in the indirect 27Al dimension, providing in ULM-3(Al) a 27Al−31P 2D NMR correlation spectrum with high resolution in both dimensions (Figure 6b). In this spectrum, the two AlVI resonances are still not resolved; however, the four AlV resonances can be distinguished. From the Al−P cross-peaks, the five-fold coordinated aluminum of the two hBUs can be distinguished: Ald and Alb belong to the hBU formed by Pa, Pc, and Pf, whereas Ala and Alc belong to the hBU formed by Pb, Pd, and Pe. However, again, the correlation patterns are too similar to allow assignment to hBU1 or hBU2. 21493



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Figure 7. (a) Experimental (i) and reconstructed (ii) 1H MAS (10 kHz) NMR spectrum of ULM-3(Al). The spectrum was recorded with wDUMBO decoupling. (b) 2D 1H→31P CP-PRESTO-II NMR correlation spectrum of ULM-3(Al) recorded with DUMBO decoupling in the indirect proton dimension. The protons from the NH3 resonances are labeled. Dashed lines indicate selected protonphosphorus proximities.

Figure 8. Representation of the building units (a) hBU1 and (b) hBU2 in ULM-4(Al), on which atoms are labeled. (c) Projection of the structure in the (a,c) plane resulting from alternate stacking of the hBU1 (yellow) and hBU2 (light green) sheets. The pores are filled with the DAP and water (red balls) molecules.

discrete distribution of 31P chemical shifts has already been observed in the microporous fluorinated aluminophosphate AlPO4−CJ2, and was assigned to the occurrence of OH/F distribution on a bridging position shared by two aluminum polyhedra.44,50 Four main peaks of similar intensity are observed on the 19F MAS NMR spectrum of ULM-4(Al) (Figure 9b), separated in two sets of two peaks. Finally, the 27Al NMR spectrum (Figure 10) shows the presence of at least two AlVI and three AlV atoms. The homonuclear 2D 19F−19F and 31P−31P DQ-SQ NMR spectra are displayed in Figure 11a and Figure 11b, respectively. Again, the 19F resonances of the two hBUs can be separated in the 19F−19F 2D NMR spectrum: lines Fa and Fc belong to one hBU, while lines Fb and Fd belongs to the second type of hBU. One can notice that the Fb resonance contains at least two contributions. This discrete distribution of 19F chemical shifts can be correlated to the distribution observed in the 31P NMR spectrum. The P atoms constituting the two inequivalent hBU can in turn be distinguished in the 31P−31P 2D NMR spectrum, and the NMR spectrum confirms that the narrower lines Pa, Pd and Pf all belong to the same hBUa, while the distributed lines Pa, Pc and Pe all belong to the second hBUb. The 19F→31P 2D NMR spectrum (Figure 12) indicates that the Fa/Fc resonances belong to hBUa, while the distributed Fb/Fd resonances belong to hBUb (which is also formed by the distributed P resonances).

Table 4. N−P Distances (below 4 Å) in ULM3-(Al) atom

atom

distance

N1

P1 P5 P6 P1 P3 P4 P2 P3 P2 P5

3.612 3.718 3.724 3.855 3.882 3.655 3.661 3.524 3.626 3.633

N2

N3 N4

molecules are inserted (Figure 8c). It is worth noticing that the water molecules are much closer to hBU1 than to hBU2. Contrary to ULM-3(Al) in which all of the six expected 31P resonances are well-resolved in the 31P MAS NMR spectrum (Figure 2a), in ULM-4(Al), the 31P MAS NMR spectrum recorded under 1H/27Al double-resonance decoupling is lessresolved. Two sets of resonances of equal intensity can nonetheless be distinguished: lines Pa, Pd, and Pf are rather narrow, with line width similar to that in ULM-3(Al); lines Pb, Pc and Pe are much broader, and, in fact, each of them contains several contributions spread over 1 to 2 ppm. Such a small 21494



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Figure 9. (a) Experimental (i) and reconstructed (ii) 31P MAS (10 kHz) NMR spectra of ULM-4(Al) recorded under 1H/27Al double-resonance decoupling. (b) Experimental (i) and reconstructed (ii) 19F MAS NMR spectrum of ULM-4(Al) recorded under 1H decoupling.

The 31P−31P correlation pattern is similar to that in ULM3(Al), thus allows the separation of the P atoms in three groups: the Pa/Pb pair corresponds to the phosphorus atoms bridging two hBUs P2 and P5 (the crystallographic site numbering in ULM-3(Al) and ULM-4(Al) are different),13,32 the Pc/Pd pair corresponds to P3 and P6 and the Pe/Pf pair corresponds to P1 and P4. It is, however, not possible to distinguish further between the two hBUs from the 1H−31P NMR correlation spectrum (See SI), as all the protons from the amines have identical chemical shifts. 3.3. Discussion. The ensemble of NMR spectra in ULM4(Al) indicates that the hBUa is fully fluorinated, while hBUb is partially hydroxylated on the bridging position(s). Since there are two bridging positions per hBUb, there are in total four possible environments for the phosphorus atoms, which intensity depends on the F/OH ratio, and each of these environment results in a different 31P chemical shift. Four resonances can clearly distinguished for the Pc peak (Figure 9a), showing that all four distributions occur in the solid network.

Figure 10. 27Al MQMAS NMR spectrum of ULM-4(Al). The top spectrum is the full projection of the MAS 27Al dimension. The right spectrum is the full projection on the isotropic dimension.

Figure 11. Sheared (a) 19F−19F DH3 and (b) 31P−31P POST-C7 2D NMR correlation spectra of ULM-4(Al). Thick lines correspond to the DQ diagonal of slope 1. Top spectra, on which lines are labeled, are the full projections on the 19F and 31P SQ dimensions. The red dashed lines show selected proximities between the F and P atoms belonging to the fluorinated hBUa, whereas the black dashed lines show selected proximities between the F and P atoms belonging to the partially hydroxylated hBUb. 21495



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fluorinated in the same network. As mentioned, the structural differences observed between ULM-3(Al) and ULM-4(Al) are related to water molecules: in ULM-3(Al), the absence of water in the structure imposes, to minimize the lattice energy, a geometrical distortion of the hBUs or of the connections between the hBUs. This is very well-reflected on the 31P MAS NMR spectrum, in which the 31P resonances between the two hBUs are significantly different (e.g., chemical shift difference of 1.2 ppm between P3 and P6) when the chemical environments are almost identical. In ULM-4(Al), in contrast, except for the small 31P chemical shift distribution due to the F/OH distributions, the 31P chemical shifts between the phosphorus atoms belonging to the two hBUs are almost identical. The presence of water in ULM-4(Al) is thus accompanied by a much smaller distortion of the two hBUs, leading to a 3D framework different from that in ULM-3(Al). Therefore, the potential role of water can be analyzed as being two-fold. First, water has a templating role in ULM-4(Al), as the association of the amine and water molecule matches the length of the water/ cation-framework distance, but water also has a role of lattice energy modification by modulating the conformation of hBUs through the F/OH distribution, as was already observed in AlPO4−CJ2.44 Therefore, the contributions to the lattice energy should be considered as the sum of the hBU connections by their inter-hBUs bonds and by the inner bridging bonds, allowing us to reach the optimal conformation matching the lowest energy network. Such conclusions can only be drawn by an in-depth structural analysis, including local F/ OH distribution.

Figure 12. 19F→31P CP-HETCOR NMR correlation spectra of ULM4(Al) recorded with 1H decoupling in the vertical indirect 19F dimension and 1H/27Al double resonance decoupling in the direct vertical 31P dimension.

The main difference between the two ULM solids presented in this study is the presence of water molecules in the structure of ULM-4(Al). ULM-4(Al) is indeed obtained at lower pH value and for higher water content in the synthesis medium than ULM-3(Al).13,32 Water molecules are incorporated in the solid network of ULM-4(Al) and were localized by singlecrystal X-ray diffraction32 in the pores close to atoms forming the hBU1 (Figure 8c). Because in ULM-3(Al) the absence of water molecules in the final network has resulted in fully fluorinated bridging positions, it might be inferred by analogy that the fully fluorinated hBU in ULM-4(Al) is the hBU2, far from the water molecules, whereas the partially hydroxylated hBU is that closer to the water molecules (hBU1). This leads to assigning hBUa to hBU2 and hBUb to hBU1 (Tables 5 and 6).

4. CONCLUSIONS In this contribution, we have reported an investigation of the localization of F atoms and OH groups in the two fluorinated aluminophosphates ULM-3(Al) and ULM-4(Al) by multinuclear and multidimensional high-resolution solid-state MAS NMR spectroscopy. First, in ULM-3(Al), we show that this compound is fully fluorinated. A whole set of state-of-the-art homonuclear and heteronuclear 2D NMR experiments (19F−31P, 19F−19F, 31P−31P, 27Al−31P, and 1H−31P) have been recorded, yielding unique resonance assignment of all the 31P and 19F resonances to the corresponding crystallographic sites. In a second part, ULM-4(Al) is shown to be a quite remarkable sample because it contains both fully and partially fluorinated hBUs. The structural differences between ULM-3(Al) and ULM-4(Al) are finally discussed in the light of the results, in terms of lattice energy and role of water as templating and lattice energy modifier.

Table 5. 31P Line, Isotropic Chemical Shift δiso (± 0.1 ppm), relative intensity (Eq. P), and line assignment in ULM-4(Al) line

δiso (ppm)

Eq. P

assignment

Pb Pa Pc Pd Pe Pf

−5.1, −7.2 −6.9 −11.1, −11.8, −12.6, −13.1 −13.9 −15.8, −16.7, −17.2 −18.2

1 1 1 1 1 1

P5 P2 P3 P6 P4 P1

■

Table 6. 19F Line, Isotropic Chemical Shift δiso (± 0.1 ppm), Relative Intensity (Eq. F), and Line Assignment in ULM4(Al) line

δiso (ppm)

Eq. F

assignment

Fb Fa Fc Fd

−109.8 −111.5 −116.8 −119.9

1 1 1 1

F2 F1 F3 F4

ASSOCIATED CONTENT

* Supporting Information S

This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +33139254476. Notes

ULM-4(Al) is a quite remarkable compound because it contains both one 100% fluorinated and one partially

The authors declare no competing financial interest. 21496



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ACKNOWLEDGMENTS We thank Prof. Nathalie Simon (ILV, Versailles − France) for providing the ULM-3(Al) sample. This study was supported by PHC EGIDE program PROCOPE N°26796SL.

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