Structural Investigation by Multinuclear Solid State NMR and X-ray

J. Phys. Chem. B 2001, 105, 9083-9092

9083

Structural Investigation by Multinuclear Solid State NMR and X-ray Diffraction of As-Synthesized, Dehydrated, and Calcined AlPO4-SOD Me´ lanie Roux,† Claire Marichal,*,† Jean-Louis Paillaud,† Christian Fernandez,‡ Christian Baerlocher,§ and Jean-Michel Che´ zeau† Laboratoire de Mate´ riaux Mine´ raux, ENSCMu, UniVersite´ de Haute Alsace, CNRS-UPRES-A 7016, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France; Laboratoire de Catalyse et Spectrochimie, ISMRA/UniVersite´ de Caen, CNRS UMR 6506, 6 bd du Mare´ chal Juin, 14050 Caen Cedex, France; and Laboratorium fu¨ r Kristallographie, ETH, CH-8092 Zu¨ rich, Switzerland ReceiVed: February 26, 2001; In Final Form: June 9, 2001

AlPO4-SOD, a microporous aluminophosphate, was synthesized in a quasi-nonaqueous medium using dimethylformamide as solvent and template. The as-synthesized, dehydrated, calcined materials and the corresponding rehydrated phases were investigated by various solid-state nuclear magnetic resonance (NMR) and powder X-ray diffraction techniques. Structural data from the NMR study are in a very good agreement with the as-synthesized AlPO4-SOD structure refined from powder synchrotron data. 31P homonuclear correlation and 31P/27Al 3QHETCOR experiments allowed the complete assignment of 31P and 27Al resonances to the corresponding crystallographic sites. 31P and 27Al NMR spectra are drastically modified after dehydration at 200 °C and calcination at 800 °C. A possible structural rearrangement of the template molecules is proposed that explains NMR and XRD data of the dehydrated material. The structure of the calcined material solved by powder XRD is in agreement with the NMR data.

1. Introduction Since their discovery in 1982 by Wilson et al.,1 aluminophosphate molecular sieves, named AlPO4-n, where n refers to a distinct structure, have been subjected to intense research activity induced by their numerous potential uses. New materials were obtained, some with structures similar to zeolites (ERI, FAU, SOD, etc.), others having original structures (AEL, VFI, ZON, etc.). Aluminum and phosphorus atoms were also partially or totally substituted by various elements (Si, Zn, Ga, etc.), leading to many chemical compositions.2,3 Recently, Vidal et al.4 synthesized a new microporous aluminophosphate in a quasi-nonaqueous medium using dimethylformamide (DMF) as solvent and structure-directing agent. This material was named AlPO4-SOD because its framework exhibits the stacking of β-cages characteristic of the SOD framework type. In AlPO4-SOD, all the phosphorus atoms are tetracoordinated, whereas aluminum atoms show both tetrahedral and octahedral coordination. The 6-fold coordinated aluminum is bonded to both a carbonyl oxygen of a DMF molecule and the oxygen of a water molecule, leading to a strong distortion of the sodalite cage. The symmetry is then lowered to monoclinic. Vidal et al.4 also suggested that water molecules are eliminated at 170 °C, whereas the removal of DMF molecules occurs between 400 and 600 °C. The aim of the present work is to fully characterize the different products by various solid-state NMR and powder X-ray diffraction techniques. The products investigated are the assynthesized phase containing both DMF and water, the inter* Corresponding author. Fax: (+33) 3 89 33 68 85. E-mail: C.Marichal@ univ-mulhouse.fr. † Universite ´ de Haute Alsace. ‡ ISMRA/Universite ´ de Caen. § ETH.

mediate phase containing only DMF and the calcined one with empty cages. Rehydration of the heated phases was also investigated. 2. Experimental Section 2.1. Sample Preparation. The aluminophosphate AlPO4SOD was synthesized following the procedure of Vidal et al. using dimethylformamide (DMF) as template and solvent.4 The gel (composition 1.0:1.0:3.5:7.8 Al2O3:P2O5:H2O:DMF) was heated at 130 °C during 7 days. Thermogravimetric analysis and variable temperature X-ray diffraction show that structural modifications occurred below 200 °C and between 200 and 800 °C. The as-synthesized material was heated in air at the desired temperature (200 or 800 °C) for 6 h in a quartz tube. Increasing the duration of heating to 24 h with or without vacuum (2 × 10-4 Torr) did not modify our data. The NMR rotors were filled in the argon atmosphere of an oxygen-free glovebox. For the X-ray analyses, identical thermal treatment was performed in capillaries that were sealed while still hot. The rehydration of the heated forms of AlPO4-SOD was obtained by exposure to moisture over a 50 wt % solution of NH4Cl for various periods of time (48 h, 43 days). 2.2. X-ray Diffraction. The as-made AlPO4-SOD was packed into a 1 mm glass capillary, and the high-resolution powder diffraction data were collected on the Swiss-Norwegian Beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, using a Debye-Sherrer diffractometer geometry and an analyzer crystal. Further details of the data collection and the crystallographic data are given in Table 1. The data collections for the thermally treated and rehydrated samples were performed at room temperature using the STOE STADI-P diffractometer in Debye-Scherrer geometry equipped with a linear position-sensitive detector (6° in 2θ) and employing Ge monochromated Cu KR1 radiation (λ ) 1.5406 Å).

10.1021/jp010723k CCC: $20.00 © 2001 American Chemical Society Published on Web 08/29/2001

9084 J. Phys. Chem. B, Vol. 105, No. 38, 2001

Roux et al.

TABLE 1: Powder Diffraction Data Collection Parameters and Crystallographic Data for As-synthesized AlPO4-SOD data collection synchrotron facility sample holder

SNBL at ESRF 1 mm glass capillary 0.64796(1) 3.78-37.00 0.005 5-15

wavelength (Å) 2θ range (°2θ) step size (°2θ) time per step (s) refinement chemical formula space group a (Å) b (Å) c (Å) β (deg) volume (Å3) Z calcd density (g/cm3) no. of observations no. of contributing reflections no. of structural parameters no. of geometric restraints no. of profile parameters Rpa ) ∑{[|yo - yc||yo - yb|/yo]/∑|yo - yb|} Rwpa ) {∑[w(yo - yc)(yo - yb)/yo]2/∑[w(yo - yb)2]}1/2 Rexpb RFb RF2 b χ2 b largest diff peak and hole (eÅ-3)

|(CH3)2NCHO‚ H2O|[Al3P3O12] Cc 12.948(1) 12.483(1) 8.639(1) 95.76(1) 1389.3(2) 4 2.185(5) 6643 764 121 103 12 0.0469 0.0595 0.0529 0.0190 0.0313 1.267 0.21, -0.21

a y , y , and y are y observed, y calculated, and y background, o c b respectively. b The definition of these residual values is given in ref 10.

2.3. NMR Measurements. 1H, 13C, 31P, and 27Al NMR experiments were performed at room temperature on a Bruker DSX 400 spectrometer operating at B0 ) 9.4T [Larmor frequencies ν0 (MHz) ) 400.1 (1H), 161.9 (31P), 104.2 (27Al), and 100.2 (13C)]. 31P magic angle spinning (MAS) NMR experiments were recorded on a standard double bearing probe with a 4 mm diameter ZrO2 rotor. Single-pulse 31P MAS NMR experiments were acquired with spinning frequency between 3.5 and 8 kHz and a π/2 pulse duration of 3.5 µs. A 31P spin lattice relaxation time (T1) of 17 s was measured on as-synthesized AlPO4-SOD with the inversion-recovery pulse sequence, indicating that a recycle delay of 85 s must be used in order to avoid saturation. 1H-31P cross-polarization magic angle spinning (CPMAS) spectra were recorded using conventional Hartmann-Hahn matching with a spinning frequency of 3.5 kHz, a 1H π/2 pulse duration of 4.8 µs, contact times ranging from 100 µs to 5 ms, and a recycle delay of 1s. The radio frequency field strength used for 1H decoupling was set to 55 kHz. 31P 2D single quantum-double quantum (SQ-DQ) homonuclear correlation experiments on as-synthesized AlPO4-SOD were performed at a spinning frequency of ωr/2π ) 10 kHz using the C7 pulse sequence.5 Periods for excitation and reconversion of DQ coherences were both taken equal to 400 µs. 27Al MAS NMR spectra were recorded using a spinning frequency of 8 kHz and a 1.2 µs single pulse, corresponding to a flip angle of π/12 in order to ensure selective excitation of the central transition. A recycle delay of 1 s was used for all 27Al experiments. The conditions for acquisition and processing of the 27Al 3QMAS spectra are described elsewhere.6 31P and 27Al chemical shifts are reported relative to H PO 3 4 (85 wt % in water) and an aqueous solution of Al(NO3)3, respectively.

Figure 1. Pulse sequence of the 3QHETCOR experiment.

The 3Q and 5QHETCOR7 experiments designed for spin I ) 5/2 (27Al) combine the MQMAS excitation scheme with cross-polarization from 27Al to 31P, to get an heteronuclear correlation spectrum with a highly resolved 27Al dimension. The excitation of three quantum (+3Q) or five quantum (-5Q) echo coherences and their conversion into -1Q coherence was achieved by applying a pair of strong (100 kHz) RF pulses as shown Figure 1 for the 3QHETCOR experiment. The isotropic echo that forms at time 19t1/12 (3Q) or 25t1/12 (5Q) after the second pulse was spin locked and used as a source of polarization for the spin 1/2 (31P) nuclei. The cross-polarization was achieved using a CP contact time of 3 ms, under constant RF field of about 10 kHz for 31P and by ramping the field between 2.5 and 5 kHz for 27Al. The acquisition of the hypercomplex 2D dataset was performed using the States method in a rotor-synchronized fashion by varying the evolution time t1 using increments equal to the rotor period.8 The experiments were performed with a 4 mm triple tuned probe under a MAS frequency of 14 kHz and lasted between 12 and 24 h. 1H MAS NMR experiments were performed with a 2.5 mm Bruker MAS probe with spinning frequencies ranging from 8 to 30 kHz, π/2 pulse duration of 3 µs, and a recycle delay of 5 s. 13C MAS NMR experiments were realized with high-power 1H decoupling. Samples were spun at 8 kHz in a 4 mm Bruker MAS probe. A π/2 pulse duration of 2 µs and a recycle delay of 80 s were used. 1H and 13C chemical shifts are relative to TMS. 3. Results and Discussion 3.1. As-Synthesized AlPO4-SOD. 3.1.1. Powder X-ray Analysis. The preliminary structure analysis of the monoclinic AlPO4-SOD from a laboratory X-ray powder diffraction pattern allowed the main part of this microporous aluminophosphate4 to be solved with the SOD framework type. However, the Rietveld refinement was not good enough and it was not possible to localize any hydrogen atoms. For this reason, a Rietveld analysis using high-resolution powder diffraction data collected at the ESRF was undertaken. In a first step, after addition of the hydrogen atoms of the DMF molecule, an energy minimization was performed with the Cerius2 software9 using the structural parameters from the previous study.4 This gave the starting model for the new Rietveld analysis. The GSAS suite of programs10 was used for the Rietveld refinement. Soft constraints were placed on the bond lengths and angles of the framework, and an adequate set of constraints to transform the organic species to an almostrigid body including the hydrogen atoms was defined. After refinement, a difference Fourier map gave two residual electronic density peaks at about 1 Å from the oxygen atom of the water molecule present in the sodalite cage. Two hydrogen atoms were placed in these two positions and bond restraints were defined for this water molecule. All the atoms were refined isotropically. Successful convergence was obtained with Rp )

Study of Dehydrated and Calcined AlPO4-SOD

J. Phys. Chem. B, Vol. 105, No. 38, 2001 9085

TABLE 2: Atomic Parameters, Isotropic Displacement Parameters (Ui), and Occupancy Parameters of As-Synthesized AlPO4-SODa atom

x

y

z

Ui

fractn

P(1) P(2) P(3) Al(1) Al(2) Al(3) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) O(dmf) N O(w1) C(1) C(2) C(3) H(co) H(11) H(12) H(13) H(21) H(22) H(23) H(w1) H(w2)

1.0433(4) 0.798525 0.9322(4) 0.9180(5) 1.0502(3) 0.7816(4) 0.8414(6) 0.8258(7) 0.6601(6) 0.9834(7) 0.8564(7) 1.0162(6) 1.0123(7) 0.9875(7) 1.0122(8) 0.6823(6) 0.8291(9) 0.8964(7) 0.7061(6) 0.6808(8) 0.7248(7) 0.5694(8) 0.7273(10) 0.7472(9) 0.8209(31) 0.547(2) 0.539(2) 0.544(2) 0.691(5) 0.717(6) 0.801(3) 0.752(7) 0.646(3)

0.3756(3) 0.6381(4) 0.0143(3) 0.5196(3) 0.1312(4) 0.8898(4) 0.0479(6) 0.7549(7) 0.8792(6) 0.6074(7) 0.5799(6) 0.0633(6) 0.4638(6) 0.0856(6) 0.2677(7) 0.6182(5) 0.5908(7) 0.0836(6) 0.1657(5) 0.2553(9) 1.0305(7) 0.2481(11) 0.3061(8) 0.2115(12) 0.223(8) 0.186(5) 0.314(4) 0.237(8) 0.381(4) 0.257(4) 0.317(6) 1.088(6) 1.014(7)

-0.5415(6) -0.5425(6) -0.3286(7) -0.7915(8) -0.5878(7) -0.5930(5) -0.4136(9) -0.5313(10) -0.4922(9) -0.8984(9) -0.4063(10) -0.7587(9) -0.6573(9) -0.4378(9) -0.6166(10) -0.5389(10) -0.6955(11) -0.1987(9) -0.2786(9) -0.5068(11) -0.6709(11) -0.5181(14) -0.6367(13) -0.3915(15) -0.395(7) -0.586(10) -0.564(10) -0.412(4) -0.659(8) -0.732(5) -0.610(5) -0.731(10) -0.693(11)

1.47(13) 1.39(13) 0.80(11) 1.56(14) 1.04(14) 1.37(13) 0.38(25) 0.27(27) 0.37(22) 0.88(24) 1.00(25) 1.07(26) 0.54(24) 0.75(28) 0.83(27) 0.38(21) 2.06(29) 0.47(24) 1.26(24) 1.63(29) 1.91(25) 2.7(4) 4.0(5) 3.4(5) 6.2(14)b 6.2(14)b 6.2(14)b 6.2(14)b 6.2(14)b 6.2(14)b 6.2(14)b 6.2(14)b 6.2(14)b

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Figure 2. Observed, calculated, and difference pattern of the Rietveld refinement of as-synthesized AlPO4-SOD using synchrotron X-ray diffraction data [λ ) 0.64796(1) Å].

a Standard deviations are given in parentheses. b Parameters with the same superscript were constrained to be equal.

0.0469, Rwp ) 0.0595, RF ) 0.0190, and χ2 ) 1.267. It must be noted that at the end of the refinement the weight of the geometric restraints on both the framework and the guest molecules could be reduced to 1 and that the refinement of the atomic positions remained stable. The final atomic parameters are given in Table 2. In Table 3 bond distances and selected bond angles are reported. The profile fit is plotted in Figure 2. In the monoclinic AlPO4-SOD with formula |(CH3)2NCHO· H2O|[Al3P3O12], the sodalite-type cage linking is preserved (Figure 3). Three crystallographically different aluminum sites that strictly alternate with the three crystallographically different phosphorus sites are present in the structure. Two aluminum atoms are in a 4-fold coordination and the third Al atom [Al(3)] is 6-fold coordinated to four framework oxygen atoms, one water molecule, and the carbonyl oxygen of the DMF (Figure 4). All phosphorus sites are tetracoordinated by framework oxygen atoms. The guest-framework hydrogen-bonding scheme involves the carbonyl hydrogen OdC-H‚‚‚O12 [distance of 2.55(5) Å] and the two water hydrogens O1,O3‚‚‚Hw1-O-Hw2‚‚‚O7,O3 [distances between 1.90(5) and 2.48(9) Å], as shown in Figure 4. The protons of the two methyl groups are farther from the framework with a mean C-H‚‚‚O distance of 3 Å, except for C-H12‚‚‚O12 and C-H13‚‚‚O9, for which the distances are 2.55 and 2.63 Å, respectively. 3.1.2. Solid-State NMR Study. 13C MAS NMR. Protondecoupled 13C MAS NMR spectra of as-synthesized AlPO4SOD (not shown) are composed of three isotropic resonances at 165.7, 39.5, and 36.1 ppm, assigned to a carbonyl and two

Figure 3. Skeletal drawing of the sodalite framework.

types of methyl groups, respectively, as observed before.4 Therefore, the nature of the organic species (DMF) occluded into the framework is confirmed. 1H MAS NMR. The experimental 1H MAS NMR spectrum of as-synthesized AlPO4-SOD is shown Figure 5a. At a spinning frequency of 30 kHz (Figure 5b), three resonances at 3.1, 6.1, and 8.2 ppm with integrated intensities of 6:2:1 can be distinguished. According to the isotropic chemical shift values and the relative intensities, resonances at 3.1 and 8.2 ppm are assigned to protons belonging to the methyl groups and to the carbonyl of the DMF, respectively. The resonance at 6.1 ppm corresponds to protons from water molecules.11 At a MAS frequency of 8 kHz pronounced spinning sideband patterns occur, indicating sizable chemical shielding anisotropies of the three proton resonances. Consequently, both template (DMF) and water molecules have a restricted mobility, which confirms their strong hydrogen bonding with the framework. 31P MAS NMR. The 31P MAS NMR spectrum of assynthesized AlPO4-SOD shown Figure 6a presents three distinct resonances located at -22.8, -27.9, and -33.9 ppm with an area ratio close to 1, in agreement with the structure. The residual broadening observed on the experimental 31P MAS


Roux et al.

TABLE 3: Selected Interatomic Distances and Bond Angles of As-Synthesized AlPO4-SODa P1 P1 P1 P1 P2 P2 P2 P2 P3 P3 O3 O3 O3 O4 O4 O7 O2 O2 O2 O5 O5 O10 O1 O1 O1 O6 O6 O8 O4 O4 O4 O5 a

O3 O4 O7 O9 O2 O5 O10 O11 O1 O6 P1 P1 P1 P1 P1 P1 P2 P2 P2 P2 P2 P2 P3 P3 P3 P3 P3 P3 Al1 Al1 Al1 Al1

1.530(7) 1.539(8) 1.514(8) 1.532(9) 1.501(8) 1.516(8) 1.529(8) 1.535(8) 1.532(8) 1.535(8) O4 O7 O9 O7 O9 O9 O5 O10 O11 O10 O11 O11 O6 O8 O12 O8 O12 O12 O5 O7 O11 O7

P3 P3 Al1 Al1 Al1 Al1 Al2 Al2 Al2 Al2

O8 O12 O4 O5 O7 O11 O6 O8 O9 O10

109.6(4) 110.7(5) 110.7(5) 108.0(5) 109.2(5) 108.6(5) 108.8(5) 112.5(5) 110.4(5) 108.4(5) 109.5(5) 107.1(5) 110.4(4) 112.6(5) 111.4(5) 104.6(5) 108.9(4) 108.6(4) 112.0(4) 105.2(5) 108.6(4) 110.2(4)

O5 O7 O6 O6 O6 O8 O8 O9 O1 O1 O1 O1 O1 O2 O2 O2 O2 O3 O3 O3 O12 O12

Interatomic distances (Å) 1.526(8) Al3 O1 1.525(7) Al3 O2 1.711(8) Al3 O3 1.733(8) Al3 O12 1.743(9) Al3 Odmf 1.731(9) Al3 Ow1 1.722(8) N C1 1.696(7) N C2 1.784(1) N C3 1.728(9) C3 Odmf Al1 Al1 Al2 Al2 Al2 Al2 Al2 Al2 Al3 Al3 Al3 Al3 Al3 Al3 Al3 Al3 Al3 Al3 Al3 Al3 Al3 Al3

Angles (deg) O11 O11 O8 O9 O10 O9 O10 O10 O2 O3 O10 Odmf Ow1 O3 O12 Odmf Ow1 O12 Odmf Ow1 Odmf Ow1

110.7(5) 109.9(5) 112.9(4) 107.9(4) 108.9(4) 106.3(5) 108.6(4) 112.1(4) 93.0(4) 86.6(4) 92.8(4) 173.7(5) 91.2(4) 93.0(4) 93.7(4) 91.7(4) 175.4(5) 173.3(5) 88.9(4) 85.4(4) 91.1(4) 87.9(4)

1.834(8) 1.839(9) 1.879(7) 1.851(8) 1.922(7) 1.996(9) 1.438(11) 1.469(11) 1.363(11) 1.290(11) Odmf Odmf Odmf N N N N H21 H21 H22 N N N H11 H11 H12 C1 Hw1

C3 Hco C2 C2 C2 C1 C1 C1 Ow1 Ow1 Al3 C3 C3 C3 C2 C2 C2 C2 C2 C2 C1 C1 C1 C1 C1 C1 N Ow1

Hco C3 H21 H22 H23 H11 H12 H13 Hw1 Hw2 Ow1 N Hco Hco H21 H22 H23 H22 H23 H23 H11 H12 H13 H12 H13 H13 C2 Hw2

0.97(4) 0.97(4) 1.06(4) 1.03(4) 0.97(4) 1.00(4) 0.97(4) 1.01(4) 0.98(4) 1.03(4) 84.0(4) 116.9(10) 124.6(22) 118.2(23) 108.7(15) 108.8(14) 110.1(14) 111.4(14) 109.3(15) 108.6(15) 108.4(15) 109.3(15) 111.1(14) 109.8(14) 109.0(15) 109.1(15) 117.4(8) 116.7(30)

Standard deviations are given in parentheses.

Figure 4. One sodalite cage of as-synthesized AlPO4-SOD showing occluded DMF and water molecules with hydrogen bonding between these species and the framework.

NMR spectrum (Figure 6a) could result from dipolar coupling to protons from water or template molecules (DMF). The protondecoupled 31P MAS NMR spectrum of as-synthesized AlPO4SOD (not shown) displays reduced line broadening of resonances at -22.8 and -27.9 ppm. Furthermore, 1H-31P crosspolarization with short contact time (200 µs) enhances the 31P resonances at -27.9 and -22.8 ppm (Figure 6a), confirming the spatial proximity of protons for those two phosphorus crystallographic sites. Protons expected to be a magnetization source for phosphorus should not be too mobile: good candidates are protons from water and/or DMF in strong

Figure 5. 1H MAS NMR spectra of as-synthesized AlPO4-SOD at νr ) 10 kHz (a) and νr ) 30 kHz (b) and dehydrated AlPO4-SOD at νr )10 kHz (c) and νr )30 kHz (d). Arrows indicate the different proton species.

interaction with the octahedral aluminum Al(3). Consequently, inspection of Hw1, Hw2, and Hco-P distances and connectivities



Figure 7. 31P homonuclear SQ-DQ correlation NMR spectrum of as-synthesized AlPO4-SOD together with the 31P MAS NMR projection.

TABLE 5: Interatomic Distances (Å) between Each Crystallographic Phosphorus Site of as-synthesized AlPO4-SODa P(1)

P(2)

P(3)

P(1)

5.319(5) × 2

Figure 6. 31P NMR spectra of as-synthesized AlPO4-SOD at νr. ) 3.5 kHz, MAS (s) (a); 1Hf31P CP/MAS (···) (a); dehydrated (b) and rehydrated after 43 days (c).

P(2)

TABLE 4: Interatomic Distances (Å) between Protons of Water and DMF Molecules and Each Crystallographic Phosphorus Site of As-Synthesized AlPO4-SODa

P(3)

4.440(5) 4.558(5) 5.104(5) 5.604(5) 5.131(4) 5.465(5) 5.587(5) 5.991(6)

4.440(5) 4.558(5) 5.104(5) 5.604(5) 5.528(6) × 2

5.131(4) 5.465(5) 5.587(5) 5.991(6) 5.274(6) 5.365(6) 5.467(5) 5.483(6) 4.334(1) × 2

Hw1 Hw2 Hco

a

P(1)

P(2)

P(3)

3.63(9) 4.24(5) 5.35(7) 2.61(8) 3.46(9)

4.42(4) 5.86(6)

2.85(9) 5.93(8)

5.21(9) 5.04(7) 5.51(5) 5.35(9) 5.84(4)

4.02(6) 4.61(6)

3.78(4) 5.09(4) 5.69(8)

3.00(9) 5.09(10) 5.77(4)

A cutoff radius of 6 Å was arbitrarily chosen.

(Table 4) allow the 31P resonance at -33.9 ppm to be assigned to P(2). It is difficult to derive any conclusion about P(1) and P(3). Thanks to the excellent spectral resolution of the 31P MAS NMR spectra, which is due to the high purity and crystallinity of the sample, several two-dimensional MAS NMR techniques12 are available for assignment purposes. In our case, the C7 SQDQ homonuclear correlation experiment was chosen because it also provides information on the crystallographically equivalent sites. The result is shown in Figure 7. Such correlation experiments evidence the spatial proximity between crystallographically equivalent (displayed along the diagonal) and nonequivalent (off-diagonal) sites. To assign the 31P resonances to the corresponding crystallographic sites, one has to consider distances between the phosphorus sites, given by the structure determination (Table 5). According to Table 5, the distances between equivalent crystallographic sites can be ranked as follow: P(3)-P(3) < P(1)-P(1) < P(2)-P(2). Consequently,

a

5.274(6) 5.365(6) 5.467(5) 5.483(6)


the most and the less intense self-connectivity correlations are expected for the 31P resonances of sites P(3) and P(2), respectively. The resonance at -27.9 ppm is assigned to phosphorus crystallographic site P(3), 31P resonance at -33.9 ppm corresponds to P(2), in agreement with the 1Hf31P CPMAS experiment; therefore, the resonance at -22.8 ppm is attributed to P(1). 27Al MAS NMR. In the spectral region of tetracoordinated aluminum (20-50 ppm13) the 27Al MAS NMR spectrum shown in Figure 8a presents a broad and complex line shape due to second-order quadrupolar interactions. Another species can be identified in the spectral region of -20 to -5 ppm 13 and can be attributed to the hexacoordinated aluminum site Al(3). Since the X-ray diffraction study suggests two tetracoordinated aluminum sites, a 27Al 3QMAS NMR experiment was performed in order to resolve the corresponding 27Al resonances. The result (not shown) unambiguously identifies two distinct tetracoordinated aluminum sites, in perfect agreement with the structure. Numerical simulations of the F2 slices (not shown) yield the quadrupolar coupling constant (CQ) and the asymmetry parameter (ηQ) for each resonance. These are then used to simulate the 27Al MAS NMR spectrum in order to determine the relative abundance of each site (Table 6). The two tetrahedral aluminum atoms have very close quadrupolar parameters, indicating similar environments. The integrated intensities for


Roux et al.

TABLE 6: 27Al Quadrupolar Interaction Parameters of Each Aluminum Site for As-Synthesized, Dehydrated, and Calcined AlPO4-SOD, Respectively

σiso (ppm) CQ (MHz) (0.1 ηQ ( 0.05

calcined (800 °C)

dehydrated (200 °C)

as-synthesized Al(1)

Al(2)

Al(3)

A(1)

A(2)

A(3)

A(4)

A(5)

Al(1)

38.47 2.3 0.79

40.98 2.4 0.79

-8.50 2.7 0.82

39.12 2.6 0.99

37.30 2.8 0.42

35.90 2.6 0.95

11.96 2.7 0.35

-11.94 2.4 0.98

36.65 3.2 0.23

Figure 9. 27Al-31P 3QHETCOR contour plot of as-synthesized AlPO4SOD. The 31P and 27Al projections appear along the F2 and F1 dimensions, respectively, together with the slices corresponding to the environment of the three aluminum atoms. Arrows indicate the different crystallographic Al and P sites.

Figure 8. 27Al MAS NMR spectra of AlPO4-SOD as-synthesized (a), dehydrated (b), and rehydrated after 43 days (c) (νr ) 8 kHz).

TABLE 7: Interatomic Distances (Å) between Aluminum and Phosphorus Atoms of As-synthesized AlPO4-SODa Al(1) Al(2) Al(3) a

P(1)

P(2)

P(3)

3.135(7) 3.116(7) 3.080(7) 5.637(4) 3.167(5) 5.542(5)

3.143(6) 3.209(6) 3.201(4) 5.637(4) 3.175(7)

5.830(4) 3.188(6) 3.055(6) 3.246(6) 3.190(6)


each 27Al resonance are 0.33:0.33:0.32, close to a 1:1:1 ratio, in agreement with the structure. Close inspection of Al/P distances and connectivities listed Table 7 shows that the Al(1)-P(3) distance is large compared to the other Al-P distances, resulting in a smaller dipolar coupling. Consequently, a 27Alf31P heteronuclear correlation (HETCOR) experiment based on dipolar coupling should give a significantly less intense correlation peak for Al(1)-P(3) than for the others. Because we needed the 27Al resonances corresponding to the two tetrahedral aluminum Al(1) and Al(2) to be resolved, we performed a 27Alf31P 3QHETCOR experiment7,8 (Figure 9). The missing correlation in Figure 9 immediately assign the 27Al resonance at σ0 ) 38.5 ppm to Al(1) and the 31P resonance at -27.2 ppm to P(3). Furthermore, the 3QHETCOR experiment confirms the other 31P resonances assignments. Indeed, Table 7 indicates that

intensities of the Al(3)-P(i) (i ) 1,3) correlation peaks should be ranked as follow: Al(3)-P(3) > Al(3)-P(1) > Al(3)-P(2), in agreement with experimental data (Figure 9). 3.1.3. Thermal Treatment. Thermogravimetric analysis of AlPO4-SOD shows two major weight losses, in agreement with the results of Vidal et al.4 The first one, occurring at 170 °C, corresponds to the release of one water molecule per unit cell. Beyond 400 °C, the template (DMF) entrapped in sodalite cages probably decomposed into dimethylamine (DMA) and carbon monoxide.4 At 800 °C, the organic template has been completely eliminated. Consequently, to study the structural modifications generated by the loss of water and template, two distinct heating temperatures were chosen, namely 200 °C (dehydrated AlPO4SOD) and 800 °C (calcined AlPO4-SOD). Rehydration of each of these two phases has also been investigated. 3.2. Dehydrated AlPO4-SOD and Rehydration. 3.2.1. SolidState NMR Study. 1H MAS NMR. To begin with, 1H MAS NMR spectra (Figure 5c,d) confirmed that AlPO4-SOD heated at 200 °C contains less than 0.3 wt % water in the framework, whereas 13C MAS NMR (not shown) proved that DMF is not decomposed at this temperature. Note that the chemical shift anisotropy of both 1H resonances assigned to the template molecules is very similar in the as-synthesized and the dehydrated samples. 31P MAS NMR. The 31P MAS NMR spectra of AlPO -SOD 4 heated at 200 °C (Figure 6b) displays two resonances at -36.4 and -32.3 ppm of relative intensities 2:3, respectively. Comparison with the 31P MAS NMR spectrum of as-synthesized AlPO4-SOD (Figure 6a) shows that the isotropic shift of these resonances moved to lower frequencies and the resolution of the 31P MAS spectrum (Figure 6b) decreased, as already noticed for dehydrated VPI-5.14,15 Rehydration of dehydrated AlPO4SOD leads to three 31P resonances, as shown in Figure 6c, at -34.3, -28.2, and -23.4 ppm of relative intensities 2:2:1. When the rehydration time was increased from 48 h to 43 days all 31P resonances sharpened and shifted to higher frequencies, as



Figure 10. Sheared 27Al 5QMAS contour plot of dehydrated AlPO4-SOD. Arrows indicate the different aluminum sites (a). 27Al-31P 5QHETCOR contour plot of dehydrated AlPO4-SOD (b). Only the spectral region of AlIV is shown.

observed for other AlPO4s.16 The intensity ratio P(3)/P(2) is recovered, whereas after 43 days of rehydration the intensity of P(1) is still lower than that of P(1) from as-synthesized AlPO4-SOD: P(1) seems less accessible to water molecules than the two others phosphorus sites. On the phosphorus side the dehydration/rehydration process is partially reversible. Such an effect was already observed for VPI-5. Indeed, rehydration of AlPO4-8 leads to the three 31P resonances characteristic for VPI-5 but with a different intensity ratio.17 27Al MAS NMR. The 27Al MAS NMR spectra of dehydrated AlPO4-SOD (Figure 8b) show the presence of three signals in the spectral regions from -20 to -10, 0 to 13, and 20 to 40 ppm, corresponding respectively to hexa-, penta- and tetracoordinated aluminum atoms. The 27Al 5QMAS NMR spectra shown in Figure 10a allowed two tetrahedral aluminum resonances to be spectrally resolved and the presence of a third AlIV to be postulated in addition to one hexa- and one pentacoordinated aluminum resonance, as observed on the 27Al MAS NMR spectra. Fortunately, thanks to the different Al-P connectivities of the AlIV, the 27Al-31P 5QHETCOR experiment (Figure 10b) confirms the existence of a third AlIV. Pentacoordinated aluminum also exists when water is removed from several other aluminophosphates such as VPI-5.17 Note that all resonances are broadened, probably because heating induce some disorder. He et al.18 also noticed that the resolution of the VPI-5 27Al MAS NMR spectra decreases when the sample is dehydrated. Removing water also implies a high-field shift of about 2 ppm for both resonances, corresponding to tetrahedral and octahedral aluminum atoms with regard to as-synthesized AlPO4-SOD. Line shape simulations of the different 27Al resonances (obtained from the corresponding slices of the 5QMAS spectra for tetrahedral aluminums) gave the quadrupolar parameters listed in Table 6. Note that A(1) and A(3) possess the same quadrupolar coupling parameters, indicating similar environments, whereas the third tetrahedral aluminum A(2) has a greater CQ and a significantly lower asymmetry parameter. Deconvolution, according to the quadrupolar parameters (Table 6), of the signal corresponding to tetrahedral aluminums was not reliable, because of insufficient resolution of the quadrupolar singularities of the 27Al MAS NMR spectrum. Nevertheless, the relative intensities of octahedral versus pentacoordinated aluminums were retrieved and showed that the signal of the octahedral aluminum decreased from 33% (for as-synthesized AlPO4-SOD) to 11%, while pentacoordinated aluminum accounts for 13% of the total 27Al signal. According to the structure of as-synthesized AlPO4-SOD, the 6-fold coordinated aluminum atom Al(3) results from bonding to one water oxygen atom and to the carbonyl oxygen of the DMF. Consequently, after complete dehydration one could expect the transformation

Figure 11. Scheme of the dehydration mechanism of AlPO4-SOD heated at 200 °C leading to the formation of three new Al sites (AlIV, AlV, AlVI).

of all octahedral aluminums Al(3) sites into pentacoordinated aluminums, as observed for other AlPO4s such as VPI-5.17 In such a case, the tetra/pentacoordinated aluminum ratio should be 0.66:0.33. A possible reason for the presence of an equal quantity of penta- and hexacoordinated aluminum could be due to a partial moving of 1/3 of the DMF molecules from a Al(3) site to neighboring Al(3) site, forming hexacoordinated aluminum Al(DMF)2O4 species, as illustrated in Figure 11. This hypothesis keeps one DMF molecule per sodalite cage. This arrangement would also lead to the formation of the third tetrahedral aluminum site that was observed on the 27Al-31P 5QHETCOR spectra. This site might correspond to the most distorted AlIV [A(2); see Table 6]. Furthermore, according to the above 27Al NMR study of the dehydrated AlPO4-SOD, the experimental ratio AlIV:AlV:AlVI 0.76:0.11:0.13 is in agreement with at least five crystallographic sites (i.e., three AlIV, one AlV, and one AlVI). Rehydration of the dehydrated sample allows recovery of an 27Al MAS NMR spectrum similar to that of the as-synthesized AlPO4-SOD, i.e., two AlIV and one AlVI with an intensity ratio of 0.66:0.33. The signals (Figure 8c) are broadened, indicating a probable disorder that can also be observed in the XRD data. Nevertheless, the framework of this rehydrated phase should be of the SOD type. The rehydration of this phase cannot be easily compared to that of other AlPO4s that occur in absence of template. 3.2.2. Powder X-ray Analysis. Two methods, ITO19 and DICVOL9120 routines, were used to index the powder X-ray pattern of dehydrated AlPO4-SOD (Figure 12). No cell was found to index all the peaks. However, elimination of the small peaks in the indexing procedure leads to monoclinic unit cell parameters close to the one of the parent product with a ) 12.805(2) Å, b ) 12.201(9) Å, c ) 8.993(2) Å, β ) 91.68(2)°, and V ) 1404.6(4) Å3. The solid-state NMR study of this dehydrated phase (see section 3.2.1) does not show the presence


Figure 12. X-ray powder diffraction pattern of dehydrated AlPO4SOD. The * indicates the peaks which need a tripling of the c unit cell parameter.

of any impurities and gave a clear indication of the probable moving of only one-third of the DMF molecules (Figure 11). Moreover, considering that this migration may occur from one Al(3) site to the only Al(3) site having one molecule of water inside the same sodalite cage and that such displacement is only possible along the c-axis, as established by the structure (Figure 11) of the as-synthesized material, a tripling of the c unit cell parameter was postulated. This allowed us to index the full pattern with a FOM of 8.7. The refined unit cell parameters are a ) 12.808(3) Å, b ) 12.189(2) Å, c ) 27.004(8) Å, β ) 91.79(2)°, and V ) 4213(1) Å3. Unfortunately, the resolution of the XRD pattern is not sufficient for a complete structural determination. Therefore, a new high-resolution synchrotron X-ray data set will be collected. Nevertheless, the systematic absences in the laboratory data indicate Cc or C2/c as possible space groups. 3.3. Calcined AlPO4-SOD and Rehydration. 3.3.1. Powder X-ray Analysis. The thermal treatment at 800 °C applied to monoclinic AlPO4-SOD under vacuum changes the monoclinic symmetry to a higher one, since the number of reflections in the powder XRD pattern decreases significantly. In fact, Werner’s trial and error indexing program21 gave a trigonal unit cell with a ) 12.571(3) Å, c ) 14.484(4) Å, and V ) 1982.2(7) Å3. The volume of this new unit cell is 1.5 times larger than that of the parent monoclinic AlPO4-SOD, and the corresponding unit cell formula for this empty aluminophosphate is Al18P18O72. The systematic absences suggest R3c (161) as the space group. The structure analysis was performed using EXPO22,23 and GSAS10 packages for structure determination and Rietveld refinement, respectively. Direct methods applied on the extracted intensities gave the atomic positions of two heavy atoms, one aluminum and one phosphorus, and four oxygen atoms, all at general positions. The connecting oxygen atoms between the aluminum and phosphorus atoms in the framework were repositioned using the DLS24 technique. Then, the ideal tetrahedral framework was used as the starting model for a Rietveld refinement. After refinement of the cell parameters and 2θ zero-point and background correction, the atomic coordinates were refined with appropriate angles and distance constraints for the aluminum and phosphorus tetrahedra. If the tetrahedra are maintained highly symmetric, the refinement is rather poor with Rp ≈ 40%. A new refinement with lower weights on the angle constraints gave the following R values, Rwp ) 0.14 and Rp ) 0.098, which suggests a possible distortion of the

Roux et al.

Figure 13. X-ray powder diffraction pattern of calcined AlPO4-SOD: experimental (crosses), calculated (full line), and difference plot (top line). The short vertical line marks the positions of possible Bragg reflections.

TABLE 8: Atomic Parameters, Isotropic Displacement Parameters Ui and Occupancy Parameters of Calcined AlPO4-SODa atom

x

y

z

Ui

fractn

Al(1) P(1) O(1) O(2) O(3) O(4)

-0.242(2) -0.011(2) -0.105(3) 0.047(5) -0.383(2) 0.104(1)

0.007(2) 0.243(2) 0.124(3) 0.366(2) -0.053(5) 0.234(2)

0.407(3) 0.404(3) 0.454(2) 0.462(3) 0.469(1) 0.387(2)

3.6 2.8 1.4 1.4 1.4 1.4

1 1 1 1 1 1

a


TABLE 9: Selected Interatomic Distances and Bond Angles for Calcined AlPO4-SODa Al1 Al1 Al1 Al1 O1 O1 O1 O2 O2 O3 a

Interatomic distances (Å) 1.739(7) P1 1.737(7) P1 1.784(7) P1 1.795(18) P1

O1 O2 O3 O4 Al1 Al1 Al1 Al1 Al1 Al1

O2 O3 O4 O3 O4 O4

Angles (deg) 121.9(27) O1 120.7(19) O1 111.7(16) O1 102.4(25) O2 94.1(11) O2 101.2(24) O2

O1 O2 O3 O4

P1 P1 P1 P1 P1 P1

O2 O3 O4 O3 O4 O4

1.547(6) 1.580(6) 1.537(6) 1.533(6) 116.9(22) 130.2(27) 107.2(27) 106.9(29) 99.4(23) 87.1(14)


Figure 14. Projection along (001) of the calcined AlPO4-SOD structure.

tetrahedra. Figure 13 represents the profile fitting. In Tables 8 and 9 are listed atomic parameters, bond distances, and bond angles. The bond angle listing shows the large distortion around the aluminum and phosphorus atoms. The projection drawn in Figure 14 shows the perfect stacking of the sodalite cages.


Figure 15.


31

P and 27Al MAS NMR spectra of calcined (a, b) and rehydrated (c, d) AlPO4-SOD.

Figure 16. Sheared 27Al 3QMAS contour plot of calcined (a) and rehydrated (b) AlPO4-SOD.

It is worth noting that this phase is different from the calcined AlPO4-20 of cubic symmetry reported by Wilson,25 although the chemical composition and the topology are similar. It is another example of the influence of the original synthesis conditions to the subsequent calcined material.26 3.3.2. Solid-State NMR Study. No 1H and 13C signals could be detected for AlPO4-SOD heated at 800 °C, proving that the organic template has been entirely removed. The 31P MAS NMR spectrum of the calcined material shows only one asymmetric resonance at -30 ppm (Figure 15a). All phosphorus atoms possess similar environment, but the slight distribution of isotropic chemical shift could result from the distortion of the tetrahedra suspected in the XRD study (section 3.3.1.). The 27Al 3QMAS NMR spectrum (Figure 16a) exhibits a unique resonance, with the quadrupolar parameters listed in Table 6 corresponding to a 4-fold coordinated aluminum. The large CQ value indicates a strong distortion of this tetrahedral aluminum site. Note that as expected with the DMF departure, no AlVI is observed (Figure 15b). Thermogravimetric analysis of the calcined rehydrated AlPO4SOD shows a unique weight loss below 150 °C corresponding to the release of the occluded water molecules. Upon hydration of the calcined material, changes are observed on the 31P MAS NMR spectra that show two resonances at -26.3 and -31.5 ppm with an area ratio of 1:4 (Figure 15c). The 27Al 3QMAS NMR spectra (Figure 16b) show two distinct resonances at 43.5

and 36.7 ppm and one at -11.6 ppm (Figures 15d and 16b) assigned respectively to two tetrahedral and one octahedral aluminum species. The AlIV/AlVI intensity ratio is 3:2. The existence of a signal at -11.6 ppm implies that some of the tetrahedral aluminum atoms changed their coordination in the presence of water molecules. Indeed, the signal intensity increases with the time the sample was exposed to moisture. Obviously, the symmetry of the rehydrated calcined sample is lowered. Such a situation has been already observed in the case of AlPO4-1127,28 and AlPO4-41.16 Further work on the calcined and rehydrated AlPO4-SOD is under progress. 4. Conclusion A detailed characterization of as-synthesized, dehydrated, and calcined AlPO4-SOD was performed by X-ray diffraction and solid-state NMR. For the as-synthesized sample, NMR confirms the XRD structure. The combined use of 31P, 1Hf31P CPMAS, 31P homonuclear correlation, and 31Pf27Al 3QHETCOR experiments allowed the assignment of all phosphorus and aluminum resonances to the corresponding crystallographic sites. The dehydration of AlPO4-SOD is peculiar because of the presence of an intermediate phase without water but with the template molecule still intact. The influence of the absence of water on the structure was investigated both by NMR and XRD.

9092 J. Phys. Chem. B, Vol. 105, No. 38, 2001 The solid-state NMR results on the dehydrated compound suggested a rearrangement of one-third of the template molecule inside the sodalite cages to form Al(DMF)2O4 species. This hypothesis allowed a tripling of the unit cell of the dehydrated material structure to be proposed, demonstrating the complementarities of the two techniques. 31P and 27Al NMR results on the calcined AlPO -SOD are 4 in perfect agreement with the X-ray structure. Acknowledgment. Dr. Loı¨c Vidal is thanked for helpful discussions on the synthesis of AlPO4-SOD. It is also a pleasure to acknowledge Dr. Luc Delmotte, Dr. Jean-Marc Le Meins, and Dr. Michel Soulard for their advice on the NMR and XRD techniques, respectively, and thermal analysis part of this work. Many thanks are also due to Dr. Thomas Wessels for collecting the synchrotron data at the ESRF. 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) Meier, W. M.; Olson, D. H.; Baerlocher, Ch. Atlas of Zeolite Structure Types, 4th ed.; Elsevier: Amsterdam, 1996. (3) Baerlocher, Ch.; McCusker, L. B. Web site: http://www.izastructure.org. (4) Vidal, L.; Paillaud, J. L.; Gabelica, Z. Microp. Mater. 1998, 24, 189. (5) Lee, Y. K.; Kurur, N. D.; Helmle, M.; Johannesen, O. G.; Nielsen, N. C.; Levitt, M. H. Chem. Phys. Lett. 1995, 242, 304. (6) Marichal, C.; Vidal, L.; Delmotte, L.; Patarin, J. Microp. Mesop. Mater. 2000, 34, 149. (7) Fernandez, C.; Morais, C.; Rocha, J.; Pruski, M. Solid State NMR Submitted.

Roux et al. (8) Massiot, D. J. Magn. Reson. 1996, 122, 24. (9) Cerius2 3.8; Molecular Simulations: Cambridge, UK, 1998. (10) Larson, A. C.; Von Dreele, R. B.; Lujan, M. L. Report MS-H 805; Los Alamos National Laboratory, 2000. (11) Hunger, M. Catal. ReV. Sci. Eng. 1997, 39, 345. (12) Dusold, S.; Sebald, A. Dipolar Recoupling under Magic-Angle Spinning Conditions; Annual Reports on NMR Spectroscopy; Academic Press: London, 2000; Vol.41, pp185-264. (13) Engelhardt, G.; Michel, D. High-Resolution Solid State NMR of Silicates and Zeolites; John Wiley & Sons: Chichester, 1987; p 317. (14) Akporiaye, D.; Sto¨cker, M. Zeolites 1992, 12, 351. (15) Maistriau, L.; Gabelica, Z.; Derouane, E. G. Appl. Catal. 1992, 81, 67. (16) Caldarelli, S.; Meden, A.; Tuel, A. J. Phys. Chem. B 1999, 103, 5477. (17) Maistriau, L.; Gabelica, Z.; Derouane, E. G. Zeolites 1991, 11, 583. (18) He, H.; Klinowski, J. Catal. Today 1996, 30, 119. (19) Visser, J. W. J. Appl. Cryst. 1969, 2, 189. (20) Boultif, A.; Lower, D. J. Appl. Cryst. 1991, 24, 987. (21) Werner, P. E.; Eriksson, L.; Westdhal, M. J. Appl. Cryst. 1995, 18, 367. (22) Altomare, A.; Burla, M. C.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G. J. Appl. Cryst. 1995, 28, 842. (23) Altomare, A.;Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Cryst. 1994, 27, 435. (24) Baerlocher, Ch.; Hepp, A.; Meier, W. M. Report DLS76; Institute for Crystallography and Petrography ETH: Zurich, 1977. (25) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. Intrazeolite chemistry. ACS Symp. Ser. 1983, 218, 79. (26) de Onate Martinez, J.; McCusker, L. B.; Baerlocher, Ch. Microp. Mesop. Mater. 2000, 40, 325. (27) Peeters, M. P. J.; de Haan, J. W.; van de Ven, L. J. M.; van Hooff, J. H. C. J. Phys. Chem. 1993, 97, 5363. (28) Khouzami, R.; Coudurier, G.; Lefebvre, F.; Vedrine, J. C. Zeolites 1990, 10, 183.

Structural Investigation by Multinuclear Solid State NMR and X-ray

Recommend Documents