Characterization of BAPO-and SAPO-Based Mesoporous Materials by

Nov 18, 2005 - For the SAPO- based mesophase, the connectivities between various P and Al sites were mapped and the chemical environments of different...
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Chem. Mater. 2005, 17, 6545-6554

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Characterization of BAPO- and SAPO-Based Mesoporous Materials by Solid-State NMR Spectroscopy Yining Huang,* Zhimin Yan, and Roger Richer The Department of Chemistry, The UniVersity of Western Ontario, London, Ontario, Canada N6A 5B7 ReceiVed February 21, 2005. ReVised Manuscript ReceiVed July 24, 2005

We have utilized several heteronuclear dipolar-coupling based 27Al/31P, 31P/11B, 27Al/11B, 27Al/29Si, and 1H/31P double-resonance solid-state NMR techniques to characterize boroaluminophosphate (BAPO)and silicoaluminophosphate (SAPO)-based mesoporous materials. For the hexagonal BAPO materials, the coordination geometry of the B atoms in the framework is exclusively tetrahedral. The fraction of B atoms that can be incorporated in the framework seems not to depend on the B content of the initial gel. For the vast majority of the framework B atoms, the coordination environment is B(-OP)4. The B-OAl linkages, however, also exist. The results indicate that the four-coordinated B atoms are not distributed randomly within the framework. Instead, they appear to be located on the channel surface. For the SAPObased mesophase, the connectivities between various P and Al sites were mapped and the chemical environments of different P sites were determined to be as follows: (HO)2P[OAl(oct)]2, (HO)P[OAl(tet)]x[OAl(oct)]3-x, and (HO)P[OAl(tet)]3. The Si-O-Al linkages were detected unambiguously.

Introduction Since Mobil researchers discovered silicate- and aluminosilicate-based mesoporous materials such as MCM-41,1 there has been increased interest in the preparation of mesostructured aluminophosphate (AlPO)-based materials2 with lamellar,3 hexagonal,4 and tubular5 structures (Chart 1). Heteroatoms such as silicon,4b,5,6 boron,7 and transition metal ions2a have been incorporated into AlPO-based mesophases, resulting in mesoporous silicoaluminophosphates (SAPOs), boroaluminophosphates (BAPOs), and metalaluminophosphates (MeAPOs). The presence of mesopores (2-10 nm) in these materials leads to distinct molecular sieving characteristics toward the adsorption of large molecules. The * Corresponding author. E-mail: [email protected].

(1) 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. (2) For some reviews see: (a) Kimura, T. Microporous Mesoporous Mater. 2005, 77, 97. (b) Tiemann, M.; Fro¨ba, M. Chem. Mater. 2001, 13, 3211. (c) Schu¨th, F. Chem. Mater. 2001, 13, 3184. (d) Sayari, A.; Liu, P. Microporous Mater. 1997, 12, 149. (3) (a) Sayari, A.; Moudrakovski, I.; Reddy, J. S.; Ratcliffe, C. I.; Ripmeester, J. A.; Preston, K. F. Chem. Mater. 1996, 8, 2080. (b) Fro¨ba, M.; Tiemann, M. Chem. Mater. 1998, 10, 3475. (c) Khimyak, Y. Z.; Klinowski, J. Chem. Mater. 1998, 10, 2258. (d) Gao, Q.; Chen, J.; Xu, R.; Yue, Y. Chem. Mater. 1997, 9, 457. (e) Tanaka, H.; Chikazawa, M. J. Mater. Chem. 1999, 9, 2923 (f) Kimura, T.; Sugahara, Y.; Kuroda, K. Chem. Mater. 1999, 11, 508. (g) Oliver, S.; Kuperman, A.; Coombs, N.; Lough, A.; Ozin, G. A. Nature 1995, 378, 47. (4) (a) Feng, P.; Xia, Y.; Feng, J.; Bu, X.; Stucky, G. D. Chem. Commun. 1997, 949. (b) Zhao, X. S.; Lu, G. Q.; Whittaker, A. K.; Drennan, J.; Xu, H. Microporous Mesoporous Mater. 2002, 55, 51. (c) Khimyak, Y. Z.; Klinowski, J. Phys. Chem. Chem. Phys. 2000, 2, 5275. (d) Kimura, T.; Sugahara, Y.; Kuroda, K. Microporous Mesoporous Mater. 1998, 22, 115 (5) Luan, Z.; Zhao, D.; He, H.; Klinowski, J.; Kevan, L. J. Phys. Chem. B 1998, 102, 1250. (6) Chakraborty, B.; Pulikottil, A. C.; Das, S.; Viswanathan, B. Chem. Commun. 1997, 911. (7) Khimyak, Y. Z.; Klinowski, J. J. Mater. Chem. 2002, 12, 1079.

ability of incorporating various heteroatoms into the framework makes these AlPO-based materials versatile heterogeneous catalysts for different types of reactions.2a Characterization of mesostructured materials is of fundamental importance since detailed structural knowledge will, in the future, enable the rational design of novel materials with desired properties. Understanding of the local environment of P, Al, and heteroatoms is particularly important for the introduction of functional groups on the internal channel surface to make these materials conducive for practical applications such as adsorption, catalysis, and chromatography8 and for modification of the framework by incorporating organic groups inside the walls to enhance thermal stability.9 It should be pointed out that characterization of local structure in mesostructured AlPO materials is not a simple task compared to AlPO4-based three-dimensional microporous materials. In the AlPO4-based crystalline microporous materials, the P sites normally are fully condensed, and there is only one type of P chemical environment, i.e., each P is bound to four Al atoms via bridging oxygen P(-OAl)4. The situation in AlPO-based mesostructured materials is, however, distinctly different. Since the degree of condensation for P atoms in these mesophases is usually low, there are many possible P chemical environments, including P(-OAl)4-x(OH)x (x ) 0-3), P(-OAl)3-x(OH)x(OR) (x ) 0-2), P(O)(-OAl)3-x(OH)x (x ) 0-2), and P(O)(-OAl)2-x(OH)x(OR) (x ) 0-1) (with R being CnH2n+1 or CnH2n+1NMe3+, depending on the template used). Further, for a given chemical environment, there may be crystallographically nonequivalent sites if the materials are crystalline. The situation is even more complicated if there are hetero(8) Gianotti, E.; Oliveira, E. C.; Coluccia, S.; Pastore, H. O.; Marchese, L. Inorg. Chim. Acta 2003, 349, 259. (9) Kimura, T. Chem. Mater. 2003, 15, 3742.

10.1021/cm050396j CCC: $30.25 © 2005 American Chemical Society Published on Web 11/18/2005

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Chart 1. Structure Types of Mesostructured AlPO-Based Materials

atoms in the framework. Solid-state NMR spectroscopy is the method of choice for characterization of local structure since it is sensitive to short-range ordering and the local coordination environment. 31P magic-angle spinning (MAS) NMR has been used to probe the local environment around P, and for a number of these mesostructured AlPO materials, their 31P MAS spectra usually exhibit multiple peaks, indicating the existence of different P local structures.3a,c,f,4c,d,5,10 The spectral assignments usually begin with a comparison of the observed 31P chemical shifts with those reported for AlPO4-based three-dimensional microporous materials whose 31 P chemical shift values often fall in the range of -19 to -30 ppm.3a,11 As previously mentioned, in those AlPO4based materials, the P sites only have one type of P chemical environment, P(-OAl)4. Thus, different 31P resonances are due to crystallographically nonequivalent sites. As discussed earlier, the AlPO-based mesostructured materials may exhibit many different 31P chemical environments due to incomplete condensation. In general, the 31P signals can be classified in two regions.3a,5 The resonances with chemical shifts in the region from -19 to -30 may be assigned to fully condensed P sites, whereas the 31P signals within the range from 0 to -19 ppm are due to the P sites with lower degrees of condensation. This classification is extremely useful for the initial spectral assignment. However, to more accurately determine the nature of each P environment in mesostructured materials, additional information is required because observing a 31P peak in the range from 0 to -19 ppm only suggests a tetrahedral P whose second coordination sphere contains less than four Al atoms. The actual number of Al neighbors, however, cannot be determined from the 31P MAS spectrum alone since the shift of 31P signals to lower field can be induced by factors in addition to the number of Al in the second coordination sphere.3a,5 A completely condensed P site can also appear at lower fields outside the range from -19 to -30 ppm if specific geometry is involved. For instance, one of the three fully condensed P sites in microporous material AlPO4-18 resonates at -12 ppm.12 Further, due to the partially disordered nature, the 31P MAS spectra of many AlPO-based mesoporous materials usually (10) Schulz, M.; Tiemann, M.; Fro¨ba, M. Jager, C. J. Phys. Chem. B 2000, 104, 10473. (11) (a) Blackwell, C. S.; Patton, R. L. J. Phys. Chem. 1988, 92, 3965. (b) Hasha, D.; Saldarriaga, L.; Hathaway, P. E.; Cox, D. F.; Davis, M. E. J. Am. Chem. Soc. 1988, 110, 2127. (12) He, H.; Klinowski, J. J. Phys. Chem. 1993, 97, 10385.

exhibit broad resonance profiles wider than 4 kHz and containing several severely overlapping signals,4c,d,5,7 making the assignment difficult. Determining the local environment of heteroatoms using simple MAS methods is also not straightforward in some cases. For the BAPO-based materials, because of the small chemical shift range, a given 11B resonance may correspond to more than one chemical environment. In this work, we have characterized several typical BAPOand SAPO-based mesoporous materials. Particular attention was directed toward obtaining more detailed information on the local structures of P, Al, B, and Si sites in these materials by solid-state NMR. The techniques employed include a number of dipolar-coupling based 31P/27Al 11B/31P, 11B/ 27 Al, 29Si/27Al, and 1H/31P double-resonance methods such as rotational echo double-resonance (REDOR),13 heteronuclear correlation spectroscopy (HETCOR),14 transfer of population in double-resonance (TRAPDOR),15 and 1H f 31 P cross-polarization (CP). They were used wherever applicable for selection of Al-O-P, B-O-P, B-O-Al, Si-O-Al, and P-O-H connectivities to solve particular problems. We have shown that a combination of the abovementioned methods provides invaluable new physical insights into the local structure of P, Al, B, and Si sites in the materials, which are not apparently available from any other techniques. Experimental Section A mesostructured SAPO-based tubular material (UHM-3)5 and the BAPO-based hexagonal phases7 were synthesized according to the procedures described in the literature. The synthesis conditions of these materials are summarized in Table 1. The powder X-ray diffraction patterns (Figure 1) of the as-made materials appeared identical to those reported in the literature, confirming the identities of the products. The NMR experiments were performed on a Varian/Chemagnetics Infinityplus 400 WB spectrometer equipped with three radio frequency (rf) channels operating at 9.4 T field strength. The Larmor frequencies of 1H, 31P, 27Al, 29Si, and 11B were 399.9, 161.6, 104.1, (13) Gullion, T.; Schaefer, J. J. Magn. Reson. 1989, 81 196. (14) (a) Fyfe, C. A.; Mueller, K. T.; Grondey, H.; Wong-Moon, K. C. J. Phys. Chem. 1993, 97, 13484. (b) Wenslow, R. M.; Fiske, K.; Mueller, K. T. In Solid-State NMR Spectroscopy of Inorganic Materials; Fitzgerald, J. J.,Ed.; ACS Symposium Series 717; American Chemical Society: Washington, DC, 1999; p 228. (15) (a) Grey, C. P.; Vega, A. J. J. Am. Chem. Soc. 1995, 117, 8232. (b) van Eck, E. R. H.; Janssen, R.; Mass, W. E. J. R.; Veeman W. S. Chem. Phys. Lett. 1990, 174, 428.

BAPO- and SAPO-Based Mesoporous Materials

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Table 1. Synthesis Conditions and Elemental Compositions elemental composition in solids sample

templatesa

BAPO BAPO

(TMA)OH + (CTA)Cl (TMA)OH + (CTA)Cl

Al:P:B ) 1:2:0.2 Al:P:B ) 1:2:4

SAPO

(TMA)OH + (CTA)Cl

Al:P:Si ) 1:2:0.25

initial gel composition

Al/P (EDX)

1:0.73

Al/Si (EDX)

Al/P/B (NMR)

structure type

1:1.64:0.02b 1:1.38:0.034c 1:1.38:0.02b

hexagonal hexagonal

1:0.24

tubular

a

(TMA)OH, tetramethylammonium hydroxide; (CTA)Cl, cetyltrimethylammonium chloride. b The boron content includes tetrahedral boron atoms only. c The boron content includes both tetrahedral and trigonal boron atoms.

Figure 1. Powder XRD patterns of as-made mesohexagonal BAPO materials prepared from an initial gel with (A) high and (B) low B contents. (C) As-made mesotubular SAPO material.

79.8, and 128.2 MHz, respectively. All the NMR spectra were acquired by using a Varian/Chemagnetics 7.5 mm T3 triple-tuned MAS probe. The spinning speed for simple MAS experiments was in the range of 4-7 kHz. For spin-1/2 nuclei, single-pulse MAS experiments with proton decoupling were carried out for quantitative spectra, 30° flip angles were employed, and the recycle times were 120 s for 31P and 300 s for 29Si. The 27Al and 11B MAS experiments were carried out using a small pulse angle (less than 15°) to ensure the quantitative spectra with a recycle delay of 0.2 and 10 s for 27Al and 11B, respectively. The rf field strength for 1H decoupling was approximately 60 kHz. Shift referencing was compared to 85% H3PO4, 1 M Al(NO3)3(aq), tetramethylsilane (TMS), and BF3‚OEt2 for 31P, 27Al, 29Si, and 11B, respectively. TRAPDOR experiment is designed to probe the heteronuclear dipolar interactions involving at least one quadrupolar nucleus (irradiated spin).15 TRAPDOR consists of two separate experiments. For 31P{27Al} TRAPDOR, the first is a control experiment in which a rotor-synchronized 31P spin-echo sequence (90°-nτr-180°-nτr) is applied with τr being one rotor period. The second (TRAPDOR) experiment is the same spin-echo as the first except that during the first half of the echo (nτr) on the observed 31P spins the 27Al spins are continuously irradiated. The continuous high-power rf irradiation of the 27Al spins under MAS conditions affects the echo intensity of 31P spins via dipolar coupling. The TRAPDOR difference spectrum (∆S) is obtained by subtracting the TRAPDOR spectrum (S) from the control spectrum (S0) and indicates dipolar coupling. The 31P 90° and 180° pulse lengths were 8 and 16 µs. The rf field strengths were 65 and 60 kHz for 27Al dephasing and 1H decoupling, respectively. The spinning speed was 6.5 kHz ( 2 Hz. All the TRAPDOR experiments were carried out under the identical spectrometer conditions. The REDOR experiment13 is also a rotor-synchronized double-resonance MAS technique. This technique involves two experiments. The first one is a normal spinecho experiment on observed spin. In the second (REDOR) experiment, during the spin-echo, a number of 180° pulses are applied to the dephasing nuclei. The echo intensity of REDOR experiments will decrease due to the nonzero average of dipolar

coupling compared to the normal echo without 180° dephasing pulses. Similar to TRAPDOR, the REDOR difference spectrum provides a measure of dipolar coupling. The spectrometer settings for the REDOR experiments for various spin pairs are summarized in Table 2. The 27Al f 31P HETCOR experiment14 is a technique based on cross-polarization. Because 27Al is a quadrupolar nucleus, the cross-polarization is complicated since the spin-locking efficiency is affected by several factors.16 The optimized strength of the 27Al spin-locking field is 14 kHz, corresponding to an 27Al 90° pulse length of 18 µs measured for the central transition. The spinning rate used was 6.5 kHz. The 27Al f 31P CP optimization was carried out using the crystalline molecular sieve VPI-5. For the 1H f 31P cross-polarization experiments, the 1H 90° pulse length was 6 µs and the Hartmann-Hahn condition was determined using (NH4)H2PO4. A repetition time of 5 s was used. All NMR measurements were performed on as-synthesized materials at room temperature. Quadrupolar parameters for various Al sites for selected samples were extracted from simulations of the 27Al MAS spectra (Supporting Information Table S1) by using the WSOLIDS software package provided by Prof. Wasylishen (University of Alberta). Powder X-ray diffraction patterns were recorded on a Rigaku diffractometer using Co KR radiation (λ ) 1.7902 Å). The EDX analysis was performed on a LEO440 scanning electron microscope equipped with an EDX analysis system.

Results and Discussion BAPO-Based Hexagonal Materials. Silicate-based MCM41 containing B has been well studied.17 Recently, boron has been incorporated into the framework of hexagonal mesoporous AlPO materials.7 Knowledge of the structure of these materials is crucial because incorporation of B into the framework can affect the acidity and hence the catalytic activities of the parent materials. For zeolites, substitution of B in aluminosilicate or silicate frameworks is relatively easy and the properties of B in zeolitic frameworks have been very well documented.18 Surprisingly, as pointed out in a recent review,19 there are very few reports of the boronsubstituted microporous AlPO4-based molecular sieves with AlPO4-5 seemingly the only well-known example with a very low B loading.20 Consequently, very little is known about the local environment of B in the open AlPO framework. (16) Vega, A. J. Solid State NMR 1992, 1, 17. (17) (a) Sayari, A.; Danumah, C.; Moudrakovski, I. L. Chem. Mater. 1995, 7, 813. (b) Sayari, A.; Moudrakovski, I.; Danumath, C.; Ratcliffe, C. I.; Ripmeester, J. A.; Preston, K. F. J. Phys. Chem. 1995, 99, 16373. (c) Oberhagemann, U.; Kinski, I.; Marler, D. B.; Gies, H. J. NonCryst. Solids 1996, 197, 145. (18) (a) Fild, C.; Shant, D. F.; Lobo, R. F.; Koller, H. Phys. Chem. Chem. Phys. 2000, 2, 3091. (b) Millini, R.; Perego, G.; Bellussi Top. Catal. 1999, 9, 13. (c) Sulikowski, B. HCR Compr. ReV. 1996, 3, 203, and references therein. (19) Cheetham, A. K.; Ferey, G.; Loiseau, T. Angew. Chem., Int. Ed. 1999, 38, 3268

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Table 2. Spectrometer Parameters for REDOR Experiments I{S} spin pair

90° pulse on I spin (µs)

180° pulse on I spin (µs)

180° pulse on S spin (µs)

recycle delay (s)

12.1 11 9.4

24.2 22 18.8 20

22 24.2 21.5 20

5 120 5 5

11B{31P} 31P{11B} 11B{27Al} 1H/29Si{27Al} a

For

29Si{27Al}

REDOR, the initial Si magnetization was prepared by 1H to

Figure 2. 31P, 27Al, and spinning sidebands.

11B

29Si

1H

decoupling field (kHz) none 60 none 50

spinning speed (Hz) 6500 ( 2 6500 ( 2 6500 ( 2 6500 ( 2

CP.

MAS spectra of the as-synthesized BAPO material prepared from an initial gel with a high B content. Asterisks (*) indicate

Therefore, the BAPO materials present an opportunity for exploring the bonding interaction and local chemical and geometrical environment of B atoms as well as the effect of substitution on P and Al sites within the framework of porous aluminophosphates. Although a previous study examined the BAPO-based materials by 11B, 31P, and 27Al MAS and 1H f 11B CP NMR,7 many fundamental questions have remained unanswered: (1) in the BAPO materials with high B content, tetrahedral and trigonal boron signals were observed in 11B MAS spectra and both were considered to be in the framework. There is, however, no direct proof for this assumption. Tetrahedral and trigonal boron sites have been identified in zeolites,18 but three-coordinated B has not been reported in microporous AlPO4-based molecular sieves. Thus, verification of the presence of trigonal B in the framework is necessary. (2) If B atoms are indeed in the framework, are they connected to P or Al? B is known to replace Al in AlPO4-5.20 If this is also the case for the meso-BAPO phase, then B-O-P linkages are expected. Recently, microporous aluminoborates have been synthesized.21 Thus, the existence of the Al-O-B connectivity cannot be ruled out. (3) Are boron sites distributed randomly within the framework or located on the channel surface? (4) For the meso-BAPO phases, their 31P MAS spectra all have many overlapping resonances with chemical shifts ranging from -2 to -19 ppm. As explained in the next paragraph, the spectral assignments are not unambiguous. To answer these and other questions, we have carefully examined the chemical environments of B, Al, and P in the BAPO materials by performing a range of solidstate dipolar-coupling based 11B/31P, 31P/ 27Al, 11B/27Al, and 1 H/31P double-resonance techniques to solve particular structural problems wherever they are needed. (20) (a) Appleyard, I. P.; Harris, R. K.; Fitch, F. R. Zeolites 1986, 6, 428. (b) Qiu, S.; Tian, W.; Pang, W.; Jiang, D. Zeolites 1991, 11, 371. (21) (a) Yu, J.; Xu, R.; Xu, Y.; Yue, Y. J. Sold State Chem. 1996, 122, 200. (b) Xiao, F.-S.; Qiu, S.; Pang, W.; Xu, R. AdV. Mater. 1999, 11, 1091, and references therein.

The 11B, 31P, and 27Al MAS spectra (Figure 2) of the sample prepared from the gel with high B content look very similar to those reported in the literature for the sample made with the same initial gel composition (Table 1).7 The 31P MAS spectrum contains a rather broad profile centered at around -13 ppm and a relatively sharp resonance at -3 ppm with relative intensity 6.7:1. These peaks were previously attributed to the P sites, which are not completely condensed (i.e. the number of Al atoms in the second coordination sphere is less than four). The exact nature of these P environments, however, cannot be unambiguously determined without further experiments for several reasons: Although the P sites were identified with a lower degree of condensation, the actual number of Al atoms bound to each P site is not known. The chemical shift values might also infer the existence of P-O-P linkages.22 It is known that the phosphate-based materials can form under similar reaction conditions.3a The assignment of the -3 ppm peak is particularly ambiguous because a recent study on microporous molecular sieve synthesis has shown that protonatedphosphate or amine-phosphate species can exist in the AlPO gel and have peaks near 0 ppm, indicating they are not connected to Al.23 The 27Al MAS spectrum exhibits peaks at +42 and -8 ppm. The +42 ppm peak was assigned to Al tetrahedrally bound to four P atoms, Al(-OP)4, and the -8 ppm resonance was assigned to octahedral Al with Al(OP)4(OH2)2 coordination.7 The simple MAS experiments, however, provide no information on how these Al sites are connected to the P resonances. To clarify the above-mentioned ambiguities, we first performed 27Al f 31P HETCOR experiments. HETCOR is a technique based on heteronuclear dipolar interaction. Since (22) (a) Jahn, E.; Mueller, D.; Richter-Mendau, J. In Synthesis of Microporous Materials, Vol. I; Occelli, M. L., Robson, H. E., Eds.; Van Nostrand Reinhold: New York, 1992; p 249. (b) Nakayama, H.; Eguchi, T.; Nakamura, N.; Yamaguchi, S.; Danjyo, M.; Tsuhako, M. J. Mater. Chem. 1997, 7, 1063. (23) Huang, Y.; Demko, B. A.; Kirby, C. W. Chem. Mater. 2003, 15, 2437.

BAPO- and SAPO-Based Mesoporous Materials

Figure 3. 27Al f 31P HETCOR spectrum of an as-synthesized BAPO material prepared from an initial gel with a high B content using a contact time of 1 ms. For each of 32 experiments in t1 5600 scans were acquired. The pulse delay is 0.25 s. Contour lines are drawn from 25 to 90% of relative intensity with a 5% increment.

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Figure 4. 31P{27Al} TRAPDOR fraction (∆S/S0) as a function of dephasing time of an as-synthesized BAPO material prepared from an initial gel with a high B content, VPI-5, and MgAPO-20 with short dephasing times. Table 3. Summary of the

the dipolar coupling is strongly dependent on internuclear distance, only the 31P nuclei that are in the proximity of the 27 Al atoms will be detected. Therefore, this technique can be used to map out Al-O-P connectivity.14 The 27Al f 31 P HETCOR spectrum (Figure 3) indicates that both tetrahedral and octahedral Al sites seen in the 27Al MAS spectrum are connected to the P signals observed in the 31P MAS spectrum. The spectrum also shows that although AlO-P linkages exist for both P resonances, they do have different connectivities. The P site at -3 ppm is only connected to the octahedral Al site. The broad envelope positioned at -13 ppm in the 31P MAS spectrum actually encompasses several overlapping signals at -11, -13, and -18 ppm. These P resonances are linked to both tetrahedral and octahedral Al. Additional information on the degree of condensation of the P sites can be obtained from the plots of 31P{27Al} TRAPDOR fraction (∆S/S0) vs dephasing time. It has been established that for several related heteronuclear dipolarcoupling based double-resonance techniques such as TRAPDOR, REDOR, and REAPDOR, in multiple-spin (ISn) systems such as ours, the initial parts of the ∆S/S0 vs evolution time plots only depend on the strength of I-S dipolar interaction (i.e. the number of S spins and internuclear I-S distances) and are independent of the specific geometry involved.24 Figure 4 and Table 3 show that the TRAPDOR curves of the two P sites at -3 and -13 ppm have different initial slopes for short dephasing times, suggesting that they experience different magnitudes of dipolar interaction with neighboring Al atoms. It has been recognized that, in aluminophosphate-based materials, including molecular sieves and glasses, the Al-O-P distance usually varies little.25 Therefore, the difference in the initial slopes of 31P/27Al (24) (a) Bertmer, M.; Eckert, H. Solid State NMR 1999, 15, 139. (b) van Wullen, L.; Muller, U.; Jansen, M. Chem. Mater. 2000, 12, 2347. (c) Chan, J. C. C.; Bertmer, M.; Eckert, H. J. Am. Chem. Soc. 1999, 121, 5238. (d) Ba, Y.; He, J.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 1999, 121, 8387. (25) van Eck, E. R. H.; Kentgens, A. P. M.; Kraus, H.; Prins, R. J. Phys. Chem. 1995, 99, 16080.

Sample BAPO SAPO MgAPO-20 VPI-5

31P{27Al}

TRAPDOR Results

δ (ppm)

slopea

-3 -13 -1 -18 -21 -28 -27

0.25 0.45 0.4 0.34 0.28 0.51 0.77

a

The initial slopes of TRAPDOR plots were obtained by linear regression of the data points with short dephasing time to show the initial dipolar dephasing behavior.

TRAPDOR and REDOR curves has been explained in terms of the average number of P (or Al) atoms coordinated to the observing spin P (or Al).25,26 In the present case, the different TRAPDOR behavior exhibited by two 31P signals implies that they have different numbers of Al atoms in their second coordination sphere. To estimate the number of Al atoms attached to the P sites, we used two model materials, VPI-5 and MgAPO-20. VPI-5 is an AlPO4-based microporous material. It has three crystallographically nonequivalent P sites with identical chemical environments, P(-OAl)4.27 MgAPO-20 is also an aluminophosphate-based molecular sieve with a SOD structure incorporating magnesium.28 Its 31P MAS spectrum has two 31P peaks at -28 and -21 ppm, corresponding to two distinct P(Mg, 3Al) and P(2Mg, 2Al) environments, respectively. We then measured the TRAPDOR curves of the P sites for the two model materials under identical spectrometer conditions. For VPI-5, the initial parts of the TRAPDOR curves for the three P sites are identical (Supporting Information Figure S1), confirming that the TRAPDOR curvature at short dephasing times is dictated mainly by the number of Al atoms bound to P. Figure 4 demonstrates that the initial slopes of the TRAPDOR curves for the three known P environments in the two model compounds have (26) (a) Gougeon, R. G.; Bodart, P. R.; Harris, R. K.; Kolonia D. M.; Petrakis, D. E.; Pomonis, P. J. Phys. Chem. Chem. Phys. 2000, 2, 5286. (b) Chan, J. C. C.; Eckert, H. J. Magn. Reson. 2000, 147, 170. (27) Davis, M. E.; Montes, C.; Hathaway, P. A.; Arhancet, J. P.; Hasha, D. L.; Grace, J. M. J. Am. Chem. Soc. 1989, 111, 3919. (28) Barrie, P. J.; Klinowski, J. J. Phys. Chem. 1989, 93, 5972.

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Figure 5. 1H f 31P CP MAS spectra of an as-synthesized BAPO material prepared from an initial gel with a high B content before (A) and after (B) D2O exchange with different contact times. Asterisks (*) indicate spinning sidebands.

the order P(OAl)4 > P(OAl)3 > P(OAl)2. As discussed earlier, following the argument of van Eck et al.,25 the observed differences in the initial slopes mainly reflect the number of Al atoms in the second coordination sphere. Figure 4 also compares the TRAPDOR curves of the -3 and -13 ppm peaks for the sample under investigation to those of the model materials. Figure 4 and Table 3 show that the initial slopes of the P at -3 and -13 ppm in the mesomaterial are much smaller than that of P(OAl)4 in VPI-5, but are close to those of P(OAl)2 and P(OAl)3 in MAPO-20 (Table 3). It appears that the number of Al atoms as next nearest-neighbors for the broad P resonance centered at -13 ppm is on average three. The initial slope of the peak at -3 ppm is close to that of P(OAl)2 in MAPO-20. Thus, the number of Al attached to this P is likely two, and this result confirms the suggestion made by Khimyak and Klinowski7 that the -3 ppm peak is due to P with a very low degree of condensation. To identify the P sites with directly attached P-OH groups, 1H f 31P CP experiments of the as-made BAPO sample were carried out. The spectra are shown in Figure 5. It is known that in mesoporous materials, the proton sources for cross-polarization are mainly hydroxyl groups in P-OH groups, organic structure directing agents, and the water molecules coordinated to the octahedral Al sites, Al(-OP)4(OH2)2.29 At room temperature, the water molecules occluded in the framework are too mobile to effectively contribute to the magnetization transfer. Since the protons in P-OH (29) Khimyak, Y. Z.; Klinowski, J. Phys. Chem. Chem. Phys. 2001, 2, 2544.

groups and coordinated water molecules can be readily exchanged, we conducted D2O exchange by stirring the sample in D2O overnight at room temperature. Compared to the spectra of the material without exchange, the absolute CP intensities of both peaks (Figure 5) for the sample with D2O exchange are much weaker at short contact times (0.1 and 0.5 ms), suggesting the existence of P-OH groups. It is also worth mentioning that for the D2O-exchanged sample (Figure 5), the intensity of the peak at -3 ppm is enhanced significantly relative to that of the broad peak at -13 ppm at short contact times of 0.1 and 0.5 ms. This observation indicates that this P is cross-polarized from the nonexchangeable protons, suggesting that, in addition to the hydroxyl group, this P site also interacts with the template molecules, inferring the existence of C16H33(CH3)3N+‚-OP groups. A combination of 1H f 31P CP, 27Al f 31P HETCOR, and 31P{27Al} TRAPDOR results suggests that the -3 and -13 ppm peaks represent the chemical environments C16H33(CH3)3N+‚-OP[OAl(oct)]2(OH) and (HO)P[O(Al(oct)]x[OAl(tet)]3-x (x ) 1-2), respectively. It seems that the peak at -3 ppm is due to the P atoms on the channel surfaces,29 and the broad peak at -13 ppm (which count for 87% of the total P) represents the P atoms in the pore wall. The 11B MAS spectrum shows a sharp signal at -0.2 ppm due to the tetrahedral B site and a very broad resonance due to three-coordinated B. Both of these B sites were previously assumed to be in the framework.7 To directly probe whether both B sites are in the framework, we carried out 11B{31P} and 31P{11B} REDOR experiments. As mentioned before, if the B coordination environment in the meso BAPO phase is

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Figure 6. 11B{31P} REDOR experiment of an as-synthesized BAPO material prepared from an initial gel with a high B content. Selected 11B spin-echo (S0), REDOR (S), and REDOR difference spectra (∆S) with a dephasing time of (A) 0.615 ms (4 rotor cycles) and (B) 1.538 ms (10 rotor cycles). (C) REDOR fraction (∆S/S0) as a function of dephasing time.

similar to what is in the microporous BAPO-5, then strong B-O-P coupling should exist. Figure 6 illustrates two sets of 11B{31P} REDOR spectra with short and long dephasing times. Seeing strong REDOR difference signals indicates the existence of B-O-P connectivity. The fact that the REDOR difference spectra contain only tetrahedral B indicates unambiguously that the four-coordinated B site is in the framework. The trigonal boron signal did not appear in the difference spectra, implying that it is not connected to P. The lack of B-O-P linkages for three-corrdinated B implies that the trigonal B is not in the frameworks and is therefore due to extraframework species. One could argue that the absence of B-O-P connectivity for three-coordinated B might be due to the formation of the islands (or clusters) of three-coordinated boron with B-O-B linkage in the framework. This is, however, an unlikely scenario, given that the B content in this material is very low (Table 1) and that it is not easy to substitute B into the framework of microporous AlPO4-based molecular sieves,19 as discussed previously. The observed REDOR effect is very strong as evidenced from a very steep slope in the REDOR curve (Figure 6C), where ∆S/S0 almost reaches unity after a dephasing time of only 1.54 ms. The very effective B-P dipolar dephasing corresponds to a large number of neighboring P atoms, suggesting a B(-OP)4 environment. To find out which P site is connected to the fourcoordinated B, 31P{11B} REDOR experiments were also performed. Although weak, we did observe a signal at -3 ppm in the 31P{11B } REDOR difference spectra for both short and long dephasing times (Figure 7). Though weak, this signal is reproducible. We also conducted control experiments to make sure the observed peak is real. There are several possible reasons to explain why the intensity of the -3 ppm resonance is weak in the REDOR difference spectra. (1) Not all the P atoms resonant at -3 ppm are connected to B. This peak also represents the coordination environment such as C16H33(CH3)3N+‚-OP[OAl(oct)]2(OH) identified by 1H f 31P CP experiments discussed earlier. (2) Some B nuclei may be in spin states other than |(1/2〉, and the natural abundance of 11B is 80%. (3) The results from 31P{27Al} TRAPDOR and 1H f 31P CP experiments suggest that the number of B atoms connected to this P is

Figure 7. 31P{11B} REDOR experiment of an as-synthesized BAPO material prepared from an initial gel with a high B content. Selected 31P spin-echo (S0), REDOR (S), and REDOR difference spectra (∆S) with a dephasing time of (A) 0.308 ms (2 rotor cycles) and (B) 3.692 ms (24 rotor cycles).

low and likely to be one: P[O(Al(oct)]2(OB)(OH) or C16H33(CH3)3N+‚-OP[OAl(oct)]2(OB). The intense broad envelope centered at -13 ppm in the 31P MAS spectrum is not observed in the difference spectra, implying that these P atoms are not connected to B. Using the REDOR intensity at a long dephasing time (24 rotor cycles), the fraction of P atoms bound to B was estimated to be 2.5%, compared to the value of 1.5% calculated from the composition of the material. The B-O-Al connectivity has not been observed in the framework of microporous AlPO4-based molecular sieves, but several microporous aluminoborates have recently been reported.21 To probe the possible Al-O-B linkage, 11B{27Al} REDOR experiments were also carried out and the REDOR difference signal (Figure 8) was indeed observed. Inspection of Figure 8 also reveals that only tetrahedral B is coupled to Al and trigonal B is not. The weak intensity in the difference spectra and the small slope of the REDOR curve (Figure 8C) infer that the average number of Al atoms connected to B is small. The results indicate that the -0.2 ppm peak in the 11B MAS spectrum represents framework

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Figure 8. 11B{27Al} REDOR experiment of an as-synthesized BAPO material prepared from an initial gel with a high B content. Selected 11B spin-echo (S0), REDOR (S), and REDOR difference spectra (∆S) with a dephasing time of (A) 1.231 ms (8 rotor cycles) and (B) 3.077 ms (20 rotor cycles). (C) REDOR fraction (∆S/S0) as a function of dephasing time.

tetrahedral B atoms in both B-O-P and B-O-Al linkages, which cannot be distinguished from chemical shifts alone due to the small chemical shift range of 11B. We also examined a meso-BAPO sample prepared from an initial gel with a low B content (Table 1). This sample only contains a tetrahedral B site as indicated by its 11B MAS spectrum (not shown). The 11B{31P}, 31P{11B}, and 11 B{27Al} REDOR results (data not shown) are very similar to those of four-coordinated B in the sample discussed above, suggesting that the local chemical environment of tetrahedral B sites in both materials are the same. The quantitative 31P, 27 Al, and 11B single-pulse MAS results show that both BAPO materials have a very low B content, implying that the fraction of total B which can be substituted into the AlPO framework is independent of the B content in the initial gel. SAPO-Based Tubular Materials. A series of SAPObased mesoporous materials (UHM-3) were first synthesized by Luan et al.5 These materials exhibit relatively disordered tubular structure with nonparallel pore systems. They are thermally stable upon calcination and have surface areas near 900 m2/g. However, due to partial disordering, only limited structural information is available. The aim of this work is to obtain the following new structural information: (1) the degree of condensation for various P sites, (2) the Al-O-P connectivities between various P and Al sites, and (3) unambiguous identification of Si-O-Al connectivity in the framework. The 31P and 27Al MAS spectra of an as-made SAPO-based mesoporous material shown in Figure 9 are comparable with those reported previously.5 The 27Al MAS spectrum contains two signals at +42 and -2 ppm. The +42 ppm peak was previously assigned to tetrahedral Al sites. The -2 ppm resonance was considered being the framework octahedral Al coordinated to water and PO4 groups. The shift value, however, also implies that this peak could be due to sixcoordinated Al in an amorphous alumina. The 31P MAS spectrum exhibits a broad envelop with a breadth of about 3.5 kHz. The profile includes two relatively sharp resonances at -0.8 ppm and -18 ppm overlapping with a broad peak centered at about -12 ppm. The peaks at -0.8 and -12 ppm were previously assigned to the P sites, which are not

Figure 9. 31P and 27Al MAS spectra of the as-synthesized mesostructured tubular SAPO material. * indicates spinning sidebands.

fully condensed5 and the -18 ppm peak to the P bonded to four Al atoms. The 27Al f 31P HETCOR spectrum (Figure 10) clearly illustrates that both +42 and -2 ppm peaks are connected to the P signals. The Al-O-P linkages exist for all three 31 P resonances in the 31P MAS spectrum, but these P sites have different connectivity. The 31P site at -18 ppm is only connected to the tetrahedral Al site. The 31P peak at -12 ppm is strongly correlated to octahedral Al, but is also weakly connected to tetrahedral Al. The -0.8 ppm resonance is linked to octahedral Al exclusively. To estimate the number of Al atoms attached to P sites, we have measured 31P{27Al} TRAPDOR curves for two welldefined peaks at -0.8 and -18 ppm. Figure 11 shows that the TRAPDOR behavior of both P sites is very similar,

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the intensity of the peak at -0.8 ppm is more enhanced by H f 31P cross-polarization relative to the peaks at -12 and -18 ppm,5 implying that the number of hydroxyl groups attached to the P at -0.8 ppm is greater than that of the peaks at -12 and -18 ppm. Combining this and the results obtained from HETCOR and TRAPDOR, we suggest that the coordination environments are (HO)2P[OAl(oct)]2, (HO)P[OAl(tet)]x[OAl(oct)]3-x (x ) 1-2), and (HO)P[OAl(oct)]3 for the P sites at -0.8, -12, and -18 ppm, respectively. 29 Si MAS spectrum (not shown) displays a single line at -93 ppm. On the basis of the chemical shift value, this resonance could be assigned to Si[(OAl)3(OSi)] and/or Si[(OAl)2(OSi)2] environments.31 However, the chemical shift might also indicate environments such as Si[(OH)2(OSi)2],32 which might form under the reaction conditions. To clarify the situation, we performed 1H/29Si/27Al CP-REDOR experiments. It should be pointed out that, in the present case, compared to 27Al/31P double-resonance experiments, the 29 Si/27Al REDOR experiment is much more difficult, due to the low sensitivity resulting from the low Si content of the sample coupled with the low Si abundance and its small magnetogyric ratio (resulting in a smaller dipolar coupling for the Si-O-Al unit). Despite these problems, we were able to observe a small REDOR effect (Figure 12). The spectra show that compared to the normal 29Si spin-echo (S0), there is a 15% reduction in peak intensity for the REDOR spectrum (S). Although weak, the observed REDOR effect is reproducible (we repeated the same experiments several times). This result confirms the existence of Si-OAl linkages. 1

Figure 10. 27Al f 31P HETCOR spectrum of the as-synthesized mesostructured tubular SAPO material with a contact time of 1 ms. For each of 32 experiments in t1 24 000 scans were acquired. The recycle delay of 0.1 was used. Asterisks (*) indicate spinning sidebands. Contour lines are drawn from 15 to 90% of relative intensity with a 5% increment.

Summary

Figure 11. 31P{27Al} TRAPDOR fraction (∆S/S0) as a function of dephasing time of the as-synthesized mesostructured tubular SAPO material, VPI-5 and MgAPO-20 with short dephasing times.

suggesting that they are both connected to the same number of Al atoms. The initial slopes of the -0.8 and -18 ppm peaks in the sample under investigation are between those of P(-OAl)3 and P(-OAl)2 in MgAPO-20. We did not carry out quantitative numerical analysis of TRAPDOR data because the polar angles defining the orientation of the EFG tensor of Al relative to the dipolar vector required for simulation are not known.30 Thus, the average number of Al atoms in the second coordination sphere can only be estimated to be between three or two. It is known that in mesostructured hexagonal and tubular materials, the remaining groups attached to the P sites that are not fully condensed are usually hydroxyl groups.4,5 Luan et al. have shown that (30) Kalwei, M.; Koller, H. Solid State Nucl. Magn. Reson. 2002, 21, 145.

We have characterized several representative BAPO- and SAPO-based mesoporous tubular and hexagonal materials by 27Al/31P, 11B/27Al, 11B/31P, 1H/31P, and 29Si/27Al doubleresonance NMR experiments including HETCOR, REDOR, TRAPDOR, and CP. For the BAPO-based hexagonal materials, the following new physical insights from the present work were obtained: Only a small fraction of B atoms can be incorporated into the meso AlPO framework regardless of the B content of the initial gel. The framework B atoms adopt tetrahedral geometry exclusively. The vast majority of the framework B atoms are connected to P in the B(-OP)4 environment. However, a small fraction of B atoms also appears to be bound to Al since the B-O-Al linkage is identified. The distribution of the B atoms seems to be nonrandom in the framework. Most of the framework B atoms are located on the channel surface rather than inside the wall with the possible surface sites being P[O(Al(oct)]2(OB)(OH) and/or C16H33(CH3)3N+‚-OP[OAl(oct)]2(OB). These results infer that the surface of the materials must be different from those of the parent AlPO mesophase. These findings imply that these BAPO materials may exhibit new catalytic behavior and novel properties for further functionalization (31) Fyfe, C. A.; Thomas, J. M.; Klinowski, J.; Gobbi, C. G. Angew. Chem., Int. Ed. Engl. 1983, 22, 259. (32) Maciel, G. E.; Sindorf, D. W. J. Am. Chem. Soc. 1980, 102, 7606.

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Figure 12. 1H/29Si/27Al CP-REDOR experiment of the as-synthesized mesostructured tubular SAPO material with 30 rotor cycles of dephasing: (A) 29Si spin-echo, S0; (B) 29Si{27Al} REDOR spectrum, S. The initial Si magnetization was prepared by 1H to 29Si CP. A set of 15 528 scans was acquired.

for specific applications. For the SAPO-based tubular material, we identified several different P chemical environments: (HO)2P[OAl(oct)]2, (HO)P[OAl(tet)]x[OAl(oct)]3-x and (HO)P[OAl(tet)]3. The existence of Si-O-Al linkages was confirmed unambiguously. Acknowledgment. Y.H. thanks the Natural Science and Engineering Research Council of Canada for a research grant and the Canada Foundation for Innovation for the award of a 400 MHz solid-state NMR spectrometer. Funding from the Canada Research Chair and Premier’s Research Excellence Award programs is also gratefully acknowledged. R.R. thanks

the province of Ontario for an OGSST scholarship. We thank Prof. R. E. Wasylishen for providing the software WSOLIDS for spectral simulation and Dr. C. Kirby for technical assistance. Supporting Information Available: Additional experimental results (PDF), including quadrapolar parameters for various Al sites (Table S1) and TRAPDOR curves for three P sites (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org. CM050396J