J. Phys. Chem. C 2007, 111, 1579-1583
1579
ARTICLES 27Al
Triple-Quantum Magic-Angle Spinning Nuclear Magnetic Resonance Characterization of Nanostructured Alumina Materials Hae Jin Kim,*,† Hyun Chul Lee,‡ and Jae Sung Lee§ Energy Nanomaterial Team, Korea Basic Science Institute, Daejeon 305-333, Korea, Samsung AdVanced Institute of Technology, Suwon 440-600, Korea, and Department of Chemical Engineering, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea ReceiVed: August 11, 2006; In Final Form: NoVember 29, 2006
The nanostructured transition aluminas were synthesized from an aluminum alkoxide precursor in the presence of various surfactants under hydrothermal conditions without any organic additives. The synthesized nanostructured alumina materials showed different morphologies depending on surfactants with high surface area, thermal stability, and aluminum sites of different coordination. The 27Al triple-quantum (3Q) magicangle spinning (MAS) NMR results showed that the samples prepared with cationic and nonionic surfactants consisted of mainly γ-Al2O3 phase and a small amount of η-Al2O3 phase due to the presence of the distorted tetrahedral site, but that the samples with anionic and neutral surfactants were predominantly in the η-Al2O3 phase. The experimental and calculated values of the electric field gradient (EFG) definitely suggested that cation vacancies were preferentially located at the octahedral sites in γ-Al2O3 whereas at the tetrahedral sites in η-Al2O3.
Introduction Alumina is most commonly used for catalysis and catalyst supports due to its thermal, chemical, and mechanical stability. Recently, enthusiastic efforts have been devoted to the synthesis of nanostructured alumina with an ordered structure. Typically, the synthetic strategy for such materials is based on templating of surfactant micelle structures. The procedure was originally developed by Mobil researchers for the synthesis of siliceous molecular sieves of the M41S family1,2 and recently has been extended to the preparation of many non-siliceous mesoporous oxides.3-5 The typical hydrothermal synthesis based on supramolecular surfactant assembly yields mesoporous alumina molecular sieves with a wormholelike channel motif.6-13 The synthesis of nanostructured alumina with other geometries has also been reported.14-18 Zhu et al.14 reported γ-alumina nanofibers prepared from aluminum hydrate with polyethyleneoxide surfactants. Zhang et al.15 synthesized porous lath- to rod-shaped nanoparticles with a nonionic surfactant. There has been no report on the synthesis of hollow alumina nanotubes based on templating of surfactants, although electrochemical anodizing methods are known to produce individual and branched alumina nanotubes.16,17 More recently, we reported a general synthesis method for unidirectional alumina nanostructures and their lithium derivatives with various surfactants without addition of any organic solvent.19,20 The synthesized nanostructured alumina * To whom correspondence should be addressed: phone +82-42-8653953; fax +82-42-865-3419; e-mail
[email protected]. † Korea Basic Science Institute. ‡ Samsung Advanced Institute of Technology. § Pohang University of Science and Technology.
materials showed different morphologies depending on the employed surfactants, yet all of them had high surface area, thermal stability, and aluminum sites of different coordinations. Applications of nanostructured alumina materials are diverse and can be extended to the field of electronics and optical science in addition to the traditional uses of separation, adsorption, and catalysis. For instance, cylindrical morphology of the nanostructured alumina materials give a possibility as matrix-mediate of molecular and quantum wire. A field emission high-resolution transmission electron microscopy (FE-HRTEM) study of the calcined materials has been carried out in order to probe the architecture of the nanostructured alumina. The representative FE-HRTEM images show different morphology depending on the surfactants.19 The powder X-ray diffraction (XRD) peaks typical of γ-alumina in these materials have been previously reported in the literature.11,13,15,19,20 However, it is very difficult to discriminate the phases between γ- and η-Al2O3, which are very similar, from the XRD powder patterns. They are both of a spinel structure and have the same oxygen arrangement but different distributions of cation vacancy between octahedral and tetrahedral sites.21 This detailed environment of Al species could be elucidated by 27Al NMR, which is the main technique employed in this study. Experimental Procedures The details of the nanostructured transition alumina with different surfactants preparation were reported elsewhere.18,19,20 The 27Al one-dimensional (1D) cross-polarization (CP) magicangle spinning (MAS) and two-dimensional (2D) triple-quantum (3Q) MAS NMR measurements were performed at 14.1 T on a
10.1021/jp0651945 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007
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Kim et al.
Figure 1. Typical 27Al MAS NMR spectra in uncalcinated nanostructured transition aluminas.
Varian Unity Inova 600 MHz spectrometer equipped with a 2.5 mm Chemagnetics MAS probe head at a sample rotation rate of 20 kHz at KBSI Taegu Center. The corresponding Larmor frequency is 156.38 MHz. For obtaining the 27Al 1D MAS spectra, a single pulse excitation of 2 µs and a recycle delay of 1 s were used. For the 27Al{1H} CP-MAS NMR experiment, optimized values for the contact time of 450 µs and a recycle delay time of 1 s were used. The 2D 3Q MAS experiments were carried out by the two-pulse z-filtered procedure with an excitation pulse of 20 µs and a conversion pulse of 3.5 µs. Typically, 256 transients consisting of 2048 points were added for each of the 64 increments of t1. The 27Al chemical shifts were determined relative to a 1.0 M [Al(H2O)6]3+ aqueous solution.
Figure 2. Typical 27Al solid-state NMR spectra of the calcined nanostructured transition alumina materials and standard γ-Al2O3 (Aldrich). (a) 27Al MAS NMR spectra. The solid lines represent the best fitted spectra. (b) 27Al CP-MAS NMR of alumina prepared with cationic and nonionic surfactants.
Results and Discussion
a From the best fitted 27Al MAS NMR spectra in the nanostructured transition alumina materials with various surfactants and standard γ-Al2O3 (Aldrich).
The solid-state 27Al NMR studies of nanostructured transition alumina materials were performed to investigate the local electronic environment around resonant Al nuclei. Figure 1 reveals 27Al MAS NMR spectra of nanostructured transition alumina materials before calcination, which showed boehmite γ-AlOOH structure in XRD analysis. All samples showed only octahedrally coordinated Al species at 0 ppm, in accord with previous works that used nonionic surfactants.11 The representative 27Al solid-state NMR spectra of calcined materials and the standard neutral γ-Al2O3 (Aldrich) are depicted in Figure 2. 27Al MAS NMR spectra (Figure 2a) show two wellresolved 27Al NMR peaks at 72 and -1 ppm, in all samples, which can be assigned to Al centers coordinated to donor atoms with tetrahedral and octahedral geometries, respectively.7,22,23 The fraction of octahedrally coordinated Al in ideal γ-Al2O3 and η-Al2O3 must be 75% and 62.5%, respectively.21 From the best fitting of 27Al MAS NMR spectra, we find that 66.06% ( 0.4% of Al cations are octahedrally coordinated in nanostructured alumina prepared with cationic and nonionic surfactants, while 62.59% ( 0.2% of Al cations are octahedrally coordinated in alumina samples prepared with anionic and neutral surfactants. The 27Al NMR results of the different fractions of octahedrally coordinated Al in all samples can be explained by the different distributions of vacancy sites in the two spinel cation sublattices. The signal intensities of the resonances are summarized in Table 1. In addition to these two peaks, alumina prepared with cationic and nonionic surfactants also show a very weak NMR signal at 33 ppm, which is assigned to a pentacoordinated aluminum site. This result implies the existence of amorphous domains as
TABLE 1: Determined Fractions of Tetrahedrally and Octahedrally Coordinated Ala fraction (%) materials
4-coord (72 ppm)
5-coord (33 ppm)
6-coord (-1 ppm)
cationic, nonionic anionic, neutral γ-Al2O3 (acidic) γ-Al2O3 (basic) γ-Al2O3 (neutral)
25.46 37.41 24.80 25.14 25.39
8.48
66.06 62.59 75.20 74.86 74.61
defects arising from distorted octahedrally coordinated Al with a poor crystallinity in these samples. Also, 27Al CP-MAS NMR spectrum (Figure 2b) of the calcined samples exhibits three wellresolved NMR peaks at 72, 33, and -1 ppm. Cross-polarization increases the relative intensity of pentacoordinated aluminum center, largely due to the magnetization transfer from proton to the aluminum center. Thus, it appears that the pentacoordinated aluminum site in these nanostructured samples contains electron acceptor functionality as defects in the wall structure. It is important to note here that alumina samples prepared with anionic and neutral surfactants do not contain pentacoordinated aluminum sites. Thus, it appears that the nature of surfactants has contributed to the formation of these defect sites. The results of the 27Al 3Q MAS NMR also provide new insight into the nature of the electronic environment around resonant aluminum nuclei in the nanostructured transition alumina. As can be seen in Figure 3a,b, 2D 3Q MAS NMR spectra of nanostructured transition alumina show well-resolved and completely different quadrupole distribution between tetrahedral and octahedral aluminum centers. However, we could not discriminate the NMR spectra at the tetrahedral coordinate Al center in neutral γ-Al2O3 (Figure 3c). This result indicates that the tetrahedrally coordinated Al site in neutral γ-Al2O3 has no electric field gradient (EFG) because of the symmetrical distribution of the electronic potential at the resonant Al site. When the nucleus spin number is greater than 1/2 (I > 1/2), the nucleus possesses an electric quadrupole moment (Q) that interacts with the EFG at the resonant nucleus. Thus, the
27Al
3Q MAS NMR of Nanostructured Alumina
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Figure 3. Two-dimensional triple-quantum 27Al MAS NMR spectra of calcined nanostructured transition alumina materials with a triple-quantum z-filter sequence. The FE-HRTEM images were taken from ref 19. (a) Samples prepared with cationic (cetyltrimethylammonium bromide) and nonionic [alkylpoly(ethylene oxide)] surfactants; (b) samples prepared with anionic (alkylcarboxylic acid) and neutral (alkyamine) surfactants; (c) samples prepared with neutral γ-Al2O3 (Aldrich).
multiple quantum (MQ) MAS efficiency strongly depends on the magnitude of the quadrupole splitting. For 3Q MAS of a spin I ) 5/2 nucleus, the center of gravity of a line shape in the MQ MAS spectrum is given by24
(δ1,δ2) ) (3δcs - /5δQ,δcs - /15δQ) 4
16
(1)
where δcs and δQ are the isotropic chemical shift and the isotropic second-order quadrupole shift, respectively. Thus, the distance of the center of gravity from the chemical shift axis along the quadrupolar induced shift direction allows us to
determine the second-order quadrupolar effect that includes the quadrupole coupling constant CQ (e2qQ/h) and asymmetry parameter η:
(
PQ ) C Q 1 +
)
η2 3
1/2
(2)
From the 27Al 3Q MAS NMR spectra, we could obtain the values of the EFGs for the tetrahedral and octahedral aluminum centers in nanostructured transition alumina and γ-Al2O3 (Aldrich).
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TABLE 2: Experimental and Calculated Electric Field Gradient for Al in Nanostructured Alumina Materialsa 3Q MAS materials cationic, nonionic anionic, neutral γ-Al2O3 η-Al2O3
PCA
WIEN-97
4-coord 6-coord 4-coord 6-coord 4-coord 6-coord 2.125
0.901
2.391
0.874
0
0.965
0 1.383
0.833 1.133
0 1.239
0.438 0.799
a Prepared with various surfactants and standard transition aluminas. The unit of EFG is 1021 V/m3.
For calculating EFG at Al sites in γ-Al2O3 and η-Al2O3, we employed the simple point charge approximation (PCA)25 and the WIEN-9726 code. Generally, the total EFG at the Al nucleus site can be expressed by25
eqt ) (1 - γ∞)eqlatt + (1 - R)eqatom
(3)
where the first term (eqlatt) is the EFG of the ionic contribution of the lattice and the second term is the atomic contribution of the cation itself. Here γ∞ and R are the Sternheimer antishielding and shielding factors, respectively. However, the transition alumina inevitably contains the defects due to the presence of the valence--unsaturated Al atoms. Thus, these vacancies destroy the cubic symmetry in the transition alumina structure. From this fact, eq 3 could be replaced by
eqt ) (1 - γ∞)eqlatt + (1 - γ∞)eqvac + (1 - R)eqatom
Conclusions
(4)
where the EFG of the valence effect (eqval), contribution of the vacancies, can be expressed by27
∑i
eqlvac ) (1 - γ∞)[Zeff - Zi(Ri - r1/2)]
3 cos2 θi - 1 Ri3
at tetrahedrally coordinated Al centers in η-Al2O3. On the basis of these results, we might conclude that the cation vacancies are located at the octahedral site in γ-Al2O3 and tetrahedral site in η-Al2O3 due to the different random distribution, respectively. This fact together with the fractions of tetrahedrally and octahedrally coordinated Al in Table 1 strongly suggest that cationic and nonionic samples consist of mainly γ-Al2O3 phase with some amorphous regions as defects and a small amount of η-Al2O3 phase due to the presence of the distorted tetrahedral site, but anionic and neutral samples are predominantly in the η-Al2O3 phase. Thus, surfactants have a significant effect on the formation of transition alumina nanostructure. Cationic and nonionic surfactants seem to deteriorate the structure to form amorphous domain as defects of γ-phase, while anionic and neutral surfactants distort the structure to change the crystal phase of alumina to the more distorted η-phase by further rapid progress of the hydrolysis reactions of remaining product. The 27Al 3Q MAS NMR results showed that the samples prepared with cationic and nonionic surfactants consisted of mainly γ-Al2O3 phase and a small amount of η-Al2O3 phase due to the presence of the distorted tetrahedral site, but that the samples with anionic and neutral surfactants were predominantly in the η-Al2O3 phase. The experimental and calculated values of the electric field gradient (EFG) definitely suggested that cation vacancies were preferentially located at the octahedral sites in γ-Al2O3 but at the tetrahedral sites in η-Al2O3.
(5)
where Zeff is the effective charge, Ri is the nth neighbor distance in the perturbed crystal, and r1 is the nearest neighbor distance in the perfect crystal. Using eqs 4 and 5 and the structure data for γ- and η-Al2O3, the obtained values of EFGs at the Al sites are eqAl(γ) ) 0.833 × 1021 V/m2, eqAl(η) ) 1.113 × 1021 V/m2 for octahedrally coordinated Al and eqAl(γ) ) 0 V/m2, eqAl(η) ) 1.383 × 1021 V/m2 for tetrahedrally coordinated Al. The experimentally determined results from 3Q MAS and calculations by employing WIEN-97 and PCA are summarized in Table 2 for comparison. The agreement between the experimental and calculated values at octahedrally coordinated Al sites with PCA and WIEN-97 is reasonable. There are many controversies as to whether the vacancies occupy the octahedral or tetrahedral sites of the cation in the transition alumina. Our calculated values, from both PCA and WIEN-97, clearly show that the EFG at the tetrahedral site in γ-Al2O3 is zero, which means the tetrahedrally coordinated Al in γ-Al2O3 still has cubic symmetry. It is in good agreement with the two-dimensional triple-quantum 27Al MAS NMR result (see Figure 3c). In other words, the vacancies in γ-Al2O3 mainly occur at the octahedral site. In case of η-Al2O3, however, the calculated EFG values, for both PCA and WIEN-97, of tetrahedrally coordinated Al center are larger than those of octahedrally coordinated Al center. This result demonstrated that the vacancies are more randomly distributed over the tetrahedral sites than the octahedral one, which induces the increased EFG
The physical properties in nanostructured transition alumina were investigated by using a variety of 27Al NMR methods. The analysis of MAS spectra showed the presence of two distinguishable sites in all samples. The 27Al 3Q MAS NMR results clearly showed that the nanostructured alumina samples have the γ-Al2O3 phase and η-Al2O3 phase due to the presence of the different distorted sites. The calculated values of the EFG definitely suggested that cation vacancies were preferentially located at the octahedral sites in γ-Al2O3 but at the tetrahedral sites in η-Al2O3. Acknowledgment. This work has been supported by HERC of Ministry of Korea Science and Technology. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) 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. (3) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (4) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (5) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813. (6) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (7) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1102. (8) Cabrera, S.; Haskouri, J. E.; Alamo, J.; Beltra´n, A.; Beltra´n, D.; Mendioroz, S.; Marcos, M. D.; Amoro´s, P. AdV. Mater. 1999, 11, 379. (9) Vaudry, F.; Khodabandeh, S.; Davis, M. E. Chem. Mater. 1996, 8, 1451. (10) Yada, M.; Machida, M.; Kijima, T. Chem. Commun. 1996, 769. (11) Gonza´lez-Pen˜a, V.; Dı´as, I.; Ma´rques-Alvarez, C.; Sastre, E.; Pe´rezPariente, J. Microporous Mesoporous Mater. 2001, 203, 44-45 (12) Valange, S.; Guth, J.-L.; Kolenda, F.; Lacombe, S.; Gabelica, Z. Microporous Mesoporous Mater. 2000, 597, 35-36.
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