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A Novel Phase Transformation Phenomenon in Mesostructured Aluminophosphate Wanling Shen,† Shenhui Li,† Jun Xu,† Hailu Zhang,† Wei Hu,† Dan Zhou,‡ Jianan Zhang,‡ Jihong Yu,‡ Wujun Xu,§ Yao Xu,§ and Feng Deng*,† State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, People’s Republic of China, State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China, and State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China ReceiVed: December 18, 2009; ReVised Manuscript ReceiVed: March 9, 2010
A novel phase transformation phenomenon that involves two successive phase transformation events was found for the first time in the synthesis of mesostructured aluminophosphate and studied by XRD, TEM, and multinuclear solid-state NMR techniques. The results showed that a hexagonal phase Hex and two lamellar phases, L1 and L2, were formed after hydrothermal treatment for 1, 3, and 50 h, respectively. The status of the surfactant was found to be arrayed interdigitated in a bilayer with a tilt angle in L1 phase but upright in L2 phase. A mechanism that the exciting of the alkane tail of the surfactant together with the condensation of aluminophosphate cooperatively promoted the phase transformation was proposed for the observed phenomenon. Additionally, a ZON microporous structure was found for the first time existing in the framework of the mesostructured aluminophosphate. Introduction In 1992, the silicate-based mesoporous materials M41S were synthesized by Mobil scientists through the utilization of supramolecular arrays of surfactants as structure-directing agents.1,2 Since then, the range of pore sizes of molecular sieves has been dramatically expanded from micropore (up to 2 nm) to mesopore (ca. 2-50 nm).3 Mesoporous silicates can exhibit several ordered mesophases, such as the hexagonal (MCM-41), cubic (MCM-48), and lamellar (MCM-50) phases. Sometimes these phases can transform to one another.4–12 In the early 1980s, some microporous aluminophosphate- and silicoaluminophosphate-based zeolite-like materials (AlPO4-n and SAPO-n so-called zeotypes) were successfully synthesized.13,14 Accordingly, there was an apparent interest for scientists in the synthesis of aluminophosphates with mesoscale ordering, regardless of whether it is mesoporous or mesostructured. Over the past few years, the concept of supramolecular structure direction has been successfully applied in the synthesis of mesostructured aluminophosphates.15,16 Analogous to the silicatebased mesostructured materials, mesostructured aluminophosphate could also present several ordered mesophases. Even more, the phase transformation phenomena found in the mesoporous silicate also exist in the synthesis of mesostructured aluminophosphate. But, so far, the phase transformation behavior reported in the literature just occurred once in a synthesis procedure (for example, from a hexagonal phase to a lamellar phase or vice versa).17–19 Here, we found an intriguing phase transformation phenomenon that involves two successive phase transformation events throughout the whole hydrothermal * To whom correspondence should be addressed. E-mail: dengf@ wipm.ac.cn. Fax: +86-27-87199291. † Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences. ‡ Jilin University. § Institute of Coal Chemistry, Chinese Academy of Sciences.
synthesis process of mesostructured aluminophosphate, which led to the formation of a novel lamellar aluminophosphate that has never been found in other mesostructured materials. To investigate the unusual phenomenon, the structure of the aluminophosphate with different mesophases was comprehensively examined by various solid-state NMR methods on the molecular/atomic scale in combination with XRD and TEM techniques on the mesoscale, and then a model for the phase transformation behavior was proposed. Experimental Section Sample Preparation. Here, the fluoride route reported by Feng et al. was used.19 In a typical synthesis procedure, 7.26 g of ethanol was added to 4.01 g of finely ground aluminum isopropoxide. After stirring for 20 min, 24.8 g of distilled water was added, and then 2.77 g of 85% H3PO4 and 0.83 g of 40% HF were added dropwise. After further stirring, 2.4 g of CTAB (cetyltrimethylammonium bromide) was added to the system. Finally, 6.04 g of 10% TMAOH was added slowly to adjust the pH of the solution to ca. 7.0. The composition of the mixture (Al2O3/P2O5/CTAB/TMAOH/HF/C2H5OH/H2O) was 1:1.22: 0.68:0.68:1.70:16.12:175.51. The mixture was stirred vigorously at room temperature for 4 h and then transferred into several autoclaves. The autoclaves were put into an oven at 110 °C for crystallization and then quenched at specific times by an ice bath. The solid phase of each autoclave was separated by filtering and then washed and dried for the later characterizations. Characterizations. X-ray diffraction (XRD) was carried out using a Siemens D5005 diffractometer with Cu KR radiation (λ ) 1.5418 Å). The step size was 0.02°. High-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL-2010 at an acceleration voltage of 200 kV. The element contents were determined by induced coupled plasma (ICP) analysis.
10.1021/jp911959u 2010 American Chemical Society Published on Web 03/26/2010
Phase Transformation in Mesostructured Aluminophosphate All NMR experiments were performed on a Varian Infinityplus 400 spectrometer operating at a magnetic field strength of 9.4 T. The resonance frequencies at this field strength were 400.1, 376.5, 161.9, and 104.2 MHz for 1H, 19F, 31P, and 27Al, respectively. A Chemagnetics 5 mm triple-resonance MAS probe was employed to acquire all the spectra with a spinning rate of 6 kHz unless specified otherwise. 27Al MAS spectra were acquired using a one-pulse sequence with a short radio frequency (rf) pulse of 0.5 µs (corresponding to a π/15 flip angle) and a pulse delay of 0.4 s. Single-pulse 31P MAS NMR experiments with 1H decoupling were performed with a 90° pulse width of 4.9 µs, a 180 s recycle delay, and a 1H decoupling strength of 40 kHz. Single-pulse 19F MAS NMR was acquired using a 2.5 mm Chemagnetics probe with a 90° pulse width of 3 µs and a 60 s recycle delay at a spinning rate of 20 kHz. Prior to the 2D 27 Al f 31P heteronuclear chemical shift correlation (HETCOR) experiment, the 1D 27Al f 31P CP/MAS experiment was first optimized on a molecular sieve sample of AlPO4-5. The modified Hartmann-Hahn matching condition for 27Al f 31P CP/MAS experiments was as follows: ωP ) 43 kHz, ωAl ) 17 kHz, and ωr ) 8 kHz, corresponding to ωP ) 3ωAl - ωr,20 where ωP and ωAl were the rf field strengths applied on the 31P and 27Al channels, respectively, and ωr was the spinning rate. The contact time for cross-polarization was 1.0 ms, and the recycle delay was 0.3 s. For the 27Al f 31P HETCOR experiments, 6000 scans were accumulated for each of the 64 time-domain data points acquired in the 27Al dimension, resulting in a total experiment time of 33 h. The TPPI method was used in the 2D data acquisition and processing.21 The 27 Al{19F} REDOR experiment was performed on the 3 h heated sample by using a 4 mm Chemagnetics probe.22,23 The spinning speed was 8 kHz ( 2 Hz and the dephasing time was 0.5 ms (4 rotor cycles). 1H f 13C CP/MAS experiments were optimized on hexamethylbenzene (HMB). The rf field strengths applied on the 1H and 13C channels were 62.5 and 55.5 kHz, respectively. The spinning rate was 7 kHz, and the contact time was 2 ms. For 1H f 31P and 1H f 27Al HETCOR experiments, the Hartmann-Hahn matching condition was optimized on ammonium dihydrogen phosphate and kaolin, respectively, and the contact time was 2.0 and 0.2 ms, respectively. The 27Al 3QMAS spectra were obtained by using a three-pulse, z-filer sequence.24 The rf field strengths were 66 kHz for the first two hard pulses and 16 kHz for the third soft pulse, and the optimized pulse lengths were 4.2, 1.4, and 10 µs for the three consecutive pulses, respectively. Single-pulse 1H MAS NMR and 2D 1H DQ MAS NMR spectra was recorded using a 4 mm Chemagnetics probe at the spinning rate of 10 kHz. The 90° pulse width is 3 µs, and the recycle delay is 5 s. The back-to-back (BABA) recoupling pulse sequence25 was used to excite the DQ coherences, and the excitation time is 200 µs (two rotor periods). The 1H, 13C, 19F, 31P, and 27Al chemical shifts were referenced to tetramethylsilane (TMS), HMB, trifluoro acetic acid (CF3COOH), 85% H3PO4, and 1 M Al(NO3)3 solution, respectively. Results and Discussion XRD and TEM Analysis. Figure 1 shows the XRD patterns for the samples with different heating times. The diffractogram of the 1 h heated solid is of a hexagonal mesophase, designated as Hex. The peak at 2θ ) 1.9° is indexed to the 100 diffraction. The corresponding d spacing of this reflection is 4.6 nm (unit cell constant a ) 5.3 nm, a ) 2d100/√3). The broad width of this peak and relatively poor signal-to-noise ratio indicate that the hexagonal arrangement is of low ordering; thus, the higher-
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Figure 1. Low-angle XRD patterns of the aluminophosphate solids with different hydrothermal treatment times, as indicated. Inset: observed high-angle XRD patterns for the aluminophosphate solid with a hydrothermal treatment time of 50 h (upper line) and calculated powder diffraction pattern of a ZON structure (lower line).
order Bragg reflections 110 and 200 are hidden in the noise and difficult to be observed. After heating for 3 h, the intensity of the broad peak at 2θ ) 1.9° is remarkably reduced, and three other diffraction peaks at 2θ ) 3.3°, 6.5°, and 9.8° appear. The three peaks are indexed to the 001, 002, and 003 reflections of a lamellar phase, designated as L1, and the d001 value is calculated as 2.7 nm. It is noted that the spacing of 2.7 nm is smaller than the calculated double length of the CTA+ cations, implying that an interdigitation of the aliphatic tails and a certain tilt angle must be present in the arrangement of surfactant molecules, which will be discussed later. XRD measurement indicates that 88.4% of Hex phase is transformed into L1 phase, and only 11.6% of Hex phase remains in the 3 h heated sample. As the heating time is extended to 50 h, a strong diffraction peak at 2θ ) 2.5° with a d spacing of 3.6 nm appears, as well as an other peak at 2θ ) 7.5° with a d spacing of 1.2 nm, which means that the L1 phase is fully transformed to another mesophase. It seems that the two peaks are the 001 and 003 reflections of a lamellar phase with the 002 reflection missing. This phenomenon is found for the first time, and the reason is unclear. Here, we ascribe this novel mesophase to a lamellar phase designated as L2, and the further proof will be presented in the HRTEM examination. When the surfactant was removed either by extraction or by calcination, the mesoscale ordering was destroyed, and the three diffraction peaks disappeared in the XRD spectrum (not shown), which was in line with the behavior of a lamellar structure. The high-angle XRD patterns for the L2 phase is also collected (Figure 1, inset). Several sharp peaks can be observed, confirming its crystalline structure with molecular-scale periodicity. The profile of the pattern is very similar to that of a ZON (a structure type code given by the structure commission of the International Zeolite Association, see Scheme 1) type microporous structure.26 Furthermore, the synthesis condition here is similar to that of the UiO-7 microporous molecular sieve, which also has a ZON structure reported by Akporiaye et al.,27,28 so it is reasonable to assume that an AlPO framework with a ZON structure and some degree of crystallinity is formed in L2 phase, which will be further supported by the following NMR experiments.
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SCHEME 1: Schematic Drawing of the Structure of the Aluminophosphate/Surfactant Aggregates and the Phase Transformation Process. Oxygen Atoms Are Omitted in the Inorganic Aluminophosphate Framework for Clarity
To elucidate the lamellar structure of L2 phase, TEM for the 50 h heated sample is performed and the result is shown in Figure 2. The image at the relatively lower magnification shows clear parallel striped patterns. The fringe spacing measured here is 3.4 nm, being close to the d spacing calculated from the peak at 2θ ) 2.5° in the XRD pattern (Figure 1). The sample has been examined by TEM either as microtomed sections or as grain mounts, and only striped patterns could be found, which is consistent with the lamellar structure of L2 phase. In the image at relatively higher magnification (Figure 2, inset), many crossed lattice fringes with different directions can be observed, corresponding to the peaks appearing in the high-angle scattering
Figure 2. TEM images of the aluminophosphate solid with a hydrothermal treatment time of 50 h at low magnification and high magnification (inset).
regime in the XRD pattern (Figure 1, inset) and indicating a crystalline framework. The XRD and TEM results above reveal the long-range ordering change of the samples. It can be clearly obtained on the mesoscale that the composites exist as different phases during the hydrothermal treatment, which are hexagonal Hex, lamellar L1, and lamellar L2 when heated for 1, 3, and 50 h, respectively. However, information about the local range ordering change is still absent, which is crucial for understanding the detailed phase transformation process. In the following, we employed various NMR experiments to study the structure evolution of the samples. Solid-State NMR. NMR is sensitive to short-range ordering at the atomic level and can be used to investigate the local bonding and close environmental changes in the composites. NMR and XRD techniques can complement each other on different length scales. 27 Al, 31P MAS NMR and 27Al MQMAS NMR. The 27Al MAS NMR spectra of the samples with different heating times are displayed in Figure 3a. Two peaks can be observed clearly in the spectrum of the 1 h heated sample. The main peak at -8 ppm and the minor peak at 46 ppm are assignable to octahedral and tetrahedral oxygen-coordinated aluminum, respectively. The signals are Gaussian-like and do not exhibit a specific quadrupolar line shape. After heating for 3 and 50 h, conspicuous changes occur in the spectra. Several signals with a chemical shift in the region of pentacoordinated and tetracoordinated Al can be found. Because 27Al is a quadrupolar nucleus, it is difficult to recognize that the asymmetric shape of the peaks results from second-order quadrupolar line shapes or overlap of multiple peaks. Further experiment is needed to resolve the complicated 27Al peaks. The 2D multiple-quantum magic-angle
Phase Transformation in Mesostructured Aluminophosphate
Figure 3. 27Al (a) and 31P (b) MAS NMR spectra of the aluminophosphate solids with different hydrothermal treatment times, as indicated. Asterisks denote spinning sidebands.
spinning (MQMAS) experiment can refocus the second-order anisotropic broadening and give isotropic spectra along the F1 dimension after a shearing transformation, whereas the F2 dimension displays the normal second-order quadrupolar line shapes. The 3QMAS spectra of the 3 and 50 h heated samples are shown in Figure 4. It can be easily discerned that there are five different peaks in each spectrum: two tetracoordinated Al peaks (denoted as Al1 and Al2 in the figure), two pentacoordinated Al peaks (denoted as Al3 and Al4 in the figure), and one hexacoordinated Al peak (denoted as Aloct in the figure). The 2D MQMAS spectra unequivocally indicate that the asymmetric signals in the 1D MAS spectra of the two samples are, in fact, a superposition of multiple quadrupolar line shapes. The quadrupolar interaction parameters and isotropic chemical shifts extracted from the simulation of corresponding slices of the 3QMAS spectra by Dmfit29 are listed in Table 1 (also see Figure S1 in the Supporting Information). It can be seen that the isotropic chemical shifts and quadrupolar coupling constants for the four Al sites in L2 phase are similar to those reported for the UiO-7 microporous molecular sieve having a ZON-type framework.30 The hexacoordinated Al can be considered as an amorphous species with a low degree of condensation, whereas the pentacoordinated and tetracoordinated Al as species with a higher degree of condensation in the crystalline structure. Integration of the peaks in the 1D 27Al MAS spectra (see Figure
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Figure 4. Sheared 27Al 3QMAS spectra of the aluminophosphate solids with hydrothermal treatment times of 3 (a) and 50 h (b).
TABLE 1: 27Al Isotropic Chemical Shifts and Quadrupole Interaction Parameters, Determined from Simulation of the Corresponding Slices of 3QMAS Spectra 3h peaks
δiso (ppm)
Al1 Al2 Al3 Al4
44.8 ( 0.5 40.4 ( 0.5 22.9 ( 0.5 23.3 ( 0.5
CQ (MHz)
50 h ηQ
3.7 ( 0.1 0.95 ( 0.05 2.7 ( 0.1 0.05 ( 0.05 3.8 ( 0.1 0.5 ( 0.1 3.0 ( 0.1 0.95 ( 0.05
δiso (ppm) 45.6 ( 0.5 40.3 ( 1 24.2 ( 0.5 23.8 ( 0.5
CQ (MHz)
ηQ
3.6 ( 0.1 0.5 ( 0.1 2.6 ( 0.3 0.8 ( 0.2 4.2 ( 0.1 0.95 ( 0.05 4.9 ( 0.1 0.3 ( 0.1
S2 in the Supporting Informaton) shows that 88, 54.5, and 22.1% of aluminum atoms are in an octahedral environment for the samples heated for 1, 3, and 50 h, respectively. It indicates that the degree of condensation increases as the heating time extends, and the condensation reaction finally leads to the formation of a crystalline structure with a ZON topology in the AlPO framework of L2 phase. Figure 3b shows the 31P MAS NMR spectra of the samples with different heating times. Only one broad peak centered at -12 ppm attributed to less condensed P species can be observed in the spectrum of the 1 h heated sample, indicating the amorphous nature of the inorganic framework in Hex phase. After heating for 3 h, the intensity of this broad peak decreases dramatically. Meanwhile, two sharp peaks at -14.5 and -19.2 ppm together with a shoulder peak at -21.1 ppm appear.
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Deconvolution of the corresponding 31P NMR spectrum (including the first-order spinning sidebands) indicates that 83.4% of Hex phase is transformed into L1 phase and 16.6% of Hex phase is left in this sample. After heating for 50 h, the peaks at -14.5 and -19.2 ppm are shifted upfield to -15.9 and -19.8 ppm, respectively, and the shoulder peak at -21.1 ppm becomes invisible, but it can be also resolved in the 2D 27Al f 31P HETCOR spectra (see the following). Meanwhile, an additional peak at -31.9 ppm appears. The four resonances at -15.9, -19.8, -21.1, and -31.9 ppm in L2 phase are similar to those observed by Akporiaye28 and Roux31 et al. on the UiO-7 microporous molecular sieve, so they can be assigned to the four crystallographically nonequivalent sites (denoted as P1, P2, P3 and P4) in the ZON structure (Scheme 1). Although our XRD measurement indicates the presence of a pure L2 phase with no Hex or L1 phase left, deconvolution of the corresponding 31P NMR spectrum reveals that 4.2% of Hex phase still remains. 27 Al f 31P HETCOR. HETCOR (heteronuclear chemical shift correlation spectroscopy) is a two-dimensional technique correlating spatially adjacent different nuclei based on CP (crosspolarization), which is mediated by direct through space heteronuclear dipolar interaction, and it has been used to detect the connectivity between Al and P atoms in AlPO gels.32–39 To ensure that the phosphorus atoms are cross-polarized only by the adjacent aluminum atoms, a short contact time of about 1 ms is adopted, so the correlation peaks in the 2D HETCOR spectra can reveal correlations between the nearest neighbors of the P and Al atoms in their second coordination sphere. Probably due to a very low degree of condensation and the disordered amorphous nature of the AlPO species in Hex phase, it is hard to obtain a 2D 27Al f 31P HETCOR spectrum of the 1 h heated sample due to its so poor sensitivity. We just got a weak signal at ca. -12 ppm in the 1D 27Al f 31P CP/MAS spectrum (not shown), which indicates the existence of some aluminophosphate species. After heating for 3 h, as the AlPO becomes more and more ordered and condensed, the CP efficiency increases, leading to strong signal intensity. Figure 5a presents the 27Al f 31P HETCOR spectrum of the 3 h heated sample. In the 27Al projection, it is hard to discern each peak of the four sites in the ZON structure but only two signals can be resolved: tetracoordinated Al (Al1 and Al2) at 35 ppm and pentacoordinated Al (Al3 and Al4) at 16 ppm, which are denoted as Altet and Alpen in the figure, respectively. It can be seen that the peaks at -14.5 and -19.2 ppm in the 31P projection are correlated to both the pentacoordinated Al and the tetracoordinated Al in the 27Al projection, whereas the 31P peak at -21.1 ppm is only correlated to tetracoordinated Al. When the heating time is extended to 50 h, the condensation reaction increases the crystallinity of L2 phase and the integral ZON structure is formed in its framework, so in the 27Al f 31P HETCOR spectrum (Figure 5b), all the P sites are correlated both to Alpen and to Altet, which is in line with the connectivity in ZON topology, as shown in Scheme 1. 19 F MAS NMR. 19F MAS NMR spectra of the samples with different heating times are shown in Figure 6. In the spectrum of the 1 h heated sample, only one broad peak can be observed at ca. -144 ppm. After hydrothermal treatment for 3 h, two sharp signals at -110 and -163 ppm arise at the expense of the intensity of the broad peak at -144 ppm. When the hydrothermal treatment time is prolonged to 50 h, the intensity of the peaks at -144 ppm decreases further. To assist the spectral assignment, an 27Al{19F} REDOR NMR experiment was performed on the selected 3 h heated sample (Figure 7)
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Figure 5. 27Al f 31P HETCOR spectra of the aluminophosphate solids with hydrothermal treatment times of 3 (a) and 50 h (b).
Figure 6. 19F MAS NMR spectra of the aluminophosphate solids with different hydrothermal treatment times, as indicated. Asterisks denote spinning sidebands.
and the result shows that only the hexacoordinated and pentacoordinated Al signals are dephased, while the tetracoordinated Al signal remains almost unchanged, indicating that both hexacoordinated and pentacoordinated Al species (rather than tetracoordinated Al) are associated with fluorine. Bailly et al.30 verified that the fluorine is in the AlPO4-ZON framework located inside the [4462] cage by measuring the internuclear distances between 19F and all 27Al sites using the MQ-REDOR technique, and the observed 19F chemical shift was -109 ppm.30,31 Therefore, we ascribe the peak at -110 ppm to the fluorine
Phase Transformation in Mesostructured Aluminophosphate
J. Phys. Chem. C, Vol. 114, No. 15, 2010 7081 TABLE 2: Results of Elemental Analysis in wt % for the Aluminophosphate Solids with Different Hydrothermal Treatment Times 1h 3h 50 h
Ala
Pa
Cb
Nb
16.7 16.2 16.3
13.8 13.6 13.8
8.5 18.3 19.5
0.4 1.0 1.5
a Inductively coupled plasma (ICP) analysis performed on a PerkinElmer Optima 3300Dv spectrometer. b Elemental analyses conducted on a PerkinElmer 2400 elemental analyzer.
Figure 7. 27Al{19F} REDOR experiment of the aluminophosphate solid with a hydrothermal treatment time of 3 h. 27Al spin-echo (S0), REDOR (S), and REDOR difference spectra (∆S).
Figure 9. 1H MAS NMR spectra of the aluminophosphate solids with different hydrothermal treatment times, as indicated.
Figure 8. 13C CP/MAS NMR spectra of the aluminophosphate solids with different hydrothermal treatment times, as indicated.
bridging two aluminum atoms within the cagelike building block and existing in 5-fold coordination, which is part of the ZON structure in the crystalline framework, as shown in Scheme 1. In SAPO-34 gels, HF exhibits peaks at ca. -159 ppm,40 and in our previous studies of mesostructured aluminophosphates containing fluorine,41 the fluorine associated with octahedral Al showed broad peaks at -147 ppm and the HF species appeared at -182 ppm in the 19F MAS NMR spectra. Here we ascribe the 19F signals at -144 ppm to amorphous Al(OH)6-(x+y)(OP)xFy species and the peaks at -163 ppm to HF species. The fluorine coordinating to octahedral Al atoms here plays a role of mineralizer to promote the condensation of AlPO species. 13 C CP/MAS NMR. Figure 8 shows the 1H f 13C CP/MAS NMR spectra of the samples with different heating times. The spectra are similar to those reported for CTA+ cations in hybrid materials.42,43 The peaks at 55 and 67 ppm are assigned to the
methyl (C1) and N-methylene (C2) groups, respectively. The peak at 28 ppm corresponds to C3 methylene groups, and the peak at 16 ppm represents the terminal methyl groups (C17). The observed peak at 24 ppm is owing to R-C atoms (C16). The peak at 31 ppm with a weak low-field shoulder at 33 ppm originates from the inner-chain methylene groups (C4-C15). It is reported that the inner-methylene carbons of the aliphatic chains of the surfactant in all-trans conformation give rise to peaks at ca. 33 ppm in mesostructured aluminophosphates and silicates, whereas that in gauche conformation appear at ca. 31 ppm.44,45 The result indicates that the surfactant in all three samples has a mixed conformation. After heating for 3 h, the intensity of the peak at 55 ppm obviously increases due to the incorporation of N(CH3)4+ into the aluminophosphate framework to compensate the negative charges from F- anions in the ZON structure site. This can be further confirmed by 1H MAS NMR presented later and the elemental analysis (Table 2), in which the molar ratio of N to polyhedral centers (Al + P) increases and that of C to N decreases. When the time of hydrothermal treatment is extended to 50 h, another peak at 58 ppm appears due to the splitting of N-methyl groups, which was ascribed to the unequivalent local environment of the three methyl carbons.43,46 The headgroups are closely spaced and partially ordered, then the rotational motion of the tail groups next to the headgroup becomes hindered and restricted, so the methyl groups are no longer in an symmetrical and identical environment and eventually cause the observed splitting of the C1 peak. The 13C NMR result reveals that the surfactant is stacked more ordered in L2 phase than L1 and Hex phases. 1 H MAS NMR and DQ MAS NMR. Figure 9 shows the 1H MAS NMR spectra of the samples with different heating times. Three peaks at 0.9, 1.3, and 3.2 ppm are discerned in the spectra, which can be attributed to alkyl chain CH3 protons, alkyl chain CH2 protons (except for N-CH2 in the R position), and polar head N-CH3/N-CH2- protons, respectively.47 It can be seen that the peak at 3.2 ppm grows up after heating for 3 h,
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Figure 10.
1
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H DQ MAS NMR spectra of the aluminophosphate solids with hydrothermal treatment times of 1 (a), 3 (b), and 50 h (c).
indicating the incorporation of N(CH3)4+ into the aluminophosphate framework (the chemical shift of N(CH3)4+ is around 3.2 ppm). This is consistent with the intensity growth of the 55 ppm peak in the 1H f 13C CP/MAS NMR spectra (Figure 8). When the integral ZON structure is formed after heating for 50 h, the N(CH3)4+ ion occupies the [466484] cage.26,28,30 For convenience, the three 1H peaks are designated as A, B, and C, respectively. To elucidate the arrangement of the organic surfactant in the mesostructured aluminophosphate in detail, we further performed the two-dimensional 1H DQ MAS NMR experiments. The 1H DQ MAS NMR experiment is a useful method for probing proton-proton proximities in various solid materials,48 and the presence of a signal in the 1H DQ MAS spectrum indicates that two protons are in close proximity (
