Crystallization of AlPO4-5 Aluminophosphate Molecular Sieve

In the present work, multinuclear solid-state NMR techniques, combined with powder X-ray diffraction (PXRD) and infrared (IR) spectroscopy, are employ...
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J. Phys. Chem. B 2007, 111, 7105-7113

7105

Crystallization of AlPO4-5 Aluminophosphate Molecular Sieve Prepared in Fluoride Medium: A Multinuclear Solid-State NMR Study Jun Xu, Lei Chen, Danlin Zeng, Jun Yang, Minjin Zhang, Chaohui Ye, and Feng Deng* State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, People’s Republic of China ReceiVed: February 5, 2007; In Final Form: April 16, 2007

In the present work, multinuclear solid-state NMR techniques, combined with powder X-ray diffraction (PXRD) and infrared (IR) spectroscopy, are employed to monitor the crystallization of AlPO4-5 aluminophosphate prepared in the presence of HF under hydrothermal condition. The crystallization process is characterized by the evolution of intermediate gels, in which the long-rang ordering arrangement is probed by PXRD, revealing the threshold of the crystallization around 120 min. The appearance of 31P signals at ca. -22 and -29 ppm due to the structural P-O-Al unit and 19F signal at -120 ppm due to the structural F-Alpen-O-P unit in the NMR spectra of the series gels indicates that the crystalline framework is starting to form. The onset of the crystallization is also evidenced by the presence of the pentacoordinated Al in the structural F-AlpenO-P unit which is considered to be associated with the ordered framework. More information about the local ordering of the gels is obtained from two-dimensional 27Al f 31P heteronuclear correlation (HETCOR) and 31P/27Al double-resonance experiments. In combination with 1H f 31P cross-polarization/magic-angle spinning (CP/MAS) experiments, two microdomains can be identified in the 120 min heated gel. A possible evolution mechanism of the gels consisting of three successive stages is proposed for the crystallization process.

Introduction Zeolites are a well-known family of microporous materials, which are usually referred to as molecular sieves, possessing regular pores or voids in the size range of 5-20 Å. After the successful synthesis of the first artificial zeolite in the 1950s, preparation of hundreds of zeolites with various structures has been achieved. Owing to their special architecture and properties, zeolites have great value in industrial applications as heterogeneous catalysts, adsorbents, ion exchange agents, and materials for molecular recognition operations. Aluminophosphates (AlPOs), first reported by Wilson et al. in 1982,1 constitute another large class of molecular sieves. Isomorphic substitution of framework Al3+ and P5+ ions by metal cations (V, Co, Mg, Ga, Fe, Zn, etc.) or silicon produces the MeAPO and SAPO family materials, respectively. These materials not only exhibit the characteristics of zeolites but also show novel physicochemical properties that are linked to their unique composition.2 Therefore, the synthesis and characterization of the new materials is a huge field. However, even though new molecular sieves are regularly being discovered, the rational “priori design” of molecular sieves now is not possible because of a lack of full understanding of their synthesis mechanism. The AlPOs or MeAPOs materials are commonly formed under hydrothermal conditions. The extreme complexity of the crystallization process and the great diversity of different reaction systems more or less hamper the understanding of synthesis mechanism. Nevertheless, during the past decade, much knowledge has been accumulated by application of advanced techniques, including X-ray diffraction,3,4 NMR,5-10 IR/Raman,11 SAXS/WAXS (small- and wide-angle X-ray scattering),12 and neutron diffraction.13 Among them, NMR is a powerful and unique tool * Corresponding author. Phone: +86-27-87198820. Fax: +86-2787199291. E-mail: [email protected].

that can probe the local or atomic environments of solid or liquid phase during the crystallization. Recently, some “in situ” NMR studies have been reported on the crystallization of aluminophosphates and relevant materials,5-7 in which the crystallization process was monitored, and the possible mechanism was proposed on the basis of the soluble aluminophosphate species in solution. Other than the studies on the soluble species, the investigation on intermediate gels that proceed to nucleate and grow into the final product may allow one to obtain some different information on the crystallization mechanism. In previous reports,8,10,14,15 several aluminosilicates and aluminophosphates were investigated by characterizing the intermediate gels during the crystallization, which provided valuable insight into the interaction between structure-directing agent and inorganic species as well as the nucleation/growth mechanism. AlPO4-5 (AFI topology) as a typical aluminophosphate molecular sieve is well documented.1 In addition to being used as an absorbent or an excellent catalyst support, large AlPO4-5 crystal prepared in the presence of fluorine ion exhibits good optical quality and low structure defect and is considered to be a novel material for optical data storage or other optical applications.16,17 It is well-established that the unit cell of AlPO4-5 possesses 12 crystallographically equivalent P and Al atoms, corresponding to only one P and Al NMR signal.18 Interestingly, in the presence of fluoride medium, the prepared AlPO4-5 product exhibits two kinds of P and Al sites due to the structural fluoride.19 Although the fluoride synthesis route was frequently reported in recent years,20-22 its synthesis mechanism is still unclear. In addition, the nucleation and growth process of AlPO4-5 in the fluoride medium is rarely investigated. In this study, our interests focus on the formation of AlPO4-5 in the fluoride medium, and the attention has been given to the intermediate gels of AlPO4-5. Various solid-state NMR techniques, coupled with powder X-ray diffraction (PXRD) and IR

10.1021/jp0710133 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007

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spectroscopy, are employed to obtain detailed information about the intermediate gels. The results shed light on the crystallization process, which would more or less allow us to improve our ability to design and modify this kind of industrially important material. Experimental Section Sample Preparation. AlPO4-5 was synthesized according to the modified procedure.21 Typically, a starting mixture with a composition of 1.0 Al(OPri)3 (triisopropylate aluminum)/1.43 H3PO4/1.0 TEA (triethylamine)/0.64 HF/211 H2O was prepared. An amount of 0.45 g of Al(OPri)3 was added to 9 mL of water under stirring, and then a mixture of 0.21 mL of phosphoric acid (85%) and 0.3 mL of TEA was added with stirring. After stirring for 2 h, 0.75 mL of hydrofluoric acid (40%) (which is highly corrosive and should be carefully handled) was added dropwise, and stirring was continued for another 2 h. The resultant homogeneous mixture was sealed in a 15 mL Teflonlined stainless-steel autoclave and heated under autogenous pressure at 180 °C for various lengths of time (60, 120, 150, 180, 240, and 720 min). According to previous works,23,24 the maximum pressure under our condition (degree of fill (about 75%), solvent (water), and temperature (180 °C)) is below 100 bar. The reaction was quenched by an ice bath. The pure crystalline AlPO4-5 product was obtained at about 16 h. The visible gel and liquid phase were separated by centrifugation. The gel was washed with distilled water and dried at room temperature for about 1 week. The dried samples were sealed in vials for later characterization. Characterizations. PXRD patterns of the solid gels were recorded on a Siemens D5005 X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å) in a 2θ range of 5-70° with a step of 0.05°. IR spectra of the samples were recorded on a Nicolet Impact 410 IR spectrometer. 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 161.9, 104.2, 100.6, 376.4, and 400.1 MHz for 31P, 27Al, 13C, 19F, and 1H, respectively. A Chemagnetics 5 mm tripleresonance MAS (magic-angle spinning) probe was employed to acquire 31P, 27Al, and 13C NMR spectra with a spinning rate of 8 kHz. 19F spectra were recorded using a 4 mm doubleresonance MAS probe with a spinning rate of 16 kHz. 27Al MAS spectra were acquired using a single-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 1.0 s. The pulse length for 27Al was measured on a 1 M Al(NO3)3 solution.25 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. 19F MAS spectra were acquired using a spin-echo pulse sequence with π/2 and π pulses of 10 and 20 µs, respectively, and a recycle delay of 20 s. The transfer of population in the double-resonance (TRAPDOR) experiment is a rotor-synchronized double-resonance technique designed to measure the correlation between two unlike spins involving at least one quadrupolar nuclei.26,27 In the 31P{27Al} TRAPDOR experiment, a spin-echo pulse sequence was applied to the 31P spins while 27Al nuclei was irradiated in an alternating fashion. The TRAPDOR fraction is defined by (1 - S/S0), where S and S0 are signal intensities in the spectra acquired with and without 27Al irradiation, respectively. All the TRAPDOR experiments were carried out under the following experimental conditions: the rf field strength for 27Al irradiation was 58 kHz, the pulse delay was 180 s, and the

Figure 1. PXRD patterns of the isolated gels throughout the hydrothermal treatment period. The inset depicts the relative crystallinity of the gels as a function of crystallization time.

irradiation time equals multiples of the rotor period (125 µs). The Hartmann-Hahn matching condition for 27Al f 31P crosspolarization/magic-angle spinning (CP/MAS) experiments was ωP ) 3ωAl ( nωr (n ) 1 or 2)28 where ωP and ωAl were the rf field strengths applied on the 31P and 27Al channels, respectively, and ωr was the spinning rate. Optimization of the 27Al f 31P CP/MAS experiment was carried out on a completely crystallized AlPO4-5 sample, and the optimized matching condition was as follows: ωP ) 43 kHz, ωAl )17 kHz, and ωr ) 8 kHz. The contact time for 27Al f 31P cross-polarization (CP) was 2.0 ms. The two-dimensional (2D) 27Al f 31P CP/MAS heteronuclear correlation (HETCOR) experiment was performed using the method reported in the literature.29 For the 1H f 31P CP/MAS experiment, the Hartmann-Hahn matching condition was optimized on NH4H2PO4. The 90° pulse length and the pulse delay for proton were 4.5 µs and 4 s, respectively. 13C MAS spectra were acquired using a single-pulse sequence with 1H decoupling. 19F T 27Al CP/MAS experiments were optimized on the completely crystallized AlPO4-5 sample, and the optimized matching condition was as follows: ωF ) 16 kHz, ωAl ) 8 kHz, and ωr ) 8 kHz. The contact time for 19F T 27Al CP was 0.3 ms. A three-pulse z-filtered pulse sequence30 was used for the 27Al 3Q MAS NMR experiment. The rf field strengths of the first two hard pulses and the third soft pulse were set to approximately 85 and 12 kHz (calibrated with Al(NO3)3 solution), respectively. The optimized three-pulse widths were 5.1, 1.7, and 7.0 µs, respectively. A total of 960 scans were accumulated, and the recycle delay was set to 0.3 s in the 3Q MAS experiment. The hypercomplex method was used in the 2D data acquisition and processing. The chemical shifts were referenced to hexamethylbenzene for 13C, 85% H3PO4 for 31P, 1 M Al(NO3)3 solution for 27Al, and trifluoroacetic acid for 19F, respectively. Results and Discussion PXRD Analysis. The PXRD data (Figure 1) obtained on the isolated gel samples with different hydrothermal treatment periods show the evolution of the long-range ordering of the gels. The inset in Figure 1 displays the crystallization curve (the relative crystallinity) as a function of the crystallization

Crystallization of AlPO4-5 Aluminophosphate

Figure 2. IR spectra of the isolated gels throughout the hydrothermal treatment period.

time. For the initial and the 60 min heated gels, their PXRD patterns are identical and contain only a broad hump between 15° and 30° without any Bragg reflections from the AFI-type structure. After heating the gel for 120 min, a weak diffraction peak appears at 2θ ) 7.4° and the broad peak is still present in the high 2θ region. The weak diffraction peak coincides with the first peak of the AFI-type framework, suggesting that the periodic framework of crystalline AlPO4-5 begins to form after 120 min of heating though a large amount of amorphous phase is still present. Further increasing the heating time to 150 min results in a remarkable increase of the diffraction peak at 7.4° and the appearance of some new Bragg reflections in the high 2θ region which are the fingerprints of the AFI-type framework. This suggests that more crystalline AlPO4-5 (65% relative crystallinity) molecular sieves are formed at the expense of the amorphous phase. After heating the gel longer than 240 min, no other crystalline product except for AlPO4-5 can be identified with the relative crystallinity increasing to more than 90%. From the above results, it is clear that the crystallization process is very fast and the PXRD data give the long-rang ordering change of the gels during the crystallization period, while more detailed information about the network formation at molecular level is still absent. In the following, IR and various NMR techniques are employed to study the nature of the gels. IR Analysis. Figure 2 shows the IR spectra of the gels throughout the crystallization period. The IR spectrum of the initial gel exhibits two strong broad bands at 3430 and 1640 cm-1, corresponding to the specificity of water molecules adsorbed on OH groups and bending vibrations of these surfaceadsorbed water molecules, respectively.31 The two bands appearing at 1092 and 550 cm-1 are characteristics of aluminophosphates. The former is due to asymmetric stretching vibration of the P-O-Al unit in the amorphous phase, and the

J. Phys. Chem. B, Vol. 111, No. 25, 2007 7107 latter arises from the P-O or Al-O bending modes,32 suggesting that an amorphous AlPOs material is formed. It is noted that no obvious vibration bands corresponding to the template molecules are observed, implying that the template molecules do not, or weakly, interact with the initial gel, and therefore they are easily washed away from the gel. After heating the gel for 60 min, several weak bands due to the C-H stretching modes of the template molecules appear in the range of 26003000 cm-1. In addition, the bands at 1477 and 1397 cm-1, due to the bending modes of the -CH2- and -CH3 groups, respectively, are present, which were considered to be associated with the protonated template.33 This is in accordance with the previous report that the TEA molecules are easily protonated by HF in the reaction mixture during the preparation of SAPO434,34 which has some effects on the nucleation and crystal growth. Heating the gel for 120 min causes an increase in the bands of the template molecules. Further increasing the crystallization time leads to the appearance of bands due to the aluminophosphate framework at 462, 590, 634, 700, 742, 837, and 1235 cm-1 (corresponding to the symmetric and asymmetric stretching vibrations of the P-O-Al units).32 Clearly, the IR results evidence the existence of P-O-Al units in the amorphous region of the heated gels sampled before 120 min, which may act as the preliminary structural units for the formation of the AFI-type framework, while the newly formed Al-O-P bonds after 150 min can be assigned to periodic structural units from the small crystalline region in the amorphous matrix. It is noteworthy that there exists an initial increase and then a decrease of the vibration intensities of TEA molecules before and after 120 min of heating, respectively. This suggests that the TEA molecules are gradually assembled in the amorphous phase and then occluded into the aluminophosphate voids acting as a “structure-directing agent or space filler” around which the AFI-type framework is formed. Solid-State NMR. 13C MAS NMR. It is highly desirable to make a more detailed investigation on the state of the template molecule during the crystallization period in order to have more insight into the crystallization process. In order to obtain quantitative results, 13C MAS instead of CP experiments were performed. Figure 3 shows the 13C MAS spectra of the selected gels. Prior to hydrothermal treatment, no obvious signals can be observed in the gel, which is in agreement with the IR result. For the 60 min heated gel, two weak signals at 48.4 and 9.9 ppm corresponding to methylene and methyl groups of TEA appear, suggesting that a small amount of template molecule interacts with the amorphous gel. It should be noted that the chemical shift of methyl group moves to high field relative to that of free TEA (at ca. 12 ppm). This can be attributed to the protonation of TEA molecules,33 which is also observed by our IR experiment. With increasing heating time, the relative intensities of the two signals are gradually increased, indicating that more and more TEA molecules are occluded in the gel. In combination with the above IR results, it is found that the TEA molecules begin to intercalate into the gel as a “structuredirecting agent or space filler” after 60 min and cannot be easily eliminated by washing, even though the formation of the aluminophosphate framework probably has not started yet at that time. With increasing crystallization time, a gradual increase in the intensity of the two signals is observed. Therefore, the crystallization trend can be deduced from the relative content of TEA calculated from the 13C spectra as a function of the crystallization time (inset of Figure 3). The curve exhibits some extent of correlation with the trend of the crystallization observed by the PXRD experiment. In addition, the decrease

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Figure 3. 13C MAS spectra of the isolated gels throughout the hydrothermal treatment period. The inset depicts the relative content of template molecule (TEA) in the gels as a function of crystallization time.

of the vibration intensity of TEA after 120 min observed by IR can be unambiguously attributed to the intercalation of TEA into the framework, which hampers the IR detection. 27Al and 31P MAS NMR. Figure 4a shows 27Al MAS spectra of the selected gels with different lengths of heating time. For the initial gel, a weak signal at 41.9 ppm and a strong signal at -6.6 ppm with a shoulder at 6.7 ppm are observed. The weak signal arises from the tetrahedral Al environments (Al(OP)4), while the strong signal is probably due to octahedral Al atoms in aluminophosphates.35,36 Our 27Al 3Q MAS spectrum (not shown) suggests the shoulder peak is an independent resonance rather than a part of the second-order quadrupolar line shape, and thus we tentatively ascribe it to Al species from unreacted starting material. The 27Al spectrum of the 60 min heated gel shows a similar feature to the initial gel except for an intensity increase of the tetrahedral Al. Heating the gel for 120 min results in almost equal intensity of the tetrahedral and the octahedral Al signals, suggesting an increase of the ordered phase at the expense of the disordered moieties. After 150 min of heating, the tetrahedral signal (shifting to 37.6 ppm) dominates the 27Al spectrum. At the same time, a new weak signal at 10.2 ppm appears, and its intensity increases with the heating time, which is probably due to the pentacoordinated Al associated with F-Al complex.19,37 In combination with the PXRD result, the 27Al spectrum of the gel heated for 180 min corresponds to the completely crystalline AlPO4-5 and further increasing heating time results in an identical 27Al NMR feature but a slight decrease of the line width of the tetrahedral Al signal, indicative of an increase in the crystallinity. It is also observed that the chemical shift of the tetrahedral Al changes from 41.9 to 37.6 ppm during the crystallization period and that of the octahedral Al gradually shifted to high field, which suggests the slight difference in chemical environment around these two types of Al atoms between the amorphous and crystalline region. 31P MAS spectra of the gels as a function of the crystallization period are shown in Figure 4b. The only one broad signal

Xu et al. observed at -11.4 ppm in the initial gel is assigned to the amorphous aluminophosphates characterized by P-O-Al units observed in IR analysis or to some free phosphate species.38 After heating the gel for 60 min, the -11.4 ppm signal shifts slightly to upfield (-12.3 ppm). When the heating time is increased to 120 min, a narrow signal appears at ca. -29 ppm. By spectral deconvolution, two other new signals at ca. -8 and -22 ppm can be found. The two signals at ca. -22 and -29 ppm can be ascribed to the tetrahedral P in the crystalline framework evidenced by the appearance of a weak diffraction peak characteristic of AlPO4-5 in PXRD.19 Further lengthening the heating time leads to a remarkable decrease of the broad signal at -12.3 ppm and the growth of the signals at ca. -29 and -22 ppm, indicative of the consumption of the amorphous or disordered moieties in favor of the formation of highly crystallized AlPO4-5, in line with the intensity increase of the new P-O-Al units observed by IR. 27Al f 31P HETCOR and TRAPDOR. HETCOR spectroscopy is a 2D technique based on CP to detect the connectivity between two different nuclei in proximity of less than 1 nm. Recently, this technique has been applied to characterize the connectivity between Al and P atoms in the AlPOs gel system.8-10 In the HETCOR spectrum of the initial gel (Figure 5a), the broad 31P signal at -12 ppm exhibits strong correlations with both tetrahedral and octahedral 27Al signals (-7.8 and 42 ppm). In addition, a sharp 31P signal at -16 ppm can be observed, which is also correlated to the two 27Al signals. The two signals at -12 and -16 ppm can be assigned to the partially condensed P with less than four Al atoms in its coordination sphere, as the fully condensed P bound to four Al atoms usually gives rise to a 31P chemical shift in the range of -19 to -31 ppm for crystalline AlPOs materials.35,36,39 The presence of P-OH groups leads to the low-field shift for the partially condensed P.40 It can be concluded that prior to hydrothermal treatment the initial gel forms amorphous aluminophosphate species immediately after mixing the reactants, which is characterized by the different partially condensed P and different Al environments. It is noteworthy that the shoulder signal at 6.7 ppm in the 27Al MAS spectrum is invisible in the HETCOR spectrum, indicating that this signal results from unreacted Al species. Figure 5b shows the HETCOR spectrum of the 120 min heated gel. It is interesting that three 31P signals at -11.6, -22.2, and -29.3 ppm with almost equal intensity appear in the 31P projection, showing distinctly different patterns compared with the corresponding 31P MAS spectrum. In addition to the relatively strong tetrahedral and octahedral Al signals, a new signal at 9.7 ppm as a shoulder of the -14.8 ppm peak is resolved in the 27Al projection. Through careful inspection, the correlations between different P and Al sites can be pictured. The two high-field 31P signals at ca. -22 and -29 ppm are correlated to both the tetrahedral Al (38.6 ppm) and the pentacoordinated Al (9.7 ppm) sites but not to the octahedral Al sites (-14.8 ppm). The 31P signal at -11.6 ppm exhibits correlations with both tetrahedral and octahedral Al sites but not with the pentacoordinated Al site. It is evident that the two high-field 31P signals at ca. -22 and -29 ppm are selectively enhanced by the 27Al f 31P CP that was used in the HETCOR experiment. In zeolites, the relative enhancement of the T site (Al or Si) produced by CP is relevant to the number of the bonding T sites. Therefore, it can be concluded that the number of Al atoms in the coordination sphere of the two P sites corresponding to the two high-field 31P signals is more than that of the P site corresponding to the low-field 31P signal, indicating that the former two P sites have a fully condensed

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Figure 4.

27Al

MAS (a) and 31P MAS (b) spectra of the isolated gels throughout the hydrothermal treatment period.

Figure 5.

27Al

f 31P HETCOR spectra of initial gel (a) and 120 min heated gel (b).

coordination sphere, whereas the latter can be considered as a partially condensed P site. However, it should be noted that sample rotation causes the first-order quadrupolar splitting to be time-dependent, which has a great effect on the spin locking of the central transition coherence of nuclei with large quadrupolar coupling constants.28 Taking this into account, the 31P{27Al} TRAPDOR technique that does not involve coherence transfer and spin locking was successfully employed to study the coordination environment around the P atom.8 Figure 6 shows the TRAPDOR spectra of 120 min heated gel with a dephasing time of 2 ms. Since the Al-O-P distance in the micro-AlPOs materials normally varies little,41 the magnitude of P-Al dipolar interaction characterized by the TRAPDOR factor depends on the average number of Al atoms bonded to the observed P atoms.42,43 The TRAPDOR factors are calculated to be 0.29, 0.55, and 0.58 for the signals at ca. -12, -22, and -29 ppm, respectively. Therefore, it can be concluded that the larger TRAPDOR effect on the -22 and -29 ppm signals definitely corresponds to more Al atoms bonded to the corresponding P atoms, consistent with the result of HETCOR

experiment. When combined with the PXRD result, it is clear that in the 120 min gel there exist two types of fully condensed P sites belonging to the crystalline region and one type of partially condensed P site belonging to the amorphous region. 1H f 31P CP NMR. From the above HETCOR and TRAPDOR experiments, two types of microstructure regions are differentiated by the partially and fully condensed P species coexisting in the 120 min heated gel. For the partially condensed P, it is supposed that instead of Al atoms, the hydroxyl group or protonated template is commonly involved in its coordination sphere. So the nature of the local environment around the P species associated with various kinds of proton sources (water molecules, template molecules, surface P-OH and Al-OH groups, etc.) can be probed by the 1H f 31P CP experiment.40 Figure 7 exhibits the 31P MAS (Figure 7a) and 1H f 31P CP MAS (Figure 7b) spectra of the 120 min heated gel. Four 31P signals at ca. -8, -12, -22, and -29 ppm, having different CP enhancement (or selectivity), can be resolved by spectral deconvolution. Obviously, this is caused by the different proton environment around the P atoms. More information about the

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Figure 6. 31P{27Al} TRAPDOR spectra of 120 min heated gel with dephasing time of 2.0 ms: spin-echo spectrum (a), TRAPDOR spectrum (b), and difference spectrum (c).

Figure 7. 31P MAS (a) and 1H f 31P CP MAS spectra with contact time of 1 ms (b) of 120 min heated gel.

nature of these P species can be extracted by analyzing the CP dynamics for each P site. It is well-known that for a two 1/2 nucleus system (namely, 31P and 1H), the CP efficiency is intimately associated with the heteronuclear dipolar-dipolar interaction that is inversely proportional to the internuclear distance and can be characterized by the time constant Tcp. The longer the internuclear distance the longer the Tcp and, thus, the lower efficiency of CP. Of course, the mobility of the proton sources will affect the CP efficiency as well. The buildup of the four 31P signals through CP as a function of contact time is shown in Figure 8. The polarization dynamics can be described by a simplified formula:44 S(t) ) Smax(1 - Tcp/T1FH)-1(exp(-t/ T1FH) - exp(-t/Tcp)). By fitting the CP data, the Tcp values are 0.21, 0.48, 0.73, and 0.92 ms for the signals at ca. -8, -12, -22, and -29 ppm, respectively. Obviously, the Tcp values of the signals at ca. -8 and -12 ppm are much smaller compared with those of the signals at ca. -22 and -29 ppm. For the 31P signal at -8 ppm, it exhibits the highest CP efficiency (the smallest Tcp) but no correlation with any 27Al signal (see the above 27Al/31P HETCOR and TRAPDOR experiments). Therefore, we ascribe it to protonated phosphates or unreacted phosphoric acid. The high CP efficiency of the -12 ppm 31P signal can be attributed to the existence of OH group in the coordination sphere of the corresponding P site, indicative of the partially condensed nature of the P site. The low CP efficiency of the two higher-field signals (-22 and -29 ppm) is likely due to the absence of OH group directly bonded to the corresponding P sites. In this case, the template (TEA) or water

Xu et al. molecules can act as the proton source for the CP that is mediated through spatial proton-proton spin diffusion, leading to a lower efficiency of polarization transfer. Therefore, the fully condensed coordination sphere around the two P sites is confirmed, in line with the above 27Al/31P HETCOR and TRAPDOR experiments. On the basis of the 1H f 31P CP experiments, besides the free P species, such as protonated phosphates or unreacted phosphoric acid characterized by the signal at ca. -8 ppm, two different domains in the 120 min heated gel can be differentiated from the Tcp values for the four P sites, namely, the fully condensed domain consisting of P species resonating at ca. -22 and -29 ppm and the partially condensed domain characterized by the signal at ca. -12 ppm. Taking into account of the PXRD result, the fully condensed domain belongs to the crystalline region though the amount of the region is very low in the 120 min heated gel. The 1H f 31P CP experiment was also performed on the 60 min heated gel. Though in PXRD no obvious diffraction peak was observed, it is not clear whether the broad 31P signal at ca. -12 ppm in the MAS spectrum hides the signal at ca. -29 ppm that results from the crystalline region. 1H f 31P CP experiments with various contact times may provide the evidence. However, even with very long contact time, no signal in the range of -19 to -31 ppm corresponding to the crystalline domains is visible (not shown), suggesting that the onset of the crystallization has not proceed yet at this moment. In addition, our 31P{27Al} TRAPDOR experiment with long dephasing time also fail to find the fully condensed 31P site in the difference spectrum (not shown), reconfirming the result deduced from the CP experiment that the crystallization begins at least after 60 min. 19F MAS and 27Al T 19F CP MAS NMR. Fluorine ion is a well-known mineralizer in the synthesis of microporous zeolites, and in some cases it plays a templating or structure-directing role. The presence of HF can reduce the solubility and supersaturation of the reaction gel, usually leading to the formation of large AlPO4-5 crystals due to the relatively lower mineralizing role of fluorine ion in comparison with that of the commonly used mineralizer such as hydroxide ion.45,46 Here, 19F MAS NMR is employed to investigate the nature of fluorine ions in the selected gels. In the 19F MAS spectra of the initial and the 60 min heated gels (Figure 9), a broad signal at -140 ppm probably due to fluorine species in an amorphous phase is observed. Heating the gel for 120 min produces a new sharp signal at -120 ppm, indicative of the homogeneous and ordered nature of the corresponding phase. Further increasing the heating time results in a gradual increase of the sharp signal at the expense of the broad one until the former dominates the 19F MAS spectrum. The -120 ppm signal was first reported to be due to the fluorine ion located in the side pocket of the AlPO4-5 framework formed by two four-membered rings,20 while regarding the unusually short distance (1.92 Å) between pentacoordinated Al and F, it was assigned to the fluorine ion that bridges two Al atoms between the two four-membered rings.19,47 Therefore, the appearance of the new signal at -120 ppm suggests the structural fluorine starts to form, a hint of the onset of the framework crystallization. The fluorine ion has a chemical shift of -109 ppm when it is associated with TMA+ (tetramethylammonium) and located in the cagelike [4462] subunits in the as-synthesized AlPO4-ZON.37 Our liquid 19F NMR (not shown) of the mixture of HF and TEA solution gives rise to a single signal at -118 ppm that is supposed to be due to the ion pair between fluorine and TEA species. So the broad signal at -140 ppm should not result form ion pairs associated with the template. Since the fluorine ion is easy to complex with other

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Figure 8. Fitting curves of 31P signals at -8 ppm (a), -12 ppm (b), -22 ppm (c), and -29 ppm (d) in the 1H f 31P CP MAS experiment on 120 min heated gel.

Figure 9. 19F MAS spectra of the isolated gels throughout the hydrothermal treatment period.

species such as Al atom, the 27Al f 19F CP experiments were performed on the gels. Unfortunately, the extremely low sensitivity for the initial gel prevents us from obtaining a reliable 27Al f 19F CP spectrum. For the 60 min heated gel, only the broad signal at -140 ppm is observable in the 27Al f 19F CP spectrum (Figure 10a). This demonstrates that fluorine ions in the gel are indeed close to the Al atoms, possibly forming the

fluoroaluminophosphate complex. It is noteworthy that the weak signal at -120 ppm is selectively enhanced compared with the broad signal at -140 ppm in the 27Al f 19F CP spectrum of the 120 min heated gel (Figure 10b), indicative of an increase of the ordered moieties in the gel. We also performed the 19F f 27Al CP experiments on the series gels. The 19F f 27Al CP spectrum of the initial gel (Figure 10c) shows only one broad 27Al signal at -8 ppm corresponding to the octahedral Al. In combination with the above 27Al f 19F CP experiment, it is clear that the broad 19F signal at -140 ppm can be ascribed to a kind of hexacoordinated fluoraluminophosphate complex. In the 19F f 27Al CP spectrum of the 120 min heated gel (Figure 10d), in addition to the hexacoordinated fluoraluminophosphate signal at -10.6 ppm, a new signal at 11.8 ppm is observed. This signal is due to pentacoordinated F-Al complex that is supposed to be transformed from the hexacoordinated fluoraluminophosphate complex. For the 240 min heated gel, the signal at 11.1 ppm arising from the pentacoordinated F-Al complex becomes the dominant peak in the 19F f 27Al CP spectrum (Figure 10e). Crystallization Process. On the basis of above experimental results, a clear crystallization process of AlPO4-5 in the presence of HF can be pictured. Prior to hydrothermal treatment, at least one type of F species in the form of amorphous fluoraluminophosphate phase exists in the initial gel, and the existence of the unreacted Al site in the gel is confirmed by 27Al f 31P HETCOR and 19F f 27Al CP spectra. The initial gel with amorphous nature can be identified by two main components, with one being composed of aluminophosphate species characterized by Altet-O-Ppar and F-Aloct-O-Ppar (par donates partially condensed) units and the other being composed of some free species, such as protonated phosphates, unreacted phosphoric acid, and unreacted Al source (Scheme 1). The amor-

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Figure 10. 27Al f 19F CP MAS spectra of 60 min heated gel (a),120 min heated gel (b), 19F f 27Al CP MAS spectra of initial gel (c), 120 min heated gel (d), and 240 min heated gel (e).

SCHEME 1

phous nature of the phase dose not change until the heating time reaches 120 min, when the formation of a semiordered network occurs. Two P sites with high-field chemical shifts (at ca. -22 and -29 ppm) are resolved by HETCOR, TRAPDOR, and CP experiments, which were believed to be 2 of the 12 crystallographically equivalent P atoms with different distances to the fluorine atom.19 This implies that the F atom has entered into the framework and the crystallization begins at least before 120 min, which is also evidenced by the appearance of the pentacoordinated Al in the F-Alpen-O-P unit (see the 19F f 27Al CP spectra). In combination with the corresponding PXRD pattern, the pentacoordinated Al can be unambiguously assigned to the crystalline framework site, confirming the onset of the crystallization before 120 min (probably after 60 min). This is also in good agreement with the 31P MAS and 1H f 31P CP NMR results, in which the signal at ca. -29 ppm appears after 60 min and becomes pronounced at 120 min. Therefore, on the basis of the local ordering arrangement, two microdomains can be clearly identified in the 120 min gel (Scheme 1). The periodic crystalline structure characterized by Altet-O-Pful and F-Alpen-O-Pful (ful donates fully condensed) units appears to be the new domain II, and amorphous fluoroaluminophosphate component (F-Aloct-O-Ppar) and aluminophosphate component (Altet-O-Ppar) found in the former stage are present as the domain I. It is noteworthy that although only one broad

tetrahedral Al signal at ca. 39 ppm (Figure 4a) was observed in the 120 min heated gel, there are two kinds of tetrahedral Al sites associated with P atoms in different condensation degrees (Altet-O-Ppar and Altet-O-Pful). As the crystal lattice grows in the semiordered network, the condensation degree of P species gradually increases, leading to the decrease of the amorphous structural units and the transformation of the Al site in the form of Altet-O-Ppar into Altet-O-Pful. Although the two kinds of tetrahedral Al sites cannot be distinguished in the 27Al NMR spectra, the two Altet-O-Ppar and Altet-O-Pful units can be identified by the 27Al f 31P HETCOR NMR. In the final stage (longer than 180 min heating), the amorphous component (domain I) and the free species are almost completely consumed in favor of the formation of the highly condensed Altet-O-Pful and F-Alpen-O-Pful units that exist in the pure crystalline aluminophosphate phase characterizing the desired AlPO4-5. It is interesting that the pentacoordinated Al in the F-AlpenO-P unit does not appear before 120 min. According to the previous study, the variation of both pH and temperature of the initial mixture during the synthesis would lead to the coordination change from hexacoordinated Al to pentacoordinated Al.6 Only in the supernatant liquid was the pentacoordinated Al observed at high temperature during the crystallization process of the microporous materials such as SAPO4-34 and AlPO4CJ2, whereas under room temperature it disappeared, and

Crystallization of AlPO4-5 Aluminophosphate hexacoordinated Al was formed.5,6 This is most likely caused by the temperature which induces a loss of water in the coordination sphere of Al from the six- to fivefold state.48,49 In case of AlPO4-CJ2, the appearance of the pentacoordinated Al was believed to be due to the crystal growth, i.e., the solid formation. Here, we also did not find any 27Al signals due to the pentacoordinated Al in the separated solution at room temperature during the whole crystallization process (the result is not shown). The observation of the pentacoordinated Al in the F-Alpen-O-P unit in the 120 min heated gel indicates the onset of the nucleation and crystalline growth of AlPO4-5. Conclusion In combination with PXRD and IR, multinuclear solid-state NMR spectroscopy can provide insights into the development and evolution of the intermediate gels during crystallization of AlPO4-5 in the presence of HF. Fast crystallization process (almost finished in 180 min) was monitored, which can be differentiated by three successive stages. During the first stage (before 60 min), amorphous gel is proved to be composed of aluminophosphate phase containing fluorine ions and some free species associated with protonated phosphates or phosphoric acid and unreacted Al source. In the second stage (60120 min), two different microdomains are identified in the gel and the appearance of the pentacoordinated fluoroaluminophosphate characterizes the onset of the crystallization. In the final stage (after 180 min), the pure crystalline phase of AlPO4-5 is present, which is characterized by the existence of both pentacoordinated fluoraluminophosphate and four-coordinated aluminophosphate. Acknowledgment. We are very grateful for the support of the National Natural Science Foundation of China (20425311). 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) Flanigen, E. M.; Patton, R. L.; Wilson, S. T. Stud. Surf. Sci. Catal. 1988, 37, 13. (3) Francis, R. I.; O’Hare, D. J. Chem. Soc., Dalton Trans. 1998, 3133. (4) Walton, R. I.; O’Hare, D. Chem. Commun. 2000, 2283. (5) Vistad, Ø. B.; Akporiaye, D. E.; Taulelle, F.; Lillerud, K. P. Chem. Mater. 2003, 15, 1639. (6) Taulelle, F.; Haouas, M.; Gerardin, C.; Estournes, C.; Loiseau, T.; Ferey, G. Colloids Surf., A 1999, 158, 299. (7) Serre, C.; Lorentz, C.; Taulelle, F.; Ferey, G. Chem. Mater. 2003, 15, 2328. (8) Huang, Y. N.; Richer, R.; Kirby, C. W. J. Phys. Chem. B 2003, 107, 1326. (9) Huang, Y. N.; Demko, B. A.; Kirby, C. W. Chem. Mater. 2003, 15, 2437. (10) Huang, Y. N.; Machado, D.; Kirby, C. W. J. Phys. Chem. B 2004, 108, 1855. (11) Twu, J.; Dutta, P. K.; Kresge, C. T. Zeolites 1991, 11, 672. (12) Grandjean, D.; Beale, A. M.; Petukhov, A. V.; Weckhuysen, B. M. J. Am. Chem. Soc. 2005, 127, 14454.

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