Investigation of Crystallization of Molecular Sieve AlPO4

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Investigation of Crystallization of Molecular Sieve AlPO4-5 by the Dry Gel Conversion Method Banghao Chen, Christopher W. Kirby, and Yining Huang* Department of Chemistry, The UniVersity of Western Ontario, London, Ontario, Canada N6A 5B7 ReceiVed: April 17, 2009; ReVised Manuscript ReceiVed: July 27, 2009

Crystallization of AlPO4-5 (AFI), a representative aluminophosphate-based molecular sieve with AFI topology, was examined under dry gel conversion, in particular, vapor phase transport (VPT) conditions. The VPT approach allows one to “freeze” crystallization at different stages and, therefore, to capture several reactive intermediates, which provide “snapshots” of the crystallization process. These intermediates were characterized by powder X-ray diffraction, Raman spectroscopy, and a combination of several solid-state NMR techniques. The results show that the reactive intermediates have two key features: (1) They bear certain structural characteristics of the target framework. (2) They are fragile and held together by weak bonding interactions. Hydrogen bonding apparently plays a key role in the formation of intermediates and their subsequent transformation to the final AFI structure. An AlPO4-5 crystallization mechanism under VPT conditions was proposed. Introduction Zeolites and aluminophosphate (AlPO4)-based molecular sieves are widely used in industry for separation and catalysis because of their unique structures.1 Recently, they have also found diverse new applications in nanotechnology2 and biotechnology.3 Because of their wide range of practical applications, it is desirable to synthesize new microporous materials with novel framework topologies. Despite tremendous advances in the understanding of the principles underlying the formation of these materials under hydrothermal synthesis (HTS) conditions,4 crystallization mechanisms are still not completely understood on a molecular level. Therefore, a rational design of novel framework structures with desired properties is still difficult. This is largely due to the fact that HTS is an extremely complicated process with multiple reactions and equilibriums occurring simultaneously in solution and solids. In the past, the structures of the intermediates or precursors to final crystalline microporous materials formed during HTS process have been mainly studied by ex situ approaches, which often involve quenching the reactions followed by isolation of solids from the liquid phase. As pointed out by several groups, many intermediates or precursors involve cooperative weak interactions, and their true structures may be altered by post synthesis treatments such as washing.4i,5,6 In some cases, it is also not easy to distinguish the true reaction intermediates from the competing phases. Recently, a number of in situ approaches have been developed, and they have provided extremely important information on molecular sieve crystallization.7 Most of the in situ techniques require a specially designed reaction vessel that can withstand high temperature and pressure, and such a cell is usually suitable for one (or in some cases two closely related) spectroscopic technique(s). However, the complex nature of the system often requires multiple spectroscopic techniques to be used for characterization. Dry gel conversion (DGC) is an alternative method6,8,9 developed for molecular sieve synthesis (Scheme 1). The approach involves treating predried gel powder at an elevated * Corresponding author. E-mail: [email protected].

SCHEME 1: Schematic of the Reaction Vessel Used for the DGC Method

temperature. It can be further classified into (i) steam-assisted conversion (SAC), where the predried gel powder with a structure directing agent (SDA) is physically separated from a very small amount of pure water in an autoclave, and (ii) vaporphase transport (VPT), which is similar to SAC except that the SDA is not contained in the initial dry gel powder. Instead, a small amount of SDA solution is placed at the bottom of the autoclave. DGC is actually a good method for studying molecular sieve crystallization because it simplifies the reaction system. The amount of bulk water used is fairly small (typically 1 g of dry gel powder needs only 0.2-0.3 mL of water), and it is not in contact with the gel before heating. The dry gel powder cannot be dissolved by this small amount of water that is brought to the surface of the dry gel via vapor. Therefore, the formation of a molecular sieve is unlikely through gel dissolution. For SAC, all of the reactive species are contained in the solid dry gel phase, which avoids direct contact between the reactive species in solution and solids, a situation occurring in HTS. For VPT, because the SDA is gradually brought in contact with the solid gel via vapor phase, this approach is well-suited for investigating the effect of a SDA on framework formation. There

10.1021/jp903549n CCC: $40.75  2009 American Chemical Society Published on Web 08/18/2009

Crystallization of Molecular Sieve AlPO4-5 SCHEME 2: Framework of AlPO4-5 (left) and ab Plane Viewed along the c Axis (right)a

a

Edges of the unit cell are shown.

have been relatively few DGC studies6,8b,9,10 that are directly focused on the crystallization process. Recently, we have investigated the crystallization of AlPO4-11 by DGC and demonstrated that DGC can indeed be used to obtain important new physical insights.10a,b Here, we present our work on investigation of AlPO4-5 crystallization by DGC. AlPO4-5 is a representative AlPO4-based molecular sieve with an AFI structure (Scheme 2). It has a unidimensional 12 member ring channel system with a pore opening of 7.3 Å. This material has a wide range of applications.2c,11 Its crystallization has been the subject of several studies under HTS conditions.12 An in situ study monitored the formation of CoAPO-5 by using Co2+ as a probe.7e A model study has suggested that AlPO4-5 could form from twodimensional layers.4d Recently, a lamellar AlPO material was synthesized in a nonaqueous solution by using an unusual P source, H3PO3 rather than conventional H3PO4 (which is used almost exclusively in molecular sieve synthesis), and this material is able to transform to AlPO4-5.13 In the present study, we present a picture of the formation of AlPO4-5 under VPT conditions by using triethylamine (TEA) as a SDA. A very brief preliminary account of this work has been published.14

J. Phys. Chem. C, Vol. 113, No. 36, 2009 15869 S-4500 scanning electron microscopy (SEM). Raman spectra were obtained on a Bruker RFS 100/S FT-Raman spectrometer equipped with an Nd3+:YAG laser operating at 1064.1 nm and a liquid nitrogen cooled Ge detector. The laser power was typically 200 mW at the sample, and the resolution was 4 cm-1. All NMR experiments were carried out on a Varian/ Chemagnetics Infinityplus 400 WB spectrometer equipped with three radio frequency (rf) channels operating at the field strength of 9.4 T. The Larmor frequencies of 1H, 31P, and 27Al were 399.9, 161.6, and 104.1 MHz, respectively. The shifts of 31P, 27 Al, and 1H were referenced to 85% H3PO4, 1 M Al(NO3)3 aqueous solution, and TMS, respectively. Depending on the requirement of the individual experiment, we used three NMR probes (Varian/Chemagnetics 7.5, 5.0, and 4.0 mm triple-tuned T3 MAS probes). For 31P MAS experiments, a 30° pulse was typically used. The recycle delay was 60 s, and the proton decoupling field was about 50 kHz. 27Al MAS spectra were acquired using short excitation (less than 12°) pluses and a pulse delay of 200 ms. The 4 mm probe was utilized to acquire the 1 H MAS experiment with a 10 s recycle time and 15 kHz spinning speed. 27Al triple-quantum magic-angle spinning (3QMAS) experiments were performed using the same 4 mm probe, and spectra were obtained by utilization of a three-pulse, z-filter sequence.16 The rf strengths of the first two hard pulses and third soft pulse were optimized individually, and the optimized pulse lengths were 4.4, 1.6, and 10.5 µs for the three consecutive pulses. The 7.5 mm probe was used for 1H f 31P cross-polarization (CP) experiments. The 1H 90° pulse length was 8 µs, and the Hartmann-Hahn condition was determined using NH4H2PO4. A repetition time of 10 s was used with a spinning speed at 6.5 kHz. For the 27Al f 31P HETCOR experiments, the 7.5 mm probe was also used. Because 27Al (I ) 5/2) is a quadrupolar nucleus, CP was carried out in the sudden passage regime.17 The modified Hartmann-Hahn matching condition is

Experimental Section Sample Preparation. Aluminum and phosphorus sources were Al(OH)3 (50% Al2O3) and 85% H3PO4. Triethylamine (TEA, 99.5%) was used as a SDA. All chemicals were purchased from Aldrich Chemical Co. Composition of the initial dry gel (Al2O3:P2O5:TEA:H2O) is 1:1:0:40. A typical procedure15 for the preparation of VPT dry gel powder is the following. An appropriate amount of Al(OH)3 was stirred with H2O at room temperature for 30 min, followed by the dropwise addition of H3PO4. The resulting mixture was dried at 353 K with constant stirring to allow evaporation of water until white solids formed. The solid sample was then ground into a fine powder, which will be, hereafter, referred to as the initial dry gel. We prepared a series of intermediates by placing 1.0 g of the above initial dry gel powder into a small Teflon cup. The cup was placed in a Teflon-lined autoclave (volume, 23 mL) with 1.2 mL of 33% aqueous TEA solution at the bottom. Crystallization was then carried out at 448 K by putting a series of autoclaves in an oven. Each autoclave was taken out of the oven at different times, and the reaction was quenched in cold water. The solid sample was divided into two parts. One half of the sample was dried in air. The other half was first washed with distilled water in a beaker and then dried in the same beaker in air without isolation from the liquid phase. This ensured that no solid particles would be washed away. The dried solid samples were then kept in tightly sealed vials for analysis. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku diffractometer using Co KR radiation (λ ) 1.7902 Å). The morphology was monitored by a Hitachi

γPB1,P ) 3γAlB1,Al ( nνr

n ) 1 or 2

The strength of the 27Al spin-locking field is typically 25 kHz, and the spinning rate was 6.5 kHz. Optimal contact time was 1 ms, and the pulse delay was 200 ms. The 4 mm probe was chosen to run the 1H f 31P HETCOR experiments, and Na2HPO4 was used as a standard for setup. A short contact time of 0.5 ms was applied to minimize the effect of spin diffusion. The recycle time was 5 s. 1H{31P} rotationecho double-resonance (REDOR),18 1H{27Al}, and 1H{14N} transfer of populations in double-resonance (TRAPDOR)19 experiments were also preformed using the same 4 mm probe with a spinning speed of 15 kHz and a recycle delay of 5 s. More spectrometer conditions for 1H f 31P, 27Al f 31P HETCOR, 1H{31P} REDOR, 1H{27Al}, and 1H{14N} TRAPDOR experiments are given in the corresponding figure captions. Results and Discussion Powder XRD was employed to follow the evolution of longrange ordering of the dry gel under the influence of a SDA. Figure 1 shows the powder XRD patterns of washed and unwashed samples at different heating times. All diffraction peaks of the 24 h samples can be assigned to the AFI structure (P6cc). The initial unwashed VPT dry gel shows only a very broad amorphous halo. The 31P MAS spectrum (Figure 2A) exhibits a broad resonance at -19 ppm, suggesting a wide

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Figure 1. Powder XRD patterns of (A) unwashed and (B) washed VPT dry gel samples (), AlPO4-5; (, AlPO4-18).

Figure 2.

31

P MAS NMR spectra of (A) unwashed and (B) washed VPT dry gel samples (* indicates spinning sideband).

distribution of P local environments. XRD and NMR data indicate that the initial dry gel powder is amorphous in nature. After 30 min VPT treatment, two strong reflections with low 2θ values and several very broad weak reflections in the middleangle region were observed, which implies that reorganization within the amorphous phase has occurred under the influence of SDA (TEA), resulting in a new intermediate phase. The two strong reflections are rather broad, suggesting this new intermediate phase is semicrystalline. Their low 2θ values imply that this semicrystalline phase may have rather large pores. Because the Raman spectra in the T-O-T (T ) tetrahedral site) bending region are sensitive to the ring systems existing in the molecular sieve frameworks,20 Raman spectroscopy was used to further probe the porosity of the intermediate. For AlPO4-5, its 12-membered ring (12-MR) channel has a char-

acteristic ring breathing mode at 262 cm-1.20a For the 30 min sample, a weak band with a maximum at 265 cm-1 was also observed (Figure 3A), indicating the existence of large pores similar to the 12-MR of AlPO4-5. The slight difference in the Raman shift of the characteristic band may be because the average Al-O-P angle of the pore in the semicrystalline phase is slightly larger than that of the 12-MR of crystalline AlPO45. It appears that the “12-MR-like pore” in the semicrystalline phase is probably elliptical in shape and not perfectly circular like the 12-MR of AlPO4-5. This argument is consistent with the fact that the position of the (100) reflection of AlPO4-5 is in between the two strong low-angle peaks of the semicrystalline phase, suggesting that these two low-angle reflections may correspond to (100) and (010) reflections of AlPO4-5, which are not equal in this intermediate but eventually become identical

Crystallization of Molecular Sieve AlPO4-5

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Figure 3. Raman spectra of (A) unwashed and (B) washed VPT dry gel samples.

when the crystallization is complete. The 31P MAS spectrum of the 30 min unwashed sample exhibits a single peak at -18 ppm (Figure 2A), and we assign it to the P site in the semicrystalline phase. The sharpness of the signal is indicative of a high degree of local ordering. Existence of a single signal means that all P sites presented in this phase have a very similar local environment. The chemical shift value further suggests that the local chemical environment is likely to be P(-OAl)3()O) and/or P(-OAl)3(-OH).21,22 Interestingly, upon washing with water, all reflections due to the semicrystalline phase disappeared from the XRD pattern completely (Figure 1B). The sharp 31P peak at -18 ppm now becomes an extremely broad envelope at -13 ppm (Figure 2B). Washing also results in disappearance of the Raman band at 265 cm-1 (Figure 3B). All of these results indicate that the semicrystalline phase transforms to an amorphous material upon washing. These results also clearly indicate that the structure of the semicrystalline intermediate is partially held by weak forces such as hydrogen bonding and van der Waals forces. Washing alters these weak interactions, resulting in the transformation to the amorphous material. Examples of the AlPO compounds with the structure held by H-bonding involving the SDA and van der Waals forces can be found in the literature,23 and so can the evidence that weak interactions are the driving forces for the self-assembly in zeolite crystallization.24 The above results imply that the semicrystalline phase, formed after 30 min VPT treatments under the direction of a SDA, has a structure containing the planes which can be related to the ab planes of AlPO4-5 as it contains 12-MR-like pores. However, the pores in the semicrystalline phase have not been developed fully into circular rings, and the structure of this intermediate is apparently held together by weak bonding interactions. For the 2 h sample, a stronger Raman band at 263 cm-1 (Figure 3A) was observed, suggesting that the large pore in the semicrystalline phase is closer to the 12-MR in AlPO4-5. This observation is consistent with the XRD pattern showing that the two low-angle reflections belonging to the semicrystalline phase shifted slightly toward each other, i.e., from 7.4° and 9.2° to 7.7° and 8.7° (Figure 1A), respectively. These results suggest that the 12-MR-like pores in the semicrystalline phase are

becoming more circular. In addition, several weak reflections (labeled as )) are emerging (Figure 1A), whose positions are akin to those (200), (210), (002), (300)/(102), (220), and (410) reflections of AlPO4-5. This indicates that the semicrystalline phase is transforming to a new phase with a three-dimensional periodic structure very similar to AlPO4-5 (hereafter, we refer this new phase as AFI-precursor). A dramatic change was observed in the 31P MAS spectrum, where two additional new sharp peaks appeared at -24 and -29 ppm (Figure 2A), which can be assigned to the AFI-precursor. Because every 31P spectrum of as-made AlPO4-5 reported in the literature exhibits a characteristic broad asymmetric peak at -29 ppm with a deshielded shoulder at around -24 ppm,25 the coordination environment and local geometry around these two new P sites in the AFI-precursor must be fairly close to those in the fully crystallized AlPO4-5. The 1H f 31P cross-polarization (CP) results (see discussion later) further confirm that the P sites at -24 and -29 ppm belong to the same phase (AFI-precursor), and this phase is different from the semicrystalline phase and fully crystallized AlPO4-5. Upon washing, the AFI-precursor and semicrystalline phase transform into an amorphous phase as evident from the XRD pattern (Figure 1B) and 31P MAS spectrum (Figure 2B). The Raman band at 263 cm-1 due to the 12-MR-like pore also disappeared (Figure 3B). It seems that treating the dry gel powder under VPT conditions for 2 h results in the formation of a new phase (AFI-precursor). This precursor is not yet a fully crystallized three-dimensional framework. It is an intermediate species between the semicrystalline phase and AlPO4-5, presumably resulting from the reorganization of the semicrystalline phase. The cohesion of the AFI-precursor is mainly due to weak bonding interactions, leading to instability toward washing. The XRD pattern of the 4 h sample is dominated by the reflections due to AlPO4-5, indicating that the majority of the semicrystalline phase has transformed into AlPO4-5 via the AFIprecursor. It is consistent with the presence of the characteristic 262 cm-1 band due to the 12-MR in the Raman spectrum, which remains unaffected by sample washing (Figure 3). There are also several weak reflections due to AlPO4-18 (labeled as (), and they remained with those of AlPO4-5 in the XRD pattern

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Figure 4. XRD patterns of selected unwashed VPT-gel samples reheated with a small amount of water (reflections labeled with ( are due to AlPO4-18).

Figure 5. SEM pictures for (A) initial dry gel, (B) 30 min, and (C) 24 h VPT gel samples.

after washing (Figure 1). It appears that a small amount of AlPO4-18 with AEI structure cocrystallized. The 31P MAS spectrum shows that the two signals due to the AFI-like precursor at -24 and -29 ppm now evolve into a broad asymmetric peak at -29 ppm with a well-defined shoulder at -24 ppm, supporting the formation of AlPO4-5. A weak peak at -11 ppm matches one of the three resonances of AlPO4-18 (Figure 2).26 The other two small AlPO4-18 peaks at -28 and -30 ppm are buried underneath the strong resonance of -29 ppm due to AlPO4-5. After 24 h, only crystalline AlPO4-5 was observed, suggesting AlPO4-18 has transformed into AlPO4-5 (a very weak peak at -9 ppm is presumably due to a small amount of impurity that is XRD invisible). The conversion between AFI and AEI structures in HTS has been reported before.27 The average particle size roughly estimated from powder XRD data using the Scherrer equation increases from 30 to 70 nm with increasing the heating time. To ensure that the dry gel samples are true intermediates, representative samples obtained by quenching the reaction at early stages of the crystallization were reheated in the Teflon cup at 448 K in the presence of a small amount of water at the bottom of the autoclave. They all transformed into AlPO4-5 (Figure 4), confirming that these gel samples are indeed reactive intermediates. The results further indicate that a small amount of AlPO4-18 can be converted to AlPO4-5 with SAC treatment. Figure 4 also shows that the 1 h sample (which is a mixture of the semicrystalline phase and AFI-precursor) can simply transform to AlPO4-5 by pure water vapor treatment without the addition of extra TEA, suggesting that both intermediates already contain enough TEA molecules for the structure directing purpose. The morphological evolution of the VPT dry gel is illustrated in Figure 5 for selected samples. To further determine the local environments of the P sites in different intermediates, 1H f 31P CP experiments were per-

TABLE 1: T1GH and TCP Values of Different P Sites in Different Phases 2 h VPT dry gel sample semicrystalline phase P site (ppm) TCP (ms) T1FH (ms)

-18 0.38 ( 0.02 4.5 ( 0.1

AFI-precursor

AlPO4-5

-24 -29 -29 0.63 ( 0.05 0.60 ( 0.05 0.66 ( 0.03 9.3 ( 0.3 10.0 ( 0.2 65.0 ( 0.2

formed on the 2 h sample (which is a mixture of the semicrystalline phase and the AFI-precursor) as well as fully crystallized AlPO4-5 (24 h sample). The CP intensities of the resonances at -24 and -29 ppm originating from the AFIprecursor as well as the -18 ppm peak of the semicrystalline phase in the 2 h gel sample together with the -29 ppm peak of the as-made AlPO4-5 (24 h sample) were plotted as a function of contact time (Figure S1 of the Supporting Information). The CP dynamics can be described by the following equation28 H -1 H S(t) ) Smax(1 - TCP /T 1F ) [exp(-τ/T 1F ) - exp(-τ/TCP)]

The initial growth of a CP signal is controlled by the crosspolarization time constant (TCP). The decay of the signal at longer contact time is governed by the proton spin-lattice relaxation time in the rotating frame of reference (T1FH). A shorter TCP corresponds to a stronger H-P dipole-dipole interaction. In this work, the T1FH was determined separately using a previously described procedure.29 TCP values were then obtained by fitting the CP data to the above equation.28b The results are shown in Table 1. The shorter TCP value for the -18 ppm site reveals that the P site in the semicrystalline phase is not fully condensed and likely to have a directly attached OH group. Thus, the local



Figure 6. 27Al f 31P HETCOR spectra of (A) 2 h sample and (B) AlPO4-5. Both spectra were obtained using a contact time of 1 ms and a pulse delay of 0.2 s. The numbers of t1 increment are 40 and 32 for panels A and B, respectively.

chemical environment of P in the semicrystalline phase is likely to be P(OAl)3(OH). The TCP values for P sites at -24 and -29 ppm of the AFI precursor are similar to that of -29 ppm P site in AlPO4-5, indicating that these two P sites in the AFI-precursor are fully condensed as well, i.e., P(OAl)4. T1FH is related to the mobility of proton and spin diffusion. A homogeneous domain tends to possess a single T1FH value because of spin diffusion, and in a heterogeneous system, different phases tend to have different T1FH values.30 The fact that the T1FH values of -24 and -29 ppm sites are very similar but are remarkably different from those of the -18 ppm site and -29 ppm site of the final product confirm that in the 2 h sample the -24 and -29 ppm peaks belong to the same phase (AFI-precursor), and this phase is different from the semicrystalline phase and fully crystallized AlPO4-5. 27 Al MAS spectra of unwashed VPT samples as a function of heating time are shown in Figure S2A of the Supporting Information. The spectrum of initial dry gel consists of several overlapping resonances, including a main peak at -13 ppm along with a shoulder at 5 ppm and a broad resonance centered at 40 ppm. The corresponding 27Al 3QMAS spectrum (Figure S2B of the Supporting Information) indicates that they are due to three distinct Al sites. Clearly, the 40 ppm peak can be attributed to tetrahedral Al, and the -13 ppm peak is due to octahedral Al, i.e., Al(OP)4(H2O)2. The 5 ppm shoulder did not appear in the corresponding 27Al f 31P HETCOR spectrum (Figure S2C of the Supporting Information), suggesting it does not belong to an AlPO species. Therefore, we assign it to the unreacted alumina. The 3QMAS spectrum also indicates distributions of chemical shifts and quadrupolar coupling constants due to the amorphous nature of the sample. The 27Al MAS spectrum of the 30 min sample (Figure S2A of the Supporting Information) mainly exhibits a peak at 44 ppm due to the tetrahedral Al site and a broad weak envelope ranging from -13 to 5 ppm, indicating that most of octahedral Al and alumina have quickly transformed into tetrahedral Al under VPT conditions. After 2 h, the tetrahedral Al peak became broadened and shifted to 39 ppm, which is due to the fact that the 2 h sample is a mixture of the semicrystalline phase and AFI-precursor. Indeed, its corresponding 27Al f 31P HETCOR spectrum (Figure 6A) shows that the broad resonance at 39 ppm in the MAS spectrum is actually composed of two components at 39 and 36 ppm. The 39 ppm Al site exclusively connects to the -18 ppm P in the semicrystalline phase, and the 36 ppm Al site correlates with the -24 and -29 ppm P sites of the

AFI-precursor. The -24 and -29 ppm P sites are also associated with the octahedral Al site at around -7 ppm, but the correlation between the octahedral Al site and -24 ppm P site is stronger than that with the -29 ppm peak. The spectrum shows that although both P sites in the AFI-precursor are fully condensed, i.e., both have a P(OAl)4 environment, they have different connectivities. The 27Al f 31P HETCOR spectrum (Figure 6B) of as-made AlPO4-5 shows that the asymmetric P peak at -29 and its shoulder at -24 ppm only correlate with tetrahedral Al at 37 ppm. The results illustrate clearly that although the AFIprecursor bears significant resemblance to AlPO4-5, and its two P sites have the same chemical shift values as those of the fully crystallized AlPO4-5, the P sites in the AFI-precursor and AlPO4-5 do have differences in the chemical environment. It appears that for the AFI-precursor, the local environment of the P at -29 ppm has almost fully developed, but a significant number of P atoms at -24 ppm are connected to octahedral Al. Because octahedral Al will undergo further hydrolysis and condensation to yield a fully crystallized three-dimensional covalent AFI framework, the local structure of the -24 ppm P connected to these Al will continue to evolve. As mentioned earlier, the SDA molecules are slowly brought in contact with the initial dry gel via vapor phase. To monitor the SDA molecules occluded in the gel phases, we examined Raman spectra. In as-made AlPO4-5, TEA occluded in the channel is found to be fully protonated with the proton being bonded to the framework oxygen.31 For protonated TEA (TEAH+), the C-H stretching frequencies are significantly shifted to higher energies relative to liquid TEA.31,32 The measured characteristic C-H stretching frequencies of the TEA molecules in different VPT intermediates were given in Table S1 of the Supporting Information, showing an apparent shift to higher frequency for TEA in all dry gel samples, indicating that the occluded TEA species in those samples strongly interact with protons. On the basis of earlier discussion, the semicrystalline phase held by weak forces such as H-bonding transforms into AlPO4-5 via the AFI-precursor. During this evolution, H-bonding must play a critical role. To gain more details on H-bonding in the intermediate phases, we acquired 1H solid-state MAS NMR spectra. Figure 7 depicts the 1H MAS spectra of selected unwashed VPT dry gel samples. For the initial dry gel powder that does not contains TEA, two peaks at 1.5 and 3.6 ppm were superimposed on top of a very broad envelope. The broad base is due to free water molecules occluded in the pores,33 and the

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Figure 8. 1H f 31P HETCOR spectrum of 30 min VPT dry gel acquired with a 5 s pulse delay and a spinning rate of 15 kHz.

SCHEME 3: Illustration of H-Bonding in the (A) Semicrystalline Phase and (B) As-Made AlPO4-5

Figure 7. 1H MAS spectra of selected unwashed VPT dry gel samples.

maxima at around 1.5 and 3.6 ppm are due to Al-OH and P-OH groups in the AlPO-based materials, respectively.34,35 In the spectrum of the 30 min sample, in addition to the three peaks due to methyl (1.3 ppm), methylene (3.2 ppm), and occluded water (4.8 ppm),33 a weak peak with a much deshielded shift at 10.2 ppm was observed. Because the proton involved in H-bonding commonly displays a large shift toward the deshielded direction,36 the resonance at 10.2 ppm is assigned to the H atom involved in hydrogen bonding. To further probe the nature of the 10.2 ppm signal, we preformed several double-resonance NMR experiments. The 1H f 31P HETCOR spectrum of the 30 min sample (Figure 8) shows that the P site in the semicrystalline phase correlates with the proton at 10.2 ppm very strongly, suggesting this proton is fairly close to the P site. This is further confirmed by the observation of the 10.2 ppm in the 1H{31P} REDOR difference spectrum (Figure 9A). This proton is also weakly coupled to the N atom in the TEA molecule as it is clearly seen in the 1 H{14N} TRAPDOR difference spectrum (Figure 9B). The 1 H{27Al} TRAPDOR difference spectrum (Figure 9C) indicates that this proton is also spatially not too far from the Al. All these results indicate that this proton is involved in H-bonding: 3(AlO)-P-O--H---N(CH2CH3)3. It appears that this proton originally belonging to the P-OH group just starts to interact with the nitrogen atom in TEA molecule coming from the vapor phase. Therefore, the TEA in the semicrystalline phase is only “partially protonated” (Scheme 3A).

The 1H MAS spectrum of the 24 h sample corresponding to fully crystallized AlPO4-5 exhibits two sharp peaks at 1.3 and 3.2 ppm as well as a weak resonance at 8.6 ppm, Their intensity ratio is 9:6:1. Thus, the resonance at 8.6 ppm can be unambiguously assigned to the proton directly attached to the nitrogen in the fully protonated SDA molecule, (CH3CH2)3NH+, which is H-bonded to the framework oxygen (Scheme 3B). At this point, a dynamic picture of the formation of AlPO4-5 under VPT conditions can be proposed (Scheme 4). The initial dry gel is an amorphous material formed in the absence of a SDA. It likely has a large number of small AlPO species with chain structures containing 4- and 6-membered rings (so-called “parent chains”4d), which are the nutrients for AlPO4-5. When TEA molecules are gradually transported via vapor phase onto the surface of the amorphous material, these AlPO species begin to reorganize around the SDA molecules via H-bonding to form a semicrystalline phase. The structure of this semicrystalline phase contains the layers with a 12-MR-like pore (the layer eventually evolves into the ab plane of AlPO4-5), and these sheets stack together along the channel direction in a disordered fashion and are held together by weak interactions such as H-bonding or van der Waals forces. As a result, this phase is very fragile. Washing with a large amount of water results in a breakdown of the structure and shifts the equilibrium back to the amorphous material. By increasing the heating time, the layers with 12-MR-like pore structures in the semicrystalline phase realign themselves under the influence of a SDA to evolve into a more ordered three-dimensional framework with many resemblances to the AFI structure in long-range (shown in the XRD pattern) and short-range ordering (shown in the 31P NMR spectrum) via hydrolysis and condensation. This is the AFIprecursor, which was captured experimentally. Although the AFI-precursor is very similar to the AFI topology in long- and



Figure 9. (A) 1H{31P} REDOR, (B) 1H{14N} TRAPDOR, and (C) 1H{27Al}TRAPDOR spectra of VPT 30 min dry gel sample. Rotor periods for dephasing are 10, 2, and 2 for panels A, B, and C, respectively. Spinning speed was 15 kHz. Recycle delay was 5 s.

SCHEME 4: Illustration of Formation of AlPO4-5 under VPT Conditions

short-range ordering, it is not yet a three-dimensional covalent bonded network, and instead its structure is held by a weak bonding interaction, at least along one dimension. AlPO4-5 eventually develops from this precursor via final hydrolysis and condensation. Conclusion This work demonstrated that the VPT approach allows one to “freeze” crystallization at different stages. This is because the reactions proceed slowly under VPT conditions. Therefore, the reactive intermediates can be captured as a function of time and provide “snapshots” of the crystallization process. Another important finding is that the reactive intermediates have two key features: (1) They bear certain similarities to the target structure. (2) These intermediates are quite fragile and are held together by weak bonding interactions, which can be easily altered or broken by washing. These properties are essential to

the intermediates formed during the VPT process. Because dry gel powder cannot be dissolved and transformations are assisted by vapor, it would be extremely difficult for a fully covalently bonded intermediate with no relationship with the final sieve to easily rearrange the local structure and then transform to the final product via breaking and reforming the covalent bond network in the absence of a bulk liquid. The weak bonding interactions in these precursors provide flexibility for reorganization of the local structure to form a three-dimensional covalent framework. Acknowledgment. Y.H. thanks the Natural Science and Engineering Research Council of Canada for a research grant and the Canada Foundation for Innovation for an equipment grant. Funding from the Canada Research Chair and Premier’s Research Excellence Award programs is also gratefully ac-

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Investigation of Crystallization of Molecular Sieve AlPO4

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