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J. Phys. Chem. C 2007, 111, 15236-15243

Examining the Self-Assembly of Microporous Material AlPO4-11 by Dry-Gel Conversion Banghao Chen and Yining Huang* Department of Chemistry, The UniVersity of Western Ontario, London, Ontario, N6A 5B7 Canada ReceiVed: March 7, 2007; In Final Form: June 24, 2007

Crystallization of a representative aluminophosphate microporous material, namely AlPO4-11 with AEL structure, has been investigated by dry-gel conversion (DGC) including steam-assisted conversion (SAC) and vapor-phase transport (VPT). The crystallization under SAC and VPT conditions appears to follow the same pathway: the initial amorphous material is converted to a semicrystalline intermediate, which transforms into AlPO4-11. The semicrystalline intermediate appears to have a three-dimensional structure that bears some similarity to AEL structure. Raman spectrum suggests that it contains the 10-ring channel. XRD patterns indicate that the structure seems to be only ordered in the layer associated with the unit cell ab plane of AlPO4-11. The layers are stacked together via H-bonding and van der Waals interactions. However, the layers are not perfectly aligned as they are in the AEL structure, resulting in disordering along the 10-ring channel direction. Quantitative 31P MAS NMR spectra confirm the amorphous to semicrystalline to AEL phase transition sequence. Since the semicrystalline phase is held together by weak intermolecular interactions, washing with water results in the conversion back to amorphous phase. The weak bonding forces in the semicrystalline phase are important, because they provide flexibility for reorganization of local bonding environment to form a three-dimensional covalent AEL framework. The VPT study provides unambiguous evidence that the formation of the semicrystalline phase is under the direction of di-n-propylamine.

Introduction Microporous materials (often referred to as molecular sieves) are crystalline, open-framework materials containing channels and cavities with molecular dimensions. The most well-known family of such materials is zeolites that are aluminosilicates. Zeolites are widely used in industry as ion exchangers, catalysts, and sorbents.1 Another important type of material are aluminophosphate molecular sieves, AlPO4s. Many of these materials have novel framework structures, and they exhibit distinct molecular sieving characteristics.2 The frameworks of AlPO4s are built upon alternating, corner-sharing PO4+ and AlO4tetrahedra. Although AlPO4s are electrically neutral, they can be made catalytically active through the isomorphic substitution of framework Al3+ and P5+ ions by divalent metal cations and silicon to produce MeAPOs (metal aluminophosphates) and SAPOs (silicoaluminophosphates), respectively.3 Recently, they have also found new uses in gas sensing application and in nanotechnology for nanocomposite materials with novel electronic and optical properties.4 Due to a wide range of practical applications, synthesis of new microporous materials is one of the major activities in materials science. Despite the tremendous progress made in the last several decades,5,6 molecular sieve crystallization is still not completely understood at a molecular level. Consequently, the rational design of a novel framework with desired properties is still difficult. AlPO4s are usually prepared by hydrothermal synthesis (HTS) from a reactive aluminophosphate (AlPO) gel containing an organic amine or quaternary ammonium ion as a structure-directing agent (SDA). The inherent difficulties in studying self-assembly arise from the fact that HTS is an extremely complicated process, involving multiple-component * Corresponding author. Phone: (+1) 519-661-2111 x86384. Fax: (+1) 519-661-3022. E-mail: [email protected].

reactions and chemical equilibria in a heterogeneous environment with both solid and liquid phases coexisting.5c,h An additional problem is the identification and subsequent characterization of the true intermediates. Usually, the intermediates or precursors to the final crystalline microporous materials are obtained by 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 the postsynthesis treatments5h,m,7 such as washing. It is sometimes uncertain if the solid phases examined are the true intermediates. In recent years, a relatively new approach, namely dry-gel conversion (DGC),7b,8 has been developed as an alternative method for molecular sieve synthesis (Scheme 1). DGC converts predried gel powder to a crystalline microporous material at elevated temperatures and pressures. It can be further classified into (1) steam-assisted conversion (SAC), in which the predried gel powder containing SDA are physically separated from a very small amount of pure water in an autoclave, and (2) vaporphase transport (VPT), which is similar to SAC except that the SDA is not contained in the initial dry gel. Instead, a small amount of SDA aqueous solution is placed at the bottom of the autoclave. DGC is well-suited for examining crystallization, because the reaction systems are simpler. For SAC, all the reactive species are contained in the solid phases. The small amount of bulk water is separated from the dry gel, avoiding the situation occurring in HTS, where the reactive species in solution are in direct contact with solid gel. The amount of water vapor is not enough to dissolve the solids. Thus, the crystallization under DGC conditions is not due to gel dissolution. The VPT method can be used to examine the role of the SDA, as it is gradually brought in contact with the solid gel via the vapor phase. The differences and similarities between dry gel conversion and

10.1021/jp071868f CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

Self-Assembly of AlPO4-11 by Dry-Gel Conversion SCHEME 1: Schematic Diagram of the Reaction Vessel Used for the SAC or VPT Method

conventional hydrothermal methods have been discussed.5a,9 Although there is the lack of an apparent liquid phase in DGC, the “dry” powder does contain some adsorbed and surface bound water. Therefore, the reactions under DGC conditions may reflect certain aspects of those occurring in a more accessible liquid phase.5a For this reason, a study on the crystallization of molecular sieves under DGC conditions may also shed some light on the understanding of the formation mechanism under HTS conditions. The potential of DGC for the study of zeolite crystallization has been previously recognized.7b,8b But there have been relatively few studies that directly focus on the crystallization process.10 Recently, we have utilized DGC to examine how AlPO4-11 self-assembles. AlPO4-11 is a representative AlPO4based molecular sieve that was first synthesized by Union Carbide.11 It has the AEL structure (Scheme 2), containing a unidimensional channel system with a noncircular 10-membered ring.12 Substitution in the AlPO4-11 framework by Si gives rise to the superior acid-catalyst SAPO-11 for alkylation,13 disproportionation, and isomerization involving aromatic compounds.14 It is also commercially used in lube oil dewaxing.15 A number of studies in literature have dealt with the synthesis of AlPO4-11 under HTS conditions with emphasis on wet gel chemistry,16 the structure of the solid gel,17 the role of amines as SDAs,16a,18 and preparation of AlPO4-11 nanocrystals.19 Recently, AlPO4-11 was also synthesized by DGC.9a Although much data have been compiled on the synthesis conditions, the mechanism of the formation is still not clear. A model involving a transformation from amorphous material to two-dimensional (2-D) layer and eventually to three-dimensional (3-D) framework for AlPO4s was proposed previously, including the AEL structure,5k but no kinetic intermediates have been directly observed for AlPO4-11. In the present work, AlPO4-11 crystallization under DGC conditions has been examined and new physical insight into the formation of AlPO4-11 gained. Experimental Section Sample Preparation. The aluminum and phosphorus sources were Al(OH)3 (50% Al2O3) and 85% H3PO4. Di-n-propylamine (DPA) was used as a SDA. All the chemicals were purchased from Aldrich Chemical Co. The initial gel compositions (Al2O3: P2O5:DPA:H2O) for SAC and VPT were 1:1:1:40 and 1:1:0: 40, respectively. A typical procedure9a for the preparation of SAC dry gel powder is the following: an appropriate amount of Al(OH)3 was mixed with distilled water, and the mixture was stirred at room temperature for 30 min, followed by dropwise addition

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15237 of 85% H3PO4. DPA was then added with continuous stirring. This mixture was dried at 80 °C 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 SAC dry gel. A series of intermediates were prepared by typically placing 1.0 g of the initial SAC dry gel powder into a series of small Teflon cups. Each cup was placed in a Teflon-lined autoclave (volume, 23 mL) with 0.3 mL of distilled water at the bottom (Scheme 1). The autoclaves were heated at 175 °C in an oven and taken out after different heating times. The reactions were quenched in cold water. The sample from each cup was divided into two parts. One-half of the sample was unwashed and directly dried in air. The other half was first washed in a beaker and then dried in the same beaker in air without isolation from the liquid phase. This ensured that no solid particles could be washed away. The dried solid samples were kept in tightly sealed glass vials. In the case of the VPT route, the initial VPT dry gel was prepared similarly to SAC, except that DPA was not included, and 1.75 g of 71% DPA solution was placed at the bottom of each autoclave. Characterization. Powder X-ray diffraction patterns were recorded on a Rigaku diffractometer using Co KR radiation (λ ) 1.7902 Å). 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 the NMR experiments were carried out on a Varian/ Chemagnetics Infinityplus 400 WB spectrometer equipped with three rf channels operating at a 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 and 27Al were referenced to 85% H3PO4 and 1 M Al(NO3)3 aqueous solution, respectively. Depending on the requirement of the experiment, two MAS (a Varian/Chemagnetics 7.5- and a 4.0-mm tripletuned) probes were used. For 31P MAS experiments, a 30° pulse was typically used and the recycle delay was 60 s. The 27Al MAS spectra were acquired using a short excitation pulse (12.5°) and a pulse delay of 200 ms. 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 condition20 was determined using NH4H2PO4. A repetition time of 10 s was used with a spinning speed at 6.5 kHz. All the transfer of population in double-resonance (TRAPDOR) experiments21 was carried out under the identical spectrometer conditions: the rf field for Al irradiation was 62.5 kHz, the spinning rate was 6.5 kHz ( 3 Hz, and the pulse delay was 120 s. Results and Discussion SAC Method. To follow the evolution of the long-range ordering in the SAC process, the powder XRD patterns of gel samples were recorded as a function of crystallization time. For the unwashed samples, the XRD pattern of the initial dry gel (Figure 1A) contains two strong reflections with 2θ values of 7.5° and 9.3°, suggesting that these peaks represent an intermediate with a certain degree of long-range ordering. In the mid-angle region, there are two features, an extremely broad halo near 2θ ) 32°, implying the coexistence of an amorphous phase, and a board peak centered at 2θ ) 26° (labeled with *), c which is superimposed on the broad halo. These two broad peaks belong to two different species (see the discussion below). Upon treating the dry gel under SAC conditions, the intensities of the two reflections at 2θ ) 7.5 ° and 9.3° increase at the

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SCHEME 2: (A) Framework of AlPO4-11 and (B) the ab Plane Viewed along the c-Axisa

a

The edges of unit cell are shown.

expenses of the broad amorphous halo near 2θ ) 32°, indicating that amorphous material is being converted to the intermediate phase with long-range ordering. After the dry gel was heated for 3 h, the broad peak at 2θ ) 26° evolved into several weak reflections whose positions correspond to (310)/(002), (231), (301), and (240) reflections of AlPO4-11, implying that the crystalline AlPO4-11 started forming. Further heating resulted in the sharpening of the above-mentioned reflections and the appearance of additional peaks belonging to AlPO4-11. These changes are accompanied by the observation that the reflection at 2θ ) 9.3° seen for the initial dry gel gradually evolves into the (110) reflection of AlPO4-11 and concomitantly the strong peak at 2θ ) 7.5° slowly vanishes with heating. It is worth noting that the d-spacing of the reflection at 7.5° is identical to that of the theoretical (100) reflection, which is not observable in fully crystallized AlPO4-11 due to systematic absences (the space group of as-made AlPO4-11 belongs to body centered orthorhombic12). The peak at 2θ ) 9.3° is highly asymmetric with a weak shoulder on the high angle side. This shoulder coincides with the (020) reflection of AlPO4-11 and is indeed resolved as a sharp (020) reflection of AlPO4-11 after 3 h of heating. It appears that the reflections at 2θ ) 7.5 ° and 9.3° as well as the broad peak near 2θ ) 26° all belong to a semicrystalline intermediate whose structure bears some resemblance to AlPO4-11. After 72 h highly crystalline AlPO4-11 was obtained. Interestingly, upon washing with water, all three reflections (at 2θ ) 7.5°, 9.3° and 26°) assigned to the semicrystalline

intermediate disappeared in the XRD patterns (Figure 1B), and only the extremely broad amorphous halo remained in the washed samples heated for less than 3 h. This result suggests that the semicrystalline intermediate transformed into an amorphous phase upon washing. It also indicates that the structure of this phase must be held by weak bonding forces (such as hydrogen bonding and van der Waals forces). Since the Raman spectra in the T-O-T (T ) tetrahedral site) bending region are very sensitive to the ring systems existing in the molecular sieve frameworks,22 they are often used to gain information on the size of the pore opening in the intermediates. The spectra of the unwashed samples (Figure 2A) exhibit a broad envelope containing several overlapping bands in the region ca. 460-400 cm-1. For the samples heated for 2h and less, these Raman bands almost completely disappeared upon washing (Figure 2B). This observation indicates that these bands originate from the semicrystalline phase, which transforms to amorphous materials upon washing. Previous work has assigned the bands in the region ca. 500-400 cm-1 to bending vibrations due to the 4- and/or 6-ring in the AlPO4-based frameworks.22 Observation of relatively strong peaks in this region suggests that a large number of 4- and/or 6-rings exist in the semicrystalline structure. The spectrum of AlPO4-11 (72 h sample) displays a band at 262 cm-1. For AlPO4-based microporous materials with unidimensional channel systems, the Raman bands in the 350-260 cm-1 region are unambiguously assigned to the “channel breathing” mode for the pore breathing vibration around the large pore channel. This particular

Figure 1. Powder XRD patterns of (A) unwashed and (B) washed SAC dry gel samples.

Self-Assembly of AlPO4-11 by Dry-Gel Conversion

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Figure 2. Raman spectra of (A) unwashed and (B) washed SAC dry gel samples.

Figure 3. Powder XRD patterns of selected SAC dry gel samples obtained by quenching the SAC reaction at different stages and reheating with a small amount of H2O under the same SAC conditions.

band at 262 cm-1 was previously assigned to the “porebreathing” mode of the 10-ring channel system of AlPO4-11.22b For the washed 3, 4, and 8 h samples, the intensity of this band increases with increasing heating time, which is consistent with the corresponding XRD patterns showing the growth of the intensities of the reflections of AlPO4-11. A band with a frequency slightly higher than 262 cm-1 also exists in the Raman spectra of the samples heated for 2 h or less. This band becomes stronger and shifts from 270 to 266 cm-1 gradually with increasing heating time, but disappears after washing. Seeing this “channel breathing” mode implies that a unidimensional 10-ring-like channel system may already exist in the semicrystalline phase. Its higher frequency suggests that, in the semicrystalline phase, the average Al-O-P angle around the channel is slightly smaller than that in the fully crystallized AlPO4-11.22b The XRD and Raman results indicate that the initial SAC dry gel contains an amorphous material and a semicrystalline intermediate with a 3-D structure. The structure of this semicrystalline phase seems to bear some similarity to the AEL structure, as its three reflections gradually evolve into the reflections belonging to AlPO4-11. The two relatively narrow low-angle reflections indicate that the periodicity initially is developed in the (100) and (110) planes. In the AEL structure, each 6-ring is connected to two neighboring 6-rings via oxygen bridges to form 4-rings between the 6-rings and linked to another 6-ring by sharing a common edge (fused 6-rings). Such an

arrangement creates noncircular 10-rings in the ab plane (Scheme 2B). These layers are stacked on top of one another to form a one-dimensional, 10-ring channel system along the c direction. Seeing several hk0 reflections for the initial dry gel and the 10-ring channel in the Raman spectrum suggests that there may be a regular arrangement in a 2-D pattern associated with the ab plane of AlPO4-11. The fact that no resolved XRD peak that can be directly related to a reflection with a nonzero l component of AlPO4-11 was observed may imply that the intermediate structure is disordered along the 10-ring channel direction. We also wish to point out that using the DGC method, the crystallization can be frozen and resume at different stages. To illustrate this point, we reheated several unwashed solid samples obtained by quenching the reaction at different stages of the crystallization for extended periods under the same SAC conditions, and the XRD patterns (Figure 3) confirm the formation of high-quality AlPO4-11. To characterize the local chemical environments of P and Al in the intermediate phases, solid-state NMR experiments were performed. Figure 4 illustrates the 31P MAS spectra of unwashed and washed SAC dry gel samples as a function of crystallization time. For the dry gel samples before 3 h, all spectra contain a relatively sharp peak at -18 ppm superimposed on a very broad peak centered at around -13 ppm. The broad resonance can be assigned to amorphous material and the narrow peak to the

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Figure 4.

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P MAS spectra of (A) unwashed and (B) washed SAC dry gel samples.

semicrystalline phase.23 The sharpness of the -18 ppm signal indicates the high local ordering around P. The assignment is directly confirmed by the corresponding 31P MAS spectra of the washed samples (Figure 4B), which contain only a very broad resonance at -13 ppm, and the narrow signal at -18 ppm disappeared completely upon washing. After heating the initial dry gel for 3 h, a signal at around -30 ppm started appearing as a shoulder, and its intensity grew with increasing heating time. This trend is seen more clearly in the spectra of the washed samples. The appearance of this new resonance, which remains after washing, is consistent with the formation of crystalline AlPO4-11, as shown in the corresponding XRD patterns. The 31P spectrum of 72 h sample looks identical to that of pure AlPO4-11 reported in the literature.24 Quantitative analysis of 31P MAS spectra (Supporting Information, Figure S1 and Table S1) clearly show that while the amorphous phase continuously decreases, the amount of the semicrystalline phase in the mixture increases in the early stage of SAC treatment and then decreases once AlPO4-11 starts forming. This suggests that, upon SAC treatment, the amorphous material is first converted to the semicrystalline material, which gradually transforms into AlPO4-11. The 27Al MAS spectra of both the initial dry gel and 2 h sample (Supporting Information, Figure S2) exhibit a strong peak at 43 and a broad peak at -8 ppm with a weak shoulder at around 7 ppm. The 43 and -8 ppm sites are assigned to the tetrahedral Al (Tet-Al) and the octahedral Al (Oct-Al) with an Al(OP)4(OH2)2 environment in the AlPO species, respectively.25 The 7 ppm shoulder is due to a small amount of unreacted aluminum oxide17 (also see Figure S3 of the Supporting Information for more discussion on the assignment). Washing the samples with water results in a significant increase in the intensity of the Oct-Al relative to the Tet-Al signal. The strong and relatively sharp peak at 43 ppm became a weak and much broader signal at the same position. The results indicate that (1) the vast majority of the Al atoms in the semicrystalline phase are Tet-Al appearing at 43 ppm. (2) The amorphous phase

contains mostly Oct-Al. Treating the initial dry gel for 3 h yields a weak shoulder at 36 ppm due to the Tet-Al in AlPO4-11 on the high-field side of the 43 ppm main peak due to the Tel-Al in the semicrystalline phase and it gradually evolved into a better resolved peak upon increasing the heating time. The change can be seen more clearly in the spectra of washed samples. Overall, the changes in the 27Al MAS spectra are parallel to those seen in the XRD patterns and 31P MAS spectra. Unlike crystalline AlPO4-11, where all the P sites are fully condensed with P(OAl)4 coordination, the P sites of the semicrystalline phase and amorphous material in the intermediates are expected to have a lower degree of condensation, i.e., the number of Al atoms attached to each P via a bridging oxygen atom is less than four. To estimate the condensation degree of the P sites in the semicrystalline and amorphous phases, 31P{27Al} TRAPDOR experiments21 were performed. This technique was recently used to estimate the number of Al atoms in the second coordination sphere for the P sites in the AlPObased material.26 It compares the initial slope of the plots of 31P{27Al} TRAPDOR fraction (∆S/S ) vs dephasing time for a 0 given P site to those three known P environments, P(OAl)4, P(OAl)3, and P(OAl)2 in two model compounds (VPI-5 and MAPO-20). Figure 5 compares the TRAPDOR fraction of the P sites in the intermediate phases and crystalline AlPO4-11 with the slopes of three known sites in the above-mentioned model compounds. It shows clearly that the initial slope of the -31 ppm P site in AlPO4-11 is almost identical to that of VPI-5, confirming that this P site is fully condensed with P(OAl)4 environment, as expected. The slope of the -18 ppm P site in the semicrystalline phase is very close to that of the P(OAl)3 environment in MAPO-20, indicating that, in this semiordered intermediate, each P is on average connected to three Al atoms. Since the slope of the resonance at -13 ppm is similar to that of the P(OAl)2 in MAPO-20, we suggest that the P sites in the amorphous material have the lowest condensation degree, with the average number of Al atom in the second coordination sphere being two.

Self-Assembly of AlPO4-11 by Dry-Gel Conversion

Figure 5. 31P{27Al} TRAPDOR fraction as a function of dephasing time. (The straight lines illustrate the slopes due to P(OAl)4, P(OAl)3, and P(OAl)2 in VPI-5 and MAPO-20).

Figure 6. 1H f 31P CP and corresponding MAS spectra of (A) initial and (B) 3 h SAC dry gel samples.

To further characterize each P site, 1H f 31P crosspolarization (CP) experiments were also carried out. For the initial dry gel without SAC treatment, the selected CP spectra with short and long contact times (CT) together with corresponding MAS spectrum are shown in Figure 6A. At a very short contact time of 0.06 ms, the intensity of the broad peak centered at -13 ppm is enhanced significantly relative to that of the sharp resonance at -18 ppm. Obviously, the P sites in the amorphous material experience stronger dipolar interaction with protons compared to the P sites in the semicrystalline phase, suggesting that the former has more hydroxyl groups attached. The CP spectra of the 3 h sample (Figure 6B) acquired at very long contact time confirm that a small amount of AlPO4-11 starts forming, which is not visible in the MAS spectrum. The CP intensities of the two peaks at -13 and -18 ppm in the initial SAC dry gel as well as the -31 ppm resonance in crystalline AlPO4-11 were plotted as a function of contact time (Supporting information, Figure S4). The CP dynamics can be described by the equation27

S(t) ) Smax(1 - TCP/T1FH)-1[exp(-τ/T1FH) exp(-τ/TCP)] The initial growth of the CP signal is controlled by the crosspolarization time constant (TCP), which is related to the second moment of the dipolar interaction between two unlike spins. 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 stronger H-P dipole-dipole interactions. The T1FH was determined by using a previously described procedure.28 TCP values were then obtained by fitting the CP data to the above equation with measured T1FH, and they are 0.46, 0.64, and 1.09 ms for the peaks at -13, -18, and -31 ppm, respectively. The fact that the P site of crystalline AlPO4-11 has the largest TCP value indicates the weakest 1H-31P dipolar interaction at the fully

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15241 condensed P(OAl)4 site. The P sites in the amorphous and semicrystalline phases have much shorter TCP values, suggesting that the P atoms in these phases are not fully condensed and have directly attached OH groups. That the -13 ppm peak has a much smaller TCP value compared to the -18 ppm peak is indicative of the P sites in the amorphous phase having more OH groups attached. From 1H f 31P CP and 31P{27Al} TRAPDOR results, the average chemical environments of P sites in the semicrystalline and amorphous phases are most likely to be P(OAl)3(OH) and P(OAl)2(OH)2. In summary, mixing of Al and P with DPA and heating at 80 °C directly results in a mixture of an amorphous material and a semicrystalline AlPO phase. Heating this initial dry gel powder under SAC conditions facilitates the conversion of the amorphous material to the semicrystalline phase. This semicrystalline phase contains a relatively ordered layer that might be related to the ab plane of AlPO4-11. The cross-link between the layers, however, is incomplete, and the structure along the channel direction is disordered. Further heating leads to the formation of proper Al-O-P linkages along the channel direction, yielding AlPO4-11 crystallites. VPT Method. In SAC study, a significant amount of the semicrystalline intermediate has already existed in the initial SAC dry gel, and thus the role of DPA is not obvious. As mentioned earlier, the VPT method is different from SAC where the initial VPT dry gel does not contain SDA. Since the SDA molecules are gradually brought in contact with the initial VPT dry gel via vapor phase, VPT should be a useful method for investigating the role of SDA. Figure 7A shows the XRD patterns of the unwashed solid samples heated for different times under the VPT conditions. For the initial dry gel prior to the VPT treatment, only two very broad envelopes were observed, indicating that only the amorphous phase exists in the absence of DPA. A brief treatment of the initial VPT dry gel for 15 min in the presence of DPA resulted in the appearance of two extremely weak low 2θ angle reflections (marked by the arrows). On the basis of their 2θ values, we assign them to the semicrystalline phase observed in the initial SAC dry gel. In the higher angle region, in addition to the amorphous halo, there are two sharp peaks (marked by /). Their 2θ values do not coincide with the reflections of AlPO4-11, indicating the existence of an additional ordered phase. Treating the initial dry gel for 50 min leads to a dramatic increase in the intensity of the two low-angle reflections at the expense of the amorphous halo and two sharp high-angle peaks, suggesting that the amorphous phase and the additional ordered phase are transforming into the semicrystalline phase in the presence of DPA. The XRD pattern of the sample heated for 5 h contains reflections of AlPO4-11 and the semicrystalline intermediate. The highly pure AlPO4-11 was obtained after 14 h of heating under VPT conditions. In addition, a control experiment showed that this initial AlPO VPT dry gel cannot transform into AlPO4-11 under the identical heating conditions when only a small amount of water without DPA (Supporting Information, Figure S5A) was placed at the bottom of the autoclave, confirming that DPA is necessary for the formation of AlPO4-11. The 31P MAS spectrum (Figure 7B) of the initial VPT dry gel illustrates a single broad profile at around -20 ppm, and the broadness of the peak is in agreement with the amorphous nature of the initial dry gel. The spectrum of the 15 min sample shows two narrower peaks at -18 and -26 ppm superimposed on the very broad envelop observed in the initial dry gel without heating. Seeing a narrow peak at -18 ppm confirms the

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Figure 7. (A) Powder XRD patterns, (B) 31P MAS spectra, and (C) Raman spectra of selected VPT dry gel samples. (All the samples are unwashed.)

formation of the semicrystalline intermediate, which is not highly visible in the corresponding XRD pattern. The -26 ppm resonance is attributed to the additional ordered phase observed in the corresponding XRD pattern. The deconvoluted spectrum (Supporting Information, Figure S5B) shows that the solid mixture contains 16% semicrystalline phase, 14% additional ordered phase, and 70% amorphous phase. The -26 ppm resonance quickly disappeared from the spectrum of the sample heated for 50 min, and the -18 ppm peak becomes dominant, indicating that the additional ordered phase is transforming into the semicrystalline phase. The peaks due to AlPO4-11 started showing up after 5 h of heating. Their intensities increase upon increasing the heating time at the expense of the -18 ppm signal, a trend very similar to that observed in the SAC synthesis. The Raman spectra of selected VPT gel samples are shown in Figure 7C. For the 15 min sample, a band at around 275 cm-1 (which is absent in the Raman spectrum of initial dry gel) indicates the existence of 10-ring-like channel that is characteristic of the semicrystalline phase. This band becomes stronger and shifts to 262 cm-1 gradually with increasing heating time. After 15 min, the evolution of the Raman spectra is almost the same as that in the SAC process. On the basis of the above discussion, we conclude that the AlPO4-11 crystallization mechanism under VPT conditions is almost the same as that of the SAC process, except that the semicrystalline intermediate forms at the beginning of the VPT treatment. As soon as the DPA molecules are introduced onto the surfaces of the solid dry gel via vapor phase, the amorphous species immediately begin to reorganize around DPA to form the semicrystalline phase, which eventually evolves into AlPO411. Since at the very beginning of the VPT treatment the DPA molecules are just being brought to the surfaces of the amorphous material, they are distributed inhomogeneously on the exterior of the initial VPT dry gel powder. Because there is no apparent liquid phase, the diffusion of the DPA molecules through the solid gel is slow. Consequently, there might be domains where the DPA concentration is either higher or lower than the optimized ratio for the semicrystalline phase, resulting in an additional short-lived ordered phase as observed by XRD and NMR. Summary We have examined the formation of molecular sieve AlPO411 as a function of heating time under DGC conditions. Both

SAC and VPT processes follow the same self-assembly mechanism, involving the formation of a key semicrystalline phase detected by XRD, which has a 10-ring-like channel system, as seen in the Raman spectrum. The XRD pattern suggests that this phase is only ordered in the layer perpendicular to the channel direction, and the layer bears resemblance to the ab plane of AlPO4-11. The structure along the channel direction appears to be disordered, and the layers are likely stacked together mainly via weak bonding interactions. Solid-state NMR results indicate an average P(OAl)3(OH) local environment in the semicrystalline phase. Existence of the OH groups allows further condensation via cross-link between the layers to form the 3-D covalent framework. We wish to emphasize that the semicrystalline phase has a significant relationship with target AlPO4-11. It contains the layers very similar to the ab planes of AlPO4-11 and the 10ring channel, which also exists in AlPO4-11. Very recently, we have used 17O-enriched water to examine the formation of AlPO4-11 under DGC conditions.10f The results clearly indicate that those joint 4- and 6-rings in the structure of AlPO4-11 have already existed in the semicrystalline phase and they remain unaffected during the transformation to the final AEL structure. All this spectroscopic evidence indicates that the semicrystalline material has the key structural signature of AlPO4-11. Therefore, it is not unreasonable to suggest that this semicrystalline phase is a structural precursor to AlPO4-11 rather than just a nutrientstoring transient. The semicrystalline phase is in dynamic equilibrium with the AEL structure. Treating the semicrystalline intermediate under DGC conditions promotes transformation to the AEL structure. The weak intermolecular interactions provide the flexibility and freedom for the structural reorganization to transfer to the AEL structure. The semicrystalline phase is also in equilibrium with the amorphous phase, and water washing destroys the weak bonding interaction and shifts the equilibrium to the amorphous phase. The amorphous material supplies nutrients such as 1-D chains or other building blocks containing 4- and 6-rings necessary for assembly of the semicrystalline framework. DPA is absolutely essential for the formation of the semicrystalline structure and therefore AlPO4-11. The evidence directly comes from the VPT study, which shows unambiguously that the AlPO-based amorphous solids can only be converted to the semicrystalline phase when DPA is present. Several early studies showed that the protonated DPA is tightly

Self-Assembly of AlPO4-11 by Dry-Gel Conversion trapped in the channel of the AEL structure.18b,c It appears that the DPA molecules stretch the two alkyl groups along the 10ring channel18b as a thread, holding the layers via H-bonding and van der Waals interactions to yield the 3-D structure. The work also reinforces the argument put forward by several others that the structure of true reaction intermediates can be very fragile; therefore, care must be exercised during the postsynthesis treatment in mechanistic studies.5h,6,7 For DGC study, both washed and unwashed samples should all be examined, since as shown in this work, the comparison reveals important structural information. 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 acknowledged. We also thank Drs. C. Kirby and N. C. Payne for technical assistance and helpful discussion. Supporting Information Available: Additional experimental results (five figures and one table). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Introduction to Zeolite Science and Practice, 2nd ed.; Van Bekkum, H., Flanigen, E. M., Jacobs, P. A., Jansen, J. C., Eds.; Elsevier: Amsterdam, 2001. (2) (a) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. (b) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S. T. Stud. Surf. Sci. Catal. 1986, 28, 103. (3) Flanigen, E. M.; Patton, R. L.; Wilson, S. T. Stud. Surf. Sci. Catal. 1988, 37, 13. (4) (a) Tuller, H. L.; Seh, H.; Hyodo, T. U.S. Pat. Appl. Publ. 2005, 24 pp. (b) Tsapatsis, M. AIChE J. 2002, 48, 654. (c) Hoffmann, K.; ReschGenger, U.; Marlow, F. In Host-Guest-Systems based on Nanoporous Crystals; Laeri, F., Schuth, F., Simon, U., Wark, M., Eds.; Wiley-VCH GmbH & Co. KgaA: Weinheim, 2003. (d) Davis, M. E. Nature 2002, 417, 813. (5) For reviews see: (a) Cundy, C. S.; Cox, P. A. Microporous Mesoporous Mater. 2005, 82, 1. (b) Cheetham, A. K.; Ferey, G.; Loiseau, T. Angew. Chem., Int. Ed. 1999, 38, 3268. (c) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 756. (d) Feng, S.; Xu, R. Acc. Chem. Res. 2001, 34, 239. (e) Gies, H.; Marler, B.; Werthmann, U. Mol. SieVes (Synth.) 1998, 1, 35. (f) Corma, A. Stud Surf. Sci. Catal. 2004, 154A, 25. (g) Zones, S. I.; Nakagawa, Y.; Lee, G. S.; Chen, C. Y.; Yuen, L. T. Microporous Mesoporous Mater. 1998, 21, 199. (h) Francis, R. J.; O’Hare, D. J. Chem. Soc., Dalton Trans. 1998, 3133. (i) Morris, R. E. J. Mater. Chem. 2005, 15, 931. (j) Epping, J. D.; Chemlka, B. F. Curr. Opin. Colloid Interface Sci. 2006, 11, 81. (k) Oliver, S.; Kuperman, A.; Ozin, G. A. Angew. Chem., Int. Ed. 1998, 37, 46. (l) Poulet, G.; Sautet, P.; Tuel, A. Actualite Chim. 2005, 282, 18. (m) Cheetham, A. K.; Mellot, C. F. Chem. Mater. 1997, 9, 2269. (n) Norby, P. Curr. Opin. Colloid Interface Sci. 2006, 11, 118 and references therein. (6) (a) O’Brien, M. G.; Beale, A. M.; Catlow, C. R. A.; Weckhuysen, B. M. J. Am. Chem. Soc. 2006, 128, 36, 11744. (b) Beale, A. M.; van der Eerden, A. M. J.; Grandjean, D.; Petukhov, A. V.; Smith, A. D.; Weckhuysen, B. M. Chem. Commun. 2006, 42, 4410. (c) Yu, J.; Li, J.; Wang, K.; Xu, R.; Sugiyama, K.; Terasaki, O. Chem. Mater. 2000, 12, 3783. (d) Norby, P. Curr. Opin. Colloid Interface Sci. 2006, 11, 118 and references therein. (e) Vistad, O. B.; Akporiaye, D. E.; Taulelle, F.; Lillerud, K. P. Chem. Mater. 2003, 15, 1639. (f) Muncaster, G.; Davies, A. T.; Sankar, G.; Catlow, C. R. A.; Thomas, J. M.; Colston, S. L.; Barnes, P.; Walton, R. I.; O’Hare, D. Phys. Chem. Chem. Phys. 2000, 2, 3523. (g) Grandjean, D.; Beale, A. M.; Petukhov, A. V.; Weckhuysen, B. M. J. Am. Chem. Soc. 2005, 127, 14454. (h) Watson, J. N.; Iton, L. E.; Keir, R. I.; Thomas, J. C.; Dowling, T. L.; White, J. W. J. Phys. Chem. B 1997, 101, 10094. (i) Sankar,

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15243 G.; Wright, P. A.; Natarajan, S.; Thomas, J. M.; Greaves, G. N.; Dent, A. J.; Dobson, B. R.; Ramsdale, C. A.; Jones, R. H. J. Phys. Chem. 1993, 97, 9550. (j) Walton, R. I.; Smith, R. I.; O’Hare, D. Microporous Mesoporous Mater. 2001, 48, 79. (k) Herrmann, R.; Schwieger, W.; Scharf, O.; Stenzel, C.; Toufar, H.; Schmachtl, M.; Ziberi, B.; Grill, W. Microporous Mesoporous Mater. 2005, 80, 1. (l) Martucci, A.; Alberti, A.; Cruciani, G.; Frache, A.; Marchese, L.; Pastore, H. O. J. Phys. Chem. B 2005, 109, 13483. (m) De Moor, P.; Beelen, T. P. M.; Van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600. (7) (a) Vistad, Ø. B.; Akporiaye, D. E.; Taulelle, F.; Lillerud, K. P. Chem. Mater. 2003, 15, 1639. (b) Kim, M.; Li, H.; Davis, M. E. Microporous Mater. 1993, 1, 191. (8) (a) Xu, W.; Dong, J.; Li, J.; Wu, F. J. Chem. Soc., Chem. Commun. 1990, 755. (b) Rao, P. R. H. P.; Matsukata, M. Chem. Commun. 1996, 1441. (c) Zhang, L.; Gavalas, G. R. Chem. Commun. 1999, 97. (9) (a) Bandyopadhyay, M.; Bandyopadhyay, R.; Kubota, Y.; Sugi, Y. Chem. Lett. 2000, 1024. (b) Bandyopadhyay, R.; Bandyopadhyay, M.; Kubota, Y.; Sugi, Y. J. Porous Mater. 2002, 9, 83. (c) Saha, S. K.; Waghmode, S. B.; Maekawa, H.; Komura, K.; Kubota, Y.; Sugi,Y.; Oumi,Y.; Sano, T Microporous Mesoporous Mater. 2005, 81, 289. (10) (a) Matsukata, M.; Osaki, T.; Ogura, M.; Kikuchi, E. Microporous Mesoporous Mater. 2002, 56, 1. (b) Matsukata, M.; Ogura, M.; Osaki, T.; Rao, P. R. H. P.; Nomura, M.; Kikuchi, E. Top. Catal. 1999, 9, 77. (c) Arnold, A.; Steuernagel, S.; Hunger, M.; Weitkamp, J. Microporous Mesoporous Mater. 2003, 62, 97. (d) Weikamp, J.; Hunger, M. Stud Surf. Sci. Catal. 2005, 155, 1 and references therein. (e) Saha, S. K.; Waghmode, S. B.; Maekawa, H.; Komura, K.; Kubota, Y.; Sugi, Y.; Oumi, Y.; Sano, T. Microporous Mesoporous Mater. 2005, 81, 289. (f) Chen, B.; Huang, Y. J. Am. Chem. Soc. 2006, 128, 6437. (11) Wilson, S. T.; Lok, B. M.; Flanigen, E. M. U.S. Patent 4,310,440, 1982. (12) (a) Bennett, J. M.; Richardson, J. W., Jr.; Pluth, J. J.; Smith, J. V. Zeolites 1987, 7, 160. (b) Richardson, J. W., Jr.; Pluth, J. J.; Smith, J. V. Acta Crystallogr. 1988, B. 44, 367. (13) (a) O’Connor, C. T.; Van, Steen, E.; Dry, M. E. Stud. Surf. Sci. Catal. 1996, 102, 323. (b) Prakash, A. M.; Chilukuri, S. V. V.; Bagwe, R. P.; Ashtekar, S.; Chakrabarty, D. K. Microporous Mater. 1996, 6, 89. (14) (a) Rabo, J. A.; Pellet, R. J.; Coughlin, P. K.; Shamshoum, W. S. Stud. Surf. Sci. Catal. 1989, 46, 1 and references therein. (b) Hung, J. K.; Colle, T. H.; Feng, X.; Mohr, G. D.; Rao, K. K. U.S. Pat. Appl. Publ. 2005, 12 pp. (c) Hammerman, J. I.; Flanigen, E. M.; Gajda, G. J.; Holmgren, J. S.; Lesch, D. A.; Patton, R. L.; Reynolds, T. M.; Roeseler, C. M. Eur. Pat. Appl. 1996, 9 pp. (15) (a) Miller, S. J. PCT Int. Appl. 1989, 32 pp. (b) Miller, S. J. PCT Int. Appl. 1990, 38 pp. (16) (a) Tapp, N. J.; Milestone, N. B.; Bibby, D. M. Zeolites 1988, 8, 183. (b) Ren, X.; Komarneni, S.; Roy, D. M. Zeolites 1991, 11, 142. (17) Huang, Y.; Richer, R.; Kirby, C. W. J. Phys. Chem. B. 2003, 107, 1326. (18) (a) Li, N.; Cao, G.; Xing, S. Stud. Surf. Sci. Catal. 2004, 154A, 1001. (b) Han, B.; Shin, C. H.; Cox, P. A.; Hong, S. B. J. Phys. Chem. B 2006, 110, 8188. (c) Prasad, S.; Vetrivel, R. J. Phys. Chem. 1992, 96, 3092. (19) Zhu, G.; Qiu, S.; Gao, F.; Wu, G.; Wang, R.; Li, B.; Fang, Q.; Li, Ya.; Gao, B.; Xu, X.; Terasaki, O. Microporous Mesoporous Mater. 2001, 50, 129. (20) Hartmann, S. R.; Hahn, E. L. Phys. ReV. 1962, 128, 2042. (21) (a) Grey, C. P.; Veeman, W. S.; Vega, A. J. J. Chem. Phys. 1993, 98, 7711. (b) Grey, C. P.; Vega, A. J. J. Am. Chem. Soc. 1995, 117, 8232. (22) (a) Dutta, P. K.; Twu, J. J. Phys. Chem. 1991, 95, 2498. (b) Holmes, A. J.; Kirkby, S. J.; Ozin, G. A.; Young, D. J. Phys. Chem. 1994, 98, 4677. (c) Dutta, P. K.; Shieh, D. C.; Puri, M. Zeolites 1988, 8, 306. (23) (a) Sayari, A.; Moudrakovski, I.; Reddy, J. S.; Ratcliffe, C. I.; Ripmeester, J. A.; Preston, K. F. Chem. Mater. 1996, 8, 2080. (b) Khimyak, Y. Z.; Kinowski, J. J. Chem. Soc., Faraday Trans. 1998, 94, 2241. (24) Barrie, P. J.; Smith, M. E.; Klinowski, J. Chem. Phys. Lett. 1991, 180, 6. (25) Blackwell, C. S.; Patton, R. L. J. Phys. Chem. 1988, 92, 3965. (26) Huang, Y.; Yan, Z. J. Am. Chem. Soc. 2005, 127, 2731. (27) Mehring, M. High-Resolution NMR Spectroscopy in Solids; SpringerVerlag: New York, 1976. (28) (a) Christiansen, S. C.; Zhao, D.; Janicke, M. T.; Landry, C. C.; Stucky, G. D.; Chmelka, B. F. J. Am. Chem. Soc. 2001, 123, 4519. (b) Gabrielles, W.; Gaur, H. A.; Feyen, F. C.; Veeman, W. S. Macromolecules 1994, 27, 5811.