Molecular Conformations of Protonated Dipropylamine in AlPO4-11

In particular, the presence of two different types of conformations of protonated dipropylamine in the circular 12-ring channels of AlPO4-31 has been ...
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J. Phys. Chem. B 2006, 110, 8188-8193

ARTICLES Molecular Conformations of Protonated Dipropylamine in AlPO4-11, AlPO4-31, SAPO-34, and AlPO4-41 Molecular Sieves Bada Han,† Chae-Ho Shin,‡ Paul A. Cox,§ and Suk Bong Hong*,† DiVision of Applied Chemistry and Biotechnology, Hanbat National UniVersity, Taejon 305-791, Korea, Department of Chemical Engineering, Chungbuk National UniVersity, Chungbuk 361-763, Korea, and School of Pharmacy and Biomedical Science, UniVersity of Portsmouth, Portsmouth PO1 2DT, United Kingdom ReceiVed: January 20, 2006; In Final Form: March 3, 2006

The host-guest interactions in AlPO4-11, AlPO4-31, SAPO-34, and AlPO4-41 molecular sieves prepared using the same organic structure-directing agent, i.e., dipropylamine, are investigated by a combination of Raman, 13C and 1H MAS NMR, and computer modeling studies. It was found that the organic molecules trapped within the pores of these four AlPO4-based materials exist as their protonated form and adopt distinct conformations in order to fit well with the pore structure of each host. In particular, the presence of two different types of conformations of protonated dipropylamine in the circular 12-ring channels of AlPO4-31 has been ascertained.

Introduction It was in the early 1980s when Union Carbide scientists announced the synthesis of a class of aluminophosphate (AlPO4) molecular sieves by using a wide range of organic amines and quaternary ammonium cations, commonly referred to as structuredirecting agents (SDAs).1 Since then, more than two dozen structure types of microporous AlPO4 materials, the majority of which have entirely novel framework topologies, have been reported.1-3 In addition, hundreds of compositions of AlPO4based molecular sieves such as silicoaluminophosphates (SAPOs) and metal-substituted aluminophosphates (MeAPOs), incorporating 1 or more of 13 elements other than Al and P from the periodic table,2-4 have been prepared. As a direct consequence, both the structural and compositional regimes of crystalline microporous materials have been greatly expanded over the past two decades. Elucidating the nature and extent of interactions between the occluded organic SDA and the framework of the crystallized molecular sieve is of fundamental importance in understanding the mechanism by which such an organic species affects the formation of a particular structure type of zeolites and related materials. Unlike the case of silicate-based frameworks,5-7 however, little attention has been devoted to the understanding of the host-guest interactions occurring within as-made AlPO4based molecular sieves.8 In particular, even less attention has been directed toward the investigation of the molecular conformations of organic SDAs in these microporous materials.9 Dipropylamine is one of the simplest organic amines and has been shown to direct the synthesis of at least 10 different * To whom correspondence should be addressed. Tel: +82-42-821-1549. Fax: +82-42-821-1593. E-mail: [email protected]. † Hanbat National University. ‡ Chungbuk National University. § University of Portsmouth.

structure types of microporous AlPO4-based materials, depending on the oxide composition of synthesis mixtures.4,10 Here we present the Raman and 13C and 1H MAS NMR spectra of as-made AlPO4-11 (AEL), AlPO4-31 (ATO), SAPO-34 (CHA), and AlPO4-41 (AFO) molecular sieves, all of which were prepared using the same organic SDA, i.e., dipropylamine. The results obtained are correlated with the optimum conformations of organic guest molecules within these four hosts that are determined via computer modeling studies. Experimental Section All syntheses of AlPO4-11, AlPO4-31, SAPO-34, and AlPO441 molecular sieves using dipropylamine (DPA) as an organic SDA were performed in Teflon-lined 23-mL Parr autoclaves. The detailed preparation conditions for these four samples are given in Table 1. Powder X-ray diffraction (XRD) patterns were measured on a Rigaku Miniflex diffractometer with Cu KR radiation. Elemental analysis for Al, P, and Si was carried out by a JarrellAsh Polyscan 61E inductively coupled plasma (ICP) spectrometer in combination with a Perkin-Elmer 5000 atomic absorption spectrophotometer. The C, H, and N contents of the samples were analyzed by using a Carlo Erba 1106 elemental organic analyzer. Thermogravimetric analyses (TGA) were performed in air on a TA Instruments SDT 2960 thermal analyzer, where the endothermic weight loss related to the desorption of water was further confirmed by differential thermal analyses (DTA) using the same analyzer. Crystal morphology and size were determined by a JEOL JSM-6300 scanning electron microscope (SEM). The Raman spectra were measured on a Bruker RFA 106/S FT-Raman spectrometer equipped with an Nd:YAG laser operating at an excitation wavelength of 1064 nm. The samples were exposed to a laser power of 200-400 mW at the spectral

10.1021/jp0604138 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/04/2006

Conformations of Protonated Dipropylamine

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TABLE 1: Synthesis Conditions and Chemical Composition Data for AlPO4-Based Molecular Sieves Prepared in This Studya pH sample

gel composnb

initial

final

unit cell composnc,d

AlPO4-11 AlPO4-31 SAPO-34 AlPO4-41

1.0DPA‚1.0Al2O3‚1.0P2O5‚40H2O 4.0DPA‚1.0Al2O3‚1.0P2O5‚40H2O 4.0DPA‚1.0Al2O3‚1.0P2O5‚0.3SiO2‚40H2O 2.0DPA‚1.0Al2O3‚1.0P2O5‚40H2O

3.5 8.8 8.9 8.0

7.1 11.0 8.0 10.8

3.1(DPA‚H+OH-)‚Al20P20O80‚3.4H2O 2.2(DPA‚H+OH-)‚Al18P18O72‚0.7H2O 3.9(DPA‚H+)‚1.1(DPA‚H+OH-)‚Al18P14.1Si3.9O72‚6.9H2O 1.4(DPA‚H+OH-)‚Al10P10O40‚0.8H2O

a The crystallization temperature and time are 473 K and 5 days, respectively. b The reagents used included dipropylamine (99%, Aldrich), aluminum isopropoxide (98+%, Aldrich), phosphoric acid (85%, Merck), colloidal silica (Ludox AS-40, DuPont), and deionized water. In the case of AlPO4-31 and SAPO-34, pseudoboehmite (Catapal B, Vista) instead of aluminum isopropoxide was used as the Al source. c Determined by elemental analysis except the water content. DPA‚H+ is the protonated DPA, and OH- has been introduced to make each sample electrically neutral. d The water content was calculated from the endothermic weight loss appearing at temperatures below 473 K in the DTA/TGA curves.

resolution of 4 cm-1. Typically, 1024 scans were accumulated for obtaining the Raman spectra. The solid-state 13C NMR spectra were measured on a Bruker DSX 300 spectrometer at a 13C frequency of 75.467 MHz with a π/2 rad pulse length of 3.0 µs. For CP and 1H-decoupled spectra, recycle delays were 3.0 and 6-12 s, respectively. Typically, 5000 pulse transients were accumulated. For MAS 13C NMR experiments, the samples were spun at 4.5 kHz. The deconvolution of the 13C MAS NMR spectra obtained was performed using the PeakFit curve-fitting program. The 1H MAS NMR spectra at a spinning rate of 13.0 kHz were recorded on the same spectrometer at a proton Larmor frequency of 300.110 MHz with a π/5 rad pulse length of 1.2 µs, a recycle delay of 3 s, and an acquisition of 32 pulse transients. Both 1H and 13C chemical shifts are referenced to TMS. Computer simulation of the energy-minimized locations and conformations of organic SDAs occluded within the pores of AlPO4-based materials studied here was carried out using a combined Monte Carlo-simulated annealing (MC-SA) approach.5c Calculations were performed using the CVFF force field as implemented in the program Discover,11 as described in our recent work.7f,12 The host molecular sieve lattice was held fixed during the simulation, while the organic SDA occluded was free to move. The unit cell parameters and atomic coordinates for molecular sieves under computer simulation were taken from the original references listed in the International Zeolite Association (IZA) tabulations,3 and their frameworks were modeled as pure AlPO4 structures, with inclusion of charge compensating OH- groups used in the SAPO-34 structure. During the initial Monte Carlo stage of the calculation, the configuration of the organic SDA was accepted only if the calculated energy was below 4200 kJ‚mol-1, otherwise the process was started again. Once an initial configuration had been accepted, successive molecular dynamics simulations were carried out at 1000, 750, 500, 300, 200, 100, and 50 K for 1000 time steps of 1 fs at each temperature, prior to a final energy minimization stage. Results and Discussion The powder XRD patterns of AlPO4-11, AlPO4-31, SAPO34, and AlPO4-41 prepared using DPA as an organic SDA can be found in Supporting Information Figure 1S. Comparison with the XRD patterns in the literature13 reveals that each material is highly crystalline and no reflections other than those from the corresponding structure are observed. SEM photographs show that these four materials are characterized by notably different morphologies from one another (Supporting Information Figure 2S). It should be noted here that the phase selectivity of the DPA-mediated synthesis of AlPO4-based molecular sieves can be notably altered according not only to the organic concentration in synthesis mixtures and the type of Al source employed but also to the introduction of heteroatoms such as

Figure 1. Raman spectra in the 200-3400 cm-1 region of (a) DPA, (b) protonated DPA (DPA‚H+) in aqueous solution, and as-made (c) AlPO4-11, (d) AlPO4-31, (e) SAPO-34, and (f) AlPO4-41. The N-H stretching mode of neutral DPA is marked with an arrow, and the structural Raman bands of each molecular sieve are indicated by asterisks.

Si into the AlPO4 framework (Table 1), which is consistent with the trend previously reported.4,10 This suggests that the structuredirecting ability of DPA itself is not strong enough to govern the phase selectivity of the crystallization. Thus, DPA appears to mainly serve as a beneficial but structurally nonspecific porefilling species. Since the initial pH of synthesis mixtures depends on the amount of DPA added, however, its role as a pH-stabilizer cannot be completely ruled out. The pore-filling and pHstabilizing roles of simple aliphatic amines are well-known in the synthesis of various zeolites.6 Figure 1 shows the Raman spectra in the 200-3400 cm-1 region of as-made AlPO4-11, AlPO4-31, SAPO-34, and AlPO441, together with those of DPA and its protonated form (i.e., DPA‚H+) in aqueous solution, prepared by adding dilute HCl to DPA. In neutral DPA one broad band, assignable to the N-H stretching mode,14 is clearly observed around 3330 cm-1. As seen in Figure 1, however, this band is completely missing in the Raman spectra of protonated DPA and any of molecular sieves prepared here. Furthermore, much weaker Bohlmann bands15 in the 2700-2830 cm-1 region, whose intensities depend mainly on the presence of a lone pair on the nitrogen atom in DPA, are observed for all four AlPO4-based molecular sieves, as well as for DPA‚H+. These results strongly suggest that most, if not all, of the organic SDA molecules trapped within the pores of each material are present as the protonated form, since there is no longer any lone pair on the nitrogen atom. The Raman signature of occluded organic species also includes notable differences in the band number, position, and/ or intensity of the C-H stretching modes at 2850-3050 cm-1, the CH3 deformation mode at 1440-1470 cm-1, and the symmetric C-N stretching mode at 840-900 cm-1. Therefore,

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Figure 2. 13C NMR spectra of (a) DPA, (b) DPA‚H+, and as-made (c) AlPO4-11, (d) AlPO4-31, (e) SAPO-34, and (f) AlPO4-41 prepared in the presence of DPA. The spectra of DPA and DPA‚H+ are 13C NMR in CDCl3 solution, while those of AlPO4-based molecular sieves are 13C MAS NMR of the solids.

it is most likely that host-guest interactions occurring within all molecular sieves studied here are significantly different from one another, which is mainly due to the difference in geometrical restrictions imposed by their particular framework topologies. Unlike the other three AlPO4 molecular sieves, on the other hand, SAPO-34 has a structure that comprises 20-hedral [4126286] cages (approximately 8.5 Å in length and 6.5 Å in diameter), separated by 8-ring windows (3.8 × 3.8 Å), inside which all organic molecules should be localized. The chemical composition data in Table 1 indicate that each of the 20-hedral cages of this CHA-type material, on an average, contains 1.7 DPA molecules in the protonated form, as well as 2.3 water molecules. At least in the case of as-made SAPO-34, therefore, we cannot rule out the possibility that the observed spectral difference can also be a result of guest-guest (DPA-DPA and/ or DPA-water) interactions. Figure 2 shows the 13C MAS NMR spectra of as-made AlPO4-11, AlPO4-31, SAPO-34, and AlPO4-41. The liquid 13C NMR spectra of DPA and its protonated form in CDCl3 solution are also given in Figure 2, and the spectral deconvolution results are summarized in Table 2. It is clear that DPA remains intact upon its occlusion into the pores of each material and has not decomposed under the crystallization conditions, in agreement with the Raman data given above. However, the observed differences in the line number, line width, and/or chemical shift of its 13C NMR resonances, which can be further supported by the 1H-13C CP MAS NMR spectra (Supporting Information Figure 3S), clearly show that the occluded organic molecule experiences the chemical environment that is distinctly different in each of these four materials. As seen in Figure 2, for example, the 13C MAS NMR spectrum of as-made AlPO4-11 is characterized by two methyl carbon resonances at 11.0 and 9.5 ppm with an intensity ratio of approximately 2:1. We note that the chemical shifts of these low- and high-field 13C NMR resonances are essentially the same as those (10.8 and 9.7 ppm) of the methyl carbon resonance observed for neutral DPA and its protonated form, respectively. However, this cannot be rational-

Han et al. ized by simply suggesting the existence of a mixture of DPA and DPA‚H+ residing in the same environment, since as-made AlPO4-11 gives no noticeable 13C NMR resonance around 21 ppm where the second methylene carbon resonance of neutral DPA should be otherwise observed. Although 13C MAS NMR spectroscopy does not allow us to unambiguously confirm whether DPA is protonated or not, we speculate that the existence of two methyl carbon resonances in the spectrum of as-made AlPO4-11 may be a characteristic of the conformationadopted by the occluded organic species (i.e., DPA‚H+ as confirmed by Raman analysis), regardless of their exact assignments. Given the relatively small and highly elliptical nature of 10-ring channels (4.0 × 6.5 Å) in this medium-pore material, in addition, it is possible to speculate that more than one degree of geometric constraints and van der Waals interactions with the molecular sieve framework could be imposed on the methyl groups of the guest molecule, leading to a split into two unequal signals of the methyl carbon resonance. Further evidence to support this speculation will be given below. The most interesting 13C MAS NMR spectrum among the spectra in Figure 2 was obtained from as-made AlPO4-31, a one-dimensional pore material consisting of circular 12-ring channels (5.4 × 5.4 Å). Unlike in the spectrum of as-made AlPO4-11, four 13C NMR lines at 12.4, 11.8, 9.8, and 9.3 ppm, all of which must be assigned to the resonances of methyl carbons of the organic species in this large-pore material, are distinguished. Also, there are signs of two components in the 47-51 and 17-22 ppm regions which correspond to the first and second methylene carbons of DPA or DPA‚H+, respectively. These results clearly show that the occluded organic molecules adopt at least two different conformations to match spatially with the circular AlPO4-31 channels. To our knowledge, asmade AlPO4-31 is the first example where the existence of more than one particular type of conformation of the organic SDA within the void spaces of a given molecular sieve is ascertained. Signs of two unequal components for all 13C NMR resonances for occluded organic SDAs are also observed from the spectra of as-made SAPO-34 and AlPO4-41, suggesting that the geometric constraints and van der Waals interactions of the two halves of the occluded organic molecule with their frameworks are no longer equivalent to each other. The curve deconvolution results in Table 2 reveal that there are small but nonnegligible differences in the chemical shift, line width, and relative intensity of the observed 13C NMR resonances from as-made SAPO-34 and AlPO4-41, which is also the case when comparing the 13C MAS NMR data for as-made AlPO4-11 or AlPO4-31. Therefore, it is clear that the organic guest molecules in all molecular sieves prepared here adopt conformations different from one another, although the 13C MAS NMR spectra in Figure 2 cannot give information on what conformations are possible inside the pores of each of these materials. Figure 3 shows the 1H MAS NMR spectra of the four asmade AlPO4-based materials employed here, as well as of the 1H NMR spectra of DPA and DPA‚H+ in CDCl solution. 3 Because of both the small range of 1H chemical shifts and the heavy line broadening caused by the strong dipolar interactions between the protons of structurally distinct 1H resonances, it would be difficult to unequivocally assign all 1H MAS NMR lines in Figure 3. However, comparison with the liquid 1H NMR data suggests that the most prominent resonance appearing around 0.9 ppm can be attributed to the methyl protons of occluded organic species, while the resonance around 2.8 ppm is mainly due to the first methylene protons. In addition, a signal that varies in the chemical shift between 4.0 and 6.5 ppm,

Conformations of Protonated Dipropylamine

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TABLE 2: Chemical Shifts, Line Widths, and Relative Intensities of the 13C MAS NMR Resonances of As-Made AlPO4-Based Molecular Sieves Prepared with Dipropylaminea 13C

sample

C1

chem shift,b,c ppm from TMS

C2

C3

DPAd 50.0 21.4 DPA‚H+ d 49.1 19.0 AlPO4-11 50.0 (220) 19.2 (160) AlPO4-31 49.2 (100) [0.55] 48.2 (80) [0.45] 19.1 (65) [0.76] 18.4 (40) [0.24] 12.4 (35) [0.17] SAPO-34 49.4 (110) [0.62] 48.7 (130) [0.38] 21.0 (150) [0.29] 19.2 (70) [0.71] AlPO4-41 49.5 (120) [0.67] 48.8 (180) [0.33] 19.3 (100) [0.55] 18.2 (150) [0.45]

10.8 9.7 11.0 (110) [0.68] 9.5 (80) [0.32] 11.8 (40) [0.30] 9.8 (40) [0.26] 9.3 (40) [0.27] 11.1 (80) [0.86] 9.6 (120) [0.14] 11.1 (60) [0.46] 10.2 (100) [0.54]

a The assignments of each resonance of DPA and DPA‚H+ are the same as those given in Figure 2. b The values given in parentheses are full widths at half-maximum (fwhm’s) of the deconvoluted components in Hz. c The relative intensities of deconvoluted components are referenced relative to the total intensity of the CH2 or CH3 13C NMR lines of the organic molecules occluded in each molecular sieve and are given in brackets. d Liquid 13C NMR data recorded in CDCl3 solution.

Figure 3. 1H NMR spectra of (a) DPA, (b) DPA‚H+, and as-made (c) AlPO4-11, (d) AlPO4-31, (e) SAPO-34, and (f) AlPO4-41 prepared with DPA. The spectra of DPA and DPA‚H+ are 1H NMR in CDCl3 solution, while those of AlPO4-based materials are 1H MAS NMR of the solids.

depending on the sample, is assignable to the water protons.16 It is clear from Figure 3 that the mobilities of the organic molecules in these four materials are significantly different from one another. Unlike in the spectra of the other three samples, for example, the resonance around 2.8 ppm is barely detectable in the 1H MAS NMR spectrum of as-made AlPO4-11. It thus appears that the spatial constraints imposed on the first methylene protons of the organic SDA upon encapsulation into the highly elliptical AlPO4-11 channels are much severer than those on the methyl protons, which may be sufficient to make these methylene protons relatively static. This is consistent with the fact that the full width at half-maximum of the first methylene carbon resonance observed for as-made AlPO4-11 is considerably larger than that of any of the other carbon resonances from the corresponding sample (Table 2). As seen in Figure 3, in contrast, the signal around 2.8 ppm in the 1H

MAS NMR spectrum of as-made AlPO4-31 is well resolved. This indicates that the mobility of the first methylene protons of organic molecules is higher in AlPO4-31 than in AlPO4-11, which must be due to the circular shape of 12-ring channels in the former material and thus to their larger value (5.4 vs 4.0 Å) of the shortest pore dimension. Figure 3 also shows that the main intensities between 0.5 and 3.0 ppm due to the resonances of the protons on the occluded organic molecules are relatively sharp for as-made SAPO-34 and AlPO4-41, when compared to those from as-made AlPO4-11 and AlPO4-31. Apparently, the 20-hedral cages in SAPO-34 are large enough to make the guest molecules much more mobile than those in the latter two materials with the onedimensional pore system and hence to permit a better spectral resolution of occluded organic species. In fact, the 1H MAS NMR spectrum of as-made SAPO-34 exhibits an additional signal at 7.4 ppm, as well as a line at 4.2 ppm most likely due to water, which is not detectable in the spectra of the other three materials. When compared with the liquid 1H NMR spectrum of protonated DPA, this signal can be attributed to the protons bonded to the nitrogen atom of occluded organic molecules. Like AlPO4-11, on the other hand, AlPO4-41 is also a onedimensional medium-pore material consisting of elliptical 10ring channels (4.3 × 7.0 Å). Although the ellipticities17 (0.788 vs 0.789) of 10-ring channels in these two materials are essentially identical with each other, the 10-ring channels in the latter are slightly larger than the analogous channels (4.0 × 6.5 Å) in the former. The 1H MAS NMR spectra in Figure 3 suggest that this discrepancy is large enough to cause a significant increase in the mobility of occluded organic SDA. To gain additional information regarding the host-guest interactions occurring within molecular sieves prepared here, we have attempted to optimize the locations and conformations of the organic guest molecules via energy minimization calculations. The energy-minimized locations for the DPA molecules occluded as their protonated form within the structures of AlPO4-11, AlPO4-31, SAPO-34, and AlPO4-41 are shown in Figure 4. It can be seen that the conformations of the molecules are different in each of the four host frameworks. The nonbonded energies calculated for the protonated DPA molecules in these AlPO4-based materials are given in Table 3. The nonbonded energy/mol of SDA was found to be highest for the AlPO4-41 structure (-142 kJ mol-1 of SDA), suggesting that the organic SDA is most strongly bound within this framework. As seen in Figure 4, the protonated DPA molecule in AlPO441 adopts a distinct ‘U-shaped’ conformation, maximizing the van der Waals interaction with the host framework. A quite similar conformation can also be observed for the SDA molecule in AlPO4-11, although its propyl arms are oriented in a manner slightly different from that found in the molecule in AlPO4-41,

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Figure 4. Energy-minimized conformations of the DPA‚H+ ion within the frameworks of (a) AlPO4-11, (b) AlPO4-31, (c) SAPO-34, and (d) AlPO4-41.

TABLE 3: Nonbonded Energies Calculated for the Dipropylammonium Ion within the Pores of Various AlPO4-Based Molecular Sieves host

nonbonded energy, kJ mol-1 of SDA

nonbonded energy, kJ mol-1 of T atoms

AlPO4-11 AlPO4-31 SAPO-34 AlPO4-41

-140.7 -135.2 -94.3 -141.6

-3.52 -3.76 -2.62 -7.08

experiencing a less symmetrical environment. As a result, the interaction energy (-141 kJ mol-1 of SDA) obtained within the AlPO4-11 structure was found to be very similar to that obtained for AlPO4-41. Another interesting result obtained from Figure 4 is that the conformations of the SDA molecules in AlPO4-31 are much more linear than those of the molecules in AlPO4-11 or AlPO441. Furthermore, two adjacent molecules adopt conformations slightly different from each other, suggesting why four methyl carbon resonances are distinguished from the 13C MAS NMR spectrum of as-made AlPO4-31 (Figure 2). This is typical of the results obtained from the MC-SA simulations, even when annealing times were increased during the simulation. We should note here that the calculated interaction energies for the two most favorable arrangements of the SDA within AlPO4-31 are essentially identical with each other (-135 kJ mol-1 of SDA), while being slightly low compared to the value obtained for AlPO4-11 or AlPO4-41. This again supports the conclusion from the experimental work that the SDA is more flexible in the AlPO4-31 framework and can adopt more than one conformation. In contrast, a significantly lower interaction energy (-94 kJ mol-1 of SDA) was calculated for the molecule in SAPO34 (Table 3). As noted earlier, the 20-hedral cage within this SAPO material is significantly larger than the SDA occluded and therefore the van der Waals interactions between the SDA and the host framework are significantly reduced. However, the optimized conformation for a single SDA within the SAPO-34 structure in Figure 4 reveals that the molecule needs to “bend” its conformation to be accommodated within the relatively short SAPO-34 cage. Computer simulation also shows that the SAPO-

34 cages can accommodate further SDA molecules that bridge between the cages, leading to a notable increase in stabilization energy. For example, the simulation performed with 1.5 molecules/cage gave stabilization energy of -129 kJ mol-1 of SDA, which is no longer much lower than the values obtained within the other three frameworks. Considering that hydrothermal synthesis of zeolites and related microporous materials is a self-assembly process involving a number of chemical equilibria and condensation steps, on the other hand, it is not surprising that there are many examples where organic SDAs with a high degree of flexibility, like DPA, can produce more than one molecular sieve structure, depending on the oxide composition of synthesis mixtures and/ or the crystallization conditions employed.4 As noted earlier, the structure-directing effect exerted by DPA is not strong enough to govern the synthesis of the four AlPO4-based materials studied here, because the phase selectivity of the crystallization is sensitive to the synthesis gel composition, especially to the DPA concentration, and hence to the initial pH of synthesis mixtures (Table 1). However, the overall results of our study reveal that the conformations of DPA molecules trapped as their proton form within the pores of these AlPO4based molecular sieves are distinctly different from one another and are closely related to the structural aspects of each material. This implies that the higher energy conformer could also serve as an SDA in its own right, especially when the energetic differences between some conformations of DPA and the barriers to interconversions at AlPO4 molecular sieve synthesis conditions are not so large. Therefore, we speculate that the flexible, linear DPA molecule could happen to be surrounded in a single or a few closely related conformations, probably depending on its concentration in the synthesis mixture, and then play an important kinetic role in determining the pore structure of the resulting AlPO4 lattice. To prove whether this speculation is correct, molecular sieve syntheses by varying the concentration of other flexible organic SDAs such as diethylamine or dibutylamine in the AlPO4 system are currently underway in our laboratory.

Conformations of Protonated Dipropylamine Conclusions AlPO4-11, AlPO4-31, SAPO-34, and AlPO4-41 molecular sieves have prepared in the presence of dipropylamine as the SDA, and the host-guest interactions occurring within these four materials have been investigated by Raman, 13C and 1H MAS NMR, and lattice energy minimization studies. The overall results of this study reveal that the organic SDA molecules occluded are present as the protonated form and adopt distinct molecular conformations, matching well with the pore structure of each material. Interestingly, the SDA molecule in AlPO4-31 was found to adopt two different types of conformation. Acknowledgment. Support for this work was provided by the Korea Science and Engineering Foundation (Grant R012006-000-10192-0) and the Carbon Dioxide Reduction and Sequestration Research Center (Grant CB2-101-1-0-1), one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of Korea. Supporting Information Available: Figures showing powder XRD patterns, SEM micrographs, and 13C NMR spectra and additional information as noted in the text (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. (b) Wilson, S. T.; Lok, B. M.; Flanigen, E. M. U.S. Patent 4,310,440, 1982. (2) (a) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, T. R.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1984, 106, 6092. (b) Flanigen, E. M.; Patton, R. L.; Wilson, S. T. Stud. Surf. Sci. Catal. 1988, 37, 13 and references therein. (3) International Zeolite Association, Structure Commission, http:// www.iza-structure.org. (4) Szostak, R. Handbook of Molecular SieVes; Van Nostrand Reinhold: New York, 1992.

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