Characterization of the amorphous phases formed during the synthesis of microporous material AlPO4-5

Microporous and Mesoporous Materials 98 (2007) 21–28 www.elsevier.com/locate/micromeso

Characterization of the amorphous phases formed during the synthesis of microporous material AlPO4-5 James G. Longstaffe, Banghao Chen, Yining Huang

*

Department of Chemistry, The University of Western Ontorio, London, Ont., Canada N6A 5B7 Received 8 February 2006; received in revised form 5 August 2006; accepted 9 August 2006 Available online 27 September 2006

Abstract We have used several solid-state NMR techniques in conjunction with powder X-ray diffraction (XRD) and Raman spectroscopy to characterize the structure of the intermediate amorphous phases of AlPO4-5 molecular sieve synthesis. The evolution of the gel phases as a function of crystallization time was followed by 31P and 27Al magic-angle spinning (MAS) NMR to obtain information on the local environments of Al and P atoms and by powder XRD to detect the long-range ordering of the gel samples. We have utilized 27Al/31P double-resonance techniques such as heteronuclear correlation spectroscopy (HETCOR) to select the 31P–O–27Al bonding connectivities and transfer of populations by double-resonance (TRAPDOR) to obtain information regarding the degree of condensation for P sites in the amorphous phases. The 1H ! 31P cross-polarization (CP) was used to identify the P sites with hydroxyl groups attached. Raman spectroscopy was also employed to follow the evaluation of the pore system. The 13C CP MAS spectra provide information regarding the chemical environment of template molecule in the gel. The work shows that a combination of the above-mentioned techniques can provide more detailed information on the structure of amorphous phases formed during hydrothermal synthesis.  2006 Elsevier Inc. All rights reserved. Keywords: Aluminophosphates; AlPO4-5; Molecular sieve synthesis; Solid-state NMR; Reaction intermediates

1. Introduction Microporous materials (often referred to as molecular sieves) are a class of inorganic solids with regular pores ˚ . Zeolites which and cavities in the size range of 5–20 A are aluminosilicates represent the most well known family of such materials [1]. They are extensively used in industry as ion-exchangers, sorbents and catalysts. Another important type of molecular sieve is aluminophosphate (AlPO4)-based materials [2]. Some of these materials have framework topologies of known zeolites, but many have novel structures. These AlPO4-based materials exhibit distinct molecular sieving characteristics and can be made catalytically active by introducing other elements (such as Si and metal centers) into the frameworks. *

Corresponding author. Tel.: +1 519 661 2111x86384; fax: +1 519 661 3022. E-mail address: [email protected] (Y. Huang). 1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.08.009

Because of the practical applications, the synthesis of new microporous materials is a major research area in materials science (see Refs. [3–8] for some reviews on molecular sieve synthesis). However, despite much data having been compiled on the synthesis conditions, understanding of the fundamental processes occurring during crystallization on a molecular level is still incomplete. Similar to zeolites, AlPO4’s are usually prepared by hydrothermal synthesis. The processes involve the formation of intermediate gels in the early stages of the reaction. The gel phases eventually transform into crystalline molecular sieves. However, the mechanisms by which the intermediate gel phases transform to the crystalline molecular sieves are not well understood. This problem arises, in part, from the fact that the structural properties of the intermediate phases are usually poorly characterized due to their amorphous nature. We have recently shown the effectiveness of solid-state NMR, in combination with Raman spectroscopy and powder X-ray diffraction, as a means to elucidate

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the structures of the intermediate gel phases formed during the hydrothermal synthesis of AlPO4 materials [9,10]. In this study we have examined the intermediate gel phases formed during the synthesis of molecular sieve AlPO4-5. Powder X-ray diffraction was utilized to follow the longrange ordering of the gel phases. 27Al and 31P magic-angle spinning (MAS) NMR was used to probe the coordination environments around Al and P. We have carried out 27 Al ! 31P heteronuclear correlation spectroscopy (HETCOR) to probe the connectivity between AlO4 and PO4 units, 31P{27Al} transfer of population double-resonance (TRAPDOR) to study the degree of condensation of phosphorus, and 1H ! 31P cross-polarization (CP) to explore hydroxyl groups attached to phosphorus. 13C CP MAS spectra were acquired to obtain the information on the template molecule occulted in the gel. Raman spectroscopy was also employed to examine the development of the porosity. AlPO4-5 has the AFI framework topology [11], which is characterized by one-dimensional channel with a pore diameter of 0.73 nm. In addition to the typical use of AlPO4 materials as an adsorption medium [12] several novel uses for AlPO4-5 have been developed where the one-dimensional channel structure is exploited as the host in nanocomposite materials [13–15]. Understandably, much attention has been directed towards the synthesis of AlPO4-5. Studies have looked at the crystallography [16], the affects of additional gel components [17,18], template interactions [19], gel pH [20], H2O molar content [20] and the isomorphous substitution of transition metals [18] among others. The focus of many studies has also been to optimize the conditions controlling the final crystal morphology [21–23]. 2. Experimental section AlPO4-5 was prepared using triethylamine (TEA) as the template [21]. Typically, 7.8 g Al(OH)3 (50% Al2O3, Aldrich) was stirred with 22.3 g deionized water for 30 min. 8.8 g of phosphoric acid (85% H3PO4) was added drop-wise and the mixture was stirred for 90 min. 3.87 g of TEA (Aldrich) was added drop-wise and the gel was aged with stirring for 120 min. The composition of this mixture was 0.1Al(OH)3:0.08(H3PO4):0.04TEA:1.24H2O. The gels were divided into several Teflon bottles, placed inside stainless steel autoclaves, and heated in an oven at 453 K. A sample of the initial gel before heating was airdried for each batch and referred to as initial gel. Reactions were quenched at different time intervals by cold water. The solid materials were dried in air at room temperature. All the solid gel samples were kept in tightly sealed glass vials for analysis. Powder X-ray diffraction patterns were obtained using a Rigaku diffractometer with Co Ka radiation with a wave˚ . Raman spectra were recorded on a length of 1.7902 A Bruker RFS 100/S FT-Raman spectrometer equipped with a Nd3+:YAG laser operating at 1064.1 nm and a liquid

nitrogen cooled Ge detector. The laser power was typically 150 mW at the sample and the resolution was 4 cm1. All NMR experiments were performed on a Varian/ Chemagnetics Infinityplus 400 WB spectrometer operating at a magnetic field strength of 9.4 T. At this field strength the 1H, 13C, 31P and 27Al resonance frequencies are 399.9, 100.46, 161.6 and 104.1 MHz, respectively. Chemical shifts were referenced to TMS, 85% H3PO4 and 1 M Al(NO3)3 (aq) for 13C, 31P and 27Al, respectively. All 13C, 31 P and 27Al MAS experiments were carried out using either a Varian/Chemagnetics 5-mm triple-tuned probe spinning at 10 kHz or with a 7.5-mm triple-tuned probe spinning at 6.5 kHz. Single pulse 31P MAS experiments were conducted with a pulse length of 1 ls corresponding to a 90 pulse length of 5 ls. The pulse delay was 90 s and the proton-decoupling power was 75 kHz. For 27Al, the central transition was probed selectively with a short rf pulse of 0.6 ls corresponding to a 90 pulse length of 4 ls. The pulse delay was 0.5 s. For 1H ! 31P cross-polarization experiments, the Hartmann–Hahn matching condition was calibrated using NH4H2PO4. The pulse delay was 10 s. 27Al ! 31P HETCOR experiments [24,25] were run using the 7.5-mm triple-tuned probe. Optimization was carried out using molecular sieve VPI-5. The optimized 27 Al spin-locking field was 25 kHz, corresponding to a central transition pulse length of 10 ls. The contact time was 1 ms and the pulse delay was 200 ms. TRAPDOR experiments [26] were performed using the same 7.5-mm probe spinning at 6.5 kHz ± 2 Hz. The strength of Al rf field was 63 kHz and the pulse delay was 150 s. The 13C CP MAS spectra were recorded by using the 5-mm probe. A contact time of 2 ms was used and the pulse delay was 10 s. All XRD, Raman and NMR experiments were performed at room temperature. For all the gel samples, the concentrations of Al and P in the liquids (from which the solid phases were recovered) were determined by ICP–AES method. From the ICP data the corresponding solid yields were estimated and they are on the range 97.92–99.99% (see Appendix A). 3. Results and discussion Powder XRD patterns of the gel samples were obtained to monitor the evolution of long-range ordering in the intermediate material as a function of crystallization time (Fig. 1). The initial gel before heating shows two extremely broad reflections centered at 2h of 7 and 30, indicating an amorphous nature for the material. After heating for 1.5 h, the broad amorphous peaks shifted to 17 and 32 2h, implying that the structure of the material has changed under hydrothermal treatment. Heating the gel for 2 h resulted in the appearance of sharp diffraction peaks matching the pattern for AlPO4-5. The intensities of the reflections due to AlPO4-5 increase with increasing the heating time at the expense of the amorphous material. The XRD pattern of the sample heated for 5 h contains only the reflections due to AlPO4-5. Previous studies have

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23

36 ppm

5.0 hrs 34 ppm

2.5 hrs

5.0 hrs 2.0 hrs 2 ppm

38 ppm

1.5 hrs -18 ppm

initial gel 2.5 hrs 200

100

0

-100

-200

ppm -30 ppm

2.0 hrs

5.0 hrs 1.5 hrs -19 ppm

initial gel

5

15

25

35

45

-29 ppm

2.5 hrs

55

-16 ppm

2 theta

2.0 hrs

Fig. 1. XRD patterns of selected intermediate gel samples.

-8 ppm

1.5 hrs

proposed that AlPO4-based microporous materials prefer to crystallize from layered intermediates [5,27] and indeed some ordered intermediate phases with layered structure were observed [10,28–30]. The XRD data indicate that under the reaction conditions employed, it is the amorphous material that is converted to crystalline AlPO4-5. To characterize the local environments of P and Al atoms in the amorphous material, 31P and 27Al MAS experiments were carried out. Fig. 2A shows the 27Al MAS spectra of the gel samples. A prominent peak at 38 ppm and two broad signals at 2 and 18 ppm were observed in the initial gel before heating. The 38 ppm resonance can be assigned to tetrahedral aluminum with four phosphorus atoms in its second coordination sphere and the 18 ppm peak is in the region of octahedral aluminum, Al(OP)4(OH2)2, in an AlPO species [31]. The assignment of 2 ppm peak is, however, ambiguous. The chemical shift suggests this resonance can be assigned to either five-coordinated aluminum, Al(OP)4(OH2), in an AlPO species or octahedral aluminum in unreacted Al2O3, both of which are possible under the reaction conditions. After hydro-

initial gel 100

50

0

-50

-100

-150

ppm Fig. 2. (A) samples.

27

Al and (B)

31

P MAS spectra of selected intermediate gel

thermal heating for 1.5 h, the spectrum shows that the 38 ppm peak has diminished and the 18 ppm peak is now the largest. The changes in the Al MAS spectra coincide with the observed changes in the XRD pattern (Fig. 1). It seems that the changes in the coordination environment of Al has taken place upon heating, resulting in a new amorphous phase containing mainly octahedral aluminum. With increasing heating time the intensity of the tetrahedral Al gradually increases and eventually becomes dominant. The spectrum of the 5-h sample corresponds to that for AlPO4-5 in the literature [32].

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Fig. 2B presents the 31P MAS spectra for selected samples. In the initial gel before heating, there is a very broad peak centered at around 16 ppm with a shoulder at 8 ppm. The broadness of the 16 ppm peak is consistent with the amorphous nature of the material. Upon hydrothermal treatment for 1.5 h, the 8 ppm resonance is absent while the 16 ppm resonance remains broad, suggesting the continued presence of an amorphous material. After heating for 2 h a new peak begins to emerge at 29 ppm. The chemical shift of this peak is very close to that of AlPO4-5 and indeed the XRD of this sample shows the presence of the AlPO4-5 framework (Fig. 1). After 2.5 h, an increase in crystallization is witnessed as an increase in the intensity of the AlPO4-5 peak at the expense of the broad amorphous peak. The spectrum of the sample heated for 5 h looks identical to that of AlPO4-5 reported in the literature [32,33]. The nature of the 16 ppm peak and the shoulder at 8 ppm is worth further commenting. In previous studies of several AlPO4-based molecular sieve synthesis, the broad 31 P peaks in the range 10 to 20 ppm were usually observed in the gel samples obtained in the early stages of the crystallization [34–37]. But there is no direct proof that these 31P peaks are actually connected to the Al sites. In addition to aluminophosphates, some phosphate and polyphosphates can also appear in this region [38–40]. To clarify this and other ambiguities, we carried out the 27 Al ! 31P HETCOR experiments. HETCOR is a twodimensional cross-polarization technique [24,25]. CP is mediated by heteronuclear dipolar coupling. Because the strength of dipolar interaction is strongly dependent on the internuclear distance, CP spectra can yield information on connectivity between two unlike spins involved. In the case of 27Al ! 31P CP, only the 31P nuclei that are in the close vicinity of Al atoms will be detected. Fig. 3A shows the HETCOR spectrum of the initial gel before heating. In the 31P projection, only the broad resonance at 16 ppm is observed, confirming that this resonance is

indeed connected to the Al sites. Thus, the amorphous material is aluminophosphate in nature. Interestingly, the 8 ppm peak seen as a shoulder in the 31P MAS spectrum is absent, clearly indicating that this P does not belong to the AlPO-based amorphous material and is likely due to an amine phosphate not directly associated with aluminum. The 27Al projection shows a strong tetrahedral resonance at 34 ppm and a weak octahedral resonance at 17 ppm. The resonance at 2 ppm is absent in the 27Al projection, indicating that this aluminum is due to unreacted alumina, rather than five-coordinated Al in the AlPO phase. The HETCOR spectrum also illustrates that the 16 ppm 31P peak mainly correlates with the tetrahedral aluminum at 34 ppm, and only weakly connected to the octahedral aluminum at 17 ppm. The results show that mixing Al and P sources together with the structure-directing agent at room temperature yields a significant amount of amorphous AlPO species. Both tetrahedral and octahedral Al sites are present in this AlPO material, and both are connected to the P sites at 16 ppm. The 27Al ! 31P HETCOR spectrum of the 2-h sample presented in Fig. 3B shows the 31P projection contains both peaks at 19 and 29 ppm due to amorphous phase and AlPO4-5, respectively. The 27Al projection shows a trend similar to that observed in the initial gel; both the tetrahedral aluminum at 36 ppm and the octahedral aluminum at 11 ppm are coupled to phosphorus while the 27Al peak at 5 ppm is not. The 29 ppm phosphorus strongly correlates with tetrahedral Al, and the amorphous peak at 19 ppm is connected to both tetrahedral and octahedral Al. Compared to the 31P MAS spectrum, the intensity of the amorphous peak in P projection reduces significantly so that now the 29 ppm resonance dominates. Fyfe and co-workers carried out the 27Al ! 29Si CP on various zeolites and found that the relative enhancement of 29Si signals is linear with the number of Al atoms in neighboring T sites (T = Si and Al) [41]. If this argument is accepted, in the present

* -50

-50

100

100

25

50

Al ppm

50

Al ppm

50

0

27

27

0

* 0 31

-25

P ppm

-50

-75

0

-20 31

-40

-60

P ppm

Fig. 3. 27Al ! 31P HETCOR spectra of (A) the initial gel before heating: 8192 scans were acquired for each of 32 experiments in t1 and (B) the 2-h sample: 5600 scans were acquired for each of 50 experiments in t1.  indicates spinning sidebands.

25

(C)

0.4

(B)

0.3

(A)

S0

0.5

0.2

0.1

0 0

0.2

0.4 ms

0.6

0.8

(A) initial gel: -16 ppm (B) 2.5 hrs: -19 ppm (C) as-made AlPO4-5: -29 ppm

Intensity units

case the larger enhancement of the 31P peak at 29 ppm might result from a larger 31P–27Al dipole–dipole interaction since this peak is due to P in AlPO4-5, which is fully condensed with a P(–OAl)4 coordination. The weak CP intensity of the 16 ppm resonance is due to that the partially condensed P sites in the amorphous phase experience a weak dipolar interaction because the number of Al atoms in the second coordination sphere is less than 4. However, care must be exercised when interpreting CP intensity since cross-polarization involving a quadrupolar nucleus such as 27Al (I = 5/2) can be inefficient due to the difficulty in spin-locking [42]. For this reason, we further preformed 31P{27Al} TRAPDOR experiments to check the CP results. TRAPDOR experiment is designed to measure dipolar interactions between two unlike spins under MAS conditions [26]. Therefore, it also provides connectivity information, but do not involve coherence transfer and spin-locking. TRAPDOR consists of two separate experiments. For 31P{27Al} TRAPDOR, the first is a control experiment, in which a rotor synchronized 31P spin-echo sequence (90–nsr–180–nsr) is applied with sr being one rotor period. The second (TRAPDOR) experiment is the same spin-echo as the first except that during the first half of the echo (nsr) on the observed 31P spins the 27Al spins are continuously irradiated. The continuous high-power rf irradiation of the 27Al spins under MAS conditions affects the echo intensity of 31P spins via dipolar coupling. The TRAPDOR difference spectrum (DS) is obtained by subtracting the TRAPDOR spectrum (S) from the control spectrum (S0) and indicates dipolar coupling. Fig. 4A shows for the selected samples the plots of the TRAPDOR fraction, (DS)/S0, vs. short Al dephasing times. It has been established that for several related heteronuclear dipolar coupling based double-resonance techniques such as TRAPDOR and REDOR, in multiple-spin (ISn) systems such as ours, the initial parts of the DS/S0 vs. evolution time plots only depend on the strength of I–S dipolar interaction (i.e. the number of S spins and internuclear I–S distances), and are independent of the specific geometry involved [43,44]. The slope for the 29 ppm peak of asmade AlPO4-5 is slightly greater than the slope for the 19 ppm peak of the 2.5 h sample (the small difference in slope is reproducible), both of which are much greater than the slope for the 16 ppm peak in the initial gel. The different slopes suggest that different P sites experience different magnitude of dipolar interaction with neighboring Al atoms. In the literature, it has been recognized that in AlPO-based materials including molecular sieves and glasses, the Al–O–P distance usually varies little [45]. Therefore, the difference in the initial slopes of 31P{27Al} TRAPDOR curves has been explained in terms of the average number of Al atoms coordinated to observing spin P [45–47]. In the present case, the different TRAPDOR behavior exhibited by the different 31P signals implies that, on average, they have different number of Al atoms in their second coordination sphere. Although it is difficult to determine the exact number of Al attached to each P site,

(S0 - S)

J.G. Longstaffe et al. / Microporous and Mesoporous Materials 98 (2007) 21–28

(A) (B) (C) 0 0

1

2 3 4 Contact time (ms)

5

(A) 2.5 hrs: -29 ppm (B) 2.5 hrs: -19 ppm (C) initial gel: -16 ppm 31

27

Fig. 4. (A) P{ Al} TRAPDOR fraction (DS/S0) as a function of dephasing time for selected gel samples. (B) Variation of the 1H ! 31P CP intensities as a function of contact time for selected gel samples.

qualitative information, however, can be extracted from the plots. The 29 ppm peak of as-made AlPO4-5 corresponds to a known environment of phosphorus with four aluminum in its second coordination sphere, P(OAl)4. Thus, for the amorphous material resonating at 19 ppm in the 2.5-h sample, the average number of Al atoms attached to the phosphorus is less than four. The fact that the slope of the 19 ppm peak is only slightly smaller than that of P(OAl)4 in AlPO4-5 suggests that the amorphous material contains many P(OAl)4 and some P(OAl)3 coordination environments. The slope of the initial gel is significantly smaller, suggesting the number of aluminum in the second coordination sphere of phosphorus here is much

26

J.G. Longstaffe et al. / Microporous and Mesoporous Materials 98 (2007) 21–28

less than four. Therefore, P(OAl)2 is probably the predominate coordination in the initial gel. The TRAPDOR results indicate qualitatively that the degree of condensation for phosphorus sites in amorphous materials increases with hydrothermal heating. Although the 31P chemical shifts of amorphous materials in the initial gel (16 ppm) and the 2.5-h samples (19 ppm) are similar, the degree of condensation for P is noticeably different. To explore further the coordination environment of phosphorus, 1H ! 31P CP experiments were carried out. As mentioned earlier, cross-polarization is governed by dipolar coupling. Because the strength of dipolar interaction decreases rapidly as internuclear distance increases, phosphorus sites with OH groups attached (P–OH) will experience stronger 1H –31P dipolar interaction. Fig. 4B shows the plots of CP intensity for the initial gel and 2.5-h sample as a function of contact time, which can be described by the following equation: SðtÞ ¼ S max ð1 

1 H T CP =T H 1q Þ ½expðs=T 1q Þ

48.4

9.9

48.1

5.0 hrs

9.4

2.5 hrs

47.2 8.7

2.0 hrs

47.2 8.8

1.5 hrs

9.6 47.4

initial gel 12.6

47.1

Pure TEA

 expðs=T CP Þ 80

where s is contact time, T H 1q the proton spin-lattice relaxation time in rotating frame of reference. TCP is the crosspolarization time constant, which is related to the second moment of the dipolar coupling between the source (I) and target (S) spins and proportional to the r6IS (rIS: internuclear distance). Fitting the curves to the equation yields TCP values of 0.59 and 0.15 ms for the 29 and 19 ppm peaks of the 2.5-h sample and 0.18 ms for the amorphous peak at 16 ppm of the initial gel, respectively. The large TCP value for the 29 ppm peak is consistent with the assignment that this resonance is fully condensed phosphorus, P(OAl)4. The shorter TCP values for the amorphous peaks at 19 and 16 ppm indicate a stronger P–H dipolar interaction, suggesting that both P sites have directly attached OH groups. Based on the results from 27Al ! 31P HETCOR, 31P{27Al} TRAPDOR and 1H ! 31P CP experiments, we suggest that the possible P coordination environments in the initial gel before heating likely are P(OH)2[OAl(tet)]2 and P(OH)2[OAl(tet)][OAl(oct)]. Upon hydrothermal treatment, these local environments evolve to become P(OH)[OAl(tet)]2[OAl(oct)], P(OH)[OAl(tet)]3 and P[OAl(tet)]4. The 13C CP MAS spectra were shown in Fig. 5. The free TEA molecule has two peaks at 47.1 and 12.6 ppm, which can be assigned to the methylene and methyl carbon, respectively. For the initial gel without heating, the methyl carbon shifted towards more shielded direction by 3.0 to 9.6 ppm. A previous study showed that the chemical shift of the methyl carbon in TEA is sensitive to protonation [19]. This high-field shift is an indication that the free amine became protonated in the initial gel. The 13C spectrum indicates that the template molecules are well distributed in the initial gel and they are all protonated. In the amorphous gel samples treated under hydrothermal treatment for 1.5 and 2 h, the methyl signal now appears at 8.7 ppm. The observed additional high-field shift suggests that the pro-

60

40

20

0

ppm Fig. 5.

13

C CP MAS spectra of selected intermediate gel samples.

tonated organic template has a slightly different chemical environment compared with the initial gel without heating. This could be due to either a different strength of hydrogen bonding between the organic TEAH+ ions and inorganic AlPO species or a different conformation of TEAH+. Interestingly, once AlPO4-5 forms, the position of the methyl carbon shifted back to 9.9 ppm. Raman spectroscopy was used to monitor the development of the pore system in the AlPO4-5 framework. A previous study has shown that for the AlPO4-based microporous materials with uni-dimensional channel systems, the ring breathing modes of the largest channels appear in the Raman spectra below 300 cm1. For the 12 membered ring channel of AlPO4-5, the frequency of this mode is at 262 cm1 [48]. Fig. 6 shows that there is no notice of the 262 cm1 peak in the early intermediate samples, and

262 cm-1

As made AlPO4–5 2.5 hrs 2.0 hrs 1.5 hrs initial gel TEA 700

500

300

100

cm-1 Fig. 6. The selected Raman spectra of the gel samples in the frequency range 700–100 cm1.

J.G. Longstaffe et al. / Microporous and Mesoporous Materials 98 (2007) 21–28

27

Table A.1 Concentrations of Al and P of selected gel samples in the liquids and corresponding solid yields determined by ICP–AES Sample Al concentration in liquid [Al]t (·104 g/ml) Solid yield for Al (%) P concentration in liquid [P]t (·104 g/ml) Solid yield for P (%)

Initial gel

1.5 h

2h

2.5 h

5h

0.06

0.86

2.2

3.4

5.5

99.99

99.86

99.65

99.46

99.12

6.4

4.4

3.8

6.4

99.17

99.39

99.48

99.10

15

97.92

this ring breathing mode first appearing in the spectrum after 2.5-h of hydrothermal heating. This tells us that the 12-ring channel only forms as a consequence of the AFI framework and not as a feature of the amorphous materials. 4. Summary The intermediate amorphous phases formed at different stages of crystallization of AlPO4-5 have been characterized by several solid-state NMR techniques in combination with powder XRD and Raman spectroscopy. The results show that upon mixing Al and P sources in the presence of the template, an amorphous aluminophosphate together with an amine phosphate was formed immediately at ambient conditions. In this amorphous AlPO phase, the vast majority of the Al atoms are tetrahedrally coordinated. The P sites have low degree of condensation with the possible coordination environments being P(OH)2[OAl(tet)]2 and P(OH)2 [OAl(tet)][OAl(oct)]. Under hydrothermal treatment, the structure of this amorphous material quickly undergoes reorganization. Upon heating most of the tetrahedral Al sites are first converted to the octahedral Al and the phosphate species is ‘‘dissolved’’. In this new amorphous phase, the P has a higher degree of condensation and the several possible P chemical environments were identified: P(OH)[OAl (tet)]2[OAl(oct)], P(OH)[OAl(tet)]3 and P[OAl (tet)]4. The 12-ring system does not exist in the intermediates. Under the reaction conditions employed, AlPO4-5 directly crystallizes from the amorphous material without going through layered intermediate with long-range ordering. The 13C CP MAS spectra suggest that upon mixing TEA with P and Al sources, the template molecule becomes completely protonated and that the chemical environment of the protonated template changes with crystallization time. We wish to point out that the formation of molecular sieve is an extremely complicated process, and the details about the crystallization mechanism of AlPO4-5 are still not known. However, being able to better characterize the structure of intermediate gel phases by using a number of solid-state NMR techniques in combination with XRD and Raman spectroscopy is a step forward towards the bet-

[X]0 (theoretical concentration of P and Al) 630

– 720

–

ter understanding of the self-assembly processes of microporous materials on a molecular level. Acknowledgements Y.H. acknowledges the financial support from the NSERC, CFI, CRC and PREA programs. We thank Drs. Kirby (NMR) and Wu (ICP–AES) for technical assistance. Appendix A See Table A.1. References [1] E.M. Flanigen, in: H. van Bekum, P.J. Jacobs, E.M. Flanigen, J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice, vol. 137, Elsevier, New York, 2001 (Chapter 2). [2] S. Wilson, B. Lok, C. Messina, T. Cannan, E. Flanigan, J. Am. Chem. Soc. 104 (1982) 1146. [3] C.S. Cundy, P.A. Cox, Chem. Rev. 103 (2003) 663. [4] A. Corma, M.E. Davis, Chem. Phys. Chem. 5 (2004) 304. [5] R. Xu, J. Yu, Acc. Chem. Res. 36 (2003) 481. [6] A.K. Cheetham, G. Ferey, T. Loiseau, Angew. Chem. Int. Ed. 38 (1999) 3268. [7] H. Gies, B. Marler, U. Werthmann, in: H.G. Karge, J. Weitkamp (Eds.), Molecular Sieves: Science and Technology, vol. 1, Springer, Berlin, 1998, pp. 35–64. [8] M.E. Davis, R.F. Lobo, Chem. Mater. 4 (1992) 756. [9] Y. Huang, R. Richer, C.W. Kirby, J. Phys. Chem. 107 (2002) 1326. [10] Y. Huang, D. Machado, C.W. Kirby, J. Phys. Chem. 108 (2003) 1855. [11] W.M. Meier, D.H. Olson, Ch. Baerlocher, Atlas of Zeolite Structure Types, fourth ed., Elsevier, USA, 1996. [12] I. Kustanovich, D. Goldfarb, J. Phys. Chem. 95 (1991) 8818. [13] N. Wang, Z.K. Tang, G.D. Li, J.S. Chen, Nature 408 (2000) 2080. [14] M. Ganschov, I. Braun, G. Schulz-Ekloff, D. Wohrle, in: F. Laeri, F. Schuth, U. Simon, M. Wark (Eds.), Host Guest Systems Based on Nanoporous Crystals, Wiley-VCH, Germany, 2003. [15] K.J. Balkus, L.J. Ball, B.E. Gnade, J.M. Anthony, Chem. Mater. 9 (1997) 380. [16] S. Qiu, W. Pang, H. Kessler, J. Guth, Zeolites 9 (1989) 440. [17] A. Iwasaki, T. Sano, T. Kodaira, Y. Kiyozumi, Micropor. Mesopor. Mater. 64 (2003) 145. [18] J. Kornatowski, G. Zadrozna, J.A. Lercher, in: R. Aiello, F. Testa, G. Giordano (Eds.), Studies in Surface Science and Catalysis, vol. 142, Elsevier, New York, 2002. [19] S. Popescu, S. Thomson, R. Howe, Phys. Chem. Chem. Phys. 3 (2001) 111.

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