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Synthesis of lamellar aluminophosphates via the supramolecular templating mechanism
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
37
Synthesis of lamellar aluminophosphates via the supramolecular templating mechanism Abdelhamid SAYARI Department of Chemical Engineering and CERPIC, Universit~ Laval, Ste-Foy, Qc, Canada G1K 7P4. The liquid-crystal templating approach for the synthesis of mesostructured materials was extended to aluminophosphates. Long chain primary and tertiary amines were used as templates. The molar gel composition was varied in a systematic manner over a wide range. Samples were thoroughly characterized using XRD, TEM, TGA, and 3~p, 27AI, 15N and lsC solid state NMR. Several lamellar phases with doo~distances in the 2 to 4 nm range were obtained. However, no three dimensional structures were detected. The gel composition was found to have a strong effect on the connectivity of aluminum and phosphorus in the final "crystalline" phase, as well as on their doo~ distances. TEM showed that some samples exhibit extended areas with unique structural features. They consisted of coaxial cylinders of alternating inorganic aluminophosphate material and organic surfactant bilayers, all wrapped around a central rodlike micelle. Such coaxial cylinders had an overall diameter of ca. 150 nm. They were aggregated into a hexagonal-like structure.
1. INTRODUCTION The crystalline mesoporous materials designated as M41S [1] have been for the last few years the subject of increasing attention. These materials are prepared hydrothermally via a supramolecular templating technique in the presence of surfactants. Synthetic methods using anionic, cationic, gemini or neutral surfactants, under either very basic or strongly acidic conditions [2-4] were developed. Thermally stable structures, particularly the so-called MCM-41 hexagonal structure have promising applications as catalysts and as advanced materials. Potential catalytic applications of such materials were reviewed recently [5]. Early investigations focussed on silicate and aluminosilicate materials [1]. Further work dealt with the incorporation of other metal cations such as Ti [6], V [7] and B [8] into MCM-41 silicates. In addition, Huo et al. [2] first reported on open-structure networks of a number of metal oxides like W, Sb, Zn, Pb, Mg, AI, Mn, Fe, Co, Ni and Zn oxides. Most of these oxides exhibited lamellar structures, except for W (hexagonal and lamellar), Sb (hexagonal and cubic) and Pb (hexagonal and lamellar) oxides. None of these oxide mesophases including the hexagonal phases was stable upon calcination. More recently, we were able to synthesize lamellar and hexagonal ZrO2
38 [9] and to stabilize the hexagonal phase [10]. Stable hexagonally packed mesoporous titania was also synthesized [11]. Abe et al. [12] prepared hexagonal vanadiumphosphorus oxides, but no information regarding their thermal stability was provided. Aluminophosphates (AIPO4) are crystalline microporous materials prepared hydrothermally, mostly in the presence of amine templates [13]. Several AIPO4s were also prepared using linear alkylene diamines [14] or cyclic diamines [15]. Recently, we extended the so-called liquid crystal templating mechanism to the synthesis of lamellar AIPO4s with d-spacings in the nanometer range [16,17]. Lamellar AIPO4s prepared in the presence of surfactants were also the subject of two other reports [18,19]. In this paper we present an overview of our findings with particular emphasis on samples prepared with the following gel composition P2Os : 0-2 AI203 : C~2H2sNH2 : 60 H20.
2. E X P E R I M E N T A L
Several series of AIPO 4 materials were prepared hydrothermally using the gel composition: x P205 : Y AI208 : z R-NR' 2 : w H20, where x = 1.0 (or 0 for P free samples), y = 0 to 2.0, z = 0.125 to 2.0 and w = 60 to 300. The template R-NR' 2 was a primary (R' = H) or a tertiary (R'= CHs) amine with a long alkyl chain (R = CnH2n§ with n = 8 to 16). However most samples were prepared in the presence of dodecylamine. These samples will be referred to as AIPO4-x:y:z:w. The following is a typical synthesis procedure of a AIPO4 sample with a molar gel composition: P2Os : AI203 : C~2H25NH2 : 60 H20. A suspension of 2.42 g of alumina (72 % pseudoboehmite alumina, Catapal B from Vista) in 5 g of water was mixed with 4 g of phosphoric acid (Fisher Scientific, 85 %) diluted with 13 g of water and stirred for about 1 h. Finally 3.2 g of dodecylamine surfactant was added to this mixture and stirred for one additional hour. The gel was then heated under autogenous pressure at 100 ~ for 24 h in a Teflon lined autoclave with no stirring. X-ray diffraction measurements were carried out on a D5000 Siemens diffractometer (CuKec radiation, ~, = 0.15418 nm). Transmission electron micrographs were obtained as reported elsewhere using a Philips CM20 instrument operated at 200 kV [17,20]. Thermogravimetric measurements were performed on a Mettler TG50 thermobalance in a flow of air. The temperature was raised at a rate of 10 ~ up to 600 ~ s~p and 27AI MAS NMR spectra were obtained on a Bruker AMX-300 (magnetic field 7.05 T, Larmor frequencies 78.18 and 121.47 MHz, respectively) and a Bruker AMX600 (magnetic field 14.1 T, Larmor frequencies 156.36 and 242.95 MHz) spectrometers. Typical MAS speeds of rotation were 10-14 kHz, and the delay times were set at 60 s for 3~p and 0.3 s for 27A1. A conventional one-pulse sequence in combination with high power proton decoupling (40 kHz) was used for both nuclei. Very short radiofrequency pulses were employed for 27AI (0.6 l~S) in order to obtain spectra for quantitative measurements [21]. A 5 mm High Speed Probe and a 5 mm Ultrasonic Speed Probe, both from DOTY Scientific were used on the AMX-300 and AMX-600, respectively. 85% H3PO4 and a 1 M solution of aluminum nitrate were used as external references. All values for the 27AI chemical shifts reported here were corrected for the second order quadrupolar interactions [22].
39 ~3C and ~SN CP MAS spectra were collected on a Bruker AMX-300 spectrometer (Larmor frequencies 75.5 and 30.1 MHz, respectively). The speed of rotation was within 3-3.5 kHz, and the CP contact time was 2 and 5 ms for ~3C and ~SN, respectively. Signals from tetramethylsilane (TMS) and the NO3" group of solid NH4NO3 were used as external references. 3. RESULTS AND DISCUSSION
Tanev et al. [6a] were the first to use long chain primary amines as supramolecular templates for the synthesis of pure and Ti-modified hexagonal mesoporous silicates. The same technique was extended to V-modified silicates [7] and to aluminophosphates [16,17]. Recently, Oliver et al. [19] prepared lamellar AIPO4s using decylamine in a non-aqueous tetraethylene glycol solvent. In the present study both primary and tertiary amines were used. ~SN and ~3C CP MAS NMR showed that amines occluded in the as-synthesized materials were actually protonated. The ~SN signal shifted from -346.0 ppm for pure dodecylamine to -340.0 + 0.4 ppm for the occluded molecule. Likewise, the ~3C chemical shift was 43.1 and 40.2 + 0.4 ppm for pure and occluded dodecylamine, respectively. The magnitude of these shifts corresponds to protonation. The effects of AI2OJP205, C~2H2s-NH2/P2Osand H20/P205 ratios as well as the effect of the alkylamine chain length were investigated systematically. All our AIPO4 materials had lamellar structures, and consequently collapsed upon high temperature calcination. The lamellar nature of these phases was inferred from the presence of only ( 001 ) XRD peaks, and also from direct TEM observations. Figure 1 shows a series of XRD patterns for AIPO4-1:y:1:60, with y = 0 to 1.8. Samples with low AI content (y = 0 to 0.4) exhibited a lamellar phase with doo~ = ca. 22.5 A. As seen below, 8~p NMR data of these samples are consistent with the occurrence of dodecylammonium dihydrogen phosphate. Recently, Oliver et al. [20] used a similar procedure to synthesize decylammonium dihydrogen phosphate. Gels with A I / P ratios higher than 0.6 gave a lamellar AIPO4 phase with doo~= 32.5 + 0.5 ,~ (Table 1). Figure 2, a representative micrograph of AIPO4-1:1:1:60, shows alternating dark and light fringes, indicative of the occurrence of layers viewed edge-on. The electron diffraction pattern is also consistent with the presence of a layered structure with a primary repeat distance of 31 A, in good agreement with XRD measurements. In addition, as shown in Figure 3, some samples exhibited extended areas with unique structural features. These areas consist of disks with an overall diameter of ca. 150 nm aggregated into a hexagonal-like army. A close-up (Figure 4) shows that each disk is comprised of alternating dark and bright concentric rings. The primary distance between dark rings was 31 A, indicating that this new mesophase is related to the layered structure shown in Figure 2. The central tubule of the disk had a diameter of ca. 36 A, consistent with the presence of a surfactant rodlike micelle. The concentric growth of rings to form self-organized, large disks with comparable diameters was interpreted as follows [17]. Because of their small head, long chain alkylammonium surfactants tend to self-organize into planar bilayers [23]. Consequently, in the presence of inorganic species, the formation of lamellar structures is strongly favored.
40
y=1.8 1.6
1.2 1.0
5
: ".';-:,.':~Y.. ~:.'Y/',,d,~ c ?;,.-x./lx.,r, fl.fJ'~ 9 . ..... ;11 ~
",<.x.,, :,.
0.8 k0.6 k.
jk o.o '
1.5
I
'
I
,
I
3.5 5.5 7.5 2 Theta (degrees)
,
I
9.5
Figure 1: XRD pattems of AIPO4-1:y:1:60 samples. Values of y are shown on the right-hand side.
Figure 2: TEM image and its corresponding selected area electron diffraction (Ref. 17).
Table 1 Properties of AIPO4-1:y:1:60 samples. l
AI/P (y)
doo1 (A)
31p ppm
27AI (Td) ppm
0 0.2
0.8 1.0 1.2 1.4 1.6 1.8 2.0 oo
32.5 32.5 33.2 32.7 32.2 32.2 32.7 am c
0.6; 2.3 s 1.0 s; -19.3 s 1.0 s; -19.0 s 0.65 s; 2.26 s -18.6 s; -19.1 s -23.8 s; -25.8 s -13.0 b -13.0 b -13.0 b -13.0 b -13.0 b -3.8 s; -13.0 b -3.0 s; -13.0 b .
-
0.6
22.3 22.6 22.6 29.7
0.4
27AI (Oh) ppm
-
47.4 sm 44.5 sm 47.7 m,b 46.8 m,b 46.5 m,b 47.5 sm,b 46.8 sm,b 46.8 sm,b 46.3 sm,b .
-
-9.8
-
-9.9 sh
-
-10.2
m,sh 10.4 s,sh 10.5 s,sh 10.4s,sh 10.7s,sh 10.4s,sh 10.0s,sh 10.3 s,sh
10.4
27AI (Oh) ppm
sh
s
-8.5 s,b -8.7 s,b -8.5 m,b -8.0 sm,b -6.8 sm,b -6.6 sm,b -6.5sm,b
(a) bulk composition; (b) framework composition; (c) amorphous. b: broad; m, medium; s: strong; sh: sharp; sm: small.
P" AI"
P" A!b
-
-
1:0.54
-
-
-
1:0.92 1:0.80 1:1.05 1:0.79 1:1.26 1:0.84 1:1.47 1:0.90 1:1.69 1:0.95 1:1.84 1:0.89 1:2.11 1:1.07
41
Figure 4: Close-up image showing altemating dark and bright concentric rings.
Fig re 3: TEM image showing disks of coP, :entdc rings packed into a he" tgonal-like array (Ref. 17).
y
1.0
20
(x = 0)
0
-20
y =2.0
D
-40 ~,.-,~
Jl\
2.0 1.8 1.6 D
1.2
(~k.~
~.o
/
_ o~
J~JL
0.6
__>.~
9 ,~
0.4
_
0.4
0.2
0.2
0.0
m
r--
20(:
'
~o
'
~
"
-1;~o"
Chemical Shift, ppm Fig re 5.27AI MAS-NMR spectra of AIF ~)4-1:y:1:60. Values of y are shown on he left-hand side.
"2()0
100
'
510
Chemical Shift, ppm Figure 6. 3~p MAS-NMR spectra of AIPO4-1 :y:1:60. Values of y are shown on the left-hand side.
42 However, in the present system, the occurrence of concentric growth suggests that the system has a tendency to form some rodlike micelles, but not enough to selfaggregate, for example into a hexagonal structure. These rodlike micelles play the role of nuclei for further concentric growth of alternating rings of inorganic AIPO4 materials (dark rings) and cylindrical vesicles of surfactant (bright rings). This unique morphology is to be regarded as an example of the occurrence of new surfactantinorganic mesophases which have no lyotropic surfactant liquid crystal counterparts. Using gemini surfactants, Huo et al. [24] also discovered a surfactant-silicate mesophase with three dimensional hexagonal symmetry which has no analog among known surfactant liquid crystal structures. Likewise, Oliver et al. [19] while studying the synthesis of lamellar aluminophosphates in the presence of decylamine in a nonaqueous tetraethylene glycol solvent, found that parts of their samples exhibit remarkable morphologies and surface patterns akin to the naturally occurring silicious skeletons of diatoms and radiolaria. Figures 5 and 6 show the 27AI and 31p NMR spectra of AIPO4-1:y:1:60 samples. Detailed data are given in Table 1. Figure 5 shows that most samples exhibit three different 27AI NMR signals. Based on literature data [25], the signal at ca. 47 ppm was assigned to tetrahedral aluminum (species A) bonded to four P atoms via oxygen bridges. In agreement with Rocha et al. [27] who found that 27AI in AI(OP)4(OH2)2 resonates between -9.5 and -12 ppm, the peak at -6 to -10 ppm was attributed to framework octahedral aluminum (species B) coordinated with water and PO4 groups. Samples with low AI content (y = 0.2 and y = 0.4) exhibited only one sharp 27AI NMR peak at ca. -10 ppm corresponding to the hydrated six-coordinated AI in AIPO4 framework. Upon vacuum treatment of the samples at room temperature, the peak of species A decreased in favor of species B. This indicates that (i) both species are related to each other, and (ii) species B is coordinated to at least two water molecules. At higher AI loading (y > 0.8) a third signal with a chemical shift of ca. 10.3 ppm developed. As seen, the amorphous P free sample exhibits only one 27AI NMR signal at 10.3 ppm. It is therefore inferred that the 10.3 ppm peak observed for AI rich samples corresponds to extraframework alumina. Figure 6 shows the 31p MAS-NMR spectra of the same AIPO4-1:y:1-60 series of samples. The aluminum free sample exhibited two 3~p NMR signals at 2.3 (40 %) and 0.6 (60 %) ppm. The anisotropy (AS = -75 + 5) and the asymmetry parameter (11 = 0.3 __. 0.1) were very similar for both species. These parameters, common for acid ammonium phosphates, were assigned to two non equivalent PO2(OH)2 anions belonging to dodecylammonium dihydrogen phosphate. Samples with AI to P ratios in the range 0.8 to 1.6 exhibited a broad 3~p NMR peak centered at -13 ppm, thus excluding the presence of P sites with P in their second coordination shells. This peak was attributed to tetrahedral P bonded to (4 - X) aluminum tetrahedra and X hydroxyl groups (where X = 1 or 2). The chemical shifts of 3~p in microporous AIPO4s generally fall in the range of -19 to -30 ppm [27]. The downfield shift observed for our samples may due to several factors, particularly for the hydrophillic nature of the materials [28]. The origin of the peak broadening is most likely attributable to the occurrence of a distribution of P sites with similar but not identical environments. This conclusion stems from the fact that at higher field (14.1 T) the resolution of the peak hardly improved. The first derivative of the 31p NMR signal
43 indicates the presence of at least five subgroups of P sites (Figure 6, inset). In addition to the -13 ppm 31p signal, samples with the highest levels of AI displayed a low intensity (< 4%), sharp peak at ca. -3.4 ppm attributed to an impurity phase. For samples with very low AI contents (y = 0.2 and y = 0.4) there was a sharp peak at -19 ppm in addition to the Sip peak close to 0 ppm observed in the AI-free sample. This -19 ppm 31p peak together with the -10 ppm 27A! peak observed for the same sample may correspond to variscite: AIPO4-2H20 with 5(~IP) = -19.5 ppm and ~(27AI) = -11 ppm [29]. If this assignment is correct, the variscite phase must be highly dispersed not to be observed by XRD. As seen in Table 1 (column 7), the overall AI/P ratios of the samples are comparable to those of the corresponding gels. The framework P to AI ratios shown in the last column represent ratios of the sum of AI species A and B to P calculated based on chemical analysis, quantitative AI NMR data and assuming complete retention of phosphorus. It is seen that AI/P is usually below one. As inferred from NMR data, this indicates the occurrence of some P-O +NH3-C12H2slinkages. Additional data on the effect of other synthesis parameters will be published elsewhere [30].
4. CONCLUSIONS A variety of lamellar aluminophosphates with doo~distances in the range of 2-4 nm were synthesized via the supramolecular templating mechanism using long chain primary and tertiary alkylamines as templates. The effects of other synthesis parameters were also studied. The occurrence of lamellar phases was inferred from XRD data and by direct TEM observations. 81p and 27AIdata were consistent with the presence of aluminophosphate. Even though the synthesis variables had strong effect on the quality of the products formed and on the connectivities of AI and P, they did not favor the formation of three dimensional AIPO4 structures. In addition to the main phase with planar lamellae, some samples exhibited extended areas consisting of coaxial cylinders of altemating inorganic aluminophosphate material and organic surfactant bilayers, all wrapped around a central rodlike micelle. Such coaxial cylinders which were aggregated into a hexagonal-like structure had an overall diameter of ca. 150 nm.
Acknowledgments Partial funding by the Natural Sciences and Engineering Research Council (NSERC) of Canada is acknowledged. I wish to thank I.L. Moudrakovski, J.S. Reddy, V.R. Karra, C.I. Ratcliffe, J.A. Ripmeester, K.F. Preston, A. Chenite and Y. Le Page for significant contributions to this work.
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44
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19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
Q. Huo, D.I. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B. Chmelka, F. Sch0th and G.D. Stucky, Chem. Mater., 6 (1994) 1176. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. S.A. Bagshaw, E. Prouzet and T.J. Pinnavaia, Science, 269 (1995) 1242. A. Sayari, Chem. Mater. (1995), submitted for publication. (a) P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. (b) A. Corma, M.T. Navarro and J. Perez-Pariente, J. Chem. Soc., Chem. Commun., (1994) 147. (c) A. Sayari, V.R. Karra and J.S. Reddy, Mat. Res. Soc. Symp. Proc., 371 (1995) 87. (a) K.M. Reddy, I.L. Moudrakovski and A. Sayari, J. Chem. Soc., Chem. Commun., (1994) 1059. (b) J.S. Reddy and A. Sayari, J. Chem. Soc., Chem. Commun., (1995) 2231. (d) J.S. Reddy and A. Sayari, Appl. Catal., (1995), submitted for publication. (a) A. Sayari, C. Danumah and I.L. Moudrakovski, Chem. Mater., 7 (1995) 813. (b) A. Sayari, I.L. Moudrakovski, C. Danumah, J.A. Ripmeester, C. Ratcliffe, C. and K.F. Preston, J. Phys. Chem., 99 (1995) 16373. J.S. Reddy and A. Sayari, Catal. Lett., (1996), in press. J.S. Reddy, P. Liu and A. Sayari, 1996 Spring Meeting of the Materials Research Society, San Diego, (1996). D.M. Antonelli and J.Y. Ying, Angew. Chem. Int. Ed. Engl., 34 (1995) 2014. T. Abe, A. Taguchi and Iwamoto, Chem. Mater., 7 (1995) 1429. (a) S.T. Wilson, B. Lok, C.A. Messina, T.R. Connan and E.M. Flanigen, J. Am. Chem. Soc., 104 (1982) 1146. (b) S.T. Wilson, Stud. Surf. Sci. Catal., 58 (1991) 137. (a) R.H. Jones, A.M. Chippindale, S. Natarajan and J.M. Thomas, J. Chem. Soc., Chem. Commun., (1994) 565, and references therein. (b) B. Kraushaar-Czametzki, W.H.J. Stork and R.J. Dogterom, Inorg. Chem., 32 (1993) 5029. P.A. Barrett and R.H. Jones, J. Chem. Soc., Chem. Commun., (1995) 1979. A. Sayari, V.R. Karra, J.S. Reddy and I.L. Moudrakovski, J. Chem. Soc., Chem. Commun., (1996), in press. A. Chenite, Y. Le Page, V.R. Karra and A. Sayari, J. Chem. Soc., Chem. Commun., (1996), in press. C.A. Fyfe, W. Achwieger, G. Fu and G.T. Kokotailo, Prepr., A.C.S. Div. Petrol. Chem., 40 (1995) 266. S. Oliver, A. Kuperman, N. Coombs, A. Lough and G. Ozin, Nature, 378 (1995) 47. A. Chenite, Y. Le Page, Y. and A. Sayari, Chem. Mater., 7 (1995) 1015. P.P. Mann, J. Klinowski, A. Trokiner, H. Zanni and P. Papon, Chem. Phys. Left., 151 (1988) 143. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, Chichester, 1987. J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, NY, 1991. Q. Huo, R. Leon, P.M. Petroff and G.D. Stucky, Science, 268 (1995) 1324. D. Muller, E. Jahn, G. Ladwig, G. and V. Haubenreisser, Chem. Phys. Lett., 109 (1984) 332. J. Rocha, W. Kolodziejski, H. He and J. Klinowski, J. Am. Chem. Soc., 114 (1992) 4884. I.L. Moudrakovski, V.P. Schmachlova, N.S. Katsarenko and V.M. Mastikhin, J. Phys. Chem. Solids, 47 (1987) 335. L.S. de Saldarriaga, C. Saldarriaga and M.E. Davis, J. Am. Chem. Soc., 109 (1987) 2686. C.S. Blackwell and R.L. Patton, J. Phys. Chem., 92 (1988) 3965. A. Sayari et al., Chem. Mater. (1996), submitted for publication.
37
Synthesis of lamellar aluminophosphates via the supramolecular templating mechanism Abdelhamid SAYARI Department of Chemical Engineering and CERPIC, Universit~ Laval, Ste-Foy, Qc, Canada G1K 7P4. The liquid-crystal templating approach for the synthesis of mesostructured materials was extended to aluminophosphates. Long chain primary and tertiary amines were used as templates. The molar gel composition was varied in a systematic manner over a wide range. Samples were thoroughly characterized using XRD, TEM, TGA, and 3~p, 27AI, 15N and lsC solid state NMR. Several lamellar phases with doo~distances in the 2 to 4 nm range were obtained. However, no three dimensional structures were detected. The gel composition was found to have a strong effect on the connectivity of aluminum and phosphorus in the final "crystalline" phase, as well as on their doo~ distances. TEM showed that some samples exhibit extended areas with unique structural features. They consisted of coaxial cylinders of alternating inorganic aluminophosphate material and organic surfactant bilayers, all wrapped around a central rodlike micelle. Such coaxial cylinders had an overall diameter of ca. 150 nm. They were aggregated into a hexagonal-like structure.
1. INTRODUCTION The crystalline mesoporous materials designated as M41S [1] have been for the last few years the subject of increasing attention. These materials are prepared hydrothermally via a supramolecular templating technique in the presence of surfactants. Synthetic methods using anionic, cationic, gemini or neutral surfactants, under either very basic or strongly acidic conditions [2-4] were developed. Thermally stable structures, particularly the so-called MCM-41 hexagonal structure have promising applications as catalysts and as advanced materials. Potential catalytic applications of such materials were reviewed recently [5]. Early investigations focussed on silicate and aluminosilicate materials [1]. Further work dealt with the incorporation of other metal cations such as Ti [6], V [7] and B [8] into MCM-41 silicates. In addition, Huo et al. [2] first reported on open-structure networks of a number of metal oxides like W, Sb, Zn, Pb, Mg, AI, Mn, Fe, Co, Ni and Zn oxides. Most of these oxides exhibited lamellar structures, except for W (hexagonal and lamellar), Sb (hexagonal and cubic) and Pb (hexagonal and lamellar) oxides. None of these oxide mesophases including the hexagonal phases was stable upon calcination. More recently, we were able to synthesize lamellar and hexagonal ZrO2
38 [9] and to stabilize the hexagonal phase [10]. Stable hexagonally packed mesoporous titania was also synthesized [11]. Abe et al. [12] prepared hexagonal vanadiumphosphorus oxides, but no information regarding their thermal stability was provided. Aluminophosphates (AIPO4) are crystalline microporous materials prepared hydrothermally, mostly in the presence of amine templates [13]. Several AIPO4s were also prepared using linear alkylene diamines [14] or cyclic diamines [15]. Recently, we extended the so-called liquid crystal templating mechanism to the synthesis of lamellar AIPO4s with d-spacings in the nanometer range [16,17]. Lamellar AIPO4s prepared in the presence of surfactants were also the subject of two other reports [18,19]. In this paper we present an overview of our findings with particular emphasis on samples prepared with the following gel composition P2Os : 0-2 AI203 : C~2H2sNH2 : 60 H20.
2. E X P E R I M E N T A L
Several series of AIPO 4 materials were prepared hydrothermally using the gel composition: x P205 : Y AI208 : z R-NR' 2 : w H20, where x = 1.0 (or 0 for P free samples), y = 0 to 2.0, z = 0.125 to 2.0 and w = 60 to 300. The template R-NR' 2 was a primary (R' = H) or a tertiary (R'= CHs) amine with a long alkyl chain (R = CnH2n§ with n = 8 to 16). However most samples were prepared in the presence of dodecylamine. These samples will be referred to as AIPO4-x:y:z:w. The following is a typical synthesis procedure of a AIPO4 sample with a molar gel composition: P2Os : AI203 : C~2H25NH2 : 60 H20. A suspension of 2.42 g of alumina (72 % pseudoboehmite alumina, Catapal B from Vista) in 5 g of water was mixed with 4 g of phosphoric acid (Fisher Scientific, 85 %) diluted with 13 g of water and stirred for about 1 h. Finally 3.2 g of dodecylamine surfactant was added to this mixture and stirred for one additional hour. The gel was then heated under autogenous pressure at 100 ~ for 24 h in a Teflon lined autoclave with no stirring. X-ray diffraction measurements were carried out on a D5000 Siemens diffractometer (CuKec radiation, ~, = 0.15418 nm). Transmission electron micrographs were obtained as reported elsewhere using a Philips CM20 instrument operated at 200 kV [17,20]. Thermogravimetric measurements were performed on a Mettler TG50 thermobalance in a flow of air. The temperature was raised at a rate of 10 ~ up to 600 ~ s~p and 27AI MAS NMR spectra were obtained on a Bruker AMX-300 (magnetic field 7.05 T, Larmor frequencies 78.18 and 121.47 MHz, respectively) and a Bruker AMX600 (magnetic field 14.1 T, Larmor frequencies 156.36 and 242.95 MHz) spectrometers. Typical MAS speeds of rotation were 10-14 kHz, and the delay times were set at 60 s for 3~p and 0.3 s for 27A1. A conventional one-pulse sequence in combination with high power proton decoupling (40 kHz) was used for both nuclei. Very short radiofrequency pulses were employed for 27AI (0.6 l~S) in order to obtain spectra for quantitative measurements [21]. A 5 mm High Speed Probe and a 5 mm Ultrasonic Speed Probe, both from DOTY Scientific were used on the AMX-300 and AMX-600, respectively. 85% H3PO4 and a 1 M solution of aluminum nitrate were used as external references. All values for the 27AI chemical shifts reported here were corrected for the second order quadrupolar interactions [22].
39 ~3C and ~SN CP MAS spectra were collected on a Bruker AMX-300 spectrometer (Larmor frequencies 75.5 and 30.1 MHz, respectively). The speed of rotation was within 3-3.5 kHz, and the CP contact time was 2 and 5 ms for ~3C and ~SN, respectively. Signals from tetramethylsilane (TMS) and the NO3" group of solid NH4NO3 were used as external references. 3. RESULTS AND DISCUSSION
Tanev et al. [6a] were the first to use long chain primary amines as supramolecular templates for the synthesis of pure and Ti-modified hexagonal mesoporous silicates. The same technique was extended to V-modified silicates [7] and to aluminophosphates [16,17]. Recently, Oliver et al. [19] prepared lamellar AIPO4s using decylamine in a non-aqueous tetraethylene glycol solvent. In the present study both primary and tertiary amines were used. ~SN and ~3C CP MAS NMR showed that amines occluded in the as-synthesized materials were actually protonated. The ~SN signal shifted from -346.0 ppm for pure dodecylamine to -340.0 + 0.4 ppm for the occluded molecule. Likewise, the ~3C chemical shift was 43.1 and 40.2 + 0.4 ppm for pure and occluded dodecylamine, respectively. The magnitude of these shifts corresponds to protonation. The effects of AI2OJP205, C~2H2s-NH2/P2Osand H20/P205 ratios as well as the effect of the alkylamine chain length were investigated systematically. All our AIPO4 materials had lamellar structures, and consequently collapsed upon high temperature calcination. The lamellar nature of these phases was inferred from the presence of only ( 001 ) XRD peaks, and also from direct TEM observations. Figure 1 shows a series of XRD patterns for AIPO4-1:y:1:60, with y = 0 to 1.8. Samples with low AI content (y = 0 to 0.4) exhibited a lamellar phase with doo~ = ca. 22.5 A. As seen below, 8~p NMR data of these samples are consistent with the occurrence of dodecylammonium dihydrogen phosphate. Recently, Oliver et al. [20] used a similar procedure to synthesize decylammonium dihydrogen phosphate. Gels with A I / P ratios higher than 0.6 gave a lamellar AIPO4 phase with doo~= 32.5 + 0.5 ,~ (Table 1). Figure 2, a representative micrograph of AIPO4-1:1:1:60, shows alternating dark and light fringes, indicative of the occurrence of layers viewed edge-on. The electron diffraction pattern is also consistent with the presence of a layered structure with a primary repeat distance of 31 A, in good agreement with XRD measurements. In addition, as shown in Figure 3, some samples exhibited extended areas with unique structural features. These areas consist of disks with an overall diameter of ca. 150 nm aggregated into a hexagonal-like army. A close-up (Figure 4) shows that each disk is comprised of alternating dark and bright concentric rings. The primary distance between dark rings was 31 A, indicating that this new mesophase is related to the layered structure shown in Figure 2. The central tubule of the disk had a diameter of ca. 36 A, consistent with the presence of a surfactant rodlike micelle. The concentric growth of rings to form self-organized, large disks with comparable diameters was interpreted as follows [17]. Because of their small head, long chain alkylammonium surfactants tend to self-organize into planar bilayers [23]. Consequently, in the presence of inorganic species, the formation of lamellar structures is strongly favored.
40
y=1.8 1.6
1.2 1.0
5
: ".';-:,.':~Y.. ~:.'Y/',,d,~ c ?;,.-x./lx.,r, fl.fJ'~ 9 . ..... ;11 ~
",<.x.,, :,.
0.8 k0.6 k.
jk o.o '
1.5
I
'
I
,
I
3.5 5.5 7.5 2 Theta (degrees)
,
I
9.5
Figure 1: XRD pattems of AIPO4-1:y:1:60 samples. Values of y are shown on the right-hand side.
Figure 2: TEM image and its corresponding selected area electron diffraction (Ref. 17).
Table 1 Properties of AIPO4-1:y:1:60 samples. l
AI/P (y)
doo1 (A)
31p ppm
27AI (Td) ppm
0 0.2
0.8 1.0 1.2 1.4 1.6 1.8 2.0 oo
32.5 32.5 33.2 32.7 32.2 32.2 32.7 am c
0.6; 2.3 s 1.0 s; -19.3 s 1.0 s; -19.0 s 0.65 s; 2.26 s -18.6 s; -19.1 s -23.8 s; -25.8 s -13.0 b -13.0 b -13.0 b -13.0 b -13.0 b -3.8 s; -13.0 b -3.0 s; -13.0 b .
-
0.6
22.3 22.6 22.6 29.7
0.4
27AI (Oh) ppm
-
47.4 sm 44.5 sm 47.7 m,b 46.8 m,b 46.5 m,b 47.5 sm,b 46.8 sm,b 46.8 sm,b 46.3 sm,b .
-
-9.8
-
-9.9 sh
-
-10.2
m,sh 10.4 s,sh 10.5 s,sh 10.4s,sh 10.7s,sh 10.4s,sh 10.0s,sh 10.3 s,sh
10.4
27AI (Oh) ppm
sh
s
-8.5 s,b -8.7 s,b -8.5 m,b -8.0 sm,b -6.8 sm,b -6.6 sm,b -6.5sm,b
(a) bulk composition; (b) framework composition; (c) amorphous. b: broad; m, medium; s: strong; sh: sharp; sm: small.
P" AI"
P" A!b
-
-
1:0.54
-
-
-
1:0.92 1:0.80 1:1.05 1:0.79 1:1.26 1:0.84 1:1.47 1:0.90 1:1.69 1:0.95 1:1.84 1:0.89 1:2.11 1:1.07
41
Figure 4: Close-up image showing altemating dark and bright concentric rings.
Fig re 3: TEM image showing disks of coP, :entdc rings packed into a he" tgonal-like array (Ref. 17).
y
1.0
20
(x = 0)
0
-20
y =2.0
D
-40 ~,.-,~
Jl\
2.0 1.8 1.6 D
1.2
(~k.~
~.o
/
_ o~
J~JL
0.6
__>.~
9 ,~
0.4
_
0.4
0.2
0.2
0.0
m
r--
20(:
'
~o
'
~
"
-1;~o"
Chemical Shift, ppm Fig re 5.27AI MAS-NMR spectra of AIF ~)4-1:y:1:60. Values of y are shown on he left-hand side.
"2()0
100
'
510
Chemical Shift, ppm Figure 6. 3~p MAS-NMR spectra of AIPO4-1 :y:1:60. Values of y are shown on the left-hand side.
42 However, in the present system, the occurrence of concentric growth suggests that the system has a tendency to form some rodlike micelles, but not enough to selfaggregate, for example into a hexagonal structure. These rodlike micelles play the role of nuclei for further concentric growth of alternating rings of inorganic AIPO4 materials (dark rings) and cylindrical vesicles of surfactant (bright rings). This unique morphology is to be regarded as an example of the occurrence of new surfactantinorganic mesophases which have no lyotropic surfactant liquid crystal counterparts. Using gemini surfactants, Huo et al. [24] also discovered a surfactant-silicate mesophase with three dimensional hexagonal symmetry which has no analog among known surfactant liquid crystal structures. Likewise, Oliver et al. [19] while studying the synthesis of lamellar aluminophosphates in the presence of decylamine in a nonaqueous tetraethylene glycol solvent, found that parts of their samples exhibit remarkable morphologies and surface patterns akin to the naturally occurring silicious skeletons of diatoms and radiolaria. Figures 5 and 6 show the 27AI and 31p NMR spectra of AIPO4-1:y:1:60 samples. Detailed data are given in Table 1. Figure 5 shows that most samples exhibit three different 27AI NMR signals. Based on literature data [25], the signal at ca. 47 ppm was assigned to tetrahedral aluminum (species A) bonded to four P atoms via oxygen bridges. In agreement with Rocha et al. [27] who found that 27AI in AI(OP)4(OH2)2 resonates between -9.5 and -12 ppm, the peak at -6 to -10 ppm was attributed to framework octahedral aluminum (species B) coordinated with water and PO4 groups. Samples with low AI content (y = 0.2 and y = 0.4) exhibited only one sharp 27AI NMR peak at ca. -10 ppm corresponding to the hydrated six-coordinated AI in AIPO4 framework. Upon vacuum treatment of the samples at room temperature, the peak of species A decreased in favor of species B. This indicates that (i) both species are related to each other, and (ii) species B is coordinated to at least two water molecules. At higher AI loading (y > 0.8) a third signal with a chemical shift of ca. 10.3 ppm developed. As seen, the amorphous P free sample exhibits only one 27AI NMR signal at 10.3 ppm. It is therefore inferred that the 10.3 ppm peak observed for AI rich samples corresponds to extraframework alumina. Figure 6 shows the 31p MAS-NMR spectra of the same AIPO4-1:y:1-60 series of samples. The aluminum free sample exhibited two 3~p NMR signals at 2.3 (40 %) and 0.6 (60 %) ppm. The anisotropy (AS = -75 + 5) and the asymmetry parameter (11 = 0.3 __. 0.1) were very similar for both species. These parameters, common for acid ammonium phosphates, were assigned to two non equivalent PO2(OH)2 anions belonging to dodecylammonium dihydrogen phosphate. Samples with AI to P ratios in the range 0.8 to 1.6 exhibited a broad 3~p NMR peak centered at -13 ppm, thus excluding the presence of P sites with P in their second coordination shells. This peak was attributed to tetrahedral P bonded to (4 - X) aluminum tetrahedra and X hydroxyl groups (where X = 1 or 2). The chemical shifts of 3~p in microporous AIPO4s generally fall in the range of -19 to -30 ppm [27]. The downfield shift observed for our samples may due to several factors, particularly for the hydrophillic nature of the materials [28]. The origin of the peak broadening is most likely attributable to the occurrence of a distribution of P sites with similar but not identical environments. This conclusion stems from the fact that at higher field (14.1 T) the resolution of the peak hardly improved. The first derivative of the 31p NMR signal
43 indicates the presence of at least five subgroups of P sites (Figure 6, inset). In addition to the -13 ppm 31p signal, samples with the highest levels of AI displayed a low intensity (< 4%), sharp peak at ca. -3.4 ppm attributed to an impurity phase. For samples with very low AI contents (y = 0.2 and y = 0.4) there was a sharp peak at -19 ppm in addition to the Sip peak close to 0 ppm observed in the AI-free sample. This -19 ppm 31p peak together with the -10 ppm 27A! peak observed for the same sample may correspond to variscite: AIPO4-2H20 with 5(~IP) = -19.5 ppm and ~(27AI) = -11 ppm [29]. If this assignment is correct, the variscite phase must be highly dispersed not to be observed by XRD. As seen in Table 1 (column 7), the overall AI/P ratios of the samples are comparable to those of the corresponding gels. The framework P to AI ratios shown in the last column represent ratios of the sum of AI species A and B to P calculated based on chemical analysis, quantitative AI NMR data and assuming complete retention of phosphorus. It is seen that AI/P is usually below one. As inferred from NMR data, this indicates the occurrence of some P-O +NH3-C12H2slinkages. Additional data on the effect of other synthesis parameters will be published elsewhere [30].
4. CONCLUSIONS A variety of lamellar aluminophosphates with doo~distances in the range of 2-4 nm were synthesized via the supramolecular templating mechanism using long chain primary and tertiary alkylamines as templates. The effects of other synthesis parameters were also studied. The occurrence of lamellar phases was inferred from XRD data and by direct TEM observations. 81p and 27AIdata were consistent with the presence of aluminophosphate. Even though the synthesis variables had strong effect on the quality of the products formed and on the connectivities of AI and P, they did not favor the formation of three dimensional AIPO4 structures. In addition to the main phase with planar lamellae, some samples exhibited extended areas consisting of coaxial cylinders of altemating inorganic aluminophosphate material and organic surfactant bilayers, all wrapped around a central rodlike micelle. Such coaxial cylinders which were aggregated into a hexagonal-like structure had an overall diameter of ca. 150 nm.
Acknowledgments Partial funding by the Natural Sciences and Engineering Research Council (NSERC) of Canada is acknowledged. I wish to thank I.L. Moudrakovski, J.S. Reddy, V.R. Karra, C.I. Ratcliffe, J.A. Ripmeester, K.F. Preston, A. Chenite and Y. Le Page for significant contributions to this work.
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