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Synthesis of mesoporous aluminophosphates and their adsorption properties1
Microporous and Mesoporous Materials 22 (1998) 115–126
Synthesis of mesoporous aluminophosphates and their adsorption properties1 T. Kimura a, Y. Sugahara a, K. Kuroda a,b,* a Department of Applied Chemistry, School of Science and Engineering, Waseda University, Ohkubo-3, Shinjuku-ku, Tokyo 169, Japan b Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda-2, Shinjuku-ku, Tokyo 169, Japan Received 23 December 1997; accepted 17 February 1998
Abstract Hexagonal mesostructured alkyltrimethylammonium–aluminophosphate materials were prepared by using hexadecyltrimethylammonium and dococyltrimethylammonium chlorides as surfactants and by utilizing 1,3,5triisopropylbenzene as an auxiliary organic additive. Calcination of these materials at 600°C yielded thermally stable mesoporous aluminophosphate materials with large surface areas above 700 m2 g−1 and with pore diameters in the range from 1.8 to 3.9 nm. The hexagonal mesostructured materials before calcination had less condensed frameworks as determined by 31P MAS NMR, which caused certain shrinkages during calcination for the formation of the porous structures. Water and benzene adsorption data indicated that the mesoporous aluminophosphate materials had comparatively hydrophilic mesopores. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous material; Aluminophosphate; Pore size; Adsorption property; Hydrophilicity
1. Introduction A class of microporous aluminophosphate materials (AlPO -n) with various crystalline structures 4 was reported as the first example of inorganic molecular sieves composed of a material other than silica [1]. Since then, a wide variety of materials which contain silicon [2] and/or various transition metals [3] have also been reported; thus, * Corresponding author. Fax: +81 3 5286 3199; E-mail: [email protected] 1Dedicated to Professor Lovat V.C. Rees in recognition and appreciation of his lifelong devotion to zeolite science and his outstanding achievements in this field. 1387-1811/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 07 2 - 9
these materials possess ion-exchange properties, acidity and catalytic activity. The physicochemical properties of these materials are very interesting and attractive because the frameworks are mainly composed of both tetrahedral AlO and PO units, 4 4 and, in particular, some of the materials have novel structures. Novel phosphate-based materials, denoted VPI-5 [4] and cloverite [5], have also been reported, and the synthesis of new porous materials with larger pore sizes is expected as matrices with uniform large pores for use in the petroleum industry, fine chemicals, and so on. Since the discovery of ordered mesoporous silicas and aluminosilicates prepared by utilizing the assemblies of surfactants [6–8], the mesoporous
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silicas incorporated with various transition metals have been synthesized for the purpose of applications to catalysts [9,10]. Adsorption [7,8,11–14] and catalytic properties [15–19] of these materials have recently been investigated. Based on water and benzene adsorption results on the ordered mesopores [7,8,20], the surface of the mesoporous silicas is known to exhibit hydrophobicity. However, for mesoporous materials composed of other inorganic units [21–38], there have been only a few reports on adsorption properties of mesoporous Al O [28,39] and Ta O [33–37]. For 2 3 2 5 mesostructured aluminophosphates (AlPOs), there have been several reports on the synthesis of lamellar mesophases [40–47] and hexagonal mesostructured and mesoporous materials [21–25,48]. Nevertheless, there have been no reports on the physical and chemical properties of mesoporous AlPOs [21–25] because the synthesis of the materials is one of the main research topics at present. We have already reported on hexagonal mesostructured AlPOs [49,50] and recently reported the successful formation of thermally stable mesoporous AlPOs with various pore diameters [51]. These materials had different compositions and framework structures from those of mesoporous and mesostructured AlPOs reported by other researchers [21–25,48]. Therefore, in this paper, we report the characterization of the hexagonal mesostructured and mesoporous AlPOs in more detail. The adsorption properties of the mesoporous AlPOs were investigated by water and benzene adsorption measurements.
2. Experimental 2.1. Materials Hexadecyltrimethylammonium chloride (C 16 TMACl, Tokyo Kasei Kogyo Co.) and dococyltrimethylammonium chloride (C TMACl ) were 22 used as the surfactants. C TMACl ( Katinal 22 BTC-80) was obtained from Toho Kagaku Co. and contains surfactants with shorter chain lengths, such as C TMACl and C TMACl at 20 18 about 15 wt% and isopropyl alcohol at about 18 wt%. Aluminum triisopropoxide (Al(OiC H ) , 3 73
Tokyo Kasei Kogyo Co.) and 85% phosphoric acid (H PO , Kanto Chemical Co.) were used 3 4 as the aluminum and phosphorous sources respectively. Tetramethylammonium hydroxide ( TMAOH, 25 wt% in water) and 1,3,5-triisopropylbenzene ( TIPBz) were obtained from Tokyo Kasei Kogyo Co. 2.2. Synthesis of hexagonal mesostructured AlPOs and mesoporous AlPOs In the preparation of all the mesostructured AlPOs, the starting mixtures were prepared by the same procedure as previously [49–51]. Alkyltrimethylammonium chloride (C TMACl ), n 25 wt% TMAOH, 85% H PO , and water were 3 4 mixed for several hours. Al(OiC H ) was added 3 73 to this mixture under vigorous stirring, and the stirring was continued for 1 day. Thus, the starting mixtures were obtained. The composition of the starting mixtures was Al O :P O : 2 3 2 5 C TMACl:2.0TMAOH:65.0H O and the pH of n 2 the suspensions was ca 10. When C TMACl was used [49,50], the starting 16 mixture was sealed in a Teflon tube and heated at 130°C for 5 days. The resultant was dispersed in distilled water at room temperature, then a white solid was obtained. Because a lamellar mesostructured product was slightly mixed as a by-product, the lamellar mesostructured product was removed by its slower sedimentation velocity. This solid was washed with distilled water repeatedly at room temperature and dried at 80°C. The as-synthesized product was denoted C –AlPO. For C TMACl 16 22 [51], the starting mixture was dispersed in distilled water at 70°C because a lamellar mesostructured product formed on heating at 130°C. When the starting mixture was dispersed in hot water, a white solid was obtained. This solid was washed with hot water (70°C ) repeatedly and dried at 80°C. The as-synthesized product was denoted C –AlPO. 22 To obtain a mesoporous AlPO with larger pore size, TIPBz was used as an auxiliary organic additive in the case of C TMACl. The preparation 22 of the starting mixture was similar to the aforementioned method except for the addition of TIPBz before the addition of Al(OiC H ) . The molar 3 73
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ratio of TIPBz/C TMACl was 1 and the pH 22 of the starting mixture was also about 10. The as-synthesized product, denoted TIPBz/ C –AlPO, was obtained by the same procedure 22 as C –AlPO. 22 Finally, all the C –AlPO, C –AlPO and 16 22 TIPBz/C –AlPO were heated at 600°C for 1 h in 22 flowing N , followed by calcination at 600°C for 2 1 h in flowing air to remove organic fractions.
2.3. Analysis Powder X-ray diffraction ( XRD) patterns were obtained by using a Mac Science M03XHF22 diffractometer with monochromated Fe Ka radiation. The lattice parameter a was employed for 0 the estimation of the size of the hexagonal units because the parameter is normally used for the hexagonal mesostructured materials [20]. For the calcined products, the a values were calculated 0 using 2d /E3 because only one peak was 100 observed after calcination in the XRD patterns, if we assume that the calcined products retain the hexagonal structures. Transmission electron micrographs (TEMs) were taken using a Hitachi H-8100A electron microscope, operated at 200 kV. From the TEM images, the periodic distances were estimated as the distance between the centers of adjacent pores (a ). The Al/P ratios and the 0 amounts of organic fractions of the as-synthesized products were determined by ICP (Jarrell ash ICAP575 Mark II ) and CHN analysis (Perkin Elmer PE-2400II ) respectively. Solid-state NMR spectra were obtained for the as-synthesized and the calcined products without pretreatment. Solid state 27Al MAS NMR measurements were performed on a JEOL GSX-400 spectrometer at a spinning rate of 5 kHz and a resonance frequency of 104.05 MHz with a 45° pulse length of 4.4 ms and a recycle time of 5 s. Solid state 31P MAS NMR was performed at a spinning rate of 5 kHz and a resonance frequency of 161.70 MHz with a 60° pulse length of 5 ms and a recycle time of 20 s. The chemical shifts were quoted from Al(H O)3+ of 0 ppm and P(C H ) 2 6 6 53 of −8.4 ppm. Adsorption measurements were performed to
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determine the porosities and the adsorption properties of the calcined products. The samples were preheated at 120°C for 3 h to a residual pressure of 1.3 Pa (10−2 Torr). Nitrogen adsorption isotherms were obtained by a BELSORP 28 (Bel Japan, Inc.) at 77 K. Corresponding pore size distributions were calculated by the Horva´th– Kawazoe method [52]. Water and benzene adsorption measurements were performed on a BELSORP 18 (Bel Japan, Inc.) at 298 K. For water and benzene adsorption measurements, the equilibrium was kept for 500 s after the change of pressure at each measurement point was below 4.0 Pa (3×10−2 Torr).
3. Results and discussion 3.1. Chacterization of hexagonal C TMA–AlPO n mesostructured materials The XRD patterns and the TEM images of C –AlPO, C –AlPO and TIPBz/C –AlPO are 16 22 22 shown in Fig. 1. The XRD peaks of C –AlPO 16 indicate that a hexagonal mesostructured hexadecyltrimethylammonium–AlPO product was obtained; the peaks at d-spacings of 4.1, 2.4 and 2.1 nm can respectively be assigned to the (100), (110) and (200) of a hexagonal phase with a lattice parameter a =4.8 nm. In the XRD patterns of 0 both C –AlPO and TIPBz/C –AlPO, the (210) 22 22 peaks were also observed. The d peaks of 100 C –AlPO and TIPBz/C –AlPO were observed at 22 22 the d-spacings of 5.1 nm and 6.2 nm respectively. The a values of C –AlPO and TIPBz/C –AlPO 0 22 22 are calculated to be 5.8 nm and 7.2 nm respectively. The TEM images of all the as-synthesized products showed relatively ordered hexagonal arrays. The periodic distances between the centers of adjacent pores of the C –AlPO, C –AlPO and 16 22 TIPBz/C –AlPO are 4.0 nm, 5.1 nm and 6.2 nm 22 respectively, being smaller than the a values calcu0 lated from the XRD results. This observation suggests that the periodic distances obtained from the TEM images might have decreased during the TEM measurements because all the as-synthesized products have less condensed frameworks, as discussed below.
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Fig. 1. XRD patterns of (a) C –AlPO, (b) C –AlPO and (c) TIPBz/C –AlPO and TEM images of (d) C –AlPO, (e) C –AlPO 16 22 22 16 22 and (f ) TIPBz/C –AlPO. 22
Both the XRD and TEM results show that hexagonal mesostructured alkyltrimethylammonium–AlPO products can be successfully prepared and that the product with a larger hexagonal array can be obtained by utilizing TIPBz as a solubilizing agent. The 27Al and 31P MAS NMR spectra of the assynthesized products are shown in Fig. 2. The 27Al MAS NMR spectra of all the products showed the presence of both four- and six-coordinated Al at around 43 ppm and 1 ppm respectively. Fourcoordinated Al can be assigned to Al(OP) and/or 4 Al(OH ) (OP) based on this chemical shift and x 4−x the asymmetric profile [40,53,54]. The six-coordinated Al may be coordinated with not only PO 4 units but also water molecules, because four-coordinated Al was mainly observed after calcination at 600°C, as discussed later. The 31P MAS NMR spectra of all the as-synthe-
sized products showed several peaks in the range from 0 to −20 ppm. It has been reported that ‘isolated PO unit (PO3− ,HPO2− ,H PO− , or 4 4 4 2 4 H PO )’, ‘H PO ligand bonded to one Al atom’, 3 4 2 4 ‘H PO ligand bonded to one Al atom’ and 3 4 ‘(HO) P{OAl(H O) } ’ are observed at >0 ppm, 2 2 5 2 −9.5 ppm, −12.6 ppm and −16.5 ppm respectively [54]. Moreover, for a lamellar mesostructured AlPO reported by us, OP(OAl ) units are observed 3 at −17.8 and −20.0 ppm [53]. In addition, the signals due to P(OAl ) units of microporous crystal4 line AlPO -n and SAPO-n materials are observed 4 in the range from −19 to −31 ppm [55]. Based on these reports, the signals in the 31P MAS NMR spectra of all the as-synthesized products were assigned by the idea that the chemical shifts of P atoms change towards higher magnetic fields depending on the number of Al atoms bonded to PO units. It is considered that the number of Al 4
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Fig. 2. 27Al MAS NMR spectra of (a) C –AlPO, (b) C –AlPO and (c) TIPBz/C –AlPO and 31P MAS NMR spectra of (d) 16 22 22 C –AlPO, (e) C –AlPO and (f ) TIPBz/C –AlPO. 16 22 22
atoms bonded to PO units is mainly one or two 4 for all the samples of C –AlPO, C –AlPO and 16 22 TIPBz/C –AlPO. These assignments indicate that 22 the hexagonal mesostructured alkyltrimethylammonium–AlPO products obtained in this study have less condensed frameworks. The composition of the as-synthesized products are shown in Table 1. The Al/P molar ratios of all the products were around 1.5. Although this value is different from unity, it has been reported that some of mesostructured surfactant–AlPO materials have Al/P ratios above 1.0 [23,48,56,57] and that 27Al MAS NMR spectra of these materials Table 1 Compositions of the as-synthesized products
C –AlPO 16 C –AlPO 22 TIPBz/ C –AlPO 22
Al/P molar ratio
C TMA/(Al+P) n molar ratio
TIPBz/C TMA n molar ratio
1.49 1.52 1.56
0.25 0.19 0.16
— — 0.67
show mainly the presence of six-coordinated Al. The Al/P molar ratio (ca 1.5) may be related to the presence of six-coordinated Al coordinated possibly with water molecules and incomplete condensation of PO units. The C TMA/(Al+P) 4 16 molar ratio was 0.25 and the values of C TMA/(Al+P) of the other two products showed n smaller values. The differences between the C TMA/(Al+P) values may mainly be related n to the washing with distilled water at different temperatures (C TMA: room temperature, 16 C TMA: 70°C ). Also, for TIPBz/C –AlPO, the 22 22 TIPBz/C TMA molar ratio was 0.67, being 22 different from that of the starting mixture. This result shows that not all the TIPBz in the assynthesized product was solubilized in the C TMA. It is probable that TIPBz molecules are 22 eliminated from the TIPBz/C –AlPO to some 22 extent in the course of washing at 70°C and drying at 80°C, because the starting mixture was homogeneous and the d value (6.4 nm) of the air100 dried product was slightly larger than that (6.2 nm) of the product dried at 80°C.
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3.2. Characterization of mesoporous AlPOs and their porous structures The XRD patterns and the TEM images of C –AlPO, C –AlPO and TIPBz/C –AlPO 16 22 22 calcined at 600°C are shown in Fig. 3, and the a 0 values of the calcined products measured by XRD and TEM are listed in Table 2. The XRD patterns of all the calcined products had XRD peaks in low diffraction angles. This result indicates that the porous structures were retained after calcination at 600°C. However, the regularities of the hexagonal arrangements decreased, judging from the broader peaks in comparison with those observed for the as-synthesized products. The d values of the C –AlPO, C –AlPO and 100 16 22 TIPBz/C –AlPO after calcination were 3.2 nm, 22 4.3 nm and 5.3 nm respectively. This shrinkage of nearly 1 nm is due to the fact that the as-synthesized products have less condensed frameworks.
This result is very similar to that observed for a porous zirconium oxo phosphate reported by Ciesla et al. [31]. As the shrinkages during calcination were constant, it is thought that these inorganic–surfactant mesostructured materials with less condensed frameworks are applicable to control of pore sizes in a micropore region by substantial shrinkage. The TEM images of all the calcined products showed disordered arrays. The periodic distances of the calcined products were not constant and varied with disordered arrays. Nevertheless, these values are almost consistent with the a values 0 measured by XRD. These XRD and TEM results indicate that thermally stable mesoporous AlPOs can be successfully prepared by calcination at 600°C. Although only one paper by Zhao et al. [21,22] has reported the preparation of thermally stable mesoporous AlPOs up to 500°C, it seems that the porosity and the structural data are not
Fig. 3. XRD patterns of (a) calcined C –AlPO, (b) calcined C –AlPO and (c) calcined TIPBz/C –AlPO and TEM images of (d) 16 22 22 calcined C –AlPO, (e) calcined C –AlPO, and (f ) calcined TIPBz/C –AlPO. 16 22 22
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Fig. 4. 27Al MAS NMR spectra of (a) calcined C –AlPO, (b) calcined C –AlPO and (c) calcined TIPBz/C –AlPO and 31P MAS 16 22 22 NMR spectra of (d) calcined C –AlPO, (e) calcined C –AlPO and (f ) calcined TIPBz/C –AlPO. 16 22 22 Table 2 Structural and pore characteristics of the calcined products a /nm 0
C –AlPO 16 C –AlPO 22 TIPBz/C –AlPO 22
XRD
TEM
3.2 4.3 5.3
2.8–3.0 3.8–4.5 4.9–5.6
BET surface area/m2 g−1
Pore volume/cm3 g−1
Pore diameter/nm
Wall thicknessb/nm
980 760 720
0.44 0.56 0.72
1.8 2.8 3.9
1.4 1.5 1.4
a a =2d /E3. 0 100 b Calculated from the XRD and N data. 2
consistent. Other papers on mesoporous AlPOs reported so far are thermally unstable [23,24]. 27Al and 31P MAS NMR measurements were performed to determine the framework structures of the calcined products and the shrinkage by calcination. The 27Al and 31P MAS NMR spectra are shown in Fig. 4. The 27Al MAS NMR spectra of the calcined products showed mainly the presence of four-coordinated Al at around 39 ppm. However, in the spectra of the calcined C –AlPO and calcined TIPBz/C –AlPO, small 22 22
peaks due to the presence of six-coordinated Al were observed at −7 ppm. This may be related to the following factors; (i) the structural change during calcination is delayed because of the large amount of organic fractions in the as-synthesized products, (ii) Al O impurity is present, and (iii) 2 3 rehydration of four-coordinated Al occurs after calcination. The 31P MAS NMR spectra of all the calcined products showed broad peaks centered around −21 ppm. This result suggests that the condensa-
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Fig. 5. 27Al MAS NMR spectra of C –AlPO calcined at (a) 200°C, (b) 400°C and (c) 600°C and 31P MAS NMR spectra of 16 C –AlPO calcined at (d) 200°C, (e) 400°C and (f ) 600°C. 16
tion of the frameworks occurred during calcination, being related to the shrinkage of the pores, because the broad peaks were observed in higher magnetic fields than those observed for the assynthesized products [53–55]. On the other hand, the 27Al and 31P MAS NMR spectra of the C –AlPO calcined at 200, 400 and 16 600°C in air without prior heating under N atmo2 sphere are shown in Fig. 5. For the 27Al MAS NMR spectra, the amount of six-coordinated Al decreased with increasing calcination temperature and, finally, the signal disappeared at 600°C. The 31P MAS NMR spectra showed a shift towards higher magnetic field with the increase in temperature; a broad peak centered at −24.5 ppm was observed for the C –AlPO calcined at 600°C. The 16 XRD pattern of the C –AlPO calcined at 600°C 16 showed a small peak at the d-spacing of 2.6 nm. These results suggest that the structural change during calcination in air occurred further than that in N and air. 2 The NMR results of the calcined C –AlPO and 22 calcined TIPBz/C –AlPO at 600°C in N flow and 22 2
the following air flow are similar to those observed for the C –AlPO calcined at around 400°C in air. 22 This fact supports the contention that the structural change was suppressed by means of calcination in N flow and the following air flow. 2 Therefore, this calcination method is favorable for removal of organic fractions from mesostructured materials composed of less condensed frameworks. The porous structures of the mesoporous AlPOs were determined by N adsorption measurements. 2 The N isotherms of the calcined products are 2 shown in Fig. 6. The N adsorption isotherm of 2 calcined C –AlPO was almost type I, indicating 16 the presence of micropores. In contrast, the isotherms of calcined C –AlPO and TIPBz/ 22 C –AlPO showed type IV behavior; the pore size 22 distributions are in a mesopore region. The BET surface areas, the pore volumes and the average pore diameters of all the calcined products are listed in Table 2. With the increase in the alkyl chain lengths of the surfactants and by using TIPBz as a solubilizing agent in the case of C TMACl, the pore diameter increased. This 22
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Fig. 6. Nitrogen adsorption isotherms of: (a) calcined C –AlPO, #; (b) calcined C –AlPO, %; (c) calcined TIPBz/C –AlPO, 6. 16 22 22 The filled symbols denote desorption.
result suggests that the pore size can be variable as well as those observed for mesoporous silicas [6–8], although the shrinkages during calcination must be taken into consideration. The BET surface areas were above 700 m2 g−1 and the pore volumes increased with the pore diameters. Although the tendency of the increase in the pore volumes is similar to those observed for mesoporous silicas [58,59], the surface areas of mesoporous silicas generally show constant values about 1000 m2 g−1. Because the wall thicknesses (1.4–1.5 nm) of the calcined products shown in Table 2 seemed to be larger than those of mesoporous silicas [59,60], the BET surface areas of the mesoporous AlPOs were above 700 m2 g−1. 3.3. Adsorption properties of mesoporous AlPOs In the preparation of typical mesoporous silicas, alkyltrimethylammonium cation surfactants have generally been used as structure-directing organic assemblies and the pore diameters in the case of
C TMA cations are around 3 nm. In the present 16 study, the calcined C –AlPO with a pore diameter 22 of 2.8 nm was employed for adsorption measurements as a representative of the mesoporous AlPOs. The water and benzene adsorption isotherms are shown in Fig. 7. The water adsorption isotherm showed type IV behavior. The benzene adsorption isotherm showed type IV behavior and the increase of adsorbed benzene due to capillary condensation was observed at a relative pressure in the range of ca 0.2–0.4. This value is higher than that observed for a mesoporous silica [20]. These adsorption data indicate that the inner surface of the mesoporous AlPO is relatively hydrophilic. The hysteresis in the benzene isotherm possibly indicates the interaction between benzene and the surface of the mesopores. For the water adsorption–desorption isotherm, the end point at P/P =0.1 in the desorption was located at about 0 0.3 g g−1, being larger than that of mesoporous silica [20]. The 27Al MAS NMR spectrum of the calcined C –AlPO after adsorption–desorption of 22
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Fig. 7. (a) Water and (b) benzene adsorption isotherms of calcined C –AlPO. Filled symbols denote desorption. Inset: 27Al MAS 22 NMR spectrum of calcined C –AlPO after water adsorption measurement. 22
water showed an intense peak at −5 ppm due to a considerable amount of six-coordinated Al. This result indicates that the mesoporous AlPOs have hydrophilic surface due to four-coordinated Al in the framework to which water molecules can access. Thamm et al. [61] speculated that a sudden increase in the water adsorption for AlPO -5 may 4 be due to coordinated water. The 27Al MAS NMR spectra of hydrated AlPO -n show not only 4 four-coordinated Al but also six-coordinated Al [62,63]. These reports support the consideration described above. In contrast, Vaudry et al. [28] reported that nheptane and neopentane are adsorbed on mesoporous alumina in a very low relative pressure (P/P ; 0.05) and Komarneni et al. [39] reported 0 that mesoporous alumina has a somewhat hydrophilic surface, based on water adsorption data. However, the amount of adsorbed water on the surface of the mesoporous alumina seems to be small. In addition, it has been reported that mesoporous silicas have hydrophobic surfaces [7,8,11– 13,20,39]. Although four-coordinated Al atoms exist in mesoporous aluminosilicates, the water adsorption isotherms are type V; the mesoporous aluminosilicates also have hydrophobic surfaces [64]. Moreover, Ta–TMS1 has been reported to have a high adsorption capacity for butane [33– 37]. The mesoporous AlPOs in this study are the
first example that mesoporous materials with ordered mesopores prepared by using surfactant assemblies have comparatively hydrophilic surfaces. We cannot discuss here the difference in the adsorption properties between the mesoporous AlPOs and microporous AlPO -n because the sur4 face structures of the mesoporous AlPOs are not clear at present. Further studies are necessary on the surface structures as well as on the surface properties of the materials doped with other units, such as SiO and various transition metal oxides. 4 Moreover, because mesoporous AlPO materials reported by other researchers [21–25] are different from the mesoporous AlPOs prepared in this study, the relationship among compositions, framework structures, and adsorption properties of those mesoporous AlPO materials should be investigated further for the purpose of enlargement of applications of mesoporous AlPO materials.
4. Conclusions Thermally stable mesoporous AlPO materials with pore sizes in the range 1.8–3.9 nm were successfully synthesized from hexagonal mesostructured alkyltrimethylammonium–AlPO materials with less condensed frameworks. From water
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and benzene adsorption measurements, the mesoporous AlPO materials exhibited hydrophilic nature of the mesopores. The method presented here, including the use of long-chain surfactant with an auxiliary additive and a controlled pyrolysis, could be applicable for the preparation of various mesoporous materials from less condensed inorganic-unit–surfactant mesostructured materials which normally afford microporous materials caused by substantial shrinkage. Although the shrinkage during calcination can be a sort of unfavorable point for the preparation of mesoporous materials, this phenomenon results in the formation of narrower pore sizes which would be suitable for some purposes such as shape-selective reactions for fine chemicals.
Acknowledgement The authors are grateful to Mr. M. Fuziwara, Materials Characterization Central Laboratory, Waseda University, for his TEM measurements and useful discussion. K.K. acknowledges the financial assistance from Grant-in-Aid for the Special Priority Area by the Ministry of Education, Science, and Culture of the Japanese Government.
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Synthesis of mesoporous aluminophosphates and their adsorption properties1 T. Kimura a, Y. Sugahara a, K. Kuroda a,b,* a Department of Applied Chemistry, School of Science and Engineering, Waseda University, Ohkubo-3, Shinjuku-ku, Tokyo 169, Japan b Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda-2, Shinjuku-ku, Tokyo 169, Japan Received 23 December 1997; accepted 17 February 1998
Abstract Hexagonal mesostructured alkyltrimethylammonium–aluminophosphate materials were prepared by using hexadecyltrimethylammonium and dococyltrimethylammonium chlorides as surfactants and by utilizing 1,3,5triisopropylbenzene as an auxiliary organic additive. Calcination of these materials at 600°C yielded thermally stable mesoporous aluminophosphate materials with large surface areas above 700 m2 g−1 and with pore diameters in the range from 1.8 to 3.9 nm. The hexagonal mesostructured materials before calcination had less condensed frameworks as determined by 31P MAS NMR, which caused certain shrinkages during calcination for the formation of the porous structures. Water and benzene adsorption data indicated that the mesoporous aluminophosphate materials had comparatively hydrophilic mesopores. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous material; Aluminophosphate; Pore size; Adsorption property; Hydrophilicity
1. Introduction A class of microporous aluminophosphate materials (AlPO -n) with various crystalline structures 4 was reported as the first example of inorganic molecular sieves composed of a material other than silica [1]. Since then, a wide variety of materials which contain silicon [2] and/or various transition metals [3] have also been reported; thus, * Corresponding author. Fax: +81 3 5286 3199; E-mail: [email protected] 1Dedicated to Professor Lovat V.C. Rees in recognition and appreciation of his lifelong devotion to zeolite science and his outstanding achievements in this field. 1387-1811/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 07 2 - 9
these materials possess ion-exchange properties, acidity and catalytic activity. The physicochemical properties of these materials are very interesting and attractive because the frameworks are mainly composed of both tetrahedral AlO and PO units, 4 4 and, in particular, some of the materials have novel structures. Novel phosphate-based materials, denoted VPI-5 [4] and cloverite [5], have also been reported, and the synthesis of new porous materials with larger pore sizes is expected as matrices with uniform large pores for use in the petroleum industry, fine chemicals, and so on. Since the discovery of ordered mesoporous silicas and aluminosilicates prepared by utilizing the assemblies of surfactants [6–8], the mesoporous
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silicas incorporated with various transition metals have been synthesized for the purpose of applications to catalysts [9,10]. Adsorption [7,8,11–14] and catalytic properties [15–19] of these materials have recently been investigated. Based on water and benzene adsorption results on the ordered mesopores [7,8,20], the surface of the mesoporous silicas is known to exhibit hydrophobicity. However, for mesoporous materials composed of other inorganic units [21–38], there have been only a few reports on adsorption properties of mesoporous Al O [28,39] and Ta O [33–37]. For 2 3 2 5 mesostructured aluminophosphates (AlPOs), there have been several reports on the synthesis of lamellar mesophases [40–47] and hexagonal mesostructured and mesoporous materials [21–25,48]. Nevertheless, there have been no reports on the physical and chemical properties of mesoporous AlPOs [21–25] because the synthesis of the materials is one of the main research topics at present. We have already reported on hexagonal mesostructured AlPOs [49,50] and recently reported the successful formation of thermally stable mesoporous AlPOs with various pore diameters [51]. These materials had different compositions and framework structures from those of mesoporous and mesostructured AlPOs reported by other researchers [21–25,48]. Therefore, in this paper, we report the characterization of the hexagonal mesostructured and mesoporous AlPOs in more detail. The adsorption properties of the mesoporous AlPOs were investigated by water and benzene adsorption measurements.
2. Experimental 2.1. Materials Hexadecyltrimethylammonium chloride (C 16 TMACl, Tokyo Kasei Kogyo Co.) and dococyltrimethylammonium chloride (C TMACl ) were 22 used as the surfactants. C TMACl ( Katinal 22 BTC-80) was obtained from Toho Kagaku Co. and contains surfactants with shorter chain lengths, such as C TMACl and C TMACl at 20 18 about 15 wt% and isopropyl alcohol at about 18 wt%. Aluminum triisopropoxide (Al(OiC H ) , 3 73
Tokyo Kasei Kogyo Co.) and 85% phosphoric acid (H PO , Kanto Chemical Co.) were used 3 4 as the aluminum and phosphorous sources respectively. Tetramethylammonium hydroxide ( TMAOH, 25 wt% in water) and 1,3,5-triisopropylbenzene ( TIPBz) were obtained from Tokyo Kasei Kogyo Co. 2.2. Synthesis of hexagonal mesostructured AlPOs and mesoporous AlPOs In the preparation of all the mesostructured AlPOs, the starting mixtures were prepared by the same procedure as previously [49–51]. Alkyltrimethylammonium chloride (C TMACl ), n 25 wt% TMAOH, 85% H PO , and water were 3 4 mixed for several hours. Al(OiC H ) was added 3 73 to this mixture under vigorous stirring, and the stirring was continued for 1 day. Thus, the starting mixtures were obtained. The composition of the starting mixtures was Al O :P O : 2 3 2 5 C TMACl:2.0TMAOH:65.0H O and the pH of n 2 the suspensions was ca 10. When C TMACl was used [49,50], the starting 16 mixture was sealed in a Teflon tube and heated at 130°C for 5 days. The resultant was dispersed in distilled water at room temperature, then a white solid was obtained. Because a lamellar mesostructured product was slightly mixed as a by-product, the lamellar mesostructured product was removed by its slower sedimentation velocity. This solid was washed with distilled water repeatedly at room temperature and dried at 80°C. The as-synthesized product was denoted C –AlPO. For C TMACl 16 22 [51], the starting mixture was dispersed in distilled water at 70°C because a lamellar mesostructured product formed on heating at 130°C. When the starting mixture was dispersed in hot water, a white solid was obtained. This solid was washed with hot water (70°C ) repeatedly and dried at 80°C. The as-synthesized product was denoted C –AlPO. 22 To obtain a mesoporous AlPO with larger pore size, TIPBz was used as an auxiliary organic additive in the case of C TMACl. The preparation 22 of the starting mixture was similar to the aforementioned method except for the addition of TIPBz before the addition of Al(OiC H ) . The molar 3 73
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ratio of TIPBz/C TMACl was 1 and the pH 22 of the starting mixture was also about 10. The as-synthesized product, denoted TIPBz/ C –AlPO, was obtained by the same procedure 22 as C –AlPO. 22 Finally, all the C –AlPO, C –AlPO and 16 22 TIPBz/C –AlPO were heated at 600°C for 1 h in 22 flowing N , followed by calcination at 600°C for 2 1 h in flowing air to remove organic fractions.
2.3. Analysis Powder X-ray diffraction ( XRD) patterns were obtained by using a Mac Science M03XHF22 diffractometer with monochromated Fe Ka radiation. The lattice parameter a was employed for 0 the estimation of the size of the hexagonal units because the parameter is normally used for the hexagonal mesostructured materials [20]. For the calcined products, the a values were calculated 0 using 2d /E3 because only one peak was 100 observed after calcination in the XRD patterns, if we assume that the calcined products retain the hexagonal structures. Transmission electron micrographs (TEMs) were taken using a Hitachi H-8100A electron microscope, operated at 200 kV. From the TEM images, the periodic distances were estimated as the distance between the centers of adjacent pores (a ). The Al/P ratios and the 0 amounts of organic fractions of the as-synthesized products were determined by ICP (Jarrell ash ICAP575 Mark II ) and CHN analysis (Perkin Elmer PE-2400II ) respectively. Solid-state NMR spectra were obtained for the as-synthesized and the calcined products without pretreatment. Solid state 27Al MAS NMR measurements were performed on a JEOL GSX-400 spectrometer at a spinning rate of 5 kHz and a resonance frequency of 104.05 MHz with a 45° pulse length of 4.4 ms and a recycle time of 5 s. Solid state 31P MAS NMR was performed at a spinning rate of 5 kHz and a resonance frequency of 161.70 MHz with a 60° pulse length of 5 ms and a recycle time of 20 s. The chemical shifts were quoted from Al(H O)3+ of 0 ppm and P(C H ) 2 6 6 53 of −8.4 ppm. Adsorption measurements were performed to
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determine the porosities and the adsorption properties of the calcined products. The samples were preheated at 120°C for 3 h to a residual pressure of 1.3 Pa (10−2 Torr). Nitrogen adsorption isotherms were obtained by a BELSORP 28 (Bel Japan, Inc.) at 77 K. Corresponding pore size distributions were calculated by the Horva´th– Kawazoe method [52]. Water and benzene adsorption measurements were performed on a BELSORP 18 (Bel Japan, Inc.) at 298 K. For water and benzene adsorption measurements, the equilibrium was kept for 500 s after the change of pressure at each measurement point was below 4.0 Pa (3×10−2 Torr).
3. Results and discussion 3.1. Chacterization of hexagonal C TMA–AlPO n mesostructured materials The XRD patterns and the TEM images of C –AlPO, C –AlPO and TIPBz/C –AlPO are 16 22 22 shown in Fig. 1. The XRD peaks of C –AlPO 16 indicate that a hexagonal mesostructured hexadecyltrimethylammonium–AlPO product was obtained; the peaks at d-spacings of 4.1, 2.4 and 2.1 nm can respectively be assigned to the (100), (110) and (200) of a hexagonal phase with a lattice parameter a =4.8 nm. In the XRD patterns of 0 both C –AlPO and TIPBz/C –AlPO, the (210) 22 22 peaks were also observed. The d peaks of 100 C –AlPO and TIPBz/C –AlPO were observed at 22 22 the d-spacings of 5.1 nm and 6.2 nm respectively. The a values of C –AlPO and TIPBz/C –AlPO 0 22 22 are calculated to be 5.8 nm and 7.2 nm respectively. The TEM images of all the as-synthesized products showed relatively ordered hexagonal arrays. The periodic distances between the centers of adjacent pores of the C –AlPO, C –AlPO and 16 22 TIPBz/C –AlPO are 4.0 nm, 5.1 nm and 6.2 nm 22 respectively, being smaller than the a values calcu0 lated from the XRD results. This observation suggests that the periodic distances obtained from the TEM images might have decreased during the TEM measurements because all the as-synthesized products have less condensed frameworks, as discussed below.
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Fig. 1. XRD patterns of (a) C –AlPO, (b) C –AlPO and (c) TIPBz/C –AlPO and TEM images of (d) C –AlPO, (e) C –AlPO 16 22 22 16 22 and (f ) TIPBz/C –AlPO. 22
Both the XRD and TEM results show that hexagonal mesostructured alkyltrimethylammonium–AlPO products can be successfully prepared and that the product with a larger hexagonal array can be obtained by utilizing TIPBz as a solubilizing agent. The 27Al and 31P MAS NMR spectra of the assynthesized products are shown in Fig. 2. The 27Al MAS NMR spectra of all the products showed the presence of both four- and six-coordinated Al at around 43 ppm and 1 ppm respectively. Fourcoordinated Al can be assigned to Al(OP) and/or 4 Al(OH ) (OP) based on this chemical shift and x 4−x the asymmetric profile [40,53,54]. The six-coordinated Al may be coordinated with not only PO 4 units but also water molecules, because four-coordinated Al was mainly observed after calcination at 600°C, as discussed later. The 31P MAS NMR spectra of all the as-synthe-
sized products showed several peaks in the range from 0 to −20 ppm. It has been reported that ‘isolated PO unit (PO3− ,HPO2− ,H PO− , or 4 4 4 2 4 H PO )’, ‘H PO ligand bonded to one Al atom’, 3 4 2 4 ‘H PO ligand bonded to one Al atom’ and 3 4 ‘(HO) P{OAl(H O) } ’ are observed at >0 ppm, 2 2 5 2 −9.5 ppm, −12.6 ppm and −16.5 ppm respectively [54]. Moreover, for a lamellar mesostructured AlPO reported by us, OP(OAl ) units are observed 3 at −17.8 and −20.0 ppm [53]. In addition, the signals due to P(OAl ) units of microporous crystal4 line AlPO -n and SAPO-n materials are observed 4 in the range from −19 to −31 ppm [55]. Based on these reports, the signals in the 31P MAS NMR spectra of all the as-synthesized products were assigned by the idea that the chemical shifts of P atoms change towards higher magnetic fields depending on the number of Al atoms bonded to PO units. It is considered that the number of Al 4
T. Kimura et al. / Microporous and Mesoporous Materials 22 (1998) 115–126
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Fig. 2. 27Al MAS NMR spectra of (a) C –AlPO, (b) C –AlPO and (c) TIPBz/C –AlPO and 31P MAS NMR spectra of (d) 16 22 22 C –AlPO, (e) C –AlPO and (f ) TIPBz/C –AlPO. 16 22 22
atoms bonded to PO units is mainly one or two 4 for all the samples of C –AlPO, C –AlPO and 16 22 TIPBz/C –AlPO. These assignments indicate that 22 the hexagonal mesostructured alkyltrimethylammonium–AlPO products obtained in this study have less condensed frameworks. The composition of the as-synthesized products are shown in Table 1. The Al/P molar ratios of all the products were around 1.5. Although this value is different from unity, it has been reported that some of mesostructured surfactant–AlPO materials have Al/P ratios above 1.0 [23,48,56,57] and that 27Al MAS NMR spectra of these materials Table 1 Compositions of the as-synthesized products
C –AlPO 16 C –AlPO 22 TIPBz/ C –AlPO 22
Al/P molar ratio
C TMA/(Al+P) n molar ratio
TIPBz/C TMA n molar ratio
1.49 1.52 1.56
0.25 0.19 0.16
— — 0.67
show mainly the presence of six-coordinated Al. The Al/P molar ratio (ca 1.5) may be related to the presence of six-coordinated Al coordinated possibly with water molecules and incomplete condensation of PO units. The C TMA/(Al+P) 4 16 molar ratio was 0.25 and the values of C TMA/(Al+P) of the other two products showed n smaller values. The differences between the C TMA/(Al+P) values may mainly be related n to the washing with distilled water at different temperatures (C TMA: room temperature, 16 C TMA: 70°C ). Also, for TIPBz/C –AlPO, the 22 22 TIPBz/C TMA molar ratio was 0.67, being 22 different from that of the starting mixture. This result shows that not all the TIPBz in the assynthesized product was solubilized in the C TMA. It is probable that TIPBz molecules are 22 eliminated from the TIPBz/C –AlPO to some 22 extent in the course of washing at 70°C and drying at 80°C, because the starting mixture was homogeneous and the d value (6.4 nm) of the air100 dried product was slightly larger than that (6.2 nm) of the product dried at 80°C.
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3.2. Characterization of mesoporous AlPOs and their porous structures The XRD patterns and the TEM images of C –AlPO, C –AlPO and TIPBz/C –AlPO 16 22 22 calcined at 600°C are shown in Fig. 3, and the a 0 values of the calcined products measured by XRD and TEM are listed in Table 2. The XRD patterns of all the calcined products had XRD peaks in low diffraction angles. This result indicates that the porous structures were retained after calcination at 600°C. However, the regularities of the hexagonal arrangements decreased, judging from the broader peaks in comparison with those observed for the as-synthesized products. The d values of the C –AlPO, C –AlPO and 100 16 22 TIPBz/C –AlPO after calcination were 3.2 nm, 22 4.3 nm and 5.3 nm respectively. This shrinkage of nearly 1 nm is due to the fact that the as-synthesized products have less condensed frameworks.
This result is very similar to that observed for a porous zirconium oxo phosphate reported by Ciesla et al. [31]. As the shrinkages during calcination were constant, it is thought that these inorganic–surfactant mesostructured materials with less condensed frameworks are applicable to control of pore sizes in a micropore region by substantial shrinkage. The TEM images of all the calcined products showed disordered arrays. The periodic distances of the calcined products were not constant and varied with disordered arrays. Nevertheless, these values are almost consistent with the a values 0 measured by XRD. These XRD and TEM results indicate that thermally stable mesoporous AlPOs can be successfully prepared by calcination at 600°C. Although only one paper by Zhao et al. [21,22] has reported the preparation of thermally stable mesoporous AlPOs up to 500°C, it seems that the porosity and the structural data are not
Fig. 3. XRD patterns of (a) calcined C –AlPO, (b) calcined C –AlPO and (c) calcined TIPBz/C –AlPO and TEM images of (d) 16 22 22 calcined C –AlPO, (e) calcined C –AlPO, and (f ) calcined TIPBz/C –AlPO. 16 22 22
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Fig. 4. 27Al MAS NMR spectra of (a) calcined C –AlPO, (b) calcined C –AlPO and (c) calcined TIPBz/C –AlPO and 31P MAS 16 22 22 NMR spectra of (d) calcined C –AlPO, (e) calcined C –AlPO and (f ) calcined TIPBz/C –AlPO. 16 22 22 Table 2 Structural and pore characteristics of the calcined products a /nm 0
C –AlPO 16 C –AlPO 22 TIPBz/C –AlPO 22
XRD
TEM
3.2 4.3 5.3
2.8–3.0 3.8–4.5 4.9–5.6
BET surface area/m2 g−1
Pore volume/cm3 g−1
Pore diameter/nm
Wall thicknessb/nm
980 760 720
0.44 0.56 0.72
1.8 2.8 3.9
1.4 1.5 1.4
a a =2d /E3. 0 100 b Calculated from the XRD and N data. 2
consistent. Other papers on mesoporous AlPOs reported so far are thermally unstable [23,24]. 27Al and 31P MAS NMR measurements were performed to determine the framework structures of the calcined products and the shrinkage by calcination. The 27Al and 31P MAS NMR spectra are shown in Fig. 4. The 27Al MAS NMR spectra of the calcined products showed mainly the presence of four-coordinated Al at around 39 ppm. However, in the spectra of the calcined C –AlPO and calcined TIPBz/C –AlPO, small 22 22
peaks due to the presence of six-coordinated Al were observed at −7 ppm. This may be related to the following factors; (i) the structural change during calcination is delayed because of the large amount of organic fractions in the as-synthesized products, (ii) Al O impurity is present, and (iii) 2 3 rehydration of four-coordinated Al occurs after calcination. The 31P MAS NMR spectra of all the calcined products showed broad peaks centered around −21 ppm. This result suggests that the condensa-
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Fig. 5. 27Al MAS NMR spectra of C –AlPO calcined at (a) 200°C, (b) 400°C and (c) 600°C and 31P MAS NMR spectra of 16 C –AlPO calcined at (d) 200°C, (e) 400°C and (f ) 600°C. 16
tion of the frameworks occurred during calcination, being related to the shrinkage of the pores, because the broad peaks were observed in higher magnetic fields than those observed for the assynthesized products [53–55]. On the other hand, the 27Al and 31P MAS NMR spectra of the C –AlPO calcined at 200, 400 and 16 600°C in air without prior heating under N atmo2 sphere are shown in Fig. 5. For the 27Al MAS NMR spectra, the amount of six-coordinated Al decreased with increasing calcination temperature and, finally, the signal disappeared at 600°C. The 31P MAS NMR spectra showed a shift towards higher magnetic field with the increase in temperature; a broad peak centered at −24.5 ppm was observed for the C –AlPO calcined at 600°C. The 16 XRD pattern of the C –AlPO calcined at 600°C 16 showed a small peak at the d-spacing of 2.6 nm. These results suggest that the structural change during calcination in air occurred further than that in N and air. 2 The NMR results of the calcined C –AlPO and 22 calcined TIPBz/C –AlPO at 600°C in N flow and 22 2
the following air flow are similar to those observed for the C –AlPO calcined at around 400°C in air. 22 This fact supports the contention that the structural change was suppressed by means of calcination in N flow and the following air flow. 2 Therefore, this calcination method is favorable for removal of organic fractions from mesostructured materials composed of less condensed frameworks. The porous structures of the mesoporous AlPOs were determined by N adsorption measurements. 2 The N isotherms of the calcined products are 2 shown in Fig. 6. The N adsorption isotherm of 2 calcined C –AlPO was almost type I, indicating 16 the presence of micropores. In contrast, the isotherms of calcined C –AlPO and TIPBz/ 22 C –AlPO showed type IV behavior; the pore size 22 distributions are in a mesopore region. The BET surface areas, the pore volumes and the average pore diameters of all the calcined products are listed in Table 2. With the increase in the alkyl chain lengths of the surfactants and by using TIPBz as a solubilizing agent in the case of C TMACl, the pore diameter increased. This 22
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Fig. 6. Nitrogen adsorption isotherms of: (a) calcined C –AlPO, #; (b) calcined C –AlPO, %; (c) calcined TIPBz/C –AlPO, 6. 16 22 22 The filled symbols denote desorption.
result suggests that the pore size can be variable as well as those observed for mesoporous silicas [6–8], although the shrinkages during calcination must be taken into consideration. The BET surface areas were above 700 m2 g−1 and the pore volumes increased with the pore diameters. Although the tendency of the increase in the pore volumes is similar to those observed for mesoporous silicas [58,59], the surface areas of mesoporous silicas generally show constant values about 1000 m2 g−1. Because the wall thicknesses (1.4–1.5 nm) of the calcined products shown in Table 2 seemed to be larger than those of mesoporous silicas [59,60], the BET surface areas of the mesoporous AlPOs were above 700 m2 g−1. 3.3. Adsorption properties of mesoporous AlPOs In the preparation of typical mesoporous silicas, alkyltrimethylammonium cation surfactants have generally been used as structure-directing organic assemblies and the pore diameters in the case of
C TMA cations are around 3 nm. In the present 16 study, the calcined C –AlPO with a pore diameter 22 of 2.8 nm was employed for adsorption measurements as a representative of the mesoporous AlPOs. The water and benzene adsorption isotherms are shown in Fig. 7. The water adsorption isotherm showed type IV behavior. The benzene adsorption isotherm showed type IV behavior and the increase of adsorbed benzene due to capillary condensation was observed at a relative pressure in the range of ca 0.2–0.4. This value is higher than that observed for a mesoporous silica [20]. These adsorption data indicate that the inner surface of the mesoporous AlPO is relatively hydrophilic. The hysteresis in the benzene isotherm possibly indicates the interaction between benzene and the surface of the mesopores. For the water adsorption–desorption isotherm, the end point at P/P =0.1 in the desorption was located at about 0 0.3 g g−1, being larger than that of mesoporous silica [20]. The 27Al MAS NMR spectrum of the calcined C –AlPO after adsorption–desorption of 22
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Fig. 7. (a) Water and (b) benzene adsorption isotherms of calcined C –AlPO. Filled symbols denote desorption. Inset: 27Al MAS 22 NMR spectrum of calcined C –AlPO after water adsorption measurement. 22
water showed an intense peak at −5 ppm due to a considerable amount of six-coordinated Al. This result indicates that the mesoporous AlPOs have hydrophilic surface due to four-coordinated Al in the framework to which water molecules can access. Thamm et al. [61] speculated that a sudden increase in the water adsorption for AlPO -5 may 4 be due to coordinated water. The 27Al MAS NMR spectra of hydrated AlPO -n show not only 4 four-coordinated Al but also six-coordinated Al [62,63]. These reports support the consideration described above. In contrast, Vaudry et al. [28] reported that nheptane and neopentane are adsorbed on mesoporous alumina in a very low relative pressure (P/P ; 0.05) and Komarneni et al. [39] reported 0 that mesoporous alumina has a somewhat hydrophilic surface, based on water adsorption data. However, the amount of adsorbed water on the surface of the mesoporous alumina seems to be small. In addition, it has been reported that mesoporous silicas have hydrophobic surfaces [7,8,11– 13,20,39]. Although four-coordinated Al atoms exist in mesoporous aluminosilicates, the water adsorption isotherms are type V; the mesoporous aluminosilicates also have hydrophobic surfaces [64]. Moreover, Ta–TMS1 has been reported to have a high adsorption capacity for butane [33– 37]. The mesoporous AlPOs in this study are the
first example that mesoporous materials with ordered mesopores prepared by using surfactant assemblies have comparatively hydrophilic surfaces. We cannot discuss here the difference in the adsorption properties between the mesoporous AlPOs and microporous AlPO -n because the sur4 face structures of the mesoporous AlPOs are not clear at present. Further studies are necessary on the surface structures as well as on the surface properties of the materials doped with other units, such as SiO and various transition metal oxides. 4 Moreover, because mesoporous AlPO materials reported by other researchers [21–25] are different from the mesoporous AlPOs prepared in this study, the relationship among compositions, framework structures, and adsorption properties of those mesoporous AlPO materials should be investigated further for the purpose of enlargement of applications of mesoporous AlPO materials.
4. Conclusions Thermally stable mesoporous AlPO materials with pore sizes in the range 1.8–3.9 nm were successfully synthesized from hexagonal mesostructured alkyltrimethylammonium–AlPO materials with less condensed frameworks. From water
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and benzene adsorption measurements, the mesoporous AlPO materials exhibited hydrophilic nature of the mesopores. The method presented here, including the use of long-chain surfactant with an auxiliary additive and a controlled pyrolysis, could be applicable for the preparation of various mesoporous materials from less condensed inorganic-unit–surfactant mesostructured materials which normally afford microporous materials caused by substantial shrinkage. Although the shrinkage during calcination can be a sort of unfavorable point for the preparation of mesoporous materials, this phenomenon results in the formation of narrower pore sizes which would be suitable for some purposes such as shape-selective reactions for fine chemicals.
Acknowledgement The authors are grateful to Mr. M. Fuziwara, Materials Characterization Central Laboratory, Waseda University, for his TEM measurements and useful discussion. K.K. acknowledges the financial assistance from Grant-in-Aid for the Special Priority Area by the Ministry of Education, Science, and Culture of the Japanese Government.
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