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Synthesis of a thermally stable mesoporous aluminophosphate by using sodium aluminate as precursor
Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 40–44
Synthesis of a thermally stable mesoporous aluminophosphate by using sodium aluminate as precursor Li-Ngee Ho, Shiro Yukushima, Rie Morikawa, Naoko Asaka, Hiroyasu Nishiguchi, Katsutoshi Nagaoka, Yusaku Takita ∗ Department of Applied Chemistry, Engineering, Oita University, Dannoharu 700, Oita 8701192, Japan Received 2 November 2004; received in revised form 17 May 2005; accepted 17 May 2005 Available online 24 August 2005
Abstract Sodium aluminate has been used as the aluminium precursor in the synthesis of mesoporous aluminophosphate where cetylpyridinium chloride (CTPC) was used as structure directing agent. Synthesis of the materials was performed in a broad range of pH from 3 to 10. The obtained materials were characterized by using XRD, TEM, ICP, nitrogen sorption isotherms, solid state NMR, TG-DTA and FTIR. The samples prepared at pH 5–8 were thermally stable up to 450 ◦ C and presented specific surface areas in the range of 420–640 m2 g−1 after calcination for removal of the surfactant material. TEM image revealed the obtained mesoporous aluminophosphate possessed disordered porous structures. DH analysis derived from the adsorption branch showed that the average pore size of the sample was 2.6 nm in the 5–8 pH range. © 2005 Elsevier B.V. All rights reserved. Keywords: Amorphous material; Aluminophosphate; Chemical synthesis; Mesoporous materials
1. Introduction Since the discovery of microporous aluminophosphate by Wilson et. al. [1], these inorganic molecular sieves have received a great deal of attention as potential catalysts. The typical AlPO molecular sieves consist of alternating tetrahedrally AlO4 and PO4 connected by shared oxygen atoms which lead to an electronically neutral framework. Recently, lots of efforts have been devoted to the development of new synthesis of mesoporous AlPO. These materials are attractive because they possess the properties of microporous analogues, which enable the incorporation of heteroatoms into the framework, whereas, in mesoporous systems, the presence of larger pores allows the access of bulky organic molecules to the inner surface [2]. The synthesis of thermally stable mesoporous aluminophosphates is very few. Most of these materials have lamellar or hexagonal structures that are thermally unstable and therefore, collapse upon template ∗
Corresponding author. Tel.: +81 97 554 7896; fax: +81 97 554 7979. E-mail addresses: [email protected], [email protected] (Y. Takita). 0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.05.015
removal by calcination [3–7]. For example, Feng et al. [7] reported the preparation of mesoporous aluminophosphate by using a cationic ammonium cetyltrimethylammonium bromide as surfactant under aqueous conditions in the presence of HF as reactant. However, the product was thermally unstable and transformed into a lamellar phase and collapsed upon calcination for template removal. Incorporation of Ti into the AlPO framework has successfully increased the thermal stability of the mesostructure. For example, the synthesis of thermally stable Ti substituted mesoporous AlPOs had been reported by both Zhao et al. [8] and Kapoor et al. [9]. Although the synthesis of the mesoporous AlPO using cationic ammonium surfactant (cetyltrimethylammonium bromide or cetyltrimethylammonium chloride) in aqueous conditions is quite well established, the synthesis conditions always involves the application of hydrothermal treatment and use of TMAOH or TEAOH in the pH adjustment [9,10]. In this study, sodium aluminate (NaAlO2 ) was used as the aluminium precursor in the synthesis of mesoporous aluminophosphate. Compared to the commonly used aluminium alkoxide which is hardly dissolved in water, sodium
L.-N. Ho et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 40–44
aluminate dissolves easily in water to form hydroxide ions, Al(OH)4 − : NaAlO2 + 2H2 O → Na+ Al(OH)4 −
(1)
Several reported Raman and infrared studies have shown that the predominant anionic species present in supersaturated caustic aluminate solutions are the monomeric tetrahydroxyaluminate ion [Al(OH)4 − ], with a dimer or polymer indicated as a minor species [11–13]. These four-coordinated ions Al(OH)4 − would react with H3 PO4 to form four-coordinated Al framework in the aluminophosphate. Besides, sodium aluminate is a very caustic liquid (pH 14) which works with H3 PO4 as an acid–base pair.
2. Experimental 2.1. Synthesis In a typical synthesis, 0.1 mol of H3 PO4 (85%, Wako) was added into 0.021 mol of cetylpyridinium chloride, CTPC (Tokyo Kasei) in 42 g of water under constant stirring. This mixture was then dropped into a beaker simultaneously with a solution containing 0.1 mol of NaAlO2 (Kishida Chemical) and 126 g of water. Initially, both acidic and alkaline precursors were dropped at the same speed until the obtained gel reached 20 ml. The pH was then adjusted by controlling the dropping speed of the precursor. For instant, in the case of acidic condition, the acidic precursor was dropped at a higher speed than the alkaline precursor in order to reach and then maintain at the desirable pH. The procedure is inverted, that is the alkaline precursor is added at a higher speed for the synthesis in alkaline conditions. The final gel was obtained when one of the precursor solutions was finished up. It was then allowed to age at ambient conditions for 24 h. This synthesis was carried out within a molar gel composition of: Al2 O3 (0.6–1.0): P2 O5 (0.6–1.0): CTPC (0.06–0.1): H2 O (28–46.5). The resultant precipitate was washed a few times with deionized water to remove all the excess precursor and filtered before it was dried at 70 ◦ C. The as synthesized sample was then heated at 450 ◦ C in N2 for 2 h, followed by calcinations in air at 450 ◦ C for another 4 h to remove all the surfactant materials. 2.2. Characterization The obtained mesoporous AlPO was characterized by using X-ray diffraction (XRD), ICP, TEM, solid state NMR, nitrogen adsorption–desorption isotherms, and thermogravimetric and differential thermal analysis (TG-DTA). The powder XRD patterns of the as synthesized and calcined samples were recorded on a standard Rigaku (RINT 2500) diffractometer. Composition of the Al and P in the sample was determined by inductively coupled plasma atomic emission spectroscopy (Perkin-Elmer 3300DV Spectrometer) of sam-
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ples dissolved by the following procedure: 0.07 g of sample was dissolved by 2 ml concentrated HNO3 and 6 ml concentrated HCl in a 100 ml volumetric flask and allowed to stand for one night. The solution was then diluted by deionized water until 100 ml before measurement of ICP. The shape and distribution of pores in the sample was observed by transmission electron microscope (JEM-2010, JEOL). 27 Al MAS NMR was measured with a JEOL CMX300 spectrometer. A resonance frequency of 78.27 MHz and magic angle spinning rate of 5 kHz were employed. All measurements were carried out at room temperature. The solution of Al(NO3 )3 was used as external standard reference for aluminium chemical shifts. Besides, nitrogen adsorption–desorption isotherm of the sample was measured by using a standard instrument, Belsorp-mini from BEL, Japan. The sample was heated at 300 ◦ C for 2 h before the measurement. The synthesized sample was investigated by TG-DTA method on a standard instrument, ThermoPlus TG8120 from Rigaku. The sample was heated in the atmosphere of air at heating rate of 10 ◦ C/min from room temperature to 800 ◦ C. Fourier transform infrared (FTIR) spectra of the sample were measured on an instrument, System 2000 FTIR, from Perkin-Elmer.
3. Results and Discussion 3.1. XRD and TEM Fig. 1a shows the XRD patterns of the as synthesized sample prepared at a range of pH from 3 to 10. There was a single diffraction peak observed at around 1.5◦ for each sample prepared at pH condition 5–8 which shows the existence of the porous structures. The set of mesopores are disordered and non-parallel pores are proposed as reported by Davis et al. [14]. This is confirmed by the TEM results where disordered porous structures were observed as shown in Fig. 2. There was no diffraction peak observed for samples prepared at pH 3 and 4 whereas the diffraction peak of the sample gradually disappeared with increasing pH from 9 to 10 indicating the absence of porous structure in these samples. Fig. 1b shows the XRD patterns of the samples obtained after calcinations. The unique diffraction peak remained for each sample prepared at the pH range of 5–8, where the porous structures still remained after calcination process indicating that the mesoporous aluminophosphates obtained are thermally stable. 3.2. ICP The compositions of Al and P in the samples were analyzed by ICP and the results were shown in Table 1. The Al/P mol ratio of the sample prepared at pH 5 was equal to one suggesting that the sample is pure AlPO4 . The Al/P ratio of samples prepared at pH 6–8 was slightly higher compare to sample prepared at pH 5 indicating, on one hand, that there
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L.-N. Ho et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 40–44 Table 1 Al/P molar ratio and BET surface areas of samples prepared at different pH Samples prepared at different pH
Molar ratio of Al/P
BET surface areas (m2 g−1 )
3 4 5 6 7 8 9 10
0.5 0.7 1.0 1.1 1.1 1.4 1.6 2.5
– – 640 525 450 420 138 –
might be an incomplete condensation of PO4 units and presence of Al2 O3 as an impurity [10]. On the other hand, it was observed that the Al/P ratio increased following the increase of pH during the synthesis. It was reported in a few studies [7,9,10,15] that synthesis of mesoporous aluminophosphate under alkaline conditions always leads to Al/P ratio above 1. This may imply that alkaline conditions favor incomplete condensation of the [PO4 ]−3 units, since the relatively high Al/P ratio is constructed of relatively more [AlO6 ]−9 structural units whereas lower Al/P ratio is composed of [AlO4 ]−5 and [PO4 ]−3 structural units and hydroxylated phosphorus species [10,15]. Samples prepared at pH <5 possessed Al/P ratio less than 1 suggesting that [AlO4 ]−5 and [PO4 ]−3 structural units alone cannot easily form an ideal hexagonal mesoporous framework arrangement as a porous structure was not formed in these samples. This is similar to the findings of Luan et al. [15] in the synthesis of mesoporous aluminophosphate. Fig. 1. (a) XRD patterns of the as synthesized samples prepared in the range of pH 3–10 and (b) XRD patterns of the calcined samples prepared in the range of pH 5–8.
3.3. Nitrogen adsorption–desorption isotherm Fig. 3 depicts the nitrogen adsorption–desorption isotherm of the sample prepared at pH 5. According to IUPAC [16], this is a typical Type H4 hysteresis featuring parallel and
Fig. 2. TEM image of the as synthesized mesoporous AlPO prepared at pH 5.
Fig. 3. Nitrogen adsorption–desorption isotherms at 77 K and DH plot based on adsorption branch for mesoporous AlPO prepared at pH 5.
L.-N. Ho et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 40–44
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Fig. 4. Nitrogen t-plot of sample prepared at pH 5.
almost horizontal branches and their occurrence has been due to adsorption–desorption in narrow slit-like pores or irregular shapes [17]. This observation is consistent with the TEM image of the sample which revealed a disordered porous structure. Spectacular effects of delayed capillary evaporation are observed for this Type H4 hysteresis loop, where desorption can be observed at much lower pressure than those attributable to the actual pore size [18,19]. Therefore, adsorption rather than desorption data should be employed in porosity determinations to avoid artifacts and to improve accuracy of the pore size determination [20]. Based on DH analysis, the pore size distribution of the sample prepared at pH 5 derived from the adsorption branch depicts a maximum at 2.6 nm indicating that average pore size of the sample is 2.6 nm. Besides, BET surface area of the samples obtained at pH 5–8 ranged from 420 to 640 m2 g−1 (Table 1). These values are compatible with the values reported by other authors who also used alkyltrimethylammonium cationic surfactant as template and removed the surfactant by calcinations [10,21]. Besides, nitrogen t-plots of the sample prepared at pH 5 (Fig. 4) has a significant upward deviation in the volume adsorbed which is the characteristic of the mesoporous materials [20,22]. 3.4.
27 Al
MAS NMR
The local structure of 27 Al ion of the sample was studied by means of MAS NMR and the results are shown in Fig. 5a and b. The 27 Al NMR spectra for the as synthesized sample and sample after calcinations gave two signals at 40 and −10 ppm. The chemical shift at 40 ppm is assigned to the tetrahedrally coordinated Al species, such as Al(OP)4 and/or Al(OP)4−x (OH)x which depends on the chemical shifts and the asymmetric profile [23,24]. However, the chemical shifts at −10 ppm indicative of Al in octahedral coordination where the coordination may completed with additional H2 O or OH groups, such as Al(OP)x (H2 O)6−x [25]. After calcinations, the relative intensity of the signal at −10 ppm was severely
Fig. 5. 27 Al MAS NMR spectra for (a) as synthesized sample and (b) sample after calcinations.
reduced, which suggests that most of the water and surfactant molecules were removed from the sample leaving more Al(OP)4 units in the mesoporous AlPO. A weak signal is observed at around −10 ppm in the spectra as shown in Fig. 5b which may be due to a slight degree of rehydration of fourcoordinated Al after calcinations [10]. 3.5. TG-DTA and FTIR TG-DTA was carried out on the as synthesized sample prepared at pH 5. The first weight loss at 30–200 ◦ C associated with an endothermic step can be assigned to the desorption of water which was physisorbed on the interparticle surface of the sample. This was followed by a second weight loss was about 12% at the range of 230–330 ◦ C associated with an exothermic peak can be assigned to the decomposition of CTPC. Finally, the weight loss above 330 ◦ C is probably due to the further calcinations of the organic fragments or carbonaceous residues. In order to confirm the complete removal of surfactant from the sample, thermal analysis was also done on the calcined sample. The first weight loss around 21% from room temperature to 200 ◦ C is ascribed to loosely bound water. Above 200 ◦ C, there are no obvious weight losses observed indicating that the surfactant was totally removed after calcinations process. Besides, FTIR spectra of the calcined sample (Fig. 6) shows that the C–H stretching bands at 2950–2850 cm−1 are absent after calcinations confirming that no surfactant remained in the final product.
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L.-N. Ho et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 40–44
minate as precursor and cetylpyridinium chloride as structure directing agent without the use of TMAOH in pH control. Porous structure was formed at the pH range of 5–8. The obtained mesoporous AlPO possessed disordered structure with average pore size at 2.6 nm. The samples are thermally stable up to 450 ◦ C and the specific surface area after calcinations ranged from 420–640 m2 g−1 .
References
Fig. 6. FTIR spectra of the as synthesized sample and calcined sample prepared at pH 5.
3.6. Formation mechanism of mesostructured aluminophosphate Based on these results, a proposal of mechanism for the mesostructured aluminophosphate can be advanced: sodium aluminate dissolves in water to form monomeric tetrahydroxy-aluminate ion [Al(OH)4 − ]: NaAlO2 + 2H2 O → Na+ Al(OH)4 −
(1)
These Al(OH)4 − will react with the mixture containing H3 PO4 and cetylpyridinium chloride where reaction started to take place to form the oligomeric AlPO species. According to the model proposed by Stucky et al. [26], in the case of ionic surfactant, formation of the mesostructured materials is mainly governed by electrostatic interactions which will then lead to either lamellar, hexagonal or cubic structure. However, only a disordered mesoporous structure could be obtained in this study. It has reported that formation of aluminophosphate mesostructured phases appears more sensitive to some of the starting materials compared to silica-based material [27]. For instant, when NaOH, KOH or NH4 OH were used as the base in place of NMe4 OH in the phosphate system, only amorphous materials were obtained. However, NaOH and NMe4 OH have been successfully used in the synthesis of silica-based mesoporous materials [27]. In this study, sodium aluminate was used as the aluminium precursor. The existence of these Na+ ions might affect the mesostructured formation in the system, which leads to a disordered structure in the materials. 4. Conclusion The synthesis of mesoporous aluminophosphate under aqueous conditions has been performed by using sodium alu-
[1] S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M. Flanigen, J. Am. Chem. Soc. 104 (1982) 1146. [2] E. Gianotti, E.C. Oliveira, S. Coluccia, H.O. Pastore, L. Marchese, Inorg. Chim. Acta 349 (2003) 259. [3] J.O. Perez, O.R.B. Borade, A. Clearfield, J. Mol. Struct. 470 (1998) 221. [4] Z.Y. Yuan, T.H. Chen, J.Z. Wang, H.X. Li, Colloids Surface: A 179 (2001) 253. [5] S. Oliver, A. Kuperman, N. Coombs, A. Lough, G.A. Ozin, Nature 378 (1995) 47. [6] Y.Z. Khimyak, J. Klinowski, Chem. Mater. 10 (1998) 2258. [7] P. Feng, Y. Xia, J. Feng, X. Bu, G.D. Stucky, Chem. Commun. (1997) 949. [8] X.S. Zhao, G.Q. (Max) Lu, Micro. Meso. Mater. 44–45 (2001) 185. [9] M.P. Kapoor, A. Raj, Appl. Catal.: Gen. 203 (2000) 311. [10] T. Kimura, Y. Sugahara, K. Kuroda, Micro. Meso. Mater. 22 (1998) 115. [11] E. Lippincott, J. Psellos, M. Tobin, J. Chem. Phys. 20 (1952) 536. [12] R.J. Moolenaar, J.C. Evans, L.D. McKeever, J. Chem. Phys. 74 (1970) 3629. [13] L.A. Carreira, V.A. Marani, J. Chem. Phys. 45 (1966) 2216. [14] C.Y. Chen, S.Q. Xiao, M.E. Davis, Micro. Mater. 4 (1995) 1. [15] Z. Luan, D. Zhao, H. He, J. Klinowski, L. Kevan, J. Phys. Chem. B 102 (1998) 1250. [16] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [17] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169. [18] H.P. Lin, S.T. Wong, C.Y. Mou, C.Y. Tang, J. Phys. Chem. B 104 (2000) 8967. [19] P.J. Kooyman, M.J. Verhoef, E. Prouzet, Stud. Surf. Sci. Catal. 129 (2000) 535. [20] R.I. Nooney, D. Thirunavukkarasu, A.E. Ostafin, Y. Chen, R. Josephs, Micro. Meso. Mater. 75 (2004) 183. [21] S. Cabrera, J.E. Haskouri, C. Guillem, A.B. Porter, S. Mendioroz, M.D. Marcos, P. Amoros, Chem. Commun. (1999) 333. [22] A. Galarneau, H. Cambon, F. Di Renzo, F. Fajula, Langmuir 17 (2001) 8328. [23] T. Kimura, Y. Sugahara, K. Kuroda, Phosphorus Res. Bull. 6 (1996) 205. [24] R.F. Mortlock, A.T. Bell, C.J. Radke, J. Phys. Chem. 97 (1993) 767. [25] M. Froba, M. Tiemann, Chem. Mater. 10 (1998) 3475. [26] Q. Huo, D.I. Margolese, U. Clesla, D.K. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Sch¨uth, G.D. Stucky, Chem. Mater. 97 (1994) 2373. [27] P. Feng, Y. Xia, J. Feng, X. Bu, G.D. Stucky, Chem. Commun. (1997) 949.
Synthesis of a thermally stable mesoporous aluminophosphate by using sodium aluminate as precursor Li-Ngee Ho, Shiro Yukushima, Rie Morikawa, Naoko Asaka, Hiroyasu Nishiguchi, Katsutoshi Nagaoka, Yusaku Takita ∗ Department of Applied Chemistry, Engineering, Oita University, Dannoharu 700, Oita 8701192, Japan Received 2 November 2004; received in revised form 17 May 2005; accepted 17 May 2005 Available online 24 August 2005
Abstract Sodium aluminate has been used as the aluminium precursor in the synthesis of mesoporous aluminophosphate where cetylpyridinium chloride (CTPC) was used as structure directing agent. Synthesis of the materials was performed in a broad range of pH from 3 to 10. The obtained materials were characterized by using XRD, TEM, ICP, nitrogen sorption isotherms, solid state NMR, TG-DTA and FTIR. The samples prepared at pH 5–8 were thermally stable up to 450 ◦ C and presented specific surface areas in the range of 420–640 m2 g−1 after calcination for removal of the surfactant material. TEM image revealed the obtained mesoporous aluminophosphate possessed disordered porous structures. DH analysis derived from the adsorption branch showed that the average pore size of the sample was 2.6 nm in the 5–8 pH range. © 2005 Elsevier B.V. All rights reserved. Keywords: Amorphous material; Aluminophosphate; Chemical synthesis; Mesoporous materials
1. Introduction Since the discovery of microporous aluminophosphate by Wilson et. al. [1], these inorganic molecular sieves have received a great deal of attention as potential catalysts. The typical AlPO molecular sieves consist of alternating tetrahedrally AlO4 and PO4 connected by shared oxygen atoms which lead to an electronically neutral framework. Recently, lots of efforts have been devoted to the development of new synthesis of mesoporous AlPO. These materials are attractive because they possess the properties of microporous analogues, which enable the incorporation of heteroatoms into the framework, whereas, in mesoporous systems, the presence of larger pores allows the access of bulky organic molecules to the inner surface [2]. The synthesis of thermally stable mesoporous aluminophosphates is very few. Most of these materials have lamellar or hexagonal structures that are thermally unstable and therefore, collapse upon template ∗
Corresponding author. Tel.: +81 97 554 7896; fax: +81 97 554 7979. E-mail addresses: [email protected], [email protected] (Y. Takita). 0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.05.015
removal by calcination [3–7]. For example, Feng et al. [7] reported the preparation of mesoporous aluminophosphate by using a cationic ammonium cetyltrimethylammonium bromide as surfactant under aqueous conditions in the presence of HF as reactant. However, the product was thermally unstable and transformed into a lamellar phase and collapsed upon calcination for template removal. Incorporation of Ti into the AlPO framework has successfully increased the thermal stability of the mesostructure. For example, the synthesis of thermally stable Ti substituted mesoporous AlPOs had been reported by both Zhao et al. [8] and Kapoor et al. [9]. Although the synthesis of the mesoporous AlPO using cationic ammonium surfactant (cetyltrimethylammonium bromide or cetyltrimethylammonium chloride) in aqueous conditions is quite well established, the synthesis conditions always involves the application of hydrothermal treatment and use of TMAOH or TEAOH in the pH adjustment [9,10]. In this study, sodium aluminate (NaAlO2 ) was used as the aluminium precursor in the synthesis of mesoporous aluminophosphate. Compared to the commonly used aluminium alkoxide which is hardly dissolved in water, sodium
L.-N. Ho et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 40–44
aluminate dissolves easily in water to form hydroxide ions, Al(OH)4 − : NaAlO2 + 2H2 O → Na+ Al(OH)4 −
(1)
Several reported Raman and infrared studies have shown that the predominant anionic species present in supersaturated caustic aluminate solutions are the monomeric tetrahydroxyaluminate ion [Al(OH)4 − ], with a dimer or polymer indicated as a minor species [11–13]. These four-coordinated ions Al(OH)4 − would react with H3 PO4 to form four-coordinated Al framework in the aluminophosphate. Besides, sodium aluminate is a very caustic liquid (pH 14) which works with H3 PO4 as an acid–base pair.
2. Experimental 2.1. Synthesis In a typical synthesis, 0.1 mol of H3 PO4 (85%, Wako) was added into 0.021 mol of cetylpyridinium chloride, CTPC (Tokyo Kasei) in 42 g of water under constant stirring. This mixture was then dropped into a beaker simultaneously with a solution containing 0.1 mol of NaAlO2 (Kishida Chemical) and 126 g of water. Initially, both acidic and alkaline precursors were dropped at the same speed until the obtained gel reached 20 ml. The pH was then adjusted by controlling the dropping speed of the precursor. For instant, in the case of acidic condition, the acidic precursor was dropped at a higher speed than the alkaline precursor in order to reach and then maintain at the desirable pH. The procedure is inverted, that is the alkaline precursor is added at a higher speed for the synthesis in alkaline conditions. The final gel was obtained when one of the precursor solutions was finished up. It was then allowed to age at ambient conditions for 24 h. This synthesis was carried out within a molar gel composition of: Al2 O3 (0.6–1.0): P2 O5 (0.6–1.0): CTPC (0.06–0.1): H2 O (28–46.5). The resultant precipitate was washed a few times with deionized water to remove all the excess precursor and filtered before it was dried at 70 ◦ C. The as synthesized sample was then heated at 450 ◦ C in N2 for 2 h, followed by calcinations in air at 450 ◦ C for another 4 h to remove all the surfactant materials. 2.2. Characterization The obtained mesoporous AlPO was characterized by using X-ray diffraction (XRD), ICP, TEM, solid state NMR, nitrogen adsorption–desorption isotherms, and thermogravimetric and differential thermal analysis (TG-DTA). The powder XRD patterns of the as synthesized and calcined samples were recorded on a standard Rigaku (RINT 2500) diffractometer. Composition of the Al and P in the sample was determined by inductively coupled plasma atomic emission spectroscopy (Perkin-Elmer 3300DV Spectrometer) of sam-
41
ples dissolved by the following procedure: 0.07 g of sample was dissolved by 2 ml concentrated HNO3 and 6 ml concentrated HCl in a 100 ml volumetric flask and allowed to stand for one night. The solution was then diluted by deionized water until 100 ml before measurement of ICP. The shape and distribution of pores in the sample was observed by transmission electron microscope (JEM-2010, JEOL). 27 Al MAS NMR was measured with a JEOL CMX300 spectrometer. A resonance frequency of 78.27 MHz and magic angle spinning rate of 5 kHz were employed. All measurements were carried out at room temperature. The solution of Al(NO3 )3 was used as external standard reference for aluminium chemical shifts. Besides, nitrogen adsorption–desorption isotherm of the sample was measured by using a standard instrument, Belsorp-mini from BEL, Japan. The sample was heated at 300 ◦ C for 2 h before the measurement. The synthesized sample was investigated by TG-DTA method on a standard instrument, ThermoPlus TG8120 from Rigaku. The sample was heated in the atmosphere of air at heating rate of 10 ◦ C/min from room temperature to 800 ◦ C. Fourier transform infrared (FTIR) spectra of the sample were measured on an instrument, System 2000 FTIR, from Perkin-Elmer.
3. Results and Discussion 3.1. XRD and TEM Fig. 1a shows the XRD patterns of the as synthesized sample prepared at a range of pH from 3 to 10. There was a single diffraction peak observed at around 1.5◦ for each sample prepared at pH condition 5–8 which shows the existence of the porous structures. The set of mesopores are disordered and non-parallel pores are proposed as reported by Davis et al. [14]. This is confirmed by the TEM results where disordered porous structures were observed as shown in Fig. 2. There was no diffraction peak observed for samples prepared at pH 3 and 4 whereas the diffraction peak of the sample gradually disappeared with increasing pH from 9 to 10 indicating the absence of porous structure in these samples. Fig. 1b shows the XRD patterns of the samples obtained after calcinations. The unique diffraction peak remained for each sample prepared at the pH range of 5–8, where the porous structures still remained after calcination process indicating that the mesoporous aluminophosphates obtained are thermally stable. 3.2. ICP The compositions of Al and P in the samples were analyzed by ICP and the results were shown in Table 1. The Al/P mol ratio of the sample prepared at pH 5 was equal to one suggesting that the sample is pure AlPO4 . The Al/P ratio of samples prepared at pH 6–8 was slightly higher compare to sample prepared at pH 5 indicating, on one hand, that there
42
L.-N. Ho et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 40–44 Table 1 Al/P molar ratio and BET surface areas of samples prepared at different pH Samples prepared at different pH
Molar ratio of Al/P
BET surface areas (m2 g−1 )
3 4 5 6 7 8 9 10
0.5 0.7 1.0 1.1 1.1 1.4 1.6 2.5
– – 640 525 450 420 138 –
might be an incomplete condensation of PO4 units and presence of Al2 O3 as an impurity [10]. On the other hand, it was observed that the Al/P ratio increased following the increase of pH during the synthesis. It was reported in a few studies [7,9,10,15] that synthesis of mesoporous aluminophosphate under alkaline conditions always leads to Al/P ratio above 1. This may imply that alkaline conditions favor incomplete condensation of the [PO4 ]−3 units, since the relatively high Al/P ratio is constructed of relatively more [AlO6 ]−9 structural units whereas lower Al/P ratio is composed of [AlO4 ]−5 and [PO4 ]−3 structural units and hydroxylated phosphorus species [10,15]. Samples prepared at pH <5 possessed Al/P ratio less than 1 suggesting that [AlO4 ]−5 and [PO4 ]−3 structural units alone cannot easily form an ideal hexagonal mesoporous framework arrangement as a porous structure was not formed in these samples. This is similar to the findings of Luan et al. [15] in the synthesis of mesoporous aluminophosphate. Fig. 1. (a) XRD patterns of the as synthesized samples prepared in the range of pH 3–10 and (b) XRD patterns of the calcined samples prepared in the range of pH 5–8.
3.3. Nitrogen adsorption–desorption isotherm Fig. 3 depicts the nitrogen adsorption–desorption isotherm of the sample prepared at pH 5. According to IUPAC [16], this is a typical Type H4 hysteresis featuring parallel and
Fig. 2. TEM image of the as synthesized mesoporous AlPO prepared at pH 5.
Fig. 3. Nitrogen adsorption–desorption isotherms at 77 K and DH plot based on adsorption branch for mesoporous AlPO prepared at pH 5.
L.-N. Ho et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 268 (2005) 40–44
43
Fig. 4. Nitrogen t-plot of sample prepared at pH 5.
almost horizontal branches and their occurrence has been due to adsorption–desorption in narrow slit-like pores or irregular shapes [17]. This observation is consistent with the TEM image of the sample which revealed a disordered porous structure. Spectacular effects of delayed capillary evaporation are observed for this Type H4 hysteresis loop, where desorption can be observed at much lower pressure than those attributable to the actual pore size [18,19]. Therefore, adsorption rather than desorption data should be employed in porosity determinations to avoid artifacts and to improve accuracy of the pore size determination [20]. Based on DH analysis, the pore size distribution of the sample prepared at pH 5 derived from the adsorption branch depicts a maximum at 2.6 nm indicating that average pore size of the sample is 2.6 nm. Besides, BET surface area of the samples obtained at pH 5–8 ranged from 420 to 640 m2 g−1 (Table 1). These values are compatible with the values reported by other authors who also used alkyltrimethylammonium cationic surfactant as template and removed the surfactant by calcinations [10,21]. Besides, nitrogen t-plots of the sample prepared at pH 5 (Fig. 4) has a significant upward deviation in the volume adsorbed which is the characteristic of the mesoporous materials [20,22]. 3.4.
27 Al
MAS NMR
The local structure of 27 Al ion of the sample was studied by means of MAS NMR and the results are shown in Fig. 5a and b. The 27 Al NMR spectra for the as synthesized sample and sample after calcinations gave two signals at 40 and −10 ppm. The chemical shift at 40 ppm is assigned to the tetrahedrally coordinated Al species, such as Al(OP)4 and/or Al(OP)4−x (OH)x which depends on the chemical shifts and the asymmetric profile [23,24]. However, the chemical shifts at −10 ppm indicative of Al in octahedral coordination where the coordination may completed with additional H2 O or OH groups, such as Al(OP)x (H2 O)6−x [25]. After calcinations, the relative intensity of the signal at −10 ppm was severely
Fig. 5. 27 Al MAS NMR spectra for (a) as synthesized sample and (b) sample after calcinations.
reduced, which suggests that most of the water and surfactant molecules were removed from the sample leaving more Al(OP)4 units in the mesoporous AlPO. A weak signal is observed at around −10 ppm in the spectra as shown in Fig. 5b which may be due to a slight degree of rehydration of fourcoordinated Al after calcinations [10]. 3.5. TG-DTA and FTIR TG-DTA was carried out on the as synthesized sample prepared at pH 5. The first weight loss at 30–200 ◦ C associated with an endothermic step can be assigned to the desorption of water which was physisorbed on the interparticle surface of the sample. This was followed by a second weight loss was about 12% at the range of 230–330 ◦ C associated with an exothermic peak can be assigned to the decomposition of CTPC. Finally, the weight loss above 330 ◦ C is probably due to the further calcinations of the organic fragments or carbonaceous residues. In order to confirm the complete removal of surfactant from the sample, thermal analysis was also done on the calcined sample. The first weight loss around 21% from room temperature to 200 ◦ C is ascribed to loosely bound water. Above 200 ◦ C, there are no obvious weight losses observed indicating that the surfactant was totally removed after calcinations process. Besides, FTIR spectra of the calcined sample (Fig. 6) shows that the C–H stretching bands at 2950–2850 cm−1 are absent after calcinations confirming that no surfactant remained in the final product.
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minate as precursor and cetylpyridinium chloride as structure directing agent without the use of TMAOH in pH control. Porous structure was formed at the pH range of 5–8. The obtained mesoporous AlPO possessed disordered structure with average pore size at 2.6 nm. The samples are thermally stable up to 450 ◦ C and the specific surface area after calcinations ranged from 420–640 m2 g−1 .
References
Fig. 6. FTIR spectra of the as synthesized sample and calcined sample prepared at pH 5.
3.6. Formation mechanism of mesostructured aluminophosphate Based on these results, a proposal of mechanism for the mesostructured aluminophosphate can be advanced: sodium aluminate dissolves in water to form monomeric tetrahydroxy-aluminate ion [Al(OH)4 − ]: NaAlO2 + 2H2 O → Na+ Al(OH)4 −
(1)
These Al(OH)4 − will react with the mixture containing H3 PO4 and cetylpyridinium chloride where reaction started to take place to form the oligomeric AlPO species. According to the model proposed by Stucky et al. [26], in the case of ionic surfactant, formation of the mesostructured materials is mainly governed by electrostatic interactions which will then lead to either lamellar, hexagonal or cubic structure. However, only a disordered mesoporous structure could be obtained in this study. It has reported that formation of aluminophosphate mesostructured phases appears more sensitive to some of the starting materials compared to silica-based material [27]. For instant, when NaOH, KOH or NH4 OH were used as the base in place of NMe4 OH in the phosphate system, only amorphous materials were obtained. However, NaOH and NMe4 OH have been successfully used in the synthesis of silica-based mesoporous materials [27]. In this study, sodium aluminate was used as the aluminium precursor. The existence of these Na+ ions might affect the mesostructured formation in the system, which leads to a disordered structure in the materials. 4. Conclusion The synthesis of mesoporous aluminophosphate under aqueous conditions has been performed by using sodium alu-
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