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Facile synthesis of thermally stable mesoporous crystalline alumina by using a novel cation–anion double hydrolysis method
Materials Letters 59 (2005) 3128 – 3131 www.elsevier.com/locate/matlet
Facile synthesis of thermally stable mesoporous crystalline alumina by using a novel cation–anion double hydrolysis method Peng Bai, Wei Xing, Zhaoxia Zhang, Zifeng Yan* State Key Laboratory for Heavy Oil Processing, PetroChina Key laboratory of Catalysis, China University of Petroleum, Dongying 257061, China Received 18 April 2005; accepted 26 May 2005 Available online 13 June 2005
Abstract By using the double hydrolysis of Al3+ and AlO2- in the presence of Pluronic P123 as the structure directing agent, mesoporous crystalline alumina was easily synthesized, which exhibits high specific surface area, narrow pore size distribution, and excellent thermal stability. D 2005 Elsevier B.V. All rights reserved. Keywords: Mesoporous; Alumina; Double hydrolysis
1. Introduction Aluminas are important industrial chemicals that have found wide application in adsorbents, ceramics, abrasives, catalysts and catalyst supports. [1 –3] However, because the traditional alumina materials only possess textural porosity and lack selective pore structure, their application is strictly limited. [4] Mesoporous materials have shown excellent structural properties, such as high surface area, narrow pore size distribution, which render them highly potential candidates as catalysts and catalysts supports for the direct activation of large organic molecules. Although several papers were reported on the synthesis of mesoporous alumina by utilizing cationic, anionic and non-ionic templates as the structural directing agents since the first in 1996, [4 –14] these methods usually use the expensive and toxic aluminum alkoxides as precursors, and/ or strictly control the synthetic conditions, which make them not convenient for the industrial scaling-up. Double hydrolysis reaction is a special kind of metathesis reaction, which involves the ionic compounds swapping their ionic partners. In the case of double hydrolysis, both the cation of one reactant and the anion of the other are readily hydrolyzed in aqueous solution. When these two kinds of * Corresponding author. Tel.: +86 546 8391527; fax: +86 546 8391971. E-mail address: [email protected] (Z. Yan). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.033
ions encounter each other, their hydrolysis would be mutually promoted. Therefore the hydrolysis balance is broken, and the reaction would irreversibly proceed to a complete extent in a real sense that other methods cannot achieve. Here, by adopting this double hydrolysis strategy, we obtained mesoporous alumina with relatively high surface area, narrow pore size distribution and excellent thermal stability using the cheap inorganic salts, Al(NO3)3 and NaAlO2, in the presence of Pluronic P123 as the structural directing agent.
2. Experimental A typical synthesis was done as follows: 3.75g Al(NO3)3.9H2O and 2.83 g of Pluronic P123 were dissolved in 30 ml of deionized water, then 20 ml solution containing 2.46 g of NaAlO2 was dropped into the former under vigorous stirring. Immediately the gel resulted. After standing at room temperature for 12 h, the gel was transferred into an autoclave and aged at 353 K for 24 h. Then a transparent gel was obtained, after washing with deionized water; the solid was recovered by suction filtration and dried at 353 K in Vacuo. The resulting white solid was then calcined at 773 K for 4 h. X-ray diffraction (XRD) measurements were carried out at a speed of 0.01- s-1 by a Bruker Axs diffractometer
P. Bai et al. / Materials Letters 59 (2005) 3128 – 3131
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(Germany) with CuK radiation generated at 40 kV, 30 mA. The nitrogen adsorption of the samples were performed on Micromeritics TRISTAR 3000 analyzer and the samples were degassed at 673 K for 4 h prior to adsorption analysis. Adsorption isotherms were measured at liquid nitrogen temperature (77.3 K) over the interval of relative pressures from 0.01 to 0.998 using nitrogen of high purity. Specific surface area of the samples was calculated using the BET method. The mesopore distribution and mean pore size of the desorption branches of nitrogen adsorption isotherms were calculated using BJH method. Thermogravimetric and differential thermal analysis (TG-DTA, Shimadzu TGA 50 H) was performed with a heating rate of 2 K/min in air. The weight of the sample was 8.5 mg. Fig. 2. Small-angle XRD pattern of the as-synthesized mesostructured alumina.
3. Results and discussions The nitrogen sorption isotherm of the calcined alumina, shown in Fig. 1, is of typical type IV (defined by IUPAC), [15] which is the characteristic of mesoporous materials. The shape of the large hysteresis loop in the isotherms indicates that ‘‘ink-bottle’’ type pores may be present in the mesoporous alumina. The specific surface area is up to 342 m2/g when the as-synthesized sample was calcined at 773 K for 4 h. When the as-synthesized sample was calcined at 973 for 2 h, the BET surface area is reduced to 280 m2/g, which can be reasonably ascribed to the slight sintering of mesoporous alumina at such high calcination temperature. However, the isotherm has the same shape as that of the one
300
0.25
dV/dD (cm3/gnm)
Quantity absorbed (cm3/g)
250
calcined at 773 K, revealing the excellent thermal stability of the alumina samples. The pore size, calculated from the desorption branch by BJH method, shown in Fig. 1 (Inset), has narrow distribution over the range from 3.0 to 5.0 nm and is centered at 3.9 nm, indicating the mesoporous alumina has uniform pore size and ordered mesostructure. Powder X-ray diffraction (XRD) patterns of the as-synthesized and calcined alumina samples are shown in Figs. 2 and 3. The presence of a single diffraction line in the small-angle region further indicates the partially ordered mesopores in the calcined
200
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0.10
0.05
0.00
0
2
4
6
8
10
12
14
16
18
20
Pore Width (nm)
150
100
50
0.0
0.2
0.4
0.6
0.8
Relative pressure (P/Po) Fig. 1. N2 adsorption/desorption isotherms at 77 K of mesoporous alumina.
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P. Bai et al. / Materials Letters 59 (2005) 3128 – 3131
Fig. 3. Wide-angle XRD patterns of the as-synthesized (Boehmite) and calcined alumina (g-Alumina).
alumina. In the wide-angle regions, the as-synthesized samples exhibit diffraction lines assignable to boehmite, while the calcined products exhibit diffraction lines characteristic of g-alumina phase, indicating the crystalline pore wall in the calcined mesoporous alumina. This is quite different from the mesoporous aluminas with amorphous pore wall reported in the previous work, [4,5], [11 – 13], which usually have bad hydrothermal stability; for instance, they quickly lose their mesostructured framework when suspended in water even at ambient temperature. [14] By contrast, the mesoporous crystalline alumina synthesized in our work is expected to have better hydrothermal stability. The thermal gravimetry and differential thermal analysis (TGDTA) profiles of the as-synthesized alumina are shown in Fig. 4. The TG profile indicates a ca. 38% loss of weight from room temperature to 773 K. The DTG profile (not shown) exhibits two main peaks at 349, 546 K, which can be assigned to the elimination of physically adsorbed water and the burning of the surfactant, respectively, corresponding to the endothermic peak at 349 and exothermic peak at 546 K in the DTA profile. The endothermic peaks from 418 to 518 K can be attributed to the decomposition of the surfactant. The existence of the endother-
Fig. 4. TG-DTA profile of as-synthesized mesostructured g-Alumina.
mic peak at 635 K may be ascribed to the continued condensation reaction of surface hydroxyls and the phase transformation of boehmite to g-Al2O3 consistent with the gradual weight loss from 623 to 773 K. No obvious endothermic peaks can be observed in the temperature range from 773 K to 1173 K, indicating that no further phase transformation (from gAl2O3 to other crystalline alumina) takes place at higher temperatures. These results further prove the high thermal stability of the mesoporous alumina. It should be pointed out that the presence of Pluronic P123 is indispensable to synthesize mesoporous alumina with high surface area, as much lower surface area of the samples synthesized in the same condition without the addition of P123 was resulted, suggesting that Pluronic P123 is really acting as the templating agent in the formation of mesoporous structure of the alumina samples. The formation of mesostructure may undergo the real supramolecular assembly mechanism described everywhere, and the detailed reaction between Al3+, AlO2- and P123 may be proceeded in the [S-Al(OH)3H+](OH)-HAlO2 route, which can be illustrated in the following two reactions: P123
Al3þ þ H2 O W AlðOHÞ3 þ Hþ
ð1Þ
AlO 2 þ H2 O W HAlO2 þ OH
ð2Þ
In reaction (1), Al3+ is hydrolyzed into Al(OH)3, which deposits on the P123 micelles and adsorbs H+, thus the P123 is protonized and the supramolecule, [S-Al(OH)3H+] is formed. In reaction (2), AlO2- is hydrolyzed into HAlO2 and OH-. When the two solutions are mixed together, the above two reactions will be mutually promoted. HAlO2 will further deposit on the supramolecule, leading to the formation of mesostructured alumina inorganic species and P123 composite. However, the mechanism of the formation of pore wall is different from the sol – gel process induced by addition of ammonia or NaOH into the Al3+ solution, [13,16] since the addition of AlO2- directly introduces the AlO4 tetrahedron promoting the formation of crystallite seed into the reaction mixture, which may eventually form the nanocrystallite of gAl2O3 in the mesostructured framework.
P. Bai et al. / Materials Letters 59 (2005) 3128 – 3131
Compared with the former methods to prepare mesoporous aluminas, this novel method has many advantages. First, by using the cheap inorganic salts, Al(NO3)3 and NaAlO2, the utilization of expensive and toxic aluminum alkoxide is avoided. Second, by mixing the stoichiometric amount of Al3+ and AlO22-, the hydrolysis reaction quickly proceeds to a complete extent, and evades the complicated pH adjusting process [13,14], [16] and the rather long hydrolysis period. [10,11] These advantages render this method highly superior over the former ones for the industrial scaling-up.
4. Conclusions To sum up, we present a simple, effective and highly reproducible way to synthesize mesoporous g-alumina. The BET surface area is up to 342m2/g. The pore size has narrow distribution over the range from 3.0 to 5.0 nm and is centered at 3.9 nm. The mesoporous structure can remain stable up to 973 K. These results render the mesoporous alumina synthesized in our work high potential in the field of catalysis, adsorption and separation.
Acknowledgement China Natural Petroleum Corporation (CNPC) is gratefully acknowledged for the project grant (20020358-1).
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References [1] C. Misra, Industrial Alumina Chemicals, ACS Monograph 184 (1986)1996 (Washington). [2] G. Tournier, M. Lecroix-Repellin, G.M. Pajonk, Stud. Surf. Sci. Catal. 31 (1987) 333. [3] H. Topsoe, B.S. Clausen, F.E. Massoth, Hydrotreating Catalysis, Springer, Berlin, 1996, p. 310. [4] S.A. Bagshaw, T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 35 (1996) 1102. [5] F. Vaudry, S. Khodabandeh, M.E. Davis, Chem. Mater. 8 (1996) 1451. [6] R.W. Hicks, T.J. Pinnavaia, Chem. Mater. 5 (2003) 78. [7] Z. Shan, J.C. Jansen, W. Zhou, Th. Maschmeyer, Appl. Catal., A 254 (2003) 339. [8] W.Z. Zhang, T.J. Pinnavaia, Chem. Comm. (1998) 1185. [9] R.W. Hicks, N.B. Castagnola, Z.R. Zhang, T.J. Pinnavaia, C.L. Marshall, Appl. Catal. A 254 (2003) 311. [10] W. Deng, P. Bodart, M. Pruski, B.H. Shanks, Microporous Mesoporous Mater. 52 (2002) 169. [11] S. Cabrera, J. El Haskouri, J. Alamo, A. Beltra´n, D. Beltra´n, S. Mendioroz, M. Dolores Marcos, P. Amoro´s, Adv. Mater. 11 (1999) 379. [12] M. Yada, H. Kitamura, M. Machida, T. Kijima, Langmuir 13 (1997) 5252. [13] S. Valange, J.-L. Guth, F. Kolenda, S. Lacombe, Z. Gabelica, Microporous Mesoporous Mater. 35 – 36 (2000) 597. [14] Z.R. Zhang, T.J. Pinnavaia, J. Am. Chem. Soc. 124 (2002) 12294. [15] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ednR, Academic Press, London, 1995. [16] N. Yao, G.X. Xiong, Y.H. Zhang, M.Y. He, W.S. Yang, Catal. Today 68 (2001) 97.
Facile synthesis of thermally stable mesoporous crystalline alumina by using a novel cation–anion double hydrolysis method Peng Bai, Wei Xing, Zhaoxia Zhang, Zifeng Yan* State Key Laboratory for Heavy Oil Processing, PetroChina Key laboratory of Catalysis, China University of Petroleum, Dongying 257061, China Received 18 April 2005; accepted 26 May 2005 Available online 13 June 2005
Abstract By using the double hydrolysis of Al3+ and AlO2- in the presence of Pluronic P123 as the structure directing agent, mesoporous crystalline alumina was easily synthesized, which exhibits high specific surface area, narrow pore size distribution, and excellent thermal stability. D 2005 Elsevier B.V. All rights reserved. Keywords: Mesoporous; Alumina; Double hydrolysis
1. Introduction Aluminas are important industrial chemicals that have found wide application in adsorbents, ceramics, abrasives, catalysts and catalyst supports. [1 –3] However, because the traditional alumina materials only possess textural porosity and lack selective pore structure, their application is strictly limited. [4] Mesoporous materials have shown excellent structural properties, such as high surface area, narrow pore size distribution, which render them highly potential candidates as catalysts and catalysts supports for the direct activation of large organic molecules. Although several papers were reported on the synthesis of mesoporous alumina by utilizing cationic, anionic and non-ionic templates as the structural directing agents since the first in 1996, [4 –14] these methods usually use the expensive and toxic aluminum alkoxides as precursors, and/ or strictly control the synthetic conditions, which make them not convenient for the industrial scaling-up. Double hydrolysis reaction is a special kind of metathesis reaction, which involves the ionic compounds swapping their ionic partners. In the case of double hydrolysis, both the cation of one reactant and the anion of the other are readily hydrolyzed in aqueous solution. When these two kinds of * Corresponding author. Tel.: +86 546 8391527; fax: +86 546 8391971. E-mail address: [email protected] (Z. Yan). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.033
ions encounter each other, their hydrolysis would be mutually promoted. Therefore the hydrolysis balance is broken, and the reaction would irreversibly proceed to a complete extent in a real sense that other methods cannot achieve. Here, by adopting this double hydrolysis strategy, we obtained mesoporous alumina with relatively high surface area, narrow pore size distribution and excellent thermal stability using the cheap inorganic salts, Al(NO3)3 and NaAlO2, in the presence of Pluronic P123 as the structural directing agent.
2. Experimental A typical synthesis was done as follows: 3.75g Al(NO3)3.9H2O and 2.83 g of Pluronic P123 were dissolved in 30 ml of deionized water, then 20 ml solution containing 2.46 g of NaAlO2 was dropped into the former under vigorous stirring. Immediately the gel resulted. After standing at room temperature for 12 h, the gel was transferred into an autoclave and aged at 353 K for 24 h. Then a transparent gel was obtained, after washing with deionized water; the solid was recovered by suction filtration and dried at 353 K in Vacuo. The resulting white solid was then calcined at 773 K for 4 h. X-ray diffraction (XRD) measurements were carried out at a speed of 0.01- s-1 by a Bruker Axs diffractometer
P. Bai et al. / Materials Letters 59 (2005) 3128 – 3131
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(Germany) with CuK radiation generated at 40 kV, 30 mA. The nitrogen adsorption of the samples were performed on Micromeritics TRISTAR 3000 analyzer and the samples were degassed at 673 K for 4 h prior to adsorption analysis. Adsorption isotherms were measured at liquid nitrogen temperature (77.3 K) over the interval of relative pressures from 0.01 to 0.998 using nitrogen of high purity. Specific surface area of the samples was calculated using the BET method. The mesopore distribution and mean pore size of the desorption branches of nitrogen adsorption isotherms were calculated using BJH method. Thermogravimetric and differential thermal analysis (TG-DTA, Shimadzu TGA 50 H) was performed with a heating rate of 2 K/min in air. The weight of the sample was 8.5 mg. Fig. 2. Small-angle XRD pattern of the as-synthesized mesostructured alumina.
3. Results and discussions The nitrogen sorption isotherm of the calcined alumina, shown in Fig. 1, is of typical type IV (defined by IUPAC), [15] which is the characteristic of mesoporous materials. The shape of the large hysteresis loop in the isotherms indicates that ‘‘ink-bottle’’ type pores may be present in the mesoporous alumina. The specific surface area is up to 342 m2/g when the as-synthesized sample was calcined at 773 K for 4 h. When the as-synthesized sample was calcined at 973 for 2 h, the BET surface area is reduced to 280 m2/g, which can be reasonably ascribed to the slight sintering of mesoporous alumina at such high calcination temperature. However, the isotherm has the same shape as that of the one
300
0.25
dV/dD (cm3/gnm)
Quantity absorbed (cm3/g)
250
calcined at 773 K, revealing the excellent thermal stability of the alumina samples. The pore size, calculated from the desorption branch by BJH method, shown in Fig. 1 (Inset), has narrow distribution over the range from 3.0 to 5.0 nm and is centered at 3.9 nm, indicating the mesoporous alumina has uniform pore size and ordered mesostructure. Powder X-ray diffraction (XRD) patterns of the as-synthesized and calcined alumina samples are shown in Figs. 2 and 3. The presence of a single diffraction line in the small-angle region further indicates the partially ordered mesopores in the calcined
200
0.20
0.15
0.10
0.05
0.00
0
2
4
6
8
10
12
14
16
18
20
Pore Width (nm)
150
100
50
0.0
0.2
0.4
0.6
0.8
Relative pressure (P/Po) Fig. 1. N2 adsorption/desorption isotherms at 77 K of mesoporous alumina.
1.0
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P. Bai et al. / Materials Letters 59 (2005) 3128 – 3131
Fig. 3. Wide-angle XRD patterns of the as-synthesized (Boehmite) and calcined alumina (g-Alumina).
alumina. In the wide-angle regions, the as-synthesized samples exhibit diffraction lines assignable to boehmite, while the calcined products exhibit diffraction lines characteristic of g-alumina phase, indicating the crystalline pore wall in the calcined mesoporous alumina. This is quite different from the mesoporous aluminas with amorphous pore wall reported in the previous work, [4,5], [11 – 13], which usually have bad hydrothermal stability; for instance, they quickly lose their mesostructured framework when suspended in water even at ambient temperature. [14] By contrast, the mesoporous crystalline alumina synthesized in our work is expected to have better hydrothermal stability. The thermal gravimetry and differential thermal analysis (TGDTA) profiles of the as-synthesized alumina are shown in Fig. 4. The TG profile indicates a ca. 38% loss of weight from room temperature to 773 K. The DTG profile (not shown) exhibits two main peaks at 349, 546 K, which can be assigned to the elimination of physically adsorbed water and the burning of the surfactant, respectively, corresponding to the endothermic peak at 349 and exothermic peak at 546 K in the DTA profile. The endothermic peaks from 418 to 518 K can be attributed to the decomposition of the surfactant. The existence of the endother-
Fig. 4. TG-DTA profile of as-synthesized mesostructured g-Alumina.
mic peak at 635 K may be ascribed to the continued condensation reaction of surface hydroxyls and the phase transformation of boehmite to g-Al2O3 consistent with the gradual weight loss from 623 to 773 K. No obvious endothermic peaks can be observed in the temperature range from 773 K to 1173 K, indicating that no further phase transformation (from gAl2O3 to other crystalline alumina) takes place at higher temperatures. These results further prove the high thermal stability of the mesoporous alumina. It should be pointed out that the presence of Pluronic P123 is indispensable to synthesize mesoporous alumina with high surface area, as much lower surface area of the samples synthesized in the same condition without the addition of P123 was resulted, suggesting that Pluronic P123 is really acting as the templating agent in the formation of mesoporous structure of the alumina samples. The formation of mesostructure may undergo the real supramolecular assembly mechanism described everywhere, and the detailed reaction between Al3+, AlO2- and P123 may be proceeded in the [S-Al(OH)3H+](OH)-HAlO2 route, which can be illustrated in the following two reactions: P123
Al3þ þ H2 O W AlðOHÞ3 þ Hþ
ð1Þ
AlO 2 þ H2 O W HAlO2 þ OH
ð2Þ
In reaction (1), Al3+ is hydrolyzed into Al(OH)3, which deposits on the P123 micelles and adsorbs H+, thus the P123 is protonized and the supramolecule, [S-Al(OH)3H+] is formed. In reaction (2), AlO2- is hydrolyzed into HAlO2 and OH-. When the two solutions are mixed together, the above two reactions will be mutually promoted. HAlO2 will further deposit on the supramolecule, leading to the formation of mesostructured alumina inorganic species and P123 composite. However, the mechanism of the formation of pore wall is different from the sol – gel process induced by addition of ammonia or NaOH into the Al3+ solution, [13,16] since the addition of AlO2- directly introduces the AlO4 tetrahedron promoting the formation of crystallite seed into the reaction mixture, which may eventually form the nanocrystallite of gAl2O3 in the mesostructured framework.
P. Bai et al. / Materials Letters 59 (2005) 3128 – 3131
Compared with the former methods to prepare mesoporous aluminas, this novel method has many advantages. First, by using the cheap inorganic salts, Al(NO3)3 and NaAlO2, the utilization of expensive and toxic aluminum alkoxide is avoided. Second, by mixing the stoichiometric amount of Al3+ and AlO22-, the hydrolysis reaction quickly proceeds to a complete extent, and evades the complicated pH adjusting process [13,14], [16] and the rather long hydrolysis period. [10,11] These advantages render this method highly superior over the former ones for the industrial scaling-up.
4. Conclusions To sum up, we present a simple, effective and highly reproducible way to synthesize mesoporous g-alumina. The BET surface area is up to 342m2/g. The pore size has narrow distribution over the range from 3.0 to 5.0 nm and is centered at 3.9 nm. The mesoporous structure can remain stable up to 973 K. These results render the mesoporous alumina synthesized in our work high potential in the field of catalysis, adsorption and separation.
Acknowledgement China Natural Petroleum Corporation (CNPC) is gratefully acknowledged for the project grant (20020358-1).
3131
References [1] C. Misra, Industrial Alumina Chemicals, ACS Monograph 184 (1986)1996 (Washington). [2] G. Tournier, M. Lecroix-Repellin, G.M. Pajonk, Stud. Surf. Sci. Catal. 31 (1987) 333. [3] H. Topsoe, B.S. Clausen, F.E. Massoth, Hydrotreating Catalysis, Springer, Berlin, 1996, p. 310. [4] S.A. Bagshaw, T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 35 (1996) 1102. [5] F. Vaudry, S. Khodabandeh, M.E. Davis, Chem. Mater. 8 (1996) 1451. [6] R.W. Hicks, T.J. Pinnavaia, Chem. Mater. 5 (2003) 78. [7] Z. Shan, J.C. Jansen, W. Zhou, Th. Maschmeyer, Appl. Catal., A 254 (2003) 339. [8] W.Z. Zhang, T.J. Pinnavaia, Chem. Comm. (1998) 1185. [9] R.W. Hicks, N.B. Castagnola, Z.R. Zhang, T.J. Pinnavaia, C.L. Marshall, Appl. Catal. A 254 (2003) 311. [10] W. Deng, P. Bodart, M. Pruski, B.H. Shanks, Microporous Mesoporous Mater. 52 (2002) 169. [11] S. Cabrera, J. El Haskouri, J. Alamo, A. Beltra´n, D. Beltra´n, S. Mendioroz, M. Dolores Marcos, P. Amoro´s, Adv. Mater. 11 (1999) 379. [12] M. Yada, H. Kitamura, M. Machida, T. Kijima, Langmuir 13 (1997) 5252. [13] S. Valange, J.-L. Guth, F. Kolenda, S. Lacombe, Z. Gabelica, Microporous Mesoporous Mater. 35 – 36 (2000) 597. [14] Z.R. Zhang, T.J. Pinnavaia, J. Am. Chem. Soc. 124 (2002) 12294. [15] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ednR, Academic Press, London, 1995. [16] N. Yao, G.X. Xiong, Y.H. Zhang, M.Y. He, W.S. Yang, Catal. Today 68 (2001) 97.