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mesoporous Al2O3 monoliths under heat-treatment
Microporous and Mesoporous Materials 153 (2012) 41–46
Contents lists available at SciVerse ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Structural evolution of hierarchically macro/mesoporous Al2O3 monoliths under heat-treatment Kuibao Zhang a,⇑, Zhengyi Fu b, Tadachika Nakayama c, Koichi Niihara c a
State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China c Extreme Energy-Density Research Institute, Nagaoka University of Technology, 1603-1 Nagaoka, Niigata 940-2188, Japan b
a r t i c l e
i n f o
Article history: Received 17 July 2011 Received in revised form 2 December 2011 Accepted 3 December 2011 Available online 30 December 2011 Keywords: Hierarchical Porous Al2O3 Monolithic Morphology
a b s t r a c t Hierarchically macro/mesoporous alumina monoliths were successfully fabricated from a facile sol–gel process by introducing PEG8000 to the reaction solution of ordered mesoporous Al2O3 (OMA, P123/isopropoxide aluminum/HNO3). The monolithic gels were subsequently calcined at different temperatures to investigate the morphological evolution. Monolithic skeletons were obtained in the dried gels with bimodal close-celled macropores. The bimodal macroporous structure is preserved after calcination at 450 °C. The macropores are interconnected with large-sized mesopores, which facilitates the monolith with high surface area of 286.9 m2/g and pore volume of 0.67 cm3/g. This hierarchical feature persists up to 1100 °C after phase-transformation from c-Al2O3 to a-Al2O3 with the shrinkage of macropores and expansion of mesopores. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Porous alumina has attracted considerable attention because of its wide applications in catalysts, catalyst supports and adsorption medias [1–4]. Many attempts have been devoted to fabricate Al2O3 with structurally controlled pores in length scale from micro- to nano-meter, especially mesoporous alumina (MA) with high surface area and narrow pore size distribution [5–7]. On the other hand, mesoporous alumina incorporated with macropores are especially desirable as the macroporous channels facilitate transportation of fluid species while mesopores provide high surface area that assists contact of the fluid with the solid surface, which can find substantial applications in catalyst supports, porous electrodes, sorption and separation [8–12]. However, it is challenging to synthesize MA with well-organized mesopores and macropores concurrently due to the fast hydrolysis–condensation rate of aluminum alkoxides. On the other hand, most of the previously synthesized mesoporous materials are pulverous samples in highly cracked morphology [13–16]. Monolithic materials with hierarchical pores incorporated in one sample should be more promising in practical applications. On the basis of ordered mesoporous alumina (OMA), Li et al. reported the synthesis of hierarchical c-Al2O3 monoliths with highly ordered 2D hexagonal mesopores in macroporous walls using PU foam as ⇑ Corresponding author. Tel./fax: +86 816 2419201. E-mail address: [email protected] (K. Zhang). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.12.014
the sacrificial macropore skeleton [17,18]. Recently, a facile sol–gel process has been established to produce alumina monoliths with multiscaled porous structures from concurrent phase-separation and sol–gel transition induced by polymerization reaction [19]. This approach was also extended for the preparation of various monolithic materials with hierarchically macro/mesoporous structure under delicate selection of the raw materials and controlling of the experimental procedures [20]. Based on the preparation of OMA, we have successfully synthesized hierarchically macro/mesoporous alumina monoliths by introducing PEG8000 to the reaction system of P123/isopropoxide aluminum/HNO3 [21,22]. PEG8000 acts as the phase-segregation initiator to generate macropores in the mesostructured matrix. In this study, the monolithic gels were heat-treated at different temperatures to elucidate the structural evolution. The formation mechanism of hierarchically porous and monolithic morphology was explored as well.
2. Experimental details Pluronic P123 (Mav = 5800, EO20PO70EO20), polyethylene glycol with average molecular weight of 8000 (PEG8000) and isopropoxide aluminum (IA) were purchased from Aldrich and employed without further treatment in this experiment. Based on the sol–gel preparation of OMA in alcoholic solution [17,21], PEG8000 was introduced to initiate phase-segregation and produce macropores. The schematic procedures of this process were illustrated in Fig. 1. Firstly,
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K. Zhang et al. / Microporous and Mesoporous Materials 153 (2012) 41–46
Fig. 1. Schematic illustration of the synthesis procedures.
2.5 g P123 was homogeneously dissolved in 50 ml ethanol under vigorous stirring of 3 h. Then 3.75 ml of 67 wt.% nitric acid and 2 g PEG8000 were added to the above mentioned solution. The solution was sealed with PE film and transferred to an 80 °C water-bath container with continued 3 h stirring. 5.1 g IA was subsequently added into the solution and stirred for 5 h. The dissolved solution was poured into designed cylindrical vessels. The vessels were sealed and putted into the water-bath container to undergo aging and gelation, which resulted in cylindrical monoliths after 2 days of aging. The monoliths were moved into a 60 °C drying cabinet to carry out the solvent evaporation process. The dried gels were firstly calcined at 450 °C for 5 h with a heating rate of 1 °C/min in a muffle furnace to eliminate the organic species. The heat-treatment was then conducted at destinated temperatures with a holding time of 2 h and heating rate of 10 °C/min. Powder X-ray diffractions of the as-synthesized and as-calcined samples were measured using a Rigaku RINT-2500HF diffractometer with Cu Ka radiation. Measurement to testify the elimination of organic species was carried out on a Thermo-Gravimetric/Differential Thermal Analyzer (TG/DTA2000SA, BRUKER) from room temperature to 500 °C under a heating rate of 5 °C/min. Macropores and mesopores were observed by scanning electronic micrograph (SEM, JEOL JSM-6700F). The parameters of mesopores were characterized by nitrogen physisorption, which were performed at 77 K on a Quantachrome Autosorb-1 instrument with the dried sample outgassed at 100 °C and calcined samples outgassed at 200 °C overnight before the measurements. The Brunauer–Emmett–Teller (BET) calculation of surface area and Barrett–Joyner–Halenda (BJH) calculation of pore size distribution were conducted from
the adsorption branch of the isotherms while the total pore volumes (Vp) were determined at a P/P0 value of 0.995. 3. Results and discussion 3.1. Elimination of the organic products Phase compositions of as-synthesized and 450 °C calcined samples were measured by powder X-ray diffraction with the spectrums presented in Fig. 2a. The as-synthesized sample is mainly composed of boehmite phase (AlOOH) [23]. Peaks around 20° are related with organic bonding groups of residual organic species from sol–gel reaction among P123, IA and PEG8000. The as-calcined sample exhibits diffraction peaks coincident with c-Al2O3 without any peaks of organic species, which evidence the complete elimination of organic products after 450 °C calcination. The weak and broad diffraction peaks indicate the c-Al2O3 is in nanocrystalline structure with low crystallinity. The crystallite size estimated by Scherrer’s formula is approximately 3.9 nm. Thermal decomposition of the as-synthesized sample is clarified by TG-DTA as the result plotted in Fig. 2b. The minor exothermic peak around 220 °C is associated with the decomposition initiation of organic species in the dried gel while the obvious exothermic peak at 360 °C can be designated as phase-transformation from boehmite (AlOOH) to c-Al2O3. TG curve reveals the weight loss starts at about 270 °C because of polymer decomposition. The discernible peak around 400 °C is associated with the weight loss of phase-transformation from boehmite to c-Al2O3. The total weight remains generally unchanged after further heat-treatment
Fig. 2. Testimony for elimination of the organic products: (a) Powder X-ray diffraction patterns of the as-synthesized and 450 °C calcined samples, (b) TG/DTA curves of the as-synthesized sample.
K. Zhang et al. / Microporous and Mesoporous Materials 153 (2012) 41–46
43
Fig. 3. N2 physisorption results of the as-synthesized and 450 °C calcined samples: (a) isotherms, (b) corresponding pore size distribution.
Table 1 Summary of pore parameters depending on the sintering temperatures. Sample sintered at (°C)
BET surface area (m2/g)
Pore volume (cm3/g)
Median pore size (nm)
Dried 450 900 1000 1100 1200
7.5 286.9 191.4 167.9 70.0 17.8
0.03 0.67 0.62 0.70 0.33 0.04
3.5 16.7 28.6 28.3 28.8 —
at temperature higher than 450 °C, which means that 450 °C of calcination can sufficiently eliminate the organic products. 3.2. Structural measurements of as-synthesized and 450 °C calcined samples Fig. 3 presents the N2 physisorption results to clarify the structural parameters of the as-synthesized and 450 °C calcined samples. The detailed data were listed in Table 1. Hysteresis loop can hardly be observed in isotherm curve of the dried gel, which can be attributed to the extremely low absorbed volume of liquid N2 as compared with the calcined sample. Actually, the dried sample does exhibit a hysteresis loop when just plot its own relative pressure with the absorbed volume. The low absorbed volume is related with the mesopores generated from the synergistic reaction are mainly blocked by organic species after drying. The blocked mesopores lead to broadly distributed mesopores with median size of 3.5 nm, accompanied with low surface area of 7.5 m2/g and pore volume of 0.03 cm3/g. The absorbed volume increases dramatically and the hysteresis loop can be clearly observed after elimination of the organic remnants. The isotherm loop exhibits a typical IV type with H2 shape according to the IUPAC classification, which is a testimony of ink-bottle shaped mesopores [24]. The pore size distribution (Fig. 3b) reveals narrowly arranged mesopores centered at 16.7 nm in the calcined sample. The large-sized mesopores are directed by the cooperative assembly of block-copolymer template and aluminum precursor accompanied with the swelling effect of PEG8000. The evaporation of organic species also results in high surface area of 286.9 m2/g and pore volume of 0.67 cm3/g. Fig. 4 demonstrates the morphological images of the as-synthesized and 450 °C calcined samples. The dried gel is in monolithic structure with a cylindrical shape as shown in Fig. 4a. The obtained
cylindrical shape is actually replicated from the pre-designed vessel. This monolithic feature is of great novelty as most of previously synthesized mesoporous materials are in pulverous state with highly cracked structure. The cracking usually results from weak strength of the gel skeleton, which could not withstand the surface tension during desiccation. The formation of monolithic feature in this experiment is associated with the incorporation of PEG8000, which acts as a regulator of phase-segregation and cross-linking in this system. On one hand, the addition of PEG8000 would reduce surface tension of the liquid solution, which depress the pressure capillary slope in the generated mesopores. On the other hand, PEG8000 increases the degree of cross-linking and polymerization, leading to higher strength of the gel skeleton. Therefore, cracking can be effectively prevented during the gel desiccation process and the monolithic structure is obtained. Unfortunately, the monoliths turn out to be cracked sample after be sintered at 450 °C for 5 h (not shown in this figure). The cracking is attributed to decreased strength of the gel skeleton after elimination of organic species. Meanwhile, the evaporation itself forms some shock to the maintenance of monolithic structure. Bimodal macropores in submicro- and micro-meter sizes appear in the dried sample as shown in Fig. 4b. These macropores are in close-celled morphology with elliptical and/or spherical shapes. The bimodal macroporous structure becomes more distinct after calcination (Fig. 4c). The macropores in the calcined sample are in much larger size than the as-synthesized one, which further proves the macropores are blocked by organic residuals in the dried gel. The generation of macroporous feature can be explained by the synergy reaction among P123, PEG8000 and aluminum alkoxide. As PEG8000 shows strong hydrogen bonding attraction with aluminum-hydroxyl group, it is preferably adsorbed to the surface of aluminum species during the polycondensation process. This adsorption reaction leads to the decrement of miscibility between the reactants and the solution system, which eventually leads to the formation of phase-segregation and macropores [25]. Meanwhile, the effect of phase-segregation is promptly freezed by the gelation process. The phase-segregation may not be formed homogeneously in the whole reaction solution, which makes the generated macropores in bimodally distributed diameters. The morphology of closed macropores prepared in this experiment is substantially different from structures induced by phase-separation as produced by Nakanishi and Tanaka [26], which are composed of penetrated macropores with mesoporous walls. This discrepancy may due to different gelation effects and solution atmospheres.
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K. Zhang et al. / Microporous and Mesoporous Materials 153 (2012) 41–46
Fig. 4. Morphology of the as-synthesized and 450 °C calcined samples: (a) Digital image of the monolithic gel, (b) SEM image of the as-synthesized gel, (c) SEM image of 450 °C calcined sample, (d) high-magnified SEM image of 450 °C calcined sample.
3.3. Phase-transformation under heat-treatment
Fig. 5. Powder X-ray diffraction spectrums of samples sintered at different temperatures.
The as-synthesized samples were further sintered for 2 h at temperatures ranging from 900 to 1200 °C after been calcined at 450 °C for 5 h. Their phase compositions were characterized by X-ray diffraction as the results presented in Fig. 5. The 900 °C and 1000 °C sintered samples exhibit the same peaks corresponding to c-Al2O3. The main phase transforms to a-Al2O3 after be sintered at 1100 °C for 2 h with a minor phase designated as c-Al2O3. 1200 °C sintering causes totally phase-transformation from cAl2O3 to a-Al2O3 with high crystallinity. This phase-transformation is normally generated around 1000 °C with a transient h-Al2O3 phase [27]. However, it takes place at much higher temperature of 1100 °C in this experiment with no peaks corresponding to hAl2O3 phase. It was reported that a-Al2O3 nuclei generate within the ultrafine h-Al2O3 matrix and then grow to produce a-Al2O3 colonies during the h-Al2O3 and a-Al2O3 transformation in sintering of c-Al2O3 powders [28]. As c-Al2O3 grains are in mesoporous arrangement with large pore size in this study, low intrinsic nucleation density is generated, leading to delayed nucleation of h-Al2O3 and subsequent precipitation of a-Al2O3. 3.4. Morphology evolution after sintering at high temperatures
The mesopores in the 450 °C calcined sample was observed by high-magnified SEM as its image typically shown in Fig. 4d. Although the image is in weak resolution because of low conductivity and crystallinity, uniform mesopores can be directly observed in this image. The calcined sample possesses well-defined mesopores with pore diameter centered at about 10 nm, which is much smaller than the calculated value from N2 physisorption. The mesopores are in disordered arrangement without distinct regularity. This result further demonstrates that the 450 °C calcined sample are in hierarchically porous structure with bimodal close-celled macropores interconnected with large-sized mesoporous walls.
Fig. 6 demonstrates the macroporous morphology of samples according to their sintering temperatures. The bimodal macroporous structure is maintained after calcination at 900 °C as shown in Fig. 6a. 1000 °C sintering results in slightly shrunk macropores with maintenance of the bimodal feature. The bicontinuous structure of macropores also exist in 1100 °C sintered sample with the accompany of obvious shrinkage in macropores. This result reveals that the macropores possess excellent thermal stability, which can withstand the phase-transformation from c-Al2O3 to a-Al2O3 at high temperature. As the resultant image presented in Fig. 6d, the macroporous morphology can hardly be observed after the
K. Zhang et al. / Microporous and Mesoporous Materials 153 (2012) 41–46
45
Fig. 6. SEM images of samples sintered at different temperatures: (a) 900 °C, (b) 1000 °C, (c) 1100 °C, (d) 1200 °C.
sample was sintered at 1200 °C for 2 h. Distinct grain-growth occurs at this temperature with coarsen grains in size range of submicrometer. Therefore, we can deduce that grain-growth exerts significant influence on the persistence of macropores. The collapsing of macropores may be related with the disappearance of mesoporous scaffold as illustrated in the following part. With temperature higher than 1100 °C, the obvious grain-growth would destroy the mesoporous organization and the structural destruction of mesostructured matrix leads to the collapse of macropores. Fig. 7 shows the N2 physisorption results of sintered samples with temperature ranging from 900 to 1200 °C. Detailed datas of the pore parameters are listed in Table 1. Hysteresis loops can be clearly observed for 900 °C and 1000 °C sintered samples. These hysteresis loops are in typical IV type with H3 shape according
to the IUPAC classification, which reveals slit-shaped mesoporous structure. The 1100 °C sintered sample exhibits a small hysteresis loop, evidencing the partial persistence of mesopores in the crystalline a-Al2O3 material. No hysteresis loop appears in the isotherm curve of 1200 °C sintered sample, which demonstrates the total collapsing of mesoporous structure induced by dramatical grain-growth. Compared with 450 °C calcined sample, the 900 °C and 1000 °C sintered ones reveal decreased surface areas of 191.4 and 167.9 m2/g while the pore volumes remain comparable of 0.62 and 0.70 cm3/g. As the mesopores are partially collapsed in 1100 °C sintered sample, the surface area and pore volume decrease drastically to 70.0 m2/g and 0.33 cm3/g, respectively. Phase-transformation and grain-growth lead to totally collapsed mesopores for the 1200 °C sintered material, which results in low
Fig. 7. N2 physisorption results for samples sintered at temperatures from 900 to 1200 °C: (a) Isotherms, (b) corresponding pore size distribution.
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surface area and pore volume of 17.8 m2/g and 0.04 cm3/g. The persisted mesopores are in broad distribution with a similar median size around 28.5 nm after high temperature sintering from 900 to 1100 °C. 4. Conclusions Hierarchically macro/mesoporous alumina monoliths were successfully fabricated from a facile sol–gel process by introducing PEG8000 to OMA reaction system. The dried gel is composed of bimodal macropores with close-celled morphology. The bimodal macroporous structure is maintained after calcination at 450 °C. The close-celled macropores are interconnected with large-sized mesostructured walls with narrowly distributed diameter centered at 16.7 nm, accompanied with high surface and pore volume of 286.9 m2/g and 0.67 cm3/g. This hierarchically porous feature can be maintained up to 1100 °C after phase-transformation from cAl2O3 to a-Al2O3. Acknowledgements The authors would like to acknowledge the financial support by the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (Southwest University of Science and Technology, No. 11zxfk26), and the project supported by State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, No. 2012-KF-15). References [1] J.C. Downing, K.P. Goodboy, Claus Catalysis and Alumina Catalysis Materials and Their Application, American Chemical Society, Washington, DC, 1990.
[2] J. Cejka, Appl. Catal., A 254 (2003) 327. [3] F. Schüth, K.S.W. Sing, J. Weitkamp, Handbook of Porous Solids, Weily, New York, 2002. [4] P. Braunstrin, H.P. Kormann, W. Meyer-Zaika, R. Pugin, G. Schmid, Chem. Eur. J. 6 (2000) 4637. [5] C. Boissière, L. Nicole, C. Gervais, F. Babonneau, M. Antonietti, H. Amenitsch, et al., Chem. Mater. 18 (2006) 5238. [6] Q. Liu, A.Q. Wang, X.D. Wang, T. Zhang, Microporous Mesoporous Mater. 100 (2007) 35. [7] G. Tai, W. Guo, Mater. Chem. Phys. 103 (2007) 201. [8] C.F. Blanford, H. Yan, R.C. Schroden, M. Al-Daous, A. Stein, Adv. Mater. 13 (2001) 401. [9] D. Grosso, G.J. de A.A. Soler-Illia, E.L. Crepaldi, B. Charleux, C. Sanchez, Adv. Funct. Mater. 13 (2003) 37. [10] D.M. Antonelli, Microporous Mesoporous Mater. 33 (1999) 209. [11] Z.Y. Yuan, B.L. Su, J. Mater. Chem. 16 (2006) 663. [12] Z.G. Shi, Y.Q. Feng, L. Xu, S.L. Da, Y. Liu, Mater. Chem. Phys. 97 (2006) 472. [13] Z. Zhang, T.J. Pinnavaia, J. Am. Chem. Soc. 124 (2002) 12294. [14] K. Niesz, P. Yang, G.A. Somorjai, Chem. Commun. 15 (2005) 1986. [15] W. Deng, B. Shanks, Chem. Mater. 17 (2005) 3092. [16] J.P. Dacquin, J. Dhainaut, D. Duprez, S. Royer, A.F. Lee, K. Wilson, J. Am. Chem. Soc. 131 (2009) 12896. [17] Q. Yuan, A.X. Yin, C. Luo, L.D. Sun, et al., J. Am. Chem. Soc. 130 (2008) 3465. [18] L.L. Li, W.T. Duan, Q. Yuan, Z.X. Li, H.H. Duan, C.H. Yan, Chem. Commun. 41 (2009) 6174. [19] Y. Tokudome, K. Fujita, K. Nakanishi, K. Miura, K. Hirao, Chem. Mater. 19 (2007) 3393. [20] J. Konishi, K. Fujita, S. Oiwa, K. Nakanishi, K. Hirao, Chem. Mater. 20 (2008) 2165. [21] K. Zhang, Z. Fu, J. Zhang, T. Nakayama, K. Niihara, S.W. Lee, Chem. Lett. 39 (2010) 980. [22] K. Zhang, Z. Fu, T. Nakayama, T. Suzuki, H. Suematsu, K. Niihara, Mater. Res. Bull. 46 (2011) 2155. [23] T.Z. Ren, Z.Y. Yuan, B.L. Su, Langmuir 20 (2004) 1531. [24] D.M. Young, A.D. Crowell, Physical Adsorption of Gases, Butterworths, London, 1962. [25] K. Nakanishi, R. Takahashi, T. Nagakane, K. Kitayama, N. Koheiya, H. Shikata, N. Soga, J. Sol-Gel Sci. Technol. 17 (2000) 191. [26] K. Nakanishi, N. Tanaka, Acc. Chem. Res. (2007) 863. [27] H.D. Kim, T. Nakayama, B.J. Hong, K. Imaki, T. Yoshimura, T. Suzuki, J. Eur. Ceram. Soc. 30 (2010) 2735–2739. [28] F.W. Dynys, J.W. Halloran, J. Am. Ceram. Soc. 65 (1982) 442.
Contents lists available at SciVerse ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Structural evolution of hierarchically macro/mesoporous Al2O3 monoliths under heat-treatment Kuibao Zhang a,⇑, Zhengyi Fu b, Tadachika Nakayama c, Koichi Niihara c a
State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China c Extreme Energy-Density Research Institute, Nagaoka University of Technology, 1603-1 Nagaoka, Niigata 940-2188, Japan b
a r t i c l e
i n f o
Article history: Received 17 July 2011 Received in revised form 2 December 2011 Accepted 3 December 2011 Available online 30 December 2011 Keywords: Hierarchical Porous Al2O3 Monolithic Morphology
a b s t r a c t Hierarchically macro/mesoporous alumina monoliths were successfully fabricated from a facile sol–gel process by introducing PEG8000 to the reaction solution of ordered mesoporous Al2O3 (OMA, P123/isopropoxide aluminum/HNO3). The monolithic gels were subsequently calcined at different temperatures to investigate the morphological evolution. Monolithic skeletons were obtained in the dried gels with bimodal close-celled macropores. The bimodal macroporous structure is preserved after calcination at 450 °C. The macropores are interconnected with large-sized mesopores, which facilitates the monolith with high surface area of 286.9 m2/g and pore volume of 0.67 cm3/g. This hierarchical feature persists up to 1100 °C after phase-transformation from c-Al2O3 to a-Al2O3 with the shrinkage of macropores and expansion of mesopores. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Porous alumina has attracted considerable attention because of its wide applications in catalysts, catalyst supports and adsorption medias [1–4]. Many attempts have been devoted to fabricate Al2O3 with structurally controlled pores in length scale from micro- to nano-meter, especially mesoporous alumina (MA) with high surface area and narrow pore size distribution [5–7]. On the other hand, mesoporous alumina incorporated with macropores are especially desirable as the macroporous channels facilitate transportation of fluid species while mesopores provide high surface area that assists contact of the fluid with the solid surface, which can find substantial applications in catalyst supports, porous electrodes, sorption and separation [8–12]. However, it is challenging to synthesize MA with well-organized mesopores and macropores concurrently due to the fast hydrolysis–condensation rate of aluminum alkoxides. On the other hand, most of the previously synthesized mesoporous materials are pulverous samples in highly cracked morphology [13–16]. Monolithic materials with hierarchical pores incorporated in one sample should be more promising in practical applications. On the basis of ordered mesoporous alumina (OMA), Li et al. reported the synthesis of hierarchical c-Al2O3 monoliths with highly ordered 2D hexagonal mesopores in macroporous walls using PU foam as ⇑ Corresponding author. Tel./fax: +86 816 2419201. E-mail address: [email protected] (K. Zhang). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.12.014
the sacrificial macropore skeleton [17,18]. Recently, a facile sol–gel process has been established to produce alumina monoliths with multiscaled porous structures from concurrent phase-separation and sol–gel transition induced by polymerization reaction [19]. This approach was also extended for the preparation of various monolithic materials with hierarchically macro/mesoporous structure under delicate selection of the raw materials and controlling of the experimental procedures [20]. Based on the preparation of OMA, we have successfully synthesized hierarchically macro/mesoporous alumina monoliths by introducing PEG8000 to the reaction system of P123/isopropoxide aluminum/HNO3 [21,22]. PEG8000 acts as the phase-segregation initiator to generate macropores in the mesostructured matrix. In this study, the monolithic gels were heat-treated at different temperatures to elucidate the structural evolution. The formation mechanism of hierarchically porous and monolithic morphology was explored as well.
2. Experimental details Pluronic P123 (Mav = 5800, EO20PO70EO20), polyethylene glycol with average molecular weight of 8000 (PEG8000) and isopropoxide aluminum (IA) were purchased from Aldrich and employed without further treatment in this experiment. Based on the sol–gel preparation of OMA in alcoholic solution [17,21], PEG8000 was introduced to initiate phase-segregation and produce macropores. The schematic procedures of this process were illustrated in Fig. 1. Firstly,
42
K. Zhang et al. / Microporous and Mesoporous Materials 153 (2012) 41–46
Fig. 1. Schematic illustration of the synthesis procedures.
2.5 g P123 was homogeneously dissolved in 50 ml ethanol under vigorous stirring of 3 h. Then 3.75 ml of 67 wt.% nitric acid and 2 g PEG8000 were added to the above mentioned solution. The solution was sealed with PE film and transferred to an 80 °C water-bath container with continued 3 h stirring. 5.1 g IA was subsequently added into the solution and stirred for 5 h. The dissolved solution was poured into designed cylindrical vessels. The vessels were sealed and putted into the water-bath container to undergo aging and gelation, which resulted in cylindrical monoliths after 2 days of aging. The monoliths were moved into a 60 °C drying cabinet to carry out the solvent evaporation process. The dried gels were firstly calcined at 450 °C for 5 h with a heating rate of 1 °C/min in a muffle furnace to eliminate the organic species. The heat-treatment was then conducted at destinated temperatures with a holding time of 2 h and heating rate of 10 °C/min. Powder X-ray diffractions of the as-synthesized and as-calcined samples were measured using a Rigaku RINT-2500HF diffractometer with Cu Ka radiation. Measurement to testify the elimination of organic species was carried out on a Thermo-Gravimetric/Differential Thermal Analyzer (TG/DTA2000SA, BRUKER) from room temperature to 500 °C under a heating rate of 5 °C/min. Macropores and mesopores were observed by scanning electronic micrograph (SEM, JEOL JSM-6700F). The parameters of mesopores were characterized by nitrogen physisorption, which were performed at 77 K on a Quantachrome Autosorb-1 instrument with the dried sample outgassed at 100 °C and calcined samples outgassed at 200 °C overnight before the measurements. The Brunauer–Emmett–Teller (BET) calculation of surface area and Barrett–Joyner–Halenda (BJH) calculation of pore size distribution were conducted from
the adsorption branch of the isotherms while the total pore volumes (Vp) were determined at a P/P0 value of 0.995. 3. Results and discussion 3.1. Elimination of the organic products Phase compositions of as-synthesized and 450 °C calcined samples were measured by powder X-ray diffraction with the spectrums presented in Fig. 2a. The as-synthesized sample is mainly composed of boehmite phase (AlOOH) [23]. Peaks around 20° are related with organic bonding groups of residual organic species from sol–gel reaction among P123, IA and PEG8000. The as-calcined sample exhibits diffraction peaks coincident with c-Al2O3 without any peaks of organic species, which evidence the complete elimination of organic products after 450 °C calcination. The weak and broad diffraction peaks indicate the c-Al2O3 is in nanocrystalline structure with low crystallinity. The crystallite size estimated by Scherrer’s formula is approximately 3.9 nm. Thermal decomposition of the as-synthesized sample is clarified by TG-DTA as the result plotted in Fig. 2b. The minor exothermic peak around 220 °C is associated with the decomposition initiation of organic species in the dried gel while the obvious exothermic peak at 360 °C can be designated as phase-transformation from boehmite (AlOOH) to c-Al2O3. TG curve reveals the weight loss starts at about 270 °C because of polymer decomposition. The discernible peak around 400 °C is associated with the weight loss of phase-transformation from boehmite to c-Al2O3. The total weight remains generally unchanged after further heat-treatment
Fig. 2. Testimony for elimination of the organic products: (a) Powder X-ray diffraction patterns of the as-synthesized and 450 °C calcined samples, (b) TG/DTA curves of the as-synthesized sample.
K. Zhang et al. / Microporous and Mesoporous Materials 153 (2012) 41–46
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Fig. 3. N2 physisorption results of the as-synthesized and 450 °C calcined samples: (a) isotherms, (b) corresponding pore size distribution.
Table 1 Summary of pore parameters depending on the sintering temperatures. Sample sintered at (°C)
BET surface area (m2/g)
Pore volume (cm3/g)
Median pore size (nm)
Dried 450 900 1000 1100 1200
7.5 286.9 191.4 167.9 70.0 17.8
0.03 0.67 0.62 0.70 0.33 0.04
3.5 16.7 28.6 28.3 28.8 —
at temperature higher than 450 °C, which means that 450 °C of calcination can sufficiently eliminate the organic products. 3.2. Structural measurements of as-synthesized and 450 °C calcined samples Fig. 3 presents the N2 physisorption results to clarify the structural parameters of the as-synthesized and 450 °C calcined samples. The detailed data were listed in Table 1. Hysteresis loop can hardly be observed in isotherm curve of the dried gel, which can be attributed to the extremely low absorbed volume of liquid N2 as compared with the calcined sample. Actually, the dried sample does exhibit a hysteresis loop when just plot its own relative pressure with the absorbed volume. The low absorbed volume is related with the mesopores generated from the synergistic reaction are mainly blocked by organic species after drying. The blocked mesopores lead to broadly distributed mesopores with median size of 3.5 nm, accompanied with low surface area of 7.5 m2/g and pore volume of 0.03 cm3/g. The absorbed volume increases dramatically and the hysteresis loop can be clearly observed after elimination of the organic remnants. The isotherm loop exhibits a typical IV type with H2 shape according to the IUPAC classification, which is a testimony of ink-bottle shaped mesopores [24]. The pore size distribution (Fig. 3b) reveals narrowly arranged mesopores centered at 16.7 nm in the calcined sample. The large-sized mesopores are directed by the cooperative assembly of block-copolymer template and aluminum precursor accompanied with the swelling effect of PEG8000. The evaporation of organic species also results in high surface area of 286.9 m2/g and pore volume of 0.67 cm3/g. Fig. 4 demonstrates the morphological images of the as-synthesized and 450 °C calcined samples. The dried gel is in monolithic structure with a cylindrical shape as shown in Fig. 4a. The obtained
cylindrical shape is actually replicated from the pre-designed vessel. This monolithic feature is of great novelty as most of previously synthesized mesoporous materials are in pulverous state with highly cracked structure. The cracking usually results from weak strength of the gel skeleton, which could not withstand the surface tension during desiccation. The formation of monolithic feature in this experiment is associated with the incorporation of PEG8000, which acts as a regulator of phase-segregation and cross-linking in this system. On one hand, the addition of PEG8000 would reduce surface tension of the liquid solution, which depress the pressure capillary slope in the generated mesopores. On the other hand, PEG8000 increases the degree of cross-linking and polymerization, leading to higher strength of the gel skeleton. Therefore, cracking can be effectively prevented during the gel desiccation process and the monolithic structure is obtained. Unfortunately, the monoliths turn out to be cracked sample after be sintered at 450 °C for 5 h (not shown in this figure). The cracking is attributed to decreased strength of the gel skeleton after elimination of organic species. Meanwhile, the evaporation itself forms some shock to the maintenance of monolithic structure. Bimodal macropores in submicro- and micro-meter sizes appear in the dried sample as shown in Fig. 4b. These macropores are in close-celled morphology with elliptical and/or spherical shapes. The bimodal macroporous structure becomes more distinct after calcination (Fig. 4c). The macropores in the calcined sample are in much larger size than the as-synthesized one, which further proves the macropores are blocked by organic residuals in the dried gel. The generation of macroporous feature can be explained by the synergy reaction among P123, PEG8000 and aluminum alkoxide. As PEG8000 shows strong hydrogen bonding attraction with aluminum-hydroxyl group, it is preferably adsorbed to the surface of aluminum species during the polycondensation process. This adsorption reaction leads to the decrement of miscibility between the reactants and the solution system, which eventually leads to the formation of phase-segregation and macropores [25]. Meanwhile, the effect of phase-segregation is promptly freezed by the gelation process. The phase-segregation may not be formed homogeneously in the whole reaction solution, which makes the generated macropores in bimodally distributed diameters. The morphology of closed macropores prepared in this experiment is substantially different from structures induced by phase-separation as produced by Nakanishi and Tanaka [26], which are composed of penetrated macropores with mesoporous walls. This discrepancy may due to different gelation effects and solution atmospheres.
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Fig. 4. Morphology of the as-synthesized and 450 °C calcined samples: (a) Digital image of the monolithic gel, (b) SEM image of the as-synthesized gel, (c) SEM image of 450 °C calcined sample, (d) high-magnified SEM image of 450 °C calcined sample.
3.3. Phase-transformation under heat-treatment
Fig. 5. Powder X-ray diffraction spectrums of samples sintered at different temperatures.
The as-synthesized samples were further sintered for 2 h at temperatures ranging from 900 to 1200 °C after been calcined at 450 °C for 5 h. Their phase compositions were characterized by X-ray diffraction as the results presented in Fig. 5. The 900 °C and 1000 °C sintered samples exhibit the same peaks corresponding to c-Al2O3. The main phase transforms to a-Al2O3 after be sintered at 1100 °C for 2 h with a minor phase designated as c-Al2O3. 1200 °C sintering causes totally phase-transformation from cAl2O3 to a-Al2O3 with high crystallinity. This phase-transformation is normally generated around 1000 °C with a transient h-Al2O3 phase [27]. However, it takes place at much higher temperature of 1100 °C in this experiment with no peaks corresponding to hAl2O3 phase. It was reported that a-Al2O3 nuclei generate within the ultrafine h-Al2O3 matrix and then grow to produce a-Al2O3 colonies during the h-Al2O3 and a-Al2O3 transformation in sintering of c-Al2O3 powders [28]. As c-Al2O3 grains are in mesoporous arrangement with large pore size in this study, low intrinsic nucleation density is generated, leading to delayed nucleation of h-Al2O3 and subsequent precipitation of a-Al2O3. 3.4. Morphology evolution after sintering at high temperatures
The mesopores in the 450 °C calcined sample was observed by high-magnified SEM as its image typically shown in Fig. 4d. Although the image is in weak resolution because of low conductivity and crystallinity, uniform mesopores can be directly observed in this image. The calcined sample possesses well-defined mesopores with pore diameter centered at about 10 nm, which is much smaller than the calculated value from N2 physisorption. The mesopores are in disordered arrangement without distinct regularity. This result further demonstrates that the 450 °C calcined sample are in hierarchically porous structure with bimodal close-celled macropores interconnected with large-sized mesoporous walls.
Fig. 6 demonstrates the macroporous morphology of samples according to their sintering temperatures. The bimodal macroporous structure is maintained after calcination at 900 °C as shown in Fig. 6a. 1000 °C sintering results in slightly shrunk macropores with maintenance of the bimodal feature. The bicontinuous structure of macropores also exist in 1100 °C sintered sample with the accompany of obvious shrinkage in macropores. This result reveals that the macropores possess excellent thermal stability, which can withstand the phase-transformation from c-Al2O3 to a-Al2O3 at high temperature. As the resultant image presented in Fig. 6d, the macroporous morphology can hardly be observed after the
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Fig. 6. SEM images of samples sintered at different temperatures: (a) 900 °C, (b) 1000 °C, (c) 1100 °C, (d) 1200 °C.
sample was sintered at 1200 °C for 2 h. Distinct grain-growth occurs at this temperature with coarsen grains in size range of submicrometer. Therefore, we can deduce that grain-growth exerts significant influence on the persistence of macropores. The collapsing of macropores may be related with the disappearance of mesoporous scaffold as illustrated in the following part. With temperature higher than 1100 °C, the obvious grain-growth would destroy the mesoporous organization and the structural destruction of mesostructured matrix leads to the collapse of macropores. Fig. 7 shows the N2 physisorption results of sintered samples with temperature ranging from 900 to 1200 °C. Detailed datas of the pore parameters are listed in Table 1. Hysteresis loops can be clearly observed for 900 °C and 1000 °C sintered samples. These hysteresis loops are in typical IV type with H3 shape according
to the IUPAC classification, which reveals slit-shaped mesoporous structure. The 1100 °C sintered sample exhibits a small hysteresis loop, evidencing the partial persistence of mesopores in the crystalline a-Al2O3 material. No hysteresis loop appears in the isotherm curve of 1200 °C sintered sample, which demonstrates the total collapsing of mesoporous structure induced by dramatical grain-growth. Compared with 450 °C calcined sample, the 900 °C and 1000 °C sintered ones reveal decreased surface areas of 191.4 and 167.9 m2/g while the pore volumes remain comparable of 0.62 and 0.70 cm3/g. As the mesopores are partially collapsed in 1100 °C sintered sample, the surface area and pore volume decrease drastically to 70.0 m2/g and 0.33 cm3/g, respectively. Phase-transformation and grain-growth lead to totally collapsed mesopores for the 1200 °C sintered material, which results in low
Fig. 7. N2 physisorption results for samples sintered at temperatures from 900 to 1200 °C: (a) Isotherms, (b) corresponding pore size distribution.
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surface area and pore volume of 17.8 m2/g and 0.04 cm3/g. The persisted mesopores are in broad distribution with a similar median size around 28.5 nm after high temperature sintering from 900 to 1100 °C. 4. Conclusions Hierarchically macro/mesoporous alumina monoliths were successfully fabricated from a facile sol–gel process by introducing PEG8000 to OMA reaction system. The dried gel is composed of bimodal macropores with close-celled morphology. The bimodal macroporous structure is maintained after calcination at 450 °C. The close-celled macropores are interconnected with large-sized mesostructured walls with narrowly distributed diameter centered at 16.7 nm, accompanied with high surface and pore volume of 286.9 m2/g and 0.67 cm3/g. This hierarchically porous feature can be maintained up to 1100 °C after phase-transformation from cAl2O3 to a-Al2O3. Acknowledgements The authors would like to acknowledge the financial support by the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (Southwest University of Science and Technology, No. 11zxfk26), and the project supported by State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, No. 2012-KF-15). References [1] J.C. Downing, K.P. Goodboy, Claus Catalysis and Alumina Catalysis Materials and Their Application, American Chemical Society, Washington, DC, 1990.
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