Journal of Colloid and Interface Science 417 (2014) 369–378
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Facile synthesis of hierarchical porous c-Al2O3 hollow microspheres for water treatment Mingyang Li a, Zhichun Si a,⇑, Xiaodong Wu a,b, Duan Weng a,b, Feiyu Kang a,b a b
Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China State Key Laboratory of New Ceramics & Fine Process, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 14 October 2013 Accepted 26 November 2013 Available online 12 December 2013 Keywords: c-Al2O3 hollow microspheres Hierarchical porous Ageing Spray-drying NH4Cl template
a b s t r a c t Hierarchical porous c-Al2O3 hollow microspheres were synthesized by a modified spray drying method. Ageing the precipitated precursor and spray-drying assisted by NH4Cl salts are considered as two key steps for the synthesis of c-Al2O3 hollow microspheres. The mechanism of the formation of hierarchical porous c-Al2O3 hollow microsphere was proposed involving phase transformation from aluminum hydroxide to laminar boehmite during ageing and a following self-assembling process with NH4Cl as the template during spray drying. The meso-/macro-pores in c-Al2O3 mainly arise from the stacking of the laminar boehmites which are obtained by ageing the precipitated precursors at 90 °C. NH4Cl, which was the byproduct from the reaction between AlCl36H2O and NH3H2O, was demonstrated to be an excellent template to act as the core and the barrier for separation of laminar boehmites. No extra NH4Cl was added. The as-synthesized hierarchical porous c-Al2O3 hollow microsphere presented remarkably higher adsorption capacity, which is thirty times higher adsorption rate for Congo Red than the solid microsphere containing only small mesopores. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The development of synthetic techniques that can be predictably manipulated to yield materials with defined and controllable features is central to modern materials science [1–3]. Especially, the routes to synthesize materials with hierarchical micro, mesoand macro-porous structures for use in applications requiring high diffusion rates and/or pressures, have attracted much attention [4–7]. The macropores and mesopores in adsorbents favor the mass transportation which makes guest molecules easy to access the active sites. Porous c-Al2O3 is widely used as absorbents and catalyst support due to its instinct microporous structures, large surface area, high stability and various acid/basic sites. c-Al2O3 with special features, such as hollow sphere [8,9] and foams with hierarchical pores [10,5], have been developed to receive the maximum benefit as the absorbent in recent years. Typically, sol–gel chemistry combined with template is employed to produce hierarchical porous frameworks; however, the preparation of meso- and macro-porous structures via sol–gel chemistry is limited due to the fragility of large-pored networks to the thermal treatments needed for crystallization. Holland et al. [11] synthesized highly organized macroporous–mesoporous
⇑ Corresponding author. Fax: +86 755 26036417. E-mail address: [email protected] (Z. Si). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.11.071
alumina by using the combination of metal alkoxide gelation and polystyrene sphere (PS) template approach. Kuemmel et al. [12] reported a method combining the block copolymer structuring way and the evaporation induced self-assembly (EISA) method for synthesizing nanocrystalline c-Al2O3 layers with contracted face-centered cubic (fcc) mesoporosity. Recently, Wu et al. [5] synthesized alumina with ordered mesostructure and hierarchical porosity via EISA method using aluminum iso-propoxide as an inorganic precursor and pluronic P123 as a template. Zhang et al. [13] fabricated hierarchically macro/mesoporous alumina monoliths through a facile sol–gel process by introducing PEG8000 to the reaction solution of ordered mesoporous Al2O3 (OMA, P123/isopropoxide aluminum/HNO3). Most of previous reports about the hierarchical porous alumina are based on the colloid crystals template, which are obviously laborious and time-consuming and the selected template materials are often expensive. Therefore, it is necessary to find a facile and simple method for the synthesis of c-Al2O3 with hierarchical pores is still a challenge. The most widely used methods for fabricating c-Al2O3 with hollow structure are nanocasting on soft or hard templates [8,4,14], chemically induced self-transformation/assembly [15,16] and spray drying or pyrolysis [9,17–19]. Dong et al. [8] reported a two-step nanocasting route to prepare monodisperse and highsurface-area mesoporous c-Al2O3 spheres, using silica spheres as primary hard templates for obtaining carbon spheres which was
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Table 1 Manipulation parameters of samples. Sample
Aging time (h)
NH4Cl concentration in aging
NH4Cl concentration In spray drying
BET (m2 g1)
Vsp (cm3 g1)
AS-1 AS-2 AS-3 AS-4 AS-5 AS-6 AS-7
– 2 4 20 20 20 20
Al:Cl = 1:3 Al:Cl = 1:3 Al:Cl = 1:3 Al:Cl = 1:3 – Al:Cl = 1:3 –
Al:Cl = 1:3 Al:Cl = 1:3 Al:Cl = 1:3 Al:Cl = 1:3 – – Al:Cl = 1:3
313 226 287 227 328 322 274
0.26 0.34 0.48 0.60 0.47 0.44 0.50
used as the secondary hard template for aluminum precursor impregnation. Monodisperse hollow spheres with a non-aggregated character were synthesized by Martins et al. [4]. Aluminum species were firstly implanted into polymeric template particles to form a core–shell structure in solvent and the template cores were then removed by thermal pyrolysis to form hollow interiors. The nanocasting methods based on the templates are advantageous for final particles with narrow size distribution, but have intrinsic disadvantages, ranging from the inherent difficulty of achieving high product yields from the multistep synthetic process to the lack of structural robustness of the shells upon template removal. Chemically induced self-transformation/assembly method for the synthesis of hollow sphere c-Al2O3 successfully avoids the use of template and related manipulation process [15,16]. Unfortunately, the mass production of uniformly hollow sphere c-Al2O3 is unresolved. Spray drying or pyrolysis methods have been developed for mass production of hollow sphere c-Al2O3 and other metal oxides in recent years. The final product or intermediates with hollow sphere structure can be continuously produced via the self tightening of particles in droplets by fast evaporation of solvent [9,17–20]. However, particles produced by traditional spray drying
or pyrolysis method have thick shells, broad size distributions and significant aggregations [9,17–20]. Therefore, mass production of c-Al2O3 hollow spheres with variable compositions, controllable morphologies, and tunable wall structures are of particular and urgency. We report a modified spray drying method for the mass production of hierarchical porous c-Al2O3 hollow spheres in this research. The boehmite precursor for spray drying was prepared via precipitation followed by aging at 90 °C. The NH4Cl, which are the byproducts of the precipitation reaction between AlCl36H2O and NH3H2O, were adopted as the template for obtaining hollow microspheres containing hierarchical pores. Congo Red (CR) was used as the target for characterizing the adsorption performances of the as-synthesized c-Al2O3.’ 2. Experimental 2.1. Materials and the synthesis of c-Al2O3 All the reagents were analytical grade. 18.93 g of AlCl36H2O (99.8%, Alladin, China) was dissolved into 200 mL of de-ionized
Fig. 1. XRD patterns of (a) AS-Xa, (b) AS-Xb, (c) AS-Xc and (d) AS-X samples.
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laboratory Spray Dryer (YC-015, China) at a feed rate of 5 ml min1. The flow rate of Spray Dryer was 90 L min1. The inlet and outlet temperature were kept at 200 °C and 105 °C, respectively. The obtained dried powders (named AS-Xb (X = 1, 2, . . .)) were further calcinated in a tubular furnace at 500 °C for 3 h under air purging (200 ml min1) to get the final c-Al2O3. The exhaust of the tubular furnace was passed through the water to retrieve the NH4Cl. To investigate the intermediates, AS-Xb powders were also calcinated at 350 °C for 3 h (named AS-Xc (X = 1, 2, . . .)). Various samples were synthesized to investigate the effects of ageing time and NH4Cl on the aged precursors and the final cAl2O3 product. The related manipulation parameters were listed in Table 1. For example, AS-5 was synthesized as follows: the precipitates were centrifuged and washed by de-ionized water for several times until no chlorides were detected in the suspension before ageing; and the other conditions were kept the same as mentioned above. Fig. 2. FTIR spectra of the AS-Xa samples.
2.2. Characterizations The powder X-ray diffraction (XRD) experiments were performed on a Japan science D/MAX-2000 diffractometer employing Cu Ka radiation (k = 0.15418 nm). The X-ray tube was operated at 40 kV and 100 mA. The X-ray powder diffractogram was recorded at 0.02° intervals in range of 5° < 2h < 80°. Scanning electron microscopy (SEM) was performed on an emission scanning electron microscopy (S-4800) at an accelerating voltage of 5 kV. Transmission electron microscope (TEM) was performed on FEI Tecnai G2 with an emission gun operating at 120 kV. Nitrogen adsorption isotherms were measured on Quantachrome NOVA apparatus. All the samples were degassed at 220 °C for 1 h prior to the nitrogen adsorption measurements. The BET surface area was determined by a multipoint BET method, using the adsorption data in the relative pressure (P P 1 0 ) range of 0.05–0.3. The adsorption branch of nitrogen adsorption–desorption isotherms was used to determine the pore size distribution by the Barret–Joyner–Halender (BJH) method, assuming a cylindrical pore model. FTIR spectra were collected on Nicolet iS10 apparatus equipped with a Smart ART appendix. The spectral resolution was 4 cm1. All the samples were washed to remove the NH4Cl before FTIR test, because the intense signal of NH4Cl can cover those of aluminum hydroxide or boehmite.
2.3. Adsorption experiments The as-synthesized c-Al2O3 samples were used as adsorbents for CR. 20 mg of c-Al2O3 were dispersed in 100 mL CR solution (pH = 7). The suspension was stirred at a constant speed of 400 rpm with a required adsorption time at room temperature. Then, the CR concentrations were analyzed by a UV–vis spectrophotometer (Scino S-4100, Korea). The characteristic absorptions of Congo red around 500 nm was used to monitor the CR concenFig. 3. The TG curves of the (a) AS-Xb samples and (b) AS-Xb samples washed by ethanol to remove NH4Cl.
Table 2 The TG results of AS-Xb samples. Sample
water at room temperature to obtained aluminum solution. Then, excess NH3H2O (25%, AR, Alladin, China) were added dropwise into the aluminum solution under magnetic stirring to get the suspension with pH at 8. The obtained suspension was aged at 90 °C for various time to get the aged precursor for spray drying (named AS-Xa (X = 1, 2, . . .)). The precursors were spray-dried in a
AS-1b AS-2b AS-3b AS-4b
Weight loss of the samples with NH4Cl (%)
Weight loss of the samples without NH4Cl (%)
RT-200 °C
200–500 °C
RT-200 °C
200–500 °C
15 8 8 8
68 72 72 74
30 20 20 15
20 15 15 15
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tration. The adsorption amount of CR (qe) on sample was calculated using the equations:
qe ¼
ðC 0 C e Þ V W
ð1Þ
where C0 is the initial concentration of CR (mg L1); Ce is the equilibrium concentration of CR after adsorption (mg L1); V is the volume of CR solution (L); W is the weight of the synthesized adsorbent (g). Adsorption isotherms were obtained by fitting the qe vs. CR concentrations according to the Langmuir equation:
qe ¼
qm bC e 1 þ bC e
The initial CR concentration is 40 mg L1 for CR adsorption ratio test. The CR adsorption ratio was defined as C/C0, in which C is the CR concentration after a certain adsorption time.
ð2Þ
where qm (mg g1) is the maximum adsorption capacity at a monolayer coverage on the sample, and b (mg1) is the equilibrium binding constant.
3. Results 3.1. Effect of aging time 3.1.1. XRD Fig. 1(a) shows the XRD patterns of the AS-Xa samples. The precursor without ageing for spray drying only presents amorphous phase (AS-1a) while these aged at 90 °C for no less than 2 h show the characteristic diffraction peaks of the boehmite. The diffraction peaks of boehmite become sharp with the increase in ageing time. And the peak of (0 2 0) shifts to higher angles. These results indicate that the ageing facilitates the phase transition of aluminum species
Fig. 4. SEM images of the as-synthesized c-Al2O3: (a-1), (a-2): AS-1; (b-1), (b-2): AS-2; (c-1), (c-2): AS-3; (d-1), (d-2): AS-4.
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from amorphous to boehmite and promotes the crystallization of boehmite. Generally, the transition from aluminum hydroxide to boehmite is very sensitive to the experimental parameters such as type of aluminum salt, pH, temperature, time of crystallization and drying method [8–10,5,11–13,4,14–19]. Very often, additional crystalline phases such as gibbsite and/or bayerite are formed beside boehmite. The present work shows that the ageing of the freshly precipitated aluminum hydroxide from AlCl3 and NH4OH reaction system was able to produce boehmite as a single phase. Fig. 1(b) and (c) shows the XRD patterns of AS-Xb and AS-Xc samples. All the samples show intense characteristic peaks of NH4Cl, of which the crystallite sizes were about 60 nm regardless of the ageing. The big crystallite sizes of NH4Cl salts lead to the difficulty for NH4Cl to act as the solid template among aluminum hydroxides (which will be discussed in the Discussion). No boehmites are detected because of the strong signals of NH4Cl. It is interesting that c-Al2O3 was the only phase in AS-1c while boehmites with a small amount of c-Al2O3 were found in those from aged precursors, indicating that the phase transition from boehmite to c-Al2O3 happened at relative higher temperatures than that for the phase transition of aluminum hydroxide to c-Al2O3. No NH4Cl is detected. Fig. 1(d) presents the XRD patterns of the final c-Al2O3 products. It is notable that all the final products show the characteristic peaks of c-Al2O3 no matter how long the precursors were aged for spray drying.
Fig. 5. (a) Nitrogen sorption isotherms and (b) BJH pore diameter distributions of the c-Al2O3.
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3.1.2. FTIR The FTIR spectra of the AS-Xa samples are shown in Fig. 2. The spectrum of AS-1a shows a broad band centered at 3370 cm1 ascribing to the water (similar to those of liquid water) adsorbed on amorphous aluminum hydroxides. And this band disappears when the precursors were aged for more than 2 h. Three bands at 3277, 3087 and 1638 cm1 are observed, assigning to the stretching and bending modes of intercalated molecular water in the crystallized boehmites [21,22]. The band at 1066 cm1 with a shoulder at 1162 cm1, which becomes intense with the increase in ageing time, originates from the structural OH groups forming H-bonds between the AlOOH octahedral sheets [21,22]. The weak band at 892 cm1 is assigned to the surface-exposed OH groups of poorly crystalline boehmite, indicating that small part of boehmites was poorly crystallized. Bands at 725 and 685 cm1 are ascribed to the OH deformation mode of the bulk OH and the vibration of Al–O–Al, respectively [21,22]. Fig. 3 shows the TG curves of AS-Xb samples and the related results are listed in Table 2. The AS-Xb samples containing NH4Cl from the aged precursors suffer identical (8%) weight loss within the temperature range of 100–200 °C, while that from non-aged precursor show higher weight loss (12%) due to the removal of more adsorbed water on aluminum hydroxides. This verifies the FTIR result that amorphous aluminum hydroxide can absorb more water than crystallized boehmites. The affinity to the surface of boehmite crystallites allows the NH4- and Cl ions to squeeze out part of the water molecules on the surface of boehmite. This
Fig. 6. XRD patterns of the (a) AS-Xb and (b) AS-X samples.
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conclusion can also be verified by the remarkable higher weight losses of the powders washed by ethanol to remove the NH4Cl at RT-200 °C (shown in Fig. 3(b)), which are ascribed to more water adsorbed on ethanol washed boehmite. All the samples with NH4Cl suffer a significant weight loss of 68–74% at 200–500 °C, ascribing to the removal of chemisorbed molecules/NH4Cl and the phase transformation from boehmite to c-Al2O3 [23]. A similar weight loss about 15% of the samples after ageing (shown in Fig. 3(b) and Table 2) occurs at 200–500 °C when only a very small quantity of NH4Cl exists in the boehmite after washing by ethanol, which is consisted with the theoretical value of the weight loss due to the transformation from well crystallized boehmite to c-Al2O3. The weight loss of the non-aged precursor (AS-1b) is 20%, which is much higher than those of the samples
from the aged precursors, ascribing to the removal of structural water in aluminum hydroxides. The weight loss at 450–1000 °C corresponds to the dehydroxylation of the surface of alumina.
3.1.3. SEM The SEM images of the c-Al2O3 from the precursors aged for various time are shown in Fig. 4. Generally, the obtained c-Al2O3 products are spherical particles of 1–10 lm in diameter. Dense surface is obtained for the sample from the precursor without ageing, while rough surface with large pores (100–500 nm) are obtained on samples from aged precursors. It is clearly seen from the amplified photos that the features of the initial units for assembling the c-Al2O3 sphere evolve from nano-dots to nano-sheets. And the
Fig. 7. SEM images of samples: (a-1), (a-2): AS-5; (b-1), (b-2): AS-6; (c-1), (c-2): AS-7; (d-1), (d-2): AS-4.
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pore diameter in the shell is enlarged by increasing the ageing time. Apparently, the features of the final c-Al2O3 products are greatly determined by the precursors for spray drying. The amorphous aluminum hydroxide precursors lead to the dense shell, while the laminar boehmites result in the rough shell of the c-Al2O3 microsphere. The pore diameters can be adjusted by controlling the ageing time of the precipitated precursors, which will be further verified by the following N2 adsorption/desorption results. 3.1.4. N2 adsorption/desorption The nitrogen sorption isotherms and the corresponding BJH pore diameter distributions of the c-Al2O3 are shown in Fig. 5 and the pore parameters are shown in Table 1. Nitrogen adsorption isotherms of all the samples are type IV with a type H3 hysteresis loop, corresponding to characteristic mesoporous
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materials of which the mesopores arise from the stacking of the laminar units. The large N2 uptake at low relative pressures reveals the presence of microporosity of all the samples. Significant changes are observed in the N2-adsorbed volume of samples from the precursors aged for various time. An uptake of N2 as a result of capillary condensation is observed on AS-1 in a relative pressure (P P0-1) of 0.4 and reaches a turning point at 0.7, suggesting the presence of small mesoporous pores in a narrow distribution on AS-1 which is confirmed by the pore diameter distribution of AS-1 in Fig. 4(b). However, the turning point of N2-adsorbed volume on AS-2, AS-3 and AS-4 ise at 0.85 and the adsorbed volume of nitrogen further increases under higher relative pressures, ascribing to the large mesorpores (>10 nm) in samples [22]. Increasing the ageing time of the precursors produces more shrinkage of pore volume, increases the pore diameter and broadens the pore size distribution.
Fig. 8. TEM of the samples: (a-1) and (a-2): AS-1; (b-1) and (b-2): AS-4; (c-1) and (c-2): AS-5.
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As listed in Table 1, the pore volume of the samples increases from 0.26 to 0.60 cm3 g1, while BET surface area of the samples decreases from 313 to 227 m2 g1 as a result of the crystallization and crystal growth of the boehmite. 3.2. Effect of NH4Cl
3.2.1. XRD The XRD patterns of the precursors for spray drying and the final c-alumina products are shown in Fig. 6(a) and (b), respectively. No mater containing NH4Cl or not, the precursors aged for 20 h show the characteristic diffraction peaks of boehmite. And the final products are poorly crystallized c-Al2O3. No significant differences are observed in the peak intensity or position of all samples from various precursors. 3.2.2. SEM and TEM The effects of NH4Cl on the features of c-Al2O3 are shown in Fig. 7. AS-4 and AS-7, which are prepared from the precursors containing NH4Cl, have similar rough surface with plenty of large holes (100–500 nm), while AS-5 and AS-6 from precursors without NH4Cl have dense shells. It is clearly that NH4Cl in the spray drying process is one of the essential parameters for obtaining products with large mesopores and macropores. The dense shells of AS-5 and AS6 are comprised of laminar units (nano-sheets), which is different from that of AS-1 (comprised of nano-dots). To further study the effects of ageing and NH4Cl on the nanostructure of final c-alumina products, the samples were characterized by TEM (shown in Fig. 8). Samples from precursors containing NH4Cl and without ageing (AS-1) are comprised of hollow spheres aggregated by nano-dots of about 5 nm. The samples from aged precursors containing NH4Cl (AS-4) are hollow spheres assembled by nano-sheets which are consisted of nano-ribbons. Without NH4Cl in the precursor, only solid spheric c-alumina (AS-5) is obtained. Again, these results prove that NH4Cl acts as the templates for fabricating the hollow c-alumina and ageing facilities the formation of laminar c-alumina units. 3.2.3. N2 adsorption/desorption Fig. 9 shows the nitrogen sorption isotherms (a) and corresponding pore diameter distributions (b) of the c-Al2O3. A strong uptake of N2 as a result of capillary condensation is observed on AS-4 and AS-7 in a relative pressure (P P0-1) of 0.4 and reaches a turning point at 0.8, suggesting that the c-Al2O3 products belong to the mesoporous materials [5,22]. However, at relative pressures larger than 0.8, the adsorbed volume of nitrogen further increases, which indicates that AS-4 and AS-7 contain an appreciable amount of secondary porosity. In addition, the corresponding pore size distribution curves of AS-4 and AS7 present a broad peak at 4 nm and a platform in large pore diameter. Moreover, AS-4 has more large mesopores (>10 nm) than AS-7. The nitrogen sorption isotherms and pore diameter distributions of AS-5 and AS-6 suggest that these two samples only have small mesopores of about 4 nm. NH4Cl in the precursors for spray drying and ageing the precursors (with NH4Cl) facilitate the formation of c-Al2O3 samples with hierarchical micro-meso-macro-pores. It is notable that AS-5 and AS-6 show significantly higher amount of mesopores in 4 nm than those of AS-4 and AS-7. NH4Cl squeezes out part of the water molecules in boehmites, and thereby decreases the generation of small mesopores which are mainly formed by dehydration of boehmite. 3.3. CR adsorption Fig. 10 presents the adsorption capacities of samples for CR from water at room temperature. AS-4 has a high adsorption rate,
Fig. 9. (a) Nitrogen sorption isotherms and (b) pore diameter distributions of the cAl2O3.
reaching the highest value in five minutes which is almost thirty times higher than that of AS-5. The maximal adsorption capacities (qm) of AS-4 are calculated to be 322 mg g1, which is 1.7 times higher than that of AS-5 (191 mg g1) under the same testing conditions. In addition, the as-prepared hierarchical porous c-Al2O3 hollow microspheres present apparently higher CR adsorption capacity than those of boehmite hollow core/shell (113 mg g1) in reference [15] and the spindle-like c-Al2O3 (176 mg g1) in reference [24]. It is clear that the adsorption performance of c-Al2O3 microspheres can be enhanced by structuring a hollow sphere with hierarchical pores, because the existing macropores and large mesopores favor the mass transportation and make CR molecules easy to access the active sites.
4. Discussion: effects of aging and NH4Cl on the nanostructure of c-Al2O3 It is well known that spray drying is an effective way to obtain spherical particles via very fast evaporation of solvent in targetcontaining aerosol droplets [20]. In our work, we try to synthesize c-Al2O3 hollow microspheres containing hierarchical pores through the preparation of boehmite sol precursors and the following spray drying. According to the experimental results, it can be concluded that the ageing treatment and NH4Cl in the precursors are two key parameters for obtaining the hierarchical porous cAl2O3 hollow microspheres. The results of N2 adsorption–desorption isotherms verify that the meso- and macro-pores in c-Al2O3
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Fig. 10. (a) Adsorption ratio of Congo red on c-Al2O3 and (b) CR sorption isotherms of the c-Al2O3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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mainly correlates with the stacking of laminar units. The presence of micropores and small mesopores comes mainly from the poorly crystallized c-Al2O3. Several studies demonstrated the close relationship between the microstructural features of the boehmite and the structure/texture of the resulting c-Al2O3 [25,26]. Since the oxygen sublattice of boehmite is cubic packing, boehmite dehydration involves only short-range rearrangements of atoms in the crystal structure which require only a small energy. The textural property of boehmite precursor will remain after transformation to c-Al2O3. Therefore, ageing the precursors for spray drying facilitates the formation of boehmite at relative low temperatures, which is beneficial for reducing the aggregation of the final c-Al2O3 products. Under the same preparation conditions, the pore structures of c-Al2O3 products depend on the crystallinity and the stacking of laminar boehmite. For the AS-1 sample, the precursors without ageing present mainly amorphous phase and poorly crystallized boehmite. This will lead to serious aggregation of primary units (nano-dots) during the long-range rearrangements of atoms in the dehydration process of aluminum hydroxides. The overall phase transformation of aluminum hydroxides to laminar boehmites at low temperatures helps to reduce the aggregation of primary unites. However, the small mesopore derived from the poorly crystalline boehmite decreases as a consequence. If the NH4Cl salts in precursors were washed before the spray drying, only dense spheres can be obtained, strongly suggesting that the NH4Cl may act as the solid templates with aerosol approach and in further thermal treatment for obtaining the hierarchical porous c-Al2O3 hollow microsphere. When the boehmite sols were spray-dried, the volume fraction of solids increased, leading to the aggregation of primary particles and finally to gel formation. The rapid crystallization of NH4Cl at the air-droplet interface may be fixed by the condensation reactions among solids. The removal of NH4Cl in the successive calcination leaves the vacancies and results in the increased pore volumes of the c-Al2O3 products (shown in Scheme 1-III). The model of the formation of the hierarchical porous hollow microsphere in the spray drying process can be presumed in Scheme 1. At first, when an aerosol droplet enters a heated zone,
Scheme 1. The mechanism of the formation of c-Al2O3 microspheres.
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it experiences a temperature gradient, with the surface of the aerosol droplets being heated first and then heat being transferred to the core. Secondly, as heating of the droplet proceeds, water containing dissolved species diffuses from the interface to the core due to the improved ion concentration at the interface because of water evaporation; simultaneously, part of boehmites aggregate as a shell and salts begin to nucleate and be fixed by primary units. Thirdly, when water evaporated completely, NH4Cl crystallized and aggregated as a core. Once the NH4Cl separates from the precursors, the structure becomes fixed by condensation reactions between the starting boehmites, inhibiting the sintering of the boehmite and the resulting c-Al2O3 by isolating the structural hydroxyl neighbors during the thermal treatment at 500 °C (Scheme 1-III). It is notable that NH4Cl did not prevent the aggregation of c-Al2O3 which leads to the dense shell of AS-1 (Scheme 1-I). From the results of XRD, the crystal diameter calculated according to the Scherrer formula is around 60 nm, which are much larger than the interspace of nano-dots (5 nm in diameter) formed in the spray process. Therefore, it is more possible that NH4Cl is formed and the aluminum hydroxides were segregated in the spray drying process. NH4Cl may present on the surface of shell and in the core of the micro-sphere, which cannot prevent the aggregation of nano-dots during the phase transformation from aluminum hydroxides to c-Al2O3. However, the large space from stacking of laminar boehmites may help to fix the crystallized NH4Cl which can act as solid template for obtaining large mesopores and macropores on the shell of c-Al2O3 particle. 5. Conclusions Hierarchical porous c-Al2O3 hollow microspheres were synthesized by a modified spray drying method, which can be divided into three steps: precipitation followed by ageing at 90 °C, spray drying assisted by NH4Cl salts and further thermal treatment at 500 °C. Ageing the precipitated precursor and spray-drying assisted by NH4Cl salts are considered as two key steps for obtaining the hollow c-Al2O3 microspheres containing micro-meso-macropores. The mechanism for obtaining the nano-structure of c-Al2O3 was proposed involving phase transformation from aluminum hydroxide to laminar boehmite during ageing, followed by a subsequent self-assembly process with NH4Cl as template via spray drying. The formation of meso-/macro-pores in c-Al2O3 was mainly correlated with the stacking of laminar boehmite. The role of NH4Cl was demonstrated to be a template to act as the core and prevent the aggregation of laminar boehmites.
Acknowledgments The authors would like to acknowledge the National Natural Science Foundation of China for financial support of Project 51202126. Moreover, we would also thank the financial support from Postdoctoral Science Foundation of China (2012M520266) and Strategic Emerging Industry Development Funds of Shenzhen (JCYJ20120619152738634).
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile synthesis of hierarchical porous c-Al2O3 hollow microspheres for water treatment Mingyang Li a, Zhichun Si a,⇑, Xiaodong Wu a,b, Duan Weng a,b, Feiyu Kang a,b a b
Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China State Key Laboratory of New Ceramics & Fine Process, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 14 October 2013 Accepted 26 November 2013 Available online 12 December 2013 Keywords: c-Al2O3 hollow microspheres Hierarchical porous Ageing Spray-drying NH4Cl template
a b s t r a c t Hierarchical porous c-Al2O3 hollow microspheres were synthesized by a modified spray drying method. Ageing the precipitated precursor and spray-drying assisted by NH4Cl salts are considered as two key steps for the synthesis of c-Al2O3 hollow microspheres. The mechanism of the formation of hierarchical porous c-Al2O3 hollow microsphere was proposed involving phase transformation from aluminum hydroxide to laminar boehmite during ageing and a following self-assembling process with NH4Cl as the template during spray drying. The meso-/macro-pores in c-Al2O3 mainly arise from the stacking of the laminar boehmites which are obtained by ageing the precipitated precursors at 90 °C. NH4Cl, which was the byproduct from the reaction between AlCl36H2O and NH3H2O, was demonstrated to be an excellent template to act as the core and the barrier for separation of laminar boehmites. No extra NH4Cl was added. The as-synthesized hierarchical porous c-Al2O3 hollow microsphere presented remarkably higher adsorption capacity, which is thirty times higher adsorption rate for Congo Red than the solid microsphere containing only small mesopores. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The development of synthetic techniques that can be predictably manipulated to yield materials with defined and controllable features is central to modern materials science [1–3]. Especially, the routes to synthesize materials with hierarchical micro, mesoand macro-porous structures for use in applications requiring high diffusion rates and/or pressures, have attracted much attention [4–7]. The macropores and mesopores in adsorbents favor the mass transportation which makes guest molecules easy to access the active sites. Porous c-Al2O3 is widely used as absorbents and catalyst support due to its instinct microporous structures, large surface area, high stability and various acid/basic sites. c-Al2O3 with special features, such as hollow sphere [8,9] and foams with hierarchical pores [10,5], have been developed to receive the maximum benefit as the absorbent in recent years. Typically, sol–gel chemistry combined with template is employed to produce hierarchical porous frameworks; however, the preparation of meso- and macro-porous structures via sol–gel chemistry is limited due to the fragility of large-pored networks to the thermal treatments needed for crystallization. Holland et al. [11] synthesized highly organized macroporous–mesoporous
⇑ Corresponding author. Fax: +86 755 26036417. E-mail address: [email protected] (Z. Si). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.11.071
alumina by using the combination of metal alkoxide gelation and polystyrene sphere (PS) template approach. Kuemmel et al. [12] reported a method combining the block copolymer structuring way and the evaporation induced self-assembly (EISA) method for synthesizing nanocrystalline c-Al2O3 layers with contracted face-centered cubic (fcc) mesoporosity. Recently, Wu et al. [5] synthesized alumina with ordered mesostructure and hierarchical porosity via EISA method using aluminum iso-propoxide as an inorganic precursor and pluronic P123 as a template. Zhang et al. [13] fabricated hierarchically macro/mesoporous alumina monoliths through a facile sol–gel process by introducing PEG8000 to the reaction solution of ordered mesoporous Al2O3 (OMA, P123/isopropoxide aluminum/HNO3). Most of previous reports about the hierarchical porous alumina are based on the colloid crystals template, which are obviously laborious and time-consuming and the selected template materials are often expensive. Therefore, it is necessary to find a facile and simple method for the synthesis of c-Al2O3 with hierarchical pores is still a challenge. The most widely used methods for fabricating c-Al2O3 with hollow structure are nanocasting on soft or hard templates [8,4,14], chemically induced self-transformation/assembly [15,16] and spray drying or pyrolysis [9,17–19]. Dong et al. [8] reported a two-step nanocasting route to prepare monodisperse and highsurface-area mesoporous c-Al2O3 spheres, using silica spheres as primary hard templates for obtaining carbon spheres which was
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Table 1 Manipulation parameters of samples. Sample
Aging time (h)
NH4Cl concentration in aging
NH4Cl concentration In spray drying
BET (m2 g1)
Vsp (cm3 g1)
AS-1 AS-2 AS-3 AS-4 AS-5 AS-6 AS-7
– 2 4 20 20 20 20
Al:Cl = 1:3 Al:Cl = 1:3 Al:Cl = 1:3 Al:Cl = 1:3 – Al:Cl = 1:3 –
Al:Cl = 1:3 Al:Cl = 1:3 Al:Cl = 1:3 Al:Cl = 1:3 – – Al:Cl = 1:3
313 226 287 227 328 322 274
0.26 0.34 0.48 0.60 0.47 0.44 0.50
used as the secondary hard template for aluminum precursor impregnation. Monodisperse hollow spheres with a non-aggregated character were synthesized by Martins et al. [4]. Aluminum species were firstly implanted into polymeric template particles to form a core–shell structure in solvent and the template cores were then removed by thermal pyrolysis to form hollow interiors. The nanocasting methods based on the templates are advantageous for final particles with narrow size distribution, but have intrinsic disadvantages, ranging from the inherent difficulty of achieving high product yields from the multistep synthetic process to the lack of structural robustness of the shells upon template removal. Chemically induced self-transformation/assembly method for the synthesis of hollow sphere c-Al2O3 successfully avoids the use of template and related manipulation process [15,16]. Unfortunately, the mass production of uniformly hollow sphere c-Al2O3 is unresolved. Spray drying or pyrolysis methods have been developed for mass production of hollow sphere c-Al2O3 and other metal oxides in recent years. The final product or intermediates with hollow sphere structure can be continuously produced via the self tightening of particles in droplets by fast evaporation of solvent [9,17–20]. However, particles produced by traditional spray drying
or pyrolysis method have thick shells, broad size distributions and significant aggregations [9,17–20]. Therefore, mass production of c-Al2O3 hollow spheres with variable compositions, controllable morphologies, and tunable wall structures are of particular and urgency. We report a modified spray drying method for the mass production of hierarchical porous c-Al2O3 hollow spheres in this research. The boehmite precursor for spray drying was prepared via precipitation followed by aging at 90 °C. The NH4Cl, which are the byproducts of the precipitation reaction between AlCl36H2O and NH3H2O, were adopted as the template for obtaining hollow microspheres containing hierarchical pores. Congo Red (CR) was used as the target for characterizing the adsorption performances of the as-synthesized c-Al2O3.’ 2. Experimental 2.1. Materials and the synthesis of c-Al2O3 All the reagents were analytical grade. 18.93 g of AlCl36H2O (99.8%, Alladin, China) was dissolved into 200 mL of de-ionized
Fig. 1. XRD patterns of (a) AS-Xa, (b) AS-Xb, (c) AS-Xc and (d) AS-X samples.
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laboratory Spray Dryer (YC-015, China) at a feed rate of 5 ml min1. The flow rate of Spray Dryer was 90 L min1. The inlet and outlet temperature were kept at 200 °C and 105 °C, respectively. The obtained dried powders (named AS-Xb (X = 1, 2, . . .)) were further calcinated in a tubular furnace at 500 °C for 3 h under air purging (200 ml min1) to get the final c-Al2O3. The exhaust of the tubular furnace was passed through the water to retrieve the NH4Cl. To investigate the intermediates, AS-Xb powders were also calcinated at 350 °C for 3 h (named AS-Xc (X = 1, 2, . . .)). Various samples were synthesized to investigate the effects of ageing time and NH4Cl on the aged precursors and the final cAl2O3 product. The related manipulation parameters were listed in Table 1. For example, AS-5 was synthesized as follows: the precipitates were centrifuged and washed by de-ionized water for several times until no chlorides were detected in the suspension before ageing; and the other conditions were kept the same as mentioned above. Fig. 2. FTIR spectra of the AS-Xa samples.
2.2. Characterizations The powder X-ray diffraction (XRD) experiments were performed on a Japan science D/MAX-2000 diffractometer employing Cu Ka radiation (k = 0.15418 nm). The X-ray tube was operated at 40 kV and 100 mA. The X-ray powder diffractogram was recorded at 0.02° intervals in range of 5° < 2h < 80°. Scanning electron microscopy (SEM) was performed on an emission scanning electron microscopy (S-4800) at an accelerating voltage of 5 kV. Transmission electron microscope (TEM) was performed on FEI Tecnai G2 with an emission gun operating at 120 kV. Nitrogen adsorption isotherms were measured on Quantachrome NOVA apparatus. All the samples were degassed at 220 °C for 1 h prior to the nitrogen adsorption measurements. The BET surface area was determined by a multipoint BET method, using the adsorption data in the relative pressure (P P 1 0 ) range of 0.05–0.3. The adsorption branch of nitrogen adsorption–desorption isotherms was used to determine the pore size distribution by the Barret–Joyner–Halender (BJH) method, assuming a cylindrical pore model. FTIR spectra were collected on Nicolet iS10 apparatus equipped with a Smart ART appendix. The spectral resolution was 4 cm1. All the samples were washed to remove the NH4Cl before FTIR test, because the intense signal of NH4Cl can cover those of aluminum hydroxide or boehmite.
2.3. Adsorption experiments The as-synthesized c-Al2O3 samples were used as adsorbents for CR. 20 mg of c-Al2O3 were dispersed in 100 mL CR solution (pH = 7). The suspension was stirred at a constant speed of 400 rpm with a required adsorption time at room temperature. Then, the CR concentrations were analyzed by a UV–vis spectrophotometer (Scino S-4100, Korea). The characteristic absorptions of Congo red around 500 nm was used to monitor the CR concenFig. 3. The TG curves of the (a) AS-Xb samples and (b) AS-Xb samples washed by ethanol to remove NH4Cl.
Table 2 The TG results of AS-Xb samples. Sample
water at room temperature to obtained aluminum solution. Then, excess NH3H2O (25%, AR, Alladin, China) were added dropwise into the aluminum solution under magnetic stirring to get the suspension with pH at 8. The obtained suspension was aged at 90 °C for various time to get the aged precursor for spray drying (named AS-Xa (X = 1, 2, . . .)). The precursors were spray-dried in a
AS-1b AS-2b AS-3b AS-4b
Weight loss of the samples with NH4Cl (%)
Weight loss of the samples without NH4Cl (%)
RT-200 °C
200–500 °C
RT-200 °C
200–500 °C
15 8 8 8
68 72 72 74
30 20 20 15
20 15 15 15
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tration. The adsorption amount of CR (qe) on sample was calculated using the equations:
qe ¼
ðC 0 C e Þ V W
ð1Þ
where C0 is the initial concentration of CR (mg L1); Ce is the equilibrium concentration of CR after adsorption (mg L1); V is the volume of CR solution (L); W is the weight of the synthesized adsorbent (g). Adsorption isotherms were obtained by fitting the qe vs. CR concentrations according to the Langmuir equation:
qe ¼
qm bC e 1 þ bC e
The initial CR concentration is 40 mg L1 for CR adsorption ratio test. The CR adsorption ratio was defined as C/C0, in which C is the CR concentration after a certain adsorption time.
ð2Þ
where qm (mg g1) is the maximum adsorption capacity at a monolayer coverage on the sample, and b (mg1) is the equilibrium binding constant.
3. Results 3.1. Effect of aging time 3.1.1. XRD Fig. 1(a) shows the XRD patterns of the AS-Xa samples. The precursor without ageing for spray drying only presents amorphous phase (AS-1a) while these aged at 90 °C for no less than 2 h show the characteristic diffraction peaks of the boehmite. The diffraction peaks of boehmite become sharp with the increase in ageing time. And the peak of (0 2 0) shifts to higher angles. These results indicate that the ageing facilitates the phase transition of aluminum species
Fig. 4. SEM images of the as-synthesized c-Al2O3: (a-1), (a-2): AS-1; (b-1), (b-2): AS-2; (c-1), (c-2): AS-3; (d-1), (d-2): AS-4.
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from amorphous to boehmite and promotes the crystallization of boehmite. Generally, the transition from aluminum hydroxide to boehmite is very sensitive to the experimental parameters such as type of aluminum salt, pH, temperature, time of crystallization and drying method [8–10,5,11–13,4,14–19]. Very often, additional crystalline phases such as gibbsite and/or bayerite are formed beside boehmite. The present work shows that the ageing of the freshly precipitated aluminum hydroxide from AlCl3 and NH4OH reaction system was able to produce boehmite as a single phase. Fig. 1(b) and (c) shows the XRD patterns of AS-Xb and AS-Xc samples. All the samples show intense characteristic peaks of NH4Cl, of which the crystallite sizes were about 60 nm regardless of the ageing. The big crystallite sizes of NH4Cl salts lead to the difficulty for NH4Cl to act as the solid template among aluminum hydroxides (which will be discussed in the Discussion). No boehmites are detected because of the strong signals of NH4Cl. It is interesting that c-Al2O3 was the only phase in AS-1c while boehmites with a small amount of c-Al2O3 were found in those from aged precursors, indicating that the phase transition from boehmite to c-Al2O3 happened at relative higher temperatures than that for the phase transition of aluminum hydroxide to c-Al2O3. No NH4Cl is detected. Fig. 1(d) presents the XRD patterns of the final c-Al2O3 products. It is notable that all the final products show the characteristic peaks of c-Al2O3 no matter how long the precursors were aged for spray drying.
Fig. 5. (a) Nitrogen sorption isotherms and (b) BJH pore diameter distributions of the c-Al2O3.
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3.1.2. FTIR The FTIR spectra of the AS-Xa samples are shown in Fig. 2. The spectrum of AS-1a shows a broad band centered at 3370 cm1 ascribing to the water (similar to those of liquid water) adsorbed on amorphous aluminum hydroxides. And this band disappears when the precursors were aged for more than 2 h. Three bands at 3277, 3087 and 1638 cm1 are observed, assigning to the stretching and bending modes of intercalated molecular water in the crystallized boehmites [21,22]. The band at 1066 cm1 with a shoulder at 1162 cm1, which becomes intense with the increase in ageing time, originates from the structural OH groups forming H-bonds between the AlOOH octahedral sheets [21,22]. The weak band at 892 cm1 is assigned to the surface-exposed OH groups of poorly crystalline boehmite, indicating that small part of boehmites was poorly crystallized. Bands at 725 and 685 cm1 are ascribed to the OH deformation mode of the bulk OH and the vibration of Al–O–Al, respectively [21,22]. Fig. 3 shows the TG curves of AS-Xb samples and the related results are listed in Table 2. The AS-Xb samples containing NH4Cl from the aged precursors suffer identical (8%) weight loss within the temperature range of 100–200 °C, while that from non-aged precursor show higher weight loss (12%) due to the removal of more adsorbed water on aluminum hydroxides. This verifies the FTIR result that amorphous aluminum hydroxide can absorb more water than crystallized boehmites. The affinity to the surface of boehmite crystallites allows the NH4- and Cl ions to squeeze out part of the water molecules on the surface of boehmite. This
Fig. 6. XRD patterns of the (a) AS-Xb and (b) AS-X samples.
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conclusion can also be verified by the remarkable higher weight losses of the powders washed by ethanol to remove the NH4Cl at RT-200 °C (shown in Fig. 3(b)), which are ascribed to more water adsorbed on ethanol washed boehmite. All the samples with NH4Cl suffer a significant weight loss of 68–74% at 200–500 °C, ascribing to the removal of chemisorbed molecules/NH4Cl and the phase transformation from boehmite to c-Al2O3 [23]. A similar weight loss about 15% of the samples after ageing (shown in Fig. 3(b) and Table 2) occurs at 200–500 °C when only a very small quantity of NH4Cl exists in the boehmite after washing by ethanol, which is consisted with the theoretical value of the weight loss due to the transformation from well crystallized boehmite to c-Al2O3. The weight loss of the non-aged precursor (AS-1b) is 20%, which is much higher than those of the samples
from the aged precursors, ascribing to the removal of structural water in aluminum hydroxides. The weight loss at 450–1000 °C corresponds to the dehydroxylation of the surface of alumina.
3.1.3. SEM The SEM images of the c-Al2O3 from the precursors aged for various time are shown in Fig. 4. Generally, the obtained c-Al2O3 products are spherical particles of 1–10 lm in diameter. Dense surface is obtained for the sample from the precursor without ageing, while rough surface with large pores (100–500 nm) are obtained on samples from aged precursors. It is clearly seen from the amplified photos that the features of the initial units for assembling the c-Al2O3 sphere evolve from nano-dots to nano-sheets. And the
Fig. 7. SEM images of samples: (a-1), (a-2): AS-5; (b-1), (b-2): AS-6; (c-1), (c-2): AS-7; (d-1), (d-2): AS-4.
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pore diameter in the shell is enlarged by increasing the ageing time. Apparently, the features of the final c-Al2O3 products are greatly determined by the precursors for spray drying. The amorphous aluminum hydroxide precursors lead to the dense shell, while the laminar boehmites result in the rough shell of the c-Al2O3 microsphere. The pore diameters can be adjusted by controlling the ageing time of the precipitated precursors, which will be further verified by the following N2 adsorption/desorption results. 3.1.4. N2 adsorption/desorption The nitrogen sorption isotherms and the corresponding BJH pore diameter distributions of the c-Al2O3 are shown in Fig. 5 and the pore parameters are shown in Table 1. Nitrogen adsorption isotherms of all the samples are type IV with a type H3 hysteresis loop, corresponding to characteristic mesoporous
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materials of which the mesopores arise from the stacking of the laminar units. The large N2 uptake at low relative pressures reveals the presence of microporosity of all the samples. Significant changes are observed in the N2-adsorbed volume of samples from the precursors aged for various time. An uptake of N2 as a result of capillary condensation is observed on AS-1 in a relative pressure (P P0-1) of 0.4 and reaches a turning point at 0.7, suggesting the presence of small mesoporous pores in a narrow distribution on AS-1 which is confirmed by the pore diameter distribution of AS-1 in Fig. 4(b). However, the turning point of N2-adsorbed volume on AS-2, AS-3 and AS-4 ise at 0.85 and the adsorbed volume of nitrogen further increases under higher relative pressures, ascribing to the large mesorpores (>10 nm) in samples [22]. Increasing the ageing time of the precursors produces more shrinkage of pore volume, increases the pore diameter and broadens the pore size distribution.
Fig. 8. TEM of the samples: (a-1) and (a-2): AS-1; (b-1) and (b-2): AS-4; (c-1) and (c-2): AS-5.
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As listed in Table 1, the pore volume of the samples increases from 0.26 to 0.60 cm3 g1, while BET surface area of the samples decreases from 313 to 227 m2 g1 as a result of the crystallization and crystal growth of the boehmite. 3.2. Effect of NH4Cl
3.2.1. XRD The XRD patterns of the precursors for spray drying and the final c-alumina products are shown in Fig. 6(a) and (b), respectively. No mater containing NH4Cl or not, the precursors aged for 20 h show the characteristic diffraction peaks of boehmite. And the final products are poorly crystallized c-Al2O3. No significant differences are observed in the peak intensity or position of all samples from various precursors. 3.2.2. SEM and TEM The effects of NH4Cl on the features of c-Al2O3 are shown in Fig. 7. AS-4 and AS-7, which are prepared from the precursors containing NH4Cl, have similar rough surface with plenty of large holes (100–500 nm), while AS-5 and AS-6 from precursors without NH4Cl have dense shells. It is clearly that NH4Cl in the spray drying process is one of the essential parameters for obtaining products with large mesopores and macropores. The dense shells of AS-5 and AS6 are comprised of laminar units (nano-sheets), which is different from that of AS-1 (comprised of nano-dots). To further study the effects of ageing and NH4Cl on the nanostructure of final c-alumina products, the samples were characterized by TEM (shown in Fig. 8). Samples from precursors containing NH4Cl and without ageing (AS-1) are comprised of hollow spheres aggregated by nano-dots of about 5 nm. The samples from aged precursors containing NH4Cl (AS-4) are hollow spheres assembled by nano-sheets which are consisted of nano-ribbons. Without NH4Cl in the precursor, only solid spheric c-alumina (AS-5) is obtained. Again, these results prove that NH4Cl acts as the templates for fabricating the hollow c-alumina and ageing facilities the formation of laminar c-alumina units. 3.2.3. N2 adsorption/desorption Fig. 9 shows the nitrogen sorption isotherms (a) and corresponding pore diameter distributions (b) of the c-Al2O3. A strong uptake of N2 as a result of capillary condensation is observed on AS-4 and AS-7 in a relative pressure (P P0-1) of 0.4 and reaches a turning point at 0.8, suggesting that the c-Al2O3 products belong to the mesoporous materials [5,22]. However, at relative pressures larger than 0.8, the adsorbed volume of nitrogen further increases, which indicates that AS-4 and AS-7 contain an appreciable amount of secondary porosity. In addition, the corresponding pore size distribution curves of AS-4 and AS7 present a broad peak at 4 nm and a platform in large pore diameter. Moreover, AS-4 has more large mesopores (>10 nm) than AS-7. The nitrogen sorption isotherms and pore diameter distributions of AS-5 and AS-6 suggest that these two samples only have small mesopores of about 4 nm. NH4Cl in the precursors for spray drying and ageing the precursors (with NH4Cl) facilitate the formation of c-Al2O3 samples with hierarchical micro-meso-macro-pores. It is notable that AS-5 and AS-6 show significantly higher amount of mesopores in 4 nm than those of AS-4 and AS-7. NH4Cl squeezes out part of the water molecules in boehmites, and thereby decreases the generation of small mesopores which are mainly formed by dehydration of boehmite. 3.3. CR adsorption Fig. 10 presents the adsorption capacities of samples for CR from water at room temperature. AS-4 has a high adsorption rate,
Fig. 9. (a) Nitrogen sorption isotherms and (b) pore diameter distributions of the cAl2O3.
reaching the highest value in five minutes which is almost thirty times higher than that of AS-5. The maximal adsorption capacities (qm) of AS-4 are calculated to be 322 mg g1, which is 1.7 times higher than that of AS-5 (191 mg g1) under the same testing conditions. In addition, the as-prepared hierarchical porous c-Al2O3 hollow microspheres present apparently higher CR adsorption capacity than those of boehmite hollow core/shell (113 mg g1) in reference [15] and the spindle-like c-Al2O3 (176 mg g1) in reference [24]. It is clear that the adsorption performance of c-Al2O3 microspheres can be enhanced by structuring a hollow sphere with hierarchical pores, because the existing macropores and large mesopores favor the mass transportation and make CR molecules easy to access the active sites.
4. Discussion: effects of aging and NH4Cl on the nanostructure of c-Al2O3 It is well known that spray drying is an effective way to obtain spherical particles via very fast evaporation of solvent in targetcontaining aerosol droplets [20]. In our work, we try to synthesize c-Al2O3 hollow microspheres containing hierarchical pores through the preparation of boehmite sol precursors and the following spray drying. According to the experimental results, it can be concluded that the ageing treatment and NH4Cl in the precursors are two key parameters for obtaining the hierarchical porous cAl2O3 hollow microspheres. The results of N2 adsorption–desorption isotherms verify that the meso- and macro-pores in c-Al2O3
M. Li et al. / Journal of Colloid and Interface Science 417 (2014) 369–378
Fig. 10. (a) Adsorption ratio of Congo red on c-Al2O3 and (b) CR sorption isotherms of the c-Al2O3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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mainly correlates with the stacking of laminar units. The presence of micropores and small mesopores comes mainly from the poorly crystallized c-Al2O3. Several studies demonstrated the close relationship between the microstructural features of the boehmite and the structure/texture of the resulting c-Al2O3 [25,26]. Since the oxygen sublattice of boehmite is cubic packing, boehmite dehydration involves only short-range rearrangements of atoms in the crystal structure which require only a small energy. The textural property of boehmite precursor will remain after transformation to c-Al2O3. Therefore, ageing the precursors for spray drying facilitates the formation of boehmite at relative low temperatures, which is beneficial for reducing the aggregation of the final c-Al2O3 products. Under the same preparation conditions, the pore structures of c-Al2O3 products depend on the crystallinity and the stacking of laminar boehmite. For the AS-1 sample, the precursors without ageing present mainly amorphous phase and poorly crystallized boehmite. This will lead to serious aggregation of primary units (nano-dots) during the long-range rearrangements of atoms in the dehydration process of aluminum hydroxides. The overall phase transformation of aluminum hydroxides to laminar boehmites at low temperatures helps to reduce the aggregation of primary unites. However, the small mesopore derived from the poorly crystalline boehmite decreases as a consequence. If the NH4Cl salts in precursors were washed before the spray drying, only dense spheres can be obtained, strongly suggesting that the NH4Cl may act as the solid templates with aerosol approach and in further thermal treatment for obtaining the hierarchical porous c-Al2O3 hollow microsphere. When the boehmite sols were spray-dried, the volume fraction of solids increased, leading to the aggregation of primary particles and finally to gel formation. The rapid crystallization of NH4Cl at the air-droplet interface may be fixed by the condensation reactions among solids. The removal of NH4Cl in the successive calcination leaves the vacancies and results in the increased pore volumes of the c-Al2O3 products (shown in Scheme 1-III). The model of the formation of the hierarchical porous hollow microsphere in the spray drying process can be presumed in Scheme 1. At first, when an aerosol droplet enters a heated zone,
Scheme 1. The mechanism of the formation of c-Al2O3 microspheres.
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it experiences a temperature gradient, with the surface of the aerosol droplets being heated first and then heat being transferred to the core. Secondly, as heating of the droplet proceeds, water containing dissolved species diffuses from the interface to the core due to the improved ion concentration at the interface because of water evaporation; simultaneously, part of boehmites aggregate as a shell and salts begin to nucleate and be fixed by primary units. Thirdly, when water evaporated completely, NH4Cl crystallized and aggregated as a core. Once the NH4Cl separates from the precursors, the structure becomes fixed by condensation reactions between the starting boehmites, inhibiting the sintering of the boehmite and the resulting c-Al2O3 by isolating the structural hydroxyl neighbors during the thermal treatment at 500 °C (Scheme 1-III). It is notable that NH4Cl did not prevent the aggregation of c-Al2O3 which leads to the dense shell of AS-1 (Scheme 1-I). From the results of XRD, the crystal diameter calculated according to the Scherrer formula is around 60 nm, which are much larger than the interspace of nano-dots (5 nm in diameter) formed in the spray process. Therefore, it is more possible that NH4Cl is formed and the aluminum hydroxides were segregated in the spray drying process. NH4Cl may present on the surface of shell and in the core of the micro-sphere, which cannot prevent the aggregation of nano-dots during the phase transformation from aluminum hydroxides to c-Al2O3. However, the large space from stacking of laminar boehmites may help to fix the crystallized NH4Cl which can act as solid template for obtaining large mesopores and macropores on the shell of c-Al2O3 particle. 5. Conclusions Hierarchical porous c-Al2O3 hollow microspheres were synthesized by a modified spray drying method, which can be divided into three steps: precipitation followed by ageing at 90 °C, spray drying assisted by NH4Cl salts and further thermal treatment at 500 °C. Ageing the precipitated precursor and spray-drying assisted by NH4Cl salts are considered as two key steps for obtaining the hollow c-Al2O3 microspheres containing micro-meso-macropores. The mechanism for obtaining the nano-structure of c-Al2O3 was proposed involving phase transformation from aluminum hydroxide to laminar boehmite during ageing, followed by a subsequent self-assembly process with NH4Cl as template via spray drying. The formation of meso-/macro-pores in c-Al2O3 was mainly correlated with the stacking of laminar boehmite. The role of NH4Cl was demonstrated to be a template to act as the core and prevent the aggregation of laminar boehmites.
Acknowledgments The authors would like to acknowledge the National Natural Science Foundation of China for financial support of Project 51202126. Moreover, we would also thank the financial support from Postdoctoral Science Foundation of China (2012M520266) and Strategic Emerging Industry Development Funds of Shenzhen (JCYJ20120619152738634).
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