Synthesis of ordered mesoporous alumina with large pore sizes and hierarchical structure

Synthesis of ordered mesoporous alumina with large pore sizes and hierarchical structure

Microporous and Mesoporous Materials 143 (2011) 406–412 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homep...

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Microporous and Mesoporous Materials 143 (2011) 406–412

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Synthesis of ordered mesoporous alumina with large pore sizes and hierarchical structure Qingling Wu, Fan Zhang ⇑, Jianping Yang, Qiang Li, Bo Tu, Dongyuan Zhao ⇑ Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, PR China

a r t i c l e

i n f o

Article history: Received 17 November 2010 Received in revised form 4 March 2011 Accepted 19 March 2011 Available online 24 March 2011 Keywords: Mesoporous materials Alumina Synthesis Self-assembly Templating

a b s t r a c t Alumina materials with ordered mesostructure and hierarchical porosity have been synthesized via a one-step process using aluminum iso-propoxide as an inorganic precursor, pluronic P123 as a template, hydrochloric acid and citric acid as the pH adjustors, and 1,3,5-trimethylbenzene (TMB) as a swelling agent. These mesoporous aluminas have relatively high surface areas (up to 309 m2/g), pore volumes (0.51 cm3/g), large pore sizes (up to 7.5 nm), and high thermal stability (up to 900 °C). In addition, the weight ratios of TMB/P123 play an important role in the synthesis process for controlling the mesostructures and pore sizes of the materials. The alumina with highly ordered 2-D hexagonal mesostructure (space group p6mm) can be synthesized with the weight ratios of TMB/P123 ranging from 0 to 3. Simultaneously, the pore sizes of ordered mesoporous aluminas were gradually enlarged with the increase of the TMB content. However, phase transformation of the mesoporous alumina from the 2-D hexagonal to hierarchical could be realized when the TMB/P123 weight ratio was increased to 5. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Alumina, a well-known industry catalyst and/or catalyst support binder, has triggered enormous research activities regarding its rational syntheses and applications in petroleum refinement, automobile emission control, and so on [1–7], due to its abundant Lewis-acid sites and excellent thermal stability. Compared with bulk alumina materials, mesoporous materials with large specific surface areas (up to 700 m2/g) possess much more abundant active sites. Meanwhile, the mesostructure is favorable for the diffusion of molecules. Therefore, many synthesis strategies such as the surfactant-directing and hard-templating methods have been used to prepare ordered mesoporous alumina with high surface area and narrow pore size distribution [8–14]. Davis and co-workers synthesized high-surface-area mesoporous alumina (ca. 710 m2/g) with small pore size (2 nm) by reacting aluminum alkoxides and carboxylic acids with controlled amounts of water in lowmolecular-weight alcoholic solvents [15]. The nanocasting method, due to the diversity of hard templates and their rigid frameworks, provides opportunities for preparing ordered mesoporous alumina materials [14,16]. However, it is an obviously laborious and industrial unfavorable method with multi-step and time-consuming procedure. Recently, Yuan et al. successfully synthesized ordered ⇑ Corresponding author. Tel.: +86 21 5163 0205; fax: +86 21 5163 0307. E-mail addresses: [email protected] (F. Zhang), [email protected] (D. Zhao). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.03.033

mesoporous aluminas with larger surface areas (ca. 400 m2/g), narrow pore size distributions (4–6 nm), and highly thermal stability (up to 1000 °C) by using surfactant-directing method [17]. Cejka and co-workers suggested a possibility to tune the composition of reaction products by proper choice of organized mesoporous alumina catalysts with different pore sizes, indicating that the pore size of mesoporous alumina was well correlated with catalytic activity of catalysts [4]. However, if the pore size of ordered mesoporous alumina can be further enlarged, it would be more in favor of the diffusion and transport of large molecules. So, it is really desirable to fabricate highly ordered mesoporous alumina with high surface area and larger pore size using a simple method. On the other hand, the fabrication of hierarchical structures at multiple length scales is highly desirable in catalysis, sorption, and separation science, since the interconnected pore networks could efficiently transport guest molecules to framework binding sites. Therefore, the synthesis of hierarchical mesoporous/macroporous alumina has attracted much interest from both a fundamental and practical viewpoint [18–24]. Recently, Dacquin and co-workers synthesized highly organized and tunable macroporous–mesoporous alumina by using the combination of surfactant and colloidal crystal templating methods [22]. In addition, a bimodal hierarchical porous alumina synthesized using alkyl carboxylates and polystyrene (PS) beads as templates has exhibited a faster adsorption rate toward anionic dye than the unimodal counterpart [25]. However, most of previous reports about the hierarchical porous alumina are based on the colloid crystals template,

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which are obviously laborious and time-consuming. Therefore, to find a facile and simple method for the synthesis of hierarchical alumina is still a challenge. In the surfactant-directing synthesis process, some hydrophobic molecules are usually used to enlarge the pore sizes of the mesoporous materials since the volume of the hydrophobic part in the surfactant could be increased by these swelling agents. Beck et al. have shown that large porous materials could be prepared by addition of auxiliary organics to MCM-41 synthesis mixtures [26]. Fan and co-workers synthesized a modified mesoporous SBA-15 with interconnecting 3-D large pore networks by introducing TMB into embryo mesostructures [27]. Mesostructured cellular foams as a new class of three-dimensional hydrothermally robust materials with ultralarge mesopores controlled by the amount of organic oil added have been synthesized [28–30]. Under the enlightenment of these reports, incorporating hydrophobic molecules in the synthesis process should be a good choice to obtain ordered mesoporous alumina with large and adjustable pore size, high thermal stability. However, to the best of our knowledge, up to now, there are few reports for the synthesis of highly ordered mesoporous alumina with tunable pore size and hierarchical mesostructure using the swelling agent. In the present work, we report the synthesis of mesoporous alumina with tailorable structure and pore size assisted by organic swelling agent TMB. The alumina products with highly ordered 2-D hexagonal mesostructure (space group p6mm) can be synthesized with the TMB/P123 weight ratios ranging from 0 to 3. Simultaneously, the pore size of ordered mesoporous alumina can be gradually enlarged to 7.5 nm with increasing TMB content in this process. Interestingly, a mesostructure transformation of the mesoporous alumina from the 2-D hexagonal to hierarchical could be realized in the large amount of TMB swelling agent (TMB/P123 weight ratio of 5). 2. Experimental 2.1. Chemicals Triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) Pluronic P123 (EO20PO70EO20, Mav = 5800 g/mol) was purchased from Aldrich Chemical Inc. Aluminum iso-propoxide, hydrochloric acid, citric acid, 1, 3, 5-trimethylbenzene (TMB), and absolute ethanol were purchased from Shanghai Chemical Co. All chemicals were used as received without any further purification. Millipore water was used in all experiments.

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maintained for 4 h and a second step to 900 °C (10 °C/min) and maintained for 1 h, respectively. The final samples were denoted as Al2O3-x-y, where x and y represent the weight ratios of TMB/ P123 (from 0 to 5) and the final pyrolysis temperature respectively. For instance, Al2O3-3-400 refers to the sample prepared with TMB/ P123 weight ratio of 3 and calcination temperature at 400 °C. And, we used hydrochloric acid and citric acid as the pH adjustors (3.2 mL of 37 wt.% HCl and 0.9 g of citric acid) in all syntheses. 2.3. Characterization Small angle X-ray scattering (SAXS) measurements were carried out on a NanoSTAR small angle X-ray scattering system (Bruker, Germany) using Cu Ka radiation (40 kV, 35 mA). The d-spacing values were calculated by the formula d = 2p/q, and the unit p cell ffiffiffi parameters were calculated from the formula: a ¼ 2d10 = 3. Wide-angle X-ray diffraction (WAXRD) patterns were collected on Bruker D8 Endeavor X-ray diffractometer using Cu Ka radiation (40 kV, 40 mA). Nitrogen sorption isotherms were measured at 77 K on a Micromeritics Tristars 3000 analyzer (USA). Before measurements, the samples were degassed in a vacuum at 180 °C for at least 6 h. The Brumauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas (SBET), using adsorption data in a relative pressure range from 0.04 to 0.2. The pore volume and pore size distributions derived from the adsorption branches of isotherms by using Barrett–Joyner–Halenda (BJH) model. The total pore volumes (Vp) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.995. Scanning electron microscopy (SEM) images were obtained on S-4800 field emission scanning electron microscope (FE-SEM, Japan) operated at 1 kV. Transmission electron microscopy (TEM) measurements were carried out on a JEOL 2011 microscope operated at 200 kV. All samples were first dispersed in ethanol and then collected by using copper grids covered with carbon films for measurements. Energy dispersive Xray spectroscopy (EDX) was performed on a JEOL 2011 EDX instrument. Thermogravimetric (TG) analysis was carried out on a Mettler Toledo TGA-SDTA851 analyzer (Switzerland) from 25 to 900 °C in an air flow of 80 mL min–1 at a heating rate of 10 °C/min. 3. Results and discussion 3.1. Hexagonal mesostructured alumina Highly ordered mesoporous alumina with tunable pore size can be synthesized by using aluminum iso-propoxide as a precursor,

2.2. Synthesis The mesoporous alumina materials were synthesized by using aluminum iso-propoxide as a precursor, Pluronic P123 as a template and TMB as an additive solvent under the evaporation induced self-assembly (EISA) approach. The weight ratios of TMB/ P123 were varied in the range of 0–5. In a typical synthesis, 2.0 g of P123 was dissolved in 40 mL of ethanol, followed by the addition of TMB (6.0 g) under stirring. To this solution, 4.08 g of aluminum iso-propoxide, 3.2 mL of 37 wt.% HCl and 0.9 g of citric acid were added under vigorous stirring at ambient temperature. After being stirred for 12 h, the mixture solution was transferred to a dish. Then, the ethanol solvent was evaporated in an oven at 60 °C for 48 h to get the as-made product. The final samples were calcined at 400, 500, 800 °C with a heating rate of 1 °C/min and held at final temperature for 4 h in air to obtain the mesoporous alumina materials. Calcination at 900 °C under air condition were performed in a stepwise manner with a first step to 400 °C (1 °C/min) and

Fig. 1. SAXS patterns of the mesoporous alumina with different TMB/P123 weight ratios and calcination temperatures in air. (a) Al2O3-3-400, (b) Al2O3-3-500, (c) Al2O3-3-800, (d) Al2O3-3-900, (e) Al2O3-0.25-500, (f) Al2O3-0.5-500, (g) Al2O3-1500, (h) Al2O3-3-500.

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Pluronic P123 as a template and TMB (varied in the range of 0–3) as a swelling agent with the evaporation induced self-assembly (EISA) approach. The SAXS pattern (Fig. 1a) of the sample Al2O3-3-400 displays two scattering peaks that can be indexed as the 1 0, 1 1, 2 0 reflections of ordered hexagonal mesostructure with the space group p6mm (the second peak is superposition of 1 1, 2 0 reflections). Its cell parameter (a0) is calculated to be 10.8 nm. After increasing the pyrolysis temperature to 500 °C, the ordered hexagonal mesostructure of Al2O3-3-500 is well-retained (Fig. 1b), and the 1 0 scattering peak has no obvious shift compared with the mesoporous alumina named Al2O3-3-400, suggesting a thermal stability. A shift of the diffraction peak to a higher angle is clearly observed, indicating a shrinkage (7.5%) of the domain size upon the high temperature treatment (800 °C) (Fig. 1c). The SAXS pattern of Al2O3-3-900 pyrolyzed at 900 °C also exhibits hexagonal mesostructure, and a further shrinkage (11.7%) is observed (Fig. 1d). Then, we investigated the influence of TMB content. With the increase of TMB/P123 weight ratio from 0.25 to 3, the hexagonal mesostructure could be retained (Fig. 1e–h). However, the 100 diffraction shows a slight shift to a low angle with the incremental addition of TMB amount. The cell parameters (a0), determined from the SAXS results, are illustrated in Table 1 for the mesoporous aluminas obtained under different synthesis conditions. From Table 1, we can conclude that incremental addition of TMB results in an obvious increase in d-spacing compared with that of the mesoporous alumina without TMB [26]. Therefore, it suggests that TMB is an effective swelling agent for the preparation of large-mesopore materials. In addition, wide-angle XRD patterns (Fig. 2a and b) reveal that the mesoporous aluminas pyrolyzed at 400 and 500 °C have amorphous frameworks. After further treatment at a temperature of 800 °C, six diffraction peaks are observed (Fig. 2c), which can be indexed to be the 2 2 0, 3 1 1, 2 2 2, 4 0 0, 5 1 1, 4 4 0 reflections of

the c-alumina (JCPDS, 10-0425). It indicates that the amorphous framework has been crystallized. The diffraction peaks (Fig. 2d) become stronger and narrower when the pyrolysis temperature increases to 900 °C. These results suggest that the crystallinity is improved upon high-temperature treatment. The combination of SAXS and wide-angle XRD data convince us that ordered mesoporous c-Al2O3 with crystalline walls are formed at beginning of 800 °C. TEM images of Al2O3-3-400 (Fig. 3a and b) and Al2O3-3-500 (Fig. 3d) show well-defined two-dimensional (2-D) hexagonal mesostructures, which are consistent with their corresponding SAXS results (Fig. 1a and b). The cell parameters (a0) estimated from the TEM images are both approximately 10.8 nm, in good agreement with the values calculated from the SAXS data. The EDX spectrum (Fig. 3c) of the ordered mesostructured alumina (Al2O3-3-400) displays strong signals from Al and O elements without the detection of other elements except Cu and C deriving from residual template and carbon film coated copper grids, indicating the formation of stoichiometric alumina with the molar ratio of Al element and O element close to 2:3. The highly ordered hexagonal arrangement of mesopores along [1 1 0] and the alignment of cylindrical pores along [0 0 1] are reflected even after being calcinated at 800 °C (Fig. 4a and b). The cell parameter (a0) calculated from the TEM images undergoes a shrinkage of about 8%, which is consistent with the SAXS results (Fig. 1c). High-resolution TEM (HRTEM) image of the sample Al2O3-3-800 (Fig. 4c) reveals the appearance of characteristic lattice fringes, further confirming the crystallization of the pore wall. The selected-area electron diffraction (SAED) pattern (Fig. 4), the insert in c also suggests that this material exhibits polycrystalline feature. After the treatment temperature increases to 900 °C, its 2-D hexagonal mesostructure can still be retained (Fig. 4d and e), and a further shrinkage (about 10%) can be estimated. The corresponding HRTEM images (Fig. 4f) show

Table 1 Textural properties of mesoporous alumina. Sample number

Weight ratio of TMB/P123

d100 spacing (nm)

Unit cell size (nm)

Pore size (nm)

Pore wall thickness (nm)

Pore volume (cm3/g)

BET surface area (m2/g)

Al2O3-3-400 Al2O3-3-500 Al2O3-3-800 Al2O3-3-900 Al2O3-0.25-500 Al2O3-0.5-500 Al2O3-1-500 Al2O3-5-500 Al2O3-5-900 Al2O3-0-500

3 3 3 3 0.25 0.5 1 5 5 0

9.4 9.4 8.7 7.6 8.4 8.5 8.7 – – 7.4

10.8 10.8 10.0 8.8 9.8 9.8 10.0 – – 8.5

7.5 7.5 6.8 5.1 6.8 6.8 7.1 5.6, 45–100 6, 45–140 5.0

3.3 3.3 3.2 3.7 3.0 3.0 2.9 – – 2.5

0.48 0.51 0.33 0.24 0.44 0.53 0.51 0.2 0.20 0.50

275 274 175 177 245 297 309 94 83 344

Fig. 2. Wide-angle XRD patterns of the mesoporous aluminas with different TMB/P123 weight ratios and calcination temperatures in air. (a) Al2O3-3-400, (b) Al2O3-3-500, (c) Al2O3-3-800, (d) Al2O3-3-900, (e) Al2O3-5-500, (f) Al2O3-5-900.

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Fig. 3. TEM images of the mesoporous alumina Al2O3-3-400 taken along the [1 1 0] (a) and [0 0 1] (b) directions, respectively. Energy dispersive X-ray (EDX) (c) of the sample Al2O3-3-400. TEM images of the sample Al2O3-3-500 taken along the [1 1 0] (d) and [0 0 1] (e) directions, respectively. (f) Low-magnification TEM image of the Al2O3-3-500 sample.

the lattice fringes of c-alumina become more resolved and clear, indicating the improvement of the crystallization. The nitrogen sorption isotherms (Fig. 5)Aa,b of the samples synthesized with the TMB/P123 weight ratio of 3 and calcinated at 400 and 500 °C both show typical type-IV curves with H1 hysteresis loop, attributed to cylindrical mesopore channels. However, the hysteresis loops of the final samples after calcinations at 800 and 900 °C (Fig. 5Ac,d) become intermediate between type H2 and H1, rather than being type H1, owing to the transformation from amorphous wall to c-alumina which lead to pore connectivity with channel-like or ink-bottle pores. The pore size distribution (Fig. 5Ba), derived from the adsorption branch of the isotherm by using BJH model, of Al2O3-5-400 is centered at 7.5 nm. With the increase of the pyrolysis temperatures (from 500 to 900 °C), the pore size shrinks from 7.5 to 5.1 nm (Fig. 5Bb, c, d). Simultaneously, the values of BET surface area drastically decrease from 274 to 177 m2/ g (Table 1). This may be related to the higher crystallinity in the latter, as shown by the wide-angle XRD patterns (Fig. 2). In our studies, we also systematically investigated the effect of TMB by nitrogen sorption analyzes (Fig. 5C). All of the isotherms (for the samples Al2O3-0.25-500 to Al2O3-3-500) exhibit type IV curves with steep H1 hysteresis loops, suggesting their uniform cylindrical pores. In addition, the hysteresis loops of the isotherms shift

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Fig. 4. TEM images of the mesoporous alumina Al2O3-3-800 taken along the [1 1 0] (a) and [0 0 1] (b) directions, and HRTEM image (c) of Al2O3-3-800 (insert c is corresponding SAED pattern). TEM images of the sample Al2O3-3-900 taken along the [1 1 0] (d) and [0 0 1] (e) directions, and HRTEM image (f) of Al2O3-3-900 (insert f is corresponding SAED pattern).

to larger p/p0 for the mesoporous alumina synthesized with TMB, suggesting that the pore sizes are larger than that of the sample without TMB (Table 1). A sharp peak from the adsorption data is found in the case of pore size distribution shown in Fig. 5D. The structural parameters deduced from these plots are summarized in Table 1. TGA–DTG curve (Fig. 6) shows four-mass-loss steps of the as-made sample with the TMB/P123 weight ratio of 3 in air atmosphere. The weight loss of 6.6 wt.% between 50 and 100 °C is attributed to desorption of physically adsorbed water and organic molecules. Two significant weight losses steps (ca. 50 wt.%) at around 150 and 260 °C are due to the decomposition of Pluronic P123 and other organics. For the evaporation of TMB, we expect an endothermic peak at 165 °C (the boiling point of TMB); however, the endothermic peak may be masked by the strong exothermic peak from Pluronic P123 decomposition that begins at 150 °C. The pronounced weight loss of 20.1 wt.% at around 375 °C is detected, which may be related to the dehydroxylation during the transformation of boehmite into alumina [31]. Above 400 °C, although the sample do not has pronounced weight loss step, the TGA curve continued descending, this may be caused by the combustion of possible residual template and the gradual loss of the hydroxyl groups in the crystalline structure. These results demonstrate that P123 and TMB are readily removed under mild conditions.

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Fig. 5. Nitrogen sorption isotherms (A, C) and corresponding pore size distributions (B, D) of the mesoporous alumina materials with different TMB/P123 weight ratios and calcination temperatures. (a) Al2O3-3-400, (b) Al2O3-3-500, (c) Al2O3-3-800, (d) Al2O3-3-900, (e) Al2O3-0-500, (f) Al2O3-0.25-500, (g) Al2O3-0.5-500, (h) Al2O3-1-500, (i) Al2O33-500. The isotherms (A, C) for Al2O3-3-400, Al2O3-3-500, Al2O3-3-800, Al2O3-0-500, Al2O3-0.25-500, Al2O3-0.5-500, Al2O3-1-500, Al2O3-3-500 were offset by 300, 140, 85, 45, 90, 155, 310 and 420 cm3/g STP on y axis, respectively.

Fig. 6. TG (a) and DTG (b) analysis of the as-made mesoporous alumina sample synthesized with TMB/P123 weight ratio of 3.

3.2. Hierarchical mesoporous alumina In this work, it is demonstrated that hierarchical mesoporous alumina can be obtained with a sufficiently large amount of TMB (TMB/P123 weight ratio of 5). The influence of the calcination temperature on the final products was investigated by wide-angle XRD patterns (Fig. 2e and f). No diffraction peaks can be observed at 500 °C (Fig. 2e), suggesting that the mesoporous alumina pyrolyzed has an amorphous framework with the low temperature treatment, which is similar to the former ordered hexagonal mesoporous alumina. By increasing the calcination temperature to 900 °C (Fig. 2f), wide-angle XRD pattern show well-resolved diffractions peaks which is well-matched to c-alumina (JCPDS, 100425), indicating that the amorphous wall is crystallized. Low-magnification FE-SEM image of the hierarchical porous alumina named Al2O3-5-500 (Fig. 7a) shows disordered large pores with diameters in the range of 100–600 nm. From high-magnification FE-SEM image (insert a), the mesostructure could be observed clearly on the frameworks with large pores, suggesting that the hierarchical pores materials is mainly composed of mesopores. The representative TEM image (Fig. 7b) further demonstrates that

Fig. 7. FE-SEM images of the hierarchical porous alumina Al2O3-5-500 (a) and Al2O3-5-900 (c). Inserts (a) and (c) are the corresponding high-magnification FESEM images. TEM images of the sample Al2O3-5-500 (b) and Al2O3-5-900 (d, e), and HRTEM images (f) of Al2O3-5-900, insert (f) is the corresponding SAED pattern.

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Fig. 8. Nitrogen sorption isotherms (A) and corresponding pore size distributions (B) of the hierarchical porous alumina: Al2O3-5-500 (a) and Al2O3-5-900 (b). The isotherm for Al2O3-5-500 (A, a) was offset by 24 cm3/g STP on y axis.

the mesostructure transition from the ordered P6mm symmetry to the hierarchical structure with multi-level interconnection at TMB/ P123 ratio of 5. With further increasing the pyrolysis temperature to 900 °C, the hierarchical structure can be well-retained (Fig. 7c– e), indicating a high thermal stability of the final material. The corresponding HRTEM images (Fig. 7f) show that the lattice fringes of c-alumina are clearly resolved, indicating that the high-temperature pyrolysis is a key factor for the formation of c-alumina. The nitrogen sorption isotherms and corresponding pore size distribution curves of the samples Al2O3-5-500 and Al2O3-5-900 are shown in Fig. 8. A strong uptake of N2 as a result of capillary condensation is observed in a relative pressure (p/p0) of 0.4 and reaches a turning point at 0.85, suggesting that the final materials belong to the mesoporous family, with pore sizes close to 6.0 nm. However, for relative pressures larger than 0.85, the adsorbed volume of nitrogen further increases, which indicates that the material contains an appreciable amount of secondary porosity. In addition, corresponding pore size distribution curves of both samples offer two broad peaks at ca. 6 nm and ca. 45–100 nm. Therefore, the SEM, TEM and N2 sorption analyzes reveal that the materials exhibit hierarchical pore distributions. So, we can conclude that TMB swollen P123 micelle templates lead to a transformation from the highly ordered p6mm mesostructure to hierarchically porous alumina. On the basis of the above observations, we speculate that the highly ordered mesostructured alumina with corresponding large pore sizes assisted by small amounts of TMB (varied from 0 to 3) may be formed from the ‘‘liquid crystal templating’’ process [32]. Liquid–crystal phases are capable of solubilization of organic molecules within hydrophobic interiors [33]. In our synthesis system, Pluronic P123 possesses a long PPO segment, two medium length PEO blocks and a relatively high molecular weight (5800 g/ mol). The combination favors the formation of micelles, which have hydrophobic PPO blocks in their cores and hydrophilic PEO segments at the micellar surface. In addition, aromatic TMB is used as the organic cosolvent because it has proven to be an effective swelling agent for the preparation of large-pore mesoporous materials [26]. So, the presence of small amounts of TMB can change the hydrophobic volumes of the P123 micellar rods, which can further affect the pore sizes. When aluminum iso-propoxide is added into Pluronic P123 and TMB ethanol solution, it can hydrolyze with water and ethanol, and yield aluminum polyoxo oligomers. As a result, alumina oligomers interact with hydrophilic PEO segments of Pluronic P123 by the hydrogen-bonding, finally form rigidity and robust 2-D mesostructure after the curing process at 60 °C. In addition, our results reveal that the pore size could be enlarged gradually with the increase of the TMB amount, which is similar to that for the MCM-41 mixture system [26]. However, large amount of TMB leads to a structure transformation from ordered hexagonal p6mm to hierarchical and disordered structure. This phenomenon

maybe explained that in the presence of the large amount TMB, the reaction system became instability with the evaporation of ethanol, which lead to the phase separation and forming multiple scaling droplets and finally obtained the hierarchical alumina. 4. Conclusions With the use of TMB as a swelling agent, we successfully synthesized the mesoporous alumina materials with ordered 2-D hexagonal structures, relatively high surface areas (309 m2/g), pore volumes (0.51 cm3/g), large pore sizes (7.5 nm), and highly thermal stability (900 °C). Simultaneously, the pore size of ordered mesoporous alumina gradually increases with the increase of the TMB content in this process. Interestingly, with further increasing the weight ratio TMB/P123 to 5, hierarchical mesostructured alumina at multiple length scales was fabricated without using secondary template. The present approach and findings not only contribute to the development of highly ordered mesoporous alumina, but also offer opportunities for further controlling the pore size and mesostructure. What is more, this method could present new possibilities to engineer mesostructured alumina for applications such as the transport of large molecule and thus attractive candidates for catalyst supports in liquid-phase chemistry. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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