CTAB-directed synthesis of mesoporous γ-alumina promoted by hydroxy carboxylate: The interplay of tartrate and CTAB

Solid State Sciences 13 (2011) 409e416

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CTAB-directed synthesis of mesoporous g-alumina promoted by hydroxy carboxylate: The interplay of tartrate and CTAB Ming Bo Yue a, b, Teng Xue a, Wen Qian Jiao a, Yi Meng Wang a, *, Ming-Yuan He a a b

Shanghai Key Lab of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai, China The Key Laboratory of Life-Organic Analysis, School of Chemistry and Chemical Engineering, Qufu Normal University, Shandong, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 September 2010 Accepted 2 December 2010 Available online 9 December 2010

Mesoporous g-aluminas are prepared via a facile one-pot hydrothermal method using aluminum sulfate as a precursor, urea as precipitating agents, sodium tartrate and CTAB (cetyltrimethylammonium bromide) as co-templates respectively. Characterizations of the intermediate aluminum oxyhydroxides by IR, XRD, element analysis and TG show that the amounts of CTAB occluded in the as-prepared mesoporous aluminum oxyhydroxides are determined by molar ratio of sodium tartrate to aluminum (ST/Al). Especially, no sodium tartrate is added, no CTAB is occluded in the mesoporous aluminum oxyhydroxides. The structures of obtained mesoporous aluminas are characterized by XRD, nitrogen adsorption analysis, and scanning electron micrographs (SEM). And the surface areas, mesopore volume and the order of meso-structure are enhanced with an increase in ST/Al molar ratio. Sodium citrate and sodium succinate are also used as additives to study the formation mechanism of organized mesoporous aluminas. The formation mechanisms are proposed that tartrate interacts with aluminum and CTAB simultaneously to form an intermediate as the building blocks of the final meso-structured hybrid. Ó 2010 Elsevier Masson SAS. All rights reserved.

Keywords: Mesoporous g-Al2O3 Hydroxy carboxylate CTAB Sodium tartrate Urea

1. Introduction Various meso-structured and mesoporous materials have been synthesized via the organiceinorganic co-assembly since the discoveries of ordered mesoporous silicas M41S [1e5]. However, the synthesis of meso-structured non-siliceous oxides, including alumina, using soft matter as templates often faces severe challenges [6e10]. The main reason is that metal oxides precursors (generally inorganic salt or alkoxides) are much more reactive, especially high sensitivity to moisture and the variation of pH value, than silicabased analogues, and the uncontrolled condensation easily yields macroscopic phase segregation. In addition, the crystallinity of the ordered mesoporous metal oxides are usually poor, probably due to the fact that the crystallization energies often tend to stabilize the inorganiceinorganic interface and disrupt the establishment of curved surfaces of ordered meso-structures [9]. However, well crystallinity is often required for metal oxides to exhibit their catalytic ability or adsorptive capacity. Therefore, the synthesis of meso-structured metal oxides with well crystallinity is much interesting.

* Corresponding author: Tel./fax: þ86 21 62232251. E-mail address: [email protected] (Y.M. Wang). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.12.003

Among the various transition aluminas known, g-alumina (g-Al2O3) is perhaps the most important with direct application as a catalyst and catalyst support in the automotive and petroleum industries [11e13]. Consequently, a variety of methods were used to prepare mesoporous g-aluminas [7,8], such as using surfactants as soft template [14e16], using mesoporous carbon as hard template [17e19], using ionic liquids as solvents [20], using other non-surfactant organic molecules as template [21e24] and microwave irradiation under acidic conditions [25]. In the soft template synthesis methodology, a generalized cooperative assembly mechanism has been recognized: soluble inorganic species and template molecules combine to form hybrid intermediates which are the building blocks of the final meso-structured hybrid [26e28]. One of key points for this organiceinorganic co-assembly is that the linkage of inorganic monomers via an oxide bridge (condensation) must be slow enough for the co-assembly with the template rather than precipitation of bulk metal oxide or disordered composites. These soft matters (micelle-forming organic molecules) come in a variety of forms, such as ionic surfactants, nonionic surfactants, and even biomolecules like phospholipids [29]. In general, charged inorganic species could interact with charged surfactant head groups or hydrophilic components of block-copolymer directly or through counter ion and hydrogen bond mediated pathways. However, as to these non-siliceous metal

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oxides precursors, the linkage of inorganic monomers is so fast that the inorganic-surfactant interaction is not strong enough for co-assembly. So, a balance must be built between the condensation of inorganic monomers and the inorganic-surfactant coassembly. In order to control the hydrolysisecondensation rates of non-siliceous oxides precursors, many strategies were adopted, such as utilizing specific pH ranges [30], controlled hydrolysis by using stabilizing ligands [31e33], non-aqueous media [34] and some combination thereof. Lowering the hydrolysisecondensation rates of non-siliceous oxides favors an appropriate and adequate co-assembly of inorganic precursor and organic template. With very lower hydrolysisecondensation rates, evaporation-induced self-assembly (EISA) had proven to be an extremely useful method for preparation of well ordered meso-structured aluminas [35,36]. In EISA method, the evaporation of a volatile solvent (usually ethanol) concentrates both the surfactant molecules and inorganic precursors, driving their co-assembly to form ordered mesophases. Subsequent aging, heat, and chemical treatments induce further condensation of inorganic precursor and lock in the meso-structure. However, EISA is time-consuming and strongly affected by acid, water, temperature, relative humidity and other factors. Besides lowering the hydrolysisecondensation rates of nonsiliceous oxides precursors to meet balance between the condensation of inorganic monomers and the co-assembly of inorganic-surfactant, enhancing the interaction of inorganic precursors with soft template may be another way to get the balance of condensation and co-assembly. If the interaction of inorganic precursors with soft template is strengthened, the hybrid intermediate of inorganic species and surfactant molecules can be formed instead of bulky inorganic phase. So, it is urgent to select an agent to enhance the interaction between inorganic species and surfactant. Previously, it was reported that hydroxy polyacids, such as citric acid, DL-malic acid and tartaric acid, could be used as a kind of organic additive for the solegel synthesis of various mesoporous materials [37,38]. These hydroxy polyacids showed strong chelation to metal species, such as aluminum, and could be used as the structure directing agents for the preparation of mesoporous alumina [21]. Unfortunately, the obtained mesoporous aluminas were always disordered when hydroxy polyacids were used as single structure director, because hydroxy polyacids, as small molecules, could not form micelles [39,40] in the same way as the surfactants [4]. But, it would be a desired agent to enhance the interaction between aluminum and cationic surfactant. In our previous report [41], hydroxy polyacids and CTAB were used as co-template to synthesize mesoporous g-Al2O3 by double hydrolysis method with Al(NO3)3 and NaAlO2 as aluminum source. One side, the hydroxy carboxylate had strong chelating or bridging ability to aluminum species in the precursor solution and the final hybrid material; in another side, carboxylate interacted with charged head groups of cationic surfactant. Here, hydroxy carboxylate were used as a bridge to link aluminum species and surfactant. Similarly, aminosilane was used as co-structure director to link anionic surfactant and silica species in the anionic surfactant templating route for synthesizing mesoporous silica [42]. However, in this double hydrolysis method, AlO 2 could also interact with CTAB through electronic interaction, which complicated the interactions among CTAþ with positive charge and different anions. In order to well elucidate the interplay between hydroxy carboxylate and CTAB, Al2(SO4)3 was chosen as sole source of aluminum in present work, which might eliminate the disturbance of AlO 2 to the interplay of hydroxy carboxylate and CTAB. In addition, succinate was used to synthesize organized mesoporous g-alumina instead of its hydroxyl derivative, tartrate. Compared with tartrate, succinate has no hydroxyl groups and owns weak chelating ability with aluminum, and thus leads to

disordered g-alumina. These differences may help to better understand the role of hydroxy carboxylate as a bridge between CTAB and aluminum species. 2. Experimental 2.1. Preparation mesoporous alumina The mesoporous alumina (named MA) was synthesized by using cetyltrimethylammonium bromide (CTAB, Aldrich) and sodium tartrate (ST, AR grade) or sodium citrate (AR grade) or sodium succinate (AR grade) as structure direct agents, Al2(SO4)318H2O (AR grade) as a source of aluminum, urea as precipitating agents. All reagents were used as received without further purification. In a typical synthesis, CTAB (0.0007 mol), Al2(SO4)318H2O (0.007 mol), CO(NH2)2 (0.028 mol), and calculated amount of sodium tartrate were dissolved in distilled water to form a clear solution (36 mL) under vigorous stirring for 0.5 h. The molar composition of the mixture Al/CTAB/sodium tartrate/urea/H2O was 1.0/0.1/x/2/140. The molar ratio of sodium tartrate/Al (x) was varied from 0 to 0.5. The solution was placed in an autoclave with a Teflon liner. The autoclave was maintained at 165  C for 3 h and then cooled to room temperature. The white precipitate was collected and washed with distilled water for several times, and then dried at 80  C for 12 h. The obtained samples were denoted as MAT-x, where x represented the molar ratio of sodium tartrate/Al (x). The as-prepared mesporous aluminum oxyhydroxides were calcined at 550  C for 3 h with a heating rate of 1  C min1 to remove template and the obtained alumina were denoted as MAT-xC. Similar experiments used sodium citrate or sodium succinate with fixed molar ratio of 0.4 (sodium citrate/Al or sodium succinate/Al) instead of sodium tartrate, and the as-prepared samples were denoted as MAC-0.4 (sodium citrate) and MAS-0.4 (sodium succinate) respectively. 2.2. Characterization The X-ray diffraction (XRD) measurements, which were used to characterize the crystalline phase and meso-structure, were carried out on a Rigaku-Ultima diffractometer with Cu Ka radiation in the 2q range from 0.5 to 5 or from 10 to 80 . Infrared tests were performed on a Nicolet Fourier transform infrared spectrometer (NEXUS 670) combined with the conventional KBr wafer technique. Nitrogen adsorptionedesorption was measured at 196  C on a Quantachrome Autosorb-3B volumetric adsorption analyzer. Before the measurements, the sample was outgassed for 4 h in the degas port of the adsorption apparatus at 300  C. The BET specific surface area was calculated using adsorption data acquired at a relative pressure (p/p0) range of 0.05e0.22. The total pore volume determined from the amount adsorbed at a relative pressure of about 0.99. The mesopore volume was calculated from the amount adsorbed at pore-size <50 nm. The pore-size distributions (PSDs) curves were calculated from the analysis of adsorption branch of the isotherm using the BarretteJoynereHalenda (BJH) algorithm respectively. Thermogravimetric analysis (TG) was performed using a PerkineElmer TGA analyzer with a heating rate of 10  C min1 up to 800  C under an air flow to discover the various decomposition steps occurring in the as-dried precursor as a function of temperature. Morphological analysis was performed by an S-4800 field emission scanning electron microscope (FESEM, Hitachi, Japan) with an acceleration voltage of 3 kV. Elemental analysis was performed on Elementar Varioe EL III type instrument (Germany).

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3. Results and discussion Powder XRD was used to monitor the changes of the phase structure of the as-prepared samples. Fig. 1A shows the wide-angle XRD patterns of the samples obtained by varying sodium tartrate/ Al molar ratio (ST/Al) from 0.0 to 0.5. MAT-0.0 sample exhibits typical XRD diffraction peaks of boehmite, and all the diffraction peaks are in good agreement with the orthorhombic boehmite (JCPDS Card No. 21-1307). So, CTAB has little influence to the formation of boehmite in this procedure. With ST/Al increasing to 0.025 and 0.05, the diffraction peaks are broadened and the intensity of diffraction peaks is reduced. Broaden diffraction peaks reveal the nanosize nature of these samples, and reduced intensity indicates their poor crystallinity of resulted boehmite. These changes are also observed from SEM images (Fig. 2). Without the addition of ST, the MAT-0.0 sample (Fig. 2, image a) shows flower like morphology assembled with flake boehmite, which is the same as the boehmites prepared without the addition of CTAB [43]. The size of primary flake of MAT-0.0 sample is about 20  200  500 nm. As to MAT-0.05 (Fig. 2b) sample, there are some amorphous spheres besides boehmite flakes in a size of10  50  100 nm. When ST/Al ratio increases to 0.1, the XRD pattern almost becomes a smooth line and the crystalline phase of boehmite are hardly observed, which indicates the amorphous nature of the MAT-0.1 sample. As shown in Fig. 2 (image c), MAT-0.1

Fig. 1. Wide-angle XRD patterns of MAT-x (A) and MAT-xC (B) samples, and smallangle XRD patterns of MAT-x (C) and MAT-xC (D) synthesized with various molar ratio of sodium tartrate/Al (x): (a) 0, (b) 0.025, (c) 0.05, (d) 0.1, (e) 0.2, (f) 0.3, (g) 0.4, (h) 0.5.

411

exhibits amorphous spheres of ∼3 mm coated with tiny boehmite crystals (Fig. 2, image c). When ST/Al molar ratio further increases to 0.2e0.5, there are no distinct diffraction peaks in XRD patterns, which indicate the amorphous nature of these samples. The surface of these samples becomes smooth and flake-like crystals disappear (Fig. 2, image def) with a further increase in ST/Al molar ratio. Therefore, the addition of sodium tartrate hinders the formation of boehmite and leads to amorphous aluminum oxyhydroxide. The reason can be ascribed to that tartrate has good chelating ability to aluminum and form a tartrateealuminum complex anion, which may inhibit Al3þ polymerization, and then retard the formation of boehmite [44]. In summary, the obtained aluminum oxyhydroxides are mainly boehmite (g-AlOOH) mixed with some amorphous phase when ST/Al ratio is smaller than 0.1, and the phase changes to amorphous solids with ST/Al ratio raising to 0.2e0.5. As we known, conventional g-alumina typically is prepared through the thermal dehydration of coarse particles of well-defined boehmite at a temperature of around 400e450  C [45]. So, as shown in Fig. 1B, g-alumina formed after calcination at 550  C, and MAT-0.0C sample gives the highest relative crystallinity as presumed. Interestingly, MAT-0.1C gives the lowest relative crystallinity among all these samples. Although a further increasing in ST/Al ratio to 0.2e0.5 results in amorphous nature, the relative crystallinity of resulted g-aluminas increases. The reason may be ascribed to the tartrate occluded in the as-prepared aluminum oxyhydroxides, which can decrease the combustion temperature of the gel and promote the phase transformation from amorphous to g-alumina as the citrate [46]. Fig. 1C gives small-angle XRD patterns of as-prepared aluminum oxyhydroxides. The MAT-0.0 sample shows no diffraction peaks in the 2q range from 0.5 to 5 , which means that no organized mesostructure formed in this sample. In other words, CTAB cannot direct the meso-structure singly in this method. In the cooperative assembly mechanism, there must be intermediates containing both inorganic and surfactant molecules as the building blocks for the final meso-structured hybrid. In this method, the pH value of the mixture is close to isoelectric point of alumina, around 9, after 3 h at 165  C and thus most aluminum species exist as neutral aluminum hydroxides [47]. And CTAB with positively charged surfactant head groups hardly interacts with neutral aluminum hydroxides [48]. Hence, CTAB can not act as meso-structure directing agents in the preparation of meso-structured silica. When ST/Al ratio increases to 0.1, MAT-0.1 shows a broad diffraction peak at 2q ¼ 1.1 (Fig. 1C, sample d), indicating the presence of some organized meso-structure. With ST/Al ratio further increasing to 0.2, the diffraction peaks become distinct and the intensity increases. This small-angle XRD peak can be related to the organized meso-structure of aluminum oxyhydroxide without a long range order but with a fairly uniform pore-size. This result proves that CTAB combined with tartrate (hydroxy carboxylate) can direct the formation of meso-structure cooperatively as illustrated in Scheme 1. Porous amorphous aluminum oxyhydroxide particles can selectively adsorb tartrate anions, resulting in the formation of hydrogen bonds between carboxyl and hydroxyl groups of tartrate ions and the surface OH groups of aluminum oxyhydroxide [49]. The adsorbed tartrate anions, which are negatively charged, may interact with positively charged head group of CTAB in favor of the formation of mesoporous aluminum oxyhydroxide. Following this directing mechanism, the diffraction peaks became more distinct with the ST/Al ratio further increasing to 0.5 (Fig. 1C, sample h). Fig. 1D demonstrates small-angle XRD patterns of g-alumina calcined at 550  C. With small ST/Al ratio (<0.1), as-made samples of MAT-0.0, MAT-0.025 and MAT-0.05 (Fig. 1C, sample aec) show no diffraction peaks in the 2q range from 0.5 to 5 , indicating that

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Fig. 2. SEM images of samples. (a) MAT-0.0, (b) MAT-0.05, (c) MAT-0.1, (d) MAT-0.2, (e) MAT-0.4, (f) MAT-0.5, (g) MAT-0.0C, (h) MAT-0.1C.

these aluminum oxyhydroxides have no organized meso-structure. Correspondingly, three calcined samples also show no mesostructure (Fig. 1D, sample aec). When ST/Al ratio further increases (0.1), MAT-0.1C and MAT-0.2C samples give a diffraction peak at 2q ¼ 1.1 (Fig. 1D, sample d and e), which means that the mesostructure is well preserved after the calcinations. However, the peaks in small-angle XRD patterns become indistinct when ST/Al ratio reaches 0.3. The reason can be ascribed to that larger amounts of ST may cause a ready sintering and the collapse of the alumina framework upon templates removal by calcinations. When ST/Al ratio is larger than 0.3, the chelating action between alumina

species and the tartrate is strengthened and thus the average AleOeAl connectivity is lowered. This low average AleOeAl connectivity is liable to result in the collapse of meso-structure during the calcination [50], whereas this complex interaction favors the formation of meso-structure. Especially, when the ST/Al ratio increases above 0.6, too many complexes lead to no precipitate and the solution keeps clear after this hydrothermal crystallization procedure. IR spectra were used to monitor the template (CTAB and ST) occluded in the as-prepared aluminum oxyhydroxides. And the IR spectra of as-prepared aluminum oxyhydroxides prepared from

M.B. Yue et al. / Solid State Sciences 13 (2011) 409e416

413

B

A

100

a b c

90

a b c

70 60

d

50

e f

40

g

30

200

400

-1

0.1 mg min

h 600

800

200

o

different ST/Al ratios are shown in Fig. 3. MAT-0.0 sample, fabricated with no tartrate added, gives the typical XRD patterns of boehmite (Fig. 1A, curve a). Accordingly, the typical IR spectrum of boehmite is shown in MAT-0.0 sample (Fig. 3 curve a): the stretching vibrations (3306 and 3098 cm1) and bending vibrations (1157 and 1067 cm1) of the OH groups; the stretching vibrations (745 and 630 cm1) and bending vibrations (480 cm1) of the AleO band. In addition, this sample shows an absorption band at 1630 cm1, which is attributed to the vibration of HeO of water adsorbed on the boehmite [51]. As to MAT-0.025 sample (Fig. 3 curve b), all the above-mentioned bands ascribed to boehmite are observed but with weaker intensity, which comes from lower crystallinity of boehmite. The bands at 2850 and 2925 cm1 ascribed to CH2 and CH3 stretching vibrations of CTAB can be observed carefully. In addition, the band around 1630 cm1 broadens and shifts to higher wavenumber of 1660 cm1. The reason can be ascribed that the C]O asymmetric stretching vibration of COO at 1660 cm1 overlaps the vibration of HeO of water (1630 cm1) when the tartrate is occluded in the samples.

a b

Transmittance

c d e f g

CTAB

3000

600

800

Temperature ( C)

Fig. 4. TG (A) and DTG (B) curves of samples in air (30 mL min1) with the rate of 10  C min1 from 25 to 800  C. (a) MAT-0.0, (b) MAT-0.025, (c) MAT-0.05, (d) MAT-0.1, (e) MAT-0.2, (f) MAT-0.3, (g) MAT-0.4, (h) MAT-0.5.

Correspondingly, the C]O symmetric stretching vibration of COO appears at 1390 cm1. With the ST/Al ratio further increasing to 0.05, the bands at 2850, 2925, 1390 cm1 are distinctly observed in the IR spectra of MAT-0.05 sample. This variation is more distinguished with the ST/Al ratio further increasing. The emergence of the bands (2850 and 2925 cm1) ascribed to CTAB means that CTAB is simultaneously occluded in the as-prepared aluminum oxyhydroxides. Especially, the increased intensity of the bands at 2850, 2925 cm1 (CTAB) is accompanied with an increase in the intensity of the bands at 1660, 1390 cm1 (tartrate), which results from an increase in ST/Al ratio. It should be noted that the CTAB/Al ratio in the prepare process was fixed at 0.1 to all samples. So, we can conclude that: no sodium tartrate is added, no CTAB is occluded (Fig. 3); more sodium tartrate is added, more CTAB is occluded (Fig. 3beh). Combined with the variation of small-angle XRD patterns of g-alumina, this cooperative directing mechanism of tartrate and CTAB are testified by IR spectra. TG and DTG analyses of as-synthesized samples are shown in Fig. 4 and the TG results of as-prepared samples are displayed in Table 1. TG and DTG curves can be divided into two zones based DTG curves of these samples. The weight loss in zone I (25e170  C) corresponds to the removal of physisorbed water. Zone II (170e800  C) is associated with the decomposition of mesostructure director (CTAB and tartrate) similar to that in meso-silica materials [52] and the release of water accompanying the phase transformation to g-Al2O3. The weight losses of zone I and II are listed in Table 1 for these samples. The theory value of weight loss Table 1 TG results of as-prepared aluminum oxyhydroxides.

h

NaTA

400

o

Temperature ( C) Scheme 1. Mechanism of tartrate and succinate interacts with aluminum and cationic surfactant.

e f g h

d

dm/dt

wt %

80

1660

1390

2000

1000 -1

Wavenumber (cm ) Fig. 3. Infrared spectra of as-synthesized samples prepared with sodium tartarate and CTAB as templates. (a) MAT-0.0, (b) MAT-0.025, (c) MAT-0.05, (d) MAT-0.1, (e) MAT-0.2, (f) MAT-0.3, (g) MAT-0.4, (h) MAT-0.5.

Sample

Zone I

Zone II

Al2O3a

Waterb

Tc

T/Ald

MATe0.0 MATe0.025 MATe0.05 MATe0.1 MATe0.2 MATe0.3 MATe0.4 MATe0.5

1.4 8.9 10.2 13.5 12.1 9.3 7.8 7.8

17.8 24.7 27.5 32.6 42 49.2 53.2 55.6

80.8 66.4 62.3 53.9 45.9 41.5 39 36.6

14.3 11.7 11.0 9.5 8.1 7.3 6.9 6.5

3.5 13.0 16.5 23.1 33.9 41.9 46.3 49.1

0.04 0.20 0.26 0.43 0.74 1.01 1.19 1.34

The weight of Al2O3 after calcination at 800  C. The theory value of water released accompanying the phase transformation from aluminum oxyhydroxides to Al2O3, Water ¼ 18  (weight percentage at 800  C) O 102. c The weight of template calculated by subtracting the theory value of water from the weight loss of zone II. d Weight ratio of template to Al2O3. a

b

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Sample

MAT-0.0

MAT-0.1

MAT-0.3

MAT-0.4

C(wt.%) H(wt.%) N(wt.%) Tartrate/CTABa

0.57 1.60 Trace e

9.60 4.46 0.39 2.43

24.10 6.99 1.08 1.76

27.12 6.70 1.26 1.53

a Mole ratio of Tartrate/CTAB was calculated from the N and C atom weight percentage. The N atom comes from CTAB and C atom comes from CTAB and tartrate. So, the mole ratio calculated as Tartrate/CTAB ¼ [(C wt.%/12  19*N wt.%/ 14)/4] O (N wt.%/14).

from aluminum oxyhydroxides (AlOOH) to Al2O3 is 15 wt.%, where just 1 mol of water is assumed to be released for each mol of Al2O3 [53]. Theoretically, the weight loss of water following phase transformation from AlOOH to Al2O3 can be calculated from the terminal weight (Al2O3) after calcination at 800  C, and the value is 14.3 (MAT-0.0), 11.7 (MAT-0.025), 11.0 (MAT-0.05), 9.5 (MAT-0.1), 8.1 (MAT-0.2), 7.3 (MAT-0.3), 6.9 (MAT-0.4) and 6.5 (MAT-0.5) wt.% for these samples respectively. Therefore, the template (CTAB and tartrate) occluded in the resulted aluminum oxyhydroxides can be obtained by deducting the theoretical water loss due to phase transformation from the weight loss of zone II. And then, the occluded template weight is 3.5 (MAT-0.0), 13.0 (MAT-0.025), 16.5 (MAT-0.05), 23.1 (MAT-0.1), 33.9 (MAT-0.2), 41.9 (MAT-0.3), 46.3 (MAT-0.4) and 49.1 (MAT-0.5) wt.% for these samples respectively. So, the weight ratio of template/Al2O3 can be calculated and shown in Table 1. With the ST/Al ratio increasing, the ratio of template/ Al2O3 increases from 0.04 (MAT-0.0) to 1.34 (MAT-0.5), which means the amounts of occluded template increases with the ST/Al molar ratio increasing. This variation accords with the IR results. However, the precise ratio of CTAB in the template can not be acquired form IR and TG. Hence, elemental analysis was performed to study the tartrate/CTAB molar ratio of the template occluded in the as-prepared aluminum oxyhydroxides. Table 2 displays the elemental analysis results of MAT-0.0, MAT-0.1, MAT-0.3 and MAT0.4 samples. MAT-0.0 shows trance amounts of N atom, which means seldom CTAB is introduced. This result consists with the results of IR and TG characterization. With the ST/Al ratio increasing from 0.1 to 0.4, the molar ratio of tartrate/CTAB occluded in the as-prepared aluminum oxyhydroxides decreases from 2.43 (MAT0.1) to 1.53 (MAT-0.4). The reason may be ascribed to that tartrate has stronger interaction with aluminum than CTAB. When small amount of sodium tartrate were added into synthesis mixture, most tartrate interacted with aluminum and seldom tartrate interacted with CTAB. With more sodium tartrate introduced, more tartrate interacted with CTAB by electronic interaction and more CTAB were occluded, which directed the formation of organized meso-alumina. In summary, the formation of intermediate through tartrate interacting with aluminum and CTAB is the key to form organized meso-structure. In order to further elucidate this mechanism, sodium citrate with one hydroxyl group and sodium succinate without hydroxyl group were used instead of sodium tartrate to assess this function of hydroxy carboxylate. Fig. 5 displays the XRD patterns of these samples. MAC-0.4, prepared by using sodium citrate as additives with molar ratio citrate/aluminum ¼ 0.4, shows a diffraction peak at 2q ¼ 1.1 (Fig. 5, sample b), indicating the presence of organized meso-structure. However, MAS-0.4, prepared by using sodium succinate as additives also with molar ratio succinate/ aluminum ¼ 0.4, shows no diffraction peaks in the 2q range from 0.5 to 5 , which means that no organized meso-structure formed in this sample. In addition, MAC-0.4 displays a weak diffraction pattern of boehmite in wide-angle XRD pattern, similar to the

a

b 0

10 20

30 40 50 60 70

80

2theta (degrees) Fig. 5. XRD patterns of samples: (a) MAS-0.4, (b) MAC-0.4.

sample prepared with sodium tartrate as additives. However, MAS-0.4 sample exhibits distinct diffraction peaks of boehmite, indicating that succinate has little influence to the formation of boehmite in this procedure. So, with same amounts of organic acid salts as additives, there are obvious difference in the form of mesostructure and boehmite. As shown in Scheme 1, sodium citrate with both carboxyl and hydroxyl group similar to sodium tartrate also has stronger interaction with aluminum than sodium succinate without hydroxyl group, which bridge the interaction between aluminum and CTAB and thus lead to the building blocks of the final mesostructured hybrid. As to sodium succinate, the interaction between succinate and aluminum is too weaker to form hybrid intermediates, and then organized meso-structure can not be formed. This formation mechanism is also testified by IR spectra of as-prepared aluminum oxyhydroxides with different organic acid salts as cotemplates (Fig. 6). MAS-0.4, MAT-0.4 and MAC-0.4 sample were prepared with sodium succinate, sodium tartrate and sodium citrate as additives respectively, and MA-0 sample was prepared with no organic acid salts added. MAS-0.4 and MA-0 sample display typical IR spectrum of boehmite (3306 and 3098 cm1,eOH stretching vibrations; 1157 and 1067 cm1, eOH bending vibrations; 745 and 630 cm1, AleO stretching vibrations; 480 cm1, the AleO bending vibrations). As to MAT-0.4 and MAC-0.4 samples, the intensity of all the above-mentioned bands of boehmite is weakened, which attributes to the chelating effect of hydroxy carboxylate. Moreover, new bands, ascribed to CTAB (2850 and 2925 cm1, CH2 and CH3 stretching vibrations) and COO (1660 and 1390 cm1, asymmetric and symmetric stretching vibration of C]O), emerge in the IR spectrum of MAT-0.4 and MAC-0.4 samples. Appearance of these bands indicates that hydroxy carboxylate exert its chelating function and form hybrid intermediate. Noticeably, no bands related to CTAB are observed in MAS-0.4, which suggests that sodium succinate without hydroxyl group may weakly interact with aluminum species and result in a poor cooperative assembly of CTAB and aluminum species.

d

Transmittance

Table 2 Elemental Analysis and the calculated Tartrate/CTAB molar ratio of the as-prepared aluminum oxyhydroxides.

Intensity (a.u.)

414

c b a

3000 2000 1000

-1

Wavenumber (cm ) Fig. 6. Infrared spectraes of as-synthesized samples. (a) MA-0.0, (b) MAS-0.4, (c) MAT-0.4, (d) MAC-0.4.

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415

on the pore structure and the BET surface area of the resulted mesoporous aluminas. The surface area increases with increasing ST/Al ratio from 172 (MAT-0.0C) to 325 (MAT-0.5C) m2 g1 and the pore-size distribution become narrower around 4 nm. Although the pore diameter decreases, the mesopore volume increases distinctly from 0.08 (MAT-0.0C) to 0.32 (MAT-0.5C) cm3 g 1. 4. Conclusions

Fig. 7. Nitrogen adsorptionedesorption isotherms and pore-size distributions of alumina. (a) MAT-0.0C, (b) MAT-0.1C, (c) MAT-0.2C, (d) MAT-0.3C, (e) MAT-0.4C, (f) MAT-0.5C. The curves are offset for clarity. The isotherms (A) for MAT-0.1C, MAT-0.2C, MAT-0.3C, MAT-0.4C, MAT-0.5C were offset by 50, 100, 150, 200 and 250 cm3 g1 STP respectively.

The nitrogen sorption isotherms and pore-size distributions of the aluminas are shown in Fig. 7. As shown of Fig. 7A, curve a, MAT-0.0C display a type II isotherm with H3 type hysteresis loop according to the IUPAC classification [54]. The type II isotherms mean non-pore solid or larger pore solid and the H3 type hysteresis loop comes from slit-shaped pores between plate-like particles, which is consistent with the SEM images of MAT-0.0C (Fig. 2g). In addition, the corresponding pore-size distributions are broader around 50 nm. So, there are no structural mesopores in the MAT0.0C sample. In other words, CTAB can not exert its mesoscopic directing function as single template. The increase of ST/Al ratio from 0.0 to 0.5 leads to a gradual change in the shape of adsorption isotherms and the corresponding pore-size distributions, implying a significant variation of pore structure. Adsorption isotherms of type IV are observed for MAT-0.1C sample. A distorted H2-type hysteresis loop is observed in the range of 0.4e0.7 P/P0, which closes at ∼0.4 P/P0, suggesting the presence of ink-bottle mesopores and/or pore constrictions. In addition, a H3 type hysteresis loop appears at the P/P0 range of 0.8e1.0 associated with slit-shaped pores. Combined with SEM image of MAT-0.1C (Fig. 2f), two types of pores are possible: mesopore formed by CTAB-tartrate as co-template and slit-shaped pores from aggregation of plate-like particles. The corresponding pore-size distributions become narrower around 5.1 nm. With ST/Al ratio further increasing to 0.2e0.5, the type IV isotherms with typical H2-type hysteresis loop are observed. Disappearance of the H3 type hysteresis loop combined with a narrow pore distribution around 4 nm indicates an organized meso-structure for these samples. Table 3 lists the textural parameters of the mesoporous alumina. These data show that the molar ratio of ST/Al has distinct effect

Table 3 Textural properties of the mesoporous aluminas prepared with various amounts of sodium tartrate. Sample

Vmeso(cm3 g1)

SBET(m2g1)

Vpore(cm3g1)

Da(nm)

MAT-0.0C MAT-0.1C MAT-0.2C MAT-0.3C MAT-0.4C MAT-0.5C

0.08 0.12 0.23 0.28 0.31 0.32

172 176 222 274 318 325

0.50 0.36 0.28 0.31 0.35 0.35

53.0 5.1 3.9 4.1 4.2 3.7

a

Pore diameter.

Organized mesoporous alumina has been successfully synthesized by the cooperative assembly of aluminum, tartrate and CTAB. With only CTAB used as template, no organized meso-structure forms and flower like boehmites are obtained, similar to that prepared without the addition of CTAB. Tartrate interacts with aluminum and CTAB simultaneously and thus builds an indirect interaction between aluminum and CTAB, which favors the formation of organized meso-structure. Sodium citrate and sodium succinate were also used to study this function of organic acid salts. The results show that only hydroxy carboxylate with well chelating interaction with aluminum can bridge the interaction between taluminum species and cationic surfactant and thus lead to organized mesoporous alumina, while succinate without hydroxyl leads to no organized meso-structure. The formation of hybrid intermediates (aluminum, hydroxy carboxylate and CTAB) is the key to prepare organized mesoporous alumina in this procedure. Acknowledgements This work is supported by National Science Foundation of China (key project No. 20890124), Shanghai Leading Academic Discipline Project (Project B409), Doctoral Foundation of Shandong Province (BS2009CL031) and Shanghai Postdoctoral Scientific Program (10R21412500). References [1] Y. Wan, D.Y. Zhao, Chem. Rev. 107 (2007) 2821e2860. [2] Y. Yamauchi, K. Kuroda, Chem. Asian J. 3 (2008) 664e676. [3] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710e712. [4] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, J. Am. Chem. Soc. 114 (1992) 10834e10843. [5] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548e552. [6] G.J.D. Soler-illia, C. Sanchez, B. Lebeau, J. Patarin, Chem. Rev. 102 (2002) 4093e4138. [7] C. Marquez-Alvarez, N. Zilkova, J. Perez-Pariente, J. Cejka, Catal. Rev. Sci. Eng. 50 (2008) 222e286. [8] J. Cejka, Appl. Catal. A 254 (2003) 327e338. [9] C.Z. Yu, B.Z. Tian, D.Y. Zhao, Curr. Opin. Solid State Mater. Sci. 7 (2003) 191e197. [10] F. Schüth, Chem. Mater. 13 (2001) 3184e3195. [11] M. Trueba, S.P. Trasatti, Eur. J. Inorg. Chem. 17 (2005) 3393e3403. [12] K. Oberlander, in: B.E. Leach (Ed.), Applied Industrial Catalysis, Academic Press, New York, 1984. [13] K. Wefers, in: L.D. Hart (Ed.), Alumina Chemicals: Science and Technology Handbook, The American Ceramic Society, Westerville, Ohio, 1990. [14] Z.R. Zhang, T.J. Pinnavaia, J. Am. Chem. Soc. 124 (2002) 12294e12301. [15] H.Y. Zhu, J.D. Riches, J.C. Barry, Chem. Mater. 14 (2002) 2086e2093. [16] M. Akia, S.M. Alavi, M. Rezaei, Z.F. Yan, Microporous Mesoporous Mater. 122 (2009) 72e78. [17] A.H. Lu, F. Schüth, Adv. Mater. 18 (2006) 1793e1805. [18] W.C. Li, A.H. Lu, W. Schmidt, F. Schüth, Chem. Eur. J. 11 (2005) 1658e1664. [19] Q. Liu, A.Q. Wang, X.D. Wang, T. Zhang, Chem. Mater. 18 (2006) 5153e5155. [20] H. Park, S.H. Yang, Y.S. Jun, W.H. Hong, J.K. Kang, Chem. Mater. 19 (2007) 535e542. [21] Q. Liu, A.Q. Wang, X.D. Wang, T. Zhang, Microporous Mesoporous Mater. 92 (2006) 10e21. [22] X.H. Liu, Y. Wei, D.L. Jin, W.H. Shih, Mater. Lett. 42 (2000) 143e149. [23] L. Ji, J. Lin, K.L. Tan, H.C. Zeng, Chem. Mater. 12 (2000) 931e939. [24] H. Zhang, G.C. Hardy, Y.Z. Khimyak, M.J. Rosseinsky, A.I. Cooper, Chem. Mater. 16 (2004) 4245e4256. [25] T.Z. Ren, Z.Y. Yuan, B.L. Su, Langmuir 20 (2004) 1531e1534.

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