Preparation and Characterization of Titania− Alumina Mixed Oxides

Dec 14, 2010 - ... N. A. , Eds.; The Royal Society of Chemistry: Cambridge, UK, 2009; ...... Jacob M. Schliesser , Rebecca E. Olsen , David B. Enfield...
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Ind. Eng. Chem. Res. 2011, 50, 883–890

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Preparation and Characterization of Titania-Alumina Mixed Oxides with Hierarchically Macro-/Mesoporous Structures Zhiming Zhou,* Tianying Zeng, Zhenmin Cheng, and Weikang Yuan State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China

A series of hierarchically macro-/mesoporous structured TiO2-Al2O3 mixed oxides with various molar ratios of Ti to Al was synthesized under different preparation conditions. The physical properties, that is, textural, morphological, and crystalline phase features of the prepared and calcined TiO2-Al2O3 materials were detailedly characterized by N2 adsorption-desorption, scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermal analysis techniques. The results showed that the preparation conditions, such as the solution pH, the water concentration, the molar ratio of Ti to Al, and the surfactant, can greatly influence the sizes of macropores and mesopores, the specific surface area, the pore volume, and the crystallization process of the mixed oxides. Calcination treatments of the prepared materials revealed their high thermal stability. The 800 °C calcined samples were thermally stable and retained the hierarchically macro-/mesoporous structure. Compared with pure TiO2 and pure Al2O3, the presence of Al2O3 in the TiO2-Al2O3 mixed oxides retarded the anatase-to-rutile phase transformation, while the incorporation of TiO2 promoted the formation of the R-Al2O3 phase in the mixed oxides. The hierarchically structured porous TiO2-Al2O3 materials had high potential applications as catalyst supports. 1. Introduction The design of porous solids with multiscale porosity has recently attracted great attention. The reduced mass-transfer limitations of species in the porous solid and the high specific surface area make these materials ideal candidates for many applications, including adsorption, separation, and catalysis. Of particular interest are materials with hierarchically macro-/ mesoporous structure.1-7 Incorporation of macropores (>50 nm) into mesoporous (2-50 nm) materials has two obvious advantages over materials with a unimodal pore-size distribution. First, the macroporous channels can greatly reduce the internal diffusion limitations of species inside the pellets, especially for large molecules such as proteins and polycyclic aromatics.8-10 Second, the narrow walls separating the macropores possess accessible mesopores, which cannot only maintain high specific surface areas similar to those of fine porous solids, but also facilitate the diffusion of species to and from the active sites due to the short diffusion distances restricted by the narrow walls.11,12 Previous investigations on hierarchically porous oxides mostly focused on single oxides, including SiO2, TiO2, ZrO2, and Al2O3, etc.1,11-22 Few published works reported binary mixed oxides with the hierarchical porosity. Yuan et al.23 made use of cationic alkyltrimethylammonium or nonionic poly(alkylene oxide)based (PEO) surfactants to prepare a series of binary mixed oxides with a hierarchically porous structure, including TiO2-ZrO2, TiO2-Al2O3, and Al2O3-ZrO2, etc. Garcı´a-Benjume et al.24 used the nonionic Tween-20 surfactant as a directing agent to synthesize the hierarchically macro-/mesoporous TiO2-Al2O3. Hosseini et al.25 prepared nanostructured macro-/mesoporous TiO2-ZrO2 catalyst supports in the presence of amine surfactants as well as the catalysts (TiO2-ZrO2 impregnated by Pd and/or Au) for oxidation of volatile organic compounds (VOCs). Chen et al.26 synthesized hierarchically bimodal porous TiO2-SiO2 and TiO2-ZrO2 composites with * To whom correspondence should be addressed. Tel: +86-2164252230. Fax: +86-21-64253528. E-mail: [email protected].

the aid of PEO surfactants and applied them for environmental photocatalysis. However, little work has systematically examined the effects of metal oxide composition and thermal calcination on the textural, morphological, and crystalline phase features of the prepared and calcined mixed oxides. Among various binary mixed oxides, TiO2-Al2O3 is undoubtedly one of the most attractive materials. In the last several decades, Al2O3 and TiO2 have been independently used as supports or catalysts in many chemical processes.27 Despite wide applications, Al2O3 and TiO2 have their own deficiencies and limitations. For example, TiO2 has relatively low surface area and poor thermal stability due to phase transformation, and Al2O3 suffers from declined catalytic activity for some applications.28-30 One of the strategies to overcome these drawbacks is to prepare TiO2-Al2O3 mixed oxides, which can retain the beneficial features of both TiO2 and Al2O3. In recent years, TiO2-Al2O3 mixed oxides have found applications in petroleum hydrotreatment,31-33 selective oxidation,34 and catalytic reduction of NOx.35,36 Taking into account the aforementioned advantages of the hierarchically macro-/mesoporous structure, it can be expected that incorporation of this unique pore structure into TiO2-Al2O3 mixed oxides will broaden the range of applications of these materials. In the present paper, a series of hierarchically macro-/ mesoporous structured TiO2-Al2O3 with different molar ratios of Ti to Al is prepared and characterized. By varying the preparation conditions, such as the solution pH, the water concentration, the molar ratio of Ti to Al, and the surfactant, we can tune to some extent the textural properties of the prepared mixed oxides. In addition, calcination treatment of these materials at different temperatures leads to variations in the textural, morphological, and crystalline phase features of the mixed oxides. To the best of our knowledge, this work systematically reports for the first time the preparation and characterization of hierarchically macro-/mesoporous TiO2-Al2O3 mixed oxides with different Ti molar fractions (0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0). Results obtained in this work

10.1021/ie101697t  2011 American Chemical Society Published on Web 12/14/2010

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Table 1. Textural Properties of TA(0.5/0.5) Samples Prepared under Different Conditionsa no.

water (mL)

ethanol (mL)

CTAB (g)

pH

SBET (m2/g)

pore volume (cm3/g)

mesopore sizeb (nm)

macropore sizec (µm)

a b c d e f g h i

35 35 35 35 35 35 50 50 15

15 15 15 15 15 15 0 0 35

0.4 0.4 0.4 0 0 0 0.4 0.4 0.4

12 7 1 12 7 1 7 1 1

338.95 506.70 326.72 510.88 456.42 316.19 463.19 496.99 434.03

0.25 0.40 0.29 0.23 0.25 0.14 0.30 0.32 0.34

5.4 5.1 5.8 2.8 3.1 3.0 4.7 4.3 4.8

0.63 1.15 0.91 0.70 1.25 0.95 1.70 1.59 0.52

a Common preparation conditions: room temperature, 350 rpm, 1 h. b BJH pore diameter determined from the adsorption branch. macropore diameter obtained from analysis of the image (the number of counts is 50).

are believed to be helpful for design and development of more high-performance catalysts in the foreseeable future. 2. Experimental Section 2.1. Materials. All chemicals were used as received without further purification. Titanium tetrabutoxide (TBOT, 98%), cetyltrimethylammonium bromide (CTAB, 99%), ethanol (99.7%), ammonia hydroxide (25%), and sulfuric acid (98%) were commercially available from Sinopharm Group Chemical Reagent Co. Aluminum tri-sec-butoxide (TBOA, 97%) was purchased from Alfa Aesar. Twice-distilled water was prepared in the authors’ laboratory. 2.2. TiO2-Al2O3 Preparation. A series of hierarchically macro-/mesoporous TiO2-Al2O3 mixed oxides with different Ti molar fractions x, defined as x ) Ti/(Ti + Al), was prepared (x ) 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1). For simplicity, the TiO2-Al2O3 with a Ti molar fraction x was denoted by TA(x/ (1 - x)) in the following text. In particular, x ) 0 and x ) 1 corresponded to pure Al2O3 and pure TiO2, respectively. In a typical synthesis of TA(0.5/0.5), 0.4 g of CTAB was added into a mixture of 35 mL of twice-distilled water and 15 mL of ethanol with slow stirring at room temperature. The pH of the solution was adjusted to 12.0 with NH3 · H2O. Meanwhile, 2.50 g of TBOA and 3.46 g of TBOT were mixed and sonicated in an ultrasonic cleaning bath for 10 min. Then, about 4 mL of the TBOA-TBOT mixture was added dropwise into the asprepared solution. After 1 h of hydrolysis-condensation reactions, the precipitates formed were separated by centrifugation, washed by Soxhlet extraction with ethanol for 30 h, and dried in air for 24 h. The differences in the preparation conditions for various TA(x/(1 - x)) samples included the ratio of Ti to Al, the ratio of water to ethanol, the solution pH, and the presence of CTAB or not. If the prepared TA(x/(1 - x)) sample need to be calcined, the calcination procedure was as follows: the sample was heated in air from room temperature to the desired temperature, with a ramp of 1 °C/min, and then it was kept at the desired temperature for 5 h. 2.3. Characterization. The nitrogen adsorption-desorption isotherm and the pore-size distribution were acquired at -196 °C on a Micromeritics ASAP 2010 instrument. All the samples were degassed at 150 °C and 1 mmHg for 6 h prior to nitrogen adsorption measurements. The pore diameter and the pore size distribution were determined by the BJH method. The morphology and the macroporous array of the TiO2-Al2O3 powders were examined with a JEOL JSM 6360 LV scanning electron microscope (SEM). The elemental composition of the sample surface was determined by energy-dispersive spectrometry (EDS, Falcon, EDAX) coupled to the SEM. High-resolution transmission electron microscopy (HRTEM) investigation was

c

The average

performed using a JEOL JEM-2010 transmission electron microscope. The sample prepared for HRTEM investigation was first dispersed in ethanol under ultrasound and then a drop of the ethanol solution was transferred onto a carbon-coated copper grid. X-ray diffraction (XRD) patterns of the prepared samples were obtained on a Rigaku D/Max 2550 VB/PC diffractiometer with Cu KR radiation scanning 2θ angles ranging from 10° to 80°. Thermal analysis was performed using a TA SDTQ600 thermogravimetric analyzer under an air flow of 100 mL/min. The sample was heated from room temperature to 1000 °C with a heating ramp of 10 °C/min. 3. Results and Discussion The TiO2-Al2O3 mixed oxides with hierarchically macro-/ mesoporous structures can be synthesized under different preparation conditions. For example, as shown in Table 1 and Figure 1, the TA(0.5/0.5) mixed oxides with the uniquely porous structure can be prepared in acidic, neutral, or basic solutions, with or without CTAB, and with or without ethanol. SEM analysis of these samples (Figure 1) reveals that all of them display monolithic macrochannels, which are parallel to each other and perpendicular to the tangent of the outer surface. Moreover, these macrochannels run through the prepared samples (see Figure S1 in the Supporting Information). HRTEM observations (Figure 2) demonstrate that the walls separating the macropores are composed of nanoparticles with accessible and interconnected mesopores of a disordered wormhole-like structure. The textural properties of the prepared samples are greatly influenced by the preparation conditions. First, the morphology of the prepared TA(0.5/0.5) samples has a strong dependence on the solution pH, which can be seen by comparison of samples a, b, and c, or samples d, e, and f. As shown in Figure 1 and Table 1, the samples prepared at pH 7.0, such as samples b and e, have larger macropores than those samples prepared at the same conditions except for the solution pH. According to investigations made by Hakim and Shanks,19 the formation of macropores is ascribed to contributions from both hydrolysis and condensation reaction rates of the transition metal alkoxides. For the titanium alkoxide (TBOT) with a high hydrolysis rate, the way to harmonize hydrolysis with condensation is to increase the solution pH because the condensation process can be accelerated in a basic solution. However, for the aluminum alkoxide (TBOA) with a relatively low hydrolysis rate, the desired balance between hydrolysis and condensation is attained in an acidic media, where the hydrolysis process is accelerated while the condensation is slowed down. This result has been verified by experimental observations19 which showed that the hierarchically macro-/mesoporous Al2O3 prepared from TBOA at higher pH value had a smaller macropore size than the

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Figure 1. SEM images of TA(0.5/0.5) samples under different preparation conditions. Scale bar: 10 µm.

Figure 2. HRTEM images of TA(0.5/0.5) samples: (T1, T2) sample a, (T3, T4) sample d.

samples synthesized at acidic or neutral pH, whereas for TiO2 prepared from TBOT the trend was reverse. Therefore, the occurrence of large macropores of the TA(0.5/0.5) material at a neutral solution would probably be the balance between the different effects of the solution pH on Al2O3 and on TiO2. Second, the macropore size of the TA(0.5/0.5) sample is affected by the water concentration of the preparation solution. Corresponding to samples i, c, and h, for which the water concentration is gradually increased, the macropore sizes of these samples are 0.52, 0.91, and 1.59 µm, respectively. It means that the higher the water concentration, the larger the macropore size. The same result is also obtained for hierarchically macro-/ mesoporous Al2O3 samples.11 As mentioned above, the formation of the macropores is greatly determined by hydrolysis and condensation rates of the alkoxide precursors. The high water concentration is believed to increase the hydrolysis-condensation rates of TBOA and TBOT, and as a result, large macropores are obtained with high water concentration. Third, the textural properties of the prepared samples are to some extent influenced by the surfactant molecules, as can be seen by comparing samples a and d, samples b and e, or samples c and f. As listed in Table 1, the addition of CTAB has almost no effect on the macropore size of the prepared samples, whereas the mesopore size is strongly affected by the presence of CTAB. As shown in Figure 3, the TA(0.5/0.5) mixed oxides prepared with CTAB, i.e., samples a, b, and c, have a bimodal pore-size distribution, consisting of smaller pores (10 nm) are measured. This result is consistent with that obtained for Al2O3.11 The reason lies in that the CTAB molecules form a bilayer structure on the surfaces of the Al2O3 and TiO2 particles, which results in the widening of the mesopore size distribution.9,11,16 On the basis of the above experimental results, a spontaneous self-assembly mechanism, which has been successfully used for explanation of formation of single oxides, can be proposed here for the formation of the hierarchically macro-/mesoporous structured TiO2-Al2O3 mixed oxides. Detailed information on this mechanism is presented elsewhere.4,11,14-16,18,19 To check the thermal stability of the prepared TiO2-Al2O3 mixed oxides, sample a listed in Table 1 is calcined at different temperatures. As show in Figure 4, the macroporous channels of the calcined TA(0.5/0.5) mixed oxides are well preserved even if the temperature is up to 1000 °C, indicating excellent thermal stability of the prepared materials. The N2 adsorptiondesorption isotherms and the corresponding pore-size distribution curves of these calcined TA(0.5/0.5) materials are shown in Figure 5. The textural properties of these samples are summarized in Table 2. With an increase in the calcination temperature, mesopores and macropores of the samples gradually increase, whereas specific surface area and pore volume increase first (until 400 °C) and then decrease. Similar results are also obtained by Linacero et al.,28 who found that the 500 °C calcined TiO2-Al2O3 mixed oxides had larger specific surface areas than the prepared samples. This is mainly caused by the development of the mesoporosity in these materials. The calcination treatment of the mixed oxide under relatively low

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Figure 5. N2 adsorption-desorption isotherms (a) and the corresponding pore-size distribution curves (b) of TA(0.5/0.5) (sample a in Table 1) under different calcination temperatures. Table 2. Textural Properties of TA(0.5/0.5) (Sample a in Table 1) under Different Calcination Temperatures

Figure 3. N2 adsorption-desorption isotherms (a) and the corresponding pore-size distribution curves (b) of samples a, d, b, e, c, and f (listed in Table 1).

temperatures (e400 °C) probably removes the organic groups occluded inside the micropores, which generates new surfaces for access of nitrogen molecules during adsorption measurements. The decrease in surface area and pore volume at high calcination temperature is related with the growth of TiO2 and Al2O3 crystallites and subsequent phase transformation of the TA(0.5/0.5) mixed oxide, because rutile and R-Al2O3 normally havesmallersurfaceareasthananataseandγ-Al2O3,respectively.12,37 At 1000 °C, the specific surface area and pore volume decrease to 8.21 m2/g and 0.04 cm3/g, respectively, indicating the collapse of the mesoporous structure. Figure 6 presents the XRD patterns of the prepared sample a and the corresponding calcined samples a1-a7. The asprepared sample a and the low temperature calcined samples

no.

temp (°C)

SBET (m2/g)

pore volume (cm3/g)

a1 a2 a3 a4 a5 a6 a7

200 400 600 650 700 800 1000

400.46 470.72 157.54 106.13 82.76 14.19 8.21

0.28 0.59 0.36 0.35 0.33 0.12 0.04

mesopore size (nm) 4.7 4.5 7.7 10.9 13.6 36.5 -

macropore size (µm) 0.58 0.78 0.92 0.98 0.90 0.94 0.99

a1 (200 °C) and a2 (400 °C) are in an amorphous phase. An EDS analysis of sample a2 (see Figure S2 in the Supporting Information) shows that this material is composed of Ti, Al, and O with the atomic percentage of 18.02, 18.52, and 63.46%, respectively, which confirms the molar ratio of Ti to Al in this sample. At 600 °C, sample a3 exhibits diffraction peaks assigned to the anatase phase (JCPDS 21-1272) and the rutile phase (JCPDS 21-1276). Upon increasing the temperature to 650 and 700 °C, the diffraction peaks of the samples (a4 and a5) are very similar to those of sample a3 except that the intensities of these peaks increase as a result of an enhancement of crystallization. Note that no noticeable peaks belonging to aluminum oxides are observed over the calcination temperature range of 200-700 °C, which implies that in these samples the alumina crystallites must be very small and dispersed so that they are not detected by XRD.23,28

Figure 4. SEM images of TA(0.5/0.5) (sample a in Table 1) under different calcination temperatures: (a) room temperature, (a1) 200 °C, (a2) 400 °C, (a3) 600 °C, (a4) 650 °C, (a5) 700 °C, (a6) 800 °C, and (a7) 1000 °C. Scale bar: 10 µm.

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Figure 8. X-ray diffraction patterns of Al2O3 and TA(0.5/0.5) under 800 °C calcination.

Figure 6. X-ray diffraction patterns of TA(0.5/0.5) (sample a in Table 1) under different calcination temperatures.

Figure 7. X-ray diffraction patterns of TiO2 and TA(0.5/0.5) under 650 °C calcination.

The mass fraction of the anatase phase (wA) in pure titanium oxide is normally determined from the intensities of the [101] (2θ ) 25.4°) and [110] (2θ ) 27.5°) reflection planes for anatase (IA) and rutile (IR), respectively, by applying the following equation:38 wA )

1 1 + 1.26(IR /IA)

(1)

As for the TiO2-Al2O3 mixed oxide, the anatase fraction can be readily obtained by multiplying the mass fraction of TiO2 in the mixed oxide by wA calculated by eq 1. For the samples calcined at 600, 650, and 700 °C, the anatase fractions are 43.3, 40.9, and 38.6%, respectively, while the rutile fractions are 17.7, 20.1 and 22.4%, respectively. It indicates that the anatase phase is gradually transformed into the rutile phase with the increased temperature. When the temperature is further up to 800 and 1000 °C, rutile and R-Al2O3 phases (JCPDS 10-0173) are observed, and no anatase peaks are detected, indicating the complete phase transformation from anatase to rutile. The effects of the calcination temperature on the phase transformation are different for pure metal oxides (TiO2 and Al2O3) and mixed oxides (TiO2-Al2O3). Figure 7 compares the XRD patterns of pure TiO2 and TA(0.5/0.5) after 650 °C calcination treatment. The TiO2 sample only exhibits the rutile phase,21 but the TA(0.5/0.5) mixed oxide displays a mixture of

anatase and rutile. This indicates that the presence of aluminum retards the anatase-to-rutile phase transformation, which is consistent with other literature reported.24,39-42 Obviously, the presence of Al2O3 as a secondary phase inhibits crystallization and consequent crystal growth of TiO2. On the contrary, the presence of titania promotes the formation of the R-Al2O3 phase. As shown in Figure 8, the 800 °C calcined Al2O3 exhibits the δ-Al2O3 phase (JCPDS 16-0394),21 while the 800 °C calcined TA(0.5/0.5) displays the R-Al2O3 phase instead of δ-Al2O3. This result is in agreement with those reported previously,28,43,44 which found that the addition of some oxides such as MgO and TiO2 into Al2O3 could facilitate the formation of the R-Al2O3 phase at a lower calcination temperature by comparison with pure Al2O3 samples. Besides the TA(0.5/0.5) material, some other TiO2-Al2O3 mixed oxides with various ratios of Ti to Al are also prepared in this work. Figure 9 shows the SEM images of different TiO2-Al2O3 mixed oxides. Among these materials, samples a and f are pure Al2O3 and pure TiO2, respectively. A regular array of macropores is clearly seen for each TA(x/(1 - x)) mixed oxide. An interesting phenomenon is that the macropore size of the prepared mixed oxide depends on the molar ratio of Ti to Al. The macropore size increases with the Ti content until x ) 0.6, which (TA(0.6/0.4)) displays the maximum macropores with the diameter of about 1.6 µm. However, when the Ti content is further increased, the macropore size decreases. As expected, the ratio of Ti to Al also affects the mesopore size, specific surface area, and pore volume of the prepared mixed oxides. As shown in Table 3 and Figure S3 in the Supporting Information, with the increase of the Ti molar fraction from 0 to 0.8, the specific surface area and pore volume of the TA(x/(1 - x)) samples gradually decrease, but the mesopore size increases. An exception is the pure TiO2 sample, which possesses larger specific surface area than the TA(x/(1 - x)) with 0.4 e x e 0.8. This is caused by the high proportion of micropores in the pure TiO2 sample, as illustrated in Figure S3. The above results show that the textural properties of the TiO2-Al2O3 mixed oxides can be adjusted and controlled to some extent by varying the molar ratio of Ti to Al. Figure 10 shows the XRD patterns of the prepared TA(x/(1 - x)) mixed oxides. The TA(x/(1 - x)) samples with 0.4 e x e 1 (including the pure TiO2) are amorphous, while the TA(0.2/ 0.8) sample exhibits weak diffraction peaks assigned to the boehmite phase (JCPDS 21-1307), which are also observed for the pure Al2O3 sample. Corresponding to the prepared TA(x/(1 - x)) mixed oxides, the 800 °C calcined samples (Figure 11b-e) show a mixture of rutile and R-Al2O3. By comparison of pure

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Figure 9. SEM images of TA(x/(1 - x)) mixed oxides with different compositions: (a) x ) 0, (b) x ) 0.2, (c) x ) 0.4, (d) x ) 0.6, (e) x ) 0.8, and (f) x ) 1. Scale bar: 10 µm. Table 3. Textural Properties of TA(x/(1 - x)) Samplesa x

SBET (m2/g)

pore volume (cm3/g)

mesopore size (nm)

macropore size (µm)

0 0.2 0.4 0.6 0.8 1

514.11 320.64 162.45 80.68 11.21 320.50

0.81 0.46 0.23 0.12 0.03 0.17

5.9 5.2 5.5 6.6 14.4 3.8

0.45 0.79 0.86 1.60 0.45 0.75

a Preparation conditions: 35 mL of H2O, 15 mL of C2H5OH, 0.4 g of CTAB, pH ) 12, room temperature, 350 rpm, 1 h.

Figure 11. X-ray diffraction patterns of 800 °C calcined TA(x/(1 - x)) mixed oxides with different compositions: (a) x ) 1, (b) x ) 0.8, (c) x ) 0.6, (d) x ) 0.4, (e) x ) 0.2, and (f) x ) 0.

Figure 10. X-ray diffraction patterns of TA(x/(1 - x)) mixed oxides with different compositions: (a) x ) 1, (b) x ) 0.8, (c) x ) 0.6, (d) x ) 0.4, (e) x ) 0.2, and (f) x ) 0.

Al2O3 (Figure 11f) with TA(x/(1 - x)), it can be seen that, just as mentioned above, the presence of Ti does promote the phase transformation from δ-Al2O3 to R-Al2O3. Moreover, Figure 11 clearly shows that with an increase in the Ti content in the mixed oxides, the diffraction peaks belonging to the rutile phase increase, while other peaks assigned to R-Al2O3 decrease accordingly. The DTA curves for the TA(x/(1 - x)) series are presented in Figure 12. For all the samples there is an endothermic peak centered at about 70 °C, which can be ascribed to the removal of the physisorbed water and ethanol. For the pure TiO2 sample, two distinct exothermic peaks are observed. The first exothermic peak centered at 264 °C is attributed to the crystallization process of the anatase phase, and the second peak centered at 447 °C corresponds to the phase transformation of anatase to rutile, which is in agreement with that reported by Linacero et

al.28 Compared with TiO2, the two peaks corresponding to anatase and rutile for the TA(x/(1 - x)) mixed oxides occur at relatively high temperatures. This result is in accord with the above observations from XRD analysis, that is, the addition of alumina into titania retards the crystallization processes of titania. For the four TA(x/(1 - x)) samples with an increase in the Ti molar fraction from 0.2 to 0.8, the temperatures of the maxima for the anatase peak are 279, 273, 268, and 267 °C, respectively. Obviously, the more the Al2O3 content in the mixed oxides is, the higher is the crystallization temperature for the anatase phase. The same result is also applicable for the rutile phase. For TA(0.6/0.4) and TA(0.8/0.2), the temperatures of the maxima for the rutile peak are 495 and 492 °C, respectively. However, for those mixed oxides with low TiO2 content, such as TA(0.2/0.8) and TA(0.4/0.6), the rutile peak is not observed from the DTA curve. This is probably caused by the overlap of the crystallization processes of the rutile phase and the γ-Al2O3 phase. According to the detailed XRD analysis on the hierarchically macro-/mesoporous Al2O3 (not presented here), the phase transformation from boehmite to γ-Al2O3 occurs between 400 and 500 °C, in which the anatase-to-rutile phase transformation also takes place. It can be expected that the component with the high content in the binary metal oxides dominates the crystallization process of the mixed oxide over this temperature range. For the pure Al2O3 sample, as shown in Figure 12, the characteristic peaks associated with phase transformations are

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properties of the prepared mixed oxides were found to be dependent on the preparation conditions: the samples prepared in neutral solutions had larger macropores than those prepared in acidic or basic solutions; a high water concentration was favorable for the formation of large macropores; surfactant molecules had no effect on the macropore size but strongly influenced the mesopore size of the prepared TiO2-Al2O3; an increase in the molar ratio of Ti to Al led to a decrease in the specific surface area and pore volume of the materials. Calcination of the mixed oxides under various temperatures showed the good thermal stability of the hierarchically structured porous TiO2-Al2O3. The crystallization process induced by calcination indicated that the presence of alumina retarded the anatase-torutile phase transformation, but titania promoted the formation of the R-Al2O3 phase. The results showed that the textural, morphological, and crystalline phase features of the hierarchically macro-/mesoporous TiO2-Al2O3 mixed oxides could be tuned to some extent by varying preparation conditions and calcination temperatures. Therefore, it can be expected that these materials would be ideal catalyst supports for many chemical processes, particularly for hydrotreatment of petroleum fractions. Acknowledgment

Figure 12. DTA curves of TA(x/(1 - x)) mixed oxides with different compositions.

not distinct, and as a result, the TA(0.2/0.8) and TA(0.4/0.6) mixed oxides with high content of alumina do not exhibit distinct peaks assigned to the anatase-to-rutile transformation. After 800 °C calcination treatment, the monolithic macropores of the TA(x/(1 - x)) mixed oxides are still well preserved, as shown in Figure S4 of the Supporting Information. The textural properties of these materials are listed in Table S1 of the Supporting Information. Although the macropores are preserved after 800 °C calcination, the mesopore size gradually increases with the Ti content. Particularly when the Ti molar fraction is above 0.6, the mesopores completely collapse. The main reason for the destruction of the mesopores is usually believed to be the excessive crystallite growth.26,41,45 Therefore, the TiO2-Al2O3 mixed oxides with high TiO2 content must possess larger crystallite sizes than the mixed oxides with low TiO2 content. Indeed, this result is confirmed by the aforementioned XRD analysis on the mixed oxides (Figure 11). In the end, the TiO2-Al2O3 mixed oxides prepared in this work exhibit excellent thermal stability, which can preserve the macro-/mesoporous structure after high temperature calcination. This is very attractive from the viewpoint of catalysis applications. In the authors’ group, the hydrodesulfurization of refractory sulfur compounds, such as thiophene, benzothiophene, and 4,6-dimethylbenzothiophene from fuels by using the Ni-Mo or Co-Mo catalysts supported on these hierarchically porousstructured TiO2-Al2O3 mixed oxides are now under way, and the relevant results will be reported in the near future. 4. Conclusions A series of hierarchically macro-/mesoporous structured TiO2-Al2O3 mixed oxides was prepared by hydrolysis and condensation of metal alkoxide precursors with various amounts of TiO2, from pure titania to pure alumina. The textural

Financial supports from the National Natural Science Foundation of China (Grant No. 20706018), the National High Technology Research and Development Program of China (Grant No. 2008AA05Z405), the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT0721), the “111” Project (Grant No. B08021), the Open Project of State Key Laboratory of Chemical Engineering (SKLChE-10C06) and the Fundamental Research Funds for the Central Universities (WA1014003) are greatly acknowledged. Supporting Information Available: SEM images of TA(0.5/ 0.5) materials; EDS analysis of the 400 °C calcined TA(0.5/ 0.5) sample; N2 adsorption-desorption isotherms and the poresize distribution curves of TA(x/(1 - x)) materials; SEM images and textural properties of the 800 °C calcined TA(x/(1 - x)) samples. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Blin, J. L.; Le´onard, A.; Yuan, Z. Y.; Gigot, L.; Vantomme, A.; Cheetham, A. K.; Su, B. L. Hierarchically mesoporous/macroporous metal oxides templated from polyethylene oxide surfactant assemblies. Angew. Chem., Int. Ed. 2003, 42, 2872–2875. (2) Yuan, Z. Y.; Su, B. L. Insights into hierarchically meso-macroporous structured materials. J. Mater. Chem. 2006, 16, 663–677. (3) Nakanishi, K.; Tanaka, N. Sol-gel with phase separation. Hierarchically porous materials optimized for high-performance liquid chromatography separations. Acc. Chem. Res. 2007, 40, 863–873. (4) Vantomme, A.; Le´onard, A.; Yuan, Z. Y.; Su, B. L. Self-formation of hierarchical micro-meso- macroporous structures: Generation of the new concept “hierarchical catalysis. Colloids Surf. A 2007, 300, 70–78. (5) Yang, X. Y.; Li, Y.; Lemaire, A.; Yu, J. G.; Su, B. L. Hierarchically structured functional materials: Synthesis strategies for multimodal porous networks. Pure Appl. Chem. 2009, 81, 2265–2307. (6) Coppens, M.-O.; Wang, G. Optimal design of hierarchically structured porous catalysts. In Design of Heterogeneous Catalysts; Ozkan, U. S., Ed.; Wiley-VCH: Weinheim, Germany, 2009; pp 25-58. (7) Wang, G.; Coppens, M.-O. Rational design of hierarchically structured porous catalysts for autothermal reforming of methane. Chem. Eng. Sci. 2010, 65, 2344–2351. (8) Ternan, M.; Rahimi, P. M.; Clugston, D. M.; Dettman, H. D. The +525 °C residue before and after hydrocracking with bimodal catalysts of varying macropore volume. Energy Fuels 1994, 8, 518–530.

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ReceiVed for reView August 10, 2010 ReVised manuscript receiVed October 28, 2010 Accepted November 27, 2010 IE101697T