Ultrasonic Synthesis of Silica-Alumina Nanomaterials with Controlled

Department of Chemical Engineering, The Hong Kong University of Science and Technology,. Clear Water Bay, Hong Kong, SAR, People's .... (11) Biz, S.; ...
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Langmuir 2002, 18, 4111-4117

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Ultrasonic Synthesis of Silica-Alumina Nanomaterials with Controlled Mesopore Distribution without Using Surfactants Nan Yao,† Guoxing Xiong,*,† King Lun Yeung,‡ Shishan Sheng,† Mingyuan He,§ Weishen Yang,† Xiumei Liu,† and Xinhe Bao† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, People’s Republic of China, Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, SAR, People’s Republic of China, and Research Institute of Petroleum Processing SINOPEC, Beijing 100083, People’s Republic of China Received October 29, 2001. In Final Form: January 29, 2002 A novel sol-gel process has been developed for the synthesis of amorphous silica-aluminas with controlled mesopore distribution without the use of organic templating agents, e.g., surfactant molecules. Ultrasonic treatment during the synthesis enables production of precursor sols with narrow particle size distribution. Atomic force microscopy analysis shows that these sol particles are spherical in shape with a narrow size distribution (i.e., 13-25 nm) and their aggregation during the gelation creates clusters containing similar sized interparticle mesopores. A nitrogen physiadsorption study indicates that the mesoporous materials containing different Si/Al ratios prepared by the new synthesis method has a large specific surface area (i.e., 587-692 m2/g) and similar pore sizes of 2-11 nm. Solid-state 27Al magic angle spinning (MAS) NMR shows that most of the aluminum is located in the tetrahedral position. A transmission electron microscopy (TEM) image shows that the mesoporous silica-alumina consists of 12-25 nm spheres. Additionally, high-resolution TEM and electron diffraction indicate that some nanoparticles are characteristic of a crystal, although X-ray diffraction and 29Si MAS NMR analysis show an amorphous material.

1. Introduction Mesoporous materials with pore sizes ranging from 2 to 50 nm have applications in shape-selective catalysis and biomolecular immobilization and separation because of their high specific surface areas and large uniform pore sizes.1-3 Numerous studies on their synthesis, characterization, and application have been reported.4-6 Since the scientists at Mobil Oil Research and Development announced the successful synthesis of mesoporous molecular sieves (M41S) in 1992,7,8 it has now been well accepted that the formation of such mesoporous materials could occur through several templating pathways such as S+I-, S-I+, S+X-I+, S-X+I-, S-I, and S0I0, where S is the surfactant, I is the inorganic precursor, and X is the mediating ions.9,10 A typical synthesis starts with the formation of organic micellar species in aqueous solution, * To whom correspondence may be addressed. E-mail: gxxiong@ ms.dicp.ac.cn. † Chinese Academy of Sciences. ‡ The Hong Kong University of Science and Technology. § Research Institute of Petroleum Processing SINOPEC. (1) Corma, A. Chem. Rev. 1997, 97, 2373. (2) Diaz, J. F.; Balkus, K. J., Jr. J. Mol. Catal. B: Enzym. 1996, 2, 115. (3) Suib, S. T. Curr. Opin. Solid State Mater. Sci. 1998, 3, 63. (4) Aguado, J.; Serrano, D. P.; Romero, M. D.; Escola, J. M. J. Chem. Soc., Chem. Commun. 1996, 765. (5) Corma, A.; Iglesias, M.; Sanchez, F. Catal. Lett. 1996, 39, 153. (6) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321. (7) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (8) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (9) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. Nature 1994, 368, 317. (10) Tanev, P. T.; Pinnavaia, T. J. Chem. Mater. 1996, 8, 2068.

followed by the polycondensation of an inorganic matrix or shell, and ends with the removal of the organic template. The nature of the interaction between the organic surfactant and the inorganic matrix is dictated by the synthesis reagents and preparation conditions and is a controlling factor in the physical and chemical properties of the mesoporous materials.11 Mesoporous silica-alumina materials possess catalytic properties similar to those of the zeolites but without the micropore restrictions. Owing to their controlled mesoporosity and Bro¨nsted acidity, these materials could be used to prepare metal bifunctional catalysts for hydroisomerization, as well as in the hydrocracking of longchain paraffins (e.g., n-alkane).12,13 Until now, none has described a method for the synthesis of mesoporous material without the aid of surfactant. This paper reports a new templateless procedure for preparing narrow pore sized mesoporous materials. The approach is based on the simple idea that the regular packing of nanometer sized sol spheres can create a network of narrow mesoporous channels. The synthesis procedure utilizes a new sol-gel process to obtain nanometer sized precursor particles of narrow size distribution from inexpensive inorganic salts. The absence of surfactant and the use of inorganic salts instead of organometallic precursors contribute to the reduction in cost and pollution during the manufacture of these materials. 2. Experimental Section 2.1. Synthesis Method. All the chemicals used in the material synthesis were A.R. grade, and the water was deionized and twice distilled. A measured amount of ammonium hydroxide (11) Biz, S.; Occelli, M. L. Catal. Rev.-Sci. Eng. 1998, 40 (3), 330. (12) Corma, A.; Martinez, A.; Pergher, S.; Peratello, S.; Perego, C.; Bellusi, G. Appl. Catal., A 1997, 152, 107. (13) Calemma, V.; Peratello, S.; Perego, C. Appl. Catal., A 2000, 190, 207.

10.1021/la0116084 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/09/2002

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(2.5%) was added dropwise to 25 mL of 0.96 M aluminum nitrate solution under vigorous stirring, bringing the pH to about 9.20. A series of centrifugation and washing steps were necessary to bring the pH of the final suspension to neutral. Nitric acid (0.94 M) was added to the aluminum hydroxide suspension until the [H+]/[Al3+] molar ratio was 0.27. The suspension was then treated by ultrasound (SB2200, Branson) for 10 min to obtain a stable alumina sol precursor for the synthesis of mesoporous silicaalumina. At the same time, the pH of a 50 mL water glass solution (0.78 mol/L SiO2, 0.25 mol/L Na2O) was adjusted to 10 through addition of 0.94 M nitric acid. The prepared alumina sol was then added to the water glass solution until the desired Si/Al molar ratio was attained. The resulting precipitate was peptized using nitric acid too. The suspension was then ultrasonically treated for 1 min to obtain a stable sol. The pH values of the stable sols with different Si/Al molar ratios are 2.55, 2.48, and 2.01 for UMSA1 (Si/Al ) 10), UMSA2 (Si/Al ) 7), and UMSA3 (Si/Al ) 3), respectively. When the sol finally formed a homogeneous and slightly opalescent gel at room temperature, 70 mL of NH4NO3 solution (1.2 M) was added to the gel to remove the sodium ions. This step was performed at room temperature in the absence of stirring. At the end of 24 h, the solution was drained off by centrifugation, and this procedure was repeated three times. Finally, the gel samples were calcined in air at 550 °C for 10 h to obtain the solid samples. In this work, the sol and solid samples prepared by the new method were named as UMSA samples. In contrast, amorphous silica-alumina samples with Si/Al ratios of 1 and 10, prepared by the conventional co-gel method using the same raw materials were referred to as GMSA. During the preparation, the precipitate was washed seven times in order to remove the sodium ions, and after drying at 50 °C for 12 h, the precipitate was calcined in air at 550 °C for 10 h. 2.2. Characterization. The pH and density of the sols were monitored using a Cole-Parmer 5986-50 pH meter and M4 density meter, respectively. A N4 plus laser scattering particle meter (Coulter) was used to measure the sol particle diameter distribution at a 90° angle to the light beam. A He-Ne laser operating at 10 mW was used as the light source. Drops of the sol were diluted in the sample cuvette with water to give the appropriate intensity for the measurement. The material structure and chemistry were characterized by X-ray diffraction (XRD, Rigaku D/MAX-RB), atomic force microscopy (AFM, Nanoscope III), highresolution transmission electron microscopy (HRTEM, JEM2010), N2 physiadsorption (Omnisorp-100CX), and 27Al and 29Si magic angle spinning (MAS) NMR (Bruker DRX400). The sol particles imaged by the AFM were prepared by depositing 4 µL of diluted sol sample (1.5 × 10-4 mol/L) onto a freshly cleaved Mica surface. The deposited sample was immediately refrigerated at 263 K for 12 h and freeze-dried (Edwards Super Modulyo 12 K) at 6 × 10-1 Torr for 12 h. This procedure prevents the aggregation of the sol particle on the Mica surface. Gel samples were obtained by depositing the same quantity of sol onto Mica, except the samples were dried at room temperature for 12 h. The solid specimen imaged by TEM and HRTEM was crushed into powder, embedded in metal copper, and polished on both sides of the specimen until the powder was exposed. Then ion etching was performed to make the specimen transparent to electrons. To keep the original structure of the powder, neither high-temperature treatment nor chemicals were used during the specimen preparation for TEM and HRTEM.14 N2 adsorption-desorption isotherms of the samples were obtained at 77 K using the static volumetric method. The solid samples were first degassed at 623 K under vacuum (10-6 Torr) for at least 3 h before recording their isotherms. The sample’s specific surface area was calculated based on the Brunauer, Emmett, and Teller theory (BET). The mesopore volume and its distribution were calculated using the Barrett, Joyner, and Halenda theory (BJH) from the adsorption isotherm. The micropore volume and its distribution were calculated according to the t-plot method and the Horvath-Kawazoe method (HK) from the adsorption isotherm. Solid-state 27Al MAS NMR spectra (14) Zhang, X. F.; Zhang, Z. Progress in Transmission Electron Microscopy, I. Concepts and Techniques; Tsinghua University Press & Springer-Verlag: Beijing, 1999.

Yao et al. of solid samples were run at 104.3 MHz at a spinning rate of 8 kHz, while the 29Si MAS NMR spectra were conducted at 79.49 MHz at a spinning rate of 4 kHz. Al2(SO4)3 and DSS (3(trimethylsiyl)propanesulfonic acid sodium salt) were used as standard references for aluminum and silicon, respectively. The nature and strength of the acid sites were studied by monitoring the thermal desorption of chemisorbed pyridine using a Nicolet Impact 410 Fourier transform infrared spectroscopy. All the IR spectra were measured in the following conditions: scan 32 times, resolution 4 cm-1, detector DTGS KBr. Selfsupported wafers (6-8 mg cm-2) used in the adsorption studies were prepared by pressing samples between 15 mm diameter dies for 2 min at 10 MPa of pressure. The wafers were mounted in homemade in situ quartz IR cells with CaF2 windows. Before the pyridine adsorption, the wafers were heated at 400 °C for 2 h in a vacuum (10-2 Pa). A reference spectrum was recorded at room temperature. For acidity measurements, 500 Pa of pyridine was introduced into the in situ cell at room temperature. The sample was then degassed at 150 °C for 30 min in a vacuum (10-2 Pa) and cooled to room temperature to record the spectrum. This process was repeated for thermal desorption at 250 and 350 °C. Only the 1700-1400 cm-1 part of the IR spectra, obtained by subtracting the absorbance reference spectrum from the absorbance spectra recorded after probe adsorption, was considered in the present work. Pyridine concentrations on Bro¨nsted and Lewis acid sites were calculated by using the equations developed by Emeis for porous aluminosilicates.15 On the basis of the method reported in the literature,16 the acid strength distribution was evaluated from the differences between the pyridine amounts present after desorption at 250 °C/150 °C, 350 °C/250 °C, and 350 °C, corresponding to the number of weak, medium, and strong sites, respectively.

3. Results and Discussion Figure 1 displays the N2 adsorption-desorption isotherms for mesoporous silica-alumina prepared by the new synthesis technique (UMSA1) and traditional co-gel method (GMSA1 and GMSA2). The samples UMSA1 and GMSA2 contained a Si/Al ratio of 10, whereas GMSA1 had a Si/Al ratio of 1. The adsorption isotherms shown in isotherms A and B of Figure 1 belong to type IV isotherm, characterized by a well-defined step in the adsorption isotherm curve. This step feature indicates the filling of the framework-confined mesopores. The plateau on the adsorption curve is associated with multilayer adsorption on the external surface of the sample. The absence of a sharp rise in nitrogen uptake near saturation pressure (P/P0 ) 1) means that there are no macropores in the samples.17 Isotherms A and B of Figure 1 show that the adsorbed volume at the step feature of GMSA1 sample is larger than UMSA1 and the latter has a broader plateau (0.78 < P/P0 < 0.98) than the GMSA1. This implies that UMSA1 has a larger external surface area but smaller mesopore volume. The presence of hysteresis loop in the desorption branch confirms the existence of a mesopore, and the shapes of the loops for UMSA1 and GMSA1 samples are classified according to IUPAC as type H2 and H1, respectively.18 It is recognized that the shape of the hysteresis loop is related to the material’s pore structure. Thus, in the case of a UMSA1 sample, the appearance of a H2 loop indicates the existence of pores with a narrow neck and a wide body. These ink-bottlelike pores are expected to occur from aggregation of globular or particulate gel structure.19 For a GMSA1 sample, an H1 type loop suggests a more open pore structure.20 (15) Emeis, C. A. J. Catal. 1993, 141, 347. (16) Perego, C.; Amarilli, S.; Carati, A.; Flego, C.; Pazzuconi, G.; Rizzo, C.; Bellussi, G. Micropor. Mesopor. Mater. 1999, 27, 345. (17) Gregg, S. J.; Sing, K. S. W. Adsorption, surface area and porosity, 2nd ed.; Academic Press: London, 1982. (18) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemientewska, T. Pure Appl. Chem. 1985, 57, 603.

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Figure 2. Pore size distribution of samples: A, UMSA1 sample; B, GMSA1 sample; C, GMSA2 sample. Table 1. Specific Surface Area and Pore Texture of the Materials as a Function of Si/Al Molar Ratio specific surface Si/Al pore size area distribution sample molar BET C VBJH Vt a b 2 no. ratio value (mL/g) (mL/g) (m /g) (nm) GMSA1 GMSA2 UMSA1 UMSA2 UMSA3

1 10 10 7 3

87.36 94.80 73.58 78.26 77.02

0.663 0.000 0.291 0.355 0.305

0.00 0.27 0.01 0.01 0.01

301.06 467.18 692.54 648.59 587.24

2.11-66.27 1.01-1.58 2.04-10.20 3.24-11.23 3.20-11.70

a Pore volume calculated by BJH method. b Pore volume calculated by t-plot method.

Figure 1. Nitrogen adsorption-desorption isotherms of samples: A, UMSA1 sample; B, GMSA1 sample; C, GMSA2 sample.

The pore size distributions of UMSA1 and GMSA1 samples were calculated from the adsorption isotherm and are shown in parts A and B of Figure 2, respectively.21 It is clear from the figure that the UMSA1 sample has a (19) Brinker, C. J.; Scherer, G. W. Sol-gel Science: the Physics and Chemistry of Sol-gel Processing; Academic Press, Inc.: Boston, MA, 1990; p 524. (20) Calvino, J. J.; Cauqui, M. A.; Cifredo, G.; Esquivias, L.; Perez, J. A.; Ramirez Del Solar, M.; Rodriguez-Izquierdo, J. M. J. Mater. Sci. 1993, 28, 2191. (21) Rouquerol, F.; Rouquerol, J.; Sing, K S. W. Adsorption by powders and porous solids; Academic Press: London, 1999.

narrower pore size distribution (i.e., 2-10 nm) when compared to GMSA1 whose pores range from 2 to 66 nm. It has been well established that the pore size distribution of porous silica-alumina materials prepared by the conventional co-gel method changes with the Si/Al ratio. This is exemplified by the GMSA2 sample. The sample displays a type I + IV isotherm that indicates the coexistence of meso- and micropores (Figure 1C). By use of HK method, the pore size distribution of the GMSA2 sample is calculated and displayed in Figure 2C. This material consists mainly of micropores with size of 1.01.6 nm instead of the desired mesopores. Table 1 summarizes the pore volumes along with the specific surface area and pore size of the different silica-alumina samples. It is seen that one of the advantages of the new synthesis method is that the samples’ pore size distribution can be maintained constant over a wide range of Si/Al ratios. This property is quite different from the literature results, which showed that the pore size distribution changes with the Si/Al molar ratio.22 Moreover, the new synthesis method also yields larger specific surface area.

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Figure 3. Schematic model proposed for the effect of ultrasonic treatment under present synthesis. The dotted lines indicates the weak bonds.

The results clearly demonstrate that mesoporous silicaaluminas with narrow pore size distribution can be successfully synthesized without the aid of organic templates or surfactants. The ability to obtain reproducible pore sizes (Table 1) for silica-alumina materials with different aluminum contents is directly related to the synthesis methodology, which involves the mixing of a stable alumina sol with silica precursor followed by peptization, gelation, and calcination to obtain the final mesoporous material. A stable sol from inorganic salt precursor was difficult to prepare and can only be successfully made using an ultrasonic method. During the ultrasonic treatment, the peptized material is subjected to microcavitation formed in rarefaction cycle when the bubbles undergo unsymmetrical collapse near the solid. This causes an inward rush of liquid known also as microstreaming during which the liquid velocity can reach as high as 100 m/s.23,24 This provides about 22.9 kJ/mol of energy ( eq 1), sufficient to overcome most of the weak bonds such as the hydrogen bonding and the van der Waal forces that exist in aggregated particles, but is much lower than the chemical bonding energy. Under the present experiment, the contributions from sample heating and stirring are small and could be safely ignored. Figure 3 illustrates the suggested formation procedure: F ) 1180 kg/m3; V ) 70 mL; M ) 18 g/mol; ν ) 100 m/s;

Wu )

1 m 2 1 VF 2 v ) v ) 22.94 kJ/mol 2M 2 M

(1)

where Wu is the molar energy provided by ultrasonic treatment, F is the density of the liquid medium, ν is the velocity, V is the volume of liquid medium, M is the molar mass of water, and m is the mass Figure 4 displays the particle size distribution as measured by laser light scattering of silica-alumina sols used in the preparation of UMSA1, UMSA2, and UMSA3. All the samples have similar narrow particle size distri(22) Witte, Bruno M. De; Uytterhoeven, Jan B. J. Colloid Interface Sci. 1996, 181, 200. (23) Mason, T. J. Chem. Soc. Rev. 1997, 26, 443. (24) Thompson, L. H.; Doraiswamy, L. K. Ind. Eng. Chem. Res. 1999, 38, 1215.

Figure 4. Particle diameter distribution of prepared silicaalumina sols with different Si/Al molar ratios: A, UMSA1; B, UMSA2; C, UMSA3.

bution. A typical AFM image of the sol particle is shown in Figure 5. The sol particles are spherical in shape with sizes between 13 and 25 nm. It is not uncommon to find that the sol particle diameter obtained by laser light scattering is larger than that obtained by AFM measurement, since the former measures the particle’s dynamic diameter, which includes the surface double layer. These results show that sols with good dynamic stability and narrow particle size distribution are obtained by ultrasonic treatment. After gelation, the sol particles maintained their size and morphology (Figure 6) but form densely packed aggregate clusters. The fluid-filled interparticle voids are the precursors for the mesoporous channel network in the final material. The size of these void spaces is dictated mainly by the size of the sol particles and their packing order. Narrow size distribution is conducive to the formation of close-packed arrays in spherical particles. Thus, silica-alumina sols of similar size and distribution (cf. Figure 4) should lead to final mesoporous materials of similar pore size distribution (cf. Table 1). It is clear that the preparation of a stable sol with a narrow particle size distribution using ultrasonic method is critical in the

Synthesis of Silica-Alumina Nanomaterials

Figure 5. AFM images of precursor silica-alumina sol: A, top view; B, surface plot.

preparation of mesoporous materials of controlled pore size distribution. After calcination, solid-state NMR and HRTEM were used to analyze the structural configuration and the particle morphology of the synthesized solids. Figures 7 and 8 display the 27Al and 29Si MAS NMR spectra of the three UMSA samples. The UMSA1 sample displays a single peak located near 50 ppm corresponding to the tetrahedrally coordinated aluminum (Figure 7a). Samples with lower Si/Al ratio (i.e., UMSA2 and UMSA3) show a smaller peak at 0 ppm (spectra b and c of Figure 7) that has been assigned to octahedrally coordinated aluminum. Five-coordinated aluminum (i.e., AlO5) is also present in the UMSA3 sample as indicated by presence of a broad band at 30 ppm (Figure 7c). It is clear from these results that the aluminum coordination is affected by the Si/Al ratio, and the present materials can contain the most percentage of tetrahedral aluminum. This is one of the reasons why researchers choose tetraalkylammonium cations as surfactants in their synthesis of mesoporous silica-alumina materials.25 Hence, the new method has the added advantage of being able to incorporate more tetrahedral aluminum even without the aid of tetraalkylammonium cations. The deconvoluted 29Si MAS NMR spectra of the UMSA samples in Figure 8 reveal three peaks at around -90, -100, and -110 ppm that have been assigned to Si(OSi)2(OAl)2, Si(OSi)3(OAl)1, Si(OSi)4, respectively.26 Furthermore, the broad line widths of the spectra indicate that the samples are amorphous,27 which is in agreement in the XRD results. Table 2 gives the (25) Corma, A.; Perez-Pariente, J.; Fornes, V.; Rey, F.; Rawlence, D. Appl. Catal. 1990, 63, 145.

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Figure 6. AFM images of the gel sample: A, top view; B, surface plot.

Figure 7. 27Al MAS NMR spectra of solid samples: (a) UMSA1; (b) UMSA2; (c) UMSA3.

amount of each silicon species present in the samples. If considering that the Si(OSi)2(OAl)2 species is capable of linking to two aluminum atoms, it is reasonable to find that the number of Si-O-Al bonds grows with increasing alumina content. This means that most of the aluminum added to the sample was successfully incorporated. Table 3 lists the UMSA1 and a commercial silica-alumina samples’ acidity sites (nature and strength) for comparison. Obviously, the developed material has stronger Bro¨nsted and Lewis acid sites than those of the commercial sample. (26) Irwin, A. D.; Holmgren J. S.; Jonas J. J. Mater. Sci. 1988, 23, 2908. (27) Fyfe, C. A.; Gobbi, G. C.; Putnis, A. J. Am. Chem. Soc. 1986, 108, 3218.

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Figure 8.

29Si

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MAS NMR spectra of different solid samples: A, UMSA1; B, UMSA2; C, UMSA3.

Table 2. Percentage of Silicon Species in the UMSA Materials sample

% Si(OSi)4

% Si(OSi)3(OAl)1

% Si(OSi)2(OAl)2

UMSA1 UMSA2 UMSA3

61.96 68.31 54.94

32.26 15.78 40.50

5.78 15.91 4.556

Table 3. Pyridine IR Data for Present Sample and Commercial Silica Alumina sample no.

acid strength

Bro¨nsted acid sites (mmol/g) 1546 cm-1 a

Lewis acid sites (mmol/g) 1454 cm-1 b

UMSA1

weak medium strong weak medium strong

0.044 0.023 0.011 0.013 0.011 0.014

0.036 0.010 0.048 0.001 0.009 0.037

CASAc

a IR band attributed to pyridine adsorbed on Bro ¨ nsted acid sites. IR band attributed to pyridine adsorbed on Lewis acid sites. c Commercial silica-alumina catalyst [Al O /(SiO + Al O ) ) 25%] 2 3 2 2 3 used in the SINOPEC China. b

Figure 9 shows a representative micrograph of the solid sample analyzed by TEM and HRTEM. The sample has three main morphological features. First, there is no apparent order in the pore arrangement unlike the highly ordered hexagonal array observed in M41S mesoporous molecular sieves. Second, the sample is made of spherical particles with narrow size distribution from 12 to 25 nm. This is in agreement with the N2 sorption experiment

(Figure 1A), which describes ink-bottle-shaped pores that are known to be created through the random packing of spherical particles. Also, a quick calculation of the void space, based on a particle size distribution, gave values between 4 and 10 nm, which is similar to that calculated from the isotherm using the BJH method. Additionally, when the results shown in Figure 4, 5, 6, and 9 are analyzed, it is found that the sol particles are able to retain their size and morphology during the sol-gel process and even after heat treatment. It is thus that the developed method can provide a new and simple way to maintain the sol particles’ size and their morphology from precursor to final product. Third, HRTEM analysis (Figure 9B) indicates that some nanoparticles feature of crystal (e.g., particle a). This result is confirmed by the electron diffraction experiment (Figure 10), because it displays a typical ring pattern, which is characteristic of polycrystalline species. In this study, it is also seen that there are some spots in the diffraction rings. This is due to the fact that the investigative nanoparticle has not enough domains to produce diffraction.28 Thus, On the basis of the HRTEM and electron diffraction observation results, it means that, under the nanoscale, some nanoparticles existing in the present material are characteristic of crystal, although XRD and 29Si MAS NMR characterization suggest an amorphous material. Furthermore, this result suggests that the synthesized material has short(28) Zhu, Y.; Zhang, C. G. The fundamental and application of electron microscopy; Peking University Press: Beijing, 1981.

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Figure 10. Electron diffraction pattern of particle a in Figure 9B.

4. Concluding Remarks This paper reported a new synthesis method for preparing mesoporous silica-alumina materials with narrow pore size distribution and large specific surface area without the use of expensive organic templates. The synthesis method is capable of incorporating a large amount of aluminum as tetrahedrally coordinated Al in the present material ensuring high acidity. The use of inorganic salt precursor in aqueous solution significantly reduces the cost of the material and decreases the amount of pollutant generated during their manufacture. The synthesis method also offers a convenient method for the preparing mesoporous materials of different pore sizes by controlling the precursor sol’s size and its distribution. By simply combining various sol materials such as alumina, silica, titania, and zirconia, mesoporous materials with different chemical and catalytic properties can be prepared.

Figure 9. (A) TEM micrograph of solid sample. (B) HRTEM micrograph of solid sample.

range ordered but long-range disordered structure, which is not contradictory to the definition of amorphous materials.

Acknowledgment. The financial support from the Chinese Academy of Sciences, the Research Institute of Petroleum Processing SINOPEC, and the National Sciences Foundation of China are acknowledged. The authors are grateful to Ms. Zhang Yan (The Materials Characterization & Preparation Faculty, HKUST) for acquiring the JEM 2010 Microscope. LA0116084