Langmuir 2002, 18, 4111-4117
4111
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.; Schüth, 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 Brönsted 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
4112
Langmuir, Vol. 18, No. 10, 2002
(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 Brönsted
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.
Synthesis of Silica-Alumina Nanomaterials
Langmuir, Vol. 18, No. 10, 2002 4113
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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Langmuir, Vol. 18, No. 10, 2002
Yao et al.
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.
Langmuir, Vol. 18, No. 10, 2002 4115
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
Brönsted 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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Langmuir, Vol. 18, No. 10, 2002
Figure 8.
29Si
Yao et al.
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
Brönsted 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 Brönsted 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.
Synthesis of Silica-Alumina Nanomaterials
Langmuir, Vol. 18, No. 10, 2002 4117
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