Facile synthesis for ordered mesoporous γ-aluminas with high thermal stability

Article abstract of DOI:10.1021/ja0764308

The facile synthesis of highly ordered mesoporous aluminas with high thermal stability and tunable pore sizes is systematically investigated. The general synthesis strategy is based on a sol-gel process associated with nonionic block copolymer as templates in ethanol solvent. Small-angle XRD, TEM, and nitrogen adsorption and desorption results show that these mesoporous aluminas possess a highly ordered 2D hexagonal mesostructure, which is resistant to high temperature up to 1000°C. Ordered mesoporous structures with tunable pore sizes are obtained with various precursors, different acids as pH adjustors, and different block copolymers as templates. These mesoporous aluminas have large surface areas (ca. 400 m²/g), pore volumes (ca. 0.70 cm³/g), and narrow pore-size distributions. The influence of the complexation ability of anions and hydro-carboxylic acid, acid volatility, and other important synthesis conditions are discussed in detail. Utilizing this simple strategy, we also obtained partly ordered mesoporous alumina with hydrous aluminum nitrate as the precursor. FTIR pyridine adsorption measurements indicate that a large amount of Lewis acid sites exist in these mesoporous aluminas. These materials are expected to be good candidates in catalysis due to the uniform pore structures, large surface areas, tunable pore sizes, and large amounts of surface Lewis acid sites. Loaded with ruthenium, the representative mesoporous alumina exhibits reactant size selectivity in hydrogenation of acetone, D-glucose, and D-(+)-cellobiose as a test reaction, indicating the potential applications in shape-selective catalysis.

Full text of DOI:10.1021/ja0764308

Published on Web 02/20/2008
Facile Synthesis for Ordered Mesoporous γ-Aluminas with
High Thermal Stability
Quan Yuan, An-Xiang Yin, Chen Luo, Ling-Dong Sun, Ya-Wen Zhang,
Wen-Tao Duan, Hai-Chao Liu,* and Chun-Hua Yan*
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth
Materials Chemistry and Applications and the PKU-HKU Joint Laboratory in Rare Earth
Materials and Bioinorganic Chemistry, State Key Laboratory for Structural Chemistry of Stable
and Unstable Species, Peking UniVersity, Beijing 100871, China
Received August 27, 2007; E-mail: yan@pku.edu.cn, hcliu@pku.edu.cn
Abstract: The facile synthesis of highly ordered mesoporous aluminas with high thermal stability and tunable
pore sizes is systematically investigated. The general synthesis strategy is based on a sol-gel process
associated with nonionic block copolymer as templates in ethanol solvent. Small-angle XRD, TEM, and
nitrogen adsorption and desorption results show that these mesoporous aluminas possess a highly ordered
2D hexagonal mesostructure, which is resistant to high temperature up to 1000 °C. Ordered mesoporous
structures with tunable pore sizes are obtained with various precursors, different acids as pH adjustors,
and different block copolymers as templates. These mesoporous aluminas have large surface areas (ca.
400 m2/g), pore volumes (ca. 0.70 cm3/g), and narrow pore-size distributions. The influence of the
complexation ability of anions and hydro-carboxylic acid, acid volatility, and other important synthesis
conditions are discussed in detail. Utilizing this simple strategy, we also obtained partly ordered mesoporous
alumina with hydrous aluminum nitrate as the precursor. FTIR pyridine adsorption measurements indicate
that a large amount of Lewis acid sites exist in these mesoporous aluminas. These materials are expected
to be good candidates in catalysis due to the uniform pore structures, large surface areas, tunable pore
sizes, and large amounts of surface Lewis acid sites. Loaded with ruthenium, the representative mesoporous
alumina exhibits reactant size selectivity in hydrogenation of acetone, D-glucose, and D-(+)-cellobiose as
a test reaction, indicating the potential applications in shape-selective catalysis.
rous alumina represents a much more complex problem3 due
to its susceptibility for hydrolysis as well as to the phase
Introduction
Since the ordered mesoporous silica material was first
reported in 1992,1 the interest in this research field has expanded
all over the world due to the potential applications of these
materials in catalysis and in other realms of chemistry.
Compared to silica, alumina is more popular in catalysis area
for its broad applications as industrial catalysts and catalyst
supports employed in petroleum refinement, automobile emis-
sion control, and others.2 With the characteristics of mesoporous
materials, such as highly uniform channels, large surface area,
narrow pore-size distribution, tunable pore sizes over a wide
range, and so on, alumina with a mesostructure should possess
much more excellent properties.
Nonsiliceous materials with ordered mesoporosity are com-
monly prepared through the sol-gel process with surfactants
as structure-directing agents (SDAs) or by utilizing the nano-
casting method with silica or carbon materials as hard templates.
However, the synthesis of ordered and thermal stable mesopo-
transitions accompanying the thermal breakdown of the ordered
structure. Although particular attentions have been devoted to
the synthesis of mesoporous aluminas, unfortunately disordered
structures with amorphous walls were fabricated in most
cases.4-9 According to previous reports,10-12 the hydrolysis
behavior of alumina is very complicated and strongly affected
by acid, water, temperature, relative humidity, and other factors,
giving rise to rather strict synthetic conditions for ordered meso-
porous aluminas.13 Pinnavaia et al.14 obtained pseudo lamellar
mesostructured γ-alumina with crystalline framework walls.
(3) Cˇ ejka, J. Appl. Catal., A 2003, 254, 327.
(4) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242.
(5) Vaudry, F.; Khodabandeh, S.; Davis, M. E. Chem. Mater. 1996. 8. 1451.
(6) Yada, M.; Ohya, M.; Machida, M.; Kijima, T. Chem. Commun. 1998, 1941.
(7) Cabrera, S.; Haskouri, J. E.; Alamo, J.; Beltra´n, A.; Betra´n, D.; Mendioroz,
S.; Marcos, M. D.; Amoro´s, P. AdV. Mater. 1999, 11, 379.
(8) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 1996, 35, 1102.
(9) Zhang, W.; Pinnavaia, T. J. Chem. Commun. 1998, 1185.
(10) Baes, C. F; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York,
1976; p 112.
(1) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli. J. C.; Beck, J.
S. Nature 1992, 359, 710. (b) 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.; Schlenkert, J. L. J.
Am. Chem. Soc. 1992, 114, 10834.
(2) (a) Tung, S. E.; Mcininch, E. J. Catal. 1964, 3, 229. (b) Topsoe, H.; Clausen,
B.S.; Massoth, F.E. Hydrotreating Catalysis; Springer: Berlin, 1996; p
310.
(11) Swaddle, T. W.; Rosenqvist, J.; Yu, P.; Bylaska, E.; Phillips, B. L.; Casey,
W. H. Science 2005, 308, 1450.
(12) Casey, W. H. Chem. ReV. 2006, 106, 1.
(13) Niesz, K.; Yang, P.; Somorjai, G. A. Chem. Commun. 2005, 1986.
(14) (a) Zhang, Z. R.; Hicks, R. W.; Pauly, T. R.; Pinnavaia, T. J. J. Am. Chem.
Soc. 2002, 124, 1592. (b) Zhang, Z. R.; Pinnavaia T. J. J. Am. Chem. Soc.
2002, 124, 12294.
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J. AM. CHEM. SOC. 2008, 130, 3465-3472
3465

A R T I C L E S
Yuan et al.
Table 1. Synthesis Conditions and Obtained Ordered Mesoporous
Aluminas
Employing aluminum tri-tert-butoxide as the main inorganic
precursor and anhydrous aluminum chloride as the pH adjustor
and hydrolysis-condensation controller, Zhao et al.15 fabricated
partly ordered mesoporous alumina. Somorjai et al.13 first
reported the synthesis of ordered mesoporous alumina with
amorphous walls through a sol-gel route under strict control
of the hydrolysis procedure as well as the condensation of
reagents. By utilizing the dip-coating method, Sanchez et al.
synthesized ordered nanocrystalline γ-Al2O3 films with con-
tracted fcc mesoporosity.16 Following the above work, Grosso,
D. et al. improved the synthesis procedure and fabricated ordered
nanocrystalline mesoporous γ-alumina powders by aerosol
generation of the initial solution using an ITS atomizer.17 After
treatment at 700 °C γ-alumina was obtained and it could be
stable up to 900 °C. Recently, Zhang and co-workers18
developed an ordered crystalline mesoporous alumina molecular
sieve with CMK-3 as hard template, which presented a new
route to obtain ordered mesoporous alumina. However, this
synthesis procedure requires multiple steps and is time-consum-
ing. From the viewpoint of synthesis, it is still a significant
challenge to obtain γ-alumina with highly ordered mesostruc-
tures via a one-step, convenient, and economic approach. More-
over, the thermal stability and catalysis properties of ordered
mesoporous alumina have not been studied in detail yet.
Herein, we present an easily accessible, reproducible, and
high-throughput method to synthesize highly ordered mesopo-
rous aluminas with amorphous and/or crystalline γ-phase
framework walls through a simple sol-gel route with block
copolymers as the soft templates. With this strategy, a series of
ordered mesoporous aluminas with 2D hexagonal structure are
readily obtained, and partly ordered mesoporous alumina is also
fabricated with hydrous aluminum nitrate as the precursor. More
important, these mesoporous aluminas exhibit a high thermal
stability up to 1000 °C, possess high surface areas, tunable pore
sizes, and large amount of surface Lewis acid sites, and show
a reactant size selectivity in hydrogenation of acetone, D-
glucose, and D-(+)-cellobiose.
d100 (nm)
alumina
sample
precursor
template
acid
400
°
C
900 °C
Al-1 Al(OPri)3
Al-2 Al(OPri)3
Al-3 Al(OPri)3
Al-4 Al(OPri)3
Al-5 Al(OPri)3
Al-6 Al(OPri)3
Al-7 Al(OPri)3
Al-8 Al(OPri)3
Al-9 Al(OBus)3
Al-10 Al(OBus)3
Pluronic P123 HNO3
9.0
9.0
7.6
9.0
8.3
8.0
8.6
Pluronic P123 HCl + C6H8O7‚H2O
Pluronic P123 HCl + C4H6O6
Pluronic P123 HCl + C4H6O5
Pluronic F127 HNO3
Pluronic F127 HCl + C6H8O7‚H2O
Pluronic F127 HCl + C4H6O6
Pluronic F127 HCl + C4H6O5
Pluronic P123 HNO3
10.7
10.8 10.0
9.8 9.2
8.0 10.5
10.8
11.0
8.2
9.4
9.6
8.3
7.0
Pluronic P123 HCl + C6H8O7‚H2O
Al-11 Al(NO3)3·9H2O Pluronic P123 C6H8O7‚H2O
7.9
process. After 2 days of aging, the solution became a light-yellow solid
(when hydrochloric acid and citric acid were used, white solids were
obtained). Calcination was carried out by slowly increasing temperature
from room temperature to 400 °C (1 °C min-1 ramping rate) and by
heating at 400 °C for 4 h in air. High-temperature treatment was carried
out in air for 1 h with a temperature ramp of 10 °C min-1
.
Catalyst Preparation. Supported ruthenium catalysts containing 2
wt % ruthenium were prepared by incipient wetness impregnation of
as-synthesized mesoporous Al2O3 and for comparison, commercial
Al2O3 (Alfa Aesar) with an aqueous solution of Ru(NO)(NO3) (Alfa
Aesar) (denoted hereinafter as ruthenium/meso-Al2O3 and ruthenium/
commerical-Al2O3, respectively) at 25 °C for 12 h. Impregnated samples
were dried in ambient air at 120 °C for 24 h and then reduced under
flowing H2/N2 (10/90 v/v) at 350 °C for 6 h.
Characterization. PXRD patterns were recorded on a Rigaku
D/MAX-2000 diffractometer (Japan) using Cu KR radiation (λ ) 1.5406
Å). TEM were taken on the Hitachi H-9000 NAR transmission electron
microscope under a working voltage of 300 kV. The nitrogen adsorption
and desorption isotherms at 78.3 K were measured using an ASAP
2010 analyzer (Micromeritics Co. Ltd.). FTIR spectra were recorded
on a Nicolet Magna-IR 750 spectrometer equipped with a Nic-Plan
Microscope. 27Al MAS NMR spectra were recorded on a Varian Unity
Inova spectrometer operating at 78.155 MHz (0.3 µs as a pulse width),
an acquisition time of 0.02 s and a pulse delay of 1 s. The acidities of
the samples were determined by the pyridine adsorption technique.
Pyridine FTIR spectra were recorded with a BIO-RAD FTS-3000 FTIR
spectrometer.
Experimental Section
Chemicals. Pluronic P123 (Mav ) 5800, EO20PO70EO20), Pluronic
F127 (Mav ) 12 600, EO106PO70EO106), and Pluronic F68 (Mav ) 8400,
EO77PO29EO77) were purchased from Aldrich and Sigma-Aldrich.
Aluminum iso-propoxide, Al(NO3)3‚9H2O, HNO3, HCl, citric acid, and
DL-malic acid were purchased from Beijing Chemical Reagents.
Aluminum tri-tert-butoxide was purchased from Acros Corp. Tartaric
acid was purchased from Sinopharm Chemical Reagent Co., Ltd.
Commercial γ-alumina was purchased from Alfa Aesar. All other
chemicals were used as received.
Synthesis. In a typical synthesis, 0.8-1.0 g of Pluronic P123 was
dissolved in 20 mL of ethanol at room temperature. Then 1.4-1.6 mL
of 67 wt % nitric acid (or 1.4-1.6 mL of 37 wt % hydrochloric acid
plus 0.5 g citric acid) and 2.04 g (10 mmol) of aluminum iso-propoxide
were added into the above solution with vigorous stirring. The mixture
was covered with PE film, stirred at room temperature for about 5 h,
and then put into a 60 °C drying oven to undergo the solvent evaporation
Catalytic hydrogenation reactions of acetone, D-glucose, and D-(+)-
cellobiose were carried out in a Teflon-lined stainless steel autoclave
(150 mL) typically at 120 °C and 4 MPa H2 using deionized water (50
mL) as a solvent. Reactants and products were analyzed by high-
performance liquid chromatography (HPLC) (Agilent 1100).
Results and Discussion
Table 1 summarizes the ordered mesoporous aluminas
synthesized with different aluminum precursors, surfactants, and
acids. Ordered mesoporous aluminas are achieved with which-
ever aluminum iso-propoxide, aluminum sec-butoxide, or hy-
drous aluminum nitrate is used as the precursor. Different acids
are added as the pH adjustors for the hydrolysis of aluminum
precursors, and the results demonstrate that it is flexible to obtain
the ordered mesostructure in a wide range of acidity and water
content. For example, sample Al-1 (synthesized with aluminum
iso-propoxide as the precursor, nitric acid as the acid adjustor,
and Pluronic P123 as the surfactant) is prepared with the
appropriate molar ratio of HNO3 to Al(OPri)3 in the range of
2.0-2.4. Compared with the synthesis of ordered mesoporous
alumina reported by Somorjai et al.,13 the ratio of HNO3 to Al-
(15) Tian, B. Z.; Yang, H. F.; Liu, X. Y.; Xie, S. H.; Yu, C. Z.; Fan, J.; Tu, B.;
Zhao, D. Y. Chem. Commun. 2002, 1824.
(16) Kuemmel, M.; Grosso, D.; Boissie`re, C.; Smarsly, B.; Brezesinski, T.;
Albouy, P. A.; Amenitsch, H.; Sanchez, C. Angew. Chem., Int. Ed. 2005,
44, 4589.
(17) Boissie`re, C.; Nicole, L.; Gervais, C.; Babonneau, F.; Antonietti, M.;
Amenitsch, H.; Sanchez, C.; Grosso, D. Chem. Mater. 2006, 18, 5238.
(18) Liu, Q.; Wang, A. Q.; Wang, X. D.; Zhang, T. Chem. Mater. 2006, 18,
5153.
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Mesoporous
γ
-Aluminas with High Thermal Stability
A R T I C L E S
Figure 1. (a) Small- and (b) wide-angle XRD patterns of Al-1 calcined at
different temperatures.
(OPri)3 does not have to be strictly fixed at a specific
composition.
Evidence for the formation of mesostructures is provided by
small-angle X-ray diffraction (XRD) patterns shown in part a
of Figure 1. The sample Al-1 calcined at 400 °C shows a very
strong diffraction peak around 1.0° and one weak peak around
1.7°, which, according to the TEM observation, can be attributed
to p6mm hexagonal symmetry. A good mesoscopic order is
reflected even after calcination at 800, 900, and 1000 °C,
respectively, indicating a high thermal stability of the mesopo-
rous structure. Part b of Figure 1 shows the wide-angle XRD
patterns of Al-1 calcined at different temperatures. Calcination
at 400 °C gives rise to the mesostructure with amorphous wall,
and then the amorphous wall is converted to γ-Al2O3 phase
(JCPDS Card No. 10-0425) after further treatment at a
temperature of 800 °C. The combination of the small- and wide-
angle XRD data convince us that the ordered mesoporous
γ-alumina with crystalline walls are prepared at 800-1000 °C.
After being calcined at 1100 °C, no diffraction peak is observed
in the low-angle range (Figure S1 in the Supporting Informa-
tion), demonstrating that the mesoporous structure has collapsed;
whereas diffraction peaks of R-Al2O3 phase (JCPDS Card No.
11-0661) phase appear in the wide-angle XRD pattern (Figure
S1 in the Supporting Information).
Figure 2. TEM images (a) and (b) of Al-1 calcined at 400 °C viewed
along [001] and [110] orientations (FFT diffractogram inset), TEM images
(c) and (d) of Al-1 calcined at 900 °C viewed along [001] and [110]
orientations (the insert in c is corresponding SAED pattern), (e) HRTEM
image of Al-1 calcined at 900 °C, (f) TEM image of Al-1 calcined at
1000 °C viewed along [001] orientation (the inset is TEM image viewed
along [110] orientation). The bars are 50 nm for insets.
the sample calcined at 1000 °C (part f of Figure 2), further
proving the high thermal stability of the mesostructure.
The nitrogen adsorption-desorption isotherms (Figure 3) of
Al-1 treated at different temperatures all yield type IV curves
with H1-shaped hysteresis loops, suggesting their uniform
cylindrical pores. Al-1 treated at 400 °C has a large BET surface
area of 434 m2/g and a pore volume of 0.80 cm3/g (Table S1 in
the Supporting Information). When transformed into γ-alumina,
it still exhibits a BET surface area of 226, 220, and 116 m2/g
at 800, 900, and 1000 °C, respectively. Narrow pore-size
distributions (4-6 nm) are maintained though the diameter
decreases as the calcination temperature increases. The large
surface areas and narrow pore-size distributions combined with
excellent thermal stability enhance the potential applications of
these ordered mesoporous aluminas in catalysis.
TEM images of Al-1 annealed at 400 °C are displayed in
parts a and b of Figure 2 with the corresponding fast Fourier
transform (FFT) patterns. The highly ordered hexagonal ar-
rangement of pores along [001] direction and the alignment of
cylindrical pores along [110] direction are observed. With the
samples prepared in different batches, more images (Figure S2
in the Supporting Information) display the similar well-ordered
pore network observed for the sample Al-1 calcined at 400 °C
and show the consistence between the same sample viewed from
different areas and between the samples prepared in different
batches. The TEM images (parts c and d of Figure 2) also
confirm to us that the ordered mesoporous structures are retained
after 900 °C treatment. The selective area electron diffraction
(SAED) pattern (the insert in part c of Figure 2) of the ordered
mesostructure domains indicates that the mesoporous wall is
crystalline with γ-Al2O3 phase. Additional proof for the crystal-
linity of the framework walls is given by high-resolution TEM
investigations (part e of Figure 2), which reveals the existence
of several crystalline nanoparticles with well-defined lattice
planes. Ordered mesostructure domains are also observed for
Hydrochloric acid is added in the system to replace nitric
acid. Unfortunately, only a broad peak is observed in small angle
XRD pattern, denoting a short range ordered mesostructure (part
a of Figure 4). If small amounts of hydro-carboxylic acids like
citric acid (CA), tartaric acid, and DL-malic acid are introduced
into this system, long range ordered mesostructures are obtained
according to the small-angle XRD patterns (part a of Figure 4).
This is confirmed by TEM observations (parts a, c, and d of
Figure 5), which clearly show the ordered hexagonally packed
cylindrical mesopores. These mesoporous structures are resistant
to the temperature up to 900 °C (part b of Figure 4 and part b
9
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A R T I C L E S
Yuan et al.
Figure 5. TEM images of (a) Al-2 (400 °C), (b) Al-2 (900 °C), (c) Al-3
(400 °C), and (d) Al-4 (400 °C). The bars are 50 nm for insets.
izability, surfactant concentration, counterion charge, and other
factors. Stucky et al.19 proposed the cooperative organization
mechanism based on the charge density matching at the
organic-inorganic interface. Regarding metal-based materials,
four different mechanisms, charge matching,20,21 ligand-assisted
templating,22,23 ion exchange/scaffolding,24 and modulation of
the hybrid interface,25 have been proposed according to the
experiment results of different spectroscopical characterizations
such as NMR, fluorescence or UV-vis absorption. On the basis
of our results and former models, in our case, the complexation
abilities of surfactants, ions, and hydro-carboxylic acids play
important roles in the mesophase formation. Alkylene oxide
segments can form crown-ether-type complexes with inorganic
ions including multivalent metal species through coordination
bonds.26 Accompanied by hydrolysis and condensation, the
metal species associate with the hydrophilic poly(ethyleneoxide)
moieties of copolymer molecules via electrostatics and van der
Waals forces to guide mesostructure formation.27 Balanced
Coulombic, hydrogen bonding, and van der Waals interactions
with charge matching at interfaces provide an effective means
of enhancing long-range periodic order. Zhao et al.28 reported
that the radius and charge of the anions such as Cl-, I-, and
NO3- impose a great effect on the reaction rate and the assembly
process of the silicate-surfactant mesophase. In our experi-
Figure 3. (a) Nitrogen adsorption-desorption isotherms and (b) pore-size
distribution curves (deduced from the desorption branches) of Al-1. For
clarity, the isotherms for 900 and 1000 °C products in (a) are offset along
the y axis by 100 and 100 cm3/g, respectively.
Figure 4. Small-angle XRD patterns of mesoporous aluminas synthesized
using HCl plus HCAs as pH adjustors at (a) 400 and (b) 900 °C.
of Figure 5). The N2 adsorpion-desorption isotherms for both
Al-2 (synthesized with aluminum iso-propoxide as the precursor,
HCl plus CA as the acid adjustor, and Pluronic P123 as the
surfactant) treated at 400 and 900 °C (Figure S3 in the
Supporting Information) show typical type IV curves with a
distinct condensation step at a p/p0 range of 0.5-0.8, suggesting
uniform mesopores (ca. 4 nm). BET analysis for Al-2 treated
at 400 °C gives a surface area of about 450 m2/g and a pore
volume of 0.57 cm3/g (Table S1 in the Supporting Information).
Transformed to γ-alumina, it still exhibits a BET surface area
of 256 m2/g and a pore volume of 0.45 cm3/g.
In the present work, a series of highly ordered mesoporous
aluminas with 2D hexagonal structures are fabricated with
different acids as pH adjustors. According to the “liquid crystal
templating” mechanism for the formation of mesoporous
structure proposed by Beck et al.,1 the ultimate liquid crystalline
phases may be related to the ionic strength, counterion polar-
(19) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walke,
S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G.
D.; Chmelka, B. F. Science 1995, 267, 1138.
(20) (a) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. K.; Feng, P.; Gier,
T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D.
Chem. Mater. 1994, 6, 1176. (b) Huo, Q.; Margolese, D. I.; Ciesla, U.;
Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky,
G. D. Nature 1994, 368, 317.
(21) Luca, V.; Hook, J. M. Chem. Mater. 1997, 9, 2731.
(22) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999,
38, 57.
(23) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. 1996, 35, 426.
(24) Hudson, M. J.; Knowles, J. A. J. Mater. Chem. 1996, 6, 89.
(25) (a) Soler-Illia, G. J. A. A.; Sanchez, C. New J. Chem. 2000, 24, 493. (b)
Soler-Illia, G. J. A. A.; Scolan, E.; Louis, A.; Albouy, P.-A.; Sanchez, C.
New J. Chem. 2001, 25, 154.
(26) Bailey, F. E., Jr; Koleske, J. V. Alkylene Oxides and Their Polymers; Marcel
Dekker: New York, 1990.
(27) Yang, P.; Zhao, D.; Margolese, I.; Chmelka, B. F.; Stucky, G. D. Nature
1998, 396, 152.
(28) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. J. Am.
Chem. Soc. 1998, 120, 6024.
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Mesoporous
γ
-Aluminas with High Thermal Stability
A R T I C L E S
-
ments, NO3 does not strongly influence the self-assembly
process because of its weak complexation ability. However,
chloride ion can strongly coordinate with aluminum ions, and
it might destroy the balance of the organic-inorganic interface
and disturb the assembly process,14b leading to long-range
disordered mesostructures. Hydro-carboxylic acids are good
coordination agents for aluminum ions via monodentate or
bridging bidentate modes.29,30 Zhang et al.31 have proposed that
hydro-carboxylic acids can act as SDA in the formation of
mesoporous γ-alumina via the coordination between the hydro-
carboxylic acid and aluminum sites of aluminum oxohydroxide
particulates. FTIR spectra of the pure citric acid as well as as-
synthesized mesoporous alumina using hydrochloric acid plus
citric acid as pH adjustors are presented in Figure S4 in the
Supporting Information. For the pure CA, the absorption band
positioned at 1724 cm-1 is ascribed to the V(CdO) carbonyl
stretching vibration of the free carboxylic acid (COOH) groups,
whereas a C-O stretching vibration absorption band at 1380
cm-1 is assigned to the free carboxylate groups of CA
moleclules.32,33 The as-synthesized alumina sample with a CA/
display that ordered mesostructures can be obtained with the
CA/Al3+ ratio in the range of 0.15 to 0.35, demonstrating that
only appropriate chelation between HCAs and alumina precur-
sors can lead to ordered assembly.
We assume that volatilization behavior of the acids is an
important factor that should be taken into account. The reactivity
of the alumina precursors can be efficiently regulated through
various methods such as careful adjustment of the pH and
dilution of solutions or the use of condensation inhibitors.37
A
subsequent slowing step is necessary to irreversibly freeze a
half-condensed structure and hence avoids the fast formation
of an inorganic network. Inorganic hydrolysis and condensation
have to be controlled to fabricate a robust mesostructure instead
of a hard solid. As for our synthesis, the starting solutions are
relatively dilute, and the inorganic polymerization can be readily
dominated by an acid, which is subsequently removed by
evaporation. In the case of this system, as the volatility of HCl
is higher than that of HNO3, the acidity of the whole system
added with HCl in evaporation process reduces more quickly
and thus consequently influences the hydrolysis behavior of
aluminum species and the self-assembly process. When non-
volatile hydro-carboxylic acids are introduced, they act as
sustained-release agents to maintain an acidic equilibrium
environment for precursors. In the beginning, their effect is not
obvious, whereas at the end of the evaporation process this effect
becomes more important as hydro-carboxylic acids’ concentra-
tion increase in the whole system.
Besides, the synthesis conditions, particularly the amount of
water, pH, temperature, and relative humidity, have a great
impact on the final mesophase. The water quantity has crucial
effects on the formation kinetics of the condensed phase and
on the chemical compatibility at the hybrid interface.37 The fine-
tuning of the water/precursor ratios should place the systems
in the right position to obtain the desired mesostructure. Herein,
a low water quantity is necessary. The species and dose of acid
do directly determine not only the sort of cations that affect the
self-assembly process through coordination interaction with the
aluminum precursor but also the pH of the medium. A small
amount of H+ slows down the hydrolysis rate of the aluminum
alkoxide, making the dissolution of the precursor molecules
difficult in ethanol, whereas higher H+ concentration results in
uncontrollable fast hydrolysis of aluminum alkoxide. Therefore,
the amount of H+ is restricted to a relatively narrow range. At
the same time, it is also necessary to control proper ambient
factors, like temperature and relative humidity. The formation
of a well-defined mesostructure depends critically on these
factors, which controls two competitive processes: solvent
evaporation and inorganic polymerization.38 In our synthesis,
high temperature and low relative humidity (<20%) are adopted
by placing the reaction vessels in a drying oven with the
temperature set at 60 °C. After evaporating for 2 days, the
product has been totally condensed to a foam-like product
without fluidity. Varying the evaporation time from 2 to 8 days,
the same 2D hexagonal mesostructures are obtained, implying
that it is the most stable mesostructure under this synthesis
condition. Two days is a proper choice for the sake of synthesis
efficiency.
Al ratio of 0.10 exhibits a new absorption band at 1450 cm-1
,
which is assigned to a C-O bond of increased single bond order
due to the monodentate coordinated carboxylate groups that are
bound with the aluminum atoms.32 The absorption bands of both
the free CA and the chelated CA exist in this as-synthesized
alumina, indicating that not all of the three carboxylate groups
of the CA are bonded to the surface of the alumina presursor.33
If the CA/Al ratios are not lower than 0.15, the band at 1724
cm-1 disappears whereas the band of 1450 cm-1 remains. This
trend suggests that the CA-Al complex is formed through the
binding of the carboxylate groups to aluminum ions. It is
believed that the complexation between carboxylic groups and
aluminum ions takes place prior to hydrolysis-condensation
of the inorganic precursors and is enhanced upon drying and
thermal treatment. CA serves as an inhibitor for hydrolysis-
condensation process of aluminum species through coordinating
with one or two aluminum sites on very small clusters of
AlOOH. In the case of alumina- or other metal-based (such as
titanium, zirconium, and niobium) oxide mesostructures, acety-
lacetone was used as an inhibitor in neutral or slightly acidic
medium in the presence of alkyl phosphate templates.34-36
Triethanolamine was employed in strongly basic media as
chelates in the synthesis of mesoporous alumina.7 In the case
with CA, the complexation interaction between HCA and
alumina precursor is also observed. These HCAs are competitors
against chloride ions, both of which coordinate with aluminum
ions through the whole evaporation process. Meanwhile, HCA
can interact with the block copolymers through hydrogen
bonding and the van der Waals force, and then protect the
aluminum ions at the organic-inorganic interface from being
affected by chloride ions. Incidentally, corresponding small-
angle XRD patterns (Figure S5 in the Supporting Information)
(29) Motekaitis, R.; Martell, A. Inorg. Chem. 1984, 23, 18.
(30) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. J. Am. Ceram. Soc. 1996, 79,
1857.
(31) Liu, Q.; Wang, A. Q.; Wang, X. D.; Zhang, T. Microporous Mesoporous
Mater. 2006, 92, 10.
(32) Thornton, D. A. Coord. Chem. ReV. 1990, 104, 173.
(33) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. J. Am. Ceram. Soc. 1996, 79,
1857.
(34) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014.
(35) Putnam, R. L.; Nakagawa, N.; McGrath, K. M.; Yao, N.; Aksay, I. A.;
Gruner, S. M.; Navrotsky, A. Chem. Mater. 1997, 9, 2690.
(36) Antonelli, D. M.; Nakahira, A.; Ying, J. Y. Inorg. Chem. 1996, 35, 3126.
(37) Soler-Illia, G. J. A. A.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV.
2002, 102, 4093.
(38) Soler-Illia, G. J. A. A.; Louis, A.; Sanchez, C. Chem. Mater. 2002, 14,
750.
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3469

A R T I C L E S
Yuan et al.
Figure 6. Small-angle XRD patterns of mesoporous aluminas using F127
as SDA calcined at (a) 400 and (b) 900 °C.
Figure 8. (a) Small-angle XRD patterns of mesoporous aluminas using
Al(OBus)3 as the precursor. TEM images of Al-9 calcined at (b) 400 and
(c) 900 °C. The bars are 50 nm for insets.
desorption isomers combined with pore-size distribution curves
are displayed in Figure S6 in the Supporting Information.
Substituting aluminum tri-tert-butoxide for aluminum iso-
propoxide as the precursor, highly ordered mesoporous aluminas
are also achieved (Figure 8). Nitrogen sorption measurements
show a narrow pore-size distribution around about 4-7 nm
(Figure S7 in the Supporting Information), BET surface area
of 350-460 m2/g, and pore volumes of 0.68-0.75 cm3/g (Table
S1 in the Supporting Information). After transformed into
γ-alumina, they still exhibit BET surface areas of 190-210 m2/g
at 900 °C.
In particular, it is worthwhile to investigate the use of hydrous
aluminum nitrate as the precursor because hydrous aluminum
nitrate is cheaper and easily acquired. In previous reports14,39
concerning the use of aluminum nitrate as the precursor, only
short range ordered mesostructures were obtained as a result of
the uncontrollable hydrolysis of aluminum nitrate. With citric
acid as the acid adjustor, we have successfully synthesized
ordered mesoporous alumina (Figure 9). Small-angle XRD
pattern shows a sharp peak at 1.1° accompanied by a weak peak
around 2.0°. Ordered domains with a 2D hexagonal mesophase
are detected, whereas wormlike structures exist as well (parts b
and c in Figure 9). It exhibits a lower BET surface area of about
250 m2/g (Table S1 in the Supporting Information) but a
narrower pore-size distribution (Figure S8 in the Supporting
Information) than that produced by using aluminum alkoxide
as the precursor. As the CA/Al3+ ratio varies from 0.4 to 0.9,
the intensity of the (100) diffraction peak together with that of
the second-order peak becomes weaker (Figure S9 in the
Supporting Information). On the basis of these results, it is
postulated that in this system the complexation ability of citric
acid is a key feature in obtaining mesostructured phases.
The 27Al MAS NMR spectra (part a of Figure 10) of Al-1
Figure 7. TEM images of mesoporous aluminas using F127 as SDA
calcined at different temperatures (a) Al-5 (400 °C), (b) Al-5 (900 °C), (c)
Al-6 (400 °C), (d) Al-6 (900 °C), (e) Al-7 (400 °C), and (f) Al-8 (400 °C).
The bars are 50 nm for insets.
Ordered mesoporous aluminas are attained using different
ethylene oxide-based surfactants via the same synthesis strategy.
treated at 400 °C present three resonance signals at 70, 35, and
Poly(alkylene oxide) triblock copolymers like P123, F127(EO106
-
1
0 ppm. These bands are assigned to the central -1/2,
/
2
PO70EO106), and F68(EO77PO29EO77) are used as SDAs. The
surfactants with different EO/PO ratios and different molecular
weights result in different structures and pore sizes. Under our
synthesis conditions, with F68 as surfactant only a wormlike
structure is prepared, whereas P123 and F127 are good SDAs
to fabricate an ordered mesostructue. Small-angle XRD patterns
(Figure 6) all indicate the ordered mesostructures with 2D
hexagonal symmetry with F127 as SDA, which is further
confirmed by TEM observations (Figure 7) showing the ordered
mesoporous structures. Corresponding nitrogen adsorption-
transition of the Al3+ ion in tetrahedral (AlO4), pentahedral
(AlO5), and octahedral (AlO6) coordination, respectively.40-42
The presence of a high concentration of pentahedrally coordi-
nated aluminum is a direct consequence of the high quantity of
(39) Liu, Q.; Wang, A. Q.; Wang, X. D.; Zhang, T. Microporous Mesoporous
Mater. 2007, 100, 1.
(40) Stebbins, J. F. In Handbook of Physical Constants; Ahrens, T. J., Ed.;
American Geophysical Union: Washington DC, 1995; Vol. 2.
(41) Akitt, J. W. Prog. Nucl. Magn. Reson. Spectrosc. 1989, 21, 127.
(42) McManus, J.; Ashbrook, S. E.; MacKenzie, K. J. D.; Wimperis, S. J. Non-
Cryst. Solids 2001, 282, 278.
9
3470 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008

Mesoporous
γ
-Aluminas with High Thermal Stability
A R T I C L E S
Table 2. Activities of Supported Ruthenium Catalysts (2 wt %
Ruthenium) in Hydrogenation of Acetone, Glucose, and
Cellobiosea
Conversion Rate (mol/mol
‚
ruthenium
glucose
cellobiosec
‚h)
rate ratio
(glucose to
cellobiose)
+
catalyst
acetoneb glucose cellobiose
ruthenium/
4207
4208
173
510
54
90 (glucose) + 3.2 (10.0)d
9 (cellobiose)
meso-Al2O3
ruthenium/
492
372 (glucose) + 1.0 (3.7)d
100 (cellobiose)
commercial-Al2O3
a Reaction conditions: 120 °C, 4 MPa H2, 0.01 mmol ruthenium, 30
min reaction time, 17.2 mmol acetone, 5.6 mmol glucose or cellobiose.
b 15 min reaction time. c 5.6 mmol glucose + 5.6 mmol cellobiose. d Data
in the parentheses are obtained from hydrogenation of an equimolar mixture
of glucose and cellobiose.
crystalline form and that these nano domains are homogeneous.17
To test the surface acidity of sample Al-1, FTIR pyridine
adsorption measurements are performed. As shown in part b of
Figure 10, the IR spectra of adsorbed pyridine display charac-
teristic absorption bands at 1620 and 1450 cm-1 corresponding
to the V8a and V19b modes of the ring-breathing vibrations
V(CCN) of pyridine, respectively, indicating pyridine adsorption
via hydrogen-bonding with the surface OH groups and the
nitrogen-lone-pair electrons interacting with the coordinatively
unsaturated Al3+ ions in octahedral and tetrahedral coordination
(Lewis acid sites).44-46 The amount of absorbed pyridine on
Lewis acid sites is estimated by integration of the band (V19b
mode) around 1450 cm-1. The numbers of Lewis acid sites
calculated for Al-1 calcined at different temperatures are very
large and decrease gradually with the calcined temperatures
elevated (Table S2 in the Supporting Information).
Figure 9. (a) Small-angle XRD patterns of Al-11 which using Al(NO3)3
as the precursor. TEM images of Al-11 calcined at (b) 400 and (c) 900 °C.
The bars are 50 nm for insets. The bars are 50 nm for insets.
These well-ordered mesoporous aluminas are examined as
catalyst supports in the hydrogenation of acetone, D-glucose,
and D-(+)-cellobiose with different molecular sizes as test
reactions. These carbonyl compounds possess different molec-
ular sizes of 0.38, 0.65, and 1.2 nm, respectively, which are
chosen to probe the pore-size effects of the mesoporous Al2O3
supports. Table 2 shows the representative results of ruthenium
supported on mesoporous γ-alumina (Al-11) with controlled
pore sizes of 3.6 nm (ruthenium/meso-Al2O3) and for compari-
son, on commercial γ-alumina (ruthenium/commercial-Al2O3)
with larger pore sizes of predominantly 10 nm (Figure S10 in
the Supporting Information). TEM characterization (Figure 11)
shows that ruthenium particles on the two Al2O3 supports
possess narrow unimodal size distributions with similar mean
diameters of 3.0 and 2.9 nm, respectively, indicating similar
ruthenium dispersions. On these two catalysts, acetone, glucose,
and cellobiose are hydrogenated exclusively to their correspond-
ing alcohols, 2-propanol, sorbitol, and 3-â-D-glucopyranosyl-
D-glucitol respectively at 120 °C and 4 MPa H2.
The conversion rates (normalized by the total ruthenium
content) depend largely on the size of the support pores and
reactants. The two catalysts are active and give identical rates
(4207-4208 mol/mol‚ruthenium‚h, Table 2) for the hydrogena-
tion of acetone, the smallest molecule chosen, demonstrating
their same intrinsic hydrogenation activities. For glucose and
cellobiose, their hydrogenation rates on ruthenium/meso-Al2O3
Figure 10. (a) 27Al MAS NMR spectra and (b) FTIR spectra of pyridine
adsorption for Al-1 at the deposition temperature of 200 °C.
defects induced by the large surface area of the network.43
Nevertheless, in the 900 and 1000 °C calcined samples no
pentahedrally coordinated aluminum is detected, whereas the
amounts of tetrahedrally and octahedrally coordinated aluminum
increase, which is strong evidence for the existence of the
γ-alumina phase and suggests that most of the product is in
(44) Abbattista, F.; Delmastro, S.; Gozzelino, G.; Mazza, D.; Vallino, M.; Busca,
G.; Lorenzelli, V.; Ramis, G. J. Catal. 1989, 117, 42.
(45) Kawai, T.; Jiang, K. M.; Ishikawa, T. J. Catal. 1996, 159, 288.
(46) Layman, K. A.; Ivey, M. M.; Hemminger, J. C. J. Phys. Chem. B 2003,
107, 8538.
(43) Chupas, P. J.; Grey, C. P. J. Catal. 2004, 224, 69.
9
J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008 3471

A R T I C L E S
Yuan et al.
controllable pore sizes and high thermal stability, provides their
potential applications in shape-selective catalysis.
Conclusions
We have demonstrated the systematic fabrication of highly
ordered mesoporous amorphous and/or crystalline γ-aluminas
with high thermal stability by a facile synthesis method. The
simplicity, versatility, and reproducibility of this synthetic
approach will facilitate the development of new mesoporous
materials. The mesostructures are stable at a temperature as high
as 1000 °C, which is an excellent advantage in high-temperature
catalytic reactions. Moreover, tunable pore sizes from 3 to 7
nm (Table S1 in the Supporting Information) make the shape-
selective catalysis become possible.47 Strong Lewis acidity,
provided by aluminum atoms in tetrahedral and octahedral
coordination sites, proves these ordered mesoporous aluminas
to be ideal supports for heterogeneous catalysis. Serving as
catalyst supports of ruthenium in the hydrogenation reaction,
these materials exhibit size selectivity for different molecules
with different sizes, promising potential applications in shape-
selective catalysis.
Figure 11. TEM images and histograms of ruthenium particle size
distribution of (a) ruthenium/meso-alumina catalyst and (b) ruthenium/
commercial alumina catalyst.
Acknowledgment. This work was supported by the MOST
of China (2006CB601104), NSFC (Grants 20221101, 20610068,
and 20423005), and the Founder Foundation of PKU.
are 173 and 54 mol/mol‚ruthenium‚h, respectively. In contrast,
glucose and cellobiose convert at similar rates (510 vs 492 mol/
mol‚ruthenium‚h) on ruthenium/commercial-Al2O3, which are
approximately three and nine times greater than those on
ruthenium/meso-Al2O3, respectively. Such differences in the
conversion rates are apparently related to the effects of steric
hindrance, imposed by the molecular sizes of the reactants
relative to the pore sizes of the Al2O3 supports, on the diffusion
and accessibility of the reactants to the ruthenium domains
confined inside the pores. Taken together, these results reflect
the features of reactant size selectivity of the ordered mesopo-
rous Al2O3 supports, which are further confirmed by hydrogena-
tion of an equimolar mixture of glucose and cellobiose. As
shown in Table 2, the coexistence of glucose and cellobiose
leads to a more preferential reaction of glucose over cellobiose
on ruthenium/meso-Al2O3, and their conversion rates are dif-
ferent by an order of magnitude, much greater than that on
ruthenium/commercial-Al2O3 (being 3.7) under the same condi-
tions and also than that obtained with individual reactants on
ruthenium/meso-Al2O3 (being 3.2). This observed size selectivity
of the ordered mesoporous aluminas, together with their
Supporting Information Available: Small- and wide-angle
XRD patterns of Al-1 calcined at 1100 °C, additional TEM
images of Al-1 calcined at 400 °C prepared in two batches,
FTIR spectra of the pure CA and the as-synthesized samples
prepared with different CA/Al3+ ratios, small-angle XRD
patterns of samples calcined at 400 °C with different CA/Al3+
ratios using aluminum iso-propoxide and hydrous aluminum
nitrate as the precursor, nitrogen adsorption-desorption iso-
therms combined with corresponding pore-size distribution
curves deduced from the desorption branches, physicochemical
properties of ordered mesoporous alumina, band position and
the number of Lewis acid sites of Al-1. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA0764308
(47) (a) Blasco, T.; Corma, A.; Navarro, M. T.; Pe´rez, Pariente, J. J. Catal.
1995, 156, 65. (b) Corma, A.; Corell, C.; Pe´rez, Pariente, J.; Guil, J. M.;
Guil Lopez, R.; Ncolopoulos, S.; Calbet, J. G.; Vallet, Regi, M. Zeolites
1996, 16, 7.
9
3472 J. AM. CHEM. SOC. VOL. 130, NO. 11, 2008