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Mesoporous Alumina: Synthesis, Characterization,
and Catalysis
Tsunetake Seki and Makoto Onaka
481
Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide
Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31794-3
15
15.1
Introduction
The pore structure is one of the most important properties of heterogeneous
catalysts that govern their activity, selectivity, and lifetime. Notably, not only the
surface reactivity but also the mass transfer of reactants and products can be tuned
by modifying the pore structure. Owing to the size of most reactant molecules
capable of access to the internal surfaces, the values of pore diameter that are of
most interest to catalyst chemists are the orders of nanometers, with a range of
approximately 0 to 50 nm. By defi nition, the pores in this range are classifi ed into
two groups: “ micropore, ” with a diameter < 2 nm; and “ mesopore, ” with diameters
in the range of 2 to 50 nm [1] . Since the mid - twentieth century, a large family of
aluminosilicates known as zeolites have been synthesized and used as catalysts
with regularly arrayed micropores [2] . On the other hand, the development of oxide
frameworks with pore diameters within the mesoporous range lagged far behind,
due to diffi culties in their synthesis. In fact, such ordered mesoporous materials
were not created successfully until 1992, when a team at the Mobil Research and
Development Corporation fi rst reported the successful synthesis of silica - based
mesoporous molecular sieves (M41S) via a liquid crystal template ( LCT ) mecha-
nism, where the surfactant liquid crystal phases served as the organic templates
[3] . Since then, investigations to prepare organized mesoporous molecular sieves
have been conducted not only for silica/aluminosilicate materials but also for
oxides of Mg, Ti, Nb, Ta, and Al [4, 5] . For this, the electrostatic interaction between
positively charged surfactants ( S
+
) and negatively charged inorganic precursors
( I
−
), charge - reversed interactions ( S
−
I
+
), counterion - mediated interactions ( S
−
M
+
I
−
and S
+
X
−
I
+
; M
+
= metal cation, X
−
= Cl
−
, Br
−
), and neutral interactions ( S
0
I
0
and N
0
I
0
; N = nonionic organics) were used to assemble series of ordered
mesopores [6] .
Because of their high surface areas and strong adsorption abilities, aluminas
with various morphologies have been widely used as catalysts and catalyst sup-
ports, both in the laboratory and in industry [7] . It seems natural, therefore, that
482 15 Mesoporous Alumina: Synthesis, Characterization, and Catalysis
catalyst chemists would be prompted to investigate the behavior of these newly
synthesized aluminas with uniform mesopores in a variety of catalytic reactions.
The most likely major advantage of using mesoporous aluminas in catalytic reac-
tions is that the regular mesoporous structures, which do not contain micropores,
are free from the pore - plug problems caused by the formation of coke and other
polymerized byproducts [8] . These problems, which have often been observed in
catalysis by conventional aluminas, include a decrease in the surface areas and a
disturbance of the diffusion of reactants and products within the porous networks.
One other advantage includes the modifi cation of surface reactivity by creating a
uniform mesopore structure within the alumina. A warped bulk structure in
mesoporous alumina may generate a unique surface reactivity (e.g., acidity and
basicity) that is considerably different from that of normal alumina, and provides
the opportunity to adjust the adsorption characteristics to optimum values by
altering the mesopore sizes.
In this chapter, attention is focused on those aluminas with a regular mesopo-
rosity, together with details of their synthesis and examples of their uses in cata-
lytic reactions. In addition, a reference base of information up to early 2007 is
included. Comparisons with conventional aluminas are frequently noted in order
to emphasize the utility of mesoporous alumina as a catalytic material.
15.2
Synthesis of Mesoporous Alumina
15.2.1
Experimental Techniques
15.2.1.1 Synthesis
The initial stage of mesoporous alumina synthesis is the hydrolysis of aluminum
precursors in the presence of a template (a structure - directing agent) in a solvent
to afford a precipitate or gel material; this material is subsequently fi ltered out,
washed, and dried to provide the as - made material. It should be noted that special
care is required when using water for any purpose in these procedures, as this
may lead to a serious structural collapse of the samples. It is recommended, there-
fore, that anhydrous ethanol or propanols be used in any washing processes [9] .
Moreover, when aluminum alkoxides are used as precursors, care must be taken
when deciding on the amount of water required for the hydrolyses, as well as how
the water should be contacted with the aluminum precursors, since the very rapid
hydrolyses may proceed without being infl uenced by the template.
The as - made products are organic/inorganic biphase composite materials in
which the assembly of a hydrophobic group of templates helps to create the cylin-
drical inorganic walls. The template group is, on the other hand, adsorbed onto
the surface of the inorganic walls by various interactions, including electrically
neutral and electrostatic interactions. The most reliable way of removing the
15.2 Synthesis of Mesoporous Alumina 483
adsorbed templates is to burn off the organic phase by calcination in air; this also
promotes dehydration of the inorganic wall, transforming the framework from
aluminum hydroxide to aluminum oxide (alumina). An example of the apparatus
suitable for the calcination of as - made samples is shown in Figure 15.1 . This
fl ow system allows a choice of calcination atmosphere simply by changing the type
of fl ow gas. A muffl e oven can also be used if the calcination steps are to be
performed under air. The required temperature for burning off the adsorbed
template molecules depends on the strength of the interaction between the mol-
ecules and the inorganic walls. Thus, whilst calcination can be carried out at lower
temperatures for nonionic templates, any ionic templates that are strongly adsorbed
onto the inorganic wall will require higher temperatures for their removal. It
should be noted that increasing the calcination temperature will enhance removal
of the templates, but also result in a structural collapse by sintering. Consequently,
the use of templates that bond too strongly to the inorganic surface should be
avoided. Some groups have experienced problems in removing the sulfate or
sulfonate surfactants, whereby substantial amounts of sulfur and organic residues
remained (even at calcination temperature) that caused the mesostructure to
collapse [5b, 10] .
Figure 15.1 Apparatus for calcination of the as - made samples
with organic/inorganic biphase. Quartz - made glass should be
used for the tubular vessel, due to its high thermal stability.
484 15 Mesoporous Alumina: Synthesis, Characterization, and Catalysis
15.2.1.2 Characterization
The structural properties of mesoporous alumina can be evaluated by N
2
adsorption – desorption, powder X - ray diffraction ( XRD ), transmission electron
microscopy ( TEM ), scanning electron microscopy ( SEM ), and
27
Al magic
angle spinning nuclear magnetic resonance ( MAS - NMR ) spectroscopy. The N
2
adsorption – desorption method represents a powerful tool for characterizing the
shape of mesoporous alumina. In Figure 15.2 the type IV isotherm is shown [11] ,
which is typically observed for aluminas with organized mesopores.
The large uptake in the lowest partial pressure ( P / P
0
) region is caused by a
monolayer coverage of the surface, followed by a steep increase in adsorbed
volume in the P / P
0
range of 0.4 – 0.8, which corresponds to the fi lling of the mes-
opores. In the latter P / P
0
region, the adsorption isotherm usually does not conform
to the desorption isotherm, giving rise to a hysteresis loop which may be accounted
for by the presence of ink - bottle - like mesopores [12] . A further increase in P / P
0
leads to no further adsorption, which is represented by a horizontal plateau in the
isotherm. In contrast, an additional steep increase is observed by the multilayer
adsorption for the low - quality samples containing larger mesopores (i.e., pore
diameter > 8 nm) and macropores. By applying the N
2
adsorption – desorption data
to the appropriate theories, it is possible to obtain the values of surface area, pore
diameter, and pore volume. The Brunauer – Emmett – Teller ( BET ) method has been
used for calculation of the surface areas [13] , whilst values of pore diameter and
pore volume have been calculated using the Barrett – Joyner – Halenda ( BJH ) or
Dollimore – Heal ( DH ) methods [14, 15] . The Horv á th – Kawazoe model, which was
employed in the characterization of M41S by Beck and coworkers [3b] , is not
recommended because the model can give overestimated pore sizes [5a] . C
ˇ
ejka
et al ., on the other hand, attempted to calculate the surface area and mesopore
volume by employing a comparison plot in which the adsorption isotherm a ( P / P
0
)
of a mesoporous alumina sample was transformed to a function a ( a
ref
) of the
amount a
ref
adsorbed on a nonporous reference solid (Degussa Aluminiumoxid
Figure 15.2 Type IV isotherm typically observed for mesoporous alumina.
15.2 Synthesis of Mesoporous Alumina 485
C) at the same relative pressure [16] . The values obtained agreed well with those
determined with the BET and BJH methods.
Powder X - Ray Diffraction Powder XRD provides information concerning the pore
and framework structure. The recommendation is to use chromium as an X - ray
source because of its longer wavelength compared to the more commonly used
copper or cobalt [9] . Aluminas with a regular mesoporous structure usually exhibit
a single intense diffraction line at a low - angle region (2 θ = 1 – 2 ° ) without further
diffraction lines, indicating a hexagonal or cubic symmetry. The position of the
line is shifted to a lower angle for samples with larger pore sizes, and also when
the sintering of a sample is promoted by calcination at higher temperatures. It
should be noted that the appearance of this single diffraction line is not suffi cient
evidence for the presence of organized mesopores [5n] . Other approaches, includ-
ing N
2
adsorption – desorption measurements and TEM imaging, are also indis-
pensable when discussing the quality of mesopores. The framework structure
of a sample may be elucidated via the XRD pattern of the wide - angle region
(2 θ = 10 – 80 ° ). The detailed assignment of the XRD diffraction lines to the frame-
work structures was described by Yanagida et al . [17] .
Transmission Electron Microscopy ( TEM ) TEM enables a direct observation of the
pore size and long - range channel order. In marked contrast to the long - range
hexagonal arrangement of channels observed for MCM - 41 [3] , the mesoporous
aluminas developed to date have exhibited a regular sponge - or worm - like channel
motif, without any discernible long - range order.
27
Al MAS NMR Spectroscopy This procedure provides information regarding
the coordination number of aluminum in mesoporous alumina. When using
this method, six - , fi ve - , and four - coordinate aluminum centers produce a peak at
δ = ca. 0, 35, and 75, respectively, although in some cases the peak from the fi ve -
coordinate form is invisible due to its low intensity. It is noteworthy that the ratio
of four - and six - coordinate Al estimated via the NMR spectrum of a sample may
provide evidence that the framework has a γ - phase structure [5i] .
15.2.2
Examples of Synthesis
An early review introducing the synthetic methods of mesoporous alumina was
provided by C
ˇ
ejka, and included those reports made up until 2002 [9] . C
ˇ
ejka clas-
sifi ed the methods into three groups – namely neutral, anionic, and cationic syn-
thesis – according to the type of electric state of the surfactant, while others used
the abbreviations, S (surfactant), I (inorganic precursors), N (nonionic organics),
with their electric charges, +, − , and 0, to express the synthetic pathways of mes-
ophases [6] . Even though the electric charge of the surfactant is known, the detailed
structure of the inorganic moiety that interacts with the surfactant has, in most
cases, not been elucidated in published reports that describe mesoporous alumina
486 15 Mesoporous Alumina: Synthesis, Characterization, and Catalysis
synthesis. For this reason, the synthetic methods are classifi ed here as four groups,
focusing only on the electric charge of the surfactant side: (i) neutral surfactant
templating; (ii) anionic surfactant templating; (iii) cationic surfactant templating;
and (iv) nonsurfactant templating. An overview of the various syntheses of mes-
oporous alumina is provided in Table 15.1 .
15.2.2.1 Neutral Surfactant Templating
The synthesis of mesoporous alumina using electrically neutral surfactant tem-
plates was fi rst developed in 1996 by Bagshaw and Pinnavaia, who used
poly(ethylene oxide) s ( PEO s) or PEO/ poly(propylene oxide) ( PPO ) copolymers as
surfactant templates [5a] . The former templates involve the use of Tergitol
®
15 - S - 9
with a formula of C
11 – 15
[PEO]
9
, while the latter templates involve Pluronic
®
L64
with a formula of [PEO]
13
[PPO]
30
[PEO]
13
. In the presence of the surfactant, an
aluminum source of aluminum sec - butoxide was hydrolyzed in 2 - butanol solvent
to give the as - synthesized surfactant – alumina mesophase. This was subsequently
washed with ethanol, dried, and calcined at 500 ° C to afford the mesoporous alu-
minas (denoted MSU - X, where X = 1 – 3 depending on the type of surfactant). The
XRD refl ections of the MSU - X aluminas at 1 – 3 ° (of which the intensity became
large after calcination) were broad, signifying a greater degree of structural disor-
der compared to the disordered MCM - 41, HMS, and MSU - X silicas (Figure 15.3 ).
Subsequent TEM images of the calcined MSU - X aluminas highlighted the
regular wormlike channel motif in which the diameters of pores were more or
less uniform (Figure 15.4 ). However, no discernible long - range order was observed,
which was indicative of the randomness of packing of the channel system.
Characterization by N
2
adsorption – desorption measurements showed hyster-
eses in the isotherms of the MSU - X aluminas that had resulted from the conden-
sation of N
2
in the mesopores, and afforded pore diameters in the range of 2.4 to
4.7 nm, as estimated by the BJH model and in agreement with values obtained
from TEM images (Figure 15.5 ). It was found that, on the basis of the BJH pore -
sizes and the d
100
values, a larger surfactant tended to give a longer pore
diameter.
The MSU - X aluminas were also characterized using
27
Al MAS NMR spectros-
copy, which provides information concerning the coordination environment of
aluminum (Figure 15.6 ). Three peaks appeared for both the as - synthesized and
calcined samples at δ 0, 35, and 75, and these were attributed to the six - , fi ve - , and
four - coordinate aluminum centers, respectively. However, the calcination
increased the peak intensities at δ 35 and 75 and decreased the intensity of the δ
0 peak through an enhancement of the dehydration and dehydroxylation.
In a succeeding study, the same group attempted to improve the thermal stabil-
ity of the MSU - X aluminas by doping with rare earth ions such as Ce
3+
and La
3+
[5c] . In this case, the hydrolysis for the surfactant – alumina mesophase formation
was performed in the presence of the corresponding rare earth nitrate. A PEO -
based surfactant of Tergitol
®
15 - S - 12 was used for synthesis of the Ce
3+
- doped
mesoporous alumina, while a PEO/PPO copolymer surfactant of Pluronic
®
P65
or P123 was employed for the La
3+
- doping. The isotherms, BJH pore - sizes, XRD
15.2 Synthesis of Mesoporous Alumina 487
Table 15.1 Overview of representative synthetic methods of mesoporous alumina.
Mesoporous
alumina denotation
Aluminum source
(solvent)
Template: name; formula
a
Surface area
b
(m
2
g
− 1
)
Pore diameter
c
(nm)
Pore volume
(ml g
− 1
)
Reference(s)
I. Neutral surfactant
templating
MSU - 1 Al[OCH(CH
3
)
CH
2
CH
3
]
3
(2 - Butanol)
Tergitol
®
15 - S - 9;
C
11 – 15
[PEO]
9
490 3.3 0.40 [5a, 5c]
MSU - 2 Al[OCH(CH
3
)
CH
2
CH
3
]
3
(2 - Butanol)
Triton
®
X - 114;
C
8
Ph[PEO]
8
460 3.3 0.35 [5a, 5c]
MSU - 3 Al[OCH(CH
3
)
CH
2
CH
3
]
3
(2 - Butanol)
Pluronic
®
L64;
[PEO]
13
[PPO]
30
[PEO]
13
430 2.4 0.21 [5a, 5c]
MSU - γ
[Al
13
O
4
(OH)
24
(H
2
O)
12
]
Cl
7
(Water)
Pluronic
®
P84;
[PEO]
19
[PPO]
39
[PEO]
19
299 6.4 0.73 [5i]
MSU - γ
Al[OCH(CH
3
)
CH
2
CH
3
]
3
(2 - Butanol)
Pluronic
®
P84;
[PEO]
19
[PPO]
39
[PEO]
19
306 9.0 1.15 [5i]
DL12 Al(NO
3
)
3
(Water; pH = 7.01)
Poly(ethylene glycol) 1540;
HO – (CH
2
CH
2
– O)
1540
– H
203 6.6 0.33 [5m]
DL54 Al
2
(SO
4
)
3
(Water; pH = 10.0)
Poly(ethylene glycol) 1540;
HO – (CH
2
CH
2
– O)
1540
– H
309 6.3 Not reported [5m]
II. Anionic
surfactant
templating
C
A383
Al[OCH(CH
3
)
CH
2
CH
3
]
3
(1 - Propanol)
Caproic acid;
C
5
H
11
COOH
530 2.1 0.31 [5b]
L
A383
Al[OCH(CH
3
)
CH
2
CH
3
]
3
(1 - Propanol)
Lauric acid;
C
11
H
23
COOH
710 1.9 0.41 [5b]
S
A383
Al[OCH(CH
3
)
CH
2
CH
3
]
3
(1 - Propanol)
Stearic acid;
C
17
H
35
COOH
700 2.1 0.41 [5b]
488 15 Mesoporous Alumina: Synthesis, Characterization, and Catalysis
Mesoporous
alumina denotation
Aluminum source
(solvent)
Template: name; formula
a
Surface area
b
(m
2
g
− 1
)
Pore diameter
c
(nm)
Pore volume
(ml g
− 1
)
Reference(s)
III. Cationic
surfactant
templating
ICMUV - 1 Al[OCH(CH
3
)
CH
2
CH
3
]
3
(Triethanolamine)
Cetyltrimethylammonium bromide;
CH CH N CH Br
32
15
3
3
()()
+
−
250 – 340 3.3 – 6.0 Not reported [5d]
I , I I A l
2
Cl(OH)
5
(Water)
1 - Methyl - 3 - octylimidazolium
chloride;
262 – 269 3.8 – 3.9 0.25 – 0.26 [5n]
IV. Nonsurfactant
templating
Al - TUD - 1 Al[OCH(CH
3
)
2
]
3
(Ethanol and
2 - propanol)
Tetra(ethylene glycol);
HO(CH
2
CH
2
O)
3
CH
2
CH
2
OH
528 4.0 0.63 [5j]
A l
2
O
3
- X
(X = 1 – 3)
Al[OCH(CH
3
)
2
]
3
(Water; pH = 4.5 – 5.5)
D - Glucose;
337 – 422 3.8 – 5.1 0.43 – 0.66 [5k]
ID 7 Boehmite sol
(Water)
Citric acid;
HOC(COOH)(CH
2
COOH)
2
320 4.1 0.44 [5l]
a Abbreviations: PEO, poly(ethylene oxide); PPO, poly(propylene oxide).
b Determined by the Brunauer – Emmett – Teller ( BET ) method.
c Determined by the Barrett – Joyner – Halender ( BJH ) method.
Table 15.1
Continued.