Three-dimensional mesoporous gallosilicate with Pm3n symmetry and its unusual catalytic performances

Article abstract of DOI:10.1002/chem.200701946

Three-dimensional, mesoporous-cage-type, GaSBA-1 materials with different n_Si/N_Ga ratios have been successfully prepared for the first time by using a low hydrochloric acid to silicon (n_HCl/n _Si) molar ratio in

Full text of DOI:10.1002/chem.200701946

FULL PAPER
DOI: 10.1002/chem.200701946
Three-Dimensional Mesoporous Gallosilicate with Pm3n Symmetry and its
Unusual Catalytic Performances
Pavuluri Srinivasu and Ajayan Vinu*[a]
Abstract: Three-dimensional, mesopo-
rous-cage-type, GaSBA-1 materials
with different nSi/nGa ratios have been
successfully prepared for the first time
by using a low hydrochloric acid to sili-
con (nHCl/nSi) molar ratio in the synthe-
sis gel by templating with cetyltriethyl-
als resemble that of SBA-1, which pos-
sesses a cubic, three-dimensional, cage-
type structure with open windows. In
addition, the specific surface area and
the specific pore volume of the
GaSBA-1 materials are much higher
than those of the SBA-1 silica, which
are very important for the catalytic ap-
plications. The amount of Ga cation in
the SBA-1 silica framework has been
found to play a critical role in control-
ling the morphology and the pore di-
ameter of the materials. Finally, the
catalytic activity in the tert-butylation
of phenol with tert-butanol as the alkyl-
G
ACHTREUNG
a
medium. The obtained materials have
been unambiguously characterized in
detail by several sophisticated tech-
niques including X-ray diffraction
(XRD), N2 adsorption, high-resolution
transmission electron microscopy, high-
resolution scanning electron microsco-
py, energy dispersive spectroscopy, ele-
mental mapping, and 29Si magic-angle-
spinning NMR spectroscopy. XRD and
nitrogen adsorption results reveal that
the structures of the GaSBA-1 materi-
Keywords: alkylation · heterogene-
ous catalysis · mesoporous materi-
als · SBA-1 · scanning probe micro-
scopy
Introduction
materials with different pore structures have been synthe-
sized by self-assembly process under basic or acidic or neu-
tral conditions by using cationic, anionic, or non-ionic sur-
Mesoporous inorganic oxide materials with well-ordered
porous structures are highly attractive and have received
considerable attention in the recent years, because of their
excellent textural parameters, such as high surface area,
pore volume, and narrow pore size distribution, and poten-
tial applications mainly in the fields of catalysis, adsorption,
and separation.[1–15] A variety of mesoporous inorganic oxide
factants,
which
are
so-called
structure-directing
agents.[1–6,16–20] One of the interesting features of the self-as-
sembly-mediated synthesis process is the easy control of the
textural parameter and pore structure of mesoporous mate-
rials; this control is achieved by altering the type of the sur-
factants and co-solvents used in the reactant mixture and
preparative conditions. There are numerous reports that
deal with the preparation of various types of one- and three-
dimensional mesoporous materials, such as MCM-41, MCM-
48, SBA-1, SBA-15, AMS, HMS, and so forth.[1–20] Among
these materials, mesoporous silica materials consisting of
three-dimensional porous networks are highly interesting
and believed to be more advantageous than the materials
that have uni-dimensional array of pores. This could be
mainly due to the fact that materials with three-dimensional
[a]Dr. P. Srinivasu, Dr. A. Vinu
International Center for Materials Nanoarchitectonics
World Premier International (WPI) Research Center
National Institute for Materials Science
1-1 Namiki, Tsukuba, 305-0044 (Japan)
Fax : (+81)29-860-4563
E-mail: vinu.ajayan@nims.go.jp
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2008, 14, 3553 – 3561
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3553

pore structures are more resistant to pore blocking, mass
transfer of the reactant molecules in the pore channels, and
provide more adsorption sites, generated from their higher
surface area and pore volume, and can be easily accessible
through three-dimensional pore channels for the reactant
molecules.[21]
highly acidic medium.[21,30,31] Further, they found that the
above materials show excellent activity in both the acid-cat-
alyzed transformations, especially for alkylation, acylation,
isomerization, and oxidation of bulky organic molecules.
Recently, gallium-substituted mesoporous materials have re-
ceived much attention, because they exhibit some interest-
ing properties and show excellent performances in many
acid-catalyzed reactions.[32,33] In addition, they provide dif-
ferent distribution of acid site strength, which is critical for
several acid-catalyzed reactions. However, unfortunately,
there has been no report available on the preparation and
catalytic performances of gallium-substituted SBA-1 materi-
als in the open literature so far. Here we extend our unique
synthetic strategy for the fabrication of GaSBA-1, which
was previously used for the synthesis of Fe, Al, and Ti-SBA-
1, in which the hydrochloric acid to silicon molar ratio in
the synthesis gel is controlled to increase the metal ion in-
corporation in the silica matrix of SBA-1.
Alkylation and acylation of aromatics have received enor-
mous attention in recent years because of the importance of
the alkylated or acylated aromatic products that are being
used as intermediates for the manufacture of antioxidants,
ultraviolet absorbers, and polymerization inhibitors.[34] One
of the interesting alkylated aromatics is 4-tert-butylphenol
(4-TBP), which plays a critical role in the manufacture of lu-
bricating oils and can be easily obtained through a simple
tert-butylation of phenol, a process that involves the use of
homogeneous catalysts such as aluminum chloride, phospho-
ric acid, sulphuric acid, silica-alumina, and cation exchange
resins.[35] Although the reaction has been extensively studied
by using homogenous catalytic systems, these catalysts pose
several problems such as toxicity, corrosiveness, and other
hurdles in the commercialization of the process due to the
tedious steps involved in their separation from the reaction
mixture. Consequently, researchers have looked for alterna-
tives, for example, heterogeneous catalysts, such as zeo-
lites,[36] zeotypes, and mesoporous materials, which are envi-
ronmentally friendly and can avoid the problems mainly in
the separation process. Recently, Selvam and co-workers re-
The structural features of three-dimensional, cage-type,
mesoporous materials resemble very much the structures of
zeolites with super cages,[3,4,18] though the cage diameter of
the former is much bigger than that of the latter. These in-
teresting textural properties make them promising candi-
dates for shape-selective catalysis, reactions involving bulky
molecules with different molecular dimensions, and adsorp-
tion experiments with differently sized hydrocarbons.[22,23]
Among mesoporous materials with a cage-type pore struc-
ture, SBA-1 is the most interesting material and was discov-
ered by Huo et al.[18] Their pore structure was elucidated by
electron crystallography and HRTEM by Kruk et al.[24]
Moreover, the SBA-1 material possesses a three-dimension-
al structure with open windows and two kinds of cage-type
porous structure, the larger one has the diameter of about
4.0 nm and the diameter of the smaller cage is 3.3 nm, with
excellent symmetry.[25] Moreover, the specific surface area of
SBA-1 is much higher than that of the three-dimensional
SBA-16 and MCM-48 materials.[3,4,18] The silica form of
mesoporous SBA-1 was prepared under acidic conditions by
A
using cationic surfactants as the structure-directing agents at
low temperature; they were formed by the cooperative self-
assembly mechanism, which involves the (S+ Xꢀ I+) pathway
(cationic surfactants (S+), halogen anions (Xꢀ) and cationic
silicic acid species (I+)).[4,18,24–26]
Although the pure silica SBA-1 materials possess interest-
ing structural features and high symmetry, their neutral
framework and poor water stability make them useless for
many applications, including catalysis and adsorption. These
problems can be circumvented by introducing di- or triva-
lent heteroatoms in the neutral silica framework. The intro-
duction of heteroatoms in the silica framework can create
negative charges in the framework, which are compensated
by the protons, the so-called Brønsted acid sites that are
vital for acid-catalyzed transformations. However, unfortu-
nately, it is very difficult to incorporate the metal atoms in
the mesoporous materials prepared under highly acidic con-
ditions, because the solubility of the metal source is high
under such conditions. In addition, the heteroatoms exist
only in the cationic forms rather than their corresponding
oxo species in acidic conditions; this suppresses the contact
between the heteroatoms and the silica species. Tatsumi
et al. have successfully reported the synthesis of V- and Mo-
containing SBA-1 by choosing the appropriate metal sour-
ces, and gained control of the crystal morphology.[27–29]
Recently, Vinu et al. have reported the direct synthesis of
Al, Fe, and TiSBA-1 with different contents of metal in the
framework by applying a novel synthesis approach that in-
volves the “control of the hydrochloric acid to silicon molar
ratio” in the synthesis mixture in order to enhance the inter-
action between the heteroatoms and the silicon species in
ported the tert-butylation of phenol over monometal-substi-
G
tuted AlMCM-41 or FeMCM-41 and proposed that the ac-
tivity and selectivity of the catalysts depend upon the nature
of acid sites present. It has been also reported that the for-
mation of 4-TBP requires catalysts with moderate acid
strength.[37]
In this manuscript, we report on the preparation of the
Ga-substituted SBA-1 with different loading of Ga by the
simple adjustment of the hydrochloric acid to silicon (nHCl
/
nSi) molar ratio[30,31] of the synthesis gel by a templation
method with cetyltriethylammonium bromide (CTEABr) as
the structure-directing agent and Ga nitrate as the Ga
source. The obtained materials have been thoroughly char-
acterized by various sophisticated techniques such as X-ray
diffraction (XRD), N2 adsorption, high-resolution transmis-
sion electron microscopy (HRTEM), high-resolution scan-
ning electron microscopy (HRSEM), energy dispersive spec-
troscopy (EDS), elemental mapping, and 29Si magic-angle-
3554
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 3553 – 3561

Mesoporous Materials
FULL PAPER
spinning (MAS) NMR spectroscopy. The XRD, N2 adsorp-
tion, and HRTEM analysis showed that the structural order
of the materials is retained even after the incorporation of
Ga in the silica framework of SBA-1. The catalytic activity
of the GaSBA-1 materials with different Ga content has
been investigated on the tert-butylation of phenol with tert-
butanol (TBA) as the alkylating agent; the results are com-
pared with those of other catalysts such as FeMCM-41,
AlMCM-41, FeAl-MCM-41, and FeSBA-1. Among the cata-
lysts studied, the GaSBA-1 with the highest Ga content reg-
isters very high phenol conversion and excellent selectivity
towards the 4-tert-butylphenol (4-TBP).
synthesis gel are not found in the samples after calcination.
It is interesting to note that the intensity of the Ga peak in-
creases with the concomitant increase of the intensity of the
Si peak in the EDX pattern, with increasing the amount of
Ga source in the synthesis mixture. Moreover, the nSi/nGa
ratios obtained from the EDS patterns are consistent with
the nSi/nGa ratio obtained from the ICP elemental analysis.
The data obtained from the ICP and EDX analyses in com-
bination with the elemental mapping clearly confirm that
Ga in SBA-1 is homogeneously distributed throughout the
samples (Figure 2). Elemental mapping of the samples also
reveal that the density of the Ga atom on the surface of the
GaSBA-1 catalysts significantly increases when the nSi/nGa
ratio in the synthesis gel is decreased.
Results and Discussion
The synthesis conditions and the elemental composition of
the calcined GaSBA-1 samples are given in Table 1. It can
be seen that the nSi/nGa ratio of the calcined GaSBA-1 sam-
ples decreases with increasing the addition of Ga source in
the synthesis mixture. However, the nSi/nGa ratio of the final
product is higher than that added in the synthesis gel. This
could be mainly due to the higher solubility of the Ga
source in the highly acidic medium, which minimizes the
contact between the Ga and Si–oxo species present in the
synthesis mixture. Figure 1 shows the EDX pattern of the
Figure 2. Elemental mapping of GaSBA-1(17) (top) and GaSBA-1
(bottom).
A
Figure 3 shows the powder XRD patterns of as-synthe-
sized GaSBA-1 samples with different Ga contents. All the
Figure 1. EDS patterns of GaSBA-1 materials with different Ga content:
a) GaSBA-1(17), b) GaSBA-1(40), c) GaSBA-1(90), and d) GaSBA-1-
(140).
calcined GaSBA-1 samples with different nSi/nGa ratios. All
the samples exhibit peaks for Ga, Si, and O, confirming that
the samples are mainly composed of these elements; impuri-
ties, such as Cl or C, which may be expected from the hy-
drochloric acid and the template, respectively, added in the
Figure 3. Powder XRD patterns of as-synthesized GaSBA-1 materials
with different Ga content: a) GaSBA-1(17), b) GaSBA-1(40), c) GaSBA-
1(90), and d) GaSBA-1(140).
N
Table 1. Textural parameters of calcined GaSBA-1 samples with different nSi/nGa ratios.[a]
Sample
a0
nSi/nGa
EDS
ABET
[m2 gꢀ1
Vp
[cm3 gꢀ1][nm]
dp(ads), BJH
Cage diameter
[nm]
Wall thickness
[nm]gel
product
G
]
A
[nm]
GaSBA-1(17)
GaSBA-1(40)
GaSBA-1(90)
7.77
7.89
7.83
7.77
7.41
10
20
40
66
–
16.9
39.6
90.3
139.6
–
16.2
40.4
93.2
135.7
–
1260
1365
1410
1485
1150
0.67
0.70
0.75
0.79
0.60
2.49
2.44
2.42
2.42
2.20
4.0
4.1
4.1
4.1
3.7
1.0
0.98
0.91
0.86
1.06
GaSBA-1
SBA-1
G
[a] ao =unit cell constant, ABET =BET surface area, dp =pore diameter, Vp =pore volume.
Chem. Eur. J. 2008, 14, 3553 – 3561
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3555

A. Vinu et al.
samples except GaSBA-1(17) exhibit three peaks which cor-
respond to the (200), (210), and (211) reflections. The peaks
are well resolved and can be associated with Pm3n space
group, indicating that the materials are highly ordered and
possess a cubic, three-dimensional, cage-type structure with
open windows. GaSBA-1(17) shows three peaks at lower
angle. However, the peaks are broad and are not well-re-
solved, indicating that the structural order of the GaSBA-1
deteriorates upon the incorporation of a larger amount of
Ga in the silica framework of SBA-1. After calcination, the
lower angle peaks are shifted toward higher angle with the
decomposition of the organic template. Moreover, the inten-
sity of the XRD patterns is also significantly improved after
the calcination (Figure 4). This indicates that the structural
framework, because of its larger size, induces a small disor-
der in the porous structure of SBA-1.
The nitrogen adsorption–desorption isotherms of the cal-
cined GaSBA-1 materials with different nSi/nGa ratios are
shown in Figure 5 and the corresponding textural properties
Figure 5. Nitrogen adsorption–desorption isotherms of calcined GaSBA-1
materials with different Ga content (open symbols: desorption; closed
!
~
&
symbols: adsorption): GaSBA-1(17), GaSBA-1(40), GaSBA-1(90),
*
and GaSBA-1
A
are given in Table 1. All the samples display typical type IV
curve, with a sharp capillary condensation step and do not
show any hysteresis loops, which is a characteristic of capil-
lary condensation in three-dimensional materials with a
small-cage-type porous structure. Moreover, the shape of
the isotherm is similar to that observed for the pure silica
SBA-1 materials, which posses a narrow pore structure con-
sisting of two types of cagelike pores. It must also be noted
that the capillary condensation step in the isotherms of all
GaSBA-1 samples is shifted toward the higher relative pres-
sure with increasing the Ga content, indicating that the ni-
trogen condensation occurs within the three-dimensional,
cage-type mesopores and the samples possess better meso-
structure ordering and pores are highly uniform even after
the Ga incorporation. The specific surface area and specific
pore volume systematically decrease with increasing the Ga
content. The specific surface area amounts to 1260 m2gꢀ1 for
GaSBA-1(17) and increases to 1485 m2gꢀ1 for GaSBA-1-
Figure 4. Powder XRD patterns of calcined GaSBA-1 materials with dif-
ferent Ga content: a) GaSBA-1(17), b) GaSBA-1(40), c) GaSBA-1(90),
and d) GaSBA-1(140).
A
order is greatly improved upon calcination which is mainly
due to the occurrence of the atomic arrangements in the
Ga-O-Si framework of the mesoporous walls during the cal-
cination process. In addition, the unit-cell constant of the
p
materials, which is calculated from the formula a0 =d210
5
and is compatible with the cubic Pm3n space group, increas-
es monotonically with decreasing the nSi/nGa ratio up to 40
in the silica framework of SBA-1, and then it decreases. The
unit cell constant of the materials increases from 7.77 nm for
(140), while the specific pore volume increases from 0.67 to
GaSBA-1
A
0.79 cm3gꢀ1, respectively, for the same samples. The specific
surface area and the specific pore volume of the GaSBA-1
samples are much higher as compared with that of pure
parent silica SBA-1, indicating that the structural order of
the parent SBA-1 is retained in all the GaSBA-1 samples.
Analysis by the Barret–Joyner–Halenda method, which is
typically used for the pore-size evaluation of mesoporous
materials, reveal that all the samples possess narrow pore
size distribution and the pore diameter of the materials in-
creases with increasing Ga content (Figure 1S in the Sup-
porting Information). However, the cage diameter of the
GaSBA-1(17) sample is smaller than that of other samples,
indicating the formation of small Ga2O3 nanoclusters in the
nanocages. These results are in quite good agreement with
the unit cell constant data obtained from the XRD analysis
(Table 1).
creases to 7.77 nm for GaSBA-1(17). It is also interesting to
note that the wall thickness of the materials increases from
0.86 to 1.0 nm with decreasing nSi/nGa ratio from 140 to 17.
The increase of the unit cell constant and the wall thickness
of the GaSBA-1 is directly related with the formation of
more Ga-O-Si bonds in the framework, as the atomic radius
of Ga3+ is much larger than that of the Si4+, assuming that
the coordination number of both the atoms is four, which
leads to a longer Ga O distance. On the other hand, the
higher amount of Ga incorporation in the GaSBA-1(17)
silica framework resulted in a slight reduction in the unit
cell constant, revealing a partial collapse of the structural
order upon Ga incorporation. From these results, it could be
concluded that the Ga atoms are directly incorporated
inside the framework, and the greater amount of Ga in the
3556
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 3553 – 3561

Mesoporous Materials
FULL PAPER
The morphology of the GaSBA-1 materials with different
Ga content is presented in Figure 6. A consistent increase in
the size of the GSBA-1 particle with increasing the Ga con-
Figure 7. HRTEM images of calcined GaSBA-1(90) (left) and GaSBA-
1(17) (right).
Figure 6. HRSEM images of calcined GaSBA-1 materials with different
Ga content: GaSBA-1(17) (top left), GaSBA-1(40) (top right), GaSBA-
Figure 8. 29Si MAS NMR spectra of calcined GaSBA-1 materials with dif-
ferent Ga content: a) GaSBA-1(17), b) GaSBA-1(40), c) GaSBA-1(90),
1(90) (bottom left), and GaSBA-1(140) (bottom right).
A
and d) GaSBA-1(140).
G
tent is clearly observed. In the case of GaSBA-1(90) and
GaSBA-1(140), a branchlike structure composed of small
G
ratios. All the samples exhibit a broad peak in the range
ꢀ115 to ꢀ95 ppm. It must be noted that the sharpness of
the peak decreases with increasing the Ga content in the
silica matrix, presumably due to the decrease of the poly-
merization degree of silica with gallo hydroxo species. The
peak can be deconvoluted into three peaks with the chemi-
cal shift of ꢀ110, ꢀ106, and ꢀ100 ppm. The peak centered
at ꢀ110 ppm can be attributed to a Q4 species, which repre-
spherical particles that are linked each other is clearly ob-
served. However, an aggregate of square-shaped particles
with large size is obtained for GaSBA-1(40) and (17). The
large difference in the size and the shape of the particles
with increasing the Ga content could be due to the presence
of a large number of heteroatoms in the synthesis mixture
that may disturb the condensation of metal–hydroxo species
and inhibit the interaction between the gallosilicate species
and the charged surfactant molecules. The interaction be-
tween the metallosilicate species and the charged surfactant
molecule is very important to obtain the materials with the
uniform particle size through liquid crystal (LC) assembly.
The LC assembly may be affected by the large number of
Ga ions in the synthesis mixture of GaSBA-1(17) and (40),
which are larger in size compared to Si in the synthesis mix-
ture, leading to particles with larger size and irregular shape.
To gain the better understanding of the topology of the
materials after Ga incorporation, GaSBA-1 materials were
characterized by HRTEM spectroscopy. Figure 7 shows the
ꢀ
sents the Si atoms bound tetragonally to four Si O groups,
whereas the peak centered at ꢀ106 ppm is attributed to Ga
[39]
ꢀ
tetragonally bound with to three Si O species.
On the
other hand, the peak at ꢀ100 pm is assigned for Q3 species,
[39]
ꢀ
that is, Si bound to three Si O groups and one OH group.
These results confirm that the Ga is indeed tetragonally in-
corporated with the silica framework of GaSBA-1 matrix.
The potential of the Ga-containing mesoporous materials
for catalytic applications has been rarely reported in the lit-
eratures. Thus, special attention has been given to investi-
gate the catalytic activity of the GaSBA-1 materials in the
acid-catalyzed reaction, tert-butylation of phenol with tert-
butanol (TBA) as the alkylating agent; the results are com-
pared with the AlMCM-41 and other metal-substituted ma-
terials (Table 2). Firstly, the catalytic cycle of the GaSBA-
1(17) was investigated. This was achieved by studying the
tert-butylation of phenol in conjunction with time on stream
studies over GaSBA-1(17) for a period of 6 h at 1758C at a
nTBA/nphenol ratio of 3 and a WHSV (weight hourly space ve-
locity) of 5.1 hꢀ1. The products were collected once every
hour. The products obtained are 2-tert-butylphenol (2-TBP),
4-TBP, and 2,4-di-tert-butylphenol (2,4-DTBP). The total
conversion of phenol and the selectivity of the products as a
HRTEM images of the GaSBA-1(140) and GaSBA-1(17).
G
Both the samples clearly exhibit the well-ordered porous
networks, which reflect the topology of the parent SBA-1
mesoporous silica. However, a slight disorder in the porous
structure was observed for GaSBA-1(17), which indicates
that the structural order of the material was affected by a
huge amount of Ga incorporated into the silica framework
of the SBA-1 materials. These results are in quite good
agreement with the data obtained from XRD and nitrogen
adsorption measurements. Figure 8 shows the 29Si MAS
NMR spectra of GaSBA-1 samples with different nSi/nGa
Chem. Eur. J. 2008, 14, 3553 – 3561
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3557

A. Vinu et al.
Table 2. Comparison of the catalytic activity of tert-butylation of phenol over GaSBA-1(17) with other mesoporous catalysts (reaction time: 1 h).
Reaction conditions/product details
GaSBA-1(17)
3
FeAlMCM-41 (20)[40]
AlMCM-41 (56)[37]
FeMCM-41 (50)[37]
FeSBA-1(36)[30]
nTBA/nphenol
3
2
2
3
WHSV [hꢀ1]5.1
reaction temperature [8C]175
phenol conversion [%]84.0
selectivity [%]
2-TBP
5.1
4.8
4.8
4.9
200
70.1
175
35.9
175
21.1
200
78.5
7.4
–
10.4
–
8.1
4.7
9.5
–
14.3
–
3-TBP
4-TBP
2,4-TBP
72.1
20.5
75.2
14.4
83.4
3.9
87
3.5
71.2
11.4
function of time are presented in Figure 9. It can be clearly
seen that after one hour of reaction, which is required to
achieve the stationary state, the conversion and the product
Interestingly, the deactivation of the catalyst is more pro-
nounced at temperatures higher than 2008C, irrespective of
the Ga content in the GaSBA-1 catalysts. The low conver-
sion at high temperature may be due to the predominant
dealkylation over alkylation at high temperature and also
due to the diminishing availability of TBA as it undergoes
side reactions, such as oligomerization or aromatization. It
can also be seen in Figure 10 that the conversion of phenol
decreases in the order of GaSBA-1(17)>GaSBA-1(40)>
GaSBA-1(90). The catalytic activity of the same reaction
over other catalysts such as FeSBA-1, FeAl-MCM-41,
AlMCM-41, and FeMCM-41 are compared with that of
GaSBA-1 catalysts (Table 2). Unfortunately, the GaSBA-
1(90), which has the highest surface area and pore volume,
was not the most active for the tert-butylation of phenol,
probably due to the lower number of active sites on the sur-
face of the catalyst.
Figure 9. Effect of time on stream on the phenol conversion and the
product selectivity over GaSBA-1(17) catalyst (TR =1758C, nTBA/nphenol
=
3, WHSV=5.1 hꢀ1): Xphenol
,
S4-TBP
,
S2-TBP, and S2,4-DTBP
.
It has been found that GaSBA-1(17) registered the high-
est phenol conversion of 84% with the selectivity to 4-TBP
of 72%, which were higher than those over FeSBA-1 cata-
lysts studied in the system. It is assumed that the higher ac-
tivity of the GaSBA-1(17) is mainly due to its high acid
strength, which originates from the Ga in the silica frame-
work of SBA-1, and is known to provide acid sites with dif-
ferent strength. Similar results have been previously report-
ed for Ga-substituted mesoporous materials, which show
higher activity in various acid-catalyzed reactions with re-
spect to those of other metal-substituted mesoporous mate-
rials.[32,33] On the other hand, the activity of AlMCM-41,
FeMCM-41, and FeAlMCM-41 was much lower, apparently
due to its unidimensional pore structure, which limits the ac-
cessibility to the acid sites, and their weaker acidity
(Table 2). As the GaSBA-1(17) showed higher conversion of
phenol and selectivity to 4-TBP, the catalyst was chosen for
further study to investigate the effect of other reaction pa-
rameters, such as the reactant feed ratio, reaction tempera-
ture, and WHSV on the catalytic activity.
~
&
*
*
selectivities to tert-butylated products barely decrease over
the five hour period of time-on-stream, indicating that the
catalyst is highly stable and not quickly deactivated under
the reaction conditions used.
The catalytic activity of tert-butylation of phenol as a
function of Ga content for GaSBA-1 at different reaction
temperatures, with a WHSV of 5.1 hꢀ1 and an nTBA/nphenol
ratio of three is shown in Figure 10. All the catalysts show a
significant rise in the conversion with increasing the temper-
ature and reaches maximum at 1758C, and then decreases.
Figure 11 shows the effect of temperature on the product
selectivity of tert-butylation of phenol over GaSBA-1(17) at
a WHSV of 5.1 hꢀ1 and nTBA/nphenol ratio of three. It is inter-
esting to note that the selectivity to 4-TBP significantly in-
creases with increasing the reaction temperature and reach-
es a maximum of 90.1% at a reaction temperature of
2758C. Similar tendency has also been observed for FeSBA-
1 catalysts.[30] The rise in the selectivity to 4-TBP with in-
Figure 10. Effect of nSi/nGa ratio and the reaction temperature on the cat-
alytic activity of tert-butylation of phenol over GaSBA-1 catalysts (nTBA
/
phenol =3, WHSV=5.1 hꢀ1):
GaSBA-1(17),
GaSBA-1(40), and
&
~
*
n
GaSBA-1(90).
3558
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 3553 – 3561

Mesoporous Materials
FULL PAPER
the blocking of active sites by the adsorption of either the
excess of TBA or the water molecules produced from the
dehydration of alcohol. As expected, the selectivity to 4-
TBP decreases with the concomitant increase of 2,4-DTBP
with increasing the concentration of TBA molecules in the
reaction feed, most probably due to the availability of
excess of the alkylating agent.
An increase in the WHSV of the feed from 1.7 to 5.1 hꢀ1
leads to an increase in the conversion of phenol from 74.1 to
84% (Figure 13). However, a reduction in the phenol con-
Figure 11. Effect of the reaction temperature on the conversion and the
product selectivity of tert-butylation of phenol over GaSBA-1(17) (nTBA
/
n
phenol =3, WHSV=5.1 hꢀ1): Xphenol
,
S4-TBP
,
S2-TBP, and S2,4-DTBP.
~
&
*
*
creasing the reaction temperature is likely a consequence of
the easy diffusion of the 4-TBP. In addition, the above re-
sults also confirm that the formation of 4-TBP is thermody-
namically favored and the higher reaction temperature does
not support the secondary alkylation, which is evident from
the reduction of selectivity to 2,4-DTBP at higher reaction
temperature. This feature is very important from the com-
mercial point of view, because the catalyst compensates the
decrease in the conversion of phenol with increasing the re-
action temperature leading to a rise in the selectivity of 4-
TBP, which is commercially important product. It can also
be seen from Figure 11 that the GaSBA-1(17) catalyst shows
higher selectivity for 2,4-DTBP and 2-TBP at lower reaction
temperatures, which decreases with a concomitant increase
of the selectivity for 4-TBP, presumably due to the fact that
some of the TBA molecules aew consumed through some
unwanted reactions such as oligomerization and cracking.
Figure 12 shows the effect of the nTBA/nphenol ratio on the
phenol conversion and product selectivity over the GaSBA-
1(17) catalyst at the reaction temperature of 1758C and
WHSV of 5.1 hꢀ1. Increasing the nTBA/nphenol ratio from 1 to 3
resulted in an increase in the conversion of phenol from
44% to 84.0%. The increase in phenol conversion with in-
creasing nTBA/nphenol ratio may be attributed to a competition
between the polar molecule TBA and phenol for adsorption
sites. However, with a further increase of the nTBA/nphenol
ratio to 4 had a negative role on the conversion of phenol,
which significantly decreases. This could be mainly due to
Figure 13. Effect of WHSV on the conversion and the product selectivity
of tert-butylation of phenol over GaSBA-1(17) (TR =1758C, nTBA/nphenol
=
~
&
*
*
3): Xphenol
,
S4-TBP
,
S2-TBP, and S2,4-DTBP.
version is observed when the WHSV is increased from 5.1
to 6.8 hꢀ1, most likely due to the shorter contact time at
higher space velocity. Interestingly, the selectivity to 4-TBP
increases from 66.3 to 73% with increasing the WHSV from
1.7 to 6.8 hꢀ1, whereas the selectivity to 2,4-DTBP signifi-
cantly decreases when the WHSV of the reactant feed is in-
creased, which is mainly due to the longer contact time that
stimulates the cracking reaction at the catalyst surface and
diminish the availability of the tert-butyl cation at lower
WHSV. These results indicate that the higher selectivity to
2,4-DTBP requires a low WHSV of reactant feed or a large
contact time between the catalyst surface and the feed,
whereas the higher WHSV facilitates the formation of
monoalkylated products, which is mainly due to the less con-
G
tact time between the alkylating agent and the actives sites
present in the surface of the GaSBA-1(17) catalyst.
Conclusion
We strikingly demonstrate for the first time the successful
preparation of novel three-dimensional gallosilicate materi-
als (GaSBA-1) with a well-ordered, cage-type porous struc-
ture and excellent textural parameters through an organic
self-assembly process with a cationic surfactant in a highly
acidic medium. XRD and nitrogen adsorption results re-
vealed that the three-dimensional, cage-type pore structure
with open windows is retained in the GaSBA-1 even after a
large amount of Ga is incorporated into the silica frame-
work. Interestingly, the specific surface area and the specific
pore volume of the GaSBA-1 materials are significantly in-
Figure 12. Effect of nTBA/nphenol on the conversion and the product selec-
tivity of tert-butylation of phenol over GaSBA-1(17) (TR =1758C,
WHSV=5.1 hꢀ1): Xphenol
,
S4-TBP
,
S2-TBP, and S2,4-DTBP.
~
&
*
*
Chem. Eur. J. 2008, 14, 3553 – 3561
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3559

A. Vinu et al.
of 10 s. Nitrogen adsorption and desorption isotherms were measured at
ꢀ1968C on a Quantachrome Autosorb 1 sorption analyzer. All samples
were outgassed for 3 h at 2508C under vacuum (p<10ꢀ5 hPa) in the
degas port of the adsorption analyzer. The specific surface area was cal-
culated by using the BET model. The pore size distributions were ob-
tained from the adsorption branch of the nitrogen isotherms by using the
corrected form of the Kelvin equation by means of the Barrett–Joyner–
Halenda method as proposed by Kruk et al. [Eq. (1)][38]
creased after Ga incorporation; these factors are very im-
portant and determine the catalytic activity of the materials.
Moreover, it has been demonstrated that particle morpholo-
gy and the pore diameter of the materials can be controlled
by incorporating Ga atoms in the silica framework of SBA-1
matrix. Finally, the catalytic activity in the tert-butylation of
phenol with tert-butanol as the alkylating agent has been in-
vestigated and the catalysts showed a remarkable activity,
with very high substrate conversion and the selectivity to 4-
tert-butylated phenol product. Moreover, the GaSBA-1 cata-
lysts are found to be superior over other catalysts such as
AlMCM-41, FeSBA-1, FeAl-MCM-41, and FeMCM-41 in
the above-mentioned reaction. Thus, we strongly believe
that the novel highly active GaSBA-1 catalysts could be uti-
lized for many acid-catalyzed organic transformations, and
offer an economic and environmentally friendly way for the
production of variety of fine chemical synthesis, as supports
for sensing, and in adsorption processes.
rðp=p0Þ ¼ 2gVL=RTln½p0=pŠ þ tðp=p0Þ þ 0:3 nm
ð1Þ
In Equation (1), VL is the molar volume of the liquid adsorbate, g is its
surface tension (8.88”10ꢀ3 Nmꢀ1),
is the gas constant
(8.314 Jmolꢀ1 Kꢀ1), T is the absolute temperature (77 K), and t
(p/p0) is
R
ACHTREUNG
the statistical film thickness of nitrogen adsorbate in pores of the SBA-1
as a function of the relative pressure p/p0.
Elementary analysis was done by using an Analyst AA 300 spectrometer.
The HRTEM images were obtained with JEOL JEM-2100F. The prepa-
ration of samples for HRTEM analysis involved sonication in ethanol for
5 min and deposition on a copper grid. The accelerating voltage of the
electron beam was 200 kV. The morphology of the materials was ob-
tained with Hitachi S-4800 HR-FESEM by using an acceleration voltage
of 10 kV, whereas the elemental mapping and EDS analysis of the sam-
ples were obtained on the above instrument by using an acceleration
voltage of 20 kV. The 29Si MAS NMR spectra were recorded at a reso-
nance frequency of 99.361 MHz at a Bruker MSL 500 spectrometer, em-
ploying a 7 mm zirconia rotor with a spinning frequency of 4.5 kHz, a
pulse length of 3 ms, delay time of 30 s, and between 1500 to 3000 scans
were accumulated to obtain a good signal to noise ratio. All spectra were
referenced against external tetramethylsilane (0 ppm).
Experimental Section
Materials: Cetyltriethylammonium bromide (CTEABr) was used as the
structure-directing reagent. Tetraethyl orthosilicate (TEOS; Aldrich) and
Ga nitrate (Wako) were used as the sources for Si and Ga, respectively.
Phenol, TBA, tert-butylphenols, and hydrochloric acid (36wt%) were ob-
tained from Wako chemicals and used as received without further purifi-
cation.
Catalytic investigations: The tert-butylation of phenol was carried out in
a fixed-bed down flow-type reactor made up of a borosil glass tube with
a length of 40 cm and an internal diameter of 2 cm. About 0.5 g of cata-
lyst was placed in the reactor and supported on either side with a thin
layer of quartz wool and ceramic beds. The reactor was heated to the re-
action temperature with the help of a tubular furnace controlled by a dig-
ital temperature controller. The catalyst was activated in air at 5508C for
6 h prior to the catalytic runs. Reactants were fed into the reactor by
using a syringe infusion pump. The bottom of the reactor was connected
to a coiled condenser and receiver to collect the products. The products
obtained in the first 20 minutes were discarded and the product collected
after one hour was analyzed for identification. The liquid products were
analyzed on a Shimadzu gas chromatograph GC-17 A by using a DB-5
capillary column. Product identification was achieved by co-injection and
GC-MS.
Synthesis of the surfactant: The surfactant (CTEABr) was synthesized by
reaction of 1-bromohexadecane with an equimolar amount of triethyl-
amine in ethanol under reflux conditions for two days. A typical synthesis
procedure for CTEABr is as follows: 1-bromohexadecane (31.5 g) was
dissolved in absolute ethanol (197.25 g). Subsequently, triethylamine
(10.2 g) was added to the reaction mixture. The resultant mixture was
heated while being stirred for 48 h under reflux conditions. The solvent
was evaporated on a rotary evaporator until a white viscous liquid was
obtained, and then placed in a water bath to get a solid white product.
The brown liquid turned to a solid upon cooling to room temperature.
The resulting solid CTEABr was purified by recrystallization from a
chloroform/ethyl acetate mixture. The mixture was cooled in a refrigera-
tor in order to increase precipitation. The resultant precipitate was fil-
tered, washed with ethyl acetate, and dried in a vacuum oven at room
temperature.
Synthesis of Ga-containing SBA-1: Ga-containing SBA-1 was synthesized
under acidic conditions by using CTEABr as the surfactant, TEOS as the
silica source and gallium nitrate as the Ga source. A typical synthesis
procedure for GaSBA-1 is as follows: CTEABr (2.44 g) was added to
HCl (78.44 mL, 3.82m). The solution thus obtained was cooled to 08C
and stirred for 30 min under cold condition. Then, TEOS (6.25 g) which
was precooled to 08C and an appropriate quantity of Ga nitrate were
added to the solution under vigorous stirring. The stirring was continued
for another 5 h at 08C. Thereafter, the reaction mixture was heated to
1008C for 1 h. The solid products were recovered by filtration and dried
in an oven at 1008C overnight. The samples were labeled GaSBA-1(x),
in which x denotes the nSi/nGa molar ratio in the final product. The molar
composition of the initial synthesis gel was 1TEOS:0.0075–
0.05Ga2O3:0.2CTEABr:10HCl:125H2O. The as-synthesized materials
were then calcined in air by raising the temperature from 20 to 5508C
with a heating rate of 1.88Cminꢀ1 and keeping the sample at the final
temperature for 10 h in oxygen.
Acknowledgement
This work was financially supported by Japan Science and Technology
under the Strategic Program for Building an Asian Science and Technol-
ogy Community Scheme.
[1]J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge,
K. D. Schmitt, C. T.-W. Che, D. H. Olson, E. W. Sheppard, S. B.
McCullen, J. B. Higgins, J. L. Schlenker, J. Am. Chem. Soc. 1992,
114, 10834.
[2]C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck,
Nature 1992, 359, 710.
[3]D. Zhao, Q. Huo, J. Feng, B. F. Chmelka, G. D. Stucky, J. Am.
Chem. Soc. 1998, 120, 6024.
[4]Q. Huo, D. I. Margolese, G. D. Stucky, Chem. Mater. 1996, 8, 1147.
[5]P. Yang, D. Zhao, D. Margolese, B. F. Chmelka, G. D. Stucky, Nature
1998, 396 152.
Physical measurements: The powder X-ray diffraction patterns of
GaSBA-1 materials were collected on a Siemens D5005 diffractometer
by using CuKa (l=0.154 nm) radiation. The diffractograms were recorded
in the 2q range of 0.8 to 108 with a 2q step size of 0.018 and a step time
3560
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 3553 – 3561

Mesoporous Materials
FULL PAPER
[6]P. Yang, D. Zhao, D. Margolese, B. F. Chmelka, G. D. Stucky, Chem.
Mater. 1999, 11, 2831.
[7]J. F. Diaz, K. J. Balkus, J. Mol. Catal. B 1996, 2, 115.
[8]Z. Luan, H. He, W. Zhou, C.-F. Cheng, J. Klinowski, J. Chem. Soc.
Faraday Trans. 1995, 91, 2955.
[9]Z. Luan, C.-F. Cheng, H. He, J. Klinowski, J. Phys. Chem. 1995, 99,
10590.
[10]Y. Yue, A. GØdØon, J.-L. Bonardet, N. Melosh, J.-B. DꢁEspinose, J.
Fraissard, Chem. Commun. 1999, 1967.
[11]Z. Luan, M. Hartmann, D. Zhao, W. Zhou, L. Kevan, Chem. Mater.
1999, 11, 1621.
[12]M. Hartmann, A. Vinu, Langmuir 2002, 18, 8010.
[13]A. Vinu, V. Murugesan, M. Hartmann, J. Phys. Chem. B 2004, 108,
7323.
[14]A. Vinu, V. Murugesan, O. Tangermann, M. Hartmann, Chem.
Mater. 2004, 16, 3056.
[15]A. Vinu, V. Murugesan, W. Bçhlmann, M. Hartmann, J. Phys.
Chem. B. 2004, 108, 11496.
[16]S. Che, A. E. G. Bennet, Y. Yokoi, K. Sakamoto, H. Kunieda, O.
Terasaki, T. Tatsumi, Nat. Mater. 2003, 2, 801.
[17]Q. Zhang, K. Ariga, A. Okabe, T. Aida, J. Am. Chem. Soc. 2004,
126, 988.
[18]Q. Huo, R. Leon, P. M. Petroff, G. D. Stucky, Science 1995, 268,
1324.
[19]P. T. Tanev, T. J. Pinnavaia, Science 1995, 267, 865.
[20]S. A. Bagshaw, E. Prouzet, T. J. Pinnavaia, Science 1995, 269, 1242.
[21]A. Vinu, T. Krithiga, V. Murugesan, M. Hartmann, Adv. Mater.
2004, 16, 1817.
[22]C. Zapilko, R. Anwander, Chem. Mater. 2006, 18, 1479.
[23]A. E. Garcia-Bennett, S. Williamson, P. A. Wright, I. J. Shannon, J.
Mater. Chem. 2002, 12, 3533.
[28]L.-X. Dai, Y.-H. Teng, K. Tabata, ; E. E. Suzuki, T. Tatsumi, Chem.
Lett. 2000, 794; E. E. Suzuki, T. Tatsumi, Chem. Lett. 2000, 794.
[29]S. Che, Y. Sakamoto, O. Terasaki, T. Tatsumi, Chem. Mater. 2001,
13, 2237.
[30]a) A. Vinu, J. Dedecek, V. Murugesan, M. Hartmann, Chem. Mater.
2002, 14, 2433; b) M. Hartmann, A. Vinu, S. P. Elangovan, V. Muru-
gesan, W. Bçhlmann, Chem. Commun. 2002, 11, 1238; c) A. Vinu, T.
Krithiga, V. V. Balasubramanian, A. Asthana, P. Srinivasu, T. Mori,
K. Ariga, G. Ramanath, P. G. Ganesan, J. Phys. Chem. B 2006, 110,
11924.
[31]a) A. Vinu, P. Srinivasu, D. P. Sawant, S. Alam, T. Mori, K. Ariga,
V. V. Balasubramanian, C. Anand, Microporous Mesoporous Mater.
2008, 110, 422; b) A. Vinu, T. Krithiga, N. Gokulakrishnan, P. Srini-
vasu, S. Anandan, K. Ariga, V. Murugesan, V. V. Balasubramanian,
T. Mori, Microporous Mesoporous Mater. 2007, 100, 87.
[32]H. Kosslick, G. Lischke, B. Parlitz, W. Storek, R. Frike, Appl. Catal.
A 1999, 184, 49.
[33]R. Birjega, C. Nenu, R. Ganea, Gr. Pop, S. Serban, T. Blasco, Stud.
Surf. Sci. Catal. 2002, 142, 1331.
[34]A. Knop, L. A. Pilato, Phenolic Resin Chemistry, Springer, Berlin,
1985.
[35]a) J. B. Niederl, S. Natelson, J. Am. Chem. Soc. 1931, 53, 272;
b) K. G. Chandra, M. M. Sharma, Catal. Lett. 1993, 19, 309.
[36]K. Zhang, C. Huang, H. Zhang, S. Xiang, S. Liu, D. Xu, H. Li, Appl.
Catal. A 1998, 166, 89.
[37]a) A. Sakthivel, S. K Badamali, P. Selvam, Microporous Mesoporous
Mater. 2000, 39, 457; b) S. K. Badamali, A. Sakthivel, P. Selvam,
Catal. Lett. 2000, 65, 153; c) A. Sakthivel, N. Saritha, P. Selvam,
Catal. Lett. 2001, 72, 225.
[38]M. Kruk„ M. Jaroniec, A. Sayari, Langmuir 1997, 13, 6267.
[39]C.-F. Cheng, H. He, W. Zhou, J. Klinowski, J. A. S. Goncalves, L. F.
Gladden, J. Phys. Chem. 1996, 100, 390.
[40]A. Vinu, K. Usha Nandhini, V. Murugesan, V. Umamaheswari, A.
Pçppl, M. Hartmann, Appl. Catal. A 2004, 265, 1.
[24]M. Kruk, M. Jaroniec, R. Ryoo, J. M. Kim, Chem. Mater. 1999, 11,
2568.
[25]S. Che, Y. Sakamoto, O. Terasaki, T. Tatsumi, Chem. Mater. 2001,
13, 2237.
[26]A. Vinu, V. Murugesan, M. Hartmann, Chem. Mater. 2003, 15, 1385.
[27]L.-X. Dai, K. Tabata, E. Suzuki, T. Tatsumi, Chem. Mater. 2001, 13,
208.
Received: December 8, 2007
Published online: March 11, 2008
Chem. Eur. J. 2008, 14, 3553 – 3561
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3561