Self-assembly of cubic phenylene bridged mesoporous hybrids from allylorganosilane precursors

Article abstract of DOI:10.1039/b607133c

The synthesis of three-dimensional (Pm3n) cubic phenylene-bridged hybrid mesoporous silica material using a novel allylorganosilane precursor 1,4-bis(triallylsilyl)phenylene and cetyltrimethylammoniumchloride (C ₁₆TMACl) as a structure-directin

Full text of DOI:10.1039/b607133c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Self-assembly of cubic phenylene bridged mesoporous hybrids from
allylorganosilane precursors
a
a
a
a
Mahendra P. Kapoor,* Masaaki Yanagi, Yuuki Kasama, Takuji Yokoyama, Shinji Inagaki,
b
c
Toyoshi Shimada, Hironobu Nanbu and Lekh R. Juneja
a
a
Received 19th May 2006, Accepted 20th June 2006
First published as an Advance Article on the web 6th July 2006
DOI: 10.1039/b607133c
The synthesis of three-dimensional (Pm3n) cubic phenylene-bridged hybrid mesoporous silica
material using a novel allylorganosilane precursor 1,4-bis(triallylsilyl)phenylene and
cetyltrimethylammoniumchloride (C16TMACl) as a structure-directing agent in acidic medium is
presented. Sulfonic acid functionalized derivatives of these materials are effective in Friedel–
Crafts acylation reactions and in controlling the atmosphere emission of volatile organic
compounds, which are responsible for ground level ozone, air toxicity and smog.
Introduction
showed the systematic control of ethane-bridged periodic
mesoporous silicas using Gemini surfactants. They have
synthesized various mesostructures including bicontinuous
cubic (Ia3d) and cubic (Pm3n) via controlling the nature of
the Gemini surfactant such as the alkyl-chain length or the
The concept of mesoporous hybrid organic–inorganic nano-
1
composites arrived in the late 1990s with an expansion of
conventional mesoporous materials. Supramolecular templat-
ing methods to derive inorganic–organic hybrid materials have
raised the general expectation that their efficient uses can be
dimensionally expanded to a number of versatile applications
as adsorbents, catalysts, separations (chromatographic or
membrane), sensors for nanoelectronics, nanocluster syntheses
9
synthesis conditions (temperature, molar composition, etc.).
Guo et al. have recently revealed that the addition of a large
amount of K SO to the synthesis mixture containing ethane
2
4
bridged silsesquioxane monomer precursor and Pluronic F127
EO106PO EO ) affords a high quality large pore ethane
(
70
106
2
and optoelectronic devices. Since the first synthesis of ethane-,
10
bridged silica with cubic Im3m symmetry. Therefore, the
most common used typical organic linker for cubic mesophase
formation is ethane. Recently, Goto and Inagaki presented the
synthesis of mesoporous phenylene-silica materials with three-
dimensional cage structures using hexadecyltrimethylammo-
nium bromide, hexadecylpyridinium chloride and triblock
phenylene-, biphenylene-, thiophene- and ferrocene-bridged
1
,3
hybrid mesoporous materials with two-dimensional hexa-
gonal symmetries (P6mm), several efforts have been made to
control the morphologies and symmetries because the three-
dimensional structures of hybrid mesoporous materials could
be more suitable for applications as catalysts or adsorbents as
they do not suffer from mass-transfer limitations and therefore
11
copolymer F88 surfactants. All materials showed spherical
particle morphologies with diameters of 0.5 to 4 mm. However,
the present report on the corresponding phenylene-bridged
mesoporous silica with three-dimensional cubic symmetry
clearly shows the cubic particle morphology. Herein, we
successfully report the synthesis of three-dimensional cubic
mesophases of phenylene-bridged hybrids using phenylene-
bridged allylorganosilane precursors via electrostatic templat-
ing pathways in acidic medium. The sulfonic acid derivatives
of the three-dimensional cubic mesophase of the phenylene-
bridged hybrids were also synthesized and examined for the
Friedel–Crafts acylation (Scheme 1) and for volatile organic
compound (VOC) elimination. The results are also compared
with those for sulfonic acid derivatives of mesoporous
4
allow the easier diffusion of reactants. Earlier, the successful
synthesis of ethane-hybrid mesoporous materials with cubic
Pm3n space group symmetries with an external decaoctahedral
morphology was reported using a cationic surfactant under
basic conditions via controlling the synthetic conditions
5
temperature, molar composition, etc.). Sayari et al. have
(
reported the more insight into the development of cubic
6
mesophases of ethane bridged hybrids. Later, Kapoor and
Inagaki also derived a hybrid mesoporous material with ethane
linkers having cubic symmetry, which is akin to SBA-1 pure
silica, using a mixture of the cationic (C TMA) and
1
8
7
oligomeric Brij-30 surfactants. Recently, Liang et al. pre-
sented the synthesis of highly ordered three-dimensional
periodic ethane silica with cubic Fm3m symmetry under basic
12
phenylene-silicas with 2d-hexagonal (P6mm) symmetry
8
conditions using a divalent structure directing agent. Lee et al
a
Taiyo Kagaku Co. Ltd., Nano-Function Division, 1-3-1 Takaramachi,
Yokkaichi, Mie, 510-0844, Japan. E-mail: mkapoor@taiyokagaku.co.jp;
Fax: +81 59 347 5417; Tel: +81 59 347 5410
Toyota Central R&D Laboratories Inc., Nagakute, Aichi, 480-1192,
b
Japan
Nara National College of Technology, 22 Yata-cho, Yamatokoriyama,
Nara, 639-1080, Japan
c
Scheme 1 Friedel–Crafts acylation over 3d-cubic phenylene-bridged
mesoporous silica.
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derived using Brij-56 surfactant under acidic conditions and
1
SBA-1 material.
phenylene-silica mesoporous material was finally collected
3a–d
after removal of the surfactant by solvent extraction.
3
Typically, 1.0 g of an as-synthesized material was extracted
with 150 mL of ethanol with 2.0 g of 36% HCl aqueous solu-
tion at 65 uC for 6 h and then the solvent extraction process
was repeated again for the complete removal of the surfactant.
Experimental
General
All reactions were carried out in a nitrogen atmosphere with
dry, freshly distilled solvents under anhydrous conditions. All
reagents were of highest purity used without further purifica-
tion unless otherwise noted. Tetrahydrofuran (THF) and
diethyl ether were distilled from sodium benzophenone.
Characterization
Materials were systematically characterized. Powder X-ray
diffraction (PXRD) patterns were recorded on a Rigaku
RINT-2200V diffractometer with CuKa radiation (40 kV,
21
1
Nitromethane was used in Friedel–Crafts acylation. For H
3
0 mA) from 1 to 10u 2h, 0.01u step size, and 1u 2h min scan
NMR, the chemical shifts are reported with respect to either
2
speed. The porosimetry experiments (N adsorption isotherms)
3
tetramethylsilane (0.00 ppm) or CHCl (7.25 ppm) as an
were performed on a BELSORP-18 sorptometer at 2196 uC.
internal standard, using the following for multiplicities: s,
singlet; d, doublet; t, triplet; q, quartet; m, multiplet; b, broad.
Prior to measurement, all samples were outgassed at 80 uC at
23
0
1
Torr. The Brunauer–Emmett–Teller (BET) surface areas
3
Solvent CDCl (77.0 ppm) was used as an internal standard
were calculated from the linear part of the BET plot. Pore size
distributions (DFT) were determined from the adsorption
29
1
3
for C NMR estimation and elemental analyses were also
performed.
branch of the isotherms. Si magic-angle spinning (MAS) and
1
3
C cross polarization (CP) NMR spectra were recorded on a
Synthesis of 1,4-bis(triallylsilyl)phenylene
JEOL spectrometer using a 4 mm zirconia rotor and a sample
spinning frequency of 4 kHz. The chemical shifts were
referenced to tetramethylsilane (TMS) at 0 ppm.
Stable allylorganosilane precursor, 1,4-bis(triallylsilyl)phenyl-
ene was synthesized and purified by silica gel column
chromatography under ambient conditions according to the
modified procedure already reported in our earlier publica-
Sulfuric acid functionalization procedure
1
4
tion. To a three-necked round-bottom flask (500 mL)
equipped with a reflux condenser were added lithium chips
The cubic mesophases of phenylene-bridged silicas were
subsequently sulfonated using concentrated H
2 4
SO . Prior to
(
15.1 g, 2.16 mol) in THF (150 mL) along with a pressure-
sulfonation, all samples were dried under evacuation at
equalizing dropping funnel and magnetic stirrer. The reaction
flask was cooled to 212 uC in a NaCl–ice water bath. Under
vigorous stirring, a solution of allyl phenyl ether (18.5 mL,
2
0
3
1
Torr for 2 h. The dispersed solution of mesoporous
hybrid materials in concentrated sulfuric acid was stirred at
0 uC for 6 h under a nitrogen environment. The dispersion
9
0
.135 mol) in THF (80 mL) was then added dropwise to the
was cooled at room temperature and diluted with a large
volume of deionized water. The solids were filtered and
repeatedly washed with deionized water. Finally, the solids
were extracted and washed with hot (boiled) deionized water
and dried under vacuum for complete dryness. The acid
content of the resultant mesoporous solids was estimated from
the acid–base potentiometric titration curve. The sulfonated
derivative of the mesoporous hybrid (100 mg) was immersed in
mixture over 3 h through the dropping funnel. After further
stirring for 2 h, the supernatant was transferred to a solution
of 1,4-bis(triethoxysilyl)phenylene (Aldrich, 5.0 g, 12.5 mmol)
in THF (120 mL) via a cannula and stirred for 2 h at 24 uC.
The resultant mixture was then poured into water and
extracted with diethyl ether. The organic extracts were dried
over MgSO for 2 h and filtered. Evaporation of solvents and
4
purification of the residual oil by column chromatography on
2
0 g of 2.0 molar NaCl aqueous solution. The resulting
silica gel (hexane–AcOEt = 40 : 1) gave a colorless viscous oil
1
3.56 g, 79%). H NMR (CDCl
suspension was stirred at room temperature for 24 h until
equilibrium was reached. The filtrate was titrated with
(
3
) d 1.76–1.78 (m, 12H), 4.82–
1
.88 (m, 12H), 5.72–5.76 (m, 6H), 7.46 (s, 4H). C NMR
3
4
2
9
0
.1 molar NaOH to produce a titration curve.
(
CDCl
d 27.63. Elemental Anal. Calcd for C24
.05. Found: C, 76.25; H, 9.01%.
3
) d 19.3, 114.5, 133.7, 133.9, 136.7. Si NMR (CDCl
3
)
H34Si
2
: C, 76.12; H,
Friedel–Crafts acylation
9
We used the sulfonated 3d-cubic mesoporous phenylene-silica
15
for Friedel–Crafts acylation reaction, an acid catalyzed
Preparation of mesoporous cubic phenylene-silica solids
carbon–carbon bond forming reaction. The reaction was
carried out in a batch reaction system using normal vessels.
To sulfonated 3d-cubic mesoporous phenylene-silica catalyst
(400 mg) was added a mixture of anisole (0.8 mol) and acetic
anhydride (1.6 mmol). The mixture was stirred for 5 minutes at
For the three-dimensional cubic mesophase preparation, 1,4-
bis(triallylsilyl)phenylene (2.65 mmol) was suspended in an
aqueous acidic solution of C TMACl (1.1 mmol in 60 g
1
6
ion-exchanged water) containing 20 g concentrated (36%)
hydrochloric acid and stirred to promote hydrolysis at 40 uC
for 24 h followed by aging at ca. 92 uC for a further 24 h under
static conditions. The cubic mesophase of phenylene bridged
mesoporous hybrids was filtered and washed thoroughly with
copious amounts (>300 mL) of deionized water and dried
under vacuum at ambient temperature. Highly ordered cubic
ambient temperature. Then LiClO
CH NO ) was added and the mixture was heated at 50 uC with
stirring and continuously stirred for another 6 h. After the
reaction the catalyst was filtered off and washed with CH CN.
The filtrate was mixed with CH Cl (30 mL) and water
4
(4.4 mol in 16 mL of
3
2
3
2
2
3
306 | J. Mater. Chem., 2006, 16, 3305–3311
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(
30 mL). The organic materials were separated, washed
repeatedly with water and concentrated under reduced
pressure using a rotary evaporator. Finally, the residue
was purified via chromatography using silica gel to yield
4
-methoxyphenyl methyl ketone (87.6%). The product struc-
1
13
ture was confirmed via H NMR and C NMR. The catalyst
was recovered quantitatively by simple filtration and could be
reused after washing and drying. The second and third runs
with the recycled catalyst gave 81.3% and 73.8% yields,
1
respectively. H NMR (CDCl ) d 2.59 (s, 3H), 3.91 (s, 3H),
3
1
3
6
.97 (d, 2H), 7.98 (d, 2H) and C NMR (CDCl
13.9, 130.6, 130.8, 163.4, 196.5.
3
) d 26.6, 55.2,
1
The recovered catalyst was activated by heating at 90 uC
under vacuum for 3 h and reused for acylation of a fresh
portion of anisole. The recovered catalyst was used for one
more consecutive acylation reaction of anisole.
Fig. 1 X-Ray diffraction patterns for 3d-cubic phenylene-bridged
mesoporous hybrids derived from phenylene-bridged allylorganosilane
precursor: (a) after extraction, and (b) after sulfonation.
Volatile organic compound (VOC) elimination
A 50 mg sample of each of the aforementioned catalyst
materials and the evaluation gas were put in a three-liter gas
container. A gas mixture containing 50 ppm acetaldehyde,
2
0.8% oxygen, and the balance as nitrogen was used as the
evaluation gas. The acetaldehyde gas was analyzed at ambient
and elevated temperatures after different time intervals by
using an acetaldehyde detector tube (GASTEC). The
acetaldehyde elimination was estimated from the equation:
2
1
acetaldehyde adsorbed (mg (g of catalyst) ) = [(acetaldehyde
concentration in gas container without catalyst) 2 (acetalde-
hyde concentration in gas container with catalyst)] 6
[
{(molecular weight of acetaldehyde) 6 (volume of the
acetaldehyde gas in container)}/22.4 6 {(reaction temperature
2
2
+
273)/273}] 6 10 .
Results and discussion
Mesostructure and porosity
The X-ray diffraction pattern of the surfactant free meso-
porous hybrid showed three main peaks at (200), (210) and
(
211), which could be well fitted to the diffraction lines for a
˚
o
Pm3n-type cubic structure with a unit cell a = 85.9 A. The
˚
d-spacing of the most intense peak (210) was 38.4 A, while the
˚
d-spacings of the (200) and (211) peaks were 42.7 and 35.1 A,
respectively (Fig. 1). No noticeable shrinkage was observed
after surfactant removal. Owing to the strong similarity
1
between this diffractogram and that of SBA-1 (Pm3n) and
3
5
–7
ethane bridged cubic hybrid silicas (Pm3n), it is inferred that
the structure of the present material was also identified as
belonging to the same space group.
Fig. 2 (a) High-resolution TEM image, (b) SEM image showing the
bulk morphology and (c) enlarged SEM image of particles for 3d-cubic
phenylene-bridged mesoporous hybrids derived from phenylene-
bridged allylorganosilane precursor under acidic conditions.
Further, transmission electron microscopy (TEM; Fig. 2a)
showing the orientation of the structure along the [241] direc-
tion and scanning electron microscopy (SEM; Fig. 2b and c)
results provide direct confirmation of the three-dimensional-
cage-type (Pm3n) symmetry and well-defined cubic particle
morphology. Such external morphology is uncommon and
could be obtained only via balanced self-assembly using allyl
derivatives of the phenylene-bridged precursor.
Brunauer–Emmett–Teller (BET) surface area, pore volume,
and NLDFT pore diameter (N adsorption at 77 K on silica
2
2
21
g , 0.314 cm g
3
21, and 37.7 A˚ ,
MCM type) were 686 m
The nitrogen adsorption–desorption isotherm was type IV,
typical of periodic mesoporous materials (Fig. 3). The
respectively. The physico-chemical and textural properties are
summarized in Table 1.
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13
Fig. 5
C NMR spectrum of 3d-cubic phenylene-bridged meso-
porous silica material.
Fig. 3 Nitrogen adsorption–desorption isotherms {adsorption ($)
and desorption (#)} and pore size distribution curves (inset) for 3d-
cubic phenylene-bridged mesoporous hybrids derived from phenylene-
bridged allylorganosilane precursor (a) after extraction and (b) after
sulfonation.
1
3
degree of framework cross-linking. The C cross-polarization
CP-MAS-NMR spectrum (Fig. 5) of the surfactant free
mesoporous hybrid displays a resonance at 133.5 ppm due to
carbons on the benzene ring; other resonance marked as (*) are
due to spinning side bands. This confirmed the phenylene
Table 1 Textural properties of phenylene-bridged mesoporous
hybrids
moieties are covalently linked to silica, composed of SiO1.5
–SiO1.5 units
Thermogravimetric analysis (Fig. 6) of the three-
–
Pore
6 4
C H
2
21
3
21
˚
Materials description
S
BET/m g
PDDFT/A vol./cm g
dimensional cubic mesophase of phenylene silica hybrids was
3
2
d-cubic (Pm3n)
d-hexagonal (P6mm)
686
1043
1017
324
489
476
37.7
41.5
34.6
27.0
30.6
22.4
0.314
0.941
0.593
0.144
0.461
0.274
performed from 25 uC to 1000 uC under air atmosphere (flow
2
1
21
SBA-1
3d-cubic (Pm3n)–SO
2d-hexagonal (P6mm)–SO
SBA-1–SO
rate, 500 mL min ; ramp 5 uC min ). A weight loss below
20 uC was due to the removal of physisorbed water. A very
3
H
1
3
H
minor weight loss between 310 and 380 uC could be assigned to
the decomposition of residual surfactant. A major weight loss
in the range of 450–700 uC with a maximum at 585 uC is due
to the decomposition of the bridged phenylene moiety. The
results revealed that the material is thermally stable up to
3
H
Compositional and structural information
2
The Si MAS NMR spectrum (Fig. 4) of the surfactant free
9
4
50 uC in the airflow atmosphere.
Therefore, we have realized the synthesis of a three-
material exhibited two peaks at 280.9 and 272.4 ppm,
3
attributable to T [SiC–(OSi) ], a fully condensed silicon, and
2
T [SiC(OH)–(OSi)
3
dimensional cubic mesophase (Pm3n) of phenylene silica under
acid conditions using the novel 1,4-bis(triallylsilyl)phenylene
monomer precursor. Both the rigidity of the precursor and the
surface polarity could be controlled in the acid medium. The
2
], a partly hydrolyzed silica species. The
species at ca. 2100 ppm
absence of any signal due to SiO
4
confirms that all Si atoms are covalently connected to carbon
atoms in the framework suggesting no Si–C bond cleavage
occurred during template extraction. The results also revealed
that the 3d-cubic mesoporous phenylene-silica exhibited a high
+
2 +
‘
‘S X I ’’ route adopted in the acid conditions provided
more flexibility in control of the anisotropic growth of
+
2
morphological symmetries in contrast to the usual ‘‘S I ’’
route under basic conditions. The exerted charge matching
forces between the cationically charged micelle surface (from
29
Fig. 4
Si NMR spectrum of 3d-cubic phenylene-bridged meso-
Fig. 6 Thermogravimetric weight loss curve for 3d-cubic phenylene-
porous silica material.
bridged mesoporous silica material under air flow.
3
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quaternary ammonium surfactant) and cationically charged
silsesquioxane precursors in very strongly acidic aqueous
media leads to an interesting lowest energy surface particle
cubic cage type macro-morphology. Further, the three-
dimensional cubic mesostructure was readily accessible upon
altering the highly reactive alkoxy group in the phenylene-
bridged precursor to an allyl group, which is unreactive
towards silica and readily leaves the reaction mixture during
hydrolysis. The reaction pathway for the deallylation of
allylorganosilane precursors under acidic conditions in the
presence of a cationic surfactant as structure directing agent
followed the well-known mechanism of protodesilylation of
agent and allow the reaction to be carried out at low
temperature to avoid potential side reactions. The very high
cost and susceptibility to aqueous media of the metal triflates
become major concerns for their industrial applications. The
replacement of the conventional acylation catalysts has been
the subject of many studies because of the safety, low cost and
ease of recovery derived from heterogeneous catalysis.
Surfactant-templated mesoporous silicas functionalized
with alkylsulfonic acid groups onto the pore surface have
been reported to be efficient solid acid catalysts for acid–base
2
0
reactions.
In addition, mercapto-functional mesoporous
silicas have received considerable attention as the anchored
thiol group can be oxidized to provide sulfonic acid func-
1
6,17
allylic silanes.
The plausible explanation is that the
2
1
combination of the above described two parameters (acid
conditions and unreactive allyl leaving groups in the alcohol
free medium) helps to provide the mesophase transformations
via changing the framework density and controlling the
micelles under the described synthetic conditions.
tionality for applications in solid acid catalysis. Sulfonic acid
functionalized periodic mesoporous hybrids are an emerging
class of new materials that hold significant promise in catalysis.
The framework hydrophobic properties of materials have been
2
2
shown to be very reactive in esterification reactions and
2
condensation of phenol with acetone to form Bisphenol A.
3
Sulfonic acid derivatives of mesoporous hybrids
Sulfonic acid functionalized hybrid mesoporous materials
could have better catalytic properties compared to triflate
3
After sulfonation with concentrated H SO , the materials
a
2
4
materials (H value 215.0) because of the comparatively lower
o
retained their mesoporosity and parent nature (see Fig. 1). The
pore volume, BET surface area, and DFT pore diameter were
negative Ho value, and could suppress potential competitive
side reactions. Anticipating that somewhat mild acidic condi-
tions might be the reason for the detrimental effect, we
propose sulfonic acid functionalized phenylene-bridged meso-
porous silicas with cubic symmetry as a replacement catalyst
for the acylation reaction.
3
21
2
21
˚
.144 cm g , 324 m g and 27.0 A, respectively. The acidic
0
strength of the sulfonic acid derivative of the cubic mesophase
of mesoporous phenylene-silica estimated by the acid–base
3
g
+
21
titration method was H = 0.973 mmol g , which is about
.8 times the acidic strength of the 2d-hexagonal mesoporous
1
2
phenylene-silica (0.594 mmol g ) derived using Brij-56
1
The results listed in Table 2 demonstrate the performance of
sulfonic acid functionalized phenylene-bridged mesoporous
silicas on Friedel–Crafts acylation. The reaction procedural
details are provided in the Experimental section. It should be
noted that sulfonic acid functionalized phenylene-bridged
mesoporous silicas with cubic symmetry gave high activity in
Friedel–Crafts acylation (yield 87.6%) compared to two-
dimensional analogous phenylene hybrid mesoporous silica
(yield 36.1%) derived using oligomeric Brij-56 surfactant under
acidic conditions. The estimated acid amounts in both
materials (see Table 2) cannot fully explain the significant
difference in the catalytic activity. However, the concentra-
1
2
surfactant. The acid strength of the sulfonic acid derivative
+
of SBA-1 sulfonated under identical conditions was H
2
=
1
.
0
.319 mmol g
Catalytic performance
Friedel–Crafts alkylation and acylation reactions are funda-
mental and important processes in organic synthesis as well as
1
5
in industrial chemistry.
While the alkylation reaction
proceeds in the presence of a catalytic amount of a Lewis
acid, the acylation reaction generally requires more than a
stoichiometric amount of Lewis acid due to the involvement of
the Lewis acid sites by coordination to the corresponding
aromatic ketone products. The utility of polymer supported
Lewis acid catalysts, microencapsulated Lewis acids, which
+
22
tions of the sulfonic acid sites (acidity, mmol H
m
) based
2
3
on the surface area of the sulfonated materials are 3.01 6 10
2
3
and 1.22 6 10 for three-dimensional and two-dimensional
sulfonic acid functionalized phenylene-bridged mesoporous
silicas, respectively. Easier access to the available reaction sites
in the event of the diffusion of the reactants and the products
during the reaction process is one of the main reasons for the
1
8
include metal triflates
3 3 3
[Bi(OTf) , In(OTf) , Sc(OTf) ,
Cu(OTf) , TMSOTf , and Sc(NTf) ], Bu P, 4-dialkylamino-
3
3
3
2
pyridines, clays, zeolites, Nafion-H, and yttria–zirconia, has
1
9
been recognized. However, there are several limitations and
disadvantages of the existing protocols. The longer reaction
time and stringent reaction conditions are the most common.
The use of halogenated solvents is also harmful.
Table 2 Friedel–Crafts acylation by sulfonic acid functionalized
3d-cubic phenylene-bridged mesoporous hybrids
Acid amount/
21
g
Acidity/
mmol H
Product
yield (%)
+
+
22
a
4
-Dialkylaminopyridines is highly toxic and Bu P is flammable
3
Material description
mmol H
m
and highly air sensitive. Both materials are hazardous.
Triflates are very expensive and water sensitive catalysts, while
special efforts are required for the synthesis of Nafion-H and
yttria–zirconia. In spite of this, triflates have emerged as the
most effective catalysts. However, the strong Lewis acid
behavior of triflates makes their use for acid-sensitive
substrates difficult and require a large excess of acylating
3
3
2
d-cubic
d-cubic–SO
d-hexagonal–SO H
3
—
—
,0.05
87.6
36.1
26.7
81.3
73.8
23
3
H
0.973
0.594
0.319
0.901
0.852
3.01 6 10
1.22 6 10
0.67 6 10
not determined
not determined
2
2
3
3
SBA-1–SO
3
H
3
3
a
d-cubic–SO
3
H-R1
H-R2
d-cubic–SO
3
4
-Methoxyphenyl methyl ketone.
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enhanced activity in the material with cubic symmetry. Also,
easier diffusion of the reactants in the three-dimensional cubic
structure of the material also greatly reduces the mass transfer
limitations and consequently gave better yields of the products.
In the case of materials with two-dimensional hexagonal
symmetry it might be possible that the reactant cannot reach
some of the sulfonic acid sites buried deep in the walls of the
materials; in contrast, all available sulfonic sites were easily
accessible in three-dimensional phenylene-bridged mesoporous
silicas. The sulfonated SBA-1 materials also gave a lower yield
Table 3 Acetaldehyde removal using various phenylene-bridged
mesoporous hybrids
a
Acetaldehyde adsorbed
2
5 uC; 30 min 60 uC; 90 min
2
mg g
1
21
mg g
Material description
%
%
3
2
d-cubic (Pm3n)
d-hexagonal (P6mm)
0.314
0.045
14.89 0.558
2.27 0.124
6.11 0.188
38.6 1.369
15.91 0.472
b
26.47
6.27
9.51
69.22
23.92
b
2d-hexagonal (P6mm)-periodicity 0.122
3
2
a
d-cubic (Pm3n)–SO
3
H
0.763
0.314
d-hexagonal(P6mm)–SO
3
H
(
26.7%) because of the low concentration of the sulfonic acid
21
Saturation limit at 25 uC = 3 mg gmaterial
.
See reference 3a.
2
3
+
22
sites (0.67 6 10 , mmol H m ) and lower hydrophobicity,
since the acylation activity could also largely depend on the
surface hydrophilic/hydrophobic properties.
mesoporous silicas with cubic symmetry are better adsorbents.
14.9% Acetaldehyde could be removed from the feed
(acetaldehyde 50 ppm in air) at ambient temperature after
30 min of reaction, while at 60 uC 26.5% acetaldehyde was
eliminated after 90 minutes of reaction which is very large
compared to phenylene-bridged mesoporous silicas (6.3%
acetaldehyde removal at 60 uC after 90 minutes) with 2d-
hexagonal symmetry (P6mm) derived using Brij-56 surfactant
under acidic conditions.
Further, the reusability of the materials was also monitored.
After, the reaction, the catalyst was filtered off and recycled
for further use (see Experimental section). Slight loss of its
original catalytic activity was observed after the first recycle
(
R1; yield 81.3%). On the second recycle (R2) the catalytic
activity was further reduced and the product yield was 73.8%.
This implies that the materials could be recovered easily by
simple filtration and reused with minimal loss of activity.
Therefore, sulfonic acid functionalized three-dimensional
phenylene-bridged mesoporous silicas are reasonable strong
protic acids for the acylation reaction and provide a more
environmentally friendly form of acylation catalysis. The
activity and selectivity of reactants diffused on the high
surface area sulfonic acid functionalized three-dimensional
phenylene-bridged mesoporous silica improve as the effective
surface area of the active reaction sites is easily available. It is
true that Lewis acid sites are mostly active for Friedel–Crafts
acylation while sulfonic acid functionalized phenylene-bridged
mesoporous silicas are rich in Brønsted acidic sites. However,
depending on the reaction parameters the Brønsted and Lewis
sites are inter-convertible, which is influenced by the structure
Also, phenylene-bridged mesoporous silicas with crystal-like
pore wall structures of 2d-hexagonal symmetry derived under
3a
basic conditions showed lower acetaldehyde elimination
(9.5% at 60 uC after 90 minutes). On the other hand, the
sulfonic acid functionalized phenylene-bridged mesoporous
silicas with cubic symmetry could effectively eliminate volatile
organic compounds at low temperature. The sulfonic acid
functionalized cubic mesoporous silica eliminates 38.6% of the
acetaldehyde from the 50 ppm acetaldehyde feed in air after
reaction at ambient temperature for 30 min and acetaldehyde
elimination was increased to 69.2% at 60 uC after 90 minutes of
2
1
reaction with a saturation limit of about 3 mg gmaterial . The
results are summarized in Table 3. More detailed studies on the
activity of materials in VOC removal are currently under way
and will be the subject of a future contribution.
(
bonding angle), reaction enthalpy and coordination and
surface changes on the particle. In the system studied, the
initial density of Brønsted acidic sites could be high due to the
removal of adsorbed water, however in the real time reaction
conditions the Brønsted acidic sites began to convert into
Lewis sites, which are the prompt active reaction sites for the
Friedel–Crafts acylation. Also catalytic results for the Friedel–
Crafts acylation reaction clearly indicate a relationship
between the catalytic activity and the structural properties.
These mesoporous hybrid silicas also have good thermal and
mechanical stability and the materials can be easily handled,
are non-corrosive and less toxic. After the reaction the
catalytic materials can be separated from the reaction mixture
through simple filtration and can be recycled and reused, and
may be useful in industrial applications. The sulfonic acid
functionalized three-dimensional phenylene-bridged meso-
porous silica could be found to have general applicability for
acylation catalysis of a wide variety of reactants.
Conclusions
In summary, we have presented a preparation of cubic meso-
phases of phenylene-bridged mesoporous hybrid silica and
functionalized them with sulfonic acid species. Friedel–Crafts
acylation proceeds smoothly to produce an aromatic ketone in
attractive yield and the aforementioned materials could also be
effective in many useful carbon–carbon bond formation
reactions. Additionally, the materials showed exceptionally
high volatile organic compound acetaldehyde elimination
ability. Therefore, the described three-dimensional cubic
mesophase of phenylene-bridegd silica could be a vital material
for acid–base catalysis, limiting air pollution mainly caused
2
4
by volatile organic compounds, and also as a solid fuel cell
electrolyte material.
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310 | J. Mater. Chem., 2006, 16, 3305–3311
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