Direct syntheses of ordered SBA-15 mesoporous materials containing arenesulfonic acid groups

Article abstract of DOI:10.1039/b110598c

SBA-15 mesoporous silica has been functionalized with arenesulfonic acid groups by means of a one-step simple synthesis approach involving co-condensation of tetraethoxysilane (TEOS) and 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS) in the presence of a poly(alkylene oxide) block copolymer (Pluronic 123) under acid silica-based catalysis. The resultant materials show hexagonal mesoscopic order and pores sizes up to 60 A, with acid exchange capacities of ca. 1.3 mequiv. H⁺ per g SiO₂ and surface areas up to 600 m² g^-1. The sulfonic groups anchored to the silica surface of the pore walls are thermally stable to temperatures up to 380 °C and resistant to leaching in organic and aqueous solutions under mild conditions. ³¹P MAS NMR measurements of chemically adsorbed triethylphosphine oxide and the catalytic properties confirm the presence of Broensted acid centers in these mesoporous materials containing arenesulfonic acid groups that are stronger than those found in propanesulfonic-modified SBA-15 and Al-MCM-41.

Full text of DOI:10.1039/b110598c

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Direct syntheses of ordered SBA-15 mesoporous materials
containing arenesulfonic acid groups
Juan A. Melero,*a Galen D. Stucky,b Rafael van Griekena and Gabriel Moralesa
aDepartment of Experimental Sciences and Technology, ESCET, Rey Juan Carlos University,
28933 Mo´stoles, Madrid, Spain. E-mail: j.melero@escet.urjc.es;
Fax: 34-91-6647490; Tel: 34-91-4887087
bDepartment of Chemistry and Materials, University of California, Santa Barbara,
California, 93106, USA
Received 19th November 2001, Accepted 20th March 2002
First published as an Advance Article on the web 16th April 2002
SBA-15 mesoporous silica has been functionalized with arenesulfonic acid groups by means of a
one-step simple synthesis approach involving co-condensation of tetraethoxysilane (TEOS) and
2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS) in the presence of a poly(alkylene oxide) block
copolymer (Pluronic 123) under acid silica-based catalysis. The resultant materials show hexagonal mesoscopic
1
˚
order and pores sizes up to 60 A, with acid exchange capacities of ca. 1.3 mequiv. H per g SiO2 and surface
areas up to 600 m2 g21. The sulfonic groups anchored to the silica surface of the pore walls are thermally
stable to temperatures up to 380 uC and resistant to leaching in organic and aqueous solutions under mild
conditions. 31P MAS NMR measurements of chemically adsorbed triethylphosphine oxide and the catalytic
properties confirm the presence of Bro¨nsted acid centers in these mesoporous materials containing arenesulfonic
acid groups that are stronger than those found in propanesulfonic-modified SBA-15 and Al-MCM-41.
2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS)
1 Introduction
using Pluronic 123 as a template in the presence or absence
of an oxidant medium under acidic conditions. The hydrolysis
of the chlorosulfonyl groups (–SO2Cl) to the corresponding
sulfonic acid groups is achieved under the conditions of acidic
condensation.
Heterogeneously catalysed chemical reactions are dramati-
cally influenced by the strength and number of acid sites as well
as the morphology of the support (surface area, pore size, etc.).
The application of acid zeolites for the production of fine
chemicals is currently an active area of research,28,29 however
an important drawback is the range of available pore sizes
The organic tailoring of the internal surface of mesoporous
materials has recently received great attention in terms of
the application of such materials in the fields of catalysis,
sensing and adsorption. This organic mesoporous functiona-
lization has been widely achieved by the grafting of functional
groups.1 Recently, direct synthesis involving co-condensation
of siloxane and organosiloxane species in the presence of
different templating surfactants has been shown to be a
promising alternative to the grafting procedures.2–13 The
covalent attachment of sulfonic acid groups to the surface
of silica,14 microporous15 and mesoporous MCM and HMS
molecular sieves16–24 has been proposed and successfully tested
in several acid-catalysed reactions.14–23 Silylation and direct
synthesis procedures have been used for the preparation of
these sulfonic functionalized silica-based materials. The active
sulfonic group is obtained post-synthetically by the oxidation of
propanethiol groups previously anchored to the surface14,17–23
or by sulfonation reactions.14,15,24 These treatments lead to
a loss of mesoscopic order and, in general, to damage of the
textural properties and an incomplete oxidation of the thiol
groups accompanied by a loss of sulfur species.
˚
(v10 A). The synthesis of Al-MCM-41 mesoporous materials
with pore sizes up to 100 A constituted a remarkable break-
˚
through in this field. Although Al-MCM-41 is applicable in
principle to reactions involving large reactants and products,
there are numerous examples showing the limitation of the acid
strength of its sites.30 In this context, mesoporous materials
containing sulfonic groups, which combine a relatively high
acid strength with a large surface area and reactant accessi-
bility, are promising acid catalysts.
Likewise, the feasibility of tuning the acid strength of the
sulfonic groups by close attachment of different functionalities
might increase their potential catalytic applications. In this
regard, a recent publication demonstrates how the attachment
of perfluorosulfonic acid [Si(CH2)3(CF2)2(O)(CF2)2SO3H] to a
silica surface generates a solid with an acid strength compar-
able to that shown by perfluorinated sulfonic acid resins.31 The
presence of the fluorine atoms increases significantly the acid
strength of the sulfonic group. However, the complexity of the
preparation of the organosilane precursor [(OH)3Si(CH2)3-
(CF2)2(O)(CF2)2SO32M1] makes this material not really cost-
effective as compared with the use of MPTMS. Herein we show
that the presence of a phenyl group close to the sulfonic group
significantly increases the acid strength in comparison to other
environments such as those with methyl groups, and hence an
enhancement of catalytic activity in acid-catalysed reactions
is easily observed.
Recently,13 we have devised an one-step direct synthesis
procedure to create periodic ordered alkanesulfonic-
˚
functionalized mesostructures with pores sizes up to 60 A
and high acid exchange capacities (1 to 2 mequiv. H1 per g
SiO2) using poly(ethylene oxide)–poly(propylene oxide)–poly-
(ethylene oxide) block copolymer species (Pluronic 123) as
the templating surfactant25,26 and acid catalysis.27 This
new procedure involves an one-step synthetic strategy based
on the co-condensation of tetraethoxysilane (TEOS) and
3-mercaptopropyltrimethoxysilane (MPTMS), in the presence
of Pluronic 123 species and H2O2 in HCl aqueous solutions.
This prompted us to report in this paper the results that we
have obtained by the same route on the preparation and the
physicochemical and catalytic properties of periodic hybrid
mesoporous materials containing arenesulfonic groups. The
synthetic strategy involves the co-condensation of TEOS and
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DOI: 10.1039/b110598c

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was allowed to equilibrate and was then titrated potentiome-
2 Experimental
trically by dropwise addition of 0.01 M NaOH (aq). The sulfur
content was determined by ICP-atomic emission spectroscopy.
Transmission electron microscopy (TEM) microphoto-
graphs were obtained on a JEOL JEM-200 FX electron
microscope operating at 200 kV. Solid-state 29Si NMR
experiments were performed on a GE-300 spectrometer
operating at a frequency of 59.70 MHz with the following
conditions: magic-angle spinning at 5 kHz; p/2 pulse, 7 ms; a
repetition delay of 600 s; and 200 scans. 29Si MAS spectra were
referenced to tetramethylsilane.
2.1 Sample preparation
Propanethiol mesoporous silica materials. These materials
were synthesised as follows: 4 g of Pluronic 123 (Aldrich) were
dissolved with stirring in 125 g of 1.9 M HCl at room
temperature. The solution was heated to 40 uC before addition
of TEOS (Aldrich). A TEOS prehydrolysis time of 45 min was
used prior to the addition of the thiol precursor [MPTMS
(Gelest)] to the mixture. The molar composition of the mixture
for 4 g of copolymer was 0.0369 TEOS : 0.0041 MPTMS : 0.24
HCl : 6.67 H2O. The resultant solution was stirred for 20 h
at 40 uC, after which time the mixture was aged at 100 uC for
24 h under static conditions. The solid product was recovered
by filtration and air-dried at room temperature overnight. The
template was removed from the as-synthesised material by
washing with ethanol under reflux for 24 h (1.5 g of
as-synthesised material per 400 mL of ethanol).
Solid-state CP MAS 13C NMR measurements were recorded
on a CMX-180 Chemagnetics spectrometer operating at a 13C
1
frequency of 45.297 MHz and H frequency of 180.129 MHz
with the following conditions: magic-angle spinning at 5 kHz;
p/2 pulse, 7 ms; a CP contact time of 2 ms; high power proton
decoupling during detection; a repetition delay of 1.25 s; and
50000 scans. The spectra were referenced to tetramethylsilane.
The acidic properties of the solid were monitored by
reference to the 31P NMR chemical shift of triethylphosphine
oxide (TEPO) chemically adsorbed on the sulfonic-functiona-
lized mesoporous silica. In a typical measurement, 0.015 g of
TEPO were dissolved in 5 ml of anhydrous pentane, and this
solution was mixed with 0.15 g of solid acid and allowed to
equilibrate under stirring for 25 min before drying at 50 uC
under vacuum. 31P NMR measurements were performed on a
GE-300 spectrometer operating at a frequency of 121.65 MHz
under the following conditions: magic-angle spinning at 5 kHz;
p/2 pulse, 5 ms; 32 scans; 30 s repetition delay. The spectra were
referenced to a 85 wt% phosphoric acid external standard.
The propanesulfonic-functionalized mesoporous silica sam-
ple. This sample was synthesised as described above for the
propanethiol mesostructural solid. Following TEOS prehy-
drolysis, MPTMS and an aqueous solution of H2O2 (30 wt%)
were added at once and the resulting mixture was stirred
at 40 uC for 20 h and aged at 100 uC for an additional 24 h
under static conditions. The solid product was recovered and
extracted as previously described. The molar composition of
the mixture for 4 g of copolymer was: 0.0369 TEOS : 0.0041
MPTMS : 0.0369 H2O2 : 0.24 HCl : y6.67 H2O.
Arenesulfonic-functionalized mesoporous silica samples. These
samples were synthesised as described above for the functiona-
lized mesostructural solid. Following TEOS prehydrolysis,
CSPTMS (Gelest) was added and the resulting mixture was
stirred at 40 uC for 20 h and aged at 100 uC for an additional 24 h
under static conditions. The solid product was recovered and
extracted as previously described. Two different materials were
prepared in the presence and absence of hydrogen peroxide
during the condensation. The molar composition of the different
mixtures for 4 g of copolymer was: 0.0369 TEOS : 0.0041
CSPTMS : Z H2O2 : 0.24 HCl : y6.67 H2O with Z ~ 0 and
0.0123, respectively.
2.3 Catalytic and stability tests
Catalytic tests of the Fries rearrangement of phenyl acetate
were carried out in a stirred batch reactor equipped with a
temperature controller and pressure gauge at 150 uC for 5
hours under autogenous pressure and using phenol as solvent.
The reactant, the solvent and the catalyst were transferred into
the teflon-lined reactor and the system was heated up to 150 uC
(phenyl acetate/acid site molar ratio ~ 7.5; phenol/phenyl
acetate mass ratio ~ 60; and mass of phenol ~ 30 g.). After
reaction an aliquot of product was dissolved in dichloro-
methane and analysed by gas chromatography (VARIAN
CP-3380) on a capillary column HP-5 30 m 6 0.25 mm.
Stability tests were carried out as follows: 0.3 g of sulfonic-
modified material was suspended in 30 ml of water or toluene
and magnetically stirred for different times and temperatures.
After treatment the solid product was filtered, washed with
ethanol several times and air-dried overnight. The decrease in
acid capacity has been used as a direct measurement of the
leaching of sulfonic groups to the solution.
Other acid solids. Amberlyst-15 was supplied by Aldrich.
Al-MCM-41 with Si/Al molar ratios of 30 was prepared
following a sol–gel approach recently described in literature.32
2.2 Sample characterisation
X-Ray powder diffraction (XRD) data were acquired on a
SCINTAG PADX diffractometer using Cu-Ka radiation. The
data were collected from 0.6 to 4u (2h) with a resolution of
0.02u.
Nitrogen adsorption and desorption isotherms at 77 K were
measured using a Micromeritics ASAP 2000 system. The data
were analysed using the Broekoff–de Boer (BdB) model.33 Both
adsorption and desorption branches were used to calculate the
pore size distribution. Good agreement between both values
was found for all of the samples. The pore volume was taken at
the P/Po ~ 0.989 single point.
A Netzsch thermoanalyser STA 409 was used for simulta-
neous thermal analysis combining thermogravimetry (TGA)
and differential thermoanalysis (DTA) at a heating rate of
10 uC min21 in air. The ion-exchange capacities of the sulfonic
mesoporous materials were determined using aqueous solu-
tions of sodium chloride (NaCl, 2 M), tetramethylammonium
chloride (TMACl, 0.05 M) and tetraethylammonium fluoride
(TEAF, 0.05 M) as exchange agents. In a typical experiment,
0.05 g of solid was added to 10 g of aqueous solution
containing the corresponding salts. The resultant suspension
3 Results and discussion
3.1 Physicochemical properties of the arenesulfonic-modified
mesoporous materials
Table 1 summarises the preparation conditions as well as
the physicochemical and textural properties of the different
organically modified SBA-15 materials. For comparison, a
propanethiol- (sample S-1; Table 1) and propanesulfonic-
modified SBA-15 (sample S-2; Table 1) have been also
included. According to results published elsewhere,13 in
which the presence of hydrogen peroxide profoundly influ-
enced the physical and chemical properties of the sulfonic acid-
functionalized mesoporous materials, the hybrid arenesulfonic
ordered materials described in this work have been synthesised
in the absence and presence of an oxidant medium (samples S-3
and S-4, respectively; Table 1).
Small-angle X-ray powder diffraction for all of the samples
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Table 1 Physicochemical and textural properties of extracted materials
Molar compositiona
XRD
Textural properties
Wall thicknessc
SBET/m2 g21
Vpe/cm3 g21
b
d
Dp /A
˚
Sample
Organosilane
Type
X
Z
d100
S-1
S-2
S-3
S-4
MPTMS
MPTMS
CSPTMS
CSPTMS
M-Alk-SH
0.0369
0.0369
0.0369
0.0369
0
97.1
102.4
106.8
107.8
57.1
45.2
60.3
61.5
55
73
63
63
c
866
820
629
646
0.95
1.40
0.72
0.78
M-Alk-SO3H
M-Ar-SO3H
M-Ar-SO3H
0.0369
0
0.0123
a4 g of copolymer : X TEOS : (0.041 2 X) organosilane : Z H2O2 : 0.24 HCl : y6.67 H2O. bd (100) spacing. Calculated by ao 2 pore size (ao
~
2 d(100)/d3). dPore size from adsorption branch applying the Broekoff–de Boer (BdB) model. eThe pore volume was taken at P/Po ~ 0.985
single point.
(XRD, Fig. 1) gave signals corresponding to hexagonally-
ordered silica SBA-15 mesophases25,26 with the unit cell
parameters (d100) shown in Table 1. The XRD peaks of the
arenesulfonic-modified SBA-15 were significantly lower in
intensity compared with samples 1 and 2 and displayed a
complete absence of long-order reflections. This feature can be
explained in terms of the presence of a larger organic molecule,
which could partially disturb the assembly of the mesophase,
although a higher contrast matching effect arising from the
higher size of the organic moiety should not be ruled out. In
these particular cases (samples S-3 and S-4) the presence of
H2O2 does not seem to have an influence on the mesoscopic
order of the final materials, as was noted in the synthesis of
propanesulfonic SBA-15 materials under analogous synthetic
conditions.13
Quantitative determinations of the functional groups after
template extraction were performed by thermogravimetric ana-
lysis in air (Fig. 2). The TGA/DTA analysis of arenesulfonic-
modified SBA-15 materials shows two desorption peaks
centred at ca. 490 and 586 uC arising from the organic group
[Fig. 2(a)]. Such thermal stability is not displayed by the
polystyrene based sulfonic resins [Amberlyst-15; Fig. 2(b)],
which show a decomposition peak starting from 260 uC and
with the much more pronounced loss at 360 uC. The quan-
tification of the desorption peaks demonstrated a high degree
of organic incorporation for arenesulfonic-modified samples,
similar to that obtained for samples 1 and 2 (ca. 90%; Table 2).
The acid capacity of the sulfonic-modified materials was
measured by means of acid–base titration using different
ion-exchange agents; the results are shown in Table 2. An
acid capacity of ca. 1.3 mmol H1 per g of SiO2 for both of the
arenesulfonic-modified SBA-15 materials is observed when
sodium chloride is used as exchange agent. This result clearly
reveals that the transformation of chlorosulfonyl moieties into
sulfonic groups is just mediated by acid-catalysed hydrolysis.
Recently, several reports have demonstrated that the structure
of SBA-15 consists not only of large uniform and ordered
channels but also of complementary micropores located on the
silica wall and providing connectivity between them.34–37 In
order to check the localisation of the arenesulfonic groups,
additional acid–base titration experiments were carried out
using larger exchange cations such as TMA1 and TEA1. The
results demonstrate a similar acid capacity to that measured
using Na1 cations, which is a strong indication that the
arenesulfonic groups are located on the surface of mesopore
walls and are not embedded within the microporous structure
of the silica walls. Moreover, the close agreement between the
ion-exchange capacities and organic loading determined by
Fig. 1 XRD spectra of organic functionalized SBA-15 samples after
extraction. (A) Alk-SH SBA-15; (B) Alk-SO3H SBA-15; (C) Ar-SO3H
SBA-15 synthesised in the absence of hydrogen peroxide; (D) Ar-SO3H
SBA-15 synthesised in the presence of hydrogen peroxide.
Fig. 2 TGA and DTA measurements. (a) Arenesulfonic functionalized
mesoporous silica, S-4 sample in Table 1. (b) Sulfonic resin, Amberlyst-15.
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Table 2 Organic incorporation and acid capacity of the organically modified SBA-15 materials
Acid capacity titrationa
Organic content
TGA/DTAb
Sulfur content
ICPc
Incorporation
(%)d
Sample
Na1
TMA1
TEA1
S-1
S-2
S-3
S-4
—
—
—
1.50
1.36
1.47
1.45
1.47
1.26
1.40
1.33
90
82
88
87
1.26
1.35
1.34
1.22
1.39
1.34
1.23
1.42
1.33
ammol of H1per g SiO2. bmmol of organic per g SiO2. cmmol of S per g SiO2. dIncorporation of the organic moiety after synthesis based on
thermogravimetric analysis (theoretical value: 1.67 mmol of organic per g SiO2).
TGA and inductively coupled plasma (ICP) is clear evidence
that most of the arenesulfonic groups effectively incorporated
in the silica network are located on the pore wall and are
accessible and useful for adsorption and catalytic reaction
processes. Likewise, the acid capacity is analogous to that
obtained for propanesulfonic-modified materials.
samples are summarised in Table 1. Arenesulfonic-modified
SBA-15 samples show a type IV isotherm, characteristic of
mesoporous solids38 (Fig. 4; inset) with a significant increase of
the pore size in comparison to sample 1 at the same organic
content. As would be expected, the incorporation of larger
groups (arenesulfonic) should diminish the pore sizes to a
greater extent than would smaller groups such as propanethiol.
The in situ formation of sulfonic groups induces significant
changes of the hexagonal cell parameter as well as an enlarge-
ment of the pore size, as described elsewhere using MPTMS as
precursor.13 The use of non-ionic poly(alkylene oxide) block
copolymers as surfactants allows the synthesis of arenesulfonic
Solid-state 29Si MAS-NMR confirmed the presence of
organo-functionalized moieties as part of the silica wall
structure for all of the organically modified SBA-15 materials
(Fig. 3). Distinct resonances can be clearly observed for the
siloxane [Qn~ Si(OSi)n(OH)42n, n ~ 2–4; Q3 at 2100 ppm; Q4
at 2110 ppm)] and organosiloxane [Tm ~ RSi(OSi)m(OH)32m
,
m ~ 1–3; T3 at 265 ppm; T2 at 257 ppm] units. All the samples
show a similar relative integrated intensity of the organosilox-
ane (Tm) and siloxane (Qn) NMR signals (Tm/Qn) of close to
0.1 in fair agreement with those theoretically expected on
the basis of the composition of the initial mixture. The
29Si NMR measurements correlate well with the TG analysis.
An important point is the enhancement of the cross-linking of
siloxane species in the presence of H2O2 as evidenced by the
increase in the Q4/Q3 ratio for samples (C) and (E) in Fig. 3.
Likewise, this effect seems to be more apparent for the
organosiloxane species when CSPTMS is used as the precursor.
The Tm signals show a narrower feature in the presence of
H2O2 [sample (E); Fig. 3] as compared to that obtained in the
absence of the oxidant [sample (D), Fig. 3], where isolated
species (T1) could be present.
˚
functionalized mesoporous materials with pore sizes up to 63 A
with relatively high surface areas. From the d(100) spacing
and the mean pore size determined by N2 adsorption, the
estimated mean thickness of the pore walls of the arenesulfonic
˚
SBA-15 is substantially thicker (wall thickness of ca. 60 A) than
the that of the walls of sulfonic functionalized MCM-41 and
HMS, made with low molecular weight surfactants.16–23 The
increase in the thickness of the walls of the hybrid mesoporous
materials leads to structures that are more stable to collapse
induced by thermal or hydrothermal treatments. The TEM
images shown in Fig. 5 confirm the high mesoscopic order of
the arenesulfonic materials synthesised in this work. These
materials exhibit a hexagonal array of uniform channels with
the typical honeycomb appearance of SBA-15 materials.25,26
The organic nature of the ordered arenesulfonic SBA-15
material has been investigated by 13C CP MAS NMR (Fig. 6).
The extracted SBA-15 material shows evidence of peaks from
residual block copolymer species, which are also observed
in the organically modified materials. The arenesulfonic func-
tionalized SBA-15 shows two peaks at 15.6 and 28.8 ppm
The results of the analysis of the adsorption–desorption
isotherms measured for the extracted arenesulfonic SBA-15
Fig. 3 29Si NMR spectra of functionalized SBA-15 samples after
extraction. (A) Pure silica SBA-15. (B) Alk-SH SBA-15. (C) Alk-SO3H
SBA-15. (D) Ar-SO3H SBA-15 synthesised in the absence of hydrogen
peroxide. (E) Ar-SO3H SBA-15 synthesised in the presence of hydrogen
peroxide.
Fig. 4 N2 adsorption–desorption isotherms and pore size distributions
of arenesulfonic-modified SBA-15 materials.
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Fig. 7 31P NMR of chemisorbed TEPO over different materials. (A)
Extracted pure silica SBA-15. (B) Alkanesulfonic SBA-15; sample S-2
in Table 1. (C) Arenesulfonic SBA-15; sample S-3 in Table 1. (D)
Arenesulfonic SBA-15; sample S-4 in Table 1. (E) Al-MCM-41; Si/Al ~
30. (F) Amberlyst-15.
Fig. 5 TEM images (a) in the direction of the pore axis and (b) in the
direction perpendicular to the pore axis of an extracted arenesulfonic-
modified SBA-15 material (S-3 sample; Table 1).
acid strength.39,40 Pure silica extracted SBA-15 displays a signal
centred at 57.9 ppm that corresponds to weak Lewis-acid
centres39,40 arising from the presence of silanol groups. The
alkanesulfonic-modified SBA-15 mesoporous material (S-2
sample; Table 1) has a peak centred at 71.1 ppm, demonstrat-
ing the presence of strong Bro¨nsted acid centres. The
arenesulfonic SBA-15 material synthesised in the absence of
H2O2 displays a 31P shift displaced to higher values indicat-
ing an increase of the Bro¨nsted acidity. The presence of an
electron-withdrawing substituent adjacent to the sulfonic
groups, such as a phenyl group, disperses the negative
charge, stabilising the anion, and thus increasing the acid
strength. Moreover, when the synthesis of the arenesulfonic
SBA-15 materials is carried out in the presence of an oxidant,
the 31P NMR shift is displaced to even higher values. The
acid strength exhibited by these materials is comparable to
that found for silica gel treated with sulfuric acid followed by
mild drying.39 The possibility of tuning the acid strength of
the sulfonic groups by the attachment of different adjacent
functionalities is readily apparent in this work.
corresponding to the methylene C2 and C3 carbons and also
signals at 148.8, 135.5 and 128.1 ppm from the aromatic
carbons.
The solid-acidity of the arenesulfonic mesostructured
materials has been characterised by means of 31P NMR of
chemisorbed triethylphosphine oxide and compared with other
acid solids (Fig. 7). Increasing values of d indicate increasing
Finally, we have compared the acid strength of these sulfonic-
modified SBA-15 mesoporous materials with two conventional
acid catalysts: Al-MCM-41 (Si/Al ~ 30) and a sulfonic resin
(Amberlyst-15; 4.8 mequiv. g21). Al-MCM-41 displays a signal
centred at 66 ppm, showing a medium acid strength in
agreement with the results of the adsorption–desorption of
pyridine and NH3 carried out on this material.41 All of the
sulfonic-modified SBA-15 materials described in this work
presented a higher acid strength than the Al-MCM-41 sample.
The arenesulfonic-modified materials synthesised in this work
show a comparable acid strength to the Amberlyst-15.
In summary, the arenesulfonic mesostructural materials
produced by the direct synthesis method have large uniform
pore sizes, high surface areas, and good mesoscopic order.
Moreover, the acid centres of these materials display an
excellent thermal stability and relatively high acid strength.
3.2 Catalytic properties of the arenesulfonic-modified
mesoporous materials
Fig. 6 CP MAS 13C NMR spectra. (A) Extracted pure silica SBA-15.
(B) Arenesulfonic functionalized SBA-15, S-3 sample in Table 1.
One of the aims of the present work is to provide a comparison
between the catalytic activity of propanesulfonic-modified
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SBA-15 and conventional acid solids (Amberlyst-15 and
Al-MCM-41) with that of the arenesulfonic-modified SBA-15
materials in order to confirm the relatively high acid strength
deduced from the 31P NMR results. We have chosen as a test
reaction the Fries rearrangement of phenyl acetate to yield
p- and o-hydroxyacetophenones. The selective Fries rearrange-
ment of esters derived from aromatic alcohols is an interesting
reaction for the production of industrial pharmaceuticals, dyes
and agrochemicals.
The catalytic results of the Fries rearrangement of phenyl
acetate in the liquid phase over different acid catalysts using
phenol as the solvent are depicted in Fig. 8. First, a blank
reaction was performed in the presence of pure silica extracted
SBA-15 free of acid centres. The results show a negligible
substrate conversion and hence thermal catalysis must be
ruled out. The beneficial influence of a phenyl ring close to the
sulfonic group (rather than methyl groups) is clearly mani-
fested by the significant enhancement of the catalytic activity
for the arenesulfonic-modified materials (S-4 vs. S-2). In com-
parison to the results of the homogeneously catalysed reaction
using methanesulfonic acid as the catalyst, the sulfonic-
modified SBA-15 materials show ‘‘shape selectivity’’ imposed
by the structure of the solid, since an enhancement of the
para : ortho molar ratio can be readily observed. It is interesting
to note that the catalytic activity for Al-MCM-41 is significant
lower than that measured for the arenesulfonic-modified
material. In all the experiments, the substrate to acid sites
molar ratio was kept constant, and therefore the higher
conversion of arenesulfonic-modified materials with respect to
Al-MCM-41 and propanesulfonic-modified SBA-15 is likely
to be ascribed to the higher acid strength of the sites present
in this material. Furthermore, the catalytic activity of these
materials has been compared with commercial sulfonic resins
(Amberlyst-15). Although the arenesulfonic-modified material
showed a slightly lower activity, their high surface area, large
uniform pore sizes and the high thermal stability of sulfonic
groups provide processing latitude not present in Amberlyst-
Fig. 9 Stability of the arenesulfonic groups towards leaching in
aqueous and organic media. (#) Toluene under reflux; (() water at
room temperature; (%) boiling water.
The plot clearly shows that the rate of leaching in an aqueous
medium at 100 uC is significantly higher than that of the same
substrate in toluene under reflux. After 24 h of treatment, the
material had lost about 50% of its initial acid capacity in an
aqueous medium at 100 uC, whereas when toluene was used as
the solvent, the material remained unaffected by the treatment.
It should be also noted that the major fraction of the overall
loss occurred during the early stages of the treatment in
agreement with the observations of other authors studying
organically bonded silica.43 Extension of the treatment to up to
72 h did not change the acid capacity of the material from that
achieved after 24 h. The results suggest that the sulfonic groups
anchored to the silica surface of the pore walls display different
hydrolytic stability. Likewise, a lowering of the temperature
leads to a remarkable enhancement of the hydrolytic stability
of the sulfonic groups.
15, which has been reported to be unstable above 90 uC.42
A
final interesting observation is related to the fair agreement
between the acid strength measured by 31P NMR and the
catalytic activity.
Another important factor in the catalytic performance of
these arenesulfonic-modified materials is their stability towards
the leaching of the sulfonic groups during reaction. To test
the stability of the material against leaching, sample S-3 was
treated in aqueous and organic solutions under different times
and temperatures. Fig. 9 shows the acid capacity measured by
acid–base titration of the sample after the different treatments.
4 Conclusions
The direct synthesis procedure allows the effective anchoring
of arenesulfonic groups on the pore surface of SBA-15 meso-
structural materials. The resultant materials had large uniform
pore sizes with high surface areas, good mesoscopic order and
thick walls leading to hydrothermally stable materials. The
acid centres display excellent thermal resistance and significant
stability towards leaching in aqueous and organic media. By
the selection of a liquid-phase reaction catalysed by acid sites of
relative high strength, the benefits of arenesulfonic-modified
SBA-15 with respect to propanesulfonic-modified SBA-15
and Al-MCM-41 are readily demonstrated. These preliminary
catalytic results illustrate the potential of the sulfonic-modified
SBA-15 for catalytic applications in the production of fine
chemicals.
Acknowledgement
This work was partially supported by the US National Science
Foundation under grant DMR 95-20971 and by the US Army
Research Office under grant DAAH04-96-1-0443. We are also
grateful for the financial support of the Comunidad Autono´mica
de Madrid through the project Grupos Estrate´gicos; project
Fig. 8 Catalytic results of the Fries rearrangement of phenyl acetate
over different acid catalytic systems.
J. Mater. Chem., 2002, 12, 1664–1670
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