Sulfonic acid functionalized crystal-like mesoporous benzene-silica as a remarkable water-tolerant catalyst

Article abstract of DOI:10.1039/b914689j

Here we report that sulfonated crystal-like benzene-silica is much more robust and active than conventionally used periodic mesoporous silica for catalyzing aqueous organic reactions. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b914689j

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Sulfonic acid functionalized crystal-like mesoporous benzene–silica
as a remarkable water-tolerant catalystw
Ayman Karam,a Joao Carlos Alonso,b Tsvetelina Ivanova Gerganova,b Paula Ferreira,b
˜
a
a
a
ˆ
´
¨
Nicolas Bion, Joel Barrault and Franc¸ ois Jerome*
Received (in Cambridge, UK) 21st July 2009, Accepted 16th September 2009
First published as an Advance Article on the web 6th October 2009
DOI: 10.1039/b914689j
Here we report that sulfonated crystal-like benzene–silica is
much more robust and active than conventionally used periodic
mesoporous silica for catalyzing aqueous organic reactions.
attentions especially in water generating reactions.8 Recently,
Fukuoka and co-workers reported that Ph–PMO–SO3H can be
successfully used directly in water.9 To the best of our knowledge
this preliminary work represents the first and unique example
involving a Ph–PMO–SO3H material for catalysis in water, thus
opening a new field to be explored.
Owing to its low cost, non-flammability and environmental
friendliness, water is now witnessing a sort of renaissance.1 In
particular, utilization of water as a solvent for acid catalyzed
reactions has become of growing interest either in industry or
academia. In this context, the search of water-tolerant acid
solid catalysts has emerged as a challenging task.2
In this context, we wish to show that, in water, sulfonated
Ph–PMO is much more active and much more robust than the
conventionally used sulfonated periodic mesoporous silica
(PMS–SO3H). In particular, whereas sulfonic sites grafted
over SBA–15 (a water-resistant siliceous support) were
progressively deactivated after prolonged reaction time in
water, we found that the Ph–PMO support was able to prevent
this deactivation, allowing us to successfully recycle the
Ph–PMO–SO3H catalyst without any loss of activity.
Amberlysts, nafions and zeolites are among the most stable
solid catalysts in water. However, the important hindrance of
their framework represents one of the main limitations
to their broad utilization in organic synthesis. In order to
circumvent this issue, many researches were directed towards the
synthesis of periodic mesoporous silica (PMS).3 Their easy
functionalization with Brønsted or Lewis acids combined to
the presence of large pore openings offer notable advantages
for catalysis. However, acid PMS-based catalysts are poorly
stable in water because of their amorphous nature.
In a first set of experiments, we investigated the acid-
catalyzed reaction of indole with benzaldehyde as a model
reaction. This reaction gives access to the bis(indolyl)methane
derivatives which constitute an important class of compounds
that display diverse pharmacological activities.10 In water,
utilization of recyclable heterogeneous catalysts for this reaction
is scarce mainly because of the huge difficulty to design a
water-resistant acid solid catalyst.11
In 1999, the groups of Inagaki, Stein and Ozin reported
independently an exciting new class of nanomaterial named
Periodic Mesoporous Organosilica (PMO).4 These meso-
structured materials were prepared using a silesquioxane of the
type (EtO)3Si–R–Si(OEt)3 as a sole siliceous precursor.5 In the
literature, it has been shown that functionalization of amorphous
ethyl or oxime bridged PMOs with palladium provided more
active solid catalysts in water than those commonly prepared
from PMSs.6 The greater hydrophobicity of PMOs is generally
invoked to explain their superior activity. However, in water, a
strong deactivation of these amorphous PMOs was still observed.
Remarkably, in 2002, Inagaki and co-workers reported
that starting from 1,2-bis(triethoxysilyl)benzene as a siliceous
precursor, mesoporous benzene–silica with crystal-like pore
walls (Ph–PMO) can be successfully prepared.7 Owing to their
crystallinity, hydrophobicity and large pore openings, sulfonated
Ph–PMO (Ph–PMO–SO3H) is now receiving more and more
In a typical procedure, benzaldehyde was mixed with 2 eq. of
indole and heated in water at 60 1C in the presence of 0.7 mol%
of sulfonic sites supported either over Ph–PMO, PMSs, resin
or carbonaceous materials. Ph–PMO–SO3H,8 HMS–SO3H,12
SBA–SO3H12 and Carb–SO3H13 were prepared as described in
the literature. After 20 min of reaction at 60 1C in water, cation
exchange resin A15 (wet) and HMS–SO3H afforded the corres-
ponding bis(indolyl)methane 3a with 35% and 40% yield,
respectively (entries 2, 3). In agreement with previous works,
owing to its higher hydrothermal stability and the presence
of a larger pore opening, SBA–SO3H was found more active
(TOF = 257 hÀ1) than HMS–SO3H (entry 4).
To our delight, we found that Ph–PMO–SO3H was far more
active (TOF = 943 hÀ1) than PMS–SO3H catalysts, affording
3a with 95% yield (entry 6). Note when comparing this result
with the one of the sulfonated carbon, Ph–PMO–SO3H was
still much more active (entries 5, 6). As already observed for
other reactions, the low activity of the carbonaceous material
(TOF = 107 hÀ1) can be attributed to its very poor dispersion
in water.14 For the moment, we are not able to discriminate
between different parameters (hydrophobicity, acid strength,
acid site content, p–p stacking, among others) to explain the
greater efficiency of Ph–PMO–SO3H over other tested solid
acid catalysts and this aspect is now topic of current studies.
a Laboratoire de Catalyse en Chimie Organique,
´
Universite de Poitiers/CNRS, 40 avenue du recteur Pineau,
86022 Poitiers, France. E-mail: franc¸ois.jerome@univ-poitiers.fr;
Fax: (+33) 549453349; Tel: (+33)549454052
b Department of Ceramics and Glass Engineering, CICECO,
University of Aveiro, 3810-193 Aveiro, Portugal.
E-mail: pcferreira@ua.pt; Fax: (+351) 234401470;
Tel: (+351) 234401419
w Electronic supplementary information (ESI) available: Catalytic
procedures, catalyst and product characterizations. See DOI:
10.1039/b914689j
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This journal is The Royal Society of Chemistry 2009
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Table 1 Aqueous synthesis of bis(indolyl)methanes over different type of solid acid catalysts
Entry
Catalyst
Surface (m2/g)a
Pore diameter (nm)b
H+ exchange capacity (mmol/g)c
TOF (hÀ1
)
Isolated yield (%)
1
2
3
4
5
6
Without catalyst
Amberlyst 15 (wet)
HMS–SO3H
SBA–SO3H
Ph–PMO–SO3H
Carb–SO3Hd
—
53
520
690
680
nd
—
30
2.8
3.5
2.3
nd
—
—
0
35
40
50
95
25
4.70
0.45
0.55
0.73
0.30
150
171
257
943
107
a
b
c
Measured by BET. Calculated from the adsorption branch of the N2 isotherm using the BJH method. Determined by potentiometric
d
titration. Vulcan was used as carbonaceous support.
the reused SBA–SO3H showed that the amount of acid sites
remained unchanged, thus confirming the superior stability of
SBA–SO3H over HMS–SO3H in water (Table 2). Note that
elemental and TDA analyses showed that there is no loss of
carbon or sulfur after 5 catalytic cycles. These results suggest
that this drop of yield can be attributed to a decrease of the
acid strength of SBA–SO3H probably caused by a solvation of
the catalytic sites by water. To support this hypothesis, fresh
SBA–SO3H catalyst was first heated at 60 1C in water for
2 hours before addition of the reactants. In this case, 3a was
obtained with only 25% yield confirming the harmful role of
Fig. 1 Catalytic recycling over ’ Ph–PMO–SO3H,
SBA–SO3H,
water towards the activity of SBA–SO3H.
HMS–SO3H (20 min, 60 1C, 0.7 mol% of supported –SO3H).
Remarkably, over Ph–PMO–SO3H, no drop of yield was
observed since, after 6 successive catalytic cycles in water, 90%
yield of 3a was still recovered (Fig. 1). We assume that the
hydrophobic environment created by the presence of the phenyl
rings in the pore wall of Ph–PMO–SO3H protects the grafted
sulfonic sites against water. XRD patterns and BET surface
area of the reused Ph–PMO–SO3H were similar than those
of the fresh Ph–PMO–SO3H confirming the remarkable
tolerance of this catalyst for water (Table 2, ESI Fig. S6w).
On the course of our work, we next investigated the scope of
the Ph–PMO–SO3H catalyst. As summarized in Table 3, in
water, various indole and aldehyde derivatives were successfully
converted to the bis(indolyl)methanes 3b–i with 65–95% yield.
Interestingly, Ph–PMO–SO3H was tolerant to the presence of
highly hydrophobic substrate since, starting from nonylaldehyde,
3h was isolated with 95% yield. Note that even the acid
sensitive furfuraldehyde derivative reacted smoothly with
indole to give 3i with 65% yield in water.
Next, we compared the stabilities of PMS–SO3H and
Ph–PMO–SO3H catalyst in water. To this end, recycling
experiments were undertaken (Fig. 1). After the first catalytic
run, 3a was selectively extracted from water with ethyl acetate.
Note that the solid catalysts remained in suspension in water
and no reactivation or purification of the catalyst was performed
between each cycle. Benzaldehyde and indole were then reloaded
for another catalytic cycle. As shown in Fig. 1, cycle after
cycle, an important drop in catalytic activity was observed
with HMS–SO3H since only 15% yield of 3a was recovered
after 5 cycles. Further inspections of the reused HMS–SO3H
catalyst revealed a total collapse of the mesoporous structure
and a decrease of the proton exchange capacity, thus showing
the instability of such catalyst in water (Table 2).
As expected, owing to a higher hydrothermal stability, the
mesoporous structure of SBA–SO3H was preserved after 5
catalytic runs in water (Table 2). However, a significant drop
of yield was still observed (50 to 22% yield of 3a). Titration of
Having these results in hand, we then investigated the
efficiency of Ph–PMO–SO3H in the conversion of glycerin,
which is an important industrial chemical platform.15 Industrially
available glycerin contains 15–20 wt% of water (residual salts
may be also present) which unfortunately causes serious
damage to the heterogeneous catalysts.
Table 2 Comparison between the fresh and reused catalysts
BET surface (m2/g)
mmol H+/g
Reuseda
Fresh
Reused
Catalyst
HMS–SO3H
SBA–SO3H
Ph–PMO–SO3H
520
690
680
20b
670b
670c
0.31
0.55
0.72c
Being aware of the promising performances of Ph–PMO–
SO3H in the presence of water, we investigated the ability of
Ph–PMO–SO3H to catalyze the conversion of crude glycerin.
To this end, the catalytic etherification of crude glycerin with
1-phenylpropan-1-ol was chosen as a model reaction. In a
typical experiment, 4 mmol of glycerin (glycerol/H2O 80/20)
a
See H+ exchange capacity of the fresh catalysts in
b
c
Table 1. Measured after 5 catalytic cycles. Measured after 6
catalytic cycles.
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Table 3 Substrate scope of the Ph–PMO–SO3H catalyst
solid catalysts (including SBA-15-based catalyst), thus increas-
ing the scope of this recently discovered acid solid catalyst for
aqueous organic reactions.
The authors are grateful to the CNRS and POCI 2010,
FEDER and FCT for financial support (POCI/CTM/ 55648/
2004 and PPCDT/CTM/55648/2004).
Notes and references
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R4
Ph
Time (min) Product Yield (%)a
3
3
3b
3c
3d
95
95
85
2
3
Ph
Ph
60
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5
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3e
3f
85
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6
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Ph–CHQCH– 10
3g
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CH3(CH2)7–
10
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8
65
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was mixed with 1 mmol of 1-phenylpropan-1-ol and stirred at
100 1C in the presence of 10 mg of Ph–PMO–SO3H (0.02 eq. of
–SO3H). After 1 h, the benzylalcohol was totally consumed
and the desired monoethers of glycerol were recovered with
85% yield (the 15% remaining are dibenzylethers of glycerol).
As observed above, using SBA–SO3H as catalyst, a strong
deactivation occurs cycle after cycle (Scheme 1). Remarkably,
in glycerin, we found that Ph–PMO–SO3H was much more
robust and can be recycled at least 5 times without any loss of
activity further demonstrating the great potential of
Ph–PMO–SO3H for catalyzing aqueous organic reactions.16
In conclusion, we have shown that, in water, Ph–PMO–SO3H
is more active and more robust than the commonly used acid
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16 Note that the Amberlyst 15 (wet) was also found to be far less
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reaction at 100 1C.
Scheme 1 Catalytic etherification of glycerin over Ph–PMO–SO3H.
7002 | Chem. Commun., 2009, 7000–7002
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This journal is The Royal Society of Chemistry 2009