Significant enhancement on selectivity in silica supported sulfonic acids catalyzed reactions

Article abstract of DOI:10.1039/b702997g

Thanks to strong hydrophilic interactions between reaction products and the catalyst surface, mesoporous silica supported sulfonic sites were found to be much more selective than homogeneous and common solid acid catalysts. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702997g

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Significant enhancement on selectivity in silica supported sulfonic acids
catalyzed reactions{
Ayman Karam,a Yanlong Gu,a Franc¸ois Je´roˆme,*a Jean-Paul Douliezb and Joe¨l Barraulta
Received (in Cambridge, UK) 27th February 2007, Accepted 22nd March 2007
First published as an Advance Article on the web 5th April 2007
DOI: 10.1039/b702997g
us to selectively and easily (no chemical modification of the silica
surface) synthesize in an one step process various polyfunctional
derivatives not accessible by conventional catalytic or organic
routes.
Thanks to strong hydrophilic interactions between reaction
products and the catalyst surface, mesoporous silica supported
sulfonic sites were found to be much more selective than
homogeneous and common solid acid catalysts.
Selective esterification of fatty hydroxylated carboxylic acids
with glycerol was first studied as a model reaction for this purpose.
This reaction is of great importance for supramolecular chemistry
since the esterified products exhibit a broad polymorphism.10 The
selectivity here is the main problem since a rapid polymerisation
reaction between reactants can occur making difficult the
formation of the targeted amphiphilic monoesters with high yield.
First, esterification of glycerol with 16-hydroxyhexadecanoic acid
(juniperic acid) was studied at 110 uC in the presence of 2.5 mol%
of p-toluene sulfonic acid (PTSA) as reference homogeneous
catalyst. As expected, using PTSA, the resulting amphiphilic
monomer was obtained with yield lower than 35% because of a
rapid polymerization of the reactants (Fig. 1).
Silica immobilized molecular catalysts (SIMCs) has become a very
promising tool for catalysis since recent researches showed a clear
synergistic effect between the immobilized catalytic entities and the
siliceous support.1 Based on this strategy new and fascinating
concepts recently emerged which gave access to multifunctional
catalysts,2 stabilisation of metal nanoparticules,3 chiral materials4
or the preparation of molecularly imprinted polymers.5 However,
the catalytic performance, activity and selectivity, of these SIMCs
is usually lower than that of the corresponding homogeneous
molecular catalysts. This lower catalytic activity is mainly due to a
poorer accessibility of the catalytic sites caused by the steric
hindrance and/or the high hydrophilicity of the inorganic solid
support. In order to circumvent these issues, few strategies were
developed such as the use of mesoporous silica framework with
high specific areas6 or the silica surface derivatization.7 However,
even if these promising methods succeded to increase the catalytic
activities of the resulting SIMCs, they do not allow to improve
their selectivity. Zeolites are one of the most spectacular examples
of siliceous heterogeneous catalysts able to reach higher selectivity
than homogeneous catalysts thanks to their shape selectivity.9
However, their small pore openings limit their applications to
weakly sterically hindered organic substrates. More recently,
reported studies showed that silica supported nanoparticles3 or
ionic liquid films8 can also afford selective processes but their long
term stability on the solid support still remain a difficult challenge.3
Here we wish to report that the hydrophilicity of mesoporous
SIMCs, usually considered as a drawback for the diffusion of
organic substrates, can be favorably used to perform highly
selective processes starting from unprotected polyfunctional
organic derivatives. To our delight, mesoporous silica supported
sulfonic groups were able to block a side polymerization process
affording monomers with (i) selectivity higher than homogeneous
and usual solid acid catalysts and (ii) high catalytic activity. The
consideration of the hydrophilicity of mesoporous SIMCs allowed
In order to increase the selectivity of this reaction, we
immobilized sulfonic groups over three different kinds of
mesoporous silica. These porous materials are known to be highly
polar11 and strong hydrophilic interactions between the silica
framework and glycerol are expected to favor a rapid desorption
of the targeted amphiphilic monomer from the catalytic surface
avoiding thus the polymerization reactions.
Synthesis of mesoporous silica-supported sulfonic groups, with
different pore diameters, was carried out according to known
procedures using tri-block P123 and hexadecylamine as templates
for mesoporous silica SBA-SO3H and HMS-SO3H respectively.11
aUniversite´ de Poitiers, Ecole Supe´rieure d’Inge´nieurs de Poitiers,
LACCO UMR 6503 CNRS-Universite´ de Poitiers, 40 avenue du recteur
pineau, 86022, Poitiers, FRANCE. E-mail: francois.jerome@
univ-poitiers.fr; Fax: +33 (0)5 49 45 33 49; Tel: +33 (0)5 49 45 40 52
bINRA Nantes, BIA e´quipe Interfaces et Syste`mes Disperse´s, rue de la
Ge´raudie`re, 44316, Nantes, FRANCE. E-mail: douliez@nantes.inra.fr;
Fax: +33 (0)2 40 67 50 84; Tel: +33 (0)2 40 67 50 83
Fig. 1 Comparison of kinetic profiles between (blue) SBA-15-SO3H,
(green) HMS1–SO3H, (red) HMS2–SO3H, (6) PTSA and (pink) blank.
From heterogeneous catalysts, maximum yields are obtained at 95%
conversion of carboxylic acid. In the case of PTSA the maximum
monomer yield is reached at only 75% conversion.
{ Electronic supplementary information (ESI) available: Experimental.
See DOI: 10.1039/b702997g
2222 | Chem. Commun., 2007, 2222–2224
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catalyst with smaller pore opening such as HMS2–SO3H (2.3 nm),
the reaction rate decreased due to a poorer accessibility of the
catalytic sites but interestingly the yield into the desired
amphiphilic monomer was increased from 81% to 92% (Fig. 1).
This selectivity improvement is probably due to a synergistic effect
between the siliceous hydrophilicity and the significant steric
hindrance of HMS2–SO3H. This effect was also clearly illustrated
at the end of the reaction since after total consumption of the
starting carboxylic acid, the monomer yield remains quite stable
for more than 8 h (Fig. 1).
The scope of this method was then studied by conducting it
starting from different functional carboxylic acids (aleuritic,
thapsic and 12-hydroxystearic acid) and the efficiency of HMS2–
SO3H was compared to other known solid acid catalysts such as
cation exchange resin (A119), zeolite (HFAU) and sulfonated
carbon (Carb-SO3H) prepared as described elsewhere.12 Whatever
the starting functional carboxylic acid, the HMS2–SO3H was
found to be the more selective solid catalyst affording the ester
derivatives with selectivity of 86–99% range at conversion levels
higher than 80% (Table 1). Replacement of the siliceous support
by a carbonaceous or polystyrene framework led to a dramatic
drop of the different targeted esters yield from 77–92% range for
HMS2–SO3H to 25–77% range for A119 and 17–59% range for
Carb-SO3H pushing forward the great contribution of the siliceous
hydrophilicty on the reaction selectivity (Table 1). HFAU zeolite is
the less selective solid acid catalyst affording ester derivatives with
only 12–68% yield (Table 1). The greater selectivity obtained with
HMS2–SO3H by comparison to the siliceous HFAU is probably
due to an easier diffusion of reactants whithin the HMS2 porous
netwok (HFAU pore opening = 0.74 nm vs 2.3 nm for HMS2–
SO3H) and to the stronger acidity of sulfonic groups. These results
clearly evidenced that the mesoporous siliceous framework
Scheme 1 Physicochemical properties of prepared acid SIMCs.
HMS1-SO3H and HMS2–SO3H differ each other from the initial
oganosilane/TEOS molar ratio (see Electronic Supplementary
Information{). Physico-chemical properties of these materials are
given in Scheme 1.
To our delight, mesoporous SBA-SO3H and HMS1–SO3H
materials with pore diameters of 3.5 nm and 2.8 nm respectively
appeared to be much more selective than homogeneous PTSA
affording more than 81% yield into the desired amphiphilic
monomer (Fig. 1). As we expected, in neat glycerol, the great
affinity of the silica framework for this hydrophilic triol allowed a
rapid desorption of the monoester from the catalyst surface
limiting thus the formation of polymers as it was the case with
PTSA. After total consumption of the starting carboxylic acid, the
monomer yield drops due to intermolecular transesterification
between two monomer units (Fig. 1). Using mesoporous silica
Table 1 selective esterification of various polyfunctional carboxylic acids over HMS2–SO3H
Acid
Catalyst
Product
Time/h
% Conversion
% Yield
% Selectivity
HMS2–SO3H
A119d
15
18
25
28
99
97
95
90
91
25
12
17
92
26
13
19
HFAUe
Carb-SO3Hf
HMS2–SO3H
A119d
4
4
6
26
94
93
90
80
92
77
68
59
98
83
75
75
HFAUe
Carb-SO3Hf
HMS2–SO3H
A119d
6
7
5
7.5
100
100
95
86
44
25
59
86
44
26
59
HFAUe
Carb-SO3Hf
99
HMS2–SO3H
A119d
6
11
15
80
80
83
77
62
39
99
77
47
Carb-SO3Hf
a
b
c
Carboxylic acid (1 mmol), glycerol (6 mmol), 110 uC, 2.5 mol% of supported sulfonic groups. Carried out at 70 uC. In the presence of
10 mol% of cetyltritylammonium bromide as a phase transfer agent. Cation exchange resin, H+ exchange capacity = 4.2 mmol/g. Zeolite
d
e
HFAU with total framework Si/Al ratio of 20, 10 wt%. H+ exchange capacity = 0.3 mmol g21
.
f
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rapid desorption of the reaction products from the catalyst surface
inhibiting thus polymerization reactions. Moreover, in the
particular case of mesoporous acid SIMCs with pore diameter of
2.3 nm, a synergystic effect between the steric hindrance of the
porous network and the siliceous hydrophilicity is observed
affording acid solid catalyst able to reach unprecedented
selectivity.
Authors thank the french Ministry of Research and the CNRS
for their financial support and the postdoctoral grant for YG. AK
is also grateful to the Syrian government for his PhD grant.
Notes and references
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Scheme 2 Selective addition of maleic acid to dicylopentadiene over
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hydrophilicity of SIMCs can be favorably used to closely control
the selective and direct transformation of unprotected polyfunc-
tional carboxylic acids. Moreover, it is noteworthy that this
selectivity improvement was not to the detriment of the catalytic
activity as it is generally the case since HMS2–SO3H exhibits
similar and, even in some cases, higher catalytic activities than
other solid acid catalysts.
In order to demonstrate that the consideration of the silica
hydrophilicity of SIMCs was not only applicable to the selective
esterification of functional carboxylic acids with glycerol, we then
moved on to the reaction of dicyclopentadiene with maleic acid as
another model reaction. Direct and selective nucleophilic addition
of functional carboxylic acids to diene instead of alkyl halide is an
attractive salt free method for organic chemists especially for the
synthesis of monomer for polyester resins.13 When maleic acid was
heated at 105 uC in the presence of three times excess of
dicyclopentadiene and 2.5 mol% of PTSA, the maleic acid was
totally consumed in less than 40 min (Scheme 2). However, in these
conditions, the formation of the targeted 10-membered ring ester
was not produced and a rapid gelification of the reaction media
was observed suggesting a polymerization of reactants. When
PTSA was replaced by HMS2–SO3H (2.5 mol% of supported
sulfonic groups), the selectivity of the reaction was considerably
improved since, even using three times excess of dicyclopentadiene,
the starting maleic acid is totally converted into more than 85% of
monoester adduct in less than 1 h (Scheme 2).
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In conclusion we report here that acid mesoporous SIMCs are
more selective than homogeneous or usual solid acid catalysts for
the direct transformation of unprotected polyfunctional substrates.
This remarkable selectivity improvement is due to the natural
hydrophilic nature of their siliceous framework which allows a
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2224 | Chem. Commun., 2007, 2222–2224
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