Pore size control and organocatalytic properties of nanostructured silica hybrid materials containing amino and ammonium groups

Article abstract of DOI:10.1039/c1jm10422e

Periodic mesoporous organosilicas containing amine and ammonium substructures were synthesized via soft templating approaches using anionic surfactants. Pore size control was achieved either using anionic surfactants containing alkyl groups of variable chain lengths or by addition of swelling agents such as mesitylene (1,3,5-trimethylbenzene, TMB) to the hydrolysis-polycondensation mixture. The addition of mesitylene allows to increase the pore size in the materials from 2 to 6 nm. The materials appear as versatile and recyclable heterogeneous organocatalysts in Knoevenagel and Henry reactions and in the formation of monoglycerides by ring opening reaction of glycidol. This study highlights the huge potential of silica hybrid materials containing ionic substructures (i-silica) materials in heterogeneous catalysis.

Full text of DOI:10.1039/c1jm10422e

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PAPER
Pore size control and organocatalytic properties of nanostructured silica
hybrid materials containing amino and ammonium groups†
ab
a
a
b
a
Samir El Hankari, Blanca Motos-P ꢀe rez, Peter Hesemann,* Ahmed Bouhaouss and Jo €e l J. E. Moreau
Received 27th January 2011, Accepted 9th March 2011
DOI: 10.1039/c1jm10422e
Periodic mesoporous organosilicas containing amine and ammonium substructures were synthesized
via soft templating approaches using anionic surfactants. Pore size control was achieved either using
anionic surfactants containing alkyl groups of variable chain lengths or by addition of swelling agents
such as mesitylene (1,3,5-trimethylbenzene, TMB) to the hydrolysis–polycondensation mixture. The
addition of mesitylene allows to increase the pore size in the materials from 2 to 6 nm. The materials
appear as versatile and recyclable heterogeneous organocatalysts in Knoevenagel and Henry reactions
and in the formation of monoglycerides by ring opening reaction of glycidol. This study highlights the
huge potential of silica hybrid materials containing ionic substructures (i-silica) materials in
heterogeneous catalysis.
6
1
. Introduction
mesoporous MCM-41 molecular sieve from 1.6 to 3.2 nm.
Another strategy to tune the pore size in template directed
hydrolysis–polycondensation procedures consists of the addition
of organic solvent to the reaction mixtures. The addition of
swellers such as mesitylene (1,3,5-trimethylbenzene, TMB)
increases the size of the micelles in the hydrolysis–poly-
condensation mixtures. This strategy allows to control the pore
size in mesoporous materials over a larger range. For MCM-41
Soft templating approaches are largely used for the generation of
nanostructured metal oxides and in particular in the field of the
1
synthesis of mesoporous nanostructured silica materials.
Conventional soft templating routes to nanostructured silica
involve hydrolysis–polycondensation procedures of molecular
silica network formers such as tetraethyl-ortho-silicate (TEOS) in
2
3,4
the presence of cationic or neutral structure directing agents.
7
type materials, obtained in the presence of cationic surfactants,
These reactions give rise to the formation of mesoporous and
nanostructured silica phases which are characterized by high
pore diameters up to 12 nm were achieved. For SBA-15 type
materials, obtained with non-ionic structure directing agents
2
surface areas (up to 1500 m g ), narrow pore size distributions
ꢀ1
such as Pluronicª P123, average pore diameters as large as
5
and regular pore arrangements on a mesoscopic length scale.
3
3
0 nm were observed.
Pore size control in soft templating approaches was intensively
studied. The simplest way to tune the pore size in micelle tem-
plated silica consists of using surfactants containing alkyl chains
with variable chain length. However, this strategy afforded pore
size control over a rather small range on a supermicroscopic or
small mesoscopic length scale. For example, increasing the alkyl
chain lengths of cationic trimethyl-alkyl ammonium templates
8–10
Periodic mesoporous organosilicas (PMOs)
recently
emerged as a new class of versatile functional materials. PMOs
are characterized by a homogeneous distribution of organic
groups throughout the whole hybrid material on a molecular
level and display regular architectures on a mesoscopic length
11–13
scale.
PMO type materials are typically synthesized by
template directed hydrolysis–polycondensation of bridged bis-
14
from C
8
to C16 increases the BJH pore diameters of the resulting
0
trialkoxysilylated precursor molecules [(R O) Si–R–Si(OR’) ].
3
3
This approach gives rise to the formation of a whole set of
a
functional organic–inorganic hybrid materials containing e.g.
20,21
Institut Charles Gerhardt de Montpellier, UMR CNRS 5253
CNRS-UM2-ENSCM-UM1, Ecole Nationale Sup eꢀ rieure de Chimie de
Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 05,
France. E-mail: peter.hesemann@enscm.fr; Fax: +33 4.67.14.43.53; Tel:
1
5–19
chiral fragments,
p-conjugated segments
entities. These materials found wide applications in catal-
or chelating
22
23,24
25
sorption and optics.
20,21,26
ysis,
Bifunctional PMOs synthe-
+
b
33 4.67.14.72.17
sized from two or more organic precursor molecules have also
been reported. The incorporation of multiple bridging groups
Laboratoire de Chimie Physique G eꢀ n eꢀ rale
I
des Mat eꢀ riaux,
Nanomat eꢀ riaux et Environnement, Universit eꢀ Mohammed V—Agdal,
Facult eꢀ des Sciences, Avenue Ibn Battouta, Rabat, Maroc
allows to fine tune the surface properties of PMOs and to achieve
27–33
†
Electronic supplementary information (ESI) available: SEM-images of
desired functionality and selectivity.
Pore size control in
materials A16A18, 1/5, 1/10; nitrogen sorption isotherms of materials
A12, A14, 1/5, 1/20, 1/40; plot of the pore–pore distance in the
materials vs. molar TMB/surfactant ratio. See DOI: 10.1039/c1jm10422e
PMO type materials can be achieved following similar strategies
34
as reported for mesoporous silicas. For example, the addition
6
948 | J. Mater. Chem., 2011, 21, 6948–6955
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of TMB to hydrolysis–polycondensation mixtures allowed to
expand the pore size diameters in ethane and ethenylene bridged
35,36
PMO type materials up to 20 nm.
The use of ionic precursors in template directed hydrolysis–
polycondensation procedures gives rise to the formation of
PMO-type materials bearing ionic groups (i-silica). These
Scheme 2 Tris(3-(trimethoxysilyl)propyl)amine precursor 1.
37–39
40
materials have high potential in catalysis,
separation and
We particularly focus on the synthesis of
41,42
electro-optics.
surfactants. The synthesis of nanostructured i-silica in the pres-
ence of sodium cetyl stearyl sulfate systematically afforded
materials displaying pore diameters in the supermicroscopic/
materials with original architectures on a mesoscopic length
scale. We especially addressed the question of whether the ionic
nature of the precursors influences the architecture of the
resulting i-silicas formed in template directed hydrolysis–poly-
condensation procedures and may result in the formation of
original materials, which are inaccessible from conventional
neutral precursors. In this field, we recently reported nano-
ꢁ
mesoscopic length scale (20–22 A). Here, we report strategies to
tune the pore size in these materials. Secondly, we monitored the
catalytic potential of these original materials in various orga-
nocatalytic reactions: Knoevenagel condensation, Henry
reactions and the monoglyceride synthesis by ring opening of
glycidol.
4
3,44
structured silica hybrid materials bearing imidazolium
and
45
ammonium halides, but also surface functionalized silicas
46
containing guanidinium sulfonimide ion pairs. We observed
that highly structured and porous silica hybrid materials are
formed from silylated cationic precursors via soft templating
approaches using long chain substituted sulfates as structure
directing agents. This study points to an unusual mechanism for
the generation of nanostructured silica hybrid materials, as
anionic surfactants are usually inappropriate structure directing
agents for the formation of nanostructured silica or silica hybrid
materials. We attributed the formation of structured i-silica
phases to ionic interactions between the anionic head groups of
the sulfate surfactants and ‘organo-cationic’ substructures of the
silylated hybrid precursors (Scheme 1).
2. Experimental
2
.1 General details
The precursor tris(3-(trimethoxysilyl)propyl)amine
1
4
was
5
synthesized following a previously described protocol. The
anionic sulfate surfactants sodium dodecyl sulfate (SDS), sodium
tetradecylsulphate (STS) and sodium cetyl stearyl sulfate (60%
sodium hexadecylsulfate/40% sodium octadecyl-sulfate) were
1
13
purchased from ABCR. H and C spectra in solution were
recorded on Bruker AC 250 or Bruker Avance 400 spectrometers
at room temperature. Deuterated chloroform was used as solvent
for liquid NMR experiments and chemical shifts are reported as
d values in parts per million relative to tetramethylsilane.
Nitrogen sorption isotherms were obtained at 77 K with
a Micromeritics ASAP 2020 apparatus. Prior to measurement,
In the field of the synthesis of functional PMO type materials,
our finding is of particular interest as it represents the first
example for the formation of nanostructured silica hybrid
materials by interactions involving the organic bridging unit of
silylated precursors. These interactions are specific for positively
charged precursors and allow to access original nanostructured
silica hybrid materials which are not accessible following stan-
ꢁ
the samples were degassed for 18 hours at 100 C. The surface
areas (SBET) were determined from BET treatment in appro-
priate p/p ranges and assuming a surface coverage of nitrogen
0
44,45
ꢁ
2
dard procedures.
In this context, Yokoi and Che already
molecule estimated to be 13.5 A . Pore size distributions were
calculated from the adsorption branch of the isotherms using the
BJH method. The pore width was estimated at the maximum of
the pore size distribution. XRD experiments were carried out
with a Xpert-Pro (PanAnalytical) diffractometer equipped with
a fast X’celerator detector using Cu-Ka radiation.
reported the so-called anionic surfactant templated mesoporous
47–51
silica (AMS),
synthesized by co-condensation reactions of
silylated ammonium precursors with silica network formers
48,49
(
TEOS). In this way, highly structured silica
and chiral
50,51
mesoporous silica
were obtained.
Here, we focused on two different aspects concerning the
processing of cationic silylated precursors and the synthesis of
nanostructured silica hybrid materials containing amine and
ammonium groups obtained from the trisilylated amino
precursor 1 (Scheme 2). Firstly, we studied the morphologies of
the materials formed via soft-templating routes using anionic
2
.2 Materials synthesis
Sodium cetyl stearyl sulfate (266 mg, 0.77 mmol) was dissolved
ꢁ
under stirring at 60 C in a solution prepared from deionized
water (17.9 g) and 2 ml of 1 M hydrochloric acid. To this solu-
tion, variable quantities x mmol of mesitylene were added (x ¼
0
.77, 3.85, 7.7, 15.4, 30.8, 61.6).
After the mixture has been stirred for 1 h, a solution of the
amine precursor 1 (1 mmol, 0.5 g), dissolved in a mixture con-
taining 1 g of water and 0.5 g of ethanol, was rapidly added to the
surfactant solution. The resulting mixture was vigorously stirred
ꢁ
at 60 C for 20 min. After this time, the mixture was heated at
ꢁ
7
0 C under static condition for 72 h. The resulting solid prod-
ꢁ
ucts were recovered by filtration and dried under air at 70 C for
15 h. The surfactant was removed by washing 0.5 g of the solid
with a solution of 100 ml ethanol/5 ml conc. hydrochloric acid or
Scheme 1 Sol–gel processing of cationic precursors using anionic
structure directing agents (from ref. 45).
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1
00 ml ethanol/5 ml NH Cl and NH (25%) to give silica hybrid
4
Table 1 Surface properties of the materials
3
materials containing ammonium or amine groups after drying.
Average
pore
diameter /A
Mesopore
volume /
c
S
m g
BET
/
2
ꢀ1
a
ꢁ
3
cm g
ꢀ1
Material
Surfactant
2
.3 Organocatalytic tests
A12
A14
A16A18
A16A18-p
Material 1/1
Material 1/5
Material 1/10
Material 1/20
Material 1/40
Material 1/80
SDS
STS
SHS
SHS
SHS
SHS
SHS
SHS
SHS
SHS
210
—
—
<20
<20
ꢃ20
42
44
54
58
60
0.15
0.32
0.58
0.45
0.67
0.99
0.95
0.90
0.96
1.04
Knoevenagel condensation. Benzaldehyde (531 mg/5 mmol) was
b
b
b
b
790
1120
970
dissolved in 5 ml of the corresponding solvent. To this solution
were added 30 mg of the catalyst A16A18 and malononitrile
(
330 mg/5 mmol). The resulting reaction mixture was stirred at
ꢁ
1180
966
915
831
795
844
5
0 C for 30 min. After this time, the catalyst was filtered and the
1
filtrate was concentrated. The product was analysed by
H
NMR. The catalyst was washed with the corresponding solvent,
dried and re-used.
a
Estimated at the maximum of the BJH pore size distribution obtained
b
Henry reactions. The reactions were carried out in the presence
of different p-substituted benzaldehydes solved in nitroethane by
using either the material A16A18 or A16A18-p as catalysts. In
a typical procedure, to a solution of the aldehyde (1 equiv.) in
nitroethane (20 equiv.) was added the catalyst (0.15 equiv.,
from the adsorption branch of the isotherm.
Determined at
values >0.05 and <0.15. Determined from adsorption isotherm
from the N -quantity at the end of the capillary condensation.
c
p/p
0
2
5
.88 mmol organic precursorper gram material, obtained from
presence of sodium cetyl stearyl sulfate yielded the highly
TGA). The same molar ratio was kept when ammonium salts
were used as catalysts. The mixture was stirred, under air, at
room temperature for the appropriate time. After that, the
material was filtered off, washed with ethanol (3 ꢂ 10 ml) and
structured supermicroporous material A16A18 showing
2
a specific surface area SBET of 1120 m g and an average pore
ꢀ1
ꢁ
45
diameter of approx. 20 A. The related materials A16A18-p
showed similar characteristics but slightly reduced specific
surface area and pore volume. In a second set of experiments, we
studied hydrolysis–polycondensation reactions of precursor 1 in
the presence of defined amounts of 1,3,5-trimethylbenzene
(TMB). These reactions were carried out with sodium cetyl
stearyl sulfate and 0.77, 3.85, 7.7, 15.4, 30.8 and 61.6 eq. TMB in
the reaction mixtures with respect to precursor 1, giving rise to
the materials 1/1, 1/5, 1/10, 1/20, 1/40 and 1/80. Nitrogen sorption
experiments with these materials indicated a strong influence of
the TMB amount in the reaction mixture on the morphology of
the formed materials. The principal trends can be seen from the
shape of the nitrogen sorption isotherms of the materials
A16A18, 1/1, 1/10 and 1/80 shown in Fig. 1. The results are
summarized in Table 1. The comparison of the isotherms shows
slightly decreasing specific surface areas for increasing TMB
amounts in the reaction mixtures as indicated by lower nitrogen
ꢁ
dried at 75–80 C overnight (18 h) for its use in a posterior cycle.
1
Conversions and syn : anti ratios were assessed by H-NMR
analysis of aliquots of the crude reaction mixture, from which the
nitroethane had been previously evaporated under vacuum.
Monoglyceride synthesis by ring opening of glycidol. Lauric
acid (500 mg, 2.5 mmol) is added to a suspension of 200 mg of the
material A16A18 or A16A18-p in toluene (10 ml). Glycidol
(
200 mg, 2.6 mmol) was added and the reaction mixture was
heated to reflux during 3 h under vigorous stirring. After cooling,
the solid catalyst was separated by filtration and dried. The
filtrate was concentrated yielding a solid product which was
1
recrystallized from hexane and finally analyzed by H NMR
spectroscopy.
uptake at low pressures (p/p < 0.3) of materials 1/10 and 1/80
0
3
. Results and discussion
compared to the materials A16A18 and 1/1. Furthermore, the
In a first series of experiments, we used sulfate surfactants
bearing alkyl chains of variable length as structure directing
agents. Hydrolysis–polycondensation reactions were performed
from the amine precursor 1, water, 1 M hydrochloric acid and
anionic surfactant [sodium dodecyl sulfate (SDS), sodium tet-
radecyl sulfate (STS) or sodium cetyl stearyl sulfate (SHS)] in the
molar composition 1/1111/2/0.77. The materials A12, A14 and
A16A18 containing amine groups were obtained after surfactant
elimination in basic media (ethanol–ammonia). A related
protonated material A16A18-p was synthesized under similar
reaction conditions except the final washing step, carried out
under acidic conditions (ethanol–hydrochloric acid). The char-
acterization of these materials by nitrogen sorption showed that
a minimum length of the alkyl chain in the anionic surfactant is
necessary for the formation of porous phases. Strongly
increasing specific surface areas can be seen in the order A12–
A14–A16A18 (Table 1). Hydrolysis–polycondensation of
precursor 1 under the above mentioned conditions in the
Fig. 1 Nitrogen adsorption–desorption isotherms of materials A16A18,
1/1, 1/10 and 1/80. The isotherms of the other materials are omitted for
clarity and given in the ESI† (Fig. S1).
6
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N -uptake in the mesopore range progressively shifts to higher
2
pressures indicating increasing pore diameters in the materials
for increasing amounts of TMB.
These results are reflected by the BJH pore size distribution
curves of the materials, showing progressively increasing average
pore diameters for increasing amounts of added TMB in the
reaction mixtures. The materials show a progressive pore size
ꢁ
increase from approx. 20 A for materials A16A18, A16A18-p and
ꢁ
/1 up to 60 A for material 1/80 (Fig. 2). The pore size distri-
1
bution becomes larger when high amounts of TMB were added
to the hydrolysis–polycondensation mixture. Finally, the meso-
pore volume of the materials obtained in the presence of TMB is
relatively constant and considerably higher than the value
observed for the supermicroporous A16A18 material. It should
be noted that further increase of the TMB quantity did not result
in further expansion of the pores and only gave less structured
materials with reduced mesopore volumes.
Fig. 3 Small angle X-ray diffractograms of materials A12, A16A18 and
1
/5, 1/10 and 1/20, formed in the presence of various amounts of TMB.
scale was already suggested by a broader pore size distribution
(vide supra) and can be seen here by the broad shape of the (100)
reflections and the absence of reflections of higher order in the
materials A12, A14, 1/5, 1/10, 1/20 and 1/40 (Fig. 3). The negative
effect of TMB addition on long-range ordering in the resulting
materials has already been reported in the case of ethenylene-
The results of X-ray diffraction measurements (Table 2) also
reflect the morphological changes in the materials. The (100)
ꢁ
reflection progressively shifts from 2q angles of 2.9 (material
ꢁ
A12) to 0.850 (material 1/40) corresponding to pore–pore
ꢁ
ꢁ
distances of 30 A in the material A12 to more than 100 A in the
case of the material 1/40. The plot of the pore–pore distance,
obtained from the XRD measurements, vs. the molar TMB/
surfactant ratio in the hydrolysis–condensation mixture shows
35
bridged PMOs.
Characterization of the materials by scanning electron
microscopy (SEM) showed that all materials have very similar
morphology and consist of agglomerated particles of 100–
200 nm in diameter (Fig. 4).
ꢁ
an asymptotic shape with a maximum value of approx. 140 A
(
ESI†, Fig. S2). However, both the use of short chain surfactants
and the addition of TMB to the hydrolysis–polycondensation
mixture result in the formation of materials with lower structural
regularity and the generation of worm-like architectures. The
formation of materials with lower regularity on a mesoscopic
4
. Organocatalysis
PMO type materials found widespread applications in hetero-
23
geneous catalysis. Here, we focus in particular on metal-free
catalytic transformations. We tested the amine containing PMO
material A16A18 as heterogeneous organocatalyst in three
different reactions: Knoevenagel condensations, Henry reactions
and the formation of monoglycerides by ring opening of glycidol.
Knoevenagel condensations
Knoevenagel reactions are often applied to monitor the catalytic
52–57
potential of amine functionalized silica.
In a preliminary
study, we screened the catalytic properties of the amine func-
tionalized PMO material A16A18 as a heterogeneous catalyst for
Knoevenagel condensations of benzaldehyde and malononitrile
(
Scheme 3), affording benzylidenemalononitrile as the sole
product. The results are summarized in Table 3.
Fig. 2 BJH pore size distribution of selected materials, obtained from
the adsorption branch of the isotherm.
Table 2 Results from XRD measurements
ꢁ
ꢁ
Pore–pore distance/A
Material
d100/2q value
d
100/A
A12
A14
A16A18
Material 1/5
Material 1/10
Material 1/20
Material 1/40
2.9
2.5
30
35
37
72
78
93
35
40
43
83.3
90.8
107
120
2.377
1.224
1.123
0.952
0.850
104
Fig. 4 SEM-image of material 1/5.
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Table 4 Conversions in Henry reactions of p-nitrobenzaldehyde and
nitroethane using the materials A16A18 and A16A18-p as heterogeneous
catalysts
Entry Catalyst
Time/h Conversion (%) Syn/anti ratio
Scheme 3 Knoevenagel condensation.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
None
SBA-15
48
4
92
72
1
4
4
4
4
4
4
4
4
0
0
18
0
85
—
—
43 : 57
—
+
MeN Et
ꢀ
a
3
Cl
ꢀ
Table 3 Conversions in Knoevenagel reactions of benzaldehyde and
malononitrile using the material A16A18 as heterogeneous catalyst
+
HN Et
3
Cl
NEt
A16A18
nd
3
61
57 : 43
56 : 44
58 : 42
47 : 53
48 : 52
45 : 55
40 : 60
40 : 60
66 : 34
52 : 48
97%
67%
>99%
>99%
>99%
47
Conversion
(%)
Conversion
(%)
A16A18 2 run
Entry Solvent
Entry Solvent
rd
A16A18 3 run
th
A16A18 4 run
a
1
2
3
4
5
6
EtOH
EtOH
MeOH
EtOH
THF
0
0
100
97
79
66
7
8
9
10
11
12
CHCl
CH Cl
2 2
nd
3
76
61
th
A16A18 5 run
0
1
2
3
4
b
c
A16A18-p
A16A18-p 2 run
A16A18
A16A18
EtOH, 2 run 96
rd
EtOH, 3 run 95
nd
34
77
33
b
th
EtOH, 4 run 94
c
4
th
EtOH, 5 run 95
CH
3
CN
a
ꢁ
b
Reaction temperature: 45 C. Reaction carried out with
p-bromobenzaldehyde. Reaction carried out with p-anisaldehyde.
a
b
Reaction without catalyst. Reaction carried out in the presence of
c
c
SBA-15 type mesoporous silica. Reaction carried out under reflux of
CH Cl
2
2
.
61
nitroalcohol in 85% yield with a syn/anti ratio of 57 : 43.
As already reported for other heterogeneous base catalyst
systems, polar protic solvents such as methanol or ethanol gave
the highest conversion rates in Knoevenagel reactions (Table 1,
entry 3,4) whereas polar aprotic solvents gave lower conversions.
Recycling experiments showed unchanged catalytic activities of
material A16A18 in up to 5 runs (entries 9–12). The results of this
preliminary study highlight the potential of these new amine
functionalized PMO materials in heterogeneous catalysis and
prompted us to study two other more ambitious test reactions.
Following these results, we tested out the amine containing
material A16A18 as a heterogeneous catalyst for Henry reac-
tions. This material showed high catalytic activity in Henry
reactions and led to the corresponding b-nitroalcohol in 97%
yield after 4 h in a first reaction cycle (entry 6). Interestingly, the
obtained product showed a syn/anti ratio of 56 : 44 which is in
nice agreement with the result obtained with free triethylamine
(
entry 5). Following these results, we tested the recycling of the
heterogeneous A16A18 catalyst. We observed a decrease of the
catalytic activity in the second reaction cycle, but a new increase
rd
of the conversion starting from the 3 cycle, giving the reaction
Henry reactions
rd
th
th
product in nearly quantitative yield in the 3 , 4 and 5 cycle.
The regioselectivity observed in the recycling experiments
displays another tendency. Whereas the syn/anti ratio in the first
reaction cycle (56 : 44, entry 6) is in nice agreement with the
result obtained with triethylamine (57 : 43, entry 5), this ratio
progressively inverts in the following cycles, and finally shows
The Henry Reaction is a base-catalyzed C–C bond-forming
reaction between nitroalkanes and aldehydes or ketones
(
Scheme 4) to yield b-nitroalcohols which are valuable synthetic
building blocks for the synthesis of pharmacologically important
58
building blocks. Amine functionalized silicas are efficient
th
promoters of Henry reactions as recently shown by Asefa et al.
a value of 45 : 55 in the 5 cycle (entry 10). These results may
5
9,60
and Wang and Shantz.
Henry reaction of benzaldehyde with nitroethane (Scheme 4, R ¼
CH ). The results of these experiments are summarized in
Table 4.
Here, we studied in particular the
indicate that the catalytic site undergoes some changes during the
cycles.
61,62
3
We therefore studied the protonated material A16A18-p as
a heterogeneous catalyst for the Henry reaction. This material
was obtained in a similar way as the material A16A18, but the
template elimination of the parent material was carried out under
acidic conditions. This material therefore contains protonated
trialkylammonium substructures. The evaluation of the catalytic
properties of the material A16A18-p showed reduced catalytic
activity with conversions of 47 and 34% in the first and second
reaction cycle, respectively. In both reactions, the formed
b-nitroalcohol showed an identical syn/anti ratio of 40 : 60
Preliminary experiments showed that neither mesoporous
SBA-15 type silica nor ammonium salts efficiently promote
Henry reactions of p-nitrobenzaldehyde and nitroethane. Only
the use of methyl triethylammonium chloride showed low
conversion and led to the formation of the desired product after
long reaction time and heating (entry 3). In contrast, the reaction
is efficiently catalyzed by tertiary amines. The reaction in the
presence of triethylamine yields the corresponding b-
(
entries 11/12).
These whole results suggest that the changes of the catalytic
activity and diastereoselecitvity observed in the recycling exper-
iments of material A16A18 may be due to a partial protonation
of the immobilized amine sites. It has to be mentioned that the
recycled material A16A18 shows a higher catalytic activity
compared to triethylamine on the one side, but also compared to
Scheme 4 Henry reaction.
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952 | J. Mater. Chem., 2011, 21, 6948–6955
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the freshly synthesized materials A16A18 and A16A18-p. These
results suggest some synergistic effects in the materials involving
both immobilized amine and ammonium substructures and the
silica hybrid network. These synergistic effects may be respon-
sible for the enhanced catalytic activity of re-used materials
together with the inversion of the regioselectivity we observed in
the recycling experiments of material A16A18.
Scheme 5 Monoglyceride synthesis by ring opening of glycidol.
Finally, it has to be mentioned that the conditions of the Henry
reaction affected neither the chemical integrity of the immobi-
lized amine substructures nor the surface properties of the
materials. We characterized the material A16A18 after 5 Henry
Table 5 Conversions in ring opening reactions of glycidol and lauric
acid using the materials A16A18 and A16A18-p as heterogeneous
catalysts
13
29
reaction cycles. C and Si solid state NMR spectra (see ESI†)
of the used material are nearly unchanged compared to those of
the material before use. Furthermore, the characterization of the
used material via nitrogen sorption shows a similar shape of the
adsorption–desorption isotherms of the material A16A18 before
use and after 5 reaction cycles indicating that the architecture did
not change significantly (Fig. 5). The slight decrease of the
Conversion
(%)
Conversion
(%)
Entry
Catalyst
Entry
Solvent
1
2
None
A16A18
<1
65
5
6
A16A18-p
A16A18-p
80
72
nd
2
A16A18-p
run
3
4
5
A16A18
nd
61
58
53
7
8
58
65
rd
2
A16A18
run
3
A16A18-p
run
2
ꢀ1
2
ꢀ1
specific surface area from 1120 m g to 790 m g can be
explained by the repeated mechanical stirring during the reac-
tions and successive filtration steps.
3
rd run
A16A18
th run
4
th
4
run
We also examined the use of other substituted aldehydes as
substrates in Henry reactions. The use of p-bromobenzaldehyde
and p-anisaldehyde gave the corresponding b-nitroalcohols in
reported to efficiently promote the formation of monoglycerides
63,64
7
7 and 33% conversion (entries 13/14). These results show that
by ring opening reactions of glycidol.
aromatic aldehydes functionalized with electron withdrawing
groups are substrates with higher reactivity. Here, p-nitro-
benzaldehyde was found to be the most reactive aldehyde in this
series. The substitution of the aromatic ring also affects the syn/
anti ratio of the formed b-nitroalcohols. However, the syn
diastereoisomer is always preferentially formed in the first reac-
tion cycles using material A16A18.
Here, we examined the catalytic activity of the amine
containing material A16A18 and the ammonium containing
material A16A18-p in this reaction. The results of these experi-
ments are summarized in Table 5. It clearly appears that both
amine and ammonium functionalized materials promote the
reaction and led to lauryl monoglyceride in 65 and 80% yield in
the first reaction cycle, respectively. Both materials can be
re-used in further reaction cycles but show decreasing catalytic
performances with yields of 53% (A16A18) and 65% (A16A18-p)
Ring opening of glycidol
th
after the 4 re-use. Globally, the ammonium functionalized
material A16A18-p shows a slightly higher catalytic activity
compared to the amine functionalized material A16A18.
Because of their emulsifying, complexing and lubricating prop-
erties, monoglycerides found applications in food, cosmetic,
pharmaceutical and textile industries. The ring opening reaction
of glycidol with fatty acids (Scheme 5) is the most convenient
route for the selective synthesis of monoglycerides. This reaction
can be catalyzed by amines and ammonium salts, metal alco-
holates or bases. Both silica supported acids and bases have been
The characterization of the recovered materials by solid state
NMR and nitrogen sorption shows that the ring opening reac-
tion of glycidol with fatty acids strongly affects the texture of the
materials. After catalysis, the recovered materials show a
2
ꢀ1
strongly reduced specific surface area (A16A18: SBET < 8 m g ;
2
ꢀ1
A16A18-p: SBET ¼ 92 m g ). Furthermore, the shape of the
nitrogen sorption isotherms of the recovered materials indicates
that the porous texture collapsed during the successive reaction
cycles. This result is supported by X-ray diffraction. The dif-
fractograms of the materials after use as heterogeneous catalysts
13
indicate reduced order in the recovered materials. C CP-MAS
solid state NMR measurements with the used material reveal
some degradation of the amine and ammonium substructures
under the conditions of the ring opening reaction. However, no
Si–C bond cleavage occurs during the reactions as indicated by
the complete absence of signals of the Q series in the
29
Si CP-MAS spectra of the materials after use. The nitrogen
sorption isotherms, X-ray diffractograms and solid state NMR
spectra of the recovered materials A16A18 and A16A18-p are
given in the ESI†.
Fig. 5 Nitrogen adsorption–desorption isotherms of material A16A18
before (-) and after 5 Henry-reaction cycles (ꢂ).
These results show that the ammonium functionalized mate-
rials show limited textural and chemical stability under the
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conditions of ring-opening reactions. The decreasing catalytic
activity of the materials in successive reaction cycles can thus be
explained by a collapsed pore structure of the materials and
slight chemical degradation of the amine and ammonium
substructures. Nevertheless, the new materials are still among the
most efficient heterogeneous catalyst systems for these
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Acknowledgements
Samir El Hankari and Peter Hesemann thank the program
AVERROES (Erasmus Mundus Programme of the European
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