Janus Interphase Organic-Inorganic Hybrid Materials: Novel Water-Friendly Heterogeneous Catalysts

Article abstract of DOI:10.1002/cctc.201900147

In this work, amphiphilic silica-based Janus-type acid and base catalysts are introduced as heterogeneous analogues of surfactants. The new interphase organic-inorganic hybrid catalysts comprise of two different groups (?C₈H₁₇, ?C

Full text of DOI:10.1002/cctc.201900147

Accepted Article
Title: Janus Interphase Organic-Inorganic Hybrid Materials: Novel
Water-Friendly Heterogeneous Catalysts
Authors: Majid Vafaeezadeh, Paul Breuninger, Philipp Lösch, Christian
Wilhelm, Stefan Ernst, Sergiy Antonyuk, and Werner R. Thiel
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ChemCatChem
10.1002/cctc.201900147
FULL PAPER
Janus Interphase Organic-Inorganic Hybrid Materials: Novel
Water-Friendly Heterogeneous Catalysts
[a]
[b]
[b]
[c]
[c]
Majid Vafaeezadeh, * Paul Breuninger, Philipp Lösch, Christian Wilhelm, Stefan Ernst, Sergiy
[b]
[a]
Antonyuk, Werner R. Thiel, *
In memory of Prof. Dr.-Ing. Stefan Ernst who died on Jan. 28, 2019.
Abstract: In this work, amphiphilic silica-based Janus-type acid and
base catalysts are introduced as heterogeneous analogues of surfac-
tants. The new interphase organic-inorganic hybrid catalysts com-
compounds in/with water. In the past, some strategies to over-
come this limitation have been worked out such as using water-
[
4]
miscible organic co-solvents
(e.g. acetone, DMF and short-
[5]
8 2 4
prise of two different groups (-C H17, -C H Ph) on the hydrophobic and
chained alcohols, etc.), surfactants or phase transfer catalysts
[6]
two different groups (-C SO H, -C
3
3
H
6
H NH ) on the hydrophilic side.
3 6 2
(PTCs).
The hydrophobic sides of the catalysts are responsible for the dif-
fusion of the organic substrates to the catalytically active sites while
the hydrophilic sides carry catalytic centers being oriented towards
the aqueous phase and thus providing sufficient connection between
water (solvent) and the organic substrates. Catalyst preparation is
based on simple synthetic procedures and leads in a first step to
hollow silica spheres with hydrophobic groups on the inner and hydro-
philic groups on the outer surface. After crushing the hollow spheres,
acid resp. base functionalized Janus nano-sheets are obtained. The
basic catalyst was tested for the Knoevenagel condensation of a
series of benzaldehydes with malononitrile in water, while the acid
catalyst was used for Fischer esterification of acetic acid with ethanol.
When heterogeneous catalysts are to be used for organic reac-
tions in water, the situation becomes even more complicated
since the heterogeneous catalyst provides a third (solid) phase in
addition to water and the organic reactants as the two liquid
phases. Desirable yields can only be achieved by sufficiently high
rates of mass transport between these three phases. Conse-
quently, such transformations often suffer from long reaction
times.
To improve the efficiency of organic reactions in water, some
modifications on heterogeneous catalysts have been carried out
[
7]
[8]
in the past: Hydrophobic and hydrophilic groups, ionic liquids
[10]
[
9]
and fluorous reverse-phase modified materials
have been
introduced onto the surface of heterogeneous catalysts. Although
these modifications could in principle improve the activities com-
pared to un-modified catalysts, a good miscibility of the two liquid
phases could in most cases not be realized. In spite of significant
achievements, the applied protocols have some further limitations
such as difficulties in the synthesis of the catalysts, expensive
starting materials and supports as well as irreversible leaching of
the immobilized groups from the surface of the catalysts or their
decomposition.
Introduction
Performing (catalytic) organic reactions in water is one of the
central goals of green chemistry, since water, is a safe and cheap
[1]
solvent and has no negative environmental impacts. Although
[2]
some organic transformations such as Diels-Alder reactions or
the Claisen rearrangement of hydrophobic reactants are facilit-
Janus-type materials are versatile compounds that comprise two
different faces or in some cases even two antithetic sides. They
have gained widespread applications in reactions that occur at
[3]
ated in aqueous environments, for most reactions the rates are
low due to the poor solubility/miscibility of hydrophobic organic
[11]
water/organic solvent interfaces. To address this issue some
typical examples shall be discussed shortly at this point: A Janus-
type, cobalt-based material has e.g. been designed as catalyst for
[a]
[b]
[c]
Dr. Majid Vafaeezadeh, Prof. Dr. Werner R. Thiel
Fachbereich Chemie, Anorganische Chemie
Technische Universität Kaiserslautern
Erwin-Schrödinger-Str. 54
[12]
the electro-splitting of water.
Other groups have investigated
[13]
applications as acid-base bifunctional catalysts, for ultra-deep
[15]
[14]
desulfurization reactions, for dye degradation,
[
for aqueous
[17]
and as micro- and nanomotors.
67663 Kaiserslautern, Germany
16]
hydrogenation reactions,
E-mail: thiel@chemie.uni-kl.de
P. Breuninger, Philipp Lösch, Prof. Dr. Ing. Sergiy Antonyuk
Fachbereich Maschinenbau und Verfahrenstechnik
Mechanische Verfahrenstechnik
Several methods have been developed and reviewed for the
synthesis of Janus-type materials as well as Janus microsphe-
[18]
res/capsules.
Technische Universität Kaiserslautern
Gottlieb-Daimler-Str. 44
For catalytic applications, the preparation of Janus-type materials
with different organic functionalities on both sides is of interest.
However, compared to classical silica-based catalysts with cataly-
tically active sites immobilized on the support’s surface via a link-
67663 Kaiserslautern
Dr. Christian Wilhelm, Prof. Dr.-Ing. Stefan Ernst
Fachbereich Chemie, Technische Chemie
Technische Universität Kaiserslautern
Erwin-Schrödinger-Str. 54
[
19]
ing chain,
the specific decoration of the Janus materials is
6
7663 Kaiserslautern, Germany
harder to achieve, since the introduction of the different functional
groups on different sides has to be controlled thoroughly. Accord-
ing to this challenge, most of the reported methods turn out not to
be easily applicable for a large-scale synthesis of such materials.
Supporting information for this article is given via a link at the end of
the document.
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Recently, inorganic Janus-type nano-sheets possessing hydro-
philic propyl amines and hydrophobic phenyl groups were synthe-
sized by Yang and coworkers by employing an oil-droplet
[
18e]
method.
Hereby, an oil/silica core/shell structure forms by a
self-assembling sol-gel process. When the alkoxysilane precur-
sors are added to an oil/water emulsion at pH 2.5, the amino
functionalized trialkoxysilane becomes protonated and can there-
fore only condensate from the water side of the droplets while the
non-polar phenyl functionalized trialkoxysilane will go inside the
oil droplet and condense from the inside of the shell. This material
shows excellent emulsifying properties for some immiscible
liquids. However, this method suffers from the fact that by
changing the ratio of the substrates, the desired morphology of
[
18c]
the material is destroyed.
Moreover, some of the starting
[
14,20]
materials are not commercially available.
The same protocol
was employed for the synthesis of a heteropolyacid-based Janus-
type material for application in catalytic oxidative desulfurization
[14]
reactions.
Here, we wish to present and discuss a new strategy for the
synthesis of interphase Janus-type nano-sheet catalysts. In our
method, hollow silica spheres with a diameter of 1-2 µm are
prepared and decorated differently with non-polar and polar
functions on their inner resp. outer surface. After crushing the
shells, the surface functionalities are oriented on opposite sides
of the resulting nano-sheets.
was carried out by treatment with H
2
O
2
. In the last step of the pro-
Results and Discussion
cess, the fully decorated shell-type particles 4a,b were crushed in
ball-milling apparatus leading to catalysts 5a
Ph/-C NH and 5b (-C SO H). Elemental
analyses (CHNS) of these materials revealed that the loading of
a
(
-C
2
H
4
3
H
6
2
)
8
H17/-C
3
H
6
3
Material synthesis and characterization
-1
amine groups in catalyst 5a is at about 1.52 mmol g and the
A schematic illustration summarizing the catalyst preparation is
shown in Figure 1. In the first step, hollow silica spheres with an
hydrophobic inner surface were created by employing a modified
Figure 1. Schematic illustration of the catalyst preparation using a modified
Stöber method.
[21]
synthesis of Stöber-type silica particles. For this, the initial mix-
ture was prepared by adding n-decane, CTAB and aqueous am-
monia to water under ultrasonication resulting in the formation of
an emulsion. Then TEOS and the non-polar organotriethoxysilane
precursor were added to the reaction mixture, which was stirred
for 24 h. In the two-phase mixture of water and n-decane, the
long-chained organotriethoxysilane is dispersed exclusively in the
organic phase leading to a non-polar decoration of the inner sur-
face of the shell. After separation of the solid materials 1a,b by
centrifugation they were washed with water and ethanol.
loading of sulfonic acid sites in catalyst 5b is at about 0.79 mmol
-1
g . The proton equivalent of 5b was measured and found to be
+
0.81 mmol H /g.
FT-IR spectra of catalysts 5a and 5b are shown in Figure 1S in
the Supporting Information. In the spectrum of catalyst 5a, the
stretching frequencies of the aliphatic and aromatic C–H bands
–
1
are observed at 2933 and 3027 cm , respectively. A further
–
1
absorption at 1665 cm belongs to C=C stretching frequency of
–
the phenyl group and an absorption at 1602 cm to the N–H
1
To remove the surfactant, the materials were heated to 60 °C in a
solution of 1 wght.-% of conc. HCl in ethanol leading to 2a,b. In
addition, this procedure strengthens the materials mechanically
by triggering further condensation in the framework while the mor-
phologies of the materials do not change significantly. A further
treatment with 6 M HCl was carried out to increase the loading of
hydroxyl (OH) groups on the outer surface (materials 3a,b). This
allows for a significantly increase of the loading of organic groups
on the surface of the silica materials in the following step. After
washing with water and ethanol and drying, the materials were
ready for the decoration of the outer surface of the shells. To
achieve this, the materials were treated with 3-aminopropyl(tri-
methoxy)silane (APTMS) resp. 3-thiopropyl(trimethoxy)silane
bending vibration of the amino group. For catalyst 5b, only a C-H
–
absorption at 2926 cm is found, which corroborates to the
1
exclusive presence of aliphatic C–H bonds in the material. A
–
broad band centered at 3375 cm observed in the spectrum of
1
5
b can be assigned to the presence of water around the
[
22]
hydrophilic sulfonic acid head groups. The absorptions of the
S=O stretching modes are overlapping with the Si–O absorptions
of the silica backbone.
The morphologies of the Janus-type catalysts 5a and 5b and their
hollow shell precursors 4a and 4b were studied by scanning elec-
tron microscopy (SEM). SEM images A and B in Figure 2 show
typical sphere-like species for 4a and 4b with rather uniform dia-
meters of 1-2 µm and some surface deformation (yellow arrows).
Rare examples of broken hollow shells can be found in Figure 2A.
(
MPTMS). In order to obtain sulfonic acid sites on the shells, the
thiol groups of the latter material had finally to be oxidized which
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They allow determining the wall thickness of the spheres to be
about 150-300 nm, which explains their high thermal and
mechanical stability. The morphologies of catalysts 5a and 5b
after crushing the spheres in a ball-mill is shown in Figures 2C
and 2D. Although a thorough grinding of the hollow spheres 4a
Figure 2. SEM images of the hollow shell precursors 4a (A) and 4b (B) before
crushing and SEM images of catalysts 5a (C) and 5b (D). The arrows assign
some deformations on the surface of the hollow silica spheres (in A) as well as
the curved structure of the catalysts (in C and D).
of the mixture. By addition of 10 mg of catalyst 5a to such a mix-
ture and shaking it for 5 min, a stable yellow colored emulsion is
obtained. In contrast, doing the same experiment with catalyst 5b
leads to a stable red emulsion. These experiments qualitatively
show the basic resp. acidic nature of catalysts 5a and 5b as well
as their ability to form stable emulsions. It is noteworthy that the
and 4b was carried out, the curved structure of the Janus-type
nano-sheets is readily observable for most of the particles. The
mechanical strength of the catalyst is an important factor for its
thermal and chemical stabilities. SEM measurement (Figure 2A),
showed that, the wall thickness of the material is 288 nm. This is
significantly higher than the Janus materials which was prepared
by other methods. For comparison, the wall thickness of
[
14]
heteropolyacid-based Janus-type material
is around 60 nm.
Moreover, the wall thickness of a Janus material which was
prepared by the oil-droplet method reached to a maximum value
[
18e]
of 95 nm
while this value for a Janus polymer−inorganic
[
18b]
layered composites was found to be 57 nm.
This clearly
indicates that the current catalyst have higher stability during the
chemical reactions.
Results of the solid-state UV analyses of catalysts 5a and 5b are
shown in Figure 3A. Since only aliphatic groups have been im-
mobilized on the surface of catalyst 5b, there is no significant and
thus detectable absorption in the UV-Vis spectrum. In contrast,
catalyst 5a shows a typical absorption at around 262 nm which
can be assigned to the B-band of the phenyl rings in the material.
The amphiphilic nature of the catalysts was investigated by emul-
sifying experiments (Figure 3B). To achieve this, a two phase
Figure 3. (A) Solid UV-Vis spectra of catalysts 5a and 5b. (B) From left to right:
samples of methyl orange dissolved in water (lower orange coloured phase) and
ethyl acetate (0.3:0.2 mL, upper colourless phase), the same mixture with 10
mg of catalyst 5a and the same mixture with 10 mg of catalyst 5b.
2
system of 0.3 mL H O and 0.2 mL ethyl acetate was prepared in
a vial and two drops of an aqueous 0.1% methyl orange solution
were added. The dye is exclusively dispersed in the aqueous part
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materials even when they come from different batches can form
materials. Less intense resonances in these spectra at around -68
2
and -59 ppm can be assigned to T [RSi(OSi)
3
2
(OH)] resp. T
2
Table 1. Surface parameters derived from N adsorption–desorption
[
RSi(OSi) ] species and provide further evidence for the success-
3
measurements of materials before and after crushing.
ful covalent immobilization of the trialkoxysilane precursors on the
surface of the materials.
material
BET surface area
2
pore volume
3
BJH pore
–1
–1
[
m .g
]
[m .g
]
diameter [nm]
4a
4b
5a
5b
949
870
729
453
0.54
0.54
0.44
0.30
2.29
2.50
2.40
2.69
Pickering emulsions and show no significant differences in
activities.
In order to study the surface parameters of the materials in more
2
details, N adsorption–desorption analyses for the fully decorated
hollow shell precursors 4a and 4b as well as for the crushed
catalysts 5a and 5b were performed (Figure 4).
2
Figure 4. N adsorption-desorption isotherms of 4a (A, ▲), 5a (A, ×), 4b (B, ▲)
and 5b (B, ×).
The adsorption-desorption isotherms of all four materials show a
type-IV profile, which is typical for mesoporous compounds. The
calculated BET surface areas, the pore size distributions (BJH
method) and the pore diameters are listed in Table 1. There is a
decrease of the specific surface area from the as-synthesized
hollow silica spheres (4a and 4b) to the final catalysts 5a and 5b,
which on one side can be explained by the mechanical stress of
the ball-milling process. The rather pronounced decrease from 4b
to 5b might in addition be due to the oxidation-acidification step
that is necessary to generate the sulphonic acid functions on the
surface. Interestingly, the average pore diameters increase from
4a and 4b to the final catalysts 5a and 5b. It seems as if smaller
channels in the materials are preferentially destroyed during the
ball-milling process.
1
3
In the C CP-MAS NMR spectra of catalysts 5a and 5b, all carbon
resonances belonging to the organic functional groups can be
assigned due to their individual chemical shifts (Figure 5). In the
spectrum of catalyst 5a (Figure 5A), the two resonances at 7.8
and 12.3 ppm are belonging to the silicon-bound carbon atoms
C1 and C7.
Further resonances in the aliphatic as well as in the aromatic
region of the spectrum can unambiguously be assigned by com-
13
parison with the high resolution C NMR resonances of the trialk-
oxysilane precursors used for the synthesis of 5a. Since there is
no pronounced difference between the chemical shifts of the
resonances of the amine bound carbon atom C9 and its congener
in APTMS, there should be no strong interaction of the amino
13
Figure 5. Solid state C CP-MAS NMR spectra of A) catalyst 5a and B) catalyst
b.* assigns a rotational side band.
5
1
3
group with the silica surface. Figure 5B, provides the C CP-MAS
NMR spectrum of 5b. Again, assignment of the resonances can
be drawn from the high resolution data of the precursor trialkoxy-
silanes. In contrast to 5a there are still resonances belonging to
silicon-bound ethoxy groups at 52.8 and 16.9 ppm. The reson-
ance that is shifted most towards lower field can be assigned to
carbon atom C13 that is directly bound to the electron withdrawing
Catalysis
The catalytic performance of the novel Janus-type basic catalyst
5a was investigated for the Knoevenagel condensation of aro-
matic aldehydes and malononitrile (Table 2). Catalyst 5a converts
aromatic aldehydes at room temperature with high rates of reac-
tion. 1-Naphtaldehyde e.g. gives quantitative yields of the accord-
ing condensation product after just 5 min (Table 2, entry 1). It
seems that neither the position nor the nature of substituents at
2
9
-
3
SO H group. The Si CP-MAS NMR spectra of both catalysts 5a
and 5b (Figure 2S, Supporting Information) exhibit two dominant
3
resonances at around -103 and -112 ppm, being attributed to Q
4
[
Si(OSi)
3
OH] and Q [Si(OSi)
4
] species in the framework of the
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the aldehyde backbone make much difference in reactivity. Gene-
rally, yields of about 90% are obtained after 30 min of reaction
(
Table 2, entries 2–6) at room temperature. It has to be mentioned
at this point, that neat water was used as the solvent in these
experiments although the Knoevenagel condensation ends up
with the elimination of a molecule of water to form the product. In
case of treating salicylic aldehyde with malononitrile, the OH
group of the substrate is able to attack malononitrile too. There-
fore a second molecule of malononitrile is incorporated in the
Table 2. Scope of the reaction of aromatic aldehyde with malononitrile over
catalyst 1.
[
a,b]
entry
substrate
product
time
min)
yield (%)
(
CN
CHO
CN
1
2
3
5
30
30
>99
reaction product by formation of a functionalized 4H-1-benzo-
[
23]
pyrane ring.
By increasing the amount of malononitrile to 2
equiv., (2-amino-3-cyano-4H-chromen-4-yl)malononitrile was for-
med in 88% yield (Table 2, entry 7). Encouraged by this result, we
additionally explored the activity of catalyst 5a in the synthesis a
four-component reaction between salicylic aldehyde, 2 equiv. of
malononitrile and thiophenol resulting in the formation of highly
functionalized benzopyranopyridine derivative in 73% yield. Such
compounds have been discussed as promising medicinal scaf-
NC
NC
CN
CN
[
c]
CHO
90 (95)
CHO
89
Cl
Cl
NC
[24]
folds in the past.
CN
4
5
6
F
CHO
30
30
30
87
84
The activity of catalyst 5b was evaluated exemplarily for the form-
F
[
25]
ation of ethyl acetate from acetic acid and ethanol. Acid cataly-
zed esterification reactions are typical equilibrium reactions. This
was confirmed for catalytically active silica surfaces bearing im-
NC
CN
CN
H
3
CO
CHO H CO
3
[26]
mobilized sulfonic acid groups by means of DFT calculations.
NC
Investigations on the conversion of acetic acid and ethanol with
catalyst 5b show that the reaction proceeds smoothly for an al-
most equimolar acid/alcohol ratio and in the absence of any other
solvent. Based this result, we investigated the outcome of this
transformation in the presence of additional amounts of water
BuO
CHO
BuO
93
NC
CHO
NC
H
H
OH
[
d]
7
CN
NH
60
88
(
Figure 6). Up to a tenfold excess of water, the conversion is
O
2
between 80 and 90%, which means, that there is no pronounced
influence of the water concentration on the equilibrium of the
esterification reaction. Slight changes in the outcome of the
reaction can be explained by changes in the emulsifying
Ph
S
NH
2
CHO
H
[
e]
CN
8
OH
240
73
O
N
NH
2
[
a] Reaction conditions: 20 mg (3 mol%, based on the nitrogen content obtained
by elemental analysis) of catalyst 5a, 1 mmol of aldehyde, 1 mmol of malono-
nitrile, 2 mL of H O, room temperature unless otherwise stated; [b] yields refer
2
1
13
to isolated pure products giving satisfactory H and C NMR spectra; [c] a
recovered catalyst was used for 10 mmol scale synthesis at the mentioned
reaction conditions; [d] 2 eqiv. of malononitrile were used; [e] reaction condi-
tions: 30 mg (4.6 mol%) of catalyst, 2 eqiv of malononitrile, 1 eqiv of thiophenol,
78° C.
[
17d]
properties of the reaction mixture.
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2
Figure 6. Effects of excess of H O on the outcome of the esterification of acetic
the agglomeration of Janus sheets of diverse size in the solution
to form temporary micro-rectors.
Figure 8. Results of laser diffraction particle size analysis of 4a, 4b, 5a and 5b.
This explains why catalyst 5b shows superior activity compared
[
to the previous reports of heterogeneous and homogeneous
8]
[22]
Brønsted acid catalysts that have been reported for “room tempe-
rature esterification”.
To further improve the reaction efficiency, we studied time and
temperature effects. According to “Le Chatelier's principle”, for
shifting the equilibrium of the reaction toward ester product, an
[28]
excess of one of the starting materials must be beneficial. Thus,
a ratio of ethanol:acetic acid of 10:1 was chosen and esterification
experiments were performed at 40, 50, 60 and 70 °C. Samples
were taken each 30 min and analyzed by gas chromatography.
The results show that improvements of the yield of ethyl acetate
significantly rises and that the equilibrium is reached after 2 h at
acid and ethanol. Reaction conditions: 2.5 mol-% of catalyst 5b (based on the
sulfur content obtained by elemental analysis), molar ratio of ethanol:acetic acid
=
1.5:1, 18 h, room temperature.
A schematic depiction of the action of the Janus-type catalyst 5b
in the esterification discussed above is given in Figure 7. The non-
polar face of 5b forms micelles with the non-polar product ethyl
acetate. Even more important is the fact, that the sulfonic acid
moieties are strongly hydrated. T. Hasegawa et al investigated
the interaction of the sulfonic acid groups of a Nafion® membrane
and water molecules by means of a combination of infrared
70 °C (Figure 9). These results support that 5b shows excellent
performance in this esterification reaction. Further details of these
experiments are listed in Table 1S in the Supporting Information.
[
27]
spectroscopy and DFT calculations. The degree of hydration
depends on both, the temperature and the water content.
Figure 9. Effects of reaction time and temperature on esterification of acetic
acid and ethanol. Reaction conditions: 2.5 mol-% of catalyst 5b, ethanol:acetic
acid = 10:1,
▲ = 40 °C, ӿ = 50 °C, ● = 60 °C, and × = 70 °C.
Figure 7. A proposal for the action of the Janus-type catalyst 5b in esterification
●
of acetic acid wit ethanol;
refers to ethyl acetate.
Conclusions
In summary, we established a simple and as we believe a general
The authors could show that there are discrete hydrate structures
with up to three water molecules in close proximity to the -SO
3
H
sites. Higher amounts of water lead to hydrated hydroxonium
species of undefined structure, which still can be well disting-
uished from bulk water by their IR signature. In our system, the
hydration of the sulfonate and the entrapping of ethyl acetate
keeps both products apart from each other and thus shifts the
equilibrium towards product formation. Additional efforts to
remove water are not necessary.
To study the particle size of materials in solvent and finding an
evidence for the proposed mechanism, laser diffraction analysis
of the particle size was employed (Figure 8). For 4a and 4b
(
hollow particles), a bimodal distribution with relatively identical
pattern and particle size between 10 and 30 µm is observable.
This increasing of the particle size in the emulsion can be
explained by an almost complete de-agglomeration of the sphe-
rical hollow particles induced by the applied protic solvent
method for the preparation of heterogeneous Janus-type organic-
inorganic hybrid catalysts with selectable properties. The scope
of this method was shown exemplarily by selecting a basic and
an acidic decoration of the material surfaces. The Janus-type
base catalyst was investigated for its activity in a Knoevenagel
condensation reaction and allowed to gain high yields after short
reaction times. This is also true for the Janus-type acid catalyst
which was evaluated for the esterification of acetic acid with
ethanol. More than 95% of ethyl acetate were obtained after 2 h
at 70 °C. In both reactions, neither the by-product water nor the
addition of excess of water have a negative impact on the catalytic
activity. We believe that the presented strategy of functionalizing
(
ethanol). According to SEM, the size of a single particle is at
about 1.2-1.8 µm. The dried samples, wherein spheres are
strongly agglomerated were treated in an ultrasonic bath before
the measurement. In our interpretation, the shearing forces in the
ultrasonic bath allows for a degradation of agglomerates down to
a size of 10 and 30 µm. This is supported by the rather sharp
particle size distribution (Figure 8). However, the results of 5a and
5b, show a broad and trimodal distribution between 0.3 and 30
µm, with a high similarity. This can be an interesting evidence for
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silica surfaces with two different covalently immobilized groups
will develop to a general tool for a rationally design of catalysts for
applications in immiscible liquid-liquid reaction mixtures. Further
investigations on the development of this concept are going on in
our group.
Synthesis of material 4b. 0.50 g of activated 3b were dispersed in 20 mL
of toluene at room temperature and stirred for 30 min. 0.37 mL (2.0 mmol)
of (3-mercaptopropyl)trimethoxysilane (MPTMS) were added to the sus-
pension. The mixture was vigorously stirred at reflux temperature for 24 h
2
under an atmosphere of N . At the end of reaction, the mixture was cooled
to room temperature. The solid was separated by centrifugation, washed
with toluene and dried under high vacuum. Then 0.3 g of the dried product
containing propyl-SH chains were added to a 25 mL round bottomed flask.
Experimental Section
10 mL of 30% H
2
O
2
and 1 mL THF were added and the mixture was stirred
for 24 h at room temperature. The resulting
under an atmosphere of N
2
suspension was centrifuged and the solid residue washed first with water
and then with ethanol. Finally, the still wet material was suspended in 50
Materials and Characterization. All chemicals and solvents were
obtained from the chemical suppliers and used without further purifications.
Brunauer, Emmett, Teller (BET) and Barrett, Joyner, Halenda (BJH)
measurements were performed by Quantachrome Autosorb-1-Instrument.
The surface area and the pore size distribution were determined employing
BET method. Solid state 13C and 29Si CP-MAS NMR were measured with
2 4
mL of a 1 M H SO solution for 2 h, separated by centrifugation and
washed several times with deionized water until pH = 7. After a final
washing with ethanol the material was dried at 80 °C for 12 h.
a 500 MHz BRUKER Avance III spectrometer under cross polarization
conditions with a spinning frequency of 11000 Hz. for both 13C and 29Si
Crushing of the hollow particles. Materials 4a and 4b were crushed
using a ball-milling apparatus (Planetary ball mill pulverisette 7 premium
line, Fritsch, Idar-Oberstein, Germany). Typically, 2 g of the material and
and scan No. 15000 and 2500, respectively. NMR spectra of the organic
products were measured in solution with a 400 MHz BRUKER DPX and
spectrometer. An ATRFT- IR spectrometer Perkin Elmer Spectrum 100
was used for measuring the infrared spectra. Elemental analyses were
performed with an Elementar Vario Micro Cube. Gas chromatography
2
50 g of 1 mm ZrO balls were added to the apparatus and were crushed
2
ten times for 5 sec at a speed of 600 rpm. The ZrO balls were separated
mechanically leaving the final catalysts 5a and 5b.
(
GC) was used for monitoring the esterification utilizing Clarus 580-
Laser diffraction particle size distribution analysis. For measurement
of the particle size a laser diffraction particle size distribution analyzer
(Retsch Horiba LA-950) was used. The measurements were performed in
the liquid cell of the particle sizer. Three independent samples were taken
from each sample container. Then a sample was taken and dispersed in
ethanol using an ultrasonic treatment. The suspension was degassed and
dispersed by a rotor-stator in the sample preparation container of the
particle sizer. Each part was measured two times and hence each sample
was measured six times.
PerkinElmer with FS-OV1701-CB-0.25 column (30 m). For crushing the
particles, a planetary ball mill pulverisette 7 premium line (Fritsch, Idar-
2
Oberstein, Germany) was used with 50 g of 1 mm ZrO balls.
Synthesis of materials 1a and 1b. 0.322 g (0.88 mmol) of cetyltri-
methylammonium bromide (CTAB) were dissolved in 0.19 mL of an aque-
ous ammonia solution (25 w.t %) and 107.8 mL of H
bottom. In a second 100 mL flask, a mixture of 0.25 mL of n-decane and
4 mL (51 g) of ethanol was prepared. The solution of the second flask
2
O in a 500 mL round
6
was added to the first one and the flask was placed in an ultrasonic bath
for 15 min at a temperature of 35 °C.[29] Then, 0.35 mL (1.6 mmol) of
trimethoxy(2-phenylethyl)silane (for 1a) or 0.50 mL (1.6 mmol) of trieth-
oxy(octyl)silane) (for 1b) and 2.00 g (9.6 mmol) of tetraethylorthosilicate
pH analysis experiment. In a 50 mL flask, 250 mg of 5b was added to 25
mL of freshly prepared 1M aqueous solution of NaCl with a primary pH
6.86. The resulting mixture was stirred for 3 h to exchange the proton and
+
Na ion. At the end, the pH of solution decreased to 2.09 which is equal to
/
+
[9]
(
TEOS) were added to this solution. The mixture was vigorously stirred at
5 °C for 24 h. At the end of reaction, the white products were collected by
a loading of 0.81 mmol H g of catalyst according to eq. (1).
3
centrifugation (2 min at 5000 rpm) and were first washed three times with
deionized water and then three times with ethanol.
pH= 4 × 25 × (10 –2.09 – 10 –6.86)= 0.81 mmol H /g of 5b
+
Eq. (1):
General procedure for the Knoevenagel condensation using catalyst
a. 20 mg of catalyst 5a, 66 mg (1 mmol) of malononitrile and 2 mL of
Synthesis of materials 2a and 2b. To remove the template CTAB, the
as-synthesized materials 1a and 1b were transferred into a 250 mL flask
containing a mixture of 120 mL of EtOH and 1 mL of concentrated
hydrochloric acid (37%) and stirred at 60 °C for 5 h. This step was repeated
two times to ensure complete removal of CTAB. At the end, the materials
were separated by centrifugation, washed first with water and then with
ethanol (three times for each solvent) and dried at 80 °C for 12 h.
5
water were filled in a 10 mL round bottom flask equipped with a magnetic
stirring bar. The mixture was stirred at room temperature for 5 min. Then
1
mmol of the corresponding aldehyde was added and the stirring was
continued for the appropriate time indicated in Table 2. At the end of
reaction, the aqueous phase was decanted and the solid residue was
treated with EtOH. The insoluble catalyst was separated by centrifugation
of the mixture. The products were recrystallized from ethanol or water/eth-
anol mixtures. The benzopyranopyridine (Table 2, entry 8) was isolated
according to the reported procedure.[30]
Synthesis of materials 3a and 3b. Materials 2a and 2b were activated by
refluxing of 5 g in 100 mL of 6 M hydrochloric acid for 24 h. At the end of
reaction, the products were isolated by centrifugation, washed thoroughly
with a 1:1 (vol.) mixture of deionized water and ethanol to adjust the pH
value of the solution to neutral and dried in an oven (80 °C) before under-
going further chemical surface modifications.
General procedure for esterification using acid catalyst 5b. To a 10
mL round bottom flask equipped with a magnetic stirring bar, 2.5 mol%
(
based on sulfur content derived by elemental analysis) of catalyst 5b, 1
mmol (0.06 mL) of acetic acid and 10 mmol (0.58 mL) of ethanol were
added and stirred for desired reaction time and temperature. At the end of
reaction, tetradecane was added to the reaction mixture as an internal
standard and the mixture was vigorously stirred for 5 min. The solid was
filtered and the solution was diluted with ~1 mL of in dichloromethane. The
samples were analyzed with gas chromatography (GC).
Synthesis of material 4a. 0.50 g of activated 3a were dispersed in 20 mL
of toluene at room temperature and stirred for 30 min. 0.35 mL (2.0 mmol)
of (3-aminopropyl)trimethoxysilane (APTMS) were added to the suspen-
sion. The mixture was vigorously stirred at reflux temperature for 24 h
2
under an atmosphere of N . At the end of reaction, the mixture was cooled
to room temperature. The solid was separated by centrifugation, washed
with toluene and dried under high vacuum.
Acknowledgements
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201900147
FULL PAPER
The authors wish to thank the Alexander von Humboldt Foun-
dation for a research grant fellowship to M. Vafaeezadeh and the
research unit NanoKat at the TU Kaiserslautern for financial
support.
Keywords: Janus interphase catalyst • organic-inorganic hybrid
material • reaction in water • esterification • Knoevenagel
condensation
[
1]
2]
P. Anastas, J. Warner, Green Chemistry: Theory and Practice, Oxford
University Press, Oxford, U.K., 1998.
2014, 26, 6944-6949; i) A. Walther, A. H. E. Müller, Chem. Rev. 2013,
113, 5194-5261; j) M. Lattuada, T. A. Hatton, Nano Today, 2011, 6, 286-
308; k) J. Hu, S. Zhou, Y. Sun, X. Fang, L. Wu, Chem. Soc. Rev. 2012,
41, 4356-4378; l) Y. Song, S. Chen, Chem. Asian J. 2014, 9, 418-430;
m) T. Zhao, X. Zhu, C. Hung, P. Wang, A. Elzatahry, A. A. Al-Khalaf, W.
N. Hozzein, F. Zhang, X. Li, D. Zhao, J. Am. Chem. Soc. 2018, 140,
10009-10015; n) Z Wua, L Li, T Liao, X. Chen, W. Jiang, W. Luo, J. Yang,
Z. Sun, Nano Today, 2018, 22, 62-82; o) A. M. Pourrahimi, M. Pumera,
Nanoscale, 2018, 10, 16398-16415; p) X. Kong, C. Wu, L. Feng, J. Qu,
P. Liu, X. Wang, X. Zhang, Chem. Commun. 2017, 53, 8054-8057.
[19] a) E. Lindner, T. Schneller, F. Auer, H. A. Mayer, Angew. Chem. Int. Ed.
1999, 38, 2154-2174; b) M. Jia, A. Seifert, W. R. Thiel, Chem. Mater.
2003, 15, 2174-2180; c) S. Shylesh, M. Jia, A. Seifert, S. Adappa, S.
Ernst, W. R. Thiel, New J. Chem. 2009, 33, 717-719; d) L. Wang, M. Jia,
S. Shylesh, T. Philippi, A. Seifert, S. Ernst, A. P. Singh, W. R. Thiel,
ChemCatChem 2010, 2, 1477–1482; e) L. Wang, S. Shylesh, D. Dehe,
T. Philippi, G. Dörr, A. Seifert, Z. Zhou, M. Hartmann, R. N. Klupp Taylor,
M. Jia, S. Ernst, W. R. Thiel, ChemCatChem. 2012, 4, 401-407; D. Dehe,
L. Wang, M. K. Müller, G. Dörr, Z. Zhou, R. N. Klupp Taylor, Y. Sun, S.
Ernst, M. Hartmann, M. Bauer, W. R. Thiel, ChemCatChem 2015, 7, 127-
136.
[
a) D. C. Rideout, R. Breslow, J. Am. Chem. Soc. 1980, 102, 7816-7817;
b) S. Narayan, J. Muldoon, M. G. Finn, V. V. Fokin, H. C. Kolb, K. B.
Sharpless, Angew. Chem. Int. Ed. 2005, 44, 3275-3279.
[
3]
4]
J. J. Gajewski, in Organic Synthesis in Water (Ed.: P. A. Grieco), Blacke
Academic and Professional, London, U.K., 1998.
[
a) P. A. Grieco, Organic Synthesis in Water, Blake Academic and
Professional, London, U.K., 1998; b) U. M. Lindström, Chem. Rev. 2002,
102, 2751-2772.
[
5]
6]
K. Manabe, Y. Mori, T. Wakabayashi, S. Nagayama, S. Kobayashi, J.
Am. Chem. Soc. 2000, 122, 7202-7207.
[
a) W. P. Weber, G. W. Gokel, in Phase Transfer Catalysis in Organic
Synthesis, Springer-Verlag, New York, USA, 1977; b) E. V. Dehmlow, S.
S. Dehmlow, in Phase Transfer Catalysis, Verlag Chemie, Weinheim,
Germany, 1980.
[
[
[
[
7]
8]
9]
I. K. Mbaraka, B. H. Shanks, J. Catal. 2005, 229, 365-373.
R. Neumann, M. Cohen, Angew. Chem. Int. Ed. 1997, 36, 1738-1740.
B. Karimi, M. Vafaeezadeh, Chem. Commun. 2012, 48, 3327-3329.
10] a) C. C. Tzschucke, C. Markert, H. Glatz, W. Bannwarth, Angew. Chem.
Int. Ed. 2002, 41, 4500-4503; b) O. Yamazaki, X. Hao, A. Yoshida, J.
Nishikido, Tetrahedron Lett. 2003, 44, 8791-8795.
[20] a) C. Tang, S. Ye, H. Liu, Polymer 2007, 48, 4482-4491; b) D. Yongquan,
W. Ming, C. Lin, L. Mingjun, Desalination 2012, 295, 53-60.
[11] D. J. Cole-Hamilton, Science 2010, 327, 41-42.
[
12] a) S. Cobo, J. Heidkamp, P. Jacques, J. Fize, V. Fourmond, L. Guetaz,
B. Jousselme, V. Ivanova, H. Dau, S. Palacin, M. Fontecave, V. Artero,
Nat. Mat. 2012, 11, 802-807; b) Z. Xue, H. Su, Q. Yu, B. Zhang, H. Wang,
X. Li, J. Chen, Adv. Energy Mater. 2017, 7, 1602355-1602361.
13] X. Wang, B. Guan, Y. He, Y. Zhang, Y. Cao, Y. Liu, Z Qiao, Q. Huo,
ChemNanoMat. 2015, 1, 562-566.
[21] W. Stöber, A. Fink, E. Bohn, J. Coll. Interf. Sci. 1968, 26, 62-69.
[22] a) R. van Grieken , J.A. Melero, G. Morales, J. Mol. Catal. A: Chem. 2006,
256, 29-36; b) B. Karimi, M. Vafaeezadeh, RSC Adv. 2013, 3, 23207-
23211; c) B. Schäfgen, O. D. Malter, E. Kaigarula, A. Schüßler, S. Ernst,
W. R. Thiel, Micropor. Mesopor. Mater. 2017, 251, 122-128.
[
[
[
[
[
[23] M. Costa, F. Areias, L. Abrunhosa, A. Venâncio, F. Proença, J. Org.
Chem. 2008, 73, 1954-1962.
14] L. Xia, H. Zhang, Z. Wei, Y. Jiang, L. Zhang, J. Zhao, J. Zhang, L. Dong,
E. Li, L. Ruhlmann, Q. Zhang, Chem. Eur. J. 2017, 23, 1920-1929.
15] X. Ji, Q. Zhang, X. Qu, Q. Wang, X. Song, F. Liang, Z. Yang, RSC Adv.
[24] a) G. Kolokythas, N. Pouli, P. Marakos, H. Pratsinis, D. Kletsas, Eur. J.
Med. Chem. 2006, 41, 71-79; b) M. A. Azuine, H. Tokuda, J. Takayasu,
F. Enjyo, T. Mukainaka, T. Konoshima, H. Nishino, G. Kapadia, J. Phar-
macol. Res. 2004, 49, 161-169; c) S.K. Srivastava, R.P. Tripathi, R. Ra-
machandran, J. Biol. Chem. 2005, 280, 30273-30281; d) H. Brötz-
Oesterhelt, I. Knezevic, S. Bartel, T. Lampe, U. Warnecke-Eberz, K.
Ziegelbauer, D. Häbich, H. Labischinski, J. Biol. Chem. 2003, 278,
39435-39442.
2
015, 5, 21877-21880.
16] S. Yan, H. Zou, S. Chen, N. Xue, H. Yang, Chem. Commun. 2018, 54,
0455-10458.
17] a) W. Gao, A. Pei, R. Dong, J. Wang, J. Am. Chem. Soc. 2014, 136,
276-2279; b) R. A. Pavlick, S. Sengupta, T. McFadden, H. Zhang, A.
1
2
Sen, Angew. Chem. Int. Ed. 2011, 50, 9374-9377.
[
18] a) B. Liu, C. Zhang, J. Liu, X. Qu, Z. Yang, Chem. Commun. 2009, 3871-
[25] E. Fischer, A. Speier, Chem. Ber. 1895, 28, 3252–3258.
[26] M. Vafaeezadeh, A. Fattahi, Comput. Theor. Chem. 2015, 1071, 27-32.
[27] T. Shimoaka, C. Wakai, T. Sakabe, S. Yamazaki, T. Hasegawa, Phys.
Chem. Chem. Phys. 2015, 17, 8843-8849.
3873; b) Y. Chen, F. Liang, H. Yang, C. Zhang, Q. Wang, X. Qu, J. Li, Y.
Cai, D. Qiu, Z. Yang, Macromolecules 2012, 45, 1460-1467; c) F. Liang,
J. Liu, C. Zhang, X. Qu, J. Li, Z. Yang, Chem. Commun. 2011, 47, 1231-
1
233; d) Q.B. Meng, P. Yang, T. Fen, X. Ji, Q. Zhang, D. Liu, S. Wu, F.
[28]
a) Y. Wang, D. Wang, M. Tan, B. Jiang, J. Zheng, N. Tsubaki, M. Wu,
ACS Appl. Mater. Interfaces 2015, 7, 26767-26775; b) C. Pirez, J. Cade-
ron, J. Dacquin, A.F. Lee, K. Wilson, ACS Catal. 2012, 2, 1607-1614; c)
D. Fang, J. Yang, C. Jiao, ACS Catal. 2011, 1, 42-47.
Liang, Z. Zheng, X. Song, J. Colloid Interface Sci. 2017, 507, 74-82; e)
F. Liang, K. Shen, X. Qu, C. Zhang, Q. Wang, J. Li, J. Liu, Z. Yang,
Angew. Chem. Int. Ed. 2011, 50, 2379-2382; f) X. Ji, Q. Zhang, F. Liang,
Q. Chen, X. Qu, C. Zhang, Q. Wang, J. Li, X. Song, Z. Yang, Chem.
Commun., 2014, 50, 5706-5709; g) A. Walther, A. H. E. Müller, Soft
Matter, 2008, 4, 663-668; h) F. Liang, C. Zhang Z. Yang, Adv. Mater.
[29]
P Xu, Z. Nan, A. Zhu, Z. Gu, Matter. Lett. 2017, 205, 20-23.
[30] N. M. Evdokimov, A. S. Kireev, A. A. Yakovenko, M. Y. Antipin, I. V. Ma-
gedova, A. Kornienko, Tetrahedron Lett. 2006, 47, 9309-9312
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201900147
FULL PAPER
Entry for the Table of Contents
FULL PAPER
A simple and practical method for the
synthesis of Janus acid and base
materials as versatile water-tolerant
and amphiphilic heterogeneous
catalysts has been developed.
Majid Vafaeezadeh,* Paul Breuninger,
Philipp Lösch, Christian Wilhelm, Stefan
Ernst, Sergiy Antonyuk, Werner R. Thiel*
Page No. – Page No.
Janus Interphase Organic-Inorganic
Hybrid Materials: Novel Water-
Friendly Heterogeneous Catalysts
This article is protected by copyright. All rights reserved.