Preparation of a Pickering emulsion by modification of an amine-functionalized graphene oxide surface with organosilane: Efficient catalyst for the Knoevenagel condensation of malononitrile with aldehydes at mild temperature

Article abstract of DOI:10.1039/c9nj06097a

In this study, a series of Pickering emulsions for catalysis of Knoevenagel condensations of malononitrile with aldehydes were prepared by surface modification of amine-functionalized graphene oxide (GO-NH₂) with trimethoxymethylsilane (MTMS).

Full text of DOI:10.1039/c9nj06097a

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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DOI: 10.1039/C9NJ06097A
ARTICLE
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Preparation of Pickering emulsion by modification of amine-functionalized
graphene oxide surface with organosilane: Efficient catalyst for Knoevenagel
condensation of malononitrile with aldehydes at mild temperature
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Received 00th
January 20xx,
Bingxu Qian, Fei Wang, Dongsheng Li, Yongxin Li *, Bo Zhang and Jie Zhu*
In this study, a series of Pickering emulsions for catalysis of Knoevenagel condensations of malononitrile with aldehydes
were prepared by surface modification of amine-functionalized graphene oxide (GO-NH2) with trimethoxymethylsilane
(MTMS). The catalytic activity of the prepared Pickering emulsion system in a Knoevenagel condensation was mainly
determined by the active sites and properties (namely droplet size, density, and distribution) of the emulsion. The results
show that the mass ratio of 3-aminopropyltrimethoxysilane to MTMS affects the active catalytic sites and properties of the
Pickering emulsion. Under conditions that provided a large number of active catalytic sites in the Pickering emulsion,
modification of the surface of amine-functionalized GO with an organosilane enhanced substrate adsorption on the
catalyst and improved the properties of water-in-oil Pickering emulsions. This increased the Picking emulsion catalytic
activity. The Pickering emulsion prepared with GO-NH2:Si = 1.8:0.2 gave the highest catalytic activity in a Knoevenagel
condensation at 40 ℃ and its activity was much higher than that of GO-NH2. The Pickering GO-NH2(1.8)-Si(0.2) still
showed high catalytic activity after six cycles, and therefore has good reusability.
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
properties of the Pickering emulsion. A hydrophilic catalyst
surface generally stabilizes oil-in-water (O/W) Pickering
emulsions. A lipophilic catalyst surface tends to stabilize
1. Introduction
The Knoevenagel condensation is one of the most important
industrial reactions for forming carbon–carbon double bonds
(C=C). The reaction products, i.e., α,β-unsaturated carbonyl
compounds, are essential intermediates in the production of
pharmaceuticals, pesticides, and fine chemicals 1-4. Generally,
Knoevenagel condensation reactions are conducted in organic
solvents5,6, which causes environmental problems. The
development of conditions that enable replacement of organic
solvents with water is therefore important. However, a major
problem with the use of water as a green solvent 7 is that most
water-in-oil (W/O) Pickering emulsions 18, and favors
formation of smaller and denser droplets in the emulsion. The
surface properties of the catalyst particles can be regulated by
surface modification 19-21. A moderate increase in the
lipophilicity of the catalyst surface promotes substrate
adsorption on the catalyst, which improves the catalytic
performance of the Pickering emulsion 22-25. However, the
catalytic activity of a Pickering emulsion depends on both the
lipophilicity and the active sites on the catalyst surface.
Amine-functionalized
GO,
in
which
3-
8
organic reactants are poorly soluble in water , which leads to
aminopropyltrimethoxysilane (APTMS) is grafted on GO via
hydroxyl groups on the GO surface, is generally used for
catalysis of Knoevenagel condensations. The surface
lipophilicity of amine-functionalized GO can be modified by
grafting trimethoxymethylsilane (MTMS) on the GO via
hydroxyl groups on the GO surface. Occupation of a large
number of active sites on the catalyst surface by organosilane
moieties (lipophilicity modification) decreases the catalytic
activity of the Pickering emulsion. The synergistic effect
between active sites and the catalyst surface lipophilicity is
therefore important in determining the catalytic activity of a
Pickering emulsion in Knoevenagel condensations. No studies
of this effect have yet been reported. In this study, a series of
Pickering emulsions for the catalysis of Knoevenagel
reactions were prepared by modification of the surface of
amine-functionalized GO by using APTMS and MTMS at
various mass ratios. The catalytic activities of the Pickering
emulsions in Knoevenagel condensations were investigated
and the synergistic effect between the active sites and
lipophilicity of the catalyst surface was examined.
mass-transfer limitations in biphasic reactions. This drawback
can be overcome by using Pickering emulsion systems. This
effectively decreases the mass-transfer resistance by
increasing the interfaces between active sites and reactants,
and has become a focus of attention in catalysis 9-14
.
Graphene oxide (GO) is an ideal Pickering emulsion
stabilizer because its lipophilic carbon skeleton and
hydrophilic oxygen-containing functional groups endow it
with good amphiphilicity 15,16. We previously reported an
efficient Pickering emulsion system, which was prepared with
bifunctional GO grafted with poly(ethylene glycol) and 3-
aminopropyltrimethoxysilane, for catalysis of nucleophilic
substitution 17. The surface properties of the catalyst particles
played an important role in determining the type and
a. Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of
Petrochemical Engineering, Changzhou University, Gehu Middle Road 21,
Changzhou, Jiangsu 213164, PR China. E-mail: liyxluck@163.com (Y. Li) ;
zhujie@cczu.edu.cn (J. Zhu) Tel.: +86-519-86330135; Fax.: +86-519-86330135
† Footnotes relating to the title and/or authors should appear here.
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2.6 Characterization of GO-NH2(x)-Si(y) and PickVeireiwnAgrticle Online
2. Experimental
DOI: 10.1039/C9NJ06097A
emulsion
2.1. Preparation of GO support
Fourier-transform infrared (FT-IR) spectra of the samples
(KBr pellets) were recorded in transmission mode at room
temperature with a Bruker Tensor 27 spectrometer at a
resolution of 4 cm−1, with 32 scans per spectrum, in the region
400–4000 cm−1. The sample:KBr mass ratio was constant in
each case, at 1:200. Thermogravimetric (TG) analysis was
performed from room temperature to 600 ℃ at a heating rate
of 20 oC min−1 under nitrogen (Perkin-Elmer TGA7 analyzer).
X-ray photoelectron spectroscopy (XPS) was used to
determine the surface elemental compositions of the samples,
utilizing the Mulfilab 2000 model. Contact Angle Test was
carried out on DSA25 optical contact angle measuring device.
The emulsion morphology photographs were microscopically
characterized by using a XSP-8CAE optical microscope
produced by Shanghai Bim Co., Ltd.
Graphite oxide was prepared by using a modified
Hummer’s method 26. Graphite oxide (0.2 g) was added to
tetrahydrofuran (THF, 100 mL) and the mixture was
ultrasonicated for 1.5 h. The mixture was washed several
times with ethanol, and the GO was collected by filtration and
dried at 50 °C in a vacuum oven.
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2.2 Preparation of GO-NH2(x)-Si(y) catalyst
GO (0.2 g) was added to a tetrahydrofuran solution (100
mL) of APTMS (x g) and MTMS (y g), where x is the mass of
APTMS, y is the mass of MTMS, and x plus y equals 2 g, to
give various APTMS:MTMS mass ratios. The mixture was
ultrasonicated three times (30 min each time). The well-
dispersed mixture was refluxed at 70 °C for 24 h and then
filtered. The solid was rinsed several times with ethanol and
dried overnight at 80 ℃. The obtained samples were denoted
by GO-NH2, GO-NH2(x)-Si(y), and GO-Si. APTMS was
grafted onto SBA-15 by using the same method and the
obtained composite was denoted by SBA-15-NH2. The
following scheme shows the process used for preparation of
GO-NH2(x)-Si(y).
3. Results and discussion
3.1 Characterizations of synthesized GO-NH2(x)-Si(y)
Scheme 1 Preparation process of GO-NH2(x)-Si(y).
2.3 Preparation of Pickering emulsion
The catalyst (20 mg), deionized water (4 mL), and
benzaldehyde (15 mmol) were sequentially added to a 50 mL
round-bottomed flask. The mixture was ultrasonicated for 5
min and the solution was vigorously stirred at 800 rpm for 5
min to form a Pickering emulsion.
Fig. 1 TG curve of GO (a), GO-Si (b), GO-NH2(1.8)-Si(0.2) (c) and GO-NH2 (d).
TG analysis of GO, GO-Si, GO-NH2 and GO-NH2 (1.8)-Si
(0.2) were respectively carried out in order to test the thermal
stability of catalytic materials. As shown in Fig. 1. TG curve
of pure GO appear two clear weight losses at 70-125 °C and
190-250 °C due to the evaporation of adsorbed water and
thermal decomposition of oxygen-containing functional
groups (OH,C=O,C-O-C) on GO, respectively 27,28. As the
temperature raise, the weight of GO continuously decline and
2.4 Catalytic evaluation
A Knoevenagel condensation was conducted in a 50 mL
round-bottomed flask. Malononitrile (15 mmol) and an
internal standard, namely n-dodecane (0.3 mL), were
sequentially added to the Pickering emulsion. The reaction
was performed under stirring at 40 ℃ for 1 h. After the
reaction, the reaction mixture was extracted with chloroform
and centrifuged to separate the water and the catalyst,
respectively. The obtained liquid phase was analyzed by using
a GC (SE-6850) system equipped with an SE-54 capillary
column and a flame-ionization detector.
weight loss is eventually arrived at about 56.7% at 600 °C
29
owing to the thermolysis of its carbon skeleton (Fig. 1a)
.
As for GO-Si, the overall weight loss is 46.0% at 600 °C,
which indicates that the silicane functionalization improves
the thermal stability of GO. For GO- NH2, the overall weight
loss gained at 600 °C is only 31.0%, much lower than those of
GO (56.7%) and GO-Si (46.0%), showing the most excellent
thermal stability. Furthermore, the overall weight losses of
GO-NH2(1.8)-Si(0.2) is 39.9%, exhibiting a higher thermal
stability than GO-Si. In addition, TG analysis of GO, GO-Si,
GO-NH2 and GO-NH2 (1.8)-Si (0.2) implies that MTMS is
2.5 Recovery of GO-NH2(x)-Si(y)
After GC analysis, the mixture was filtered and washed by
ethanol for several times, and then the solid was dried at 80 ℃
overnight.
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successfully grafted onto the surface of amine-functionalized
GO 30
of the epoxy group (C–O–C, 287.4 eV) and carboxyl group (–
DOI: 10.1039/C9NJ06097A
COOH, 288.6 eV) peaks in the C1s spectrum of GO-
View Article Online
.
The chemical groups in the materials were identified by FT-
IR. The spectra are shown in Fig. 2. The absorption peaks in
the spectrum of pure GO, at 1724, 1616, 1259, and 1068 cm–1,
are attributed to the stretching vibrations of C=O, C=C, C–O–
C, and C–OH, respectively, and are consistent with those
reported in the literature 31,32. The C–OH absorption peak in
the GO-Si spectrum is weaker than the corresponding peak in
the GO spectrum. Two new bands emerged in the GO-Si
spectrum, at 1120 and 1036 cm−1, and can be ascribed to the
stretching vibrations of C–O–Si and Si–O–Si, respectively, in
MTMS 33. The absorption peaks at 1569 and 773 cm−1 in the
GO-NH2 spectrum are ascribed to the stretching vibration and
out-of-plane bending vibration, respectively, of N–H bonds in
the amine group. The absorption peaks at 1650, 1036, and 693
cm−1 are assigned to the stretching vibration of amide N–H,
stretching vibration of Si–O–Si, and bending vibration of
C−O−Si, respectively 34. The C–OH peak was clearly weaker
and a C–O–Si absorption peak appeared at 1120 cm–1, which
indicates that APTMS was grafted onto the GO surface. The
appearance of a C–O–Si absorption peak in the spectrum of
GO-NH2(1.8)-Si(0.2) shows that the C–O–Si groups in
MTMS and APTMS reacted with C–OH on GO, and GO-
NH2(1.8)-Si(0.2) contained all the functional groups of GO-Si
and GO-NH2. This confirms that MTMS was successfully
grafted onto the surface of amine-functionalized GO.
NH2(1.8)-Si(0.2) were lower than those in the C1s spectrum of
GO. This indicates that the amine group (–NH2) in APTMS
underwent a covalent reaction with the C–O–C and –COOH
groups on the GO surface during grafting; this is in agreement
with the FT-IR spectroscopic results. To sum up, these results
verify that MTMS-modified amine-functionalized GO was
successfully synthesized by a facile one-step method.
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Fig. 3 Survey XPS spectra and C 1s ones of GO (a, c) and GO-NH2 (1.8)-Si (0.2) (b,
d).
3.2 Characterizations of Pickering emulsion
Fig. 4 Pickering emulsion stabilized by GO-NH2(1.8)-Si(0.2) was dispersed in
water (a) and benzaldehyde (b); Optical micrograph and droplets size
distribution of Pickering emulsion stabilized by GO-NH2(1.8)-Si(0.2) (c).
The type and properties of the Pickering emulsion prepared
with GO-NH2(1.8)-Si(0.2) were investigated. Fig. 4 shows
that dispersion of the emulsion prepared with GO-NH2(1.8)-
Si(0.2) was better in BAL than in water. This confirms that the
Pickering emulsion was water-in-BAL (W/O) type (Fig. 4a
and b). This Pickering emulsion (Fig. 4c) had smaller, denser
droplets, with a narrower size distribution. Statistical analysis
showed that the proportion of droplets of size 15–30 μm was
about 78% of the total and the mean diameter of the droplets
in the Pickering emulsion was about 24 μm. All the prepared
Pickering emulsions were W/O type.
Fig. 2 FT-IR spectra of GO (a), GO-Si (b), GO-NH2(1.8)-Si(0.2) (c) and GO-NH2 (d).
XPS was used to investigate the surface chemical
environments of the materials. The survey XP spectrum of GO
showed that the pure GO surface consisted of only carbon and
oxygen elements (Fig. 3a). For the GO-NH2(1.8)-Si(0.2)
sample (Fig. 3b), nitrogen and silicon were detected in
addition to carbon and oxygen. This indicates that ATPMS
and MTMS had been immobilized on the GO surface.
Deconvolution of the C1s spectrum of GO (Fig. 3c) shows
that the spectrum can be divided into four independent peaks.
The peaks at 284.6, 286.5, 287.4, and 288.6 eV are ascribed to
sp2 C=C, C–OH, C–O–C, and O–C=O, respectively. In the
C1s spectrum of GO-NH2(1.8)-Si(0.2) (Fig. 3d), the intensity
of the C–OH peak decreased sharply and a new peak emerged
at 285.6 eV, which is assigned to C–O–Si 35,36. The intensities
3.3 Catalytic activity of Pickering emulsion
A series of Pickering emulsion systems were prepared with
GO-NH2(x)-Si(y) and used in Knoevenagel condensations of
benzaldehyde with malononitrile. The catalytic activities of
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the emulsions were evaluated; the results are shown in Table
1.
mass ratio from 1.8:0.2 to 0.6:1.4. The BAL conversion
DOI: 10.1039/C9NJ06097A
decreased from 97.6% with GO-NH2(1.8)-Si(0.2) as the
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catalyst to 45.5% with GO-NH2(0.6)-Si(1.4) as the catalyst.
Generally, the properties of a Pickering emulsion, e.g., the
size, density, and size distribution of the droplets, are closely
related to its catalytic activity. On the basis of the above
results, the Pickering emulsions prepared with GO-NH2(x)-
Si(y) were examined by optical microscopy. Fig. 5 shows that
the Pickering emulsion prepared with GO-NH2(1.5)-Si(0.5)
had the smallest and densest droplets; the mean droplet size
was 17 μm and droplets of size 10–25 μm accounted for about
83% of the total (the narrowest size distribution). The droplets
of the Pickering emulsion prepared with GO-NH2(1.5)-Si(0.5)
were smaller and denser than those of the emulsion prepared
with GO-NH2(1.8)-Si(0.2), but the Pickering emulsion
prepared with GO-NH2(1.8)-Si(0.2) had the highest catalytic
activity. This indicates that the amount of active sites is an
important factor in the synergistic effect between the active
sites and lipophilicity of the GO surface. When the
APTMS:MTMS mass ratio was 0.6:1.4, the prepared
Pickering emulsion contained only a few droplets, which
resulted in a decline in the catalytic activity. Because of the
synergistic effect between the active sites and lipophilicity of
the GO surface, the catalytic activity of the Pickering
emulsion prepared with GO-NH2(1.8)-Si(0.2) was much
higher than that of the emulsion prepared with GO-NH2 in
Knoevenagel condensations at mild temperature (40 °C).
The synergistic effect between the active sites and
lipophilicity of the GO surface was investigated by measuring
the contact angles of water and BAL on GO-NH2(x)-Si(y).
Fig. 6 shows that the contact angles of water on GO-
NH2(1.5)-Si(0.5), GO-NH2(1.2)-Si(0.8), GO-NH2(0.9)-Si(1.1),
and GO-NH2(0.6)-Si(1.4) were 47°, 56°, 67°, and 79°,
respectively. The contact angles of BAL on GO-NH2(1.5)-
Si(0.5), GO-NH2(1.2)-Si(0.8), GO-NH2(0.9)-Si(1.1), and GO-
NH2(0.6)-Si(1.4) were 17°, 16°, 14°, and 11°, respectively.
These results indicate that the lipophilicity of the GO surface
increased with increasing organosilane mass. More active sites
on the GO surface were occupied by organosilane species
because both APTMS and MTMS were grafted on the GO
surface via the hydroxyl groups on GO, and this decreased the
catalytic activity of the Pickering emulsion. However, a
significant increase in the lipophilicity destroys the
amphiphilicity of GO. In a certain range, a moderate increase
in the lipophilicity of the GO surface promotes substrate
adsorption on the catalyst and stabilizes the W/O Pickering
emulsion. The results show that the Pickering emulsion
prepared with GO-NH2(1.8)-Si(0.2) had the highest catalytic
activity in the Knoevenagel condensation at mild temperature
(40 °C) and its activity was much higher than that of the
emulsion prepared with GO-NH2. This indicates a synergistic
effect between the active sites and lipophilicity of the GO
surface.
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Table 1 Catalytic activity of Pickering emulsion for Knoevenagel condensation a.
O
CN
CN
Cat.
H
+
+
H2O
9
H2O, 1h
CN
CN
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11
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Entry
Catalysts
Conv. (%)
Sel. (%)
1
2
Blank
13.7
15.4
66.2
20.8
74.1
45.5
57.2
72.5
86.3
97.6
>99.5
>99.5
95.1
GO
3
SBA-15-NH2
4
GO-Si
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
5
GO-NH2
6
GO-NH2(0.6)-Si(1.4)
GO-NH2(0.9)-Si(1.1)
GO-NH2(1.2)-Si(0.8)
GO-NH2(1.5)-Si(0.5)
GO-NH2(1.8)-Si(0.2)
7
8
9
10
a Reaction conditions: benzaldehyde (15 mmol), malononitrile (15 mmol), H2O
(4 mL), catalyst (20 mg), n-dodecane (0.3 mL); reaction temperature: 40 ℃,
reaction: time 1 h.
In the catalyst-free Knoevenagel condensation of
benzaldehyde with malononitrile, the BAL conversion was
only 13.7%. The BAL conversion over GO was 15.7%, which
is almost the same as that for the blank. The BAL conversion
over GO-Si was slightly higher, i.e., 20.8%. The BAL
conversion clearly increased when GO-NH2 was used as the
catalyst. This indicates that amine groups are the active
catalytic active sites for the Knoevenagel condensation.
compared to the SBA-15-NH2, The catalytic activity of the
Pickering emulsion prepared with GO-NH2 was higher than
that of SBA-15-NH2. This is because SBA-15-NH2 is non-
amphiphilic and does not stabilize the Pickering emulsion.
The use of Pickering emulsions prepared with organosilane-
modified amine-functionalized GO as catalysts for
Knoevenagel condensations was investigated. The catalytic
activity of the Pickering emulsion prepared with GO-
NH2(1.8)-Si(0.2) in a Knoevenagel condensation at mild
temperature (40 °C) was much higher than that of the catalyst
prepared with GO-NH2. The BAL conversion increased from
74.1% with GO-NH2 as the catalyst to 97.6% with GO-
NH2(1.8)-Si(0.2) as the catalyst. This indicates that the
synergistic effect between the active sites and lipophilicity of
the GO surface played an important role in determining the
catalytic activity of the Pickering emulsion in Knoevenagel
condensations. The results show that the APTMS:MTMS
mass ratio is an important factor in the catalytic activity of the
Pickering emulsion. The catalytic activity of the Pickering
emulsion clearly decreased with decreasing APTMS:MTMS
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Fig. 5 Optical micrograph and droplets size distribution of Pickering emulsion stabilized by GO-NH2(x)-Si(y) with various the mass ratio of APTMS to MTMS. a: GO-NH2;
b: GO-NH2(0.6)-Si(1.4); c: GO-NH2(0.9)-Si(1.1); d: GO-NH2(1.2)-Si(0.8); e: GO-NH2(1.5)-Si(0.5); f: GO-NH2(1.8)-Si(0.2).
Fig. 6 Contact angles of water on surface of GO-NH2, GO-NH2(1.8)-Si(0.2), GO-NH2(1.5)-Si(0.5), GO-NH2(1.2)-Si(0.8), GO-NH2(0.9)-Si(1.1) and GO-NH2(0.6)-Si(1.4) (a1-
f1); Contact angles of BAL on surface of GO-NH2 and GO-NH2(1.8)-Si(0.2), GO-NH2(1.5)-Si(0.5), GO-NH2(1.2)-Si(0.8), GO-NH2(0.9)-Si(1.1) and GO-NH2(0.6)-Si(1.4) (a2-f2).
favored the formation of smaller and denser droplets with a
narrower size distribution in the Pickering emulsion. This
effectively enhances the interfaces between active sites and
reactants and increases the catalytic activity of the Pickering
emulsion. For example, the BAL conversion was only 76.8%
over 10 mg of GO-NH2(1.8)-Si(0.2) (Table 2, entry 1) because
the droplets were relatively large and less dense, with a broad
size distribution. The BAL conversion reached 97.6% over 20
mg of GO-NH2(1.8)-Si(0.2) (Table 2, entry 3); the Pickering
emulsion had smaller and denser droplets with a narrower size
distribution. When the amount of GO-NH2(1.8)-Si(0.2) was
25 mg, the BAL conversion was 99.8% (Table 2, entry 4); the
Pickering emulsion had the smallest and most dense droplets,
with the narrowest size distribution.
3.4 Effect of amount of catalyst on the catalytic activity of
Pickering emulsion
Table 2 Effect of amount of GO-NH2(1.8)-Si(0.2) on catalytic activity a.
Entry
Catalyst weight (mg)
Conv. (%)
Sel. (%)
1
2
3
4
10
15
20
25
76.8
87.3
97.6
99.8
>99.5
>99.5
>99.5
>99.5
a
Reaction conditions: benzaldehyde (15 mmol), malononitrile (15 mmol), H2O
(4 mL), n-dodecane (0.3 mL) and GO-NH2(1.8)-Si(0.2) (10-25 mg); reaction
temperature: 40 °C, reaction time: 1 h.
The effects of the amount of catalyst on the catalytic
activity of the Pickering emulsion was studied. The results are
shown in Table 2. The BAL conversion gradually increased
with increasing amount of GO-NH2(1.8)-Si(0.2). Fig. 7 shows
that an increase in the amount of GO-NH2(1.8)-Si(0.2)
These results show that for catalysts with the same
APTMS:MTMS mass ratio on GO, the Pickering emulsion
properties are related to the amount of catalyst, and this is a
key factor in determining its catalytic activity.
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Fig. 7 Optical micrograph and droplets size distribution of Pickering emulsions prepared using different mass of GO-NH2(1.8)-Si(0.2) (a): 10 mg, (b): 15 mg, (c): 20 mg,
(d): 25 mg.
example, the maximum 3-fluorobenzaldehyde conversion over
the emulsion system was 99.8% (Table 4, entry 1), and 4-
3.5 Comparison of Pickering emulsion and other solvents
fluorobenzaldehyde conversion reached 99.6% (Table 4, entry
2), p-Tolualdehyde conversion over this system reached 97.5%
(Table 4, entry 3), and p-methoxybenzaldehyde conversion was
95.1% (Table 4, entry 4). The high conversions of aromatic
aldehydes over this catalytic system can be attributed to π–π
Table 3 Catalytic activity of GO-NH2(1.8)-Si(0.2) in different solvents a.
Entry
Solvent
Conv. (%)
Sel. (%)
1
2
3
4
5
6
THF
Toluene
Benzene
Ethyl acetate
CHCl3
41.4
30.6
16.9
19.6
14.9
28.1
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
interactions between GO and aromatic molecules 18
.
Table 4 Catalytic activity of Pickering emulsion under various substrates a.
CH3CN
Entry
1
Aldehyde
Nitrile
Conv. (%)
99.8
Sel. (%)
>99.5
a Reaction conditions: benzaldehyde (15 mmol), malononitrile (15 mmol), solvent
(4 mL), n-dodecane (0.3 mL) and GO-NH2(1.8)-Si(0.2) (20 mg); reaction
temperature: 40 °C, reaction time: 1 h.
F
H2
C
CHO
The Knoevenagel condensation catalytic activities of GO-
NH2(1.8)-Si(0.2) in various organic solvents were investigated.
The highest BAL conversion was achieved in THF (41.4%,
Table 3, entry 1). This is attributed to good dispersion of GO-
NH2(1.8)-Si(0.2) in THF 37. The BAL conversions in toluene,
benzene, ethyl acetate, chloroform, and acetonitrile were
30.6%, 16.9%, 19.6%, 14.9%, and 28.1%, respectively (Table
3, entries 2–6). The catalytic activity of the Pickering emulsion
prepared with GO-NH2(1.8)-Si(0.2) was much higher than
those of the organic solvents tested. This is because the reaction
in organic solvents is a heterogeneous catalytic reaction and
therefore has high mass-transfer resistance. The Pickering
emulsion effectively enhances the interfaces between active
sites and reactants and clearly decreases the mass-transfer
resistance, which leads to much higher catalytic activity. In
particular, the Pickering emulsion prepared with GO-NH2(1.8)-
Si(0.2) showed excellent catalytic activity because of its
smaller and denser droplets, and narrower size distribution.
NC
CN
CHO
H2
C
2
3
4
99.6
97.5
95.1
>99.5
98.7
NC
NC
NC
CN
CN
CN
F
CHO
H2
C
CHO
H2
C
>99.5
MeO
H2
C
5
6
87.5
83.1
>99.5
98.5
CHO
NC
NC
CN
CN
O
H2
C
CHO
3.6 Exploration of substrates scope
Various aldehydes were used in Knoevenagel condensations
with malononitrile in the Pickering emulsion systems to explore
the compatibility of the catalyst with different substrates; the
results are shown in Table 4. The catalytic activities of the
Pickering emulsion systems were higher with aromatic
aldehydes than with heterocyclic and aliphatic aldehydes. For
a
Reaction conditions: aldehyde (15 mmol), malononitrile (15 mmol), H2O (4 mL),
n-dodecane (0.3 mL) and GO-NH2(1.8)-Si(0.2) (20 mg); reaction temperature: 40
°C, reaction time: 1 h.
The catalytic activities in reactions with aromatic aldehydes
with electron-withdrawing groups were higher than those in
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reactions with aldehydes with electron-donating groups. This is
ARTICLE
The catalytic activity of a Pickering emulsion system for
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because the strongly electropositive carbonyl carbon in a Knoevenagel condensations was mainlDyOId:e10te.1r0m39in/Ce9dNJb0y609th7Ae
compound with electron-withdrawing groups facilitates attack active sites and lipophilicity of the catalyst surface. The
of the carbanions of active methylene compounds such as APTMS:MTMS mass ratio was important in determining the
malononitrile. When other aldehydes were used as substrates, catalytic activity of the Pickering emulsion. The catalytic
the catalytic activity of the Pickering emulsion system clearly performance of the Pickering emulsion prepared with GO-
declined. For example, the conversion of 2-furaldehyde and NH2(1.8)-Si(0.2) in the Knoevenagel condensation of
valeraldehyde were 87.5% and 83.1%, respectively (Table 4, benzaldehyde with malononitrile at mild temperature was much
entries 5 and 6). In summary, GO-NH2(1.8)- Si(0.2) gave good better than that of the catalyst prepared with GO-NH2. This
catalytic activity with the above six substrates, and this indicates a synergistic effect between the active sites and
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indicates that it has excellent substrate compatibility.
lipophilicity of the GO surface.
3.7 Catalyst recycling
Conflicts of interest
There are no conflicts to declare.
Reusability is an important indicator in evaluating the
performance of a catalyst. The reusability of GO-NH2(1.8)-
Si(0.2) was therefore examined. Fig. 9a shows that GO-
NH2(1.8)-Si(0.2) retained high activity after six runs, and BAL
conversion reached 90.7%. Fig. 9b shows that after six runs the
Pickering emulsion still had a small droplet size, high droplet
density, and narrow droplet size distribution, i.e., 10–45 μm.
Statistical analysis showed that the proportion of droplets of
size 15–30 μm was approximately 73% of the total and the
droplet size in this Pickering emulsion was about 25 μm.
Acknowledgements
This work was supported by the Natural Science Foundation of
China (No.21673024，No. 21676029).
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