Amine-functionalized go as an active and reusable acid-base bifunctional catalyst for one-pot cascade reactions

Article abstract of DOI:10.1021/cs400761r

The amine-functionalized graphene oxide was prepared by a facile one-step silylation approach and used as an acid-base bifunctional catalyst in one-pot cascade reactions containing successive acetal hydrolysis and Knoevenagel condensation owing to the separate coexistence of original carboxylic acid on the edge of the GO sheet and the postgrafted amine groups on the GO basal surface. This catalyst exhibited much higher activity than either amine-functionalized active carbon, amine-functionalized SBA-15, or amine-functionalized Al₂O₃ due to the enriched surface acid sites and the diminished diffusion limitation as well as high catalyst dispersion in liquid solution due to the unique two-dimensional structure. More importantly, this catalyst could be easily recycled and used repetitively, showing potential application in industry.

Full text of DOI:10.1021/cs400761r

Research Article
pubs.acs.org/acscatalysis
Amine-Functionalized GO as an Active and Reusable Acid−Base
Bifunctional Catalyst for One-Pot Cascade Reactions
Fang Zhang,* Huangyong Jiang, Xiaoyan Li, Xiaotao Wu, and Hexing Li*
The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai
Normal University, Shanghai 200234, People’s Republic of China
S
* Supporting Information
ABSTRACT: The amine-functionalized graphene oxide was
prepared by a facile one-step silylation approach and used as
an acid−base bifunctional catalyst in one-pot cascade reactions
containing successive acetal hydrolysis and Knoevenagel
condensation owing to the separate coexistence of original
carboxylic acid on the edge of the GO sheet and the
postgrafted amine groups on the GO basal surface. This
catalyst exhibited much higher activity than either amine-
functionalized active carbon, amine-functionalized SBA-15, or
amine-functionalized Al2O3 due to the enriched surface acid sites and the diminished diffusion limitation as well as high catalyst
dispersion in liquid solution due to the unique two-dimensional structure. More importantly, this catalyst could be easily recycled
and used repetitively, showing potential application in industry.
KEYWORDS: acid−base bifunctional catalyst, one-pot cascade reactions, acetal hydrolysis, Knoevenagel condensation,
amine-functionalized graphene oxide
often suffer from tedious protection−deprotection procedures
and/or exhibit unsatisfactory catalytic efficiencies due to the
enhanced steric hindrance and the change of the chemical
microenvironment, mainly due to the weak surface acidity.12−14
Recently, graphene has received increasing interest owing to
its unique structural and surface properties.15 Accordingly, a
graphene derivative, such as graphene oxide, offers a wide range
of possibilities to synthesize the novel functional catalysts
owing to its abundant oxygen-containing functionalities.16 The
structure of graphene oxide sheets consists of a basal plane of
the sheet, decorated with hydroxyl and epoxy functional groups,
while carbonyl groups are present as carboxylic acids along the
sheet edge. Recently, several research groups successfully
demonstrated that GO could be an ideal carbocatalyst for use
in a variety of chemical transformations such as hydration,
oxidation, C−H activation, and polymerization.17 Up to now,
GO has been used only as a monofunctional catalyst based on
its intrinsic chemical properties of either high acidity or strong
oxidization. Herein, we report for the first time a novel acid−
base bifunctional catalyst by using the intrinsic carboxylic acids
on the edges of GO and the amine groups postgrafted onto the
GO basal surface (see Scheme 1), which exhibited excellent
activity in one-pot deacetalization−Knoevenagel cascade
reactions owing to the unique two-dimensional structure with
low mass transfer limitation and suitable surface acidity of GO.
Meanwhile, it could be easily recycled and reused repetitively.
INTRODUCTION
■
The unparallel ability of enzymes to realize the complicated but
efficient metabolic processes is mostly through highly specific
but multistep synergic catalytic processes.1 However, the
present industrial production of fine chemicals and pharma-
ceuticals usually needs iterative synthetic strategies,2 which are
time- and cost-demanding as well as waste-producing.3 “One-
pot” cascade reactions represent the development trend of
modern organic synthesis owing to the simplified process with
low cost and reduced waste.4 Until now, most studies have
focused on homogeneous catalysts.5 Despite the high activity
and selectivity, homogeneous catalysts are difficult to recycle
and reuse, which will inevitably enhance the cost and even
cause pollution of the environment and/or final product by
heavy metallic ions.6 Heterogeneous catalysts are easily recycled
and reused.7 More importantly, the immobilized active sites
could avoid their cross-poisoning effect.8 The key problem is
the poor catalytic efficiency due to the reduced dispersion of
active sites, the enhanced diffusion limit, and the damaged
chemical microenvironment of the active center.
The combination of acid and base catalysts for the cascade
process is recognized as a more attractive approach since the
acidic and basic functions can activate electrophiles and
nucleophiles, respectively.9 Obviously, such cascade reactions
could not be performed in one pot by using homogeneous
catalysts due to the acid−base neutralization, but they could be
realized by using heterogeneous catalysts since the acid and
base groups are fixed and cannot contact directly.10 To date,
acid−base bifunctional catalysts hosted in the solid matrix, such
as silica, clay, and polymer, etc., have been reported,11 but they
Received: September 2, 2013
Revised: November 18, 2013
© XXXX American Chemical Society
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Scheme 1. Illustration of the Amine-Functionalized Graphene Oxide Preparation
ments was performed on flow apparatus (Micrometrics TP-
5080). The samples were saturated with a pure NH3 flow, and
then the physisorbed NH3 was removed through purging with
argon gas at 40 °C for 0.5 h. The sample was then heated to
750 °C at a speed of 10 °C/min. The amounts of acidic site
were measured by calculating the ammonia absorption per
gram of the samples through a TCD detector.
Activity Test. One-pot cascade reactions were carried out in
a 10 mL round-bottomed flask. In a typical run of reactions, a
catalyst containing 0.040 mmol nitrogen species, 0.50 mmol
benzaldehyde dimethyl acetal, 0.60 mmol ethyl cyanoacetate,
5.0 mL anhydrous toluene, and 50 μL water was mixed and
allowed to stir at 80 °C for 3.0 h. The product analysis was
performed on a high-performance liquid chromatograph with
mass spectrometry (HPLC/MS, Agilent 6410B). The reaction
conversion was calculated based on benzaldehyde dimethyl
acetal since there was excess ethyl cyanoacetate. The
reproducibility was checked by repeating experiments at least
three times and was found to be within acceptable limits
( 5%).
In order to determine the catalyst durability, the catalyst was
centrifuged after each run of the reactions and the clear
supernatant liquid was decanted slowly. The residual solid
catalyst was washed thoroughly with toluene and dichloro-
methane, followed by vacuum drying at 80 °C for 6.0 h. Then,
the catalyst was reused with a fresh charge of solvent and
reactants for subsequent recycle runs under the same reaction
conditions.
EXPERIMENTAL PROCEDURES
■
Catalyst Preparation. The parent graphene oxide (GO)
was prepared by the previously reported Hummers method.18
Then, 0.50 g of GO was dispersed in 20 mL of anhydrous
toluene. After ultrasonication for 2.0 h, a certain amount of 3-
aminopropyltriethoxysilane (APTES) was added and stirred for
12 h via reflux. The solid was collected by filtration, washed
with dichloromethane, and vacuum-dried at 60 °C for 10 h.
The final catalyst could be defined as amine-functionalized
graphene oxide: NH2-GO-1, NH2-GO-2, and NH2-GO-3,
corresponding to 100, 200, and 400 μL of APTES used in
the initial mixture, respectively.
For comparison, the NH2-AC was also synthesized according
to the following procedure. One gram of activated carbon (AC)
was mixed with 40 mL of concentrated nitric acid and stirred at
80 °C for 2.0 h. Then the mixture was filtered and washed with
ethanol, followed by grafting amine groups according to the
aforementioned way using 200 μL of APTES for 0.50 g of AC.
The NH2-SBA-15 was prepared by directly grafting amine
groups onto the SBA-1519 according to the aforementioned
way using 200 μL of APTES for 0.50 g of SBA-15. The NH2-
Al2O3 was prepared through the same procedure with the NH2-
SBA-15 sample.
The control samples acidified NH2-GO-2 (NH2-GO-2-A)
and basified NH2-GO-2 (NH2-GO-2-B) were prepared by
immersing NH2-GO-2 in 1.0 mol/L HCl and NaOH aqueous
solution for 1.0 h, respectively. The obtained solid powders
were washed by water and dried in vacuum at 60 °C for 10 h,
resulting in the final product.
Characterization. The nitrogen, silicon, carbon, and
hydrogen contents were determined in a Vario EL III elemental
analysis analyzer. Fourier transform infrared spectroscopy
(FTIR) spectra were collected through a Nicolet Magna 550
spectrometer using the standard KBr pellet method. Solid-state
13C CP MAS NMR, 15N CP MAS NMR, and 29Si MAS NMR
spectra were recorded on a Bruker AV-400 spectrometer using
100.6, 40.5, and 79.5 MHz, respectively. Thermogravimetric
analysis (TGA) was carried out on Shimadzu DTG-60
thermogravimetric analyzer with a ramping rate of 10 °C/min
in 50 mL/min of nitrogen flow. The surface electronic states
were analyzed by X-ray photoelectron spectroscopy (XPS,
Perkin-Elmer PHI 5000C ESCA). All the binding energy values
were calibrated by using C1S = 284.6 eV as a reference. X-ray
diffractions (XRD) were performed on a Rigaku D/MAX B
diffraction system with Cu Kα radiation. Atomic force
microscope (AFM) images were observed through a Multi-
mode IIIa electron microscope. Scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) images
were obtained on a JSM-6380LV electron microscope and a
JEOL JEM2011 electron microscope, respectively. The temper-
ature-programmed desorption of ammonia (NH3-TPD) experi-
RESULTS AND DISCUSSION
Structural Characteristics. As shown in Table 1, the
nitrogen loading increased up to 2.3 wt % after amino-silylation
■
Table 1. Elemental Analysis of GO and Amine-
Functionalized GO, AC, SBA-15, and Al2O3
C (wt
%)
H (wt
%)
N (wt
%)
Si (wt
%)
mole ratio (N/
Si)
sample
GO
52
48
46
45
74
2.7
3.3
1.8
1.9
2.0
2.1
1.6
2.1
1.8
0
0
NH2-GO-1
NH2-GO-2
NH2-GO-3
NH2-AC
0.53
1.0
2.3
1.0
1.1
1.3
1.1
2.2
4.9
2.1
1:1.01
1:1.05
1:1.06
1:1.05
NH2-SBA-15
NH2-Al2O3
2.7
1:1.03
with the enhanced APTES amount, indicating the successful
immobilization of amine groups onto the GO. Meanwhile, the
mole ratios between nitrogen and silicon content of NH2-GO,
NH2-AC, and NH2-Al2O3 samples were close to 1:1, which
indicated that there were no other nitrogen sources
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contaminating these samples in our catalyst preparation
process. The existence of amine species could be further
confirmed by FTIR and NMR. The FTIR spectra in Figure 1
material. In comparison with the pristine GO, the 13C CP MAS
NMR spectrum of NH2-GO-2 displayed three additional
resonance signals at 14, 27, and 47 ppm, corresponding to
C1, C2, and C3 atoms in the aminopropyl functional group,24
confirming that the molecular structure of organic amino
groups on the GO surface could be well-retained. Moreover,
the peak intensity of two signals at 65 and 75 ppm remarkably
decreased due to the reaction between epoxy or hydroxyl
groups on the edge of GO and triethoxy groups of 3-
aminopropyltriethoxysilane, in good accordance with the FTIR
characterization. Furthermore, we also tried to employ 15N CP
MAS NMR to directly investigate the molecular structure of the
amine group in our representative catalyst NH2-GO-2.
However, we did not collect the valuable information from
the obtained 15N NMR spectrum (Supporting Information
Figure S1) because the natural abundance of 15N is very low
(0.37%).
The XRD patterns in Figure 3 demonstrated that the original
sharp diffraction peak at 2θ = 10.2° indicative of the (001)
Figure 1. FTIR spectra of GO and NH2-GO-2 samples.
revealed that the pristine GO displayed three peaks at 3420,
1640, and 1060 cm−1 characteristic of the hydroxyl, carboxyl,
and epoxy groups, respectively.20 The representative NH2-GO-
2 exhibited two additional peaks at 2934 and 2863 cm−1,
indicative of the asymmetric and symmetric stretching modes
of C−H bonds from CH2−CH2 groups connecting with the
NH2 group. Two other additional peaks at 1569 and 801 cm−1
could be ascribed to the symmetric N−H stretching band and
the N−H out-of-plane bending vibration from the NH2 group,
respectively.21 Besides, the appearance of two new peaks at
1109 and 1031 cm−1 were assigned to the Si−O−Si asymmetric
stretching and Si−O−C stretching vibration.22 Moreover, the
intensity of the absorption peak around 3400 cm−1 character-
istic of the surface OH groups significantly decreased,
suggesting that the amino-silylation mainly occurred via
reaction with surface OH. As shown in Figure 2, the 29Si
MAS NMR spectrum of NH2-GO-2 displayed one intense peak
at 66 ppm corresponding to the T3 signal (T3 = RSi(OSi)3),23
indicating that the Si species were covalently bonded with GO
support, leading to the formation of GO/organosilica hybrid
Figure 3. XRD patterns of GO and NH2-GO-2 samples.
plane of the pristine GO25 almost completely disappeared and
turned into a broad peak in NH2-GO-2. This revealed that,
during the silylation process, the regularly aggregated GO
sheets were efficiently exfoliated to basal sheets bridged with
amine groups,26 corresponding to the transformation from
Figure 2. 29Si CP NMR of NH2-GO-2 (a) and 13C CP MAS NMR of GO and NH2-GO-2 samples (b).
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Figure 4. SEM images of GO (a) and NH2-GO-2 (b), TEM image (c) and AFM image (d) of NH2-GO-2 (c). The insets in (c) and (d) are the
SAED image and the line scan profile of the NH2-GO-2 flake.
crystalline state to amorphous state. As shown in Figure 4a,b,
the SEM images demonstrated that the amine functionalization
of GO resulted in the decrease in the degree of sheet
aggregation. Meanwhile, the TEM image of NH2-GO-2 (Figure
4c) confirmed that the amino-silylation caused no significant
damage to the GO sheet structure, and the attached selected
area electron diffraction (SAED) pattern displayed only
diffraction rings characteristic of amorphous state, which was
consistent with XRD characterization. Furthermore, the AFM
image of NH2-GO-2 in Figure 4d displayed irregular sheets
with an average thickness around 1.5 nm (see the attached line
scan profile of the NH2-GO-2 flake). Taking into account that
the thickness of single-layer graphene was about 0.34 nm,27 we
concluded that the enhanced sheet thickness observed from
NH2-GO-2 (1.5 nm) was possibly ascribed to the multiple
layers bridged with organic amine groups.
Catalytic Performances. Table 2 summarizes the catalytic
performances of NH2-GO in one-pot cascade reactions
containing sequential acetal hydrolysis and Knoevenagel
condensation. First, it was found that NH2-GO-2 catalyst
displayed absolute selectivity toward (E)-ethyl-2-cyano-3-
phenylacrylate product regardless of the reaction conditions.
We investigated the catalyst loading and reaction temperature
in the NH2-GO-2-catalyzed one-pot cascade reaction. The
benzaldehyde dimethyl acetal conversion increased with the
amount of the NH2-GO-2 catalyst added in the reaction system.
Slight diffusion limit was present because the increasing rate of
conversion was slightly lower than that of the catalyst amount.
Meanwhile, the conversion also remarkably increased with the
increase of reaction temperature, which could be attributed to
the enhanced molecular motions. The result revealed that NH2-
GO-2 catalyst could obtain high yield (95%) and excellent
Table 2. Catalytic Performances of NH2-GO in One-Pot
Cascade Reactions Containing Hydrolysis and Knoevenagel
Condensation
a
N content
(mmol)
T
(°C)
conversion of yield of B yield of C
catalyst
A (%)
(%)
(%)
NH2-GO-2
NH2-GO-2
NH2-GO-2
NH2-GO-2
NH2-GO-2
NH2-GO-2
NH2-GO-2
NH2-GO-2
NH2-GO-1
NH2-GO-3
0.040
0.020
0.040
0.080
0.020
0.040
0.080
0.020
0.040
0.040
80
30
30
30
60
60
60
80
80
80
95
35
47
66
52
74
90
66
95
89
0
0
95
35
47
66
52
74
90
66
66
87
0
0
0
0
0
0
29
2
a
Reaction conditions: benzaldehyde dimethyl acetal (0.50 mmol),
ethyl cyanoacetate (0.60 mmol), toluene (5.0 mL), deionized water
(50 μL), 3.0 h.
selectivity (100%) with 0.04 mmol N loading at 80 °C for 3.0 h.
Next, we compared the catalytic reactivity for NH2-GO catalyst
with different amine content. It was found that the yield of (E)-
ethyl-2-cyano-3-phenylacrylate first increased and then de-
creased with the increase of N loading from 0.53 to 1.0 to 2.3
wt %, corresponding to NH2-GO-1, NH2-GO-2, and NH2-GO-
3 catalysts, respectively. For NH2-GO-1 catalyst, it displayed
high catalytic reactivity in the acetal hydrolysis reaction.
However, the decreased conversion of the Knoevenagel
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condensation reaction caused the unsatisfactory yield. More-
over, NH2-GO-3 catalyst also displayed inferior catalytic
performance owing to the reduced conversion of the acetal
hydrolysis reaction. To explain this phenomenon, we measured
the amounts of acidic site and basic site of the NH2-GO
catalysts by NH3-TPD (Figure S2) and elemental analysis,
respectively. As shown in Table 3, for NH2-GO-1 catalyst, the
revealing that the carboxylic acids on the edges of GO
participated in the acetal hydrolysis reaction. Similarly, no
significant reaction occurred by using pure propylamine as a
homogeneous catalyst since it could not catalyze the hydrolysis
of A to B due to the absence of acid catalyst. Interestingly, we
found that the physical mixture of GO and propylamine
exhibited significant efficiency decrease in both the first
hydrolysis of A to B and the subsequent Knoevenagel
condensation. The increased amount of GO sample led to
the enhanced conversion of the acetal hydrolysis reaction and
also caused the decreased catalytic efficiency in the
Knoevenagel reaction. This phenomenon could be attributed
to the cross-poisoning effects between acid and base catalysts
due to the neutralization reaction between carboxyl acid on the
GO sheets and free propylamine. We furthermore tested the
catalytic activity of the acidified NH2-GO-2 (NH2-GO-2-A)
and basified NH2-GO-2 (NH2-GO-2-B) catalysts. As shown in
Table 4, NH2-GO-2-B catalyst lost its activity completely
because it could not catalyze the first step reaction. In the other
case, NH2-GO-2-A catalyst preceded the hydrolysis reaction
well; however, the remarkable decrease yield confirmed that the
acidified process destroyed the basic sites. Also, the mixture of
GO and NH2-GO-2-B catalysts showed the reduced catalytic
efficiency in comparison with NH2-GO-2 catalyst, which could
be attributed to the enhanced transfer limitation owing to the
mutual layer−layer interaction between GO and NH2-GO-2-B
samples. These results demonstrated that acidic and basic sites
existed separately in a single flake of NH2-GO-2 catalyst,
resulting in the excellent catalytic activity.28 To explore the
mechanism for the catalyst behavior of NH2-GO-2 catalyst, we
changed toluene solvent to protic ethanol solvent, and the
result showed that NH2-GO-2 catalyst displayed almost the
same catalytic reactivity for the acetal hydrolysis reaction
(Table 4). However, it exhibited decreased catalytic reactivity
for the Knoevenagel reaction, which was maybe due to the
formation of the NH3+ group resulting from a H+ proton in the
carboxylic acid groups.29 Thus, this phenomenon could be
attributed to the combination of NH2-GO-2 catalyst and
anhydrous toluene that could not shuttle the proton effectively
and created isolated regions of acid−base functionality that
reacted independently with different reactants.30
Table 3. Active Site Amount of Different Amine-
Functionalized Catalysts
sample
acidic site amount (mmol/g) basic site amount (mmol/g)
NH2-GO-1
NH2-GO-2
NH2-GO-3
NH2-AC
2.59
1.66
1.03
1.02
0.234
0.633
0.378
0.714
1.64
0.714
0.786
0.928
NH2-SBA-15
NH2-Al2O3
highest acidic site amount led to the good conversion in the
first step, and meanwhile, the lowest basic site amount caused
the uncompleted conversion in the Knoevenagel reaction. It
could be attributed that the high dispersion of amine active sites
with long distance between each other was unfavorable to this
second cross-coupling reaction between benzaldehyde and
ethyl cyanoacetate adsorbed on two neighboring active sites.
However, very high N loading in the NH2-GO-3 catalyst was
also harmful for the present reaction due to the decreased
acidic site amount and the enhanced steric hindrance for the
diffusion and adsorption of reactant molecules onto amine
active sites.
To furthermore understand the unique catalytic performance
of NH2-GO-3 catalyst, several control experiments were carried
out in the same reaction conditions (Table 4). The pristine GO
could efficiently catalyze the hydrolysis of A to B, but it could
not catalyze the subsequent Knoevenagel condensation to
produce target product (C) due to the absence of base catalyst,
Table 4. Catalytic Performances in “One-Pot” Cascade
Reactions Containing Hydrolysis and Knoevenagel
a
Condensation
To furthermore demonstrate the advantage of NH2-GO-2
catalyst, we compared the performances of different NH2-
functionalized catalysts in the one-pot cascade reaction.
Compared to NH2-AC, NH2-SBA-15, and NH2-Al2O3 with
almost the same N loadings (∼1.0 wt %), the NH2-GO-2
exhibited much higher activity in either the first acetal
hydrolysis or the subsequent Knoevenagel condensation
(Table 4), leading to the high yield and absolute selectivity
toward target product C, which could also be confirmed by the
reaction profiles of the Knoevenagel reaction (Figure S3). As
shown in Figure 5, the XPS analysis demonstrated that NH2-
GO-2, NH2-AC, and NH2-SBA-15 catalysts showed similar
features and chemical microenvironment of base active sites,
corresponding to the principal peak with binding energy
around 399.1 eV characteristic of amine groups.31 Unlike either
the NH2-AC or the NH2-SBA-15, the NH2-GO-2 displayed a
well-resolved peak at binding energy around 289.1 eV indicative
of the O−CO bond, indicating the presence of carboxylic
acid groups (see Scheme 1).32 The NH3-TPD curves in Figure
6 revealed that, besides the desorption peak at 550 °C due to
the decomposition of aminopropyl groups terminally bonded to
the supports, the NH2-GO-2 displayed much stronger
conversion of A
catalyst
(%)
yield of B (%) yield of C (%)
b
GO
94
0
94
0
0
0
c
C3H7NH2
d
e
f
GO + C3H7NH2
GO + C3H7NH2
GO + C3H7NH2
NH2-GO-2-A
NH2-GO-2-B
23
56
73
99
0
5
18
49
51
28
0
7
22
71
0
GO + NH2-GO-2-B
82
96
36
44
52
12
26
8
70
70
28
31
46
g
NH2-GO-2
NH2-AC
NH2-SBA-15
NH2-Al2O3
13
6
a
Reaction conditions: benzaldehyde dimethyl acetal (0.50 mmol),
ethyl cyanoacetate (0.60 mmol), a catalyst containing 0.040 mmol N
content, toluene (5.0 mL), deionized water (50 μL), 80 °C, 3.0 h.
b
c
d
With 50 mg of GO. With 0.040 mmol C3H7NH2. With 25 mg of
e
GO and 0.040 mmol C3H7NH2. With 50 mg of GO and 0.040 mmol
f
g
C3H7NH2. With 75 mg of GO and 0.040 mmol C3H7NH2. Using 5
mL of ethanol instead of toluene as solvent.
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Figure 5. XPS spectra of NH2-GO-2, NH2-AC, and NH2-SBA-15 samples (a, N1s; b, C1s).
and NH2-Al2O3 catalysts.35 Besides, the SEM and TEM images
(Figure S2) revealed that NH2-AC and NH2-SBA-15 were
present in three-dimensional shapes with microporous and
mesoporous structure, respectively. However, the NH2-GO-2
displayed two-dimensional sheets, which favored diffusion and
adsorption of reactant molecules onto the active sites.36
Although the NH3-TPD curves in Figure 6 revealed that the
NH2-AC contained much more acid sites than the NH2-SBA-
15, it still exhibited lower activity than the NH2-SBA-15,
possibly due to the enhanced diffusion limit in irregular
micropores than that in ordered mesoporous channels.
Moreover, Figure 7 revealed that both the NH2-AC and the
NH2-SBA-15 easily settled down from the solution, while the
NH2-GO-2 still had high dispersion in solution within 1.0 h
owing to the unique two-dimensional structure, which also
promoted catalytic activity by enhancing the contacting
probability between the reactant molecules and the catalyst.
To demonstrate this advantage of the combination of suitable
carboxylic acid intensity and unique two-dimension structure of
the NH2-GO-2 catalyst, we calculated the apparent activation
energy over different catalysts. The plots of the concentration
of benzaldehyde dimethyl acetal versus reaction time and the
Arrhenius plots for NH2-GO-2, NH2-AC, and NH2-SBA-15
catalysts in the temperature range from 60 to 80 °C are shown
in Figures S5−S7. The results indicated that the overall reaction
Figure 6. NH3-TPD profiles of NH2-GO, NH2-AC, and NH2-SBA-15
samples.
desorption peaks than either NH2-AC, NH2-SBA-15, or NH2-
Al2O3 in the temperature range from 150 to 400 °C, implying
the presence of more carboxylic acid sites,33 which could
sufficiently account for its higher activity.34 The quantitative
results (Table 3) demonstrated that NH2-GO-2 had the highest
acidic site amount in comparison with NH2-AC, NH2-SBA-15,
Figure 7. Picture of NH2-GO (a), NH2-AC (b), and NH2-SBA-15 (c) dispersed in toluene after ultrasound for 10 min and the following static
settlement for 1.0 h.
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might be a second-order reaction of benzaldehyde dimethyl
acetal since the calculated concentrations of benzaldehyde
dimethyl acetal (1/Ct) in the time courses increased linearly
with the reaction time (t) according to the reaction rate
equation, 1/Ct = kt + 1/C0.37 Accordingly, the calculated
activation energies for NH2-GO-2, NH2-AC, and NH2-SBA-15
catalysts were 63.9, 67.3, and 66.6 kJ/mol, respectively. Thus,
the lowest activation energy for NH2-GO-2 in this cascade
reaction was responsible for its highest catalytic efficiency.
To ensure that the catalytic activity originated from the NH2-
GO-2 sample other than from a dissolved amine group either
from the absorbed amine or from catalyst leaching, the
following procedure proposed by Sheldon et al. was carried
out.38 After reaction for 1.5 h, where the conversion exceeded
45% in this cascade reaction, the mixture was filtered to remove
the solid catalyst and then allowed the mother liquor to react
for another 3.0 h under the same reaction conditions. No
significant acetal hydrolysis nor increased yield of (E)-ethyl-2-
cyano-3-phenylacrylate was observed, demonstrating that the
active species were not the dissolved amine leached from NH2-
GO-2 catalyst. Therefore, it was reasonable to suggest that the
present catalysis was heterogeneous in nature.
comprising sequential acetal hydrolysis and Knoevenagel
condensation owing to the enriched surface carboxylic acid
sites and the diminished diffusion limit on the unique two-
dimensional sheets as well as the high catalyst dispersion in the
liquid phase. Other acid−base bifunctional catalysts could be
further designed in this way and could be used in various one-
pot cascade reactions catalyzed by both acid and base catalysts,
which might offer a general route for simple and green organic
synthesis with high efficiencies.
ASSOCIATED CONTENT
■
S
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*Tel: +86-21-64322272. Fax: +86-21-64322272. E-mail:
zhangfang@shnu.edu.cn.
*Tel: +86-21-64322272. Fax: +86-21-64322272. E-mail:
hexing-li@shnu.edu.cn.
An important merit of heterogeneous catalyst is that they
could be conveniently recycled and reused.39 As shown in
Figure 8, the NH2-GO-2 catalyst could be used repetitively for
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by Natural Science Foundation of
China (21107071 and 51273112), PCSIRT (IRT1269) and
Shanghai Government (10dj1400100 and 13QA1402800).
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