General acid and base bifunctional graphene oxide for cooperative catalysis

Article abstract of DOI:10.1039/c4ta01446d

In this study, a bifunctional graphene oxide material (GO-AEP-UDP) with a general acid, the ureidopropyl (UDP) group, and a general base, the 3-[2-(2-aminoethylamino)ethylamino]-propyl (AEP) group, was synthesized by silylanization of graphene oxide (GO)

Full text of DOI:10.1039/c4ta01446d

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Materials Chemistry A
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General acid and base bifunctional graphene oxide
for cooperative catalysis†
Cite this: J. Mater. Chem. A, 2014, 2,
10239
Wenfeng Zhang, Hanying Gu, Zhen Li, Yuanzhi Zhu, Yang Li, Guoliang Zhang,
*
Fengbao Zhang and Xiaobin Fan
In this study, a bifunctional graphene oxide material (GO–AEP–UDP) with a general acid, the ureidopropyl
(UDP) group, and a general base, the 3-[2-(2-aminoethylamino)ethylamino]-propyl (AEP) group, was
synthesized by silylanization of graphene oxide (GO) with organoalkoxysilane precursors.
Characterization of the obtained GO–AEP–UDP was carried out by Fourier transform infrared
spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive
spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron
spectroscopy (XPS) and zeta potential measurements. Systematic studies demonstrated that GO–AEP–
UDP has superior catalytic performance in the classic Henry reaction of 4-nitrobenzaldehyde with
nitromethane. It was found that the relative concentration and spatial arrangement of cooperative
functional groups were very important factors in the synergistic catalysis.
Received 26th March 2014
Accepted 29th April 2014
DOI: 10.1039/c4ta01446d
www.rsc.org/MaterialsA
general acid and base groups immobilized on graphene has yet
to be demonstrated in chemical reactions.
1. Introduction
Herein, we report the rst graphene supported general acid
and base bifunctional catalytic system that comprises a general
acid, the ureidopropyl (UDP) group, and a general base, the 3-[2-
(2-aminoethylamino) ethylamino]propyl (AEP) group. We also
found that this catalytic system has an excellent catalytic
performance in the cooperative Henry reaction of 4-nitro-
benzaldehyde with nitromethane.
Organocatalysis is a powerful methodology in organic synthesis,
and organocatalysts have remarkable advantages over tradi-
tional transition-metal catalysts such as insensitivity to mois-
ture and oxygen, ready availability and low toxicity.1–9 However,
the application of organocatalysts in the chemical industry is
still limited, attributed to not only their relatively low catalytic
activities (especially compared with many metal complexes),
but also the difficulty in separation from the reaction mixtures.
Therefore, development of a recyclable organocatalyst with high
catalytic performance is of great importance due to its practical
application on an industrial scale. Among the strategies that
have been developed for recycling the organocatalysts, the
preparation of heterogeneous organocatalysts is the most
promising one. So far, lots of organic acid/base catalysts and
their bifunctional systems have been successfully immobilized
on various solid supports and have shown impressive catalytic
performances in a variety of reactions.10–30 Recently, we and
others found that graphene supported phenyl–SO3H groups,
amine groups and phosphotungstic acid show excellent cata-
lytic activities and robustness in the hydrolysis rate of ethyl
acetate, nitroaldol reaction and one-pot tandem reaction.31–33
However, to the best of our knowledge, the cooperation of
2. Experimental
2.1. Preparation of the general acid/base–graphene oxide
catalyst
Graphene oxide (GO) was prepared and puried by the
Hummers method.34 The resulting GO suspension (15 mL, 19.4
mg mLꢀ1) was sonicated for 30 min. It was then added to the
mixture of ethanol (150 mL), ureidopropyltrimethoxysilane
(0.8877 g), and 3-[2-(2-aminoethylamino)ethylamino]propyl-
trimethoxysilane (0.2228 g) in a three-necked round ask with a
condenser and a stirrer. Aer 24 h reuxing at 78 ꢁC, the black
solid was ltered and washed with ethanol and deionized
water successively, followed by drying under vacuum to get GO–
AEP–UDP.
2.2. Catalytic ability
State Key Laboratory of Chemical Engineering, School of Chemical Engineering &
Technology, Tianjin University, Tianjin, China. E-mail: xiaobinfan@tju.edu.cn; Fax:
+86-22-27408778; Tel: +86-22-27408778
The performance of the catalysts was examined by the Henry
reaction. Into a round ask were placed the catalyst (10 mg),
4-nitrobenzaldehyde (1 mmol) and nitromethane (10 mL). The
resulting mixture was vigorously stirred for 24 h at 90 ꢁC. Aer
† Electronic supplementary information (ESI) available: Si 2p XPS spectrum and
HRTEM spectrum of GO–AEP–UDP, photographs and GC-MS results of the
product, and mechanism of the catalytic reaction. See DOI: 10.1039/c4ta01446d
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the catalyst was separated from the mixture by ltration, the
ltrate was analyzed by GC.
2.3. Characterization
The samples were characterized by Fourier transform infrared
spectroscopy (FTIR) (Thermo-Nicolet 380), Raman spectroscopy
(NT-MDT NTEGRA Spectra), scanning electron microscopy
(SEM) (Hitachi S4800), energy dispersive spectroscopy (EDS)
(Hitachi S4800), high-resolution transmission electron micros-
copy (HRTEM) (Philips Tecnai G2 F20), zeta potential
measurements (Malvern, nano ZS) and X-ray photoelectron
spectroscopy (XPS) (PerkinElmer, PHI 1600 spectrometer). The
reactions were monitored using a GC (Agilent 6890N GC-FID
system) and GC-MS (Agilent 6890-5973).
Fig. 1 FTIR spectra of GO and GO–AEP–UDP.
3. Results and discussion
seen in Fig. 2. GO shows absorption at 1584 cmꢀ1 correspond-
ing to the G-band. While the G-band of GO–AEP–UDP shis to
1594 cmꢀ1 aer the silylation reaction, implying the increased
compressive local stress induced by the attached AEP and UDP
groups. Moreover, the I(D)/I(G) intensity ratio of GO–AEP–UDP
(1.895) is slightly higher than that of GO (1.478); this means that
abundant AEP and UDP groups were introduced to the sp2
carbon network; this conclusion was further supported by our
XPS analysis.
The X-ray photoelectron spectroscopy (XPS) spectra of the
samples of GO and GO–AEP–UDP are shown in Fig. 3. In
comparison to GO, the increased signals of C, O, N, and Si were
observed in GO–AEP–UDP (Fig. 3a). Furthermore, a signicant
decrease of the C–O–C & C–OH bonds and a slight increase of
the carboxyl groups in the C 1s peak (Fig. 3c and d), together
with the formation of Si–O–C observed at 102.4 eV in the Si 2p
XPS spectrum (Fig. S1 of ESI†), indicate the successful hydro-
lysis and dehydration condensation of the hydroxyl and epoxy
groups on GO with the AEP– and UDP–trimethoxysilane
precursors. Moreover, the peak at 399.1 eV corresponds to
C–NH–C of AEP groups, another one at 399.8 eV represents
The bifunctional graphene oxide material with ureidopropyl
(UDP) and 3-[2-(2-aminoethylamino)ethylamino]propyl (AEP)
groups was synthesized by introducing UDP–, AEP–trimethox-
ysilane precursors to the silylanization reaction that is based on
the hydrolysis and dehydration condensation of the silanes with
the hydroxyl and epoxy groups on GO (Scheme 1).
The catalysts were characterized aer the functionalization.
Fig. 1 shows Fourier transform infrared spectroscopy (FTIR)
spectra of the samples of GO and GO–AEP–UDP. Compared with
GO, the peaks at 1639 cmꢀ1 and 1565 cmꢀ1 are associated with
C]O stretching vibrations of the UDP group and N–H bending
vibrations of the AEP group, respectively. The increased
doublets at 2927 cmꢀ1 and 2873 cmꢀ1, together with a new band
at 1442 cmꢀ1, represent the stretching and bending vibrations
of C–H from the methyl groups of UDP and AEP groups. The
signal of O–H becomes smaller aer formation of GO–AEP–
UDP. In addition, the strong stretching vibrations of Si–O at
1041 cmꢀ1, as well as the obvious stretching and bending
vibrations of Si–O–C at 1095 and 690 cmꢀ1, demonstrate that
the silylanization reaction proceeded successfully.
Raman spectroscopy was used to characterize the structural
change of graphene oxide in the chemical process, as can be
Scheme 1 Illustration of the preparation of the general acid/base
bifunctional catalyst.
Fig. 2 Raman spectra of GO and GO–AEP–UDP.
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Fig. 5 shows the scanning electron microscopy (SEM) images
and the corresponding quantitative energy dispersive X-ray
spectroscopy (EDS) mapping of GO–AEP–UDP. Fig. 5a reveals
that GO–AEP–UDP shows a crumpled structure that can also be
observed in the HRTEM image (Fig. S2 of ESI†). Element
distribution analysis demonstrates that elements O, Si, and N
(Fig. 5b–d) are homogeneously distributed on the whole surface
of GO–AEP–UDP, which indicates that AEP and UDP groups are
anchored on GO uniformly. These features of GO–AEP–UDP
make active sites utilize efficiently, because all the active sites
located on the surface of the layer are accessible, which will
render GO–AEP–UDP efficient for the catalytic reaction.
The catalytic efficiency of the resultant GO–AEP–UDP catalyst
was investigated by using the Henry reaction of 4-nitro-
benzaldehyde and nitromethane to p,b-dinitrostyrene (Scheme
2). The rst step of this reaction is the classic nucleophilic
addition of aldehyde and nitromethane to nitroalcohol,35–37
followed by intramolecular dehydration of the –OH with the
ortho-H of nitroalcohol at high temperature (GC-MS results of
the nal products can be seen in Fig. S3 of ESI†). Note that
equivalent amounts of GO, GO–UDP and GO–AEP were also
examined for comparison purposes, and the results are pre-
sented in Table 1.
Fig. 3 XPS spectra of GO and GO–AEP–UDP (a), N 1s XPS spectra of
GO–AEP–UDP (b), C 1s XPS spectra of (c) GO and (d) GO–AEP–UDP.
Yields of the products catalyzed by GO, GO–UDP, GO–AEP
and GO–AEP–UDP are 3.2%, 12.2%, 6.7% and 88.4%, respec-
tively (entries 1, 2, 3, and 5 in Table 1), which is in accordance
with the product color (Fig. S4 of ESI†); a synergistic effect
between the AEP and UDP groups was clearly observed. This
result can be explained by the fact that the secondary amines of
the AEP group were responsible for the deprotonation of
CH3NO2,38 and the UDP group could activate the carbonyl group
of 4-nitrobenzaldehyde to nucleophilic attack through double
–NH2 and O]C–NH of UDP groups in the N 1s spectrum
(Fig. 3b). Calculated by the two integral areas, the molecule ratio
of AEP and UDP groups is about 1 : 4. Note that the element
atomic ratios of N in GO–AEP–UDP are 9.62%, That is, the
density of the introduced AEP and UDP groups is about 0.62 and
2.48 mmol gꢀ1, respectively.
Fig. 4 shows the zeta potential of GO and GO–AEP–UDP. GO
displays a zeta potential of ꢀ26.6 mV under neutral conditions
because of the negatively charged carboxylate groups. However,
GO–AEP–UDP shows positively charged surfaces with a zeta
potential of 26.8 mV, which is assigned to the abundant graed
amine groups. The successful silylation reaction was veried
again.
Fig.
5 SEM image of (a) GO–AEP–UDP and the corresponding
Fig. 4 The zeta potentials of GO and GO–AEP–UDP.
quantitative EDS element mapping of (b) O, (c) Si and (d) N.
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should be attributed to the two-dimensional GO that not only
provides a large surface to support the catalytically active sites
but also facilitates the mass transport process.
To further evaluate the synergistic effect of GO–AEP–UDP,
catalysts with different relative concentrations, initial molar
ratios of the organoalkoxysilane precursors as AEP/UDP ¼ 4/1,
1/1, 1/4 and 1/8, were synthesized and systematically studied for
their catalytic performances. As summarized in Fig. 6, the
catalyst with an AEP/UDP ratio of 1 : 4 shows the highest cata-
lytic activity with a yield of 88.4% towards the product.
Scheme
2 The catalytic preparation of p,b-dinitrostyrene from
4-nitrobenzaldehyde and nitromethane.
Table 1 Performances of different catalysts in the Henry reactiona
Rank
Catalyst
Yield (%)
TONb
1
2
3
4
GO
GO–UDP
GO–AEP
3.2
12.2
6.7
—
4. Conclusions
16.9
9.0
2.5
In conclusion, we have successfully prepared a bifunctional
graphene oxide material that comprises a general acid group,
UDP, and a general base group, AEP, by silylanization.
Systematic studies demonstrated that these active groups could
be readily loaded on both sides of graphene oxide sheets with
homogeneous distribution. Notably, the obtained bifunctional
graphene oxide materials (GO–AEP–UDP) show excellent cata-
lytic performances and synergistic catalytic effect in the Henry
reaction. By adjusting the relative concentrations and spatial
arrangement of the cooperative functional groups, our bifunc-
tionalized graphene-based catalyst could have better applica-
tion prospects in other organic reactions.
Heterogeneous
1.8
mixturec
5
6
GO–AEP–UDPd
Homogeneous
mixture
88.4
11.5
124.5
6.78
a
Reaction conditions: 4-nitrobenzaldehyde (1 mmol), CH3NO2 (10 mL),
b
catalyst (10 mg), 90 ꢁC, 24 h. TON (turnover number) ¼ mmol
product per mmol catalyst for a 24 h reaction time with 10 mg of
c
d
catalysts. GO–AEP (2 mg) + GO–UDP (8 mg). The initial molar ratio
of AEP/UDP ¼ 1/4.
hydrogen bonding39 (the mechanism can be seen in Fig. S5 of
ESI†). The yield of the product by using the heterogeneous
mixture with GO–AEP and GO–UDP (1.8%, entry 4 in Table 1)
was clearly lower than that by using GO–AEP–UDP (88.4%); this
result suggests that spatial arrangements between these active
molecules are crucial for this reaction. Note that the homoge-
neous mixture of AEP and UDP gives a poor yield of 11.5% (entry
6 in Table 1) due to a large amount of byproducts. The turnover
numbers (TONs) of these catalysts give similar results; GO–AEP–
UDP gives the best performance with a TON of 124.5. Compared
with the existing studies on silica-based catalysts,40–43 the TON
of our catalyst GO–AEP–UDP is comparable to (if not better
than) most of the existing catalysts. This promising result
Acknowledgements
This study was supported by the National Natural Science Funds
for Excellent Young Scholars (no. 21222608), Program for New
Century Excellent Talents in University (no. NCET-12-0392),
Research Fund of the National Natural Science Foundation of
China (no. 21106099), Foundation for the Author of National
Excellent Doctoral Dissertation of China (no. 201251) and the
Programme of Introducing Talents of Discipline to Universities
(no. B06006).
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