Amines functionalized C60 as solid base catalysts for Knoevenagel condensation with high activity and stability

Article abstract of DOI:10.1039/c5ra16011a

Four kinds of amines functionalized C₆₀ were prepared through an amination reaction of C₆₀ with organic amines. Their surface structures and basic properties were studied by Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) with CO₂ as the probe molecule. When used as solid base catalysts, N-aminoethylpiperazine functionalized C₆₀ (C₆₀-AEP) showed the best catalytic activity in the Knoevenagel condensation reaction due to the highest basic strength. In addition, the C₆₀-AEP catalyst also showed impressive catalytic stability. All these features endowed C₆₀-AEP with characteristics of both homogeneous and heterogeneous catalysts. More interestingly, chiral l-lysine functionalized C₆₀ (C₆₀-L-L) can be used for asymmetric intermolecular aldol reactions.

Full text of DOI:10.1039/c5ra16011a

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Amines functionalized C60 as solid base catalysts for Knoevenagel
condensation with high activity and stability
Received 00th January 20xx,
Accepted 00th January 20xx
Yongbin Sun, Changyan Cao*, Peipei Huang, Shuliang Yang and Weiguo Song*
Four kinds of amines functionalized C60 were prepared through an amination reaction of C60 with organic amines. Their
surface structures and basic properties were studied by Fourier transform infrared spectrometer (FTIR), X-ray
photoelectron spectroscopy (XPS) and Temperature-programmed desorption (TPD) with CO2 as probe molecule. When
used as solid base catalysts, N-aminoethylpiperazine functionalized C60 (C60-AEP) showed best catalytic activity in
Knoevenagel condensation reaction due to the highest basic strength. In addition, C60-AEP catalyst also showed impressive
catalytic stability. All these features enabled C60-AEP with characteristics of both homogeneous and heterogeneous
catalyst. More interesting, chiral L-lysine functionalized C60 (C60-L-L) can be used for asymmetric intermolecular Aldol
reactions.
DOI: 10.1039/x0xx00000x
www.rsc.org/
group reported that C60 fullerenol, a simple derivative of C60,
was an outstanding catalyst for Henry reaction, Aldol reaction
and the cycloaddition between CO2 and epoxides.13 We
envision that the amines functionalized C60 may also be used
as effective heterogeneous base catalysts.
In this manuscript, we report four kinds of amines
functionalized C60 through simple amination reaction of C60
with organic amines. Fourier transform infrared spectrometer
(FTIR), X-ray photoelectron spectroscopy (XPS) and elemental
analysis indicated the presence of abundant amine groups.
When used as solid base catalysts, they showed excellent
catalytic activity and stability in Knoevenagel condensation
reaction. More interesting, chiral L-lysine functionalized C60
can be used for asymmetric intermolecular Aldol reactions.
1. Introduction
Basic-type catalysts play a decisive role in a large number of
chemical processes.1 Various solid base catalytic systems have
been developed in the literatures, such as layered double
hydroxides,2 zeolites,3 oxides,4 porous metal-carboxylates,5
graphene based materials,6 carbon nitride,7 and so on.
Typically, organic bases immobilized on different supports, for
example, amino-grafting onto silica (NH2-SiO2)8 and amino-
grafting onto metal-organic frameworks (NH2-MOF),9 have
been actively investigated due to their unique textural and
physicochemical properties. However, most of these solid base
catalysts suffer from deactivation due to the instability of the
supports, or their preparation methods are complicated.10 It is
very desirable to develop a convenient and effective route to
prepare highly active and stable heterogeneous base catalysts.
Compared to silica, zeolite and MOF, carbon-based
materials are more suitable as supports for amino grafting due
to their chemical stability. The most common carbon supports
are carbon nanotubes and graphene. Fullerenes are also one
kind of carbon materials, which are aromatic molecules
containing double bonds, so they can undergo a wide variety
of reactions characteristic of alkenes.11 Over the last two
decades, a great diversity of reactions had been developed to
functionalize C60 or C70 for organic reactions.12 Recently, our
2. Experimental Section
2.1 Materials. All solvents were purchased from Sinopharm
Chemical Reagent Beijing Co., Ltd. Organic amines were
purchased from Adama's Reagent Co., Ltd. C60 was purchased
from Suzhou Dade Carbon Nanotechnology Co., Ltd. All the
reagents used for Knoevenagel Condensation reaction were
purchased from Alfa Aesar. All the reagents were used without
further purification.
2.2 Catalyst preparation. The ethanediamine (EDA)
functionalized C60 were synthesized by following the reported
procedure.14 200 mg of C60 (2.8 mmol) was added into a round
bottom flask which contained 6 mL of ethanediamine, the
CAS Key Laboratory of Molecular Nanostructures and Nanotechnology, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
E-mail: cycao@iccas.ac.cn, wsong@iccas.ac.cn;
o
mixture was stirred at 50 C for 24 h, then 20 mL of toluene
was added, the solid was collected by centrifugation, and
washed three times with tetrahydrofuran. The product was
dried at 60 oC for 12 h. The N-aminoethylpiperazine (AEP)
Fax&Tel: +86 10-62557908
†
Electronic supplementary information (ESI) available: Experimental details, XRD
analysis, elemental analysis, FT-IR spectra of C60-AEP after being reused 14 times,
CD spectrum of C60-L-L and CO2 TPD analysis. See DOI: 10.1039/x0xx00000x
functionalized
C60
and
diethylenetriamine
(DETA)
functionalized C60 were prepared in the same way except that
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the N-aminoethylpiperazine or diethylenetriamine was used
instead of ethanediamine. The synthesis of L-lysine (L-L)
functionalized C60 was different as L-lysine was not liquid and
there was acid site in it. The preparation method was as
follows. 108 mg of C60 (0.15 mmol) was dissolved in 50 mL of
toluene, 500 mg of L-lysine (25 equiv.) and 150 mg of NaOH
(25 equiv.) was added, the mixture was stirred at 110 oC for 6 h,
and the toluene became colorless. After that, the toluene was
removed, and the residue solid was dissolved with water, and
precipitated by adding 5 times volume ethanol. This process
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DOI: 10.1039/C5RA16011A
o
repeated 5 times. The product was dried at 60 C for 12 h. The
N-aminoethylpiperazine functionalized carbon nanotube was
prepared in the similar method with that of C60. 100 mg of
multi-wall carbon nanotube or few-wall carbon nanotube was
added into a round bottom flask which contained 6 mL of N-
Scheme 1 Preparation of amines functionalized C60
.
N-aminoethylpiperazine (AEP), diethylenetriamine (DETA),
ethanediamine (EDA) and L-lysine (L-L). The corresponding
products were denoted as C60-AEP, C60-DETA, C60-EDA and C60-
L-L, respectively. During the amination reaction, organic
amines acted as nucleophilic reagents reacted with C60 to
obtain the respective amine functionalized C60 according to the
literatures.14 The amines functionalized C60 were multi-
addition products, and elemental analysis was difficult to
reconcile with rational formulae.14 The average number of
organic amines attached to one C60 molecule was determined
by the mass ratio of carbon and nitrogen, and the results were
listed in Table S1 (ESI†).
o
aminoethylpiperazine. The mixture was stirred at 50 C for 72
h, after that 20 mL of toluene was added. The solid was
collected by centrifugation, and washed three times with
tetrahydrofuran. The product was dried at 60 oC for 12 h.
2.3 Catalyst Characterizations. X-ray diffraction (XRD) pattern
was recorded on a Rigaku model D/MAX-2500V system (Cu Kα
radiation) at 40 kV and 200 mA. Fourier transform infrared
spectrum was performed on a TENSOR-27 type infrared
spectrometer. X-ray photoelectron spectroscopy data were
obtained with an ESCALab220i-XL electron spectrometer from
VG Scientific using 300 W Al Kα radiation. Elemental analysis
was carried out on a Flash EA 1112 type elemental analyzer.
X-ray diffraction analyses showed that C60 was crystalline,
but amines functionalized C60 were amorphous (Fig. S1). It
meant that the crystal structure of C60 was destroyed after
nucleophilic addition, and the products were not physical
mixture of C60 and organic amines. In order to confirm that
organic amines were attached onto C60, Fourier transform
infrared spectrometer (FTIR) analyses were conducted (Fig. 1).
The bands at around 3400 cm-1 and 1630 cm-1, 2920 cm-1 and
1460 cm-1, 1110 cm-1 and 1040 cm-1 can be assigned to amine
groups (-NH-),15 alkyl groups (-CH2-)16 and C-N groups,14
respectively. The spectrum of C60-L-L showed a COO- peak at
1580 cm-1, suggesting the existence of L-lysine.16 These peaks
cannot be found in the spectrum of C60 (Fig. S2), it indicates
the presence of organic amines.
X-ray photoelectron spectroscopy was further used to
characterize the nitrogen forms in amines functionalized C60.
The spectra (Fig. 2) showed a small difference among these
samples, indicating the chemical environments of nitrogen in
these samples were different. The peak with a binding energy
at around 398.8 eV could be assigned to amine.17 C60-EDA
exhibited the lowest binding energy (398.6 eV), C60-AEP and
C60-DETA showed a slightly higher value (398.8 eV), which
could be attributed to the influence of alky groups. C60-L-L
showed the highest value among these samples (398.9 eV),
which may be due to the electron withdrawing effect of
carboxyl group. All these results indicated that organic amines
were successfully attached to C60. The C1s spectra of C60
before and after the functionalization were nearly the same,
which meant the electron structure of C was not changed
greatly (Fig. S3). C60 might act as a carrier for organic amines.
Thermogravimetric (TG) analysis was conducted on
a
SHIMADZU DTG-60 thermogravimetric analyzer. Temperature-
programmed desorption (TPD) experiments were carried out
on a Micromeritics Auto Chem II 2920 type instrument.
Circular dichroism (CD) spectrophotometry was collected on a
J815 type spectropolarimeter.
2.4 Catalytic activity test
2.4.1 Knoevenagel condensation reaction. Benzaldehyde (1.0
mmol), ethyl isocyanoacetate (1.2 mmol), para-xylene (1 mmol,
as internal standard), ethanol (3 mL) and catalyst (30 mg) were
o
added into a glass reactor. The temperature was kept at 60 C.
The products were analyzed by GC (SHIMADZU, GC-2010 Plus)
and GC-MS (SHIMADZU, GCMS-QP2010s). To study the
reusability of the catalyst, the catalyst was washed one time
with ethanol before next cycle.
2.4.2 Asymmetric aldol reaction. 4-nitrobenzaldehyde (0.5
mmol), cyclohexanone (3.0 mmol) and catalyst (100 mg of C60-
L-L) were added into a glass reactor which contained 1 mL of
water. The mixture was stirred at room temperature for stated
time. Ethyl acetate was then added, and the organic solvent
was evaporated under vacuum. The crude aldol product was
purified by silica-gel column chromatography. The ee value of
the aldol product was determined by chiral-phase HPLC
(SHIMADZU, LC-15C) analysis.
3. Results and discussion
As shown in Scheme 1, four kinds of amines functionalized
C60 were prepared through amination reaction of C60 with
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In order to test the basicity of amines functVioiewnaAlritizceledOnClin6e0
catalysts, temperature-programmed desDoOrpI:t1io0.n103w9/itCh5RCA1O6201a1As
probe molecule was conducted. As shown in Fig. 3b, CO2
o
o
desorption bands were observed between 100 C and 250 C
for all amines functionalized C60 catalysts. C60-AEP displayed
o
o
two bands concentrated at 147 C and 206 C, corresponding
to secondary amines and tertiary amines, respectively. While
the maximum desorption temperature for C60-DETA, C60-EDA
and C60-L-L were 140 oC, 157 oC and 167 oC, respectively.
Typically, the desorption temperature is an indication of the
basic site strength.18 Higher desorption temperature suggests
stronger basicity. Therefore, C60-AEP exhibited the highest
basicity among these four kinds of amines functionalized C60
catalysts, which was in agreement with the total amounts of
adsorbed CO2 (Table S2).
Fig. 1 FTIR spectra of amines functionalized C60
.
Organic amines as non-ionic bases can be the catalysts for
many reactions, such as esterification reaction,19 Baylis-
Hillman reaction,20 Aldol reaction,21 Knoevenagel condensation,
and Henry reaction.22 The amines functionalized C60 have
amine groups, which are similar to organic amines. We expect
that they would be advantageous for catalysis. Knoevenagel
condensation of benzaldehyde and ethyl cyanoacetate was
then chosen as model reaction for testing their catalytic
activities. As shown in Table 1, high yields were obtained when
DMF and ethanol were used as solvents (Table 1, entries 1-6).
Polar solvent was favorable for the reaction, so ethanol was
used as solvent in the following reactions. Four kinds of amines
functionalized C60 showed different catalytic activity under the
same condition. C60-AEP and C60-EDA exhibited the highest
catalytic activity (Table 1, entries 6 and 8), while C60-L-L
showed the lowest activity (Table 1, entry 9). The catalytic
activity results were consistent with their basic density.
Knoevenagel condensation between various aldehyde
derivatives and ethyl cyanoacetates were tested. As shown in
Table 2, excellent yields were obtained with C60-AEP as catalyst
when benzaldehyde derivatives reacted with ethyl
cyanoacetate (Table 2, entries 1-4). For cyclohexanone
derivatives (Table 2, entries 5-6), alkyl aldehyde (Table 2, entry
7) and alkenyl aldehyde (Table 2, entry 8), C60-AEP also showed
high activity. In addition, other kinds of substrates such as
dimethyl malonate and 2, 4-pentanedione could be activated
as well (Table 2, entries 9-10).
Fig. 2 XPS spectra of the N1s of amines functionalized C60
.
The thermal stability of amines functionalized C60 was
analysed by thermogravimetric analysis. As shown in Fig. 3a,
the weight loss profiles of C60-AEP, C60-DETA and C60-EDA were
o
similar. They all displayed a rapid weight loss at around 175 C,
which can be attributed to the decomposition of amines
functionalized C60. This result also indicated that amines
functionalized C60 were thermally stable at below 170 oC,
ensuing their applications in a variety of catalytic reactions.
The impressive tolerance of functional groups and the
excellent catalytic activity shown in Table 2 confirmed that C60-
AEP was a highly active and effective catalyst for Knoevenagel
condensation. In addition, due to the insolubility of C60-AEP in
ethanol, C60-AEP can be separated easily from the reaction
solution through centrifugation. As shown in Figure 4, no
obvious decrease of yield was observed after 14 cycles,
indicating the catalyst was very stable. FTIR spectrum of C60-
AEP after being used for 14 times was nearly the same as that
of the fresh one (Fig. S4).
With abundant amine groups, C60-AEP was dispersed well in
reaction system and it showed high activity like homogeneous
catalyst. At the same time, due to the insolubility of C60-AEP in
ethanol, it can be recycled as heterogeneous catalyst. These
Fig. 3 (a) TG curves and (b) Carbon dioxide TPD spectra of amines functionalized C60
.
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Table 1. Catalytic activity of amines functionalized C60 for Knoevenagel condensation in
View Article Online
DOI: 10.1039/C5RA16011A
different solvents.a
a Reaction conditions: amines functionalized C60 (30 mg), benzaldehyde (1.0 mmol),
ethyl cyanoacetate (1.2 mmol), solvent (3 mL), 60 oC, 1 h.
Fig 4. Cycle performance of the prepared C60-AEP catalyst.
Table 2. Knoevenagel condensation with C60-AEP as the catalyst.a
Scheme 2. Asymmetric Aldol reaction with C60-L-L as catalysts.
Amino acids have been used as catalysts for asymmetric
intermolecular Aldol reactions.23 L-lysine is a chiral amino acid,
we envision that L-lysine functionalized C60 (C60-L-L) may also
be a chiral catalyst for asymmetric Aldol reaction. In order to
confirm this, circular dichroism spectrum was first conducted.
The result showed C60-L-L possessed CD peaks (Fig. S5). When
it was used as catalyst for asymmetric Aldol reaction between
cyclohexanone and 4-nitrobenzaldehyde (Scheme 2), 98 %
yield with ee value of 13 % was obtained. Although the
observed ee value was low at present, this chiral amino acid
functionalized C60 may provide an effective way for
preparation of heterogeneous chiral catalysts.
Conclusions
In summary, four kinds of amines functionalized C60 were
prepared through an amination reaction of C60 with organic
amines. When used as solid base catalysts, N-
aminoethylpiperazine functionalized C60 (C60-AEP) showed best
catalytic activity in Knoevenagel condensation reaction due to
the highest basic strength. In addition, no obvious decrease of
yield was observed after 14 times, indicating the impressive
catalytic stability of C60-AEP catalyst. All these features enabled
C60-AEP with characteristics of both homogeneous and
heterogeneous catalyst. More interesting, chiral L-lysine
functionalized C60 (C60-L-L) can be used for asymmetric
intermolecular Aldol reaction.
aReaction conditions : C60-AEP (30 mg), aldehyde (1.0 mmol), ethyl cyanoacetate (1.2
mmol, entries 1-8), dimethyl malonate (2.0 mmol, entry 9), diacetone (2.0 mmol, entry
10), ethanol (3.0 mL), 60 oC.
two features enabled C60-AEP with characteristics of both
homogeneous and heterogeneous catalyst.
The amines functionalized C60 fullerene can be
advantageous for catalysis, we think that carbon nanotube
may also work. The
catalytic activity of
N-
aminoethylpiperazine functionalized
few-wall carbon
nanotube was higher than that of multi-wall carbon nanotube
however, all the yields were low. Elemental analysis showed
that the nitrogen content in N-aminoethylpiperazine
functionalized carbon nanotube was far lower than that of C60-
AEP (Table S3). We supposed that carbon nanotube was more
difficult to be functionalized by amines compared to C60
fullerene.
，
Acknowledgements
We thank the National Natural Science Foundation of China
(NSFC 21273244, 21333009, and 21121063) and the Chinese
Academy of Sciences (KJCX2-YW-N41) for financial support.
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