Synthesis of sulfonated carbon nanocage and its performance as solid acid catalyst

Article abstract of DOI:10.1007/s10562-010-0526-6

A carbon nanocage material (CKT) was first successfully sulfonated by introducing sulfophenyl groups on the surface of pore channels through benzenesulfonic acid-containing aryl radical in situ generated from the reaction of 4-aminobenzenesulfonic acid and isoamyl nitrite in water. The sulfonated carbon nanocage material (S-CKT) was characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, powder small-angle X-ray diffraction and nitrogen sorption measurements. The results showed that the S-CKT still possess the high specific surface area (787 m²/g) and uniform mesoporous (pore diameter 4.7 nm) structures, although the structure of S-CKT is slightly disorder, compared with its unsulfonated precursor. S-CKT, as a carbon-based solid acid catalyst, showed good catalytic performance and reusability in the cross-Aldol condensation of ketones with aromatic aldehydes under solvent-free condition.

Full text of DOI:10.1007/s10562-010-0526-6

Catal Lett (2011) 141:459–466
DOI 10.1007/s10562-010-0526-6
Synthesis of Sulfonated Carbon Nanocage and Its Performance
as Solid Acid Catalyst
•
Pei Lin Baolin Li Jiangtao Li Huichun Wang
•
•
•
•
Xiaobing Bian Xiaomei Wang
Received: 11 September 2010 / Accepted: 29 November 2010 / Published online: 14 December 2010
Ó Springer Science+Business Media, LLC 2010
Abstract A carbon nanocage material (CKT) was first
successfully sulfonated by introducing sulfophenyl groups
on the surface of pore channels through benzenesulfonic
acid-containing aryl radical in situ generated from the
reaction of 4-aminobenzenesulfonic acid and isoamyl
nitrite in water. The sulfonated carbon nanocage material
(S-CKT) was characterized by Fourier transform infrared
spectroscopy, transmission electron microscopy, powder
small-angle X-ray diffraction and nitrogen sorption mea-
surements. The results showed that the S-CKT still possess
the high specific surface area (787 m2/g) and uniform
mesoporous (pore diameter 4.7 nm) structures, although
the structure of S-CKT is slightly disorder, compared with
its unsulfonated precursor. S-CKT, as a carbon-based solid
acid catalyst, showed good catalytic performance and
reusability in the cross-Aldol condensation of ketones with
aromatic aldehydes under solvent-free condition.
1 Introduction
The principles of green chemistry and increasing concerns
about environmental issues have stimulated the search for
recyclable strong solid acids to replace conventional toxic
and corrosive acid catalysts, such as sulfuric acid [1]. In the
past few years, various types of solid acids have been
developed, such as sulfonated ordered mesoporous silicas
(OMSs) [2–8] and sulfonated silica/carbon nanocomposites
[9]. In 2010, Jacobs’ group reported that the sulphonation
of carbon–silica hybrid materials and its use in the
hydrolysis of cellulose into high yields of glucose in mild
conditions [9]. In addition to sulfonated OMSs and sulfo-
nated silica/carbon hybrids, carbon-based solid acids have
been recently reported [10–13]. For example, by sulfona-
tion of incompletely carbonized sugars, Hara et al. obtained
a series of carbon-based solid acid catalysts with acid
densities ranging from 0.37 to 1.34 mmol/g [11, 12].
However, these materials were non-ordered [13] or non-
porous [11, 12] and exhibited low surface area, which lead
to modest catalysis activity. Recently, ordered mesoporous
carbons (OMCs) have been synthesized through the
nanocasting technique using OMSs as templates [14–16] or
via self-assembly of phenolic resins and block copolymers
[17–19]. These carbon materials exhibit high surface area,
narrow pore size distribution (PSD) and large pore volume.
Thus the functionalization of OMCs with sulfonic acid
groups is expected to allow high densities of functional
groups and ensure good accessibility to active sites.
Keywords Carbon nanocage Á Mesoporous carbon Á
Sulfonated carbon material Á Solid acid catalyst Á
Cross-Aldol condensation
P. Lin Á B. Li (&) Á J. Li Á H. Wang Á X. Wang
Key Laboratory of the Ministry of Education for Medicinal
Resources and Natural Pharmaceutical Chemistry, and School of
Chemistry and Materials Science, Shaanxi Normal University,
Xi’ an 710062, China
Significant efforts have been devoted to the sulfonation
of OMCs [20–23], and their potential applications in
catalysis have been reported [24, 25]. But those sulfonation
processes were reported with two-dimensional hexagonal
carbon as precursor. In 2005, Vinu and co-workers reported
a series of three-dimensional cage type mesoporous
e-mail: baolinli@snnu.edu.cn
X. Bian
School of Physical Science, Shaanxi Normal University,
Xi’ an 710062, China
123

460
P. Lin et al.
carbons (CKTs) using face centered cubic Fm3m meso-
porous silicas (KITs) as inorganic templates synthesized
from Pluronic F127 (EO106PO70EO106) as a structure-
directing agent [26]. These materials possess a well ordered
pore structure, high specific pore volume, specific surface
area and tunable pore diameter, and have been using in
adsorptions for biomolecules such as proteins, vitamins,
amino acids etc. [27, 28] and separation of tea components
such as catechin and tannic acid [29]. However, the
hydrophobic and inert nature of mesoporous carbons can be
unfavorable for direct applications. In addition, the three-
dimensional carbon nanocages, as new mesoporous carbon
materials, are made up of connected cage of each other.
The connected structure of pore walls make for mass
transport in chemical reaction, and each cage is like a
micro-reactor. Surface sulfonation of carbon nanocage
materials may not only change the character of materials
from hydrophobicity to hydrophilicity, but also enable
them available as a good Brønsted acid catalyst. Consid-
ering these, herein we report the preparation of sulfonated
carbon nanocages through benzenesulfonic acid-containing
aryl radical in situ generated from the reaction of 4-am-
inobenzenesulfonic acid and isoamyl nitrite to introduce
sulfophenyl groups on the surface of pore channels. The
structure of sulfonated carbon nanocages were investigated
with electron microscopic and spectroscopic method etc.
Furthermore the catalytic performance, as a carbon-based
solid acid catalyst, was evaluated through cross-Aldol
condensation of ketones with aromatic aldehydes.
The transmission electron microscopy (TEM) experiments
were conducted on a JEM-2100 electron microscope
operated at 200 kV. The samples for TEM measurements
were suspended in ethanol and supported onto a holey
1
carbon film on a Cu grid. The H NMR spectra were col-
lected on Super-conducting Fourier Digital NMR spec-
trometer (AVANCE 300 MHz) with TMS as the internal
reference. The melting points of compounds were mea-
sured by X-5 microscopic melting point apparatus. The
acid capacity of sample was determined by ZD-2 automatic
potentiometric titrator. All chemicals used were of ana-
lytical grade.
2.2 The preparation and Sulfonation of Carbon
Nanocage Material
The carbon nanocage material, CKT-3(A), was synthesized
with cage type mesoporous silica KIT-5-150 (where 150
denote the synthesis temperature of the silica material) as
inorganic template and sucrose as the carbon source using
literature method [26]. In the typical sulfonation process of
carbon material, 0.12 g of CKT-3(A) was mixed with
15 mL deionized water in a glass flask. The contents were
homogenized under vigorous stirring. The reaction flask
was heated to 80 °C in an oil bath. 0.48 g of 4-amino-
benzenesulfonic acid and 400 lL isoamyl nitrite were then
added, and a condenser was attached. The mixture was
stirred vigorously for 12 h at 80 °C. Then the contents
were allowed to cool to room temperature and filtered on a
Teflon filter (0.45 lm). The filter cake was repeatedly
washed with NaOH (1 M) and absolute alcohol to remove
any remaining unreacted organic matters, and then neu-
tralized to excessive NaOH with HCl (1 M), and dried at
40 °C overnight in a vacuum oven. The obtained sample
was denoted as S-CKT. Acid capacity of sulfonated sample
was determined by titration with NaOH aqueous solution
[30]. Briefly, 50 mg of S-CKT was dispersed in 15 g of
aqueous solution of NaCl (2 M) with stirring at room
temperature overnight. The dispersion was then titrated
potentiometrically with 0.1 M NaOH. Titration was repe-
ated for 3 times and the average number was reported.
2 Experiment Procedures
2.1 Measurement Instrument and Chemicals
The IR spectra of samples were collected on an Ava-
tar360E.S.P.FTIR spectrometer using KBr pellets. The
powder small-angle X-ray diffraction (SAXD) patterns of
samples were recorded by a Rigalcu (D/Max2550VB)
diffractometer with Ni-filtered Cu Ka radiation (40 kV,
40 mA). The step width was 0.02° at an acquisition time of
8 s per step. N2 adsorption isotherms were measured using
a Micromeritics ASAP 2020 M adsorption analyzer under
liquid N2 temperature. Prior to the measurements, the
samples were degassed for 6 h in the degas port of the
adsorption apparatus at 200 °C. The specific surface areas
(SBET) were calculated using the Brunauer–Emmett–Teller
(BET) method based on adsorption data in the relative
pressure (P/P0) range 0.05–0.35, the total pore volumes
(Vt) were determined based on the amount of the N2
adsorbed at a relative pressure of 0.992 (P/P0), and the
PSDs were derived from the desorption branches of iso-
therms by using the Barrett–Joyner–Halenda (BJH) model.
2.3 General Procedure for Cross-Aldol Condensation
of Ketones with Aromatic Aldehydes
1 mmol of Ketone, 2 mmol (or 1 mmol) of aromatic
aldehyde, and certain amount of S-CKT were mixed
thoroughly, placed in a glass flask equipped with a mag-
netic stirrer and a reflux condenser. The mixture was
heated at 70 °C with stirring for 30–120 min until aromatic
aldehyde was exhausted (monitored with TLC). After the
mixture was cooled to room temperature, dichloromethane
was added and heated for few minutes to sufficiently
123

Synthesis of Sulfonated Carbon Nanocage
461
dissolve the products. The catalyst (S-CKT) was removed
by filtration, and can be recycled after washing with
dichloromethane and drying. The filtrate was concentrated
in normal pressure to recycle the dichloromethane, and the
solid residue was recrystallized from acetone or ethanol to
give pure target product.
Ar–H), 7.59 (m, 3H, Ar–H), 7.76–7.81 (d, J = 15.6 Hz,
1H, = CH), 7.99–8.02 (d, J = 7.1 Hz, 2H, Ar–H).
4-Nitrochalcone: mp 157–158 °C (lit. 158–160 °C) [33].
1H NMR (300 MHz, CDCl3)d 7.54–7.61 (m, 5H, Ar–H),
7.62–7.67 (d, J = 15.6 Hz, 1H), 7.80–7.85 (d, J =
15.6 Hz, 1H), 8.03–8.05 (d, J = 6.6 Hz, 2H, Ar–H),
8.27–8.29 (d, J = 6.6 Hz, 2H, Ar–H).
2,6-Dibenzylidenecyclohexanone: mp 116–117 °C (lit.
116–117 °C) [31]. 1H NMR (300 MHz, DMSO-d6)d
1.81–1.85 (t, J = 6.4 Hz, 2H, CH2), 2.94–2.98 (t, J =
6.4 Hz, 4H, 2CH2), 7.34–7.59 (m, 10H, Ar–H), 7.83 (s, 2H,
=CH).
3 Results and Discussion
2,6-Di(4-nitrobenzylidene)cyclohexanone: mp 159–160 °C
(lit. 159 °C) [31]. H NMR (300 MHz, CDCl3)d 1.84–1.88
3.1 Materials Characteristics
1
(t, J = 6.2 Hz, 2H, CH2), 2.93–2.95 (t, J = 6.2 Hz, 4H,
2CH2), 7.59–7.61 (d, J = 8.2 Hz, 4H, Ar–H), 7.81 (s, 2H,
=CH), 8.26–8.29 (d, J = 8.2 Hz, 4H, Ar–H).
Sulfonated carbon materials, such as sulfonated hexagonal
tube-like mesoporous carbon [23, 26], sulfonated carbon
nanotubes [34, 35] and sulfonated amorphous carbon
material [11] etc., have attracted considerable attention due
to their applications in catalysis as Brønsted acid catalysts,
and fuel cells [35]. We attempted to explore the function-
alization of carbon nanocage CKT-3(A) with sulfophenyl
group and its catalysis effect as solid Brønsted acid. Sul-
fonation of CKT-3(A) was carried out using the method
reported by Tour’s group for sulfonating carbon nanotubes
[22], this method generates benzenesulfonic acid-contain-
ing aryl radicals in situ from the reaction of 4-aminoben-
zenesulfonic acid and isoamyl nitrite in water. In order to
check the structural change of carbon material after sul-
fonation treatment, the CKT-3(A) and S-CKT were char-
acterized by nitrogen isothermal adsorption, TEM images,
SAXD and IR spectra. Nitrogen gas adsorption–desorption
isotherm tests were conducted to investigate the pore-
structural character for the carbon materials (Fig. 1).
According to the International Union of Pure and Applied
Chemistry (IUPAC) classification [36], the samples of
2,6-Di(4-methoxybenzylidene)cyclohexanone: mp 202–
203 °C (lit. 203–204 °C) [31]. 1H NMR (300 MHz,
CDCl3)d 1.80–1.83 (t, J = 6.0 Hz, 2H, CH2), 2.92 (t, J =
6.0 Hz, 4H, 2CH2), 3.84 (s, 6H, 2OCH3), 6.92–6.95 (d, J =
8.1 Hz, 4H, Ar–H), 7.44–7.47 (d, J = 8.1 Hz, 4H, Ar–H),
7.76 (s, 2H, =CH).
2,6-Di(4-hydroxybenzylidene)cyclohexanone: mp 294–
295 °C (lit. 294–295 °C) [31].1H NMR (300 MHz, CDCl3)
d 1.71 (t, J = 6.2 Hz, 2H, CH2), 2.85 (t, J = 6.2 Hz, 4H,
2CH2), 6.82–6.85 (d, J = 8.1 Hz, 4H, Ar–H), 7.39–7.42
(d, J = 8.1 Hz, 4H, Ar–H), 7.54 (s, 2H, =CH), 9.93 (s, 2H,
2OH).
2,6-Di(3,5-dimethoxybenzylidene)cyclohexanone: mp 136–
137 °C. 1H NMR (300 MHz, CDCl3) d 1.71 (t, J=6.2 Hz, 2H,
CH2), 2.89 (t, J = 6.2 Hz, 4H, 2CH2), 3.77 (s, 12H, 4OCH3),
6.54 (s, 2H, Ar–H), 6.67 (s, 4H, Ar–H), 7.55 (s, 2H, =CH); 13
C
NMR (300 MHz, DMSO-d6) d 22.8, 28.3, 55.7, 101.4, 108.6,
136.3, 137.2, 137.6, 160.8, 189.5.
2,6-Di(3,4,5-trimethoxybenzylidene)cyclohexanone: mp
1
211–213 °C (lit. 208–210 °C) [32]. H NMR (300 MHz,
CDCl3) d 1.84 (t, J=6.2 Hz, 2H, CH2), 2.97 (t, J=6.2 Hz,
4H, 2CH2), 3.89 (s, 18H, 6OCH3), 6.72 (s, 2H, Ar–H), 7.74
(s, 2H, =CH).
2,6-Di[4-(dimethylamino)benzylidene]cyclohexanone: mp
250–252 °C (lit. 250–252 °C) [31]. 1H NMR (300 MHz,
CDCl3) d 1.79 (t, J=5.4 Hz, 2H, CH2), 2.93 (t, J=5.4 Hz, 4H,
CH2), 3.02 (s, 12H, 4CH3), 6.71–7.44 (m, 8H, Ar–H), 7.75
(s, 2H, =CH).
1
Chalcone: mp 55–57 °C (lit 57–58 °C) [33]. H NMR
300 MHz, CDCl3)d 6.12 (d, J = 16.0 Hz, 1H, =CH), 7.26
(d, J = 16.0 Hz, 1H, =CH), 7.10–7.32 (m, 5H, Ar–H),
7.42–7.92 (m, 5H, Ar–H).
4-Methoxychalcone: mp 74–75 °C (lit. 75–77 °C) [33].
1H NMR (300 MHz, CDCl3)d 3.86 (s, 3H, OCH3),
6.92–6.95(d, J = 8.2 Hz, 2H, Ar–H), 7.38–7.44 (d,
J = 15.6 Hz, 1H, =CH), 7.49–7.52 (d, J = 7.1 Hz, 2H,
Fig. 1 Nitrogen absorption isotherms and pore size distribution of
CKT-3(A) and S-CKT
123

462
P. Lin et al.
Fig. 2 TEM images of CKT-
3(A) (a) and S-CKT (b)
CKT-3(A) and S-CKT present type IV isotherms with quite
significant H2-type hysteresis loops, which is a character-
istic of mesoporous materials with ‘‘ink-bottle’’ type pores
[36], and the PSD curves (inserted in Fig. 1) exhibit sharp
peaks centered at 5.1 and 4.7 nm, respectively. These data
indicated that S-CKT holds the mesoporous characteristics
of CKT-3(A), except for a slight decrease of the pore
diameter. The specific surface area (787 m2/g) and pore
volume (0.91 cm3/g) of S-CKT present obviously decrease
compared with that of CKT-3(A) (1368 m2/g and
2.10 cm3/g). These results showed that the sulfonation
process obviously changes the textural parameters of car-
bon material, which may be attributed to some block of
pore by the introduced sulfophenyl groups on the surface of
pore channels, and structural destruction and collapse
partially in the sulfonation reaction. These changes of
structure can be also observed from TEM images (Fig. 2).
The collapse and disorder of some pore structure appear in
the TEM image of S-CKT (Fig. 2b), although the period-
ical pore structure of CKT-3(A) is represented in Fig. 2a.
However, S-CKT still possesses higher specific surface
area (787 m2/g), and the mesoporous structure of CKT-
3(A) is mainly kept in S-CKT deduced from the similarity
of the SAXD patterns between CKT-3(A) and S-CKT as
shown in Fig. 3.
Fig. 3 SAXD patterns of CKT-3(A) and S-CKT
The introduction of sulfophenyl group on the surface of
the pore of S-CKT was successful. It was proved by the
significant difference between the FTIR spectra of CKT-
3(A) and S-CKT (Fig. 4). Nearly no absorption peak was
observed in the FTIR spectrum of CKT-3(A), in contrast the
peaks appear obviously in the FTIR spectrum of S-CKT.
The –SO2OH groups can be identified by the peak at 3426
(–OH, and –OH from adsorptive water, stretching vibra-
tion), 1391 and 1183 cm-1 (–SO2-asymmetric and sym-
metric stretching modes). When adsorptive water combines
with the sulfonic acid group, the asymmetric and symmetric
stretching modes shift to 1121 and 1038 cm-1. The peak at
630 cm-1 also indicate the S=O stretching mode of –SO3H
Fig. 4 FTIR spectra of CKT-3(A) (a) and S-CKT (b)
[37]. The absorption peaks of phenyl group present at 1600,
1571 and 1462 cm-1 (aromatic C=C), 1009 cm-1 (=CH),
and 835 cm-1 (C–H para-aromatic out-of-plane vibration)
123

Synthesis of Sulfonated Carbon Nanocage
463
Fig. 5 Dispersibility of CKT-3(A) (a) and S-CKT (b) in H2O, DMF, EtOH, CHCl3, CH3COCH3 and C6H5CH3 (from left to right) after disperse
by vibration and standing for 24 h
[35]. In low frequency range, the peak at 553 cm-1 was
assigned to C–S stretching mode, suggesting the existence
of covalent sulfonic acid groups. These representative
absorption peaks indicate that sulfophenyl groups were
Scheme 1 The cross-Aldol condensation of cyclohexanone and
benzaldehyde
successfully modified on the surface of cage type meso-
porous carbon S-CKT.
The density of the SO3H group was measured using
NaOH (0.10 mol/L) as titrant by acid-base potentiometric
resulting from the TLC tracking, and gave a target com-
titration [30]. The amount of SO3H loaded to the S-CKT
pound in 88.5% yield (entry 1 in Table 1). Decreasing the
was 1.53 0.09 mmol/g. The sulfonation process has
catalyst amount from 0.75 to 0.10 g/mmol of cyclohexa-
changed the surface property of the carbon materials. As
none, the time of reaction completion, yield and formation
shown in digital graph (Fig. 5a), CKT-3(A) does not
rate of target product were not changed obviously, but the
exhibit a good suspension in water and various organic
turnover frequency (TOF) at acid sites increased from 0.026
solvents after vibration, while S-CKT (Fig. 5b) can form
to 0.195 (from entry 1 to entry 4 in Table 1). When the
a homogeneous suspension in solvents, especially in water
catalyst amount was further decreased to 0.05 g/mmol
and highly polar solvents (EtOH, DMF), and these
of cyclohexanone, the time of reaction completion was
homogeneous suspension systems were retained even
increased to 60 min, the yield was reduced dramatically
after 6 months. The S-CKT exhibits hydrophobic and
to 68.6% and TOF was decreased to 0.149 (entry 5 in
hydrophilic properties because it possesses a hydropho-
Table 1). These results indicated the amount of catalyst at
bic substrate and hydrophilic functional groups. This
0.10 g/mmol of cyclohexanone is minimum dose in the
amphiphilic material may allow S-CKT to be an effi-
reaction. p-CH3C6H4SO3H is used as Brønsted acid catalyst
in high frequency. When 0.05 g (0.29 mmol) of p-CH3C6
cient solid catalyst in both hydrophobic and hydrophilic
environments.
H4SO3H instead of SCKT was used to catalyze this reac-
tion, the reaction could complete in 20 min, but the yield of
3.2 Catalytic Effect of S-CKT as Solid Acid on Aldol
Condensation Reaction
target product was only in 70.0%, and the reaction exhibited
lower selectivity compared to the reaction catalyzed by
S-CKT. It is noteworthiness the recovery of p-CH3C6
H4SO3H from the reaction mixture is tedious and difficult.
Though sulfonic acid supported on solid materials such as
non-mesoporous carbon [40], nanoporous silica [41] and
polystyrene [42] have been applied to catalyze the cross-
Aldol condensation of cyclohexanone and benzaldehyde,
they needed longer reaction time, larger amount of catalyst
or noxious solvent. Herein S-CKT exhibited a higher
efficiency to catalyze this reaction under solvent-free
condition.
The Aldol condensation reaction is recognized to be one of
the most important carbon–carbon bond-forming processes
in organic synthesis [38, 39]. It is well known that Aldol
condensation can be catalyzed not only by base, but also by
acid. In order to explore the catalytic properties of this
sulfonated carbon material as a solid acid, herein, the cross-
Aldol condensation of cyclohexanone and benzaldehyde
was used as a model reaction under solvent-free condition
(Scheme 1). Initially, 0.75 g of S-CKT was used to catalyze
the reaction of cyclohexanone (1 mmol) and benzaldehyde
(2 mmol) at 70 °C. The reaction could be over in 30 min
The recycling of catalyst is very important. The S-CKT
can stably suspend in water and highly polar solvents
123

464
P. Lin et al.
Table 1 Condensation of cyclohexanone and benzaldehyde under different catalyst loading
Entry
Sample
Catalyst loading
(g/mmol)a,b
Time of reaction
completion (min)
Yield
(%)c
Formation rate of
product (lmol/min)
TOF
(min-1 d
)
1
2
3
4
5
6
a
0.75
0.35
0.20
0.10
0.05
0.05
30
30
30
30
60
20
88.5
87.2
90.8
89.7
68.6
70.0
29.5
29.1
30.3
29.9
11.4
35.0
0.026
0.054
0.099
0.195
0.149
0.121
S-CKT
p-CH3C6H4SO3H
Catalyst loading is the amount of S-CKT used in the reaction to 1 mmol of cyclohexanone
The density of –SO3H on the S-CKT was 1.53 0.09 mmol/g
The isolated yield of target compound
b
c
d
TOF = moles product formed per mole of acid site per minute
the –SO3H group of S-CKT plays an important role in
the cross-Aldol condensation of cyclohexanone and
benzaldehyde.
Table 2 Reusability of catalyst on condensations of cyclohexanone
and benzaldehydea
NO. of run
Time of reaction
completion (min)
Yield (%)b
This sulfonated carbon material should also be able to
catalyze the Aldol condensation of other compounds. We
expanded reaction to the condensation of various substi-
tuted aromatic aldehydes with ketones in the same condi-
tion. The results shown in Table 3 indicate that the key
advantages of this catalyst are shorter reaction times,
higher yield and simple workup. The catalytic activity
of S-CKT may be attributed to its large surface area and
cage-type mesoporous besides higher density of the SO3H
groups. The catalyst with cage type mesoporous is benefit
to mass transport of molecules in reaction [45], and may
act as numerous connected micro-reactors to help the
reaction to proceed in a short time.
1
2
3
4
5
6
7
a
30
30
30
30
30
30
40
89.7
84.6
82.2
83.8
77.0
77.4
70.0
The amount of catalyst used was 0.2 g/mmol of cyclohexanone
The isolated yield of target compound
b
(EtOH, DMF), but it can still be recovered easily from the
solution of dichloromethane by a simple filtration. After
reaction, dichloromethane was added to the reaction
mixture and heated for few minutes to sufficiently dis-
solve the mixture. Then, the catalyst was recovered by
reduced pressure filtration, washed with dichloromethane
and drying in vacuum for recycling. Table 2 shows the
catalyst after being recycled six times still present a high
catalytic activity for this Aldol condensation reaction, this
run could give yields of 77.4% within the same reaction
time. As we can see in Table 2, the yields after recycling
are gradually decrease, it may be attributed to the loss of
catalyst during recycling and the loss of active sites.
Considering unfunctionalized carbon material can be also
used as a catalyst in some reactions itself [43, 44], when
we used the same amount of unfunctionalized carbon
CKT-3(A) instead of S-CKT to test whether the conden-
sation reaction of cyclohexanone and benzaldehyde
occurred under the same conditions, no target compound
was found in the reaction mixture. This result indicated
4 Conclusions
In summary, a nanocage carbon bearing benzene sulfonic
acid, S-CKT, is prepared by contacting the CKT parent
material with an aqueous solution of sulfophenyl radi-
cals, which are easily generated by reacting 4-aminoben-
zenesulfonic acid with isoamyl nitrite in water. The
sulfonated nanocage carbon material possesses higher
surface area (787 m2/g), narrower PSD and higher den-
sity of the SO3H group (1.53 mmol/g). S-CKT presents
good suspension in various solvents, especially in some
highly polar solvents such as water, EtOH and DMF.
These homogeneous suspension systems were maintained
even after 6 months. When using S-CKT to the cross-
Aldol condensations of ketones and aromatic aldehydes,
it exhibited good catalytic activity under solvent-free
condition. We believe this new carbon-based Brønsted
123

Synthesis of Sulfonated Carbon Nanocage
465
Table 3 Cross-Aldol condensation of ketones with aromatic aldehydes in the presence of S-CKTa
a
Reaction condition: the mixture of 1 mmol of ketone, 2 mmol (entry 1–7) or 1 mmol (entry 8–10) of aromatic aldehyde and 0.2 g of S-CKT
was heated at 70 °C with stirring
b
The isolated yield of product
Acknowledgments The authors are thankful to financial support
from the Natural Science Foundation of China (No. 20972090), and
grateful to Dr. Kai-qiang Liu for TEM measurements and Dr. Ming-
zhen Wang for NMR measurements.
acid catalyst can be also used extensively to catalyze
other reaction because of its numerous potential benefits,
such as high efficiency, easy recovery and reuse, and low
environmental impact.
123

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