Design and synthesis of sulfonated carbons with amphiphilic properties

Article abstract of DOI:10.1039/c4ta01836b

A new type of sulfonated carbon material with amphiphilic properties was synthesized by the hydrothermal carbonization of a mixture of furfural-sodium dodecylbenzene sulfonate at 180 °C in an autoclave. The addition of SDBS is necessary for the production of materials with long carbon chains and is possibly used to improve the solubilization of long carbon-chain and steric compounds such as pivalic acid. The resulting material was characterized by N₂ adsorption, XPS, ¹³C NMR, XRD and FTIR. The synthesized material was proven to be a highly efficient solid-acid catalyst in reactions such as the esterification of pivalic acid with alcohols, and catalytic performance much better than that of conventional solid acid catalysts, e.g. Amberlyst-15 and Nafion resin, was observed.

Full text of DOI:10.1039/c4ta01836b

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Materials Chemistry A
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Design and synthesis of sulfonated carbons with
amphiphilic properties
Cite this: J. Mater. Chem. A, 2014, 2,
11195
*
Rong Jia, Jiawen Ren, Xiaohui Liu, Guanzhong Lu and Yanqin Wang
A new type of sulfonated carbon material with amphiphilic properties was synthesized by the hydrothermal
carbonization of a mixture of furfural–sodium dodecylbenzene sulfonate at 180 ꢀC in an autoclave. The
addition of SDBS is necessary for the production of materials with long carbon chains and is possibly
used to improve the solubilization of long carbon-chain and steric compounds such as pivalic acid. The
resulting material was characterized by N2 adsorption, XPS, 13C NMR, XRD and FTIR. The synthesized
material was proven to be a highly efficient solid-acid catalyst in reactions such as the esterification of
pivalic acid with alcohols, and catalytic performance much better than that of conventional solid acid
catalysts, e.g. Amberlyst-15 and Nafion resin, was observed.
Received 15th April 2014
Accepted 8th May 2014
DOI: 10.1039/c4ta01836b
www.rsc.org/MaterialsA
production of highly efficient solid acids, in addition to the
drawbacks that a high energy cost and a ow of inert gas were
1 Introduction
required for carbonization.
Homogeneous Brønsted acids such as H2SO4 and H3PO4 are
essential catalysts for many reactions in the chemical industry.1
However, such liquid acid catalysts require energy-inefficient
processing for separation and neutralization. Therefore, princi-
ples of “green chemistry” and “green technology” have stimu-
lated research for reusable, insoluble and easily separable solid
acids, such as zeolites,2–5 heteropolyacids,6,7 metal oxides or
phosphates,8,9 sulfonated resins10–12 and sulfonated carbon.13–29
Sulfonated carbon is one type of a solid acid catalyst that has
been well studied because of its easy preparation and tunable
surface properties and has been reported to replace sulfuric acid
in many reactions.22–29 There are several methods for preparing
sulfonated carbon; one is the sulfonation of active carbon,
mesoporous carbon and pretreated char by using concentrated
acid or oleum. However, the catalytic performance of sulfonated
carbon materials synthesized by this method strongly depends
on the properties of the precursor materials employed,24,30,31 and
among these precursors, an incompletely carbonized ‘char’ was
oen used.1,13–15,25,32–34 These chars were normally produced by
the carbonization of saccharides or polycyclic aromatic hydro-
Another method for preparing sulfonated carbon is the
direct hydrothermal carbonization (HTC) of sugars, such as
glucose with p-toluene sulfonic acid (TsOH).29 Actually, HTC is a
useful method and had attracted signicant interest due to its
simplicity, low operation temperature (433 K–473 K) and low
cost.35,36 More importantly, HTC is a green method for the
generation of carbonaceous materials with tunable surface
functionality such as that achieved with imidazole, sulfonic acid
and carboxylic acid groups.29,37,38
Amorphous carbon bearing –SO3H groups can incorporate
other hydrophilic functional groups bonded to the exible
carbon sheets, such as carboxylic and hydroxyl groups. Such
incorporation provides good access by reactants in solution to
the –SO3H groups or has a synergistic effect, which gives rise to
high catalytic performance for industrially important reactions,
despite the small surface area of the material.1,13,26 However, for
many reactions, such as esterication and dehydration, the
hydrophilic surface of carbon materials makes the sorption of
bulky substrates non-effective, and the removal of the byprod-
uct water is difficult, thus leading to the deactivation of
sulfonated carbon catalysts.14,24,34 Very recently, Xiao's group
developed a mesoporous polymer-based solid acid catalyst with
excellent hydrophobicity and extraordinary catalytic activity in
the esterication and dehydration of fructose.10,39 In addition,
in many reactions involving bulky molecules, steric hindrance
also hinders the application of solid acids, such as sulfonated
resin and sulfonated carbon, and thus, the synthesis of
sulfonated carbons with exible long carbon chains and
hydrophobic or hydrophilic properties, which can solubilize
more substrates such as surfactants, is desirable for more
applications.1,22,37
ꢀ
carbons at high temperature (e.g. 400 C) under an inert atmo-
sphere for a long time (>15 h) to form polycyclic aromatic carbon
sheets. The structure or properties of these ‘polycyclic aromatic
carbon sheets’ are key for the production of a highly efficient
solid acid catalyst and are dependent on the carbon precursor
and carbonization temperature. Consequently, the properties of
these sulfonated carbon materials, e.g. acid density, were not
easy to tune due to the narrow acceptable temperature for the
Key Lab for Advanced Materials, Research Institute of Industrial Catalysis, East China
University of Science and Technology, Shanghai 200237, P. R. China. E-mail:
wangyanqin@ecust.edu.cn; Fax: +86 21 64252923; Tel: +86 21 64253824
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water was close to neutral and nally dried at 373 K for 8 h
again. The prepared sulfonated carbon was denoted as F-S. As a
comparison, 8 g of furfural and 10 g of p-toluenesulfonic acid
were also treated in a similar way and denoted as F-T.
2.2 Characterization
Powder X-ray diffraction (XRD) patterns were collected in the
q–2q mode using a Bruker D8 Focus diffractometer (Cu Ka1
˚
radiation, l ¼ 1.5406 A), operated at 40 kV and 40 mA. Fourier
transform infrared spectroscopy (FTIR) was carried out on a
Nicolet Nexus 670 FT-IR spectrometer in the range of 400–4000
cmꢁ1. The samples were ground along with KBr and then
pressed into thin wafers before analysis. X-ray photoelectron
spectroscopy (XPS) was performed using a Thermo ESCA LAB-
250 spectrometer with monochromatic Al Ka radiation, and the
Scheme
catalysis.
1 Mechanism of esterification under conditions of acid
XPS results were calibrated using the C 1s peak at 284.8 eV. 13
C
Esterication is a type of a typical acid-catalyzed reaction and
thus used here as a model reaction. The reaction mechanism is
shown in Scheme 1. Accordingly, the organic acid is rst
adsorbed onto hydrophilic groups and activated by protons and
then it reacts with alcohols. At the same time, the in situ
generated water should be removed as soon as possible during
heterogeneous catalytic esterication. In addition, when bulky
molecules are involved in esterication, which is always the case
in the production of cosmetics, sulfonated carbons with
amphiphilic properties would better enhance the reactivity
through solubilizing more starting materials. This type of a
catalyst can be prepared by incorporating long carbon chains
with hydrophilic groups such as –SO3H, –COOH and –OH in the
carbon framework. Previously, we reported the synthesis of a
carbon-based solid acid with glucose as the carbon source and
p-toluene sulfonic acid as the sulfonating agent by a hydro-
thermal method.23,29 Here we extended this method and used
sodium dodecylbenzene sulfonate (SDBS) instead of p-toluene-
sulfonic acid as the source of sulfonic acid to obtain sulfonated
carbon with exible carbon chains and hydrophobicity. Its
surface properties and catalytic activity in esterication were
investigated. This new type of a sulfonated carbon catalyst
exhibited remarkable catalytic performance for various reac-
tions involving acids, especially for esterication with long
carbon chains or steric hindrance, and can be reused several
times without a signicant drop in activity aer the initial
deactivation.
MAS NMR was performed on a BRUKER AV400 apparatus
purchased from BRUKER. Nitrogen sorption isotherms were
measured at 77 K with a NOVA 4200e sorption analyzer. Before
the measurements, the samples were degassed at 373 K in
vacuum for 8 h. The Brunauer–Emmett–Teller (BET) method
was utilized to calculate the apparent surface areas. Thermal
gravimetric (TG) analysis and differential thermal gravimetry
(DTA) were performed at a heating rate of 10 K minꢁ1 from 40 to
800 ꢀC in owing air using a WCT-2 thermal analyzer (Perki-
nElmer Diamond TG-DTA). The acidity of catalysts was
measured by titration.40 Typically, 0.1 g catalyst was put into
10 mL (0.05 mol Lꢁ1) of an aqueous base solution (sodium
hydroxide, sodium bicarbonate or sodium chloride) and stirred
for 1 h at room temperature. Aer washing with deionized water
and ltration, the supernatant solution was titrated with an
aqueous solution of hydrochloric acid or sodium hydroxide
(0.05 mol Lꢁ1) using phenolphthalein as an indicator. Sodium
chloride, sodium bicarbonate and sodium hydroxide were used
for the calculation of the –SO3H density, –SO3H plus –COOH
density and total acid site density (–SO3H plus –COOH, lactonic
and phenolic groups), respectively.
2.3 Catalytic test
In a typical catalytic experiment, 10 mmol of pivalic acid
(1.02 g), 50 mmol of methanol (1.60 g) and 0.25 g of solid-acid
catalyst were added to a 25 mL round-bottom ask with a reux
condenser and then heated to 343 K under stirring. A few drops
of the reaction mixture were withdrawn periodically for GC
analysis. Products were analyzed by GC-MS (Agilent 7890A-
5975C) with a HP-5 column. The substrate was determined by
titration with NaOH (0.05 mol Lꢁ1). Other reactions were carried
out in a similar way. The reaction conditions are listed in
section 3.2.
2 Experimental
2.1 Catalyst preparation
The carbon-derived solid-acid catalyst was prepared via the
thermal treatment of the mixture of furfural–sodium dode-
cylbenzene sulfonate (SDBS)–sulfuric acid at 453 K. Typically, 8
g of furfural, 8 g of SDBS and a little amount of sulfuric acid
were mixed in a 100 mL Teon-sealed autoclave and stirred for
30 min. Then, the mixture was hydrothermally treated at 453 K
3 Results and discussion
3.1 Structure and surface analysis of F-S and F-T samples
for 24 h. The obtained black product was vacuum-oven dried at XRD, XPS, FTIR and 13C MAS NMR techniques were used to
423 K for 8 h to remove small molecules and then thoroughly characterize the structures and surface properties of these two
washed with water and ethanol until the pH of the washing samples. The XRD patterns of samples F-T and F-S (Fig. 1) show
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Fig. 1 XRD patterns of F-T and F-S samples.
Fig. 3 S 2p XPS spectra of F-S and F-T samples.
that both samples display amorphous carbon nature, as indi-
cated by the broad peaks of (002) at 2q of 10–30ꢀ.
synthesis instead of DBSA (dodecylbenzene sulfonic acid) and a
small amount of sulfuric acid was added to generate sulfonic
acid, it is reasonable that the by-product sodium sulfonate was
generated and may be tightly wrapped in carbon materials,
presenting a S 2p peak at 169.5 eV.
The FTIR spectra of F-T and F-S samples are shown in Fig. 2.
The bands attributed to –SO3H group stretching at 1032, 1007
and 1118 cmꢁ1 are observed for these two samples, indicating
that the –SO3H groups considered as the acid sites were
successfully incorporated into the carbon framework.
Compared to that for the F-T sample, the stronger absorption
band at 2922 cmꢁ1 attributed to C–H stretching for the F-S
sample would originate from the long carbon chains from
SDBS. In addition, the absorption bands at 698 and 730 cmꢁ1
can be attributed to the C–H bending vibrations of benzene, and
the bands at 1495 and 1602 cmꢁ1 can be assigned to the C]C
stretching vibration in the benzene ring. According to FTIR
spectra, the two samples have almost the same functional
groups except for the difference in the amount of –CH2–.
Besides the FTIR spectra, the X-ray photoelectron spectra of
F-T sample reveal a single S 2p peak at 168.2 eV (Fig. 3), which is
attributed to the sulfur in the SO3H groups.29,30,41 This result
indicated that –SO3H groups were successfully introduced into
the carbon framework in the F-T sample. For the F-S sample,
besides the peak at 168.2 eV, another S 2p peak at 169.5 eV is
present, which could be attributed to sodium sulfonate.
Because SDBS was used as the sulfonate source during the
To conrm that the –CH2– chains were really linked on the
F-S sample, 13C CP/MAS NMR was used, and the spectra for
these two samples are presented in Fig. 4. It can be seen that the
F-T sample has NMR resonance bands at 128 and 141 ppm,
which were attributed to the aromatic carbons (sp2 carbon) and
polycyclic aromatic carbon coordinated with SO3H groups (Ar–
SO3H) in the framework.15,41 For the F-S sample, for which SDBS
was used as the sulfonate source, a group of broad peaks
appeared at 0–40 ppm, which can be assigned to methylene or
other CHx groups (sp3 carbon), in addition to the resonance
bands at 128 and 141 ppm.14,41–43 This conrmed the presence of
a few types of long carbon chains bonded onto the polycyclic
carbon ring. In addition, the aromatic carbon provided ample
room for many –OH groups to bind.
The acid site densities of the catalysts were determined by
acid–base back titration44 and are summarized in Table 1. The
results show that the F-S sample, which has a surface rich in
long carbon chains, contains almost the same acid density as
the F-T sample, according to the Boehm titration (Table 1). The
abundant –OH and –COOH groups would be due to partial
oxidation by concentrated sulfonic acid (H2SO4 and TsOH).
Fig. 2 FTIR spectra of F-S and F-T samples.
Fig. 4 13C CP/MAS NMR spectra of F-S and F-T samples.
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Table 1 Structural parameters and acid distribution of various solid Table 2 Comparison of various organic acid esterification reactions
acids
over two catalysts
Acid density (mmol gꢁ1
–SO3Hb –COOHc –OHd
)
Yield over Yield over
Substratea
Conditions F-S/%
F-T/%
SBET
Total acid
Catalyst
(m2 gꢁ1
)
(mmol gꢁ1 a
)
Methanol Acetic acid
Oleic acid
338 K/5 h
338 K/4 h
86.2
87.3
95.7
73.6
81.6
63.8
58.3
23.5
F-S
F-T
40
34
2.3
1.6
4.7
0.9
1.1
1.2
3.8
0.9
0.5
0.3
—
0.7
0.1
—
Phenylacetic acid 338 K/10 h
Pivalic acid 338 K/8 h
Amberlyst-15 53
Naon-50
<1
—
—
a
Catalyst, 0.25 g; molar ratio of organic acid and methanol: 1 : 5;
a
Determined by titration using NaOH. b Determined by titration using
methanol: 50 mmol.
NaCl exchange and subsequent titration. c Obtained by subtracting the
d
titration results of NaCl from the value of NaHCO3. Obtained by
subtracting the titration results of NaCl and NaHCO3.
3.2.1 Comparison of various carbon-based solid acids on
the esterication of pivalic acid with methanol. The inuence of
the molar ratio of methanol to pivalic acid on the conversion of
pivalic acid was investigated rst, and we found the maximum
conversion was achieved at a ratio of 5 : 1. Therefore, this ratio
was used in the following study. Here, methanol was used not
only as the reaction substrate, but also as the solvent in this
reaction. It should be noted that there were no by-products
detected, and thus, the yield of methyl trimethylacetate (MTMA)
was the same as that for the conversion of pivalic acid.
For comparison, commercial catalysts Amberlyst-15(A-15)
and Naon-50 were used in the esterication of pivalic acid, and
the results are shown in Fig. 5. The results show that F-S was
really an excellent solid acid catalyst and not only better than
F-T, but also better than Amberlyst-15 and Naon-50. The
inuence of the catalyst amount on the activity also conrmed
the good performance of F-S (Fig. 6).
3.2.2 Adsorption of pivalic acid and water on every solid
acid. The solid acid Amberlyst-15 (3.8 mmol gꢁ1) has a larger
density of –SO3H groups than F-S (1.1 mmol gꢁ1), and Naon
has stronger acidity (H0 ¼ ꢁ11 to ꢁ13) than F-S. This indicates
that the catalytic performance in the esterication reaction is
not related to the density and amount of acid sites, although
suitable acidity is necessary. One possible explanation for the
excellent performance of F-S is that pivalic acid is much more
easily adsorbed by F-S, which has both hydrophilic and
Combining all of the characterizations described above, we
believe that SDBS really supplied long carbon chains to the F-S
sample. The long exible carbon chains are hydrophobic and
can work together with the hydrophilic part (–SO3H, –COOH,
–OH) to solubilize more bulky organic acids (Scheme 2B) and
therefore enhance the reactivity over that of the F-S catalyst,
although the F-S sample has functional groups, an acid density
and a carbon framework similar to those of F-T synthesized by
TsOH (Scheme 2A).
3.2 Catalytic activity
The catalytic activities of the prepared solid acid (F-S) with
amphiphilic properties were demonstrated through the esteri-
cation of various organic acids with methanol, and the results
are presented in Table 2. For comparison, the catalytic activities
over the F-T sample were also determined and are summarized
in Table 2. The results show that the two samples have similar
yields for ethyl acetate, because acetic acid is a small molecule.
However, F-S has catalytic activities much higher than those of
F-T for bulky molecules, which proved the advantages of
sulfonated carbon with long carbon chains. One advantage is
the access of long-chain carboxylic acid by solubilization and
the other may be due to surface hydrophobicity, which removes
the in situ generated water. Because of the bulky and steric
structure, pivalic acid was selected as the substrate for further
studies.
Fig. 5 Conversion of pivalic acid over various solid acid catalysts at
343 K for different reaction times (catalyst, 0.5 g; pivalic acid, 1.02 g;
Scheme 2 Possible structures of these two carbon-based solid acids. methanol, 1.60 g).
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Table 3 TG and titration analysis results before and after 200 ꢀC
treatment
Mass (mg)
Total
Total weight Total Acid weight MC
Per.
(%)
Sample weight loss
acid
resta
(mg)
F-S
F-T
2.806
2.527
0.352
0.435
0.346 0.262
0.284 0.255
0.268
0.406 16.07
9.55
a
Titration of the acid density of the sample F-S aer thermal treatment
at 473 K under N2 gas ow.
Fig. 6 Conversion of pivalic acid over various solid acid catalysts at
343 K for 8 h (catalyst, different weights; pivalic acid, 1.02 g; methanol,
1.60 g).
bottom for 48 h at room temperature, The data from the TG
analysis are summarized in Table 3. It should be noted that in
the TG curve, there is a weight loss from the adsorbed water and
from the decomposition of –SO3H groups. Thus, the adsorbed
amount of moisture was calculated according to the equation
below by titrating the total acids of the two samples aer
treatment at 473 K:
hydrophobic properties to solubilize more pivalic acid. Another
possibility is that the hydrophobicity of F-S facilitates the
removal the in situ generated water from the active sites.
The adsorption of pivalic acid on every solid acid catalyst is
shown in Fig. 7, in terms of the decrease in the pivalic acid
concentration in methanol solution. In all experiments, 0.25 g
of the solid acid was added to 1.02 g pivalic acid with 1.60 g
methanol solution, and the mixtures were stirred at room
temperature (298 K) for different times. The results revealed
that pivalic acid can be absorbed by all solid acids. However, the
adsorption capacity is obviously different, although the surface
areas of these solid acids are similar. These results show that
F-S has the highest adsorption capacity, while A-15 has the least,
which is consistent with the catalytic reactivity ndings in
Fig. 5, indicating that the adsorption of pivalic acid really played
a crucial role. The ability of F-S to adsorb pivalic acid was due to
the presence of long carbon chains and the hydrophilic groups
on its surface.
MC ¼ M(total weight loss) ꢁ (M(total acid) ꢁ M(acid weight rest)
)
The results shown in Table 3 revealed that the long carbon
chains from SDBS really prevent the adsorption of water
(16.07% vs. 9.55%), which would be helpful for the removal of
the in situ generated water and would prompt the reaction to go
to the right side.
3.2.3 Reusability of the F-S catalyst in the esterication of
pivalic acid. The stability of the catalyst is of great importance
for practical usage. Therefore, the cycle usage of F-S was eval-
uated, and the results are shown in Fig. 8. Aer each reaction,
the used catalyst was separated from the solution by centrifu-
gation, washed with ethanol, dried at 363 K overnight and then
put into a fresh mixture of pivalic acid–methanol, maintaining
the same weight ratio of catalyst/pivalic acid/methanol. We can
see from Fig. 8 that the conversion of pivalic acid in ve cycles
was greater than 70% in every cycle, indicating good stability
aer initial deactivation.
To conrm the hydrophobicity of F-S, which would prompt
the reaction by removing the in situ generated water, TG analysis
was used to measure the amount of adsorbed water on F-S and
F-T samples. During the experiment, F-T and F-S samples were
kept in the same two airtight containers with water in the
Fig. 7 Concentrations of pivalic acid in methanol solution in the
presence of various solid acids at room temperature (298 K).
Fig. 8 Conversion of pivalic acid in the cycle usage.
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Table 4 Conversion of pivalic acid with different alcoholsa
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Pivalic acid
Methanol
t-Butanol
Cyclohexanol
Octanol
343 K
353 K
413 K
413 K
73.8
48.8
39.9
59.8
a
Reaction time: 8 h, catalyst: 0.25 g, pivalic acid: alcohol ¼ 0.01 mol:
0.05 mol.
3.2.4 Extension to other alcohols. To demonstrate that
sample F-S is a widely applicable catalyst in esterication, we
chose four different alcohols: methanol (simple alcohol),
cyclohexanol (cyclic alcohol), t-butanol (branched alcohol) and
octanol (long chain alcohol). Table 4 shows the conversion of
pivalic acid during esterication with four alcohols. We can see
that the high conversion of pivalic acid still can be obtained
when using long chain and cyclic alcohols, although the
temperature is relatively high, indicating that F-S is a really good
solid acid catalyst.
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4 Conclusions
Sulfonated carbon bearing long carbon chains was synthesized
successfully by the hydrothermal carbonization of furfural,
SDBS and H2SO4, and was shown to be highly active in the
esterication of reactions involving bulky organic acids. The
long carbon chains from SDBS provide hydrophobic domains
and play two roles in esterication: one is working together with
–SO3H, –COOH and –OH groups to solubilize more substrates
like amphiphilic surfactants to enhance the contact of the
investigated organic acid with the catalytic active sites and the
other is removing the in situ generated water and prompting the
reaction to proceed further. The cycling usage indicated that the
catalyst prepared by this method was relatively stable.
Furthermore, the prepared catalyst can also be widely used in
other types of esterication reactions, and this carbon-based
solid acid can also be synthesized using other cheaper carbon
precursors, such as sugars, starch, cellulosic polymers and
anything that can be carbonized.
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