Transesterification of Tributyrin and Dehydration of Fructose over a Carbon-Based Solid Acid Prepared by Carbonization and Sulfonation of Glucose

Article abstract of DOI:10.1002/cplu.201500261

A series of carbon-based sulfonated solid acid (CS) catalysts can be prepared through a facile method of direct simultaneous carbonization and sulfonation of d-glucose with concentrated sulfuric acid without a hydrothermal process. Temperature is a key factor in obtaining CS catalysts with higher amounts of -SO₃H acid sites. The transesterification of tributyrin with methanol and the dehydration of fructose to 5-hydroxymenthylfurfural (5-HMF) are investigated over the CS catalysts. The CS-150 catalyst, which is the CS catalyst prepared at 150C, shows much higher catalytic activity to give a tributyrin conversion of 99.4 %. The yield of methyl butyrate is 97.2 % and a remarkably high yield of 5-HMF (93 %) can be obtained efficiently; these yields result from the high acid density of -SO₃H. Good linearity is displayed between the total acid density and yield of methyl butyrate/5-HMF. The CS-150 catalyst has high stability and good recyclability for the dehydration of fructose.

Full text of DOI:10.1002/cplu.201500261

DOI: 10.1002/cplu.201500261
Full Papers
Transesterification of Tributyrin and Dehydration of
Fructose over a Carbon-Based Solid Acid Prepared by
Carbonization and Sulfonation of Glucose
Chuanxu Wang,[a] Fulong Yuan,[a] Lijing Liu,[a] Xiaoyu Niu,*[b] and Yujun Zhu*[a]
A series of carbon-based sulfonated solid acid (CS) catalysts
can be prepared through a facile method of direct simultane-
ous carbonization and sulfonation of d-glucose with concen-
trated sulfuric acid without a hydrothermal process. Tempera-
ture is a key factor in obtaining CS catalysts with higher
amounts of -SO3H acid sites. The transesterification of tributyr-
in with methanol and the dehydration of fructose to 5-hydrox-
ymenthylfurfural (5-HMF) are investigated over the CS cata-
lysts. The CS-150 catalyst, which is the CS catalyst prepared at
1508C, shows much higher catalytic activity to give a tributyrin
conversion of 99.4%. The yield of methyl butyrate is 97.2%
and a remarkably high yield of 5-HMF (93%) can be obtained
efficiently; these yields result from the high acid density of
-SO3H. Good linearity is displayed between the total acid densi-
ty and yield of methyl butyrate/5-HMF. The CS-150 catalyst has
high stability and good recyclability for the dehydration of
fructose.
Introduction
With growing concerns about energy consumption and issues
of environmental pollution, the utilization of environmentally
friendly catalytic materials has become a top priority in the
chemical industry.[1] In recent decades, acid-catalyzed reactions,
such as transesterification and dehydration of carbohydrates,
have attracted much attention in the production of chemi-
cals.[2] Most acid-catalyzed processes are performed in the
presence of homogeneous acids, such as H2SO4, H3PO4, and
HCl; however, corrosion, toxicity, and separation problems limit
their industrial application.[3] With the development of catalytic
science and advances in environmental awareness, searching
for recoverable and reusable solid acid catalysts to replace ho-
mogenous acid catalysts has become a hot research field. To
date, several researchers have reported a variety of solid acid
catalysts, such as zeolites, resins, oxides, and phosphates,[4] for
the production of a variety of chemical products.[5]
tions and subsequent sulfonation. These SO3H-bearing carbons
exhibited remarkable catalytic properties for various acid-cata-
lyzed reactions, such as esterification, transesterification, hydra-
tion, hydrolysis, and dehydration of sugars.[3c,4d,5c] Hara et al.
prepared a carbon-based solid acid by incomplete carboniza-
tion of sulfonated aromatic compounds.[6] Later, they prepared
sulfonated carbon by the sulfonation of incompletely carbon-
ized sugars and proved that the activity was comparable to
that of liquid sulfuric acid in the esterification of higher fatty
acids.[7] The particular amorphous carbon materials with SO3H
groups were also considered as new types of solid acids;[3b]
these exhibited remarkable catalytic performances for various
acid-catalyzed reactions, such as the esterification of higher
fatty acids,[7] hydration,[8] and hydrolysis.[9] There are two routes
for the preparation of amorphous carbon with SO3H groups:
incomplete carbonization of sulfoaromatic compounds[6] and
sulfonation of incompletely carbonized organic matter.[8,9]
Fraile and co-workers reported that sulfonated hydrothermal
carbons were highly active for esterification of palmitic acid
with different alcohols.[10] Qi et al. prepared cellulose-derived
carbonaceous solids by hydrothermal carbonization of cellu-
lose followed by sulfonation. The obtained carbonaceous solid
acids exhibited good activity in fructose dehydration to give 5-
hydroxymenthylfurfural (5-HMF).[11] Recently, Ji et al. prepared
a sulfonated graphene acid catalyst that had high catalytic ac-
tivity and could be repeatedly used as a water-tolerant solid
acid catalyst.[12] Xiao’s group also demonstrated a facile synthe-
sis of sulfated graphene (GÀSO3H) from hydrothermal sulfona-
tion by using fuming sulfuric acid at a relatively high tempera-
ture (1808C).[13] More importantly, GÀSO3H exhibited much
better activity and recyclability. In addition, a series of sulfonic
acid functionalized carbon materials, including poly(p-styrene-
sulfonic acid)-grafted carbon nanotubes (CNT-PSSA), poly(p-
In recent years, with research into carbon materials, sulfonat-
ed carbon materials have shown promising applications as
new, cheap, and environmentally friendly solid acid catalysts.
One type of sulfonated carbon catalyst was prepared by pyro-
lytic treatment of biomass feeds under hydrothermal condi-
[a] C. Wang, Prof. F. Yuan, L. Liu, Prof. Y. Zhu
Key Laboratory of Functional Inorganic Material Chemistry
Ministry of Education School of Chemistry and Materials
Heilongjiang University, 74 Xuefu Road, Harbin 150080 (P. R. China)
E-mail: yujunzhu@hlju.edu.cn
[b] Dr. X. Niu
Key Laboratory of Chemical Engineering Process
& Technology for High-efficiency Conversion
College of Heilongjiang Province, School of Chemistry and Materials
Heilongjiang University, Harbin 150080 (P. R. China)
E-mail: niuxiaoyu2000@126.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201500261.
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styrenesulfonic acid)-grafted carbon nanofibers (CNF-PSSA),
benzenesulfonic acid grafted CMK-5 (CMK-5-BSA), and benze-
nesulfonic acid grafted carbon nanotubes (CNT-BSA), have
been studied for fructose dehydration to give 5-HMF and fruc-
tose alcoholysis to give alkyl levulinate.[14] Sulfonated carbon
nanotubes and porous carbon also displayed significant activi-
ties as acid catalysts owing to high acidities and -SO3H densi-
ties.[15] However, it must be pointed out that the methods to
prepare the above-mentioned carbon-based solid acids are
complex and expensive; thus, it is interesting to explore
a novel facile synthetic method for carbon-based solid acids
with high catalytic activities and low costs.
ing Information). Among the various synthetic conditions in-
vestigated, temperature plays a key role in the formation of
a CS catalyst with a much higher acid site density. Thus,
herein, we describe in detail the effect of different preparation
temperatures on the catalytic performance of CS catalysts. The
CS catalysts were prepared at 130, 150, and 1708C, and are de-
noted as CS-130, CS-150, and CS-170, respectively, hereafter.
Figure 1 shows the XRD patterns of the CS-T (T=130, 150,
and 1708C) catalysts. All XRD patterns exhibit a broad diffrac-
tion peak in the 2q range of 15 to 378 that is assigned to the
C(002) plane of the amorphous carbon composed of aromatic
carbon. The C(101) plane, which gives rise to a weak and
broad diffraction peak at 37–508, can be distinguished in these
samples, although it is not clear. This peak is attributed to the
axis of the graphite structure.[16] As a result of these two peaks,
we can deduce that the CS catalysts consist of a carbon sheet
and amorphous carbon with aromatic carbon rings.
Herein, we report on a one-pot, facile synthetic method for
a carbon-based solid acid (CS) catalyst by direct simultaneous
carbonization and sulfonation, without a hydrothermal process,
with glucose and concentrated sulfuric acid as raw materials.
The obtained catalyst has a high acid density and acid
strength; moreover, it is low cost. Their catalytic performances
have been investigated by using the transesterification of trib-
utyrin with methanol to give methyl butyrate and the dehydra-
tion of fructose to give 5-HMF as model reactions. Various pa-
rameters, such as reaction temperature, time, and ratio of glu-
cose to H2SO4, were varied to optimize the preparation condi-
tions to achieve a solid acid catalyst with much higher activity
by using transesterification of tributyrin as a probe reaction.
Among these synthetic conditions, temperature plays a key
role in the formation of CS catalyst with much higher acid site
density, resulting in higher activity for the direct simultaneous
carbonization and sulfonation of glucose with H2SO4.
Table 1 shows the elemental composition of the CS catalysts.
The carbon content is about 57% for the CS-130, CS-150, and
CS-170 catalysts; moreover, the ratio of carbon to hydrogen is
above 1.35 for these CS catalysts. The amount of sulfur is
3.890, 4.081, and 2.960% for CS-130, CS-150, and CS-170, re-
spectively. The sulfur content decreases in the following order:
CS-150>CS-130>CS-170. It is noted that about 36% of the
composition is oxygen, which indicates that some oxygen-con-
taining groups may be formed during the preparation process.
Based on the elemental analysis results, the elemental compo-
sition is CH0.74O0.47S0.026, CH0.69O0.47S0.027, and CH0.62O0.49S0.019 for
Results and Discussion
The CS catalysts were prepared, without using a hydrothermal
process, by direct simultaneous carbonization and sulfonation
with glucose and concentrated sulfuric acid as raw materials in
a low-cost, facile synthetic process. To achieve a CS catalyst
with much higher activity, the effects of various preparation
parameters, such as reaction temperature, time, and ratio of
glucose to H2SO4, on the catalytic activity were optimized by
using the transesterification of tributyrin with methanol as
a model reaction. The optimized synthetic conditions involved
a carbonization and sulfonation time of 20 min (Figure S1 in
the Supporting Information) and a ratio of the amount of glu-
cose (g) to H2SO4 volume (mL) of 1:6 (Figure S2 in the Support-
Figure 1. XRD patterns of the CS catalysts: a) CS-130, b) CS-150, and
c) CS-170.
Table 1. Texture properties of the CS catalysts.
[a]
Catalyst
BET surface area
ID/IG
Element composition [%]
Acid site density [mmolgÀ1
]
[m2 gÀ1
]
C
H
S
O
atomic ratio
H+ (SO3H)
H+ (other)
total
CS-130
CS-150
CS-170
used CS-150[b]
used CS-150[c]
11.0
9.2
8.4
7.8
8.7
0.86
0.80
0.75
0.78
0.81
57.05
56.92
57.12
55.28
56.78
3.498
3.270
2.955
2.551
3.305
3.890
4.081
2.960
2.298
3.986
35.56
35.73
36.96
39.87
35.93
CH0.74O0.47S0.026
CH0.69O0.47S0.027
CH0.62O0.49S0.019
CH0.55O0.54S0.015
CH0.70O0.47S0.026
1.22
1.28
0.92
0.71
1.25
0.14
0.31
0.40
0.02
0.30
1.36
1.59
1.32
0.73
1.55
[a] Data from Raman spectra; ID/IG is the intensity ratio of the D band to G band. [b] For the transesterification of tributyrin with methanol after one cycle.
[c] For the dehydration of fructose to 5-HMF after three cycles.
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CS-130, CS-150, and CS-170, respectively. Combined with the
results from XRD, these results suggest that the structure of
the CS catalyst may include an incompletely carbonized
carbon sheet with aromatic carbon rings.
Raman spectroscopy was used to investigate the degree of
crystallinity of the carbon materials. For the CS catalysts, two
prominent bands can be observed in Figure 2, at n˜ ꢀ1365 and
1590 cmÀ1, which are usually assigned to the breathing mode
Figure 3. FTIR spectra of the CS catalysts: a) CS-130, b) CS-150, c) CS-170,
and d) glucose.
samples.[20] The band at n˜ =628 cmÀ1 is attributed to vibration
of the CÀS bond.[21] The results indicate that the carbonation
reaction proceeds simultaneously with sulfonation. In other
words, sulfonic groups can be formed to accompany the car-
bonation of glucose. The thermal decomposition experiments
also prove these results. Figure 4 displays the results of tem-
perature-programmed decomposition of the CS-150 catalyst.
The results indicate that the peak at 2068C can be assigned to
the formation of SO2, namely, the decomposition of the sulfon-
ic acid group on the surface of the CS-150 catalyst. This con-
firms that the sulfonic acid group can be grafted onto the sur-
face of the CS catalysts and remain stable below 2068C in air.
The results from elemental analysis, XRD, Raman spectrosco-
py, and IR spectroscopy verify that the structure of the CS cata-
lyst is composed of an incompletely carbonized carbon sheet
and amorphous carbon containing aromatic carbon rings with
sulfonic acid groups (-SO3H).
Figure 2. Raman spectra of the CS catalysts: a) CS-130, b) CS-150, and
c) CS-170.
of k-point phonons of A1g symmetry and the E2g phonons of
sp2-carbon atoms, respectively.[12,17] The band at n˜ ꢀ1590 cmÀ1
is attributable to the G band commonly observed in graphitic
materials. The band at n˜ =1365 cmÀ1 is assigned to the
D band, which is commonly associated with the presence of
defects in the graphite layer.[3b] It is well known that an in-
crease in the intensity ratio of the D band to G band (ID/IG) in-
dicates a decrease in the graphite crystallinity of the carbon
materials.[18] In our case, the ID/IG values for the CS-130, CS-150,
and CS-170 samples are 0.86, 0.80, and 0.75, respectively
(Table 1). The results suggest that the obtained CS samples
have a poor degree of crystallinity and carbonation, which is in
agreement with XRD analyses. The higher ID/IG with increasing
preparation temperature results in a lower degree of graphiti-
zation and carbonation. It can be deduced that temperature
has a greater impact on the degree of carbonization of the CS
catalysts.
Table 1 shows the surface acidity of the CS catalysts. The
total acid-site density of the CS samples was estimated by the
acid–base titration method, and the content of -SO3H was cal-
culated by elemental analysis based on the sulfur component.
The other H+ amount is that of the difference between the
FTIR spectroscopy was employed as an additional probe to
provide evidence for the presence of -SO3H groups and other
organic species in the CS catalysts (Figure 3). It is clear that the
IR spectra of the CS catalysts (Figure 3a–c) have undergone
drastic changes compared with glucose (Figure 3d). The vibra-
tion bands at n˜ =1701 and 1623 cmÀ1 in Figure 3a–c are attrib-
uted to the stretching vibrations of C=O in the carboxylic
acid[19] and C=C in the aromatic ring,[10] respectively. This indi-
cates that glucose is carbonized through dehydration with
concentrated sulfuric acid at different temperatures. The broad
absorption band at n˜ =1250–1150 cmÀ1 and the sharp band at
n˜ =1030 cmÀ1 are assigned to the asymmetric and symmetric
stretching vibrations, respectively, of the SO2 group of the CS
Figure 4. Spectrum obtained by temperature-programmed decomposition
of the CS-150 catalyst.
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acid-site density and -SO3H content. The total acid-site density
is 1.36, 1.59, and 1.32 mmolgÀ1 for CS-130, CS-150, and CS-
170, respectively. Accordingly, the amount of SO3H is 1.22,
1.28, and 0.92 mmolgÀ1, respectively. The amount of -SO3H
groups dominates the total acid-site density for the CS sam-
ples; meanwhile, the CS-150 catalyst possesses the highest
concentration of sulfonic groups. For the CS-170 sample, the
amount of -SO3H is low, which may be attributed to the en-
hanced degree of carbonization, leading to hydrolysis of the
sulfonic acid groups by water produced during the carbona-
tion process because the temperature (1708C) is too high. The
amount of other H+ acid increases from 0.14 mmolgÀ1 to 0.31
and 0.40 mmolgÀ1 for the CS-130, CS-150, and CS-170 cata-
lysts, which reveals that more -OH and -COOH groups are
formed when the temperature increases from 130 to 1708C.
However, the amount of acid of the other H+ from -OH and
-COOH groups is clearly less than the amount of -SO3H. This
demonstrates that the acidity of the CS catalysts is derived
from the SO3H groups. The results indicate that the appropri-
ate preparation temperature should be selected to obtain a CS
catalyst with the highest acid density.
Figure 6. Activity of the transesterification of tributyrin with methanol over
the CS catalysts; 20 mg of catalyst, ratio of catalyst to tributyrin is 20
(mggÀ1), molar ratio of methanol to tributyrin is 20:1, reaction temperature
808C, reaction time 8 h.
obtained for this reaction, although the BET surface area was
low as 8.4–11.0 m2 gÀ1 (Table 1). Over 95% tributyrin conversion
and nearly 85% yield of methyl butyrate could be obtained
with all CS catalysts. In particular, 97.8% selectivity and 97.2%
yield of methyl butyrate were obtained with the CS-150
sample; this corresponded to 99.4% conversion of tributyrin.
The effects of the various reaction conditions, including tem-
perature, reaction time, molar ratio of methanol to tributyrin,
and the amount of catalyst, on the activity were also investi-
gated for the CS-150 catalyst based on its good catalytic activi-
ty. The results are presented in Figures S3–S6 in the Supporting
Information, and suggest that the CS-150 catalyst shows excel-
lent catalytic performance for the transesterification of tributyr-
in in a 20:1 molar ratio of methanol to tributyrin with 20 mg of
catalyst and 1.0 g of tributyrin at 808C for 8 h.
The above results indicate that the prepared CS catalysts
possess the characteristics of amorphous carbon with a high
acid-site density of SO3H groups. In particular, the CS-150 cata-
lyst exhibits the highest acid-site density.
The hydrophobic and lipophilic properties of the CS catalysts
were measured. Figure 5 gives the results from contact-angle
(CA) measurements for the CS catalysts. From CA data for
water (CAꢀ1428; Figure 5a1–3), the CS catalysts are poorly hy-
drophilic. In comparison with results for the lipophilicity (Fig-
ure 5b1–3, the CA of all CS catalysts is approximately 08, which
indicates that the CS samples have good lipophilicity proper-
ties.
Figure 6 shows the transesterification of tributyrin with
methanol over the CS catalysts. High catalytic activities were
Figure 7 is a plot of the yield of methyl butyrate versus the
total acid density for the CS catalysts in Table 1. The results
reveal good linearity between the yield of methyl butyrate and
the total acid density of the CS catalysts. According to the
order from CS-170 to CS-130 and CS-150, the total acid density
increases, and the yield of methyl butyrate is also markedly en-
Figure 5. Contact angles of a) a water droplet and b) a soybean oil droplet
Figure 7. Plot of the yield of methyl butyrate versus total acid density for
on the catalyst surface: a1,b1) CS-130; a2,b2) CS-150; and a3,b3) CS-170.
the CS catalysts.
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hanced. These results prove that all of the surface protonated
acids are the active acid sites of the CS catalysts for the trans-
esterification of tributyrin with methanol. The sulfonic acid
groups dominate the total acid density. Thus, the CS-150 cata-
lyst, which has the highest concentration of sulfonic acid
groups, is the most active catalyst for the transesterification of
tributyrin with methanol. In addition, the lipophilicity of the CS
catalysts leads to the enhanced local adsorption of tributyrin,
which should promote the catalytic transesterification reac-
tion.[22] A similar result was reported by Wang et al.,[23] who de-
veloped a carbon-based solid acid catalyst through the sulfo-
nation of carbonized vegetable oil asphalt and petroleum as-
phalt. This carbon-based solid acid exhibited high catalytic ac-
tivity for the simultaneous esterification and transesterification
of a waste vegetable oil containing large amounts of free fatty
acids. The high catalytic activity is ascribed to the high acid-
site density.[2a,23] Additionally, Keggin-type heteropolyacids, in-
cluding H3PW12O40, H4SiW12O40, H3PMo12O40, and H4SiMo12O40,
have been applied in the transesterification of rapeseed oil
with methanol and ethanol, and they exhibit outstanding cata-
lytic activity comparable to that of H2SO4.[2a] This excellent
transesterification activity is explained in terms of the high
acid strength of the heteropolyacids compared with that of
H2SO4. Thus, the CS catalyst, with the much higher concentra-
tion of -SO3H, is favorable in the transesterification reaction.
Based on our previous studies and results reported in the lit-
erature,[24] the transesterification reaction proceeds according
to pseudo-first-order kinetics, and the kinetic model is given
by Equation (1), where gives the yield of methyl butyrate at
time t (min).
Figure 8. a) Plots of Àln(1Àyieldmethyl butyrate) versus time at different tempera-
tures. b) Arrhenius plot of lnk versus 103/T for the reaction of tributyrin with
methanol. Reaction conditions: 20 mg of CS-150 catalyst, the ratio of catalyst
to tributyrin is 20 (mggÀ1), molar ratio of methanol to tributyrin=20:1.
The reusability of the CS-150 catalyst has also been investi-
gated and the results are depicted in Figure 6. The used CS-
150 catalyst was separated from the reaction mixture by filter-
ing and washing with methanol several times, and then reused
under the same reaction conditions as the original reaction.
The conversion of tributyrin decreased to 62.7%; however, the
selectivity of methyl butyrate maintained a much higher value
(90.2%). The yield of methyl butyrate decreased from 91.3 to
56.6%, which indicated that the catalyst was partially deacti-
vated during the recycling process. The BET surface area
(7.8 m2 gÀ1) and ID/IG value (0.78) obtained from the Raman
spectra of the used CS-150 catalyst are not significantly
changed compared with the fresh CS-150 catalyst (Table 1);
however, the acid-site density of the used CS-150 catalyst de-
creases to 0.71 and 0.02 mmolgÀ1 for H+(SO3H) and H+(other),
respectively, and a decrease of 0.86 mmolgÀ1 for the total
acid-site density from fresh CS-150 to the used CS-150 catalyst
ð2Þ (Table 1). It is interesting that the linear relationship is also
present between the yield of methyl butyrate and the total
Àln ð1Àyieldmethyl butyrateÞ ¼ kt
ð1Þ
A plot of Àln(1Àyieldmethyl butyrate) as a function of time will be
linear, with a slope equal to the reaction rate constant, k.
The kinetics of the CS-150-catalyzed transesterification of
tributyrin has been studied in the temperature range of 333–
353 K. A graph of Àln(1Àyieldmethyl butyrate) versus t is given in
Figure 8a. The plots are represented by straight lines, which
validate the first-order reaction model. The rate constants were
calculated from these plots and found to be 0.00317, 0.00441,
and 0.00597 minÀ1 at 333, 343, and 353 K, respectively. The ac-
tivation energy (Ea) and pre-exponential factor (A) for the same
reaction were determined by the Arrhenius equation [Eq. (2)].
ln k ¼ ÀEa=RT þ ln A
A plot of lnk versus 1/T is shown in Figure 8b. The slopes of acid-site density for the used CS-150 and fresh CS catalysts
the lines provide the activation energy (Ea) by using Equa- (Figure 7). Therefore, it can be concluded that the deactivation
tion (2); in this case, the activation energy calculated from the of the CS-150 catalyst is related to the loss of H+(SO3H). The
slope of the Arrhenius plot is 30.7 kJmolÀ1 and the correspond- formation of sulfonate esters may account for the deactivation
ing pre-exponential factor is 209 minÀ1. The activation energy behavior in reactions taking place in alcohols as solvents, espe-
for the CS-150 solid acid catalyst is much lower than that cially with methanol, owing to its higher reactivity. Fraile and
of the 25% KF/La2O2CO3 (55.03 kJmolÀ1
) and KF/La2O3 co-workers reported a general picture of the interaction be-
(128.9 kJmolÀ1) solid base catalysts used in our previous tween HCÀSO3H and alcohols (Scheme 1), in which the forma-
study.[25]
tion of hydrogen bonds was favored between the alcohol and
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Scheme 1. Deactivation of the CS catalysts (R=CH3).
protonated groups of different materials (sulfonic and carbox-
ylic acids, hydroxyls of alcohols, and phenols) at room temper-
ature.[10] The formation of sulfonate esters through the sulfonic
groups reacting with the alcohol at reaction temperature ex-
plains the strong deactivation effect of the CS-150. In other
words, the CS-150 catalyst interacts with methanol, so the
total acid-site density decreases under the reaction conditions.
Thus, the catalytic activity of the reused CS-150 catalyst de-
clined significantly in the transesterification of tributyrin with
methanol.
Figure 9. Effect of the amount of catalyst on the dehydration of fructose to
5-HMF over CS-150; reaction conditions: 200 mg of fructose, reaction time
30 min at 1408C.
The product 5-HMF is mainly synthesized by the acid-cata-
lyzed dehydration of monosaccharides.[2c,d,14] The dehydration
of fructose is closely associated with the acidity of the catalyst,
especially the acid sites.[2d,14,26] Because the CS-150 catalyst ex-
hibited the highest acid density and -SO3H concentration, as
well as excellent acid catalysis performance for transesterifica-
tion of tributyrin with methanol, the dehydration of fructose to
5-HMF was evaluated over the CS-150 catalyst. The effects of
various reaction conditions, including temperature, time, and
the amount of catalyst on the activity of the dehydration of
fructose to 5-HMF, were investigated over the CS-150 catalyst.
The results are presented in Figures 9–11 below. In addition to
the effectiveness of the catalyst for high yield and highly selec-
tive HMF production, solvent, including water, isopropanol, sul-
folane, and DMSO, also plays an important role in enhancing
the HMF yield. DMSO can suppress undesired side reactions
and increase the selectivity of 5-HMF.[2c,d,27] Therefore, DMSO
was used as the solvent in this study for the dehydration of
fructose to give 5-HMF. The main byproducts of these reac-
tions are humins. Currently, the humins cannot be quantified
owing to their composition. Thus, the conversion of fructose,
as well as the selectivity and yield of 5-HMF, were considered.
The effect of the amount of catalyst on the dehydration of
fructose to 5-HMF was investigated over the CS-150 catalyst
(Figure 9). The 5-HMF yield was 75% when the amount of CS-
150 used was 10 mg. The yield of 5-HMF improved to 93%
when the amount of catalyst was augmented to 30 mg. The in-
crease in the yield of 5-HMF with the catalyst dosage could be
attributed to an increase in the availability and number of
active sites during the dehydration of fructose. However, a fur-
ther increase in the amount of catalyst to 50 mg led to a slight-
ly reduced 5-HMF yield of 87%; this indicated acid-catalyzed,
rapid oligomerization of 5-HMF over the surface of the CS-150
catalyst under a higher acid-site concentration.[14] Thus, a large
amount of catalyst is not necessary to optimize the reaction.
The effect of the reaction time on the dehydration of fruc-
tose to 5-HMF over the CS-150 catalyst was investigated and
the results are depicted in Figure 10. The 5-HMF yield in-
creased from 15 to 45 min and thereafter remained constant.
The maximum 5-HMF yield (93%) was obtained at 1408C after
a reaction time of 30 min; prolonged reaction, however, led to
a slight reduction in the yield of 5-HMF. Therefore, the results
in Figure 10 show that 30 min is the optimum period to obtain
a high yield of 5-HMF.
Figure 11 shows the influence of the reaction temperature in
the range of 100 to 1608C on the dehydration of fructose cata-
lyzed by CS-150. The reaction temperature greatly affects the
conversion of fructose and formation of 5-HMF. The conversion
of fructose and selectivity of 5-HMF were 34 and 28% at
1008C, respectively. Through increasing the temperature, the
conversion sharply improved and nearly 100% conversion of
fructose was achieved at 1208C and thereafter remained con-
stant. The yield of 5-HMF was only 9.6% at 1008C, whereas it
increased quickly to 83% at 1208C; this confirmed that increas-
Figure 10. Effect of reaction time on the dehydration of fructose to 5-HMF
over CS-150; reaction conditions: 200 mg of fructose, 30 mg of CS-150, at
1408C.
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tions, and it can be observed that the catalytic activity was
well maintained, and only a slight decline in selectivity and
yield of 5-HMF was observed. After a three-cycle experiment,
the 5-HMF yield was still nearly 90%. The elemental composi-
tions, BET surface areas, ID/IG values from the Raman spectra,
and acid-site densities of the CS-150 catalyst after three cycles
were measured and the results are shown in Table 1. The re-
sults reveal that the BET surface area (8.7 m2 gÀ1), ID/IG value
(0.81), percentage of sulfur component, and acid-site density
are almost unchanged for the used CS-150 catalyst. Thus, the
CS-150 catalyst does not interact with DMSO under at 1408C.
In addition, the results of thermal decomposition experiments
also show that the CS catalysts are stable below 2068C in air
(Figure 4). So, the -SO3H groups on the surface of the CS-150
catalyst are not decomposed at 1408C. Thus, the CS-150 cata-
lyst has higher stability for the dehydration of fructose to 5-
HMF with much higher activity.
Figure 11. Effect of temperature on the dehydration of fructose to 5-HMF
over CS-150; reaction conditions: 200 mg of fructose, 30 mg of CS-150, reac-
tion time 30 min.
To investigate the relationship between activity and acid
density, the catalytic activities were also evaluated over the CS-
130 and CS-170 catalysts, as shown in Figure S7 in the Sup-
porting Information. The results show that the conversion of
fructose is nearly 100%; however, the yield of 5-HMF is slightly
different: 90.1, 93.3, and 89.1% for the CS-130, CS-150, and CS-
170 catalysts, respectively. Figure 13 shows a plot of the yield
of 5-HMF versus the acid density for the CS catalysts in Table 1.
It further provides deeper insight into the relationship be-
tween the catalyst structure and performance, showing a strik-
ing linearity between the yield of 5-HMF and the total acid
density of the CS catalysts. These results can also prove the
surface acids are the active acid sites for the dehydration of
fructose to 5-HMF under these reaction conditions. Several het-
erogeneous catalytic materials have been developed for the
dehydration of fructose to 5-HMF, including ion-exchange
resin, metal oxides, zeolites, sulfonic acid functionalized carbon
materials (CÀSO3H), and MOFÀSO3H.[14,26,28] Among these cata-
lysts, CNT-PSSA, CNF-PSSA, CMK-5-BSA, and CNT-BSA showed
good catalytic activity for fructose dehydration to 5-HMF, in
which the highest yield of 5-HMF was 89% under optimal con-
ditions over the CNT-PSSA catalyst.[14] For the MOFÀSO3H cata-
lyst, the highest yield of 5-HMF under optimal conditions
ing the reaction temperature improved the conversion of fruc-
tose to 5-HMF. The maximum 5-HMF yield of 93% with full
fructose conversion was obtained at 1408C; no further appreci-
able improvement in the yield was obtained upon further in-
creasing the temperature to 1608C. On the contrary, increasing
the temperature further led to a slightly reduced 5-HMF yield,
which indicated that CS-150 could catalyze 5-HMF degradation
at higher reaction temperatures. Therefore, the optimum reac-
tion conditions are 200 mg of fructose and 30 mg of CS-150
catalyst at 1408C for 30 min.
Heterogeneous acid catalysts are advantageous owing to
easy recovery from the reaction medium. Thus, apart from
good catalytic activity, reusability is an important criterion for
the solid catalysts. To demonstrate the reusability of the CS-
150 catalyst for the dehydration of fructose to 5-HMF, the cata-
lyst was separated by centrifugation, washed with deionized
water, and dried at 808C; a three-cycle experiment was per-
formed. Figure 12 shows the results of four successive reac-
Figure 12. Reusability tests for the CS-150 catalyst; reaction conditions:
200 mg of fructose, 30 mg of CS-150, reaction time 30 min at 1408C.
Figure 13. Relationship between yield of 5-HMF and total acid density for
the CS catalysts.
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reached 90%.[26] Compared with the above catalysts, the CS-
150 catalyst in our study showed excellent activity for fructose
dehydration to 5-HMF: 93% yield of 5-HMF could be obtained
at 1408C in 30 min. Moreover, the CS-150 catalyst has the ad-
vantage of high stability, good recycling capabilities, low cost,
and a facile preparation method.
by combustion in a VarioMICROV1.9.0 elemental analyzer equipped
with a TCD detector. The total acid-site density was estimated by
acid–base titration: the CS catalyst (0.100 g) was added to a saturat-
ed aqueous solution of NaCl (20 mL) with ultrasonic treatment for
2 h. The mixture was separated by filtration and the filtrate was ti-
trated with a 50 mmolLÀ1 solution of KOH. The CA values were
measured on a G-1erma, Kyowa Company, Japan, instrument.
Conclusion
Catalytic reaction evaluation
The CS catalysts were prepared by a facile direct simultaneous
carbonization and sulfonation method by using glucose and
concentrated sulfuric acid as raw materials without the need
for a hydrothermal process. The carbonization and sulfonation
temperature played an important role among various synthesis
conditions on carbon-based solid acid catalysts with high acid
density. The optimal preparation conditions were a carboniza-
tion and sulfonation time of 20 min and a ratio of the amounts
of glucose (g) to H2SO4 (mL) of 1:6 at 1508C. The characteriza-
tion results showed that the CS catalysts contained amorphous
carbon with a high -SO3H acid density. The CS-150 sample ex-
hibited excellent catalytic properties for the transesterification
of tributyrin with methanol owing to its high acid-site density;
97.8% selectivity and 97.2% yield of methyl butyrate were ob-
tained, which corresponded to 99.4% conversion of tributyrin.
The results displayed good linearity between the yield of
methyl butyrate and the total acid density for fresh CS-170, CS-
130, and CS-150 catalysts and used CS-150 catalyst. In addition,
the dehydration of fructose to 5-HMF was also evaluated over
the CS-150 catalyst, and a maximum 5-HMF yield of 93% with
full fructose conversion (100%) was obtained. Moreover, the
CS-150 catalyst had high stability with much higher activity for
the dehydration of fructose to 5-HMF. There was also good lin-
earity between the yield of 5-HMF and the total acid density
for the CS catalysts.
Transesterification reactions
All catalytic experiments were performed in a 30 mL double-lay-
ered glass reactor equipped with a reflux condenser, a magnetic
stirrer, and a superthermostat, as previously described.[25] Standard
conditions were as follows: tributyrin (1.00 mL, 3.42 mmol) and
methanol (17.1–102.4 mmol) were added to the flask. After the
mixture was heated to the desired temperature (313–353 K), cata-
lyst (5–40 mg) was added, and then the reaction was performed
under these conditions for 4–8 h. The liquid products were ana-
lyzed quantitatively by GC (SP-2100 P.R. China, FID detector) with
an OV-1 capillary column (30 m”0.25 mm”0.33 mm) by using cali-
bration curves and cyclohexanone as an internal standard, accord-
ing to a procedure reported in the literature.[25] The yield of methyl
butyrate was defined as the number of moles of methyl butyrate
produced divided by the number of moles of tributyrin reacted di-
vided by three (since three moles of methyl butyrate are produced
per mole of tributyrin at complete conversion).
Fructose conversion into 5-HMF
In a typical run, the procedure for fructose dehydration was as fol-
lows: fructose (200 mg), CS-150 catalyst (10–50 mg), and DMSO
(20 mL) were added to a 100 mL three-necked flask. The mixture
was heated at 100–1508C with vigorous stirring for 15–45 min.
After the reaction, the mixture was recovered by centrifugation
and then decanted into a volumetric flask by using pure water as
the diluent. The mixture was analyzed by HPLC. The quantitative
analysis of fructose was monitored by using a HPLC instrument
(Shimadzu 20A) equipped with a refractive index detector and
a Bio-Rad Aminex HPX-87H column. The 5-HMF product was ana-
lyzed by using a HPLC instrument (DIONEX Ultimate 3000) with an
InterSustain C18 column and a l=284 nm UV detector.
Experimental Section
Catalyst preparation
The CS catalysts were prepared by a one-pot facile method of
direct simultaneous carbonization and sulfonation with glucose
and concentrated sulfuric acid (98% H2SO4). In brief, H2SO4 (20–
40 mL) was added to glucose (5.00 g) in a 250 mL three-necked
flask. The mixture was stirred for 0–60 min at different tempera-
tures (130, 150, and 1708C) in air, and the solid was separated by
filtration. The obtained solid product was washed thoroughly with
Acknowledgements
This study is supported by the Major Foundation of Educational
Commission of Heilongjiang Province of China (11531Z11), the
Foundation of Educational Commission of Heilongjiang Province
of China (11531286), the Postdoctoral Science-Research Develop-
mental Foundation of Heilongjiang Province of China (LBH-
Q12022), the Innovative Research Team in Heilongjiang University
(Hdtd2010-10), and the Program for Innovative Research Team in
University (IRT-1237). We also thank the Project Sponsored by the
Scientific Research Foundation for the Returned Overseas Chinese
Scholars, State Education Ministry (2013-1792), and Ministry of
Human Resources and Social Security (2013-277).
2À
hot distilled water (about 808C) until neutral pH and no SO4 in
the filtrate was measured by the formation of BaSO4 precipitate
upon the addition of Ba(NO3)2 and a dilute solution of HNO3. Final-
ly, the obtained CS samples were dried at 808C for 12 h.
Catalyst characterization
XRD patterns of the catalysts were obtained with a D/MAX-3B X-
ray diffractometer (Rigaku Co.) by using CuKa radiation combined
with a Ni filter. Raman measurements were recorded on a JobinY-
von HR800 spectrometer (laser l=459.9 nm). The FTIR spectra
were recorded on a PE Spectrum One FTIR spectrometer as KBr
disks at room temperature. The C, H, and S contents were analyzed
Keywords: acid catalyst
catalysis · sulfur · transesterification
· dehydration · heterogeneous
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Manuscript received: June 14, 2015
Final Article published: July 17, 2015
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