Preparation of biomass hydrochar derived sulfonated catalysts and their catalytic effects for 5-hydroxymethylfurfural production

Article abstract of DOI:10.1039/c3ra23314f

Amorphous carbon-based sulfonated catalysts were generated from four kinds of biomass (lignin, cellulose, wood meal and d-xylose) by hydrothermal carbonization at various temperatures (225, 245 and 265 °C) followed by sulfonation, with a yield of 36-56%. All of these catalysts showed aromatic structure, hydroxyl and carboxyl groups, with a density of SO₃H groups between 0.56 and 0.87 mmol g^-1. 5-Hydroxymethylfurfural (HMF) was produced from inulin in ionic liquids (ILs) in one step with the addition of carbon-based sulfonated catalysts, with a factual yield of 47-65% at 100 °C, 60 min. Moderate extension of reaction time (from 30 to 90 min) and increase of temperature (from 80 to 120 °C) promoted HMF production. Ethyl acetate was used as extractant, and about 39-55% of HMF can be recovered from ILs. One problem with these carbon-based sulfonated catalysts was that they would be partly deactivated in ILs for separate reuse, however, they can be easily regenerated by dilute sulfuric acid treatment. The carbon-based sulfonated catalysts exhibited good catalytic activity compared with traditional solid acid catalysts, and the carbon-based sulfonated catalyst/ILs reaction system showed high reusability. In consideration of the renewable as well as the high catalytic activity abilities, these biomass derived carbon-based sulfonated catalysts would be promising for industrial application. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra23314f

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Preparation of biomass hydrochar derived sulfonated
catalysts and their catalytic effects for
Cite this: RSC Advances, 2013, 3,
7
360
5-hydroxymethylfurfural production
Shimin Kang, Jun Ye, Yu Zhang and Jie Chang*
Amorphous carbon-based sulfonated catalysts were generated from four kinds of biomass (lignin,
cellulose, wood meal and D-xylose) by hydrothermal carbonization at various temperatures (225, 245 and
2
65 uC) followed by sulfonation, with a yield of 36–56%. All of these catalysts showed aromatic structure,
21
hydroxyl and carboxyl groups, with a density of SO
3
H groups between 0.56 and 0.87 mmol g
.
5
-Hydroxymethylfurfural (HMF) was produced from inulin in ionic liquids (ILs) in one step with the addition
of carbon-based sulfonated catalysts, with a factual yield of 47–65% at 100 uC, 60 min. Moderate
extension of reaction time (from 30 to 90 min) and increase of temperature (from 80 to 120 uC) promoted
HMF production. Ethyl acetate was used as extractant, and about 39–55% of HMF can be recovered from
ILs. One problem with these carbon-based sulfonated catalysts was that they would be partly deactivated
in ILs for separate reuse, however, they can be easily regenerated by dilute sulfuric acid treatment. The
carbon-based sulfonated catalysts exhibited good catalytic activity compared with traditional solid acid
catalysts, and the carbon-based sulfonated catalyst/ILs reaction system showed high reusability. In
consideration of the renewable as well as the high catalytic activity abilities, these biomass derived carbon-
based sulfonated catalysts would be promising for industrial application.
Received 13th December 2012,
Accepted 1st March 2013
DOI: 10.1039/c3ra23314f
www.rsc.org/advances
1
1,12
polymeric materials, and biofuels.
In recent years, dehy-
1
. Introduction
dration of biomass-based carbohydrates for HMF preparation
has received much attention, especially in the acid-catalyzed
dehydration of fructose by eliminating three moles of water
molecules. Moreover, considerable interest has emerged in the
application of ionic liquids (ILs) in the field of HMF
production, due to the good solubility of carbohydrates in
Biomass derived carbon-based sulfonated catalysts have
become a research hotspot recently, and this kind of catalyst
has been widely studied in biodiesel production, hydrolysis
1
–3
3
–5
5–9
of cellulose,
advantages of this kind of catalyst is that the raw material
biomass) is abundant, widely existent, and renewable.
and some other applications.
One of the
(
11
ILs. However, it remains a challenge to develop an efficient
and green catalyst to obtain HMF from carbohydrates. Inulin
Biomass derived carbon-based sulfonated catalysts can be
prepared by sulfonation of a carbon-based precursor with
concentrated H SO , while the precursor can be obtained from
2 4
biomass through carbonization methods, including pyrolysis,
gasification, hydrothermal carbonization, and flash carboniza-
tion. In summary of the former works, most of the carbon-
based sulfonated catalysts were prepared by biomass pyrolysis
followed by sulfonation. Hydrothermal carbonization could be
a good method to get a carbon-based precursor (hydrochar)
due to its relative low temperature and lack of necessity for
predrying as water is used as the solvent. However, detailed
information on the effects of various biomass hydrochar in
preparing carbon-based sulfonated catalyst is very lacking.
13
is a natural polysaccharide consisting of linear chains of
fructose units linked by a b (2 A 1) linkage with the polymer
chains terminating in a glucose unit, and it exists in many
plants, such as chicory, dahlia and jerusalem artichoke, etc.
Inulin would be a promising biomass material for HMF
production in two steps, hydrolysis for fructose production
and then dehydration of fructose for HMF production.
In this paper, wood meal, cellulose, lignin, and D-xylose
were first hydrothermally carbonized, and then these hydro-
thermal carbonization products were sulfonated to obtain four
kinds of biomass derived carbon-based sulfonated catalysts;
the object was to test and compare the characterization
properties and catalytic effects of these four kinds of catalysts.
Moreover, in contrast to the typical two step processes,
catalytic one step in ILs with these carbon-based sulfonated
catalysts was explored to obtain HMF from inulin.
1
0
1
0
5-Hydroxymethylfurfural (HMF) is expected to be a versatile
and key precursor for the production of fine chemicals,
State Key Laboratory of Pulp Paper Engineering, School of Chemistry and Chemical
Engineering, South China University of Technology, Guangzhou, 510640, China.
E-mail: changjie@scut.edu.cn; Fax: +86 20 87112448
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2
.4. Catalytic reaction procedure
In a typical run, 0.1 g of inulin, 0.1 g of water and 2 g of
AMIM]Cl were added into the reactor with or without 0.1 g of
2
. Materials and methods
2.1. Materials
[
1
-Allyl-3-methylimidazolium chloride ([AMIM]Cl, >99%) was
catalyst. Before the addition, the catalysts were ground and
filtered by a 200 mesh strainer. The reaction mixture was
agitated with electromagnetic stirring at the desired tempera-
ture and desired time in an oil bath. All experiments were
conducted in duplicate.
obtained from The Chinese Academy of Sciences Lanzhou
Institute of Chemical Physics. Inulin (Biochemical) was
obtained from Aladdin-reagent Co., Ltd. Lignin (dealkali)
was obtained from Tokyo Chemical Industry Co., Ltd., and
cellulose (microcrystalline) was obtained from Sinopharm
Chemical Reagent Co., Ltd., China. D-Xylose was obtained
from Aladdin Chemistry Co., Ltd. Wood meal (Pine) was
obtained from Guangdong province, China, and which was
2.5. Separation and analyses
After reaction, two methods were used to detect the content of
the products. Method 1: the products were extracted by 30 mL
(4 6 7.5 mL) of ethyl acetate, then the content of HMF was
detected by GC (Shimadzu QP 2010), and the yield of HMF was
defined as the ethyl acetate extracted yield.
Method 2: the products after reaction was diluted with 30
mL of ultra pure water, then the product contents were
analyzed with Ion Chromatography (Dionex, ICS-3000). The
yield of HMF detected by method 2 was defined as the factual
yield.
3
about 3 6 1 6 0.5 mm in size, consisting of 26.1% lignin,
50.3% cellulose, and 22.3% hemicellulose on a dry basis. The
moisture contents in lignin, cellulose, wood meal, and D-xylose
were 10.3, 2.6, 4.6, and 0%, respectively. HZSM-5 molecular
sieve was obtained from Tianjin Kaimeisite Technology Co.,
Ltd. Strong-acid 732 cation resin was obtained from Shanghai
Lingfeng Chemical Reagent Co., Ltd. Amberlyst-15 resin was
obtained from Aladdin Chemical Co., Ltd. Sulfated zirconia
1
4
was synthesized according to Yee et al.
2
.2. Preparation of carbon-based catalyst
3
. Results and discussion
Wood meal, cellulose, lignin and D-xylose were first hydro-
thermally carbonized at various temperatures (225, 245, 265
uC) to get corresponding hydrochars (carbon-based precursor)
according to Kang et al.,
sulfonated according to Suganuma et al. Briefly, these
hydrochars were sulfonated by concentrated H SO at 150 uC
3.1. Yields of carbon-based sulfonated catalysts
As shown in Fig. 1, the yield of these carbon-based sulfonated
catalysts is among 36–56%, and with a trend of lignin derived
catalyst > wood meal derived catalyst ¢ cellulose derived
catalyst > D-xylose derived catalyst at the same preparation
temperature. The catalyst yield got a maximum value at 245 uC,
which was an integrated result of both hydrothermal
carbonization (hydrochar preparation from biomass) and
sulfonation (catalyst preparation from hydrochar) processes.
The yield of carbon-based precursor (hydrochar) prepared
from the raw materials decreased as temperature increased
from 225 to 265 uC, while the hydrochar produced at a low
temperature resulted in a low yield of carbon-based sulfonated
1
5
then these hydrochars were
4
2
4
for 12 h, followed by a washing process with excess hot water
>80 uC). The washed sulfonated solid products were dried at
05 uC to constant weight in a vacuum drying oven to form the
(
1
carbon-based sulfonated catalysts. The yield of catalyst was
calculated as (weight of catalyst)/(weight of biomass raw
material). The lignin, cellulose, wood meal, and D-xylose
derived hydrochar are labeled as LX, CX, WX, DX, respectively,
where X (X = 225, 245, 265 uC) is the hydrothermal
carbonization temperature. The lignin, cellulose, wood meal,
and D-xylose derived carbon-based sulfonated catalysts are
labeled as LX–SO
3
H, CX–SO
3
3 3
H, WX–SO H, DX–SO H, respec-
tively. For example, L225–SO
3
H means the lignin derived
carbon-based sulfonated catalyst, and the carbon-based pre-
cursor (lignin hydrochar) was produced at 225 uC.
2.3. Characterization
The obtained sulfonated carbon catalyst was characterized by
X-ray diffraction (XRD) (D8 Advance, Bruker), thermogravi-
metric (TG) and derivative thermogravimetric (DTG) (TGAQ
5
6
000), Fourier transform infrared spectroscopy (FTIR) (Nexus
70, Nicolet), scanning electron microscopy (SEM) and energy
dispersive spectrometer (EDS) (S-3700N, Hitachi), X-ray photo-
electron spectroscopy (XPS) (Kratos AXis Ultra), elemental
analyzer (Vario EL III) and Brunauer-Emmett-Teller (BET)
surface area analyzer (ASAP 2020 V3.03 H, Micromeritics).
The hydrogen ion exchange capacity was estimated from the
+
exchange of Na in aqueous NaCl solution. The densities of
SO
3
H groups were estimated based on XPS analysis and the
Fig. 1 Yield of carbon-based sulfonated catalysts. The values are averages of
sulfur content determined by elemental analysis.
two duplicate experiments.
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Fig. 3 XRD spectra of carbon-based sulfonated catalysts.
As shown in Fig. 3, broad peaks are located between 10 and
0u (2h) for D245–SO H, L245–SO H, C245–SO H and WM245–
SO H, which can be ascribed to the diffraction of amorphous
3
3
3
3
3
4
,16
carbon,
indicating that all of these biomass feedstock have
become corresponding amorphous carbon catalysts after the
hydrothermal carbonization and sulfonation processes.
The TG/DTG curves are shown in Fig. 4, a weight loss in the
TG curve occurs before 100 uC, which should be caused by the
loss of water adsorbed on the catalysts. The catalysts seem
somewhat stable between 100–200 uC, and the weight
decreased obviously with increasing temperature from 250
uC. Moreover, according to the DTG curve peak and TG
remaining weight, the thermal stability of these catalysts
increases with increased carbon-based precursor preparation
temperature.
Fig. 2 SEM spectra of hydrochars and the corresponding carbon-based
sulfonated catalysts.
The functional groups are shown in Fig. 5, the FTIR spectra
of the catalysts produced by the same feedstock are similar
and they differ only in the intensity of some peaks, indicating
that hydrothermal carbonization at various temperatures (225,
245 and 265 uC) followed by sulfonation did not change the
functional group composition. The FT-IR spectrum shows that
all of these catalysts have aromatic structure (around 1600
catalysts during the sulfonation process. This is probably
because the low temperature results in high hydrochar yield
but a relatively soft and loose structure of the hydrochar, and
some of the relatively soft and loose hydrochar particles would
be easily lost during the sulfonation and washing process.
3.2. Characterization and properties of the carbon-based
21
21
cm ), OH groups (around 3400 cm ), SO H groups (around
3
catalysts
21
21
1
190 and 1040 cm ) and CLO groups (around 1720 cm ).
The SEM spectra of carbon-based sulfonated catalysts are
compared with those of the corresponding hydrochars in
Fig. 2. The surface topography of cellulose and wood meal
derived hydrochars (C245, W245) have changed a lot after
sulfonation, and the fiber skeletons are destroyed in both
C245–SO H and W245–SO H. The D245–SO H keeps the
The whole element contents, and C, O and S contents on
and near the surface were detected by elemental analysis and
EDS, respectively, as shown in Table 1. It shows that the C
content on and near the surface was lower than that in the
whole body of these catalysts, while the O and S contents on
and near the surface were higher than those in the whole body
of these catalysts. The results indicated that sulfonation
mainly acted on the surface of these hydrochars. While the
added O and S contents on and near the surface should be due
3
3
3
accumulation status of microspheres, however, these micro-
spheres in D245–SO H are accumulated much closer than the
3
corresponding hydrochar D245. Compared with the corre-
sponding hydrochar L245, there are obvious cracks and holes
to the SO
analysis show that the S exists in the forms of SO
(168 eV). According to the S content in the whole body of the
catalyst and the XPS analysis results, the SO H group contents
were calculated and shown in Table 2 to be between 0.56 and
3
H groups. As shown in Fig. 6, the results of XPS
in the surface of L245–SO
3
H, which meant that sulfonation
3
H groups
has destroyed the polymeric surface of lignin derived hydro-
char. All of the above results indicated that sulfonation
reaction has occurred on all of the biomass derived hydro-
chars.
3
2
1
+
0.87 mmol g . Besides, the H exchange value (from 1.11 to
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Fig. 4 TG/DTG curves of carbon-based sulfonated catalysts.
2
1
1
.44 mmol g , Table 2) was more than the SO
3
H group
3.3. Catalytic reactions
content in all of the carbon-based sulfonated catalysts, and
according to the above FTIR spectra analysis, there should be
some other acid groups in these catalysts, e.g. carboxyl groups.
As shown in Table 1, the whole carbon content increased as
the hydrochar preparation temperature increased for all of the
catalysts. The sulfur content in cellulose, wood meal and D-
xylose derived catalysts become lower at a lower hydrochar
preparation temperature. The possible reason is that these
hydrochars produced at a relatively low temperature would
possess a relatively soft but loose structure. Though the soft
structure is of benefit to sulfonation, many sulfonated carbon
particles on the surface of the catalyst would be run off in the
sulfonation and excess hot water washing process due to its
loose structure. In contrast, the sulfur content in lignin
derived catalysts becomes higher at a lower hydrochar
preparation temperature. This phenomenon maybe explained
by the phenolic polymeric surface structure of lignin derived
HMF can be obtained from inulin by a possible two-step
reaction pathway: hydrolysis of inulin to fructose followed by
dehydration of fructose (Fig. 7). Both of the two steps can be
catalyzed by acid catalysts, so it seems promising for the
production of HMF from inulin by an acid-catalysed one pot
reaction, however, details of the one pot reaction process with
various solid acid catalysts are very limited. In this work,
[AMIM]Cl was used as the solvent for the hydrolysis of inulin
and for dehydration of fructose. All of these cellulose, lignin,
wood meal and D-xylose derived carbon-based catalysts, as well
as four traditional solid acid catalysts (HZSM-5, amberlyst-15
resin, 732 cation resin, sulfated zirconia) were tested in the
one pot reactions (Fig. 7).
3.3.1. Catalytic activities of carbon-based catalysts. As shown
in Fig. 8, all of the carbon-based catalysts showed effective
catalytic effects in the HMF production compared with the
control experiment, with a factual yield of 47–65% at 100 uC,
1
5
hydrochar, which should be relative stable in the sulfonation
process and hard to be washed off. While the relatively soft
structure of lignin derived hydrochar produced at relatively
60 min. It was surprising that fructose was not found in the
reaction system, one possible reason is that hydrolysis of
inulin to fructose is a rate-determining step, while dehydration
of fructose is fast in the ionic liquid of [AMIM]Cl. Comparing
Fig. 8 with Table 2, there seems certain relationship between
low temperatures would generate more SO H groups during
3
the sulfonation process.
SO
3
H content and catalytic effects for the same biomass
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Fig. 5 FTIR spectra of carbon-based sulfonated catalysts.
derived carbon catalysts (e.g. D225–SO
3
H, D245–SO
3
H and
HMF. These results indicated that SO
3
H content should not be
D265–SO H), the high SO H content would result in high
factual yield of HMF, while for the carbon-based catalysts
derived from different biomass there seems no obvious
the only factor affecting the reactions. The BET surface area
values of all of these biomass derived carbon catalysts were low
(,2 m g ), and there was no obvious difference among these
3
3
2
21
correlation between SO H content and the factual yield of
3
Table 1 Elemental content of the carbon-based sulfonated catalysts
Detected by elemental analyzer
Detected by EDS
a
C (%) H (%) S (%) O (%) C (%) O (%) S (%)
C225–SO
C245–SO
C265–SO
W225–SO
W245–SO
W265–SO
D225–SO
D245–SO
D265–SO
3
3
3
H
H
H
51.29
52.65
52.99
3.44
3.46
3.57
3.29
3.22
3.13
3.47
3.70
3.60
3.44
3.56
3.49
2.17
2.22
2.27
1.79
1.91
1.98
1.81
1.94
2.02
2.79
2.72
2.37
43.10
41.67
41.17
40.03
39.98
39.97
45.02
44.41
44.37
41.65
41.58
40.99
42.07 55.43 2.50
41.99 55.47 2.54
42.22 55.00 2.78
41.50 56.15 2.35
41.70 55.72 2.58
41.87 55.51 2.62
41.59 56.25 2.15
42.25 55.47 2.28
42.06 55.37 2.56
41.47 54.88 3.65
42.19 54.7 3.11
43.08 54.02 2.90
3
3
3
H 54.89
H 54.89
H 54.92
3
H
49.70
49.95
50.01
52.19
52.07
53.15
3
H
H
3
L225–SO
L245–SO
L265–SO
3
H
H
H
3
3
a
O content was calculated by difference.
3
Fig. 6 XPS spectra of D245–SO H.
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+
Table 2 SO
3
H content and H exchange value of the carbon-based sulfonated catalysts. The values are averages of two duplicate experiments
+
+
SO
(mmol g
3
H content
H exchange value
SO
(mmol g
3
H content
H exchange value
21
2
1
21
21
Catalyst
)
(mmol g
)
Catalyst
)
(mmol g
)
C225–SO
C245–SO
C265–SO
W225–SO
W245–SO
W265–SO
3
3
3
H
H
H
0.68
0.69
0.71
0.56
0.60
0.62
0.57
0.61
1.41
1.42
1.44
1.11
1.20
1.25
1.42
1.42
D265–SO
3
H
0.63
0.87
0.82
0.74
—
—
—
1.43
1.21
1.21
1.15
2.06
0.61
4.20
2.20
L225–SO
L245–SO
L265–SO
3
H
H
H
3
3
3
H
H
H
3
3
732 Resin
HZSM-5
Amberlyst-15
Sulfated zirconia
D225–SO
D245–SO
3
H
3
H
—
values. So there may be some other factors in influencing the
catalytic ability of these biomass derived carbon catalysts,
possibly such as the dispersibility in ILs, affinity of these
catalysts with reactants, etc.; further work on these fields is
needed. Compared with amberlyst-15 resin and 732 cation
resin (the best two of the four traditional solid acid catalysts),
by ethyl acetate. Further work on the improvement of the
extraction efficiency seems necessary.
3.3.4. Reusability. Reuse of D265–SO H in [AMIM]Cl was
3
tested at a reaction condition of 100 uC, 60 min as following
1
7,18
steps.
After a typical reaction, the [AMIM]Cl solution was
extracted by excess ethyl acetate until the extracting ethyl
acetate looks like the fresh ethyl acetate. The remaining
the D265–SO
comparative factual yield of HMF.
.3.2. Effects of temperature and reaction time. D265–SO
3 3 3
H, L225–SO H, and W265–SO H etc. showed a
sublayer products (mainly [AMIM]Cl and D265–SO H) after
3
3
3
H
extraction were distilled at 70 uC in a rotary evaporator to
was selected to test the effects of reaction time and
temperature for HMF production. As shown in Fig. 9, the
HMF yield increases as reaction time increases from 30 to 90
min, and the yield remains stable as the reaction time
increases from 90 to 120 min. This is similar to the former
results that the HMF yield could be sustained at a level with
remove the remaining water and ethyl acetate, and little HMF
was found in the distilled fraction (mainly water and ethyl
acetate). The remaining [AMIM]Cl and D265–SO
into weighed fresh inulin to start a reuse reaction. The result
was that the [AMIM]Cl and D265–SO H system kept good
3
H were added
3
reusability for HMF production, the factual yield remained at
61% even after being used five times. Impurities, if present in
the reaction mixture, did not seem to have any significant
1
7
prolonged reaction time. High temperature is of benefit to
the HMF production; the factual yield increases from 47% to
66% as the temperature increases from 80 to 120 uC.
3
effect on the HMF yields in the recycling of the D265–SO H/
3
.3.3. Extraction effects of ethyl acetate. Some publications
[AMIM]Cl mixture. Thus, this recycled catalyst/ILs mixture
system retained good activity for conversion of inulin to HMF.
However, for some real biomass in the catalyst/ILs mixture, the
have reported that HMF could be separated from ILs by
extraction through some special organic solvents, including
1
1
ethyl acetate, toluene, etc. Ethyl acetate is a potential
extracting solvent due to its immiscibility with the ILs but
good solvency for HMF, as well as its low boiling point which is
hopefully via evaporation. In this work, ethyl acetate (4 6 7.5
mL) was used to extract HMF from [AMIM]Cl. As shown in
Fig. 8 and 9, the ethyl acetate extracted yield was much lower
than the factual yield, with 39–55% of the HMF being extracted
Fig. 8 The yield of HMF with the addition of various catalysts in a reaction
condition of 100 uC, 60 min. The values are averages of two duplicate
experiments.
Fig. 7 The pathways for acid-catalyzed hydrolysis and dehydration of inulin to
HMF.
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D265–SO
3
H/ILs reaction system showed good effects in the
reusability study. Considering its renewable property, high
catalytic activity, as well as good reusability in the ILs mixture
system, this biomass derived carbon-based catalyst would have
great potential in industry applications.
Acknowledgements
We acknowledge the financial supports from the Doctoral
Fund of Ministry of Education (20120172110011), National
Basic Research Program of China (973 Program) (No.
2
013CB228104 and 2010CB732205), and National High
Technology Research and Development Program of China
863 Program) (No.2012AA051801).
(
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D265–SO
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H addition. The values are averages of two duplicate experiments.
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2
9
1
inactivated D265–SO
3
H was conducted by dipping into 10%
1
1
H
2
SO solution for about 10 h, and then being washed by pure
4
2
water. Subsequently, the regenerated catalyst was dried and
resupplied for the next reaction. The results showed that with
1 M. E. Zakrzewska, E. Bogel-Łukasik and R. Bogel-Łukasik,
Chem. Rev., 2011, 111, 397–417.
the addition of regenerated D265–SO
reached 63%, which was nearly the same to the HMF factual
yield with the addition of fresh D265–SO H.
3
H, the HMF factual yield
1
1
1
2 P. Gallezot, Chem. Soc. Rev., 2012, 41, 1538–1558.
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3
1
1
1
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4
. Conclusions
Four kinds of hydrochars were successfully sulfonated,
yielding carbon-based sulfonated catalysts catalytically active
for HMF preparation from inulin in one step in [AMIM]Cl. The
carbon-based sulfonated catalysts, including D265–SO H,
3 3
L225–SO H and W265–SO H, have showed competitive cataly-
7 H. Xie, Z. K. Zhao and Q. Wang, ChemSusChem, 2012, 5,
901–905.
3
18 Y. Gong, M. Liu, S. Jia, J. Feng, C. Song and X. Guo, Acta
Phys.-Chim. Sin., 2012, 28, 686–692.
tic activities over four traditional solid acid catalysts (HZSM-5,
amberlyst-15 resin, 732 cation resin, sulfated zirconia), and the
19 R. Rinaldi, N. Meine, J. V. Stein, R. Palkovits and F. Sch u¨ th,
ChemSusChem, 2010, 3, 266–276.
7
366 | RSC Adv., 2013, 3, 7360–7366
This journal is ß The Royal Society of Chemistry 2013