Conversion of carbohydrates into 5-hydroxymethylfurfural using polymer bound sulfonic acids as efficient and recyclable catalysts

Article abstract of DOI:10.1039/c3ra41912f

52-94% yields of 5-hydroxymethylfurfural (HMF) from carbohydrates such as monosaccharides, disaccharides and polysaccharides were achieved using PEG-OSO₃H or PS-PEG-OSO₃H as reusable catalysts in a DMSO/H₂O reaction system. Using polymer-supported catalysis, corn stover could also be effectively transformed into HMF and furfural.

Full text of DOI:10.1039/c3ra41912f

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Conversion of carbohydrates into
5
-hydroxymethylfurfural using polymer bound sulfonic
Cite this: RSC Advances, 2013, 3, 9201
acids as efficient and recyclable catalysts3
Received 16th January 2013,
Accepted 30th April 2013
Zhang Zhang,* Bin Du, Li-Jia Zhang, Yu-Xia Da, Zheng-Jun Quan, Liang-Jie Yang
and Xi-Cun Wang*
DOI: 10.1039/c3ra41912f
www.rsc.org/advances
5
5
2–94% yields of 5-hydroxymethylfurfural (HMF) from carbohy-
5-hydroxy-4-keto-2-pentenoic acid (Scheme 1). Numerous impor-
tant scientific groups are carrying out studies on the synthesis and
applications of HMF and its derivatives. Glucose conversion to
HMF proceeds via two steps: isomerization of glucose to fructose,
followed by dehydration of fructose to HMF in the presence of
drates such as monosaccharides, disaccharides and polysacchar-
ides were achieved using PEG-OSO
3 3
H or PS-PEG-OSO H as
reusable catalysts in a DMSO/H O reaction system. Using poly-
2
mer-supported catalysis, corn stover could also be effectively
transformed into HMF and furfural.
6–10
enzyme, metal chloride or base catalysts
(Scheme 2). Among
these catalysts, metal chlorides are favored because they effectively
At the beginning of the 21st century, the utilization of renewable
raw material will gain significant importance in the industrial
conversion of chemicals. This is a consequence of diminishing
fossil fuel reserves, which will urge the development of new
methodologies to make use of sustainable sources for chemical
catalyze isomerization and dehydration. Zhao et al. have shown
that using CrCl
2
in ionic liquid leads to HMF yields of 67% at 100
10a
uC. Hu and co-workers reported a catalytic conversion of glucose
10b
using SnCl
Interestingly, Kerton transformed chitosan into HMF using
SnCl ?5H O as a catalyst in microwave irradiation and the yield
of HMF was only 10%. Abu-Omar and partners have employed
AlCl ?6H O as a catalyst to obtain HMF from carbohydrates in
AlCl ?6H
was found to efficiently convert carbohydrates into HMF by Wang
4
, in which HMF was formed in 64% yield.
1
production in the near future. Since biomass is renewable and
4
2
11
abundant in nature, it is a promising alternative for the
sustainable supply of valuable intermediates and platform
3
2
2
12
chemicals to the chemical industry.
3
2 2
O–NaCl–H O/THF biphasic medium. Sn–Mont catalyst
Carbohydrates are among the most abundant organic com-
pounds on earth and represent the majority of the world’s
annually renewable biomass. Sources of carbohydrates include
conventional forestry, wood processing by-products, agricultural
crops and surpluses, and plants grown on degraded soils. The
bulk of the carbohydrate-biomass comprises monosaccharides
13
et al. These catalyst systems, however, have drawbacks, as Cr and
Sn are toxic, ionic liquids and expensive due to several separation
processes. The biphasic medium is harsh while the Sn–Mont
(e.g. glucose and fructose), disaccharides (e.g. sucrose and maltose)
and polysaccharides (e.g. starch and cellulose), which by Brønsted
3
or Lewis acid catalyst hydrolysis form 5-hydroxymethylfurfural.
HMF has been hailed as the central biorenewable chemical from
which liquid fuel and other important chemicals can be
4
produced. HMF is very useful, not only as an intermediate for
the production of the biofuel dimethylfuran (DMF) and other
molecules, but also for important molecules such as levulinic acid
(LA), 2,5-furandicarboxylic acid (FDCA), 2,5-diformylfuran (DFF),
2,5-dihydroxymethylfuran, 5-chloromethyl furfural (CMF) and
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of
Education, Gansu Key Laboratory of Polymer Materials, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, P. R.
China. E-mail: wangxicun@nwnu.edu.cn; zhangz@nwnu.edu.cn;
Fax: +86-931-7971971; Tel: +86-931-7971971
3
Electronic supplementary information (ESI) available: experimental section and
experimental data. See DOI: 10.1039/c3ra41912f
Scheme 1 5-Hydroxymethylfurfural (HMF) as a bio-platform molecule.
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Scheme 2 Schematic representation of glucose to HMF with an acid catalyst.
catalyst preparation is complex and their application on an
industrial scale remains prohibitive.
In recent years, an abundant and cheap catalyst was widely
14
used in many organic reactions. For instance, Fu et al. reported
that Fe O -SBA-SO H (a magnetic solid acid) could be used as a
3
4
3
cheap and nontoxic catalyst for fructose dehydration into HMF.
However, there is still no report on using the polymer bound
sulfonic acids to catalyze the dehydration of carbohydrates
towards HMF. The utility of polymer-supported acid catalysts is
well-recognized because of their ease of workup and separation of
products and catalysts, from an economical point of view, and in
Fig. 1 Effect of different solvents on the yield of HMF from glucose.
product HMF in 65% yield. Subsequently, we investigated different
temperatures and found that the temperature has a significant
effect on the yield of HMF. In the temperature range 80–140 uC,
the maximum yield occurred at 120 uC. At lower temperatures (80–
15
application to industrial processes. We previously reported the
preparation and utilization of polyethylene glycol (PEG)-bound
16
3
sulfonic acid (PEG-OSO H) and polystyrene-poly (ethylene glycol)
17
(
PS-PEG) resin-supported sulfonic acid (PS-PEG-OSO H) in the
3
110 uC) and higher temperatures (130–140 uC), the yield of HMF
Biginelli reaction, the Beckmann rearrangement and other multi-
was low. The main reason was that the isomerization and
dehydration reactions of glucose were accelerated by increasing
the temperature, which was favorable to achieving a high yield.
However, the side reactions were also enhanced by rising
temperature. The competition of the two opposite factors led to
the largest yield at 120 uC.
In addition, various co-catalysts were investigated; the results
are summarized in Fig. 2. We can see from Fig. 2 that glucose is
converted to HMF in higher yields from metal chloride (LiCl, NaCl,
18
component reactions. We found that these two polymer bound
19
catalysts are mild, non-volatile and non-corrosive organic acids
and can efficiently convert carbohydrates to HMF in a one-pot
fashion. In this report, we describe a one-pot synthesis of HMF
from carbohydrate and lignocellulosic biomass using these
2
polymer bound sulfonic acids catalysts in a DMSO/H O medium.
Glucose can be obtained from starch, cellulose, sucrose,
maltose, and even biomass. Therefore, we initially chose the
reaction of glucose as a typical reaction to optimize the reaction
conditions (Scheme 3).
2 2 3 2 2 3 2 3 2
KCl, NiCl , CrCl , BiCl , CuCl , ZnCl , FeCl ?6H O, AlCl ?6H O,
CoCl ?6H O and FeCl /CuCl ). Increased yields up to 76% can be
2
2
3
2
We began by exploring the reactivity of glucose with PEG-
obtained through catalysis by LiCl. However, metal catalysts (Se,
Sn, Fe, Cu and Cu/Fe) showed a lower yield (e.g. Cu gave 31%
yield). We also investigated two kinds of rare earth metal (Rb O
2 5
and Nb O ), for which no product was found. The existence of LiCl
gave HMF in higher yield; this improvement might very well be
attributed to the decrease in pH upon introduction of LiCl (pH =
OSO H as a catalyst. The reaction was performed in the presence
3
of 2 mmol glucose, 0.3 g lithium chloride (LiCl), 0.7 g PEG-OSO
0.23 mmol –SO H), 2 mL H O and 4 mL various solvents (water,
n-butyl alcohol, THF, DMA, DMF, DMSO, 1-butyl-3-methylimida-
zolium hexafluorophosphate [BMIM]PF , glycerol and PEG-300) in
3
H
2
(
3
2
6
Fig. 1. From the results we can see that the reaction did not occur
when only water, glycerol or PEG-300 was used as the solvent and
good HMF yields (35–76%) were obtained in organic solvents. The
highest yield of HMF (76%) was obtained in DMSO because it
favours the furanoid form and the side reactions are remarkably
3.0–3.6 versus pH = 2.0–2.4). In the acid medium, it promoted
20
suppressed. Lower yields were obtained when the load of catalyst
was reduced, such as 0.4 g (0.13 mmol) of catalyst, yielding the
Scheme 3 Conversion of glucose into HMF catalyzed by polymer-bound sulfonic
acids.
Fig. 2 The yield of HMF from glucose by various co-catalysts in H O/DMSO systems.
2
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3 3
Table 1 Recycling study of PEG-OSO H and PS-PEG-OSO H for glucose
converted into HMF
PEG-
OSO
No.
m g
Recovery
Fresh
0.70
—
1
2
3
4
5
6
a
21
3
H
0.68 0.60 0.57 0.53 0.48 0.43
97
72
1
88
64
2
81
58
3
76
50
4
69
43
5
61
38
6
b
(%)
Yield
76
c
(%)
PS-PEG- No.
Fresh
0.20
—
d
21
OSO
3
H
m g
Recovery
0.19 0.18 0.16 0.15 0.13 0.12
95
90
80
75
65
46
60
41
Fig. 3 Possible reaction mechanism for glucose isomerisation to fructose following
e
(%)
3
dehydration to HMF on PEG-OSO H catalyst.
Yield
84
77
69
60
52
f
(%)
a
Reaction conditions: molar ratio of glucose/PEG-OSO
3
H/LiCl =
b
isomerization of glucose to fructose, followed by dehydration of
fructose to HMF as shown in Fig. 3. The influence of the amount
of LiCl in the system on dehydration of glucose to HMF was
investigated at 120 uC; the yield of HMF increased to a maximum
of 76% with an increase in the amount of LiCl from 0 to 0.3 g and
then slowly decreased to 60% with a further increase in the
amount of LiCl from 0.3 to 1.0 g. Thus, we decided to use 2 mmol
of glucose, 0.3 g LiCl, PEG-OSO H (0.7 g, 0.23 mmol), in H O (2
2 : 0.23 : 7), H O (2 mL), DMSO (4 mL), 120 uC, 1.5 h. Based on 0.7
2
c
d
g PEG-OSO
conditions: molar ratio of glucose/PS-PEG-OSO
O (2 mL), DMSO (4 mL), PS-PEG-OSO H (0.2 g, 0.66 mmol), 120
3
H. Isolated yield based on glucose. Reaction
3
H/LiCl = 2 : 0.66 : 7),
H
2
3
e
f
3
uC, 1 h. Based on 0.2 g PS-PEG-OSO H. Isolated yield based on
glucose.
6
0%, 52%, 46%, 41%, respectively), and the results are shown in
3
2
Table 1.
mL) and DMSO (4 mL) at 120 uC for 1.5 h as the optimal reaction
conditions for the preparation of HMF.
These results indicate that the polymer bound sulfonic acids
catalysts exhibited excellent activity for glucose conversion.
However, the PS-PEG-OSO
than the PEG-OSO H catalyst. The poor activity of PEG-OSO
might very well be attributed to the gradual leaching of its surface
The reaction between glucose (2 mmol), LiCl (0.3 g) and H
2
O (2
3
H catalyst showed a higher activity
mL) using PS-PEG-OSO H 0.2 g (0.66 mmol) as the catalyst in
3
3
3
H
various solvents was tested. As shown in Fig. 1, the yield of HMF
was increased and the best yield (84%) of HMF was obtained by
carrying out the reaction at 120 uC. It can be indicated that high
polarity solvents (such as THF and DMSO) are more effective than
–SO H groups in every repeatable process.
3
The excellent activity of polymer bound sulfonic acid
catalysts in synthesizing HMF from glucose opens up the
possibility of using more complex carbohydrates, such as
disaccharides (sucrose and maltose) and polysaccharides
the non-polar solvents. When the load of catalyst PS-PEG-OSO H
3
was reduced to 0.1 g (0.33 mmol), the yield of HMF was 77%.
Thus, we decided to use 0.2 g (0.66 mmol) catalyst in subsequent
reactions. The ratio of substrates and temperatures were also
screened and we found that the optimal reaction conditions for
(starch and cellulose) as renewable raw materials. Herein,
sucrose, maltose, starch and cellulose could be converted into
HMF with our catalysts and the yields of HMF were lower than
monosaccharide (glucose and fructose). Moderate to good
yields of HMF were obtained from glucose, fructose, sucrose,
the preparation of HMF were glucose (2 mmol), LiCl (0.3 g), H O (2
2
mL), DMSO (4 mL) and PS-PEG-OSO
DMSO at 120 uC for 1 h.
Recyclability is of great importance for applying a catalyst in
industrial processes. The reusability of the catalysts PEG-OSO H
3
H (0.2 g, 0.66 mmol) in
maltose, starch and cellulose using PEG-OSO
3
H (yield: 78%,
90%, 75%, 67%, 60%, and 52%, respectively) or PS-PEG-OSO H
3
3
as the catalyst (yield: 86%, 94%, 76%, 68%, 66%, and 54%,
respectively). The results are illustrated in Table 2.
Finally, we applied the polymer bound sulfonic acid catalysts to
the raw biomass variant. When corn stover was treated with PS-
and PS-PEG-OSO
through seven repeated reactions (Table 1). Firstly, the catalytic
activity and repeatability of PEG-OSO H was tested in the
reaction for glucose, the yields of HMF were 76%, 72%, 64%,
8%, 50%, 43% and 38%, respectively. After the reaction, the
catalyst was extracted by dichloromethane. The white PEG-
3
H were studied in this work and evaluated
3
3
PEG-OSO H at 130 uC for 6 h, a mixture, which contained HMF
5
and furfural in a ratio of 15 : 82 with 3% of other side products,
was obtained in 36% yield (w/w). It is notable that the major
product was furfural, which is also an important chemical and
OSO H catalyst can be obtained by adding cooled diethyl ether
3
21,22
into the concentrated organic phase. The recovered catalyst can
be used directly in the next run.
synthetic material,
and can be obtained from corn stover
under these reaction conditions. Furfural can be the starting
material for the synthesis of a series of derivatives including
furfuryl alcohol, furoic acid, furan, tetrahydrofuran, 2-methylte-
trahydrofuran, and related resins.
The recyclability of PS-PEG-OSO
same reaction. To recover the catalyst, the resulting mixture after
reaction was filtered, the PS-PEG-OSO H was filtered off, washed
3
H was also investigated in the
3
with EtOH (3 6 10 mL) and dried at 50 uC. The used catalyst was
added to the new substrates directly for the next run. The highest
yield of HMF (84%) was obtained from fresh one. Subsequently,
the yield of HMF was checked in recycle 1 to recycle 6 (77%, 69%,
Conclusions
Glucose or glucose-based carbohydrates were efficiently trans-
formed into HMF co-catalyzed by polymer bound sulfonic acids
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Table 2 Carbohydrate conversion and HMF yield catalyzed by the polymer bound sulfonic acid catalysts
a
b
PEG-OSO
3
H
PS-PEG-OSO
Con. (%)
3
H
c
Carbohydrates
Con. (%)
HPLC yield (%)
Isolated yield (%)
HPLC yield (%)
Isolated yield (%)
Glucose
Fructose
Sucrose
Maltose
Starch
99
.99
98
98
99
78
90
75
67
60
52
76
85
71
64
52
47
98
99
97
98
98
96
86
94
76
68
66
54
84
91
73
63
55
50
Cellulose
98
a
Reaction conditions: carbohydrates (2.0 mmol), LiCl (0.3 g), H
Reaction conditions: carbohydrates (2.0 mmol), LiCl (0.3 g), H
Detected by HPLC.
2
2
O (2 mL), DMSO (4 mL), PEG-OSO
O (2 mL), DMSO (4 mL), PS-PEG-OSO
3
H (0.7 g, 0.23 mmol), 120 uC, 1.5 h.
H (0.2 g, 0.66 mmol), 120 uC, 1 h.
b
c
3
with LiCl. It displayed high activity and good yield. For glucose, the
yield of HMF can mount to 86% in DMSO/H O mixed systems. It
Acknowledgements
2
We are thankful for the financial support from the National
Nature Science Foundation of China (Nos. 20902073 and
is confirmed that the key to successfully achieving the direct
conversion of the carbohydrates to HMF is that the catalytic
system contains both Brønsted acid (polymer bound sulfonic
acids) and Lewis acid (LiCl) sites and combines the isomerization
process with the dehydration step in a one-pot reaction system.
Hence, our chemical process uses simple, inexpensive catalysts to
transform carbohydrates into a valuable product in an ample
yield. Due to its economy, non-corrosive, highly effective and high
reusability, this catalyst system has excellent potential for the
conversion of biomass into biofuels and platform chemicals.
21062017), the Natural Science Foundation of Gansu Province
(No. 1208RJYA083), and the Scientific and Technological
Innovation Engineering program of Northwest Normal
University (Nos. nwnu-kjcxgc-03-64, nwnulkqn-10-15).
Notes and references
1
2
3
Y. Rom ´a n-Leshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic,
Nature, 2007, 447, 982–985.
Y. Rom ´a n-Leshkov, J. N. Chheda and J. A. Dumesic, Science,
2006, 312, 1933–1937.
Experimental procedure
J. N. Chheda, Y. Rom ´a n-Leshkov and J. A. Dumesic, Green
Chem., 2007, 9, 342–350.
D-Glucose, D-fructose, sucrose, maltose and starch were purchased
from J&K Scientific Ltd (Beijing, China), a-cellulose was obtained
from Aladdin Reagent Co., Ltd (Shanghai, China). All chemicals
were analytical grade, and used as received without further
purification, Corn stover was obtained from our test field. PEG-
4 (a) A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107,
2411–2502; (b) A. A. Rosatella, S. P. Simeonov, R. F. M. Frade
and C. A. M. Afonso, Green Chem., 2011, 13, 754–793.
5
(a) J. M. R. Gallo, D. M. Alonso, M. A. Mellmer and J.
A. Dumesic, Green Chem., 2013, 15, 85–90; (b) M. Balakrishnan,
E. R. Sacia and A. T. Bell, Green Chem., 2012, 14, 1626–1634; (c)
A. A. Rosatella, S. P. Simeonov, R. F. M. Frade and C. A.
M. Afonso, Green Chem., 2011, 13, 754–793; (d) Z. T. Du, J.
P. Ma, F. Wang, J. X. Liu and J. Xu, Green Chem., 2011, 13,
16
17
OSO H and PS-PEG-OSO H were prepared according to our
3
3
previous procedures.
All reaction products were analyzed by HPLC with a Kromasil-
C -5m column at 30 uC, P98-I pump, UV98-I detector at 254 nm.
18
Acetonitrile and water (45 : 90) were used as the mobile phase at a
5
54–557.
R. L. Huang, W. Qi, R. X. Su and Z. M. He, Chem. Commun.,
010, 46, 1115–1117.
A. Takagaki, M. Ohara, S. Nishimura and K. Ebitani, Chem.
Commun., 2009(41), 6276–6278.
21
flow rate of 0.8 mL min . HMF was quantified with calibration
curves generated from commercially available standards.
Following a typical experimental procedure for the glucose
reaction: A mixture of glucose (2 mmol), LiCl (0.3 g) and PS-
6
7
2
PEG-OSO
3
H (0.2 g, 0.66 mmol –SO
3
H) in DMSO aqueous solution
8 M. Watanabe, Y. Aizawa, T. Iida, T. M. Aida, C. Levy, K. Sue and
H. Inomata, Carbohydr. Res., 2005, 340, 1925–1930.
was stirred at 120 uC for 1 h. After completion monitored by TLC,
the resulting mixture was diluted with a known mass of deionized
water. The concentrations of products were calculated from HPLC-
peak integrations and were used to calculate molar yield, and 86%
yield of HMF was achieved. H NMR (400 MHz, CDCl ): 3.00 (s,
9 (a) Y. Roman-Leshkov, M. Moliner, J. A. Labinger and M.
E. Davis, Angew. Chem., Int. Ed., 2010, 49, 8954–8957; (b)
H. Jadhav, C. M. Pedersen, T. Sølling and M. Bols,
ChemSusChem, 2011, 4, 1049–1051; (c) R. S. Assary and L.
A. Curtiss, J. Phys. Chem. A, 2011, 115, 8754–8760.
1
3
1
(
(
1
H, OH), 4.72 (s, 2H, CH
d, J = 3.6 Hz, 1H, Furan-H-3), 9.58 (s, 1H, CHO) ppm. C NMR
101 MHz, CDCl ): 57.39 (CH ), 109.93, 123.18 (Ar), 152.15 (Ar),
60.83 (Ar), 177.69 (CHO) ppm. FTIR (KBr) (n, cm ): 1026 (C–O–
C), 1520 (CLC), 1674 (–CHO), 2992, 2908 (–CH ), 3412 (–OH).
): calculated for C H O : [M + H] 127.0390; found:
2
), 6.52 (d, J = 3.6 Hz, 1H, Furan-H-4), 7.23
13
10 (a) H. Zhao, J. E. Holladay, H. Brown and Z. C. Zhang, Science,
2007, 316, 1597–1600; (b) S. Hu, Z. Zhang, J. Song, Y. Zhou and
3
2
B. Han, Green Chem., 2009, 11, 1746–1749; (c) B. R. Caes, M.
J. Palte and R. T. Raines, Chem. Sci., 2013, 4, 196–199.
21
2
11 K. W. Omari, J. E. Besaw and F. M. Kerton, Green Chem., 2012,
+
HRMS (ESI
3
6
6 3
14, 1480–1487.
127.0392. See ESI
3
for further experimental details.
9
204 | RSC Adv., 2013, 3, 9201–9205
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1
2 Y. Yang, C. W. Hu and M. M. Abu-Omar, Green Chem., 2012, 14,
09–513.
3 J. J. Wang, J. W. Ren, X. H. Liu, J. X. Xi, Q. E. Xia, Y. H. Zu, G.
Z. Lu and Y. Q. Wang, Green Chem., 2012, 14, 2506–2512.
4 Z. Z. Yang, J. Deng, T. Pan, Q. X. Guo and Y. Fu, Green Chem.,
012, 14, 2986–2989.
5 L. A. Thompson and J. A. Ellman, Chem. Rev., 1996, 96,
55–600.
6 X. C. Wang, Z. J. Quan, F. Wang, M. G. Wang, Z. Zhang and
Z. Li, Synth. Commun., 2006, 36, 451–456.
7 Z. J. Quan, Y. X. Da, Z. Zhang and X. C. Wang, Catal. Commun.,
009, 10, 1146–1148.
18 X. C. Wang, L. Li, Z. J. Quan, H. P. Gong, H. L. Ye and X. F. Cao,
Chin. Chem. Lett., 2009, 20, 651–655.
19 (a) A. Hasaninejad, A. Zare, M. Shekouhy and J. Ameri-Rad,
Green Chem., 2011, 13, 958–964; (b) A. Hasaninejad, A. Zare and
M. Shekouhy, Tetrahedron, 2011, 67, 390–400.
20 (a) J. J. Wang, W. J. Xu, J. W. Ren, X. H. Liu, G. Z. Lu and Y.
Q. Wang, Green Chem., 2011, 13, 2678–2681; (b) M. Bicker,
D. Kaiser, L. Ott and H. Vogel, J. Supercrit. Fluids, 2005, 36,
5
1
1
2
1
5
1
18–126; (c) F. W. Lichtenthaler and S. Ronninger, J. Chem. Soc.,
Perkin Trans., 1990, 2, 1489–1497.
1 H. Guo and G. Yin, J. Phys. Chem. C, 2011, 115, 17516–17522.
1
2
1
22 J.-P. Lange, E. V. D. Heide, J. A. Buijtenen and R. Price,
2
ChemSusChem, 2012, 5, 150–166.
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