Catalytic conversion of biomass-derived carbohydrates to formic acid using molecular oxygen

Article abstract of DOI:10.1039/c4gc00145a

Here we report a catalytic process for the efficient production of formic acid (FA) from common carbohydrates via VO₂⁺ formed by dissolving sodium metavanadate in acidic water. The polysaccharides undergo a hydrolysis reaction and an immediate oxidation reaction successively to produce FA in the same reaction system. The hydrolysis reaction decomposes the polymerized structures to produce monosaccharides, which are readily oxidized to FA under catalysis of VO₂⁺. Typically, formic acid mole yields are 64.9% from cellulose and 63.5% from xylan (hemicellulose). The separation of FA from the catalytic system and recycling of the catalyst are demonstrated. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00145a

Green Chemistry
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Catalytic conversion of biomass-derived
carbohydrates to formic acid using
molecular oxygen†
Cite this: Green Chem., 2014, 16,
2614
Wenhua Wang,a Muge Niu,a Yucui Hou,b Weize Wu,*a Zhenyu Liu,a Qingya Liu,a
Shuhang Rena and Kenneth N. Marsh*c
Here we report a catalytic process for the efficient production of formic acid (FA) from common carbo-
+
hydrates via VO2 formed by dissolving sodium metavanadate in acidic water. The polysaccharides
undergo a hydrolysis reaction and an immediate oxidation reaction successively to produce FA in the
same reaction system. The hydrolysis reaction decomposes the polymerized structures to produce mono-
saccharides, which are readily oxidized to FA under catalysis of VO2+. Typically, formic acid mole yields
are 64.9% from cellulose and 63.5% from xylan (hemicellulose). The separation of FA from the catalytic
system and recycling of the catalyst are demonstrated.
Received 27th January 2014,
Accepted 10th February 2014
DOI: 10.1039/c4gc00145a
www.rsc.org/greenchem
Various strategies for producing chemicals and fuels from 0.044 hydrogen.12–14 It can be easily and selectively decom-
biomass are being researched in the interest of developing posed to H2 and CO2 through catalysis under mild con-
alternatives to fossil fuels. Carbohydrates, representing ditions.15,16 Thus FA from biomass offers
a mild and
the largest fraction of biomass, are attracting the major sustainable way to produce H2.
attention.1–3 At present, compounds of commercial interest
Many researchers have studied the production of FA from
that can be obtained directly from carbohydrates by non- carbohydrates using thermal cracking,17 SCW/H2O2,18
fermenting approaches are very few and limited mainly to simple Fe2(SO4)3/O2 or CuSO4/O2,19,20 OH−/H2O2,21 Keggin-type
carbohydrates such as glucose and fructose. These include the H5PV2Mo10O40/O2 22,23 and the recent H5PV2Mo10O40 + p-toluene-
production of gluconic acid and sorbitol from glucose by oxi- sulfonic/O2 systems.24 Due to its unique properties, high
dation and hydrogenation, respectively, and 5-hydroxymethyl- temperature water (HTW) is suitable as a solvent for the oxi-
furfural (HMF) from glucose and fructose using acidic dation of carbohydrates to FA. However, the attainment of a
catalysts.4–6 The ultimate goal of producing chemicals by high FA yield with a satisfactory reaction rate is still a great
direct reaction with cellulose or hemicellulose is still far away challenge, especially from polysaccharides such as cellulose
because they are not soluble in conventional solvents and are and hemicellulose, because the oxidative pathway is intricate
very resistant to chemical and biological transformations.7–9
Formic acid (FA) is a basic chemical feedstock widely used
and difficult to control.
Because of the stability of polysaccharides, the catalytic oxi-
in chemical, textile, leather, pharmaceutical, rubber and other dation of cellulose to FA in HTW must undergo two reactions:
industries. Recent research has demonstrated that FA has the (1) acid hydrolysis of cellulose to give glucose or other small
potential to power fuel cells for electricity generation and molecular intermediates and (2) selective oxidation of glucose
automobiles.10,11 Additionally, with respect to a possible or other small molecular intermediates to produce FA.23,24 The
hydrogen economy, FA has been proposed as a suitable conversion conditions should satisfy simultaneously the two
hydrogen storage compound, having a mass fraction of reactions, hydrolysis and oxidation, because they occur in one
solution.
For hydrolysis of polysaccharides in acidic HTW, the by-
aState Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, 100029 Beijing, China. E-mail: wzwu@mail.buct.edu.cn;
Fax: +86 10 64427603
product humin is always produced from glucose or other small
molecular intermediates, resulting in low selectivity towards
the target product. Hence, the oxidation should be faster than
hydrolysis in order to avoid producing humin.
bDepartment of Chemistry, Taiyuan Normal University, 030031 Taiyuan, China
cCentre for Energy, School of Mechanical and Chemical Engineering, The University
of Western Australia, Crawley, WA 6009, Australia. E-mail: ken.marsh@uwa.edu.au
†Electronic supplementary information (ESI) available: Spectroscopic data and
further details. See DOI: 10.1039/c4gc00145a
Initially, Wasserscheid et al.24 performed the selective cata-
lytic conversion of biobased carbohydrates to formic acid
using molecular oxygen, and reported that cellulose could be
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oxidized in HTW using H5PV2Mo10O40 as a catalyst at 80 °C for
26 h with a low FA yield of 1%. And then they added p-toluene-
sulfonic acid as an additive to improve the FA yield to 22% at
90 °C for 66 h. The time was very long, such as 26 h or 66 h,
and the FA yield was still very low. Fu et al.23 also used
H5PV2Mo10O40 as a catalyst and added H2SO4 as an additive to
oxidize cellulose with a FA yield of 28% at 170 °C and the cel-
lulose was completely converted in 9 h. The increase in temp-
erature and acidity (by adding H2SO4) increased the FA yield,
and the time was shortened to 9 h, but it was still too long for
practical application, which suggests that the conversion rate
is low. It is reported that higher temperatures and higher
acidities are necessary for hydrolysis of cellulose in a short
time (less than 0.5 h25) which hints that the temperature or
acid amount or both of them used previously are too low to
reach completion in a short time.
Table 1 Conversion of carbohydrates in an aqueous sodium metavana-
date solution with and without sulfuric acid under an initial oxygen
pressure of 3 MPa at time. Substrate (100 mg), sodium metavanadate
(22 mg), and water (6 g) were stirred in an autoclave at 160 °C. FA yields
are calculated on a carbon basis and given as mean values. The key
experiments were repeated five times (see Table S3). Conversion is cal-
culated based on the amount of unconverted carbohydrates. The
content of sulfuric acid is calculated based on the mass fraction of
sulfuric acid in water
H2SO4/
wt%
Time/
min
Entry
Substrate
Conversion/%
FA yield/%
1a
2
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Glucose
Glucose
Fructose
Cellose
0
0
120
120
120
120
120
120
120
120
120
120
120
120
1
1
1
10
5
1
30
30
1
3
<0.5
<1
3a
4
0.1
0.1
0.2
0.4
0.5
0.7
0.7
0.9
1.5
2.0
0
0.7
0.7
0.7
0.7
0.7
0
72.1
74.3
83.5
92.1
94.6
100
95.2
100
100
100
96.8
100
100
100
100
100
6
13.1
31.8
41.2
49.9
56.8
64.9
20.6
63.7
62.8
61.2
42.8
68.2
61.5
65.3
64.6
66.2
2.3
5
6
7
8
On the other hand, pentavalent
V in the catalyst
9a
10
11
12
13
14
15
16
17
18
19
20
H5PV2Mo10O40 plays a key role in the oxidation of glucose or
intermediates to FA.22–24 The question arises whether NaVO3,
which is readily available and inexpensive, has an efficient cata-
lytic performance for the same oxidation.
It is well known that V–O species in an aqueous solution of
NaVO3 exists in various forms depending on the acidity of the
Sucrose
Xylose
Xylan
solution (pH).26,27 Among them VO2 showed a high oxidative
+
potential for selective oxidations, such as benzene oxidation to
phenol28 and alkane oxidation.29 VO2 mainly appears under
+
Xylan
0.7
100
63.5
a The reaction was carried out without NaVO3.
strong acidic conditions (pH < 1). Therefore, it is possible that
FA selectivity is high even under drastic hydrolysis conditions
(pH < 1) because VO2 is capable of oxidizing glucose or inter-
+
mediates readily.
cellose, xylose, sucrose and xylan were also examined in the
According to previous work, the simple catalyst sodium same reaction system, and excellent performance was attained
metavanadate (NaVO3) with sulphuric acid as an additive in under non-optimized conditions (Table 1, entries 13–20). For
HTW was employed to convert carbohydrates into FA. We xylan (hemicellulose) and cellulose, the most abundant
found that the catalyst system could satisfy the two reactions biomass in nature, the FA yield can reach 63.5% and 64.9%
of hydrolysis and oxidation, and monosaccharides, disacchar- based on carbon, or 79.4% and 77.9% based on hydrogen,
ides and polysaccharides could be converted readily into FA respectively. It is worth noting that 30% yield of FA from cellu-
with high selectivity under an oxygen pressure of about 3 MPa. lose has been the highest recorded in the literature to date,
The typical results are the FA yield of 64.9% based on carbon which was obtained in the H5PV2Mo10O40 + p-toluenesulfonic/
(77.9% based on hydrogen) with cellulose as the substrate, and O2 system.24 Moreover, the reaction time was shortened drasti-
63.5% (79.4% based on hydrogen) FA yield with xylan (hemi- cally in the suggested system.
cellulose) as the substrate. The produced FA could be easily
The effect of temperature on the conversion of substrate
separated by solvent extraction after the reaction. The catalytic and the yield of FA was studied (shown in Fig. S7†). When the
system could be reused directly after simple purification. This temperature is increased from 140 °C to 160 °C, the conversion
route has great potential for industrial application.
of cellulose increases from 28.0% to 100%, and the yield of FA
Table 1 shows the conversion of carbohydrates and the increases from 13.4% to a maximum value of 64.9%, which
yield of FA for different catalytic systems. Cellulose conversion means that the rate of cellulose oxidation increases with
and FA yield are very low in the absence of sulfuric acid increasing temperature. But when the temperature is further
(Table 1, entries 1, 2). When only sulfuric acid is used, the increased to 190 °C, the yield of FA decreases gradually to
yield of FA reaches 13.1% with a cellulose conversion of 72.1% 49.7%, resulting from the over-oxidation of FA at high temp-
(Table 1, entry 3). When NaVO3 and sulfuric acid are used simul- eratures. The effect of the amount of initial substrate on the
taneously, the yield of FA increases significantly with an elev- reaction was also investigated (as shown in Fig. S8†). At low
ated cellulose conversion (Table 1, entries 4–12). Interestingly, amounts of cellulose, not more than 0.1 g, the conversion of
the yield of FA increases with an increase of sulfuric acid con- cellulose is 100% and the yield of FA increases with the
centration until a selectivity plateau of 64.9% is reached at a amount of cellulose and reaches a maximum value of 64.9% at
sulfuric acid mass fraction equal to or greater than 0.007. 0.1 g of cellulose. Then both the conversion and the yield of
Several other carbohydrates including glucose, fructose, FA decrease with increasing cellulose. The low yield of FA at
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<0.1 g of cellulose results from the over-oxidation of FA, and conditions is only 40–50%.33,34 The yields of FA from cellulose
the low yield of FA at >0.1 g of cellulose is due to the low con- and glucose with sulfuric acid and NaVO3 are comparable
version of cellulose.
(64.9% vs. 68.2%). Thus, with the participation of NaVO3 and
The byproducts were determined by HPLC analysis; however, oxygen, the glucose produced from hydrolysis of cellulose is
no other products were detected with a mass fraction of sulfuric readily oxidized to FA, thus resulting in a comparable FA yield
acid of 0.007 (Fig. S9†). Besides, CO2 is the sole product in the and avoiding the production of byproducts. In other words,
gas phase as detected by GC, and the carbon balance shows the generation of humin can be completely avoided because
that FA and CO2 are exclusive in the products (Table S2†).
the glucose produced from hydrolysis cannot exist under the
The role of vanadium–oxygen catalyst in redox reactions is drastic oxidation conditions in the present system. The hydro-
well known, such as V2O5 for SO2 to SO3,30 V–Mo oxides for lysis reaction is the rate-determining step with cellulose as a
naphthalene to phthalic anhydride26 and H7PMo8V4O40 for reactant because of the significant increase of oxidation reac-
benzene to phenol,28 etc. During these oxidation reactions, the tion rate with an increase in the mass fraction of sulfuric acid
vanadium–oxygen (V–O) species are reduced and then oxidized (Fig. S5 and S6†).
+
by an oxidant such as oxygen or hydrogen peroxide to com-
To obtain the reaction pathway under catalysis by VO2 ,
plete a catalytic cycle, or the vanadium–oxygen species acti- several two-carbon and three-carbon containing compounds
vates the oxidant to form an intermediate that oxidizes the that may be the intermediates were investigated under the
reactants. The selectivity of the products is highly dependent same reaction conditions as those for glucose (see Table 1,
on the V–O species present in these reactions. In the aqueous entry 14), following the method suggested by Wasserscheid
sodium metavanadate solution, the V–O species varies with and coworkers.22 Table 2 shows the reaction results for these
the acidity of the solution (pH)26,27 and consequently deter- compounds. FA is very stable under the reaction conditions,
mines the selectivity for FA. The experimental results in indicating that CO2 is derived from reaction processes other
Table S1† show that the pH of the solution is lower than 1 than FA oxidation. According to the results of carbohydrate
when the mass fraction of sulfuric acid reaches 0.007 (this conversion, the intermediates should meet three criteria: they
should be the case under the reaction conditions due to evap- should be oxidized readily, have high selectivity to FA and
4−
oration of water at higher temperature). Generally, V4O12 is exclusive selectivity to FA and CO2. Since glycolaldehyde,
5−
predominant with a small amount of V5O15 at pH around 7; glyoxal and glyceraldehyde meet these criteria, they are most
6−
V10O28 appears at pH < 6 and becomes dominant in a pH likely to be the oxidation intermediates of a monosaccharide
+
range of 4.1–3.0; VO2 appears at pH < 2 and becomes domi- such as glucose, and are formed via oxidative C–C scission.
nant at pH < 1. This suggests that the high FA selectivity at a The oxidation of polysaccharides such as cellulose and xylan
sulfuric acid mass fraction greater than 0.007 may be attribu- starts with hydrolysis first to yield a monosaccharide. These
+
ted to catalysis via VO2 , which is different from the V–O
species at sulfuric acid mass fractions lower than 0.007. The
high FA selectivity at sulfuric acid mass fractions greater than
or equal to 0.007 in comparison with the many byproducts
Table 2 The oxidation of possible intermediates in the carbohydrate
oxidation process in the acidic aqueous sodium metavanadate solution
by oxygen. Substrate (100 mg), sodium metavanadate (22 mg), sulfuric
produced at sulfuric acid mass fractions lower than 0.007
(Fig. S3 and S4†) indicates an exclusive reaction pathway to FA
acid (42 mg, mass fraction of 0.007 in water), water (6 g) and oxygen
(initial pressure 3 MPa) were stirred in an autoclave at 160 °C and the
reaction time of 1 min. FA yields were calculated on a carbon basis and
given as mean values. Conversion of substrates is calculated based on
the amount of unconverted substrate. AC represents acetic acid
+
under catalysis by VO2 . 51V-NMR and XPS analyses have esta-
+
blished that VO2 is the only V–O species in the solution at a
+
sulfuric acid mass fraction of 0.007 and VO2 can be reduced
to a tetravalent form by carbohydrates in the absence of
+
oxygen (Fig. S10 and S11†). Thus, it is considered that VO2
Entry Substrate
Conversion/% FA yield/% Others
oxidizes carbohydrates by giving an oxygen atom first, and
then is regenerated through oxidation by molecular oxygen.
The low cellulose conversion in the absence of sulfuric
acid, in comparison with that of glucose (see Table 1, entries
2, 13) which can be converted in 96.8% within 1 min, indicates
the inertia of the polymeric structure of carbohydrates and
the activeness of the monosaccharide structure with respect
to oxidation. Thus the role of sulfuric acid in converting
polysaccharides appears not only to transform the V–O
1
2
3
4
Formic acid
Formaldehyde
Acetic acid
Glycol
Oxalic acid
Glycolaldehyde
Glycolic acid
Glyoxylic acid
Glyoxal
Glycerol
Glyceraldehyde
Lactic acid
<1
2
0
67.5
85.2
100
90.2
100
100
84
100
100
100
100
70.1
15.8
—
0
—
16.7
0
62.6
30.1
30.2
85.6
40.3
65.3
15.1
20.9
57.3
16.5
2.1
—
CO2
—
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
AC 25.2%
AC 50.4%
AC 21.1%
AC 17.8%
AC 3.0%
5
6a
7
8
9
10
11
12
13
14b
15
16
+
species to VO2 , but also to initiate and accelerate the hydro-
Methylglyoxal
1,3-Dioxyacetone
1,2-Propylene glycol
1,3-Propylene glycol
lysis reaction. The acid catalyzed hydrolysis of cellulose, an
industrial process, produces glucose first and ultimately
results in a mixture of glucose, HMF, levulinic acid, FA
and humin of unknown structure.31,32 The normal yield of
a The substrate was the glycolaldehyde dimer. b The substrate was the
glucose from cellulose under sulfuric acid-catalyzed reaction 1,3-dioxyacetone dimer.
2616 | Green Chem., 2014, 16, 2614–2618
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Fig. 1 Proposed reaction pathways for the conversion of cellulose into FA in an aqueous sodium metavanadate solution with sulfuric acid under
oxygen pressure.
reaction pathways are shown in Fig. 1, including the catalytic various monosaccharides, disaccharides and polysaccharides
cycle of the vanadium catalyst.
to formic acid (FA) using an aqueous solution containing two
The separation of FA from water has been reported to be common chemicals, sodium metavanadate and sulfuric acid,
easy using the rectification method and energy-efficient extrac- as catalysts. The carbohydrates undergo hydrolysis and an
tion methods.24,35,36 In this work the extraction method was immediate oxidation to produce FA in the suggested system.
investigated using butyl ether, dichloromethane, diethyl ether, High FA selectivity and production efficiency are obtained due
+
ethyl acetate and n-octanol, and the results show that diethyl to the excellent catalytic performance of VO2 . FA can be easily
ether performs the best, offering a 99.9% extraction efficiency separated by an extraction method. The catalyst solution can
in five operation cycles (Table S4†). The diethyl ether and be reused multiple times without substantial change in the FA
water mixture can be easily separated and recovered by distilla- selectivity. This route can be an efficient and sustainable way
tion or gas sweeping (Fig. S12†). The recovered catalyst solution to produce FA and hydrogen from biomass in the future.
was reused five times with no substantial change in the cellu-
lose conversion efficiency and FA selectivity as shown in Fig. 2.
This work was supported by the Natural Science Foundation
of China (21076138), the National Basic Research Program of
In conclusion, we have proposed a mild and efficient China (2011CB201303), and the Program for New Century
method to convert biomass-based carbohydrates containing Excellent Talents in University (NCET-08-0710).
Notes and references
1 A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107,
2411–2502.
2 G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006,
106, 4044–4098.
3 M. S. Holm, S. Saravanamurugan and E. Taarning, Science,
2010, 328, 602–605.
4 H. B. Zhao, J. E. Holladay, H. Brown and Z. C. Zhang,
Science, 2007, 316, 1597–1560.
5 Y. R. Leshkov, J. N. Chheda and J. A. Dumesic, Science,
2006, 312, 1933–1937.
6 M. Bicker, J. Hirth and H. Vogel, Green Chem., 2003, 5,
280–284.
7 C. O. Tuck, E. Pérez, I. T. Horváth, R. A. Sheldon and
M. Poliakoff, Science, 2012, 337, 695–699.
■
Fig. 2 Reuse of the catalyst system for cellulose conversion ( ) and FA
▲
selectivity ( ). Reaction conditions: cellulose, 100 mg; NaVO3, 22 mg;
8 E. L. Kunkes, D. A. Simonetti, R. M. West, J. C. S. Ruiz,
C. A. Gärtner and J. A. Dumesic, Science, 2008, 332, 417–421.
sulfuric acid, 42 mg (mass fraction of 0.007 in water); water, 6 g; oxygen
(initial pressure 3 MPa); temperature, 160 °C; reaction time, 120 min.
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 2614–2618 | 2617

View Article Online
Paper
Green Chemistry
9 M. E. Himmel, S. Y. Ding, D. D. Johnson, W. S. Adney, 23 J. Li, D. J. Ding, L. Deng, Q. X. Guo and Y. Fu, Chem-
M. R. Nimlos, J. W. Brady and T. D. Foust, Science, 2007,
315, 804–807.
10 S. Uhm, S. T. Chung and J. Lee, J. Power Sources, 2008, 178,
34–43.
11 M. Weber, J. T. Wang, S. Wasmus and R. F. Savinell,
J. Electrochem. Soc., 1996, 143, 158–163.
12 Y. H. P. Zhang, Energy Environ. Sci., 2009, 2, 272–277.
SusChem, 2012, 5, 1313–1318.
24 J. Albert, R. Wolfel, A. Bosmann and P. Wasserscheid,
Energy Environ. Sci., 2012, 5, 7956–7962.
25 B. Girisuta, L. P. B. M. Janssen and H. J. Heeres, Ind. Eng.
Chem. Res., 2007, 46, 1696–1708.
26 B. M. Weckhuysen and D. E. Keller, Catal. Today, 2003, 78,
25–46.
13 D. G. Vlachos and S. Caratzoulas, Chem. Eng. Sci., 2010, 65, 27 M. Jian, L. F. Zhu, J. Y. Wang, J. Zhang, G. Y. Li and
18–29. C. W. Hu, J. Mol. Catal. A: Chem., 2006, 253, 1–7.
14 N. Z. Muradov and T. N. Veziroglu, Int. J. Hydrogen Engery, 28 M. Tani, T. Sakamoto, S. Mita, S. Sakaguchi and Y. Ishii,
2008, 33, 6804–6839. Angew. Chem., Int. Ed., 2005, 44, 2586–2588.
15 C. Fallay, P. J. Dyson and G. Laurenczy, Angew. Chem., 29 M. Engeser, M. Schlangen, D. Schroder and H. Schwarz,
Int. Ed., 2008, 47, 3966–3968. Organometallics, 2003, 22, 3933–3943.
16 S. Fukuzumi, T. Kobayashi and T. Suenobu, J. Am. Chem. 30 J. P. Dunn, R. R. Koppula, H. G. Stenger and I. E. Wachs,
Soc., 2010, 132, 1496–1497. Appl. Catal., B, 1998, 19, 103–117.
17 N. Taccardi, D. Assenbaum, M. E. M. Berger, A. Bosmann, 31 A. Orozco, M. Ahmad, D. Rooney and G. Walker, Trans.
F. Enzenberger, R. Wolfel, S. Neuendorf, V. Goeke,
N. Schodel, H. J. Maass, H. Kistenmacher and
P. Wasserscheid, Green Chem., 2010, 12, 1150–1156.
18 D. Yu, M. Aihara and J. Antal, Energy Fuels, 1993, 7,
574–577.
19 G. D. Mcginnis, S. E. Prince, C. J. Biermann and
J. T. Lowrimore, Carbohydr. Res., 1984, 128, 51–60.
20 G. D. Mcginnis, W. W. Wilson and C. E. Mullen, Ind. Eng.
Chem. Prod. Res. Dev., 1983, 22, 352–357.
IChemE, Part B, Process. Saf. Environ. Prot., 2007, 85,
446–449.
32 Y. Sun, L. Lin, C. S. Pang, H. B. Deng, H. Peng, J. Z. Li,
B. H. He and S. J. Liu, Energy Fuels, 2007, 21, 2386–2389.
33 A. Onda, T. Ochi and K. Yanagisawa, Green Chem., 2008, 10,
1033–1037.
34 W. N. Liu, Y. C. Hou, W. Z. Wu, Z. Y. Liu, Q. Y. Liu,
S. D. Tian and K. N. Marsh, Ind. Eng. Chem. Res., 2012, 51,
15503–15508.
21 F. M. Jin, J. Yun, G. M. Li, A. Kishita, K. Tohji and 35 H. Hohenschutz, J. E. Schmidt and H. Kiefer, US Pat.,
H. J. Enomoto, Green Chem., 2008, 10, 612–615. 4217460, 1980.
22 R. Wolfel, N. Taccardi, A. Bosmanna and P. Wasserscheid, 36 P. Eaglesfield, B. K. Kelly and J. F. Short, Ind. Chemist.,
Green Chem., 2011, 13, 2759–2763. 1953, 29, 147–152.
2618 | Green Chem., 2014, 16, 2614–2618
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