Experimental and theoretical studies on imidazolium ionic liquid-promoted conversion of fructose to 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c2gc35845j

A combined experimental and computational study on the imidazolium ionic liquid-promoted conversion of fructose to 5-hydroxymethylfurfural (HMF) was performed. In particular, 1-butyl-3-methyl-imidazolium bromide (BMImBr) was found to be unexpectedly effective for conversion of fructose into HMF without utilizing any other additive or catalyst. Under the optimized conditions, nearly 100% conversion of fructose with a 95% yield of HMF could be obtained. In addition, BMImBr could be easily recovered and reused over 6 times without significant loss of activity. This protocol represents a simple, recyclable and environmentally friendly pathway for HMF production. Furthermore, the detailed mechanism of the BMImBr-promoted conversion of fructose into HMF was also studied through an in situ FT-IR technique, NMR and density functional theory calculations, and demonstrated that the hydrogen bond interaction between BMImBr and fructose could play an important role in promoting the dehydration of fructose. This work also provides further understanding at the molecular level of the reaction process for ionic liquid-promoted conversion of fructose to HMF.

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PAPER
Experimental and theoretical studies on imidazolium ionic liquid-promoted
conversion of fructose to 5-hydroxymethylfurfural†
Yu-Nong Li,a Jin-Quan Wang,*b Liang-Nian He,*a Zhen-Zhen Yang,a An-Hua Liu,a Bing Yua and
Chao-Ran Luana
Received 31st May 2012, Accepted 19th July 2012
DOI: 10.1039/c2gc35845j
A combined experimental and computational study on the imidazolium ionic liquid-promoted conversion
of fructose to 5-hydroxymethylfurfural (HMF) was performed. In particular, 1-butyl-3-methyl-
imidazolium bromide (BMImBr) was found to be unexpectedly effective for conversion of fructose into
HMF without utilizing any other additive or catalyst. Under the optimized conditions, nearly 100%
conversion of fructose with a 95% yield of HMF could be obtained. In addition, BMImBr could be easily
recovered and reused over 6 times without significant loss of activity. This protocol represents a simple,
recyclable and environmentally friendly pathway for HMF production. Furthermore, the detailed
mechanism of the BMImBr-promoted conversion of fructose into HMF was also studied through an
in situ FT-IR technique, NMR and density functional theory calculations, and demonstrated that the
hydrogen bond interaction between BMImBr and fructose could play an important role in promoting the
dehydration of fructose. This work also provides further understanding at the molecular level of the
reaction process for ionic liquid-promoted conversion of fructose to HMF.
Introduction
The impending exhaustion of fossil resources associated with
climate change has prompted increasing research for better path-
ways to produce chemicals and fuels from renewable sources.
Particularly biomass could be regarded as a potentially abundant
and renewable source to produce energy, chemicals and
materials.1,2 In this respect, 5-hydroxymethylfurfural (HMF),
which is a platform compound for the preparation of fine chemi-
cals,3 pharmaceuticals, polymers, liquid transportation fuels and
biofuels, can be prepared through the dehydration of mono-
saccharides instead of petroleum.4 The monosaccharides can
finally be obtained from hydrolysis of the biomass-derived
carbohydrates.
Various catalysts have been developed for the formation of
HMF from fructose, including mineral acids and organic acids,
salts, solid acid catalysts and zeolites.3 It is also worth mention-
ing that water,5 polar organic solvents,6 and even sub-/supercriti-
cal7 or multiphase media8 are effective solvents in conjugation
with multifarious acidic catalysts.9 Ionic liquids (ILs) have
recently been reported to be promising solvents for dissolving
Scheme 1 IL-assisted formation of HMF from fructose.
various carbohydrates,10 offering environmentally friendly
benefits in comparison with hazardous volatile organic counter-
parts.11 For example, Moreau and Lansalot-Matras successfully
performed the conversion of fructose using ILs and DMSO as a
binary solvent by employing Amberlyst-15 or para-toluene sul-
fonic acid as the catalyst.12 To date, the acid-catalyzed HMF for-
mation in ILs has already been well studied (Scheme 1a).13
However, Brønsted acids, including acidic ion-exchange resins,
concentrated H2SO4, concentrated HCl solution or a metal cata-
lyst is generally required.14 In addition, the protic acidic IL, such
as 1-H-3-methylimidazolium chloride (HMImCl), can be suc-
cessfully utilized as both solvent and catalyst for dehydration of
fructose into HMF.15 The supported acidic ILs have also been
developed for this purpose.16 Moreover, Lewis acidic catalysts in
conjunction with the ILs were also found to be effective.17
aState Key Laboratory and Institute of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, P.R. China.
Interestingly,
a
few comparative trials have recently
E-mail: heln@nankai.edu.cn
bKey Laboratory of Green Process and Engineering, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing 100190, People’s
Republic of China. E-mail: jqwang@home.ipe.ac.cn
†Electronic supplementary information (ESI) available: copies of
NMR, FT-IR, MS, HPLC, GC-MS spectra and DFT study. See DOI:
10.1039/c2gc35845j
showed that HMF can be produced from fructose in only
BMImCl.14b,c,18 Although the HMF yield and selectivity were
not satisfied, those trials could provide a simple promising
process for the conversion of fructose into HMF, thus inspiring
us to investigate the process in a deep-going way. In the
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Table 1 Imidazolium-based ILs promoted formation of HMFa
Table 2 Effect of temperature and reaction timea
Entry
IL
Conv. (%)
Yieldd (%)
Entry
T (°C)
t (h)
Conv. (%)
Yieldb (%)
1b
2
3
4
5
HMImHSO4
BMImHSO4
(CH2)4SO3HMImHSO4
BMImH2PO4
(HOOC)CH2MImCl
HMImCl
HPyCl
HPyBr
HPyI
BMImCl
BMImBr
BMImI
BMImCl
BMImBr
95
98
73
94
81
93
90
94
75
42
>99
96
23
84
63
>99
21
16
28
9
8
40
0
1
2
3
4
5
80
120
100
100
100
1
1
0.5
1
2
>99
>99
81
>99
>99
81
71
79
95
76
50
34
66
67
81
60
5
92
65
<1
75
44
68
0
6b
7b
8b
9b
10
11
12
13c
14c
15c
16
17
18
19
20
21
a Reaction conditions: fructose (0.5 mmol, 0.09 g), BMImBr (0.3 g).
b HPLC yield.
Table 3 Effect of amount of BMImBra
BMImI
Entry
MILs (g)
Conv. (%)
Yieldb (%)
C8MImBr
C12MImBr
BMImPF6
C6MImPF6
C6MImBF4
C6MImNTf2
1
2
3
4
5
0.1
0.2
0.3
0.5
0.7
94
>99
>99
>99
>99
63
76
95
92
94
0
0
0
0
0
a Reaction conditions: fructose (0.5 mmol, 0.09 g), IL (0.5 g), 100 °C,
1 h. b H refers to hydrogen atom. c Reaction time: 0.5 h. d HPLC yield.
a Reaction conditions: fructose (0.5 mmol, 0.09 g), BMImBr, 100 °C,
1 h. b HPLC yield.
framework of our continuous efforts on the development of the
ILs to perform biomass conversion18 and other organic reac-
tions,19 we herein systemically investigated the dependence of
cations and anions of the imidazolium ILs on the conversion
of fructose into HMF, and found a simple and effective process
for HMF formation from fructose using BMImBr alone
(Scheme 1b). In particular, the mechanism of the BMImBr-
promoted conversion of fructose into HMF was also studied
through an in situ FT-IR technique, NMR and density functional
theory (DFT) calculations, and demonstrated that the hydrogen
bond could play an important role in promoting the dehydration
of fructose. Therefore, this work provides a simple, environmen-
tally friendly pathway for the conversion of fructose into HMF
and also a molecular level understanding of the reaction process
for the IL-promoted biomass conversion.
on the reaction in comparison with Cl− and I− (entries 8 vs. 7, 9;
11 vs. 10, 12, Table 1). BMImBr thus showed the best perform-
−
ance even within 0.5 h (entries 13–15, Table 1). However, PF6
and BF4− anions gave poor activity (entries 18–20, Table 1), and
C6MimNTf2 (1-hexyl-3-methyl imidazolium trifluoromethane
sulfonate) was also found to be inactive (entry 21, Table 1).
On the other hand, the alkyl group at the imidazolium ring also
has a remarkable impact on the reaction outcome. The catalytic
activity decreased as the alkyl length of substituents increased
(entries 14, 16 and 17, Table 1), partially being attributed to the
increased viscosity of the IL.20
Subsequently, the influence of the reaction parameters was
investigated in the presence of BMImBr. The conversion was
almost invariant but the HMF yield was decreased when the
temperature rose from 80 to 120 °C (entries 1 and 2, Table 2).
In other words, higher temperatures have an adverse effect on
the selectivity, possible due to facile formation of side-products
originating from condensation reactions and the instability of
HMF at high temperatures (Fig. S1, see ESI†).3a In addition, the
HMF yield increased as the reaction time increased within 1 h
(entries 3–5, Table 2). Further extending the reaction time to 2 h
could cause a decrease in yield, possibly owing to instability of
HMF (entry 5, Table 2), which was confirmed by in situ FT-IR
study. Accordingly, 1 h is required to complete the reaction.
In addition, 0.3 g of BMImBr is sufficient to perform the reac-
tion smoothly (Table 3). Furthermore, the IL, i.e., BMImBr, can
be readily recovered and reused over 6 sequential runs without
significant loss in activity (Fig. 1).
Results and discussion
The preliminary study was started by performing the reaction in
various imidazolium based ILs. The results are summarized in
Table 1. Although the Brønsted acidic ILs can work for the con-
version of fructose, very low yields of HMF were obtained
(entries 1–5, Table 1) possibly due to decomposition of HMF
and other side reactions, like the formation of degradation pro-
ducts or humins under such acidic conditions.3a,14b Protic ILs,
for example, HMImCl and pyridine hydrochloride (HPyCl),
gave good yields with significant selectivity (entries 6 and 7,
Table 1). HPyBr and HPyI were also effective (entries 8 and 9,
Table 1). In addition, 1-butyl-3-methyl-imidazolium chloride
(BMImCl) was found to be poorly active, with only a 5% HMF
yield (entry 10, Table 1). To our delight, BMImBr gave un-
expected results in terms of both fructose conversion and HMF
yield (entry 11, Table 1). In this context, Br− had a better effect
Mechanism insights
The mechanism of imidazolium ILs-promoted conversion of
fructose to HMF was investigated through experimental and
theoretical studies. In addition to an in situ FT-IR technique,
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NMR and DFT studies were also employed to gain insight into
why BMImBr alone worked so well without any co-catalyst/
co-solvent. Recent work by Pan and Shuai demonstrated that
the hydrolysis of cellulose could be significantly promoted by
chloride ions through hydrogen bond interactions with hydroxyl
groups.21 In situ FT-IR spectroscopy was first used to get infor-
mation about the hydrogen bond-promoted effect of BMImBr as
depicted in Fig. 2 and Fig. S2 (see ESI†). OH-stretching
vibrations of fructose approaches the lower frequency region
in the presence of BMImBr. This is understandable because the
formation of hydrogen bonds could weaken the OH-stretching
vibration.22 Whereafter, the absorption shifts to high-frequency
due to HMF generation.
presumably due to hydrogen bond interactions between Br− and
hydroxyl groups. The split signals of fructose in the 13C NMR
would be also ascribed to the interaction between fructose and
BMImBr (Fig. S3†).23
1
The changes in the H NMR and 13C NMR spectrum could
1
also be seen in Fig. 3. The H NMR signals of H1 and H6 of
fructose in the reaction mixture shift to the up-field region,
Fig. 2 Results of in situ FT-IR spectroscopy. Reaction conditions: fruc-
tose (10 mmol, 1.8 g), BMImBr (6.0 g), 100 °C. (a) Hydroxyl absorp-
tion peaks monitoring at various reaction times (min); 3380 cm−1 could
be assigned to the absorption of hydroxyl groups of fructose; 3443 cm−1
could be assigned to the absorption of hydroxyl groups of HMF. (b)
Curves of reagents and the product with prolongation.
Fig. 1 Six consecutive cycles of BMImBr.
Fig. 3 1H NMR spectrum of the reaction mixture in D2O (400 MHz). (a) 1H NMR spectrum of fructose for comparison; (b) 1H NMR signals of fruc-
tose in the reaction mixture. Reaction conditions: 100 °C, 3 min.
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Fig. 4 Overview of the reaction pathway for the dehydration of fructose into 5-hydroxymethylfurfural in the presence of BMImBr.
Scheme 2 Overview of the reaction pathway for dehydration of fructose into 5-hydroxymethylfurfural in the presence of BMImBr. The distance is
described as Å.
In addition, the possible HBr formation from BMImBr
decomposition was also investigated. The pH values of the
BMImBr aqueous solution (0.27 M) and the heated BMImBr
(100 °C) aqueous solution (0.27 M) were measured to be 6.25
and 6.38, respectively. Deionized water was measured as a com-
parison with the results of pH = 6.30. Obviously, the pH value
of BMImBr was found to be unchanged before and after being
heated, the same as the deionized water. Then, different concen-
trations of the aqueous solution of HBr were measured as a com-
were interested in determining how the presence of BMImBr
affected the reaction pathway. From the literature, the most feas-
ible mechanisms go through three elementary steps with three
transition states.25 We initially studied the reaction process in the
presence of BMImBr, and found that the reaction also involved
three elementary steps with three transition states. As shown in
Fig. 4 and Scheme 2: (1) The attachment of two hydroxyl
groups at C1 and C2 to the Br− to form the fructose–BMImBr
complex 1a, then loses a water molecule via TS1 with a relative
energy of 51 kJ mol−1 to yield an enediol intermediate 1b and
rapidly isomerizes into its aldehyde 1c via barrierless reaction
pathways; (2) 1c releases a second water molecule upon the
coordination of oxygen atom at C3 to the BMImBr via inter-
mediate TS2 via the transition state of TS2 with a relative energy
of 50 kJ mol−1 to produce 1d; (3) The formation of 1d upon the
coordination of the oxygen atom at C4 to the BMImBr, followed
by the removal of a third water molecule and through TS3 via
the transition state of TS3 with a relative energy of 57 kJ mol−1
to give the final product HMF. The hydrogen bond between Br−
1
parison (pH0.0027 M = 3.37; pH0.27 M = 1.77). H NMR spectra
of the heated BMImBr was also consistent with the normal one.
All of these results show that decomposition of BMImBr to HBr
did not occur under the reaction conditions, which is also expa-
tiated in other papers.24
Furthermore, a molecular level understanding of the reaction
mechanism was investigated to elucidate the results obtained
from the experimental work through a DFT study by use of the
B3PW91 functional with the 6-311++G (d,p) basis set as
implemented in Gaussian 09 program package. In particular, we
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prevent the formation of undesired products through hydrogen
bond interactions during the reaction. On the other hand, the
bromide anion could act as a base to induce the dehydration
through hydrogen bond interactions, but its function as a nucleo-
phile can not be excluded because of the possible formation of
organic acid, like levulinic acid or 5-hydroxymethyl-furan-
2-carboxylic acid, from further conversion of HMF.
Experimental
Materials and equipment
D-Fructose was obtained commercially from Aladdin without
further purification. Some of the ILs were obtained commercially
from J&K Chemica and dried prior to use. Others were
prepared according to the published reports.11,15 1H NMR
spectra was recorded at Bruker 300 and 400 or Varian Mercury-
Plus 400 spectrometer in CDCl3 or D2O and TMS (0 ppm)
was used as internal reference. 13C NMR was recorded at
75 MHz or 100.6 MHz in CDCl3 or D2O, and CDCl3
(77.0 ppm) was used as internal reference. GC-MS were
recorded on a Thermo Finnigan Polaris Q GC/MS. ESI-MS were
recorded on a Thermo Finnigan LCQ Advantage spectrometer in
ESI mode with a spray voltage of 4.8 kV. The products were ana-
lyzed on a high performance liquid chromatograph (ComHPLC)
equipped with a ComatexTM ODS1 Column (250 × 4.6 mm)
using 6000 PVW UV/VIS detector, 6000 LDI pump and
Column Oven Co-IV. The fructose was analyzed on Bio-Rad
Aminex HPX-87H column equipped with refractive index
detector.
Fig. 5 Optimized geometries for 1a and TS1, the distances are in
angstroms.
Table 4 The activation energy for different chain lengtha
ILs
E1
E2
E3
EMImBr
BMImBr
C6MImBr
50.97
51.45
51.40
50.47
54.00
50.40
57.30
57.35
57.39
a The calculations were carried out by performing DFT by use of the
B3PW91 functional with the 6-311++G (d,p) basis set as implemented
in Gaussian 09 program package.
and the hydroxyl group in fructose exists in all the intermediates
and transition states, and thus could possibly enhance the dehy-
dration of fructose and stabilize the intermediates and states. For
example in the case of the first elementary step, the optimized
geometries for 1a and TS1 are shown in Fig. 5. The distance of
the hydrogen bond between the Br− and hydroxyl group in C2
was 2.299 Å, which might help to induce the dehydration of the
first H2O. The leaving hydroxyl group in TS1 has hydrogen
bond interactions with Br− and also the hydroxyl group at C4,
which might help to stabilize the TS1. In addition, we were also
interested in determining how the alkyl group at the imidazolium
ring affect the relative activation energies along the reaction
pathway. Here, 1-ethyl-3-methylimidazolium bromide and 1-hexyl-
3-methylimidazolium bromide were chosen as representatives to
study the mechanism. The results indicated that the ILs with
different chain lengths have little influence on the relative ener-
gies (Table 4, Fig. S4 and S5, see ESI†). The catalytic activity
decreases as the alkyl length of the substituents increases, poss-
ibly being attributed to solvent effects, which are not taken into
account in the present calculations.
Computational method
The calculations were carried out by performing DFT by use of
the B3PW91 functional with the 6-311++G (d,p) basis set as
implemented in Gaussian 09 program package. All geometries
for the isolated reactants, products, intermediates, and transition
states involved in the cycloaddition reaction have been fully opti-
mized. Vibrational frequency calculations, from which the zero-
point energies (ZPEs) were derived, have been performed for
each optimized structure at the same level to identify the nature
of all the stationary points (local minimum or first-order saddle
point). The intrinsic reaction coordinate (IRC) pathways have
been traced in order to verify that each saddle point links two
desired minima.
General procedure for the synthesis of HMF
Based on this experiment, results from the DFT calculations
and results from previous work,15,21 we could propose a putative
mechanism for the complete process of glucose to HMF pro-
moted by BMImBr, as shown in Scheme 2. Firstly, fructose (1a)
dehydrates to form 1b via transition state TS1, and then isomeri-
sation of 1b gives intermediate 1c, which proceeds via a barrier-
less reaction pathways. After further induction by BMImBr, 1c
can be dehydrated to produce the intermediate 1d via transition
state TS2, followed by dehydration of 1e to gives 1f via tran-
sition state TS3. In addition, the role of BMImBr could also be
to help stabilize the intermediates and transition states, and thus
The reaction was carried out in a 5 mL reaction tube with a mag-
netic stirrer. A mixture of fructose (0.5 mmol) and the IL (0.3 g)
was placed in the tube, then heated and stirred at 100 °C for 1 h.
After reaction, the resultant mixture was cooled in the ice bath,
then dissolved in methanol and analyzed by HPLC. To further
purify the desired product, the solvent was removed by rotary
evaporator, and the residue was subjected to column chromato-
graphy with ethyl acetate–petroleum ether 60–90 °C (ethyl
acetate–petroleum ether 2 : 1) as the eluent, and a yellow viscous
liquid was obtained.
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5 Typical examples for HMF synthesis in aqueous media: (a) C. Carlini,
P. Patrono, A. M. R. Galletti and G. Sbrana, Appl. Catal., A, 2004, 275,
111–118; (b) G. W. Huber, J. N. Chheda, C. J. Barrett and J. A. Dumesic,
Science, 2005, 308, 1446–1450; (c) H. Ishida and K. Seri, J. Mol. Catal.
A: Chem., 1996, 112, L163–L165; (d) H. E. Van Dam, A. P. G. Kieboom
and H. Van Bekkum, Starch, 1986, 38, 95–101; (e) T. Armaroli,
G. Busca, C. Carlini, M. Giuttari, A. M. R. Galletti and G. Sbrana,
J. Mol. Catal. A: Chem., 2000, 151, 233–243.
6 For synthesis of HMF in organic solvents, see: (a) D. W. Brown,
A. J. Floyd, R. G. Kinsman and Y. Roshan-Ali, J. Chem. Technol. Bio-
technol., 1982, 32, 920–924; (b) K. Seri, Y. Inoue and H. Ishida, Chem.
Lett., 2000, 22–23; (c) G. A. Halliday, R. J. Young, Jr. and V. V. Grushin,
Org. Lett., 2003, 5, 2003–2005; (d) L. Lai and Y. G. Zhang, Chem-
SusChem, 2011, 4, 1745–1748.
The procedure for recycling
After the product was extracted, the remaining BMImBr system
was washed by ethyl acetate and ether, and the washing liquids
were decanted respectively. The residual washing liquids and
possible impurities were removed by rotary evaporator, and then
BMImBr was dried at 70 °C under vacuum for 6 hours. The
recovered BMImBr was directly used for the next run by introdu-
cing new substrate, where the dehydration reactions were per-
formed under identical reaction conditions.
7 For synthesis of HMF in sub-/supercritical conditions, see:
(a) F. S. Asghari and H. Yoshida, Ind. Eng. Chem. Res., 2007, 46, 7703–
7710; (b) M. Bicker, J. Hirth and H. Vogel, Green Chem., 2003, 5, 280–
284; (c) Y. Li, X. Lu, L. Yuan and X. Liu, Biomass Bioenergy, 2009, 33,
1182–1187.
Conclusions
In summary, we have developed an acid-free and metal-free
process for the highly efficient conversion of fructose into HMF
in the single IL medium of BMImBr. In particular, the role of
BMImBr was further investigated by employing an in situ FT-IR
technique and DFT study. This simple protocol represents an
environmentally friendly pathway for the facile synthesis of
HMF. Furthermore, hydrogen bond interactions between
BMImBr and fructose could play an important role in promoting
the dehydration of fructose and could also stabilize the inter-
mediates through hydrogen bond interactions. Therefore, this
work also provides a molecular level understanding of the reac-
tion process for the IL-promoted conversion of fructose to HMF,
and also a new direction in the search for catalyst systems allow-
ing efficient HMF formation from renewable sources. We believe
that the yield and selectivity of HMF could be enhanced with
extensive experimental work and further modeling on different
IL-based catalytic systems, which would be a strong competitor
to other catalysts in future biochemical industry. Presently,
further development and modification of ILs are ongoing in our
laboratory to improve the efficiency for biomass conversion to
fine chemicals.
8 Catalysis in multiphase systems for HMF formation, see: (a) Y. Román-
Leshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic, Nature, 2007, 447,
982–985; (b) C. Moreau, R. Durand, S. Razigade, J. Duhamet,
P. Faugeras, P. Rivalier, P. Ros and G. Avignon, Appl. Catal. A: Gen.,
1996, 145, 211–224; (c) Y. Román-Leshkov, J. N. Chheda and
J. A. Dumesic, Science, 2006, 312, 1933–1937; (d) V. V. Ordomsky,
J. van der Schaaf, J. C. Schouten and T. A. Nijhuis, J. Catal., 2012, 287,
68–75; (e) F. R. Tao, H. L. Song and L. J. Chou, RSC Adv., 2011, 1, 672–
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B. H. Shanks and J. A. Dumesic, ACS Catal., 2012, 2, 930–934.
9 Selected examples for HMF formation via acidic catalysis, see:
(a) T. Ståhlberg, S. Rodriguez-Rodriguez, P. Fristrup and A. Riisager,
Chem.–Eur. J., 2011, 17, 1456–1464; (b) X. Qi, M. Watanabe,
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109–114; (d) X. H. Qi, H. X. Guo and L. Y. Li, Ind. Eng. Chem. Res.,
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X. H. Wang, Chem. Commun., 2011, 47, 2176–2178; (f) K. De Oliveira
Vigier, A. Benguerba, J. Barrault and F. Jérôme, Green Chem., 2012, 14,
285–289; (g) S. Lima, M. M. Antunes, M. Pillinger and A. A. Valente,
ChemCatChem, 2011, 3, 1686–1706.
10 Selected examples for carbohydrates dissolved in ionic liquids, see:
(a) R. P. Swatloski, S. K. Spear, J. D. Holbrey and R. D. Rogers, J. Am.
Chem. Soc., 2002, 124, 4974–4975; (b) A. P. Abbott, T. J. Bell, S. Handa
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and M. Nunes da Ponte, Ind. Eng. Chem. Res., 2010, 49, 4850–4857;
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Acknowledgements
We are grateful to the National Natural Science Foundation of
China (no. 21172125, 21003129), Ministry of Science and Tech-
nology (2012BAD32B10), the “111” Project of Ministry of Edu-
cation of China (project no. B06005), and the Committee of
Science and Technology of Tianjin for financial support.
12 C. Lansalot-Matras and C. Moreau, Catal. Commun., 2003, 4, 517–
520.
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