Effect of different ionic liquids on 5-hydroxymethylfurfural preparation from glucose in DMA over AlCl3: Experimental and theoretical study

Article abstract of DOI:10.1002/cjoc.201500030

In this work, effect of different ionic liquids (ILs) on 5-hydroxymethylfurfural (HMF) preparation from glucose in N,N-dimethylacetamide (DMA) over AlCl₃ was revealed by a combined experimental and computational study. ILs used as cocatalysts in this work included N-methyl-2-pyrrolidone hydrogen sulfate ([NMP]HSO₄), N-methyl-2-pyrrolidone methyl sulfate ([NMP]CH₃SO₃), N-methyl-2-pyrrolidone chlorine ([NMP]Cl) and N-methyl-2-pyrrolidone bromide ([NMP]Br) which were endowed with the same cation but different anions. According to the conclusion that fructose was intermediate product from glucose to HMF, we found fructose was transformed to more by-products by [NMP]HSO₄, making HMF yield decline significantly when glucose was treated as substrate. Neither glucose nor fructose could be converted by [NMP]CH₃SO₃ efficiently, leading to its no influence on glucose conversion to HMF. [NMP]Br had a higher selectivity for HMF from fructose than [NMP]Cl and AlCl₃. Besides, Al³⁺ preferred to combine with Br^-, slightly decreasing both the overall free energy barrier for glucose isomerization and activation barrier for H-shift at 393.15 K. So a high HMF yield of 57% was obtained from glucose catalyzed by AlCl₃ together with [NMP]Br under mild conditions. A series of ionic liquids which possess the same cation and different anions were synthesized to explore the effect of anions on 5-hydroxymethylfurfural (HMF) preparation from glucose over AlCl₃. Systematic experiments showed that N-methyl-2-pyrrolidone bromide ([NMP]Br) could promote selective conversion of fructose. Theoretical study indicated that Br^- could also combine with Al³⁺, accelerating process of H-shift during glucose isomerization. This discovery can help us choose cocatalysts for low toxic metal, realizing selective conversion of cellulosic biomass conversion.

Full text of DOI:10.1002/cjoc.201500030

FULL PAPER
DOI: 10.1002/cjoc.201500030
Effect of Different Ionic Liquids on 5-Hydroxymethylfurfural
Preparation from Glucose in DMA over AlCl3:
Experimental and Theoretical Study
Yan Yang, Wentao Liu, Ningning Wang, Haijun Wang,* Wei Li, and Zhanxin Song
Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material En-
gineering, Jiangnan University, Wuxi, Jiangsu 214122, China
In this work, effect of different ionic liquids (ILs) on 5-hydroxymethylfurfural (HMF) preparation from glucose
in N,N-dimethylacetamide (DMA) over AlCl3 was revealed by a combined experimental and computational study.
ILs used as cocatalysts in this work included N-methyl-2-pyrrolidone hydrogen sulfate ([NMP]HSO4), N-methyl-
2-pyrrolidone methyl sulfate ([NMP]CH3SO3), N-methyl-2-pyrrolidone chlorine ([NMP]Cl) and N-methyl-2-pyrro-
lidone bromide ([NMP]Br) which were endowed with the same cation but different anions. According to the con-
clusion that fructose was intermediate product from glucose to HMF, we found fructose was transformed to more
by-products by [NMP]HSO4, making HMF yield decline significantly when glucose was treated as substrate. Nei-
ther glucose nor fructose could be converted by [NMP]CH3SO3 efficiently, leading to its no influence on glucose
conversion to HMF. [NMP]Br had a higher selectivity for HMF from fructose than [NMP]Cl and AlCl3. Besides,
＋
Al3 preferred to combine with Br−, slightly decreasing both the overall free energy barrier for glucose isomeriza-
tion and activation barrier for H-shift at 393.15 K. So a high HMF yield of 57% was obtained from glucose cata-
lyzed by AlCl3 together with [NMP]Br under mild conditions.
Keywords anions, quantum chemistry, complex, glucose isomerization, H-shift
Introduction
mium(0) nanoparticles,[16] Sn-Mont catalysts,[17] phos-
[18]
phate-immobilized anatase TiO2
and so on. Among
With the increase in population, the rapid develop-
ment of global economy and depletion of fossil fuels,
the utilizing of renewable resources is becoming more
and more urgent. Biomass is abundant, CO2 neutral and
can be treated as feedstock for the production of chemi-
cals and fuels.[1-3] 5-Hydroxymethylfurfural (HMF) is
listed as one of the top building-block chemicals ob-
tained from biomass.[4] It is considered as an important
intermediate for preparing fine chemicals, pharmaceuti-
cals, plastic resins, liquid transportation fuels, etc.[5]
Carbohydrates are major constituents of lignocellulosic
biomass and their conversion to the platform chemicals
has gained significant attention. Fructose has been
widely studied as substrate for HMF preparation and
high yield has been obtained over acids or ionic liquids
(ILs).[6-9] While high price of fructose prevents HMF
from large scale production. Glucose, monomer of
cellulose,[10,11] seems to be more attractive as feedstock.
Unfortunately, its conversion to HMF is still a challenge.
So far various catalysts have been developed for the
production of HMF from glucose, including Lewis acid
catalysts,[12] ionic liquids (ILs),[13,14] zeolites,[15] chro-
all catalytic systems, IL/Lewis acid system is the most
efficient. Zhang and co-workers[19] demonstrated a sig-
nificant HMF yield of 70% from the dehydration of
glucose by CrCl2 in 1-ethyl-3-methylimidazolium chlo-
ride ([EMIM]Cl) ionic liquid. Han et al.[12] reported that
SnCl4 in 1-ethyl-3-methylimidazolium tetrafluoroborate
([EMIM]BF4) enabled synthesis of HMF from glucose
in good yield of 61%. What’s more, SnCl4 could show
the best result only in IL with BF- rather than Cl−,
4
Tf2N−, TFA−, Trif−, Sacc− as anion. In other words, an-
ion of IL has a great influence on activity of metal
chloride.
Molecular level insight into the structural and coor-
dination properties of copper(II) and chromium(II)
chlorides, their complexes with glucose in dialkylimi-
dazolium chloride ([RMIM]Cl) ionic liquids and the
mechanism of glucose dehydration in MCl2/[EMIM]Cl
(M＝Cr, Cu and Fe) system were researched by combi-
nation of kinetic experiments and density functional
theory (DFT) calculations.[20-23] [MCl4]2− complexes
were demonstrated as the reactive species for glucose
isomerization. Catalytic activities of MCl3 (M＝W, Mo
*
E-mail: wanghj329@outlook.com
Received January 14, 2015; accepted February 11, 2015; published online March 30, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500030 or from the author.
Chin. J. Chem. 2015, 33, 583—588
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Yang et al.
FULL PAPER
and Fe) in 1-butyl-3-methyl-imidazolium chloride
([BMIM]Cl) were also compared using DFT calcula-
tions.[24] However, current theoretical studies are mostly
based on the systems which are not environmentally
friendly or not effective for preparation of HMF. What’s
more, how different anions besides Cl− affect glucose
isomerization has been rarely studied.
AlCl3 is a cheap, nontoxic and abundant chemical. It
showed high activity for conversion of glucose into
HMF under microwave heating in a H2O/THF solvent
system.[25] So in this paper, AlCl3 was chosen as catalyst
ethyl acetate (10 mL×3) and dried at 60 ℃ in vacuum
for 2 d.[26-30] 1H NMR (D2O, 400 MHz) δ: 2.01－2.09
(m, 2H), 2.46 (t, J＝8.0 Hz, 2H), 2.84 (s, 3H), 3.52 (t,
J＝8.0 Hz, 2H). Synthetic processes of [NMP]CH3SO3,
[NMP]Cl and [NMP]Br were similar to [NMP]HSO4.
[NMP]CH3SO3: 1H NMR (D2O, 400 MHz) δ: 2.0－2.08
(m, 2H), 2.44 (t, J＝8.0 Hz, 2H), 2.80 (s, 3H), 2.83 (s,
3H), 3.51 (t, J＝8.0 Hz, 2H).
Glucose dehydration test
The catalytic activity experiments were performed in
50 mL round bottom flask that was heated in the oil-
bath with magnetic stirrer at 120 ℃. At different time
intervals, 50 µL samples were extracted and quenched
immediately with deionized water (×100) for analysis.
and
N-methyl-2-pyrrolidone
hydrogen
sulfate
([NMP]HSO4), N-methyl-2-pyrrolidone methyl sulfate
([NMP]CH3SO3), N-methyl-2-pyrrolidone chlorine
([NMP]Cl), N-methyl-2-pyrrolidone bromide ([NMP]Br)
were treated as cocatalyst, respectively. We mainly dis-
cussed effect of anions. The advantages of ILs that were
used in this study are easy preparation and less dosage.
Based on above green system, experimental and com-
putational approaches were combined to unravel cata-
lytic mechanism of different catalysts and how different
anions of ILs affect glucose dehydration by AlCl3 in
N,N-dimethylacetamide (DMA).
Product analysis
The products were analyzed by high performance
liquid chromatography (HPLC) on an Agilent Alliance
1100 series chromatograph equipped with refractive
index detector and Shodex SURGER SP-0810 columns
(300 mm×8.0 mm). The mobile phase was ultrapure
water at a flow rate of 0.7 mL•min−1 and the column
temperature was 70 ℃. The concentrations of products
were calculated based on the standard curve that was
obtained with known concentrations of the standard
substance.
O
O
O
S
O
S
+
N
+
N
-O
CH3
-O
OH
H
H
O
O
N-methyl-2-pyrrolidone methyl
N-methyl-2-pyrrolidone hydrogen
Confirmation of initial form of AlCl3 in DMA
sulfate ([NMP]CH3SO3)
sulfate ([NMP]HSO4)
A certain amount of anhydrous AlCl3 was mixed
with moderate DMA and stirred at 120 ℃ for 2 h. The
mixed solution was analyzed by electron spray ioniza-
tion mass spectrometry (ESI-MS) under negative ion
mode.
O
O
+
+
N
Cl-
N
Br-
H
H
N-methyl-2-pyrrolidone bromide
N-methyl-2-pyrrolidone chlorine
([NMP]Cl)
([NMP]Br)
Density functional theory calculations
The geometries of all structures, including glucose,
AlCl-4 , DMA, the four anions ( HSO-4 , CH3SO3- , Cl−
and Br−) and various complexes were optimized using
density functional theory (DFT) with hybrid Becke
3-Lee-Yang-Parr (B3LYP)[31,32] exchange-correlation
functional using Gaussian 03 program.[33] The
6-311G(d,p) basis set was used for C, H, O, N, Al, Cl
and S atoms, while the Br atom was treated with the
LanL2DZ basis set.[34] In order to be consistent with
experiment condition, all of the compounds were opti-
mized using the polarizable continuum model (PCM)
for DMA. Vibrational frequency calculations, from
which the zero-point energies (ZPEs) were derived had
been performed for each optimized structure at the same
level to verify that the computed structures corre-
sponded to energy minima. Various initial geometries of
glucose were optimized and the most stable one was
used for subsequent research. As for the study of reac-
tion mechanism, DFT calculations were carried out to
obtain energy barriers for reactants, intermediates (IMs),
transition states (TSs) and products by the same method
which was described above. Minima in potential energy
Figure 1 Structures of ILs prepared and used in this paper.
Experimental
Materials
D-Glucose (Glu, AR), anhydrous AlCl3 (AR),
N,N-dimethylacetamide (DMA, AR), N-methyl pyrroli-
done and other chemicals (AR) were all commercial
product from Sinopharm Chemical Reagent Co., Ltd
(Shanghai, China). AlCl3 was wax seal stored in a cool,
dry and well-ventilated cupboard. Sulfuric acid (98%)
and other chemicals (AR) are commercially available
and used without further purification unless otherwise
stated.
Synthesis of cocatalysts
[NMP]HSO4 was prepared by dropping concentrated
sulfuric acid (0.12 mol) slowly into N-methyl-2-pyr-
rolidone (0.1 mol) at 0－5 ℃ and stirring for 4 h at
room temperature. The unreacted chemical reagents and
other impurities were removed by vacuum distillation.
The obtained product [NMP]HSO4 was washed with
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Chin. J. Chem. 2015, 33, 583—588

Effect of Different Ionic Liquids on 5-Hydroxymethylfurfural Preparation
surfaces that corresponded to stable molecular species
and saddle points that corresponded to transition states
were obtained at the same time. The intrinsic reaction
coordinate (IRC) pathways have been traced in order to
verify that the saddle point links two desired minima.
Results and Discussion
Examination of various catalytic systems
A set of glucose dehydration experiments were per-
formed in DMA at 120 ℃ for different times. The re-
sults are summarized in Figure 2. From Figure 2a, we
can see glucose is converted to HMF in moderate yield
(39%) by AlCl3. Decreased yield (ca. 20%) can be ob-
tained by the addition of [NMP]HSO4 (7 mol% to glu-
cose) while other ILs have no effect on the yield. After
increasing the dosage of ILs (Figure 2b), there is still no
change in HMF yield for [NMP]Cl and [NMP]CH3SO3.
While to our surprise, generation rate of HMF and the
maximum yield increase significantly (57%) after addi-
tion of [NMP]Br.
In order to reveal this phenomenon, supplementary
experiments were carried out based on the conclusion
that HMF was obtained by glucose isomerization to
fructose and fructose dehydration (Table 1). As can be
seen from Table 1, HMF can not be obtained from glu-
cose just over ILs. But ILs except [NMP]CH3SO3 can
promote fructose dehydration to HMF. In another word,
[NMP]CH3SO3 has neither positive nor negative effect
on the reaction. However, [NMP]HSO4 shows so low
selectivity for HMF. That may be one of the reasons
why addition of [NMP]HSO4 suppresses HMF forma-
tion. Large amounts of [NMP]Br shows higher HMF
selectivity than [NMP]Cl,[35] increasing the yield of
HMF. All of the above tells us ionic liquids except
[NMP]CH3SO3 are mainly responsible for fructose de-
hydration in DMA. That corresponds to previous re-
port.[36]
Figure 2 Glucose conversion to HMF in DMA for various reac-
tion times using AlCl3 and AlCl3＋IL as catalysts. Reaction con-
ditions: 5 mL DMA, 1 mmol glucose, AlCl3 (7 mol% to glucose),
T＝120 ℃.
ity and coordinating ability, enabling unparalleled trans-
formation of glucose into HMF.[35] [MCl4]2− complexes
were demonstrated as the reactive species for glucose
isomerization.[21] In order to reveal whether [NMP]Br
acts as ligand of AlCl3 and how it affects the activity of
AlCl3 for glucose isomerization in DMA at the molecu-
lar level, the action mechanism was explored with quan-
titative method.
But Raines and Binder speculated that the halide ad-
ditives may also serve as ligands for metal and bromide
potentially offered the optimal balance of nucleophilic-
Table 1 Dehydration of glucose and fructose in DMAa
Catalyst (Dosage/mol%) t/min Conversion/%
60 48.7
Substrate
Glucose
Glucose
Glucose
Glucose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
HMF yield/%
[NMP]HSO4 (7)
[NMP]CH3SO3 (7)
[NMP]Cl (7)
0
0
60
60
60
60
60
60
60
5
0
37.7
54.1
86.0
55.8
93.6
91.6
97.0
98.7
0
[NMP]Br (7)
0
[NMP]HSO4 (7)
[NMP]CH3SO3 (7)
[NMP]Cl (7)
41.4
0
70.2
72.1
60.3
80.3
[NMP]Br (7)
[NMP]Cl (28)
[NMP]Br (28)
5
a Reaction conditions: 5 mL DMA, 1 mmol glucose or fructose, catalyst (7 or 28 mol% to substrate), T＝120 ℃.
Chin. J. Chem. 2015, 33, 583—588
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
585

Yang et al.
FULL PAPER
The form of anhydrous AlCl3 in DMA
lation (Table 2).
The smaller the value of ΔE or ΔG is, the more eas-
ily the corresponding complex forms. Data in Table 2
show that four-coordinated complexes are more stable.
In DMA/glucose/AlCl3 system (Entries 1－5), com-
plexes of AlCl-4 and solvent molecules are formed
more easily than that of AlCl-4 and glucose. In DMA/
glucose/AlCl3/[NMP]Br system, AlCl-4 can hardly
bind with cation of IL (Entry 6), just as what was dis-
cussed in previous literature,[21] cations provide an effi-
cient charge-compensating environment for the anionic
metal chloride complexes. Complexes of AlCl-4 and
Due to the structure characteristics of anhydrous
AlCl3, it is necessary to discuss the initial form of AlCl3
before getting a better understanding of the catalytic
reaction process. ESI-MS of AlCl3 in DMA (Figure 3)
explains that form of AlCl3 before addition of glucose
and ILs is mainly AlCl-4 and this structure is used for
the next research.
Br− are formed facilely. Taking value of ΔE, ΔG298.15K
,
ΔG393.15K and steric hindrance into account, AlCl3-DMA;
AlCl2-2Br, AlCl-3Br and Al-4Br were chosen as reac-
tive species in DMA/glucose/AlCl3 and DMA/glucose/
AlCl3/[NMP]Br system, respectively. The correspond-
ing configurations are shown in S2 of Supporting In-
formation. In order to simplify the simulation, cation of
every IL was not considered during the later mechanism
research.
Mechanism of glucose isomerization with different
complexes as active species
According to previous literature,[23,38] the mechanism
＋
of Al3 complex-catalyzed isomerization of glucose to
Figure 3 ESI-MS of AlCl3 in DMA.
fructose is proposed as Scheme 1. The reaction is initi-
Possible combinations of AlCl－4 and DMA or
[NMP]Br
ated by the direct coordination of glucose with the
＋
Lewis acidic Al3 center and deprotonation of O(1)H
moiety of the carbohydrate by free Cl− (intermediate 2).
The subsequent protonation of O(5) opens the gluco-
pyranose ring and O(2)H is deprotonated at the same
time (intermidate 4). What happens next is H-shift from
C(2) to C(1) which is a rate-determing step and also the
focus of our discussion (TS-H-shift). Fructose is formed
by protonation of intermidate 5 at the anionic O(1) site
Raines et al.[35] proposed that in DMA-LiCl solvent,
lithium ions could associate with DMA to form
DMA-Li＋ macrocations, resulting in a high concentra-
tion of weakly ion-paired chloride ions. Lin et al.[37]
inferred that in CPL-LiCl, lithium ions associated with
CPL to form CPL•Li＋ macrocations. So we speculated
＋
that weak interaction existed between Al3 and solvent
and closure of the furanose ring. In an attempt to ex-
molecules (DMA) or cocatalyst (IL). The species of
＋
plain the effect of Br− on activity of Al3 , mechanism of
＋
Al3 in DMA/glucose/AlCl3 and DMA/glucose/AlCl3/
glucose isomerization with different active species
[NMP]Br systems were speculated by theoretical calcu-
Table 2 Possible combinations of Al3+ and DMA or [NMP]Br before glucose transformationa
Entry Solvent
Ligand
DMA
DMA
DMA
DMA
glucose
NMPd
Br−
Br−
Br−
Br−
Br−
Species of Al3+
AlCl3-DMA＋Clb
AlCl2-2DMA＋2Cl−
AlCl-3DMA＋3Cl−
AlCl4-DMA
ΔEc/(kJ•mol−1)
ΔG298.15K/(kJ•mol−1)
ΔG393.15K/(kJ•mol−1)
1
2
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
−9.15
6.42
17.58
−11.06
−2.78
23.02
50.39
3
222.31
48.13
88.64
4
0.09
67.32
5
AlCl3-glucose＋Cl
AlCl3-NMP＋Cl−
AlCl3-Br＋Cl−
AlCl2-2Br＋2Cl−
AlCl-3Br＋3Cl−
Al-4Br＋4Cl−
74.42
91.24
108.30
70.69
6
120.48
−4.55
267.45
−4.54
7
−4.03
8
−8.67
−8.67
−8.73
9
−12.34
−15.48
31.85
−12.29
−9.27
−11.64
−7.81
10
11
AlCl4-Br
57.62
72.31
a Optimizations and frequencies were calculated at the B3LYP/6-311G(d,p) level using the polarizable continuum model (PCM) for DMA.
b
AlCl-4 combined with one solvent molecules and released one Cl−: AlCl-4 ＋DMA; AlCl3-DMA＋Cl−. c ΔE＝Ecomplex＋nCl−−EAlCl4
−
nEDMA or anion (n＝1, 2, 3, 4), the similar method for ΔG298.15K and ΔG393.15K. d Cation of IL.
586
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Chin. J. Chem. 2015, 33, 583—588

Effect of Different Ionic Liquids on 5-Hydroxymethylfurfural Preparation
(AlCl3-DMA, AlCl2-2Br, AlCl-3Br, Al-4Br,) was stud-
ied by theoretical calculation. AlCl3-DMA presents
DMA/glucose/AlCl3 system and AlCl2-2Br, AlCl-3Br,
Al-4Br present DMA/glucose/ AlCl3/[NMP]Br system.
isomerization can be accelerated by the lower activation
barrier for H shift that is a rate-controlling step.[20] From
Figure 4 we can also see that the activation free energy
barriers for H-shift decrease in the order AlCl3-DMA
(71.2 kJ/mol) ＞ AlCl2-2Br (43.3 kJ/mol) ＞ AlCl-3Br
(38.5kJ/mol)＞Al-4Br (34.4 kJ/mol). Configurations (b),
Scheme 1 Mechanism of glucose isomerization to fructose
catalyzed by Al3+ complex in DMA ([Al3+] represents different
complexes of Al3+ and anions or DMA.)
(c) and (d) in Figure 5 reveal that Br− is released com-
＋
pletely by Al3 to interact with H during H-shift, which
may facilitate migration of the H. That is in agreement
with the experimental results that addition of [NMP]Br
accelerates HMF generation and the yield rises with in-
creasing of IL dosage. What’s more, in report of Xu et
al.[39] AlBr3 showed higher selectivity for HMF than
AlCl3 in DMA. So we conjectured that Br− not only pro-
OH
O6H
OH
4
O
O5 [Al3+], Cl-
O
-HClHO
HO
HO
HO
HO 3
HO
OH
[Al]
OH
[Al]
O2H
O1H
O
O
H
Cl
α-D-glucose
2
1
+HCl
OH
O
moted selective conversion of fructose but also strength-
OH
＋
ened activity of Al3 for glucose isomerization slightly
OH
OH
H
OH
H
OH
O
-HCl
by serving as ligand.
OH
OH
OH
OH
OH
OH
O
O
O
O
Cl
[Al]
[Al]
[Al]
3
TS-H-shift
4
H
OH
OH
OH
OH
O
O
OH
O
+HCl
O
[Al]
Cl
OH
OH
HO
OH
OH
Cl
O
O
O
[Al]
H
OH
H
[Al]
5
fructose
6
Optimized structures of reaction intermediates and
transition-state during glucose isomerization to fructose
over different complexes in DMA were obtained (see
section S3 of Supporting Information). The correspond-
ing free energy diagrams are shown in Figure 4. The
＋
overall barrier decreases when Br− combines with Al3
Figure 5 Optimized structures of transition-state during H-shift
over (a) AlCl3-DMA, (b) AlCl2-2Br, (c) AlCl-3Br and (d) Al-4Br.
Selected bond lengths are given in Å.
center. What’s more, the barrier decreases as more and
＋
more Br− coordinating with Al3 . In addition, glucose
Conclusions
How different anions of ILs (Brönsted acids) affect
HMF preparation from glucose in DMA over AlCl3 was
researched with experimental and theoretical methods.
Brönsted acids except [NMP]CH3SO3 are mainly re-
sponsible for fructose dehydration. HSO-4 can debase
the selectivity of HMF from fructose. Compared with
HSO-4 and Cl−, Br− can promote selective conversion
of fructose. On the other hand, Br− can combine with
＋
Al3 , forming different complexes and slightly lowering
the overall free energy barrier. Process of H-shift is
considered as a rate-controlling step. Optimized struc-
tures of transition-state during H-shift over these com-
＋
plexes show that Br− is released completely by Al3 ,
interacting with H which is transferring from C(2) to
C(1). As a result, this decreases the activation free en-
ergy barriers for rate-controlling step. So we speculate
[NMP]Br may also promote glucose isomerization. In
general, because of favourable nucleophilicity and co-
ordinating ability for Br−, it acts as two roles: facilitat-
Figure
4
The DFT-computed free energy diagrams of
AlCl3-DMA, AlCl2-2Br, AlCl-3Br and Al-4Br catalyzed process.
All of the compounds were optimized using the polarizable con-
tinuum model (PCM) for DMA at 393.15 K. [Al] presents com-
plex of Al3+.
Chin. J. Chem. 2015, 33, 583—588
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587

Yang et al.
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Acknowledgement
The authors are grateful to the National Natural Sci-
ence Foundation of China (No. 21206057), the Natural
Science Foundation of Jiangsu Province (Nos.
BK2012118, BK2012547), and MOE & SAFEA for the
111 Project (No. B13025) for financial support.
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(Zhao, C.)
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