Effect of organic solvent and Bro?nsted acid on 5-hydroxymethylfurfural preparation from glucose over CrCl3

Article abstract of DOI:10.1039/c5ra02057c

In the present work, the influence of organic solvent on the mechanism of 5-hydroxymethylfurfural (HMF) preparation from glucose over CrCl₃ and the role of Bronsted acid (N-methyl-2-pyrrolidone hydrogen sulfate ([NMP]HSO₄), N-methyl-2-pyrrolidone bromide ([NMP]Br), N-methyl-2-pyrrolidone chlorine ([NMP]Cl), H₂SO₄, HBr and HCl) during the reaction were researched by a complementary computational and experimental study. It was found that dimethyl sulfoxide (DMSO) gave the lowest conversion of glucose by surrounding CrCl₃, forming a six-coordinated structure (CrCl₃-3DMSO). Glucose conversion in N,N-dimethylformamide (DMF) was not selective. N,N-Dimethylacetamide (DMA) and n-butyl alcohol exhibited superior selectivity towards HMF from glucose. Then the role of different Bronsted acids in DMA was expounded. On increasing the dosage of [NMP]HSO₄, glucose conversion decreased. A computational study found that HSO₄^- could also combine with CrCl₃, forming six-coordinate complexes. Addition of [NMP]Br and [NMP]Cl accelerated the generation of HMF significantly but didn't increase the yield. An experimental method preliminarily confirmed that they were mainly responsible for fructose dehydration to HMF. A subsequent computational study further verified that the two kinds of ILs had no effect on glucose isomerization.

Full text of DOI:10.1039/c5ra02057c

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PAPER
Effect of organic solvent and Brønsted acid on 5-
hydroxymethylfurfural preparation from glucose
over CrCl3†
Cite this: RSC Adv., 2015, 5, 27805
Yan Yang, Wentao Liu, Ningning Wang, Haijun Wang,* Zhanxin Song and Wei Li
In the present work, the influence of organic solvent on the mechanism of 5-hydroxymethylfurfural (HMF)
preparation from glucose over CrCl
sulfate ([NMP]HSO ), N-methyl-2-pyrrolidone bromide ([NMP]Br), N-methyl-2-pyrrolidone chlorine
[NMP]Cl), H SO , HBr and HCl) during the reaction were researched by a complementary computational
and experimental study. It was found that dimethyl sulfoxide (DMSO) gave the lowest conversion of
glucose by surrounding CrCl , forming a six-coordinated structure (CrCl –3DMSO). Glucose conversion
3
and the role of Brønsted acid (N-methyl-2-pyrrolidone hydrogen
4
(
2
4
3
3
in N,N-dimethylformamide (DMF) was not selective. N,N-Dimethylacetamide (DMA) and n-butyl alcohol
exhibited superior selectivity towards HMF from glucose. Then the role of different Brønsted acids in
DMA was expounded. On increasing the dosage of [NMP]HSO
4
, glucose conversion decreased. A
4ꢀ
3
computational study found that HSO
could also combine with CrCl , forming six-coordinate
Received 2nd February 2015
Accepted 5th March 2015
complexes. Addition of [NMP]Br and [NMP]Cl accelerated the generation of HMF significantly but didn’t
increase the yield. An experimental method preliminarily confirmed that they were mainly responsible for
fructose dehydration to HMF. A subsequent computational study further verified that the two kinds of ILs
had no effect on glucose isomerization.
DOI: 10.1039/c5ra02057c
www.rsc.org/advances
mentioned above contain a glucose monomer, thus the devel-
opment of a simple and effective process to transform glucose
1
. Introduction
With the rapid development of the global economy and into HMF would be of great signicance. Up to now, organic
16
17,18
19
increasing demands for energy, the utilization of biomass solvents, ionic liquids (IL)
and even water have been used
which is abundant, renewable, CO neutral and can be treated as reaction medium. In addition, various catalysts have been
2
1
as a feedstock for the production of platform chemicals has developed for the production of HMF from the dehydration of
20
attracted wide attention. It is important to seek bio-based glucose, including Lewis acid catalysts, basic ionic liquids
2
1
22
7
chemicals from carbohydrate chemistry and the mineral oil- (ILs), zeolites, chromium(0) nanoparticles, Sn-Mont cata-
23
24
based industrial chemistry. Furan derivatives, such as 5- lysts, phosphate-immobilized anatase TiO
,
and so on.
2
hydroxymethylfurfural (HMF), are known as important inter- Among all the catalytic systems studied, organic solvent–CrCl
3
mediates for preparing ne chemicals, pharmaceuticals, plastic or CrCl
resins, liquid transportation fuels, etc. Recently, cellulosic and co-workers demonstrated a signicant HMF yield of 70%
2
, IL–Lewis acid systems are the most efficient. Zhang
2
2
biomass conversion to platform chemicals has gained signi- from glucose by use of CrCl in 1-ethyl-3-methylimidazolium
3–6
7,8
9–11
25
20
cant attention. Fructose, glucose and cellulose have been chloride ([EMIM]Cl) ionic liquid (IL). Han et al. reported
used for the preparation of HMF successively. High yields of that SnCl
HMF could be easily obtained from dehydration of fructose in ([EMIM]BF
4
in 1-ethyl-3-methylimidazolium tetrauoroborate
) enabled the synthesis of HMF from glucose in a
4
12
ionic liquid, and using organic solvents especially dimethyl good yield of 61%. All of the systems seem to be closely
13
sulfoxide (DMSO). However, sucrose, starch and cellulose are dependent on organic solvent, IL and metal halide. Each of
more widely available raw materials for HMF production. these has different effects on HMF preparation. For example,
2
6
Among these materials cellulose is highly attractive because of DMSO can stabilize HMF, a metal chloride especially CrCl
3
or
14,15
25
its low price and inedibility.
All the polysaccharides CrCl can promote glucose isomerization to fructose. It is
2
necessary to have an insight into the structural and coordina-
tion properties of the catalytically relevant species, as well as
their complexation with carbohydrates at a molecular level to
design novel and green catalytic systems for chemical trans-
formations of lignocellulosic biomass. Qian investigated the
mechanisms and energetics for the acid catalyzed b-D-glucose
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School
of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, Jiangsu,
China. E-mail: wanghj329@outlook.com; Tel: +86 13382888162
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra02057c
1
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27
conversion to HMF in water. Hensen and co-workers have concentrations of 50 and 0.1 mmol, respectively. Their Ham-
specialized in investigating the structural and coordination mett acidity functions (H ) were calculated using the equation
0
+
properties of copper(II) and chromium(II) chlorides, their H ¼ pK (I) + log([I]/[IH ]) where pK (I) is the pK value of the
0
a
aq
a
a
+
complexes with glucose in dialkylimidazolium chloride p-nitroaniline (0.99) indicator solution, [I] and [IH ] are the
(
[RMIM]Cl) ionic liquids and the mechanism of glucose dehy- molar concentrations of the unprotonated and protonated
dration in a MeCl
–[EMIM]Cl (Me ¼ Cr, Cu and Fe) system by a forms of the indicator, respectively.
combination of kinetic experiments and density functional
2
2
8–31
theory (DFT) calculations.
They demonstrated that
2
ꢀ
[
MeCl
4
]
complexes were the reactive species in the MeCl -
2
2
.4 Glucose dehydration test
catalyzed glucose dehydration in dialkylimidazolium chloride
The catalytic activity experiments were performed in a 25 mL
reaction vial that was heated in an oil-bath with a magnetic
stirrer. In a standard experiment, CrCl , cocatalyst (7–28 mol%
3
ionic liquid. The catalytic activity of MeCl (Me ¼ W, Mo and Fe)
3
in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) was also
32
compared using DFT calculations.
to glucose or fructose) and substrate were dissolved in organic
solvent (5 mL). Then the vial was placed in an oil-bath preheated
to a certain temperature with stirring. At different time inter-
vals, 50 mL samples were extracted and quenched immediately
with deionized water (ꢂ100) for the product analysis.
However, the effect of organic solvent on glucose conversion
and whether acidic ILs affect glucose isomerization have been
rarely studied. So in our work, N,N-dimethylformamide (DMF),
dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMA) and
n-butyl alcohol were chosen as reaction medium, and CrCl3
served as catalyst. N-Methyl-2-pyrrolidone hydrogen sulfate
(
[NMP]HSO ), N-methyl-2-pyrrolidone bromide ([NMP]Br), N-
4
2 4
methyl-2-pyrrolidone chlorine ([NMP]Cl), H SO , HBr, and HCl 2.5 Product analysis
were treated as cocatalysts. The advantages of the ILs used in
this study are easy preparation and low dosage. Experimental
and computational approaches were combined to unravel the
mechanism of action of organic solvent and Brønsted acid.
The products were analyzed by high performance liquid chro-
matography (HPLC) on an Agilent Alliance 1100 series chro-
matograph equipped with a refractive index detector and
Shodex SURGER SP-0810 columns (300 mm ꢂ 8.0 mm). The
ꢀ1
mobile phase was ultrapure water at a ow rate of 0.7 mL min
ꢁ
2. Experimental section
and the column temperature was 70 C. The concentrations of
products were calculated based on the standard curve obtained
with known concentrations of the standard substance.
2
.1 Materials
D-Glucose (Glu, AR), CrCl
AR), N,N-dimethylacetamide (DMA, AR), dimethyl sulfoxide
DMSO, AR), n-butyl alcohol (AR), N-methylpyrrolidone and
other chemicals (NMP, AR) were all commercial products from
Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). The geometries of all structures involved, including glucose,
3
(AR), N,N-dimethylformamide (DMF,
(
2.6 Density functional theory calculations
ꢀ
ꢀ
ꢀ
Sulfuric acid (98%), hydrochloric acid (38%), hydrobromic acid CrCl
3
, DMA, DMSO, the three anions (HSO
4
, Cl and Br ) and
(
40%) and other chemicals (AR) are commercially available and various complexes were optimized using density functional
38,39
used without further purication unless otherwise stated.
theory (DFT) with hybrid Becke 3-Lee–Yang–Parr (B3LYP)
exchange–correlation functional using the Gaussian 03
program. The 6-311G(d,p) basis set was used for C, H, O, N, Cl
and S atoms, while the Cr, Br atom was treated with the
LanL2DZ basis set. In order to be consistent with the experi-
ment conditions, all of the compounds were optimized using
the polarizable continuum model (PCM) for DMA or DMSO at
2
.2 Synthesis of cocatalysts
[NMP]HSO4 was prepared by dropping concentrated sulfuric
40
acid (0.12 mol) slowly into N-methyl-2-pyrrolidone (0.1 mol) at
ꢁ
0–5 C and stirring for 4 h at room temperature. The unreacted
chemical reagents and other impurities were removed by
vacuum distillation, and the obtained product [NMP]HSO
393.15 K. Vibrational analyses on all optimized structures reveal
4
was
a lack of imaginary frequencies, ensuring the presence of a true
minimum. The interaction energies are also corrected by zero-
point energy (ZPE). Various initial geometries of glucose were
optimized and the most stable one was used for subsequent
research. As for the study of reaction mechanism, DFT calcu-
lations were carried out to obtain energy barriers for reactants,
intermediates (IMs), transition states (TSs) and products by the
same method described above and to get minima in potential
energy surfaces corresponding to stable molecular species and
ꢁ
washed with ethyl acetate (3 ꢂ 10 mL) and dried at 60 C under
33–35
1
vacuum for 2 days.
H NMR (400 MHz, D
2
O): d
H
(ppm) ¼
2
2
.01–2.09 (m, 2H), 2.42–2.48 (t, 2H), 2.84 (s, 3H), 3.50–3.54 (t,
H). The synthetic processes for [NMP]Cl, [NMP]Br were similar
to [NMP]HSO
Fig. 1.
4
. The structures of the ILs prepared are shown in
2
.3 UV-vis acidity evaluation
The Brønsted acidity was evaluated from the determination of saddle points that corresponded to transition states. The
the Hammett acidity functions, using UV-visible spectros- intrinsic reaction coordinate (IRC) pathways have been traced
36,37
copy.
Herein, p-nitroaniline was chosen as the basic indi- in order to verify that the saddle point links two desired
cator. IL and indicator were dissolved in water with minima.
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Fig. 1 Structures of ILs prepared and used in this paper.
boiling point. So DMA was chosen as the solvent for the
following research.
3
. Results and discussions
3.1 Glucose dehydration in different organic solvents
Time, temperature, catalyst loading and substrate concen-
A set of glucose dehydration experiments were performed in tration are key factors for chemical reactions. Fig. 3 shows that
ꢁ
ꢁ
DMF, DMA, DMSO and n-butyl alcohol over CrCl
The results are summarized in Fig. 2. HMF is obtained in mol% CrCl
3
at 120 C. the yield of HMF reaches its highest at 120 C for 70 min with 7
when the glucose concentration is 1.0 mmol/5 mL.
3
moderate yield from glucose in DMA and n-butyl alcohol. More The further research was performed under the optimal
unidentied by-products were formed in DMF. In DMSO, less conditions.
glucose was converted even if the time was prolonged (Fig. 2b).
So we assume that the DMSO prevented CrCl from attacking
3
3.2 Glucose dehydration with different Brønsted acids as
glucose in some way. It is not possible to adjust the tempera-
cocatalyst in DMA
ture of n-butyl alcohol over a wide range because of its low
The IL is considered as not only a solvent but also a cocatalyst
which may have synergy with the metal chloride for the
20
conversion of glucose. So in this work, we chose three kinds of
ILs ([NMP]HSO , [NMP]Cl, [NMP]Br) which possessed the same
cation and different anions to investigate their effect on HMF
yield in DMA. Fig. 4a shows that addition of [NMP]HSO (7
4
4
mol% to glucose) lowers the yield of HMF slightly. What’s more,
as the dosage increases, HMF yield decreases signicantly
(
Fig. 4b). [NMP]Cl and [NMP]Br both accelerate HMF generation
but don’t increase the highest yield. To conrm the crucial role
of anions, H SO , HCl and HBr were chosen to replace the
corresponding ILs (Fig. 4c and d). In a similar way HMF yield
decreases as H SO dosage increases and the reaction is
2
4
2
4
promoted by HCl and HBr but without the HMF yield
increasing.
3.3 Exploration of mechanism of action of the catalysts
under different experimental conditions
In order to reveal why little HMF was obtained with addition of
4 2 4
[NMP]HSO or H SO , we paid attention to the conversion of
glucose. As can be seen from Fig. 5, glucose conversion reduces
with increasing concentration of [NMP]HSO or H SO . So we
4
2
4
ꢀ
inferred that HSO4 lowered the activity of CrCl3.
The mechanism for glucose dehydration to HMF involves
25
fructose formation as a crucial reaction intermediate. So the
question is which step do [NMP]Br, [NMP]Cl, HBr and HCl
promote, accelerating the generation of HMF? To answer the
question, a series of reactions were carried out with glucose and
fructose as substrate, respectively. As is shown in Table 1, no
HMF formed from glucose while high HMF yield was obtained
from fructose over Brønsted acids. So we conrm Brønsted
acids can’t realize the isomerization of glucose but mainly
convert fructose to HMF. In addition, [NMP]Br and HBr show
higher selectivity than [NMP]Cl and HCl, which is consistent
Fig. 2 (a) Glucose conversion and HMF yield in different solvents over
CrCl
3
. Reaction conditions: 5 mL solvent, 1 mmol glucose, CrCl
3
(7
ꢁ
mol% to glucose), T ¼ 120 C, t ¼ 30 min. (b) Glucose conversion and
3
HMF yield in DMSO for different reaction times over CrCl .
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Fig. 3 Effect of different reaction conditions.
41
with Binder’s treatise. While CrCl can catalyze both glucose reach an equal yield of HMF from fructose. That’s why a certain
3
isomerization and fructose dehydration to HMF. But compared amount of Brønsted acid accelerates HMF formation from
with Brønsted acids, it takes a longer time for CrCl (7 mol%) to glucose (Fig. 4).
3
Fig. 4 Effect of different Brønsted acids on HMF yield. Conditions: DMA (5 mL), glucose (1 mmol), CrCl
3
(7 mol%), CrCl
3
: Brønsted acids ¼ 7 : 7 or
ꢀ1
ꢁ
7
: 28 (mol mol ), T ¼ 120 C.
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Fig. 5 Effect of [NMP]HSO
acids ¼ 7 : 7 or 7 : 28 (mol mol ), T ¼ 120 C.
4
and H
2
SO
4
on glucose conversion. Conditions: DMA (5 mL), glucose (1 mmol), CrCl
3
(7 mol%), CrCl
3
: Brønsted
ꢀ
1
ꢁ
3
.4 Determination of H
0
values of Brønsted acidic ILs
halide particularly bromide not only promote selective trans-
formation of fructose but also serve as a ligand for chromium?
Do [NMP]Br, [NMP]Cl, HBr and HCl improve the reactivity of
41
Due to the fact that dehydration of fructose is associated with the
acidity of the catalyst, the acidic scale of these Brønsted acidic ILs
was investigated with a basic indicator. The maximal absorbance
of the unprotonated form of the indicator was observed at 379 nm
CrCl , promoting the glucose to fructose isomerization step that
3
determines the overall HMF selectivity? To solve these prob-
lems, a computational method was employed.
2
in H O. When an acidic IL was added, the absorbance of the
Binder et al. have reported that in DMA–LiCl solvent, lithium
unprotonated form of the indicator decreased. As shown in Fig. 6
the absorbance of the unprotonated form of the indicator on three
acidic ILs decreased as follows: [NMP]Cl > [NMP]Br > [NMP]HSO4.
Ultimately, we obtained the acidity order of the three ILs with the
following H values (Table 2): [NMP]HSO (1.15) > [NMP]Br (1.31) >
+
41
ions could associate with DMA to form DMA–Li , Hensen and
co-workers conrmed in an MeCl
/[EMIM]Cl (Me ¼ Cr, Cu and
complexes were the reactive species
2
2
ꢀ
Fe) system, that [MeCl ]
4
in the MeCl -catalyzed glucose dehydration by a combination of
2
0
4
kinetic experiments and density functional theory (DFT) calcu-
[NMP]Cl (1.41). Table 1 shows selectivity from fructose to HMF
28
lations. So in our work, the structural and coordination
properties of complexes formed upon the interaction of chro-
mium(III) chlorides with solvent, ionic liquids and glucose were
decreasing in the order [NMP]Br > [NMP]Cl > [NMP]HSO . This
implies that too strong acidity isn’t benecial for fructose dehy-
dration to HMF.
4

rst studied by density functional theory (DFT) calculations
(Table 3). The smaller the value of DG393.15K is, the easier the
3.5 Investigation of the mechanism of inuence of solvents
corresponding species forms. Entry 1–4 in Table 3 represent the
and Brønsted acids using a computational method
DMA–CrCl –glucose system. There is no obvious difference in
3
values of DG393.15K. That is to say, CrCl can combine with both
3
How do DMSO, [NMP]HSO and H SO suppress glucose
conversion? Does, just as Binder and Raines conjectured, a
4
2
4
glucose and DMA. When combining with DMA, CrCl –DMA is
3
a
Table 1 Dehydration of glucose and fructose in DMA
ꢁ
Substrate
Catalyst/mol%
t/min
T/ C
Conversion/%
HMF yield/%
Glucose
Glucose
Glucose
Glucose
Glucose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
[NMP]Cl (7)
[NMP]Br (7)
HCl (7)
60
60
60
60
60
5
5
5
5
5
40
40
40
40
40
40
40
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
37.7
54.1
43.2
60.4
93.5
97.0
98.7
95.6
98.3
94.9
<100
<100
<100
<100
<100
<100
96.1
0
0
0
0
HBr (7)
CrCl
3
(7)
59.6
60.5
80.2
59.4
70.4
10.2
55.3
74.9
40.3
71.1
74.2
41.0
67.8
[NMP]Cl (28)
[NMP]Br (28)
HCl (7)
HBr (7)
CrCl
3
(7)
[NMP]Cl (28)
[NMP]Br (28)
4
[NMP]HSO (28)
HCl (7)
HBr (7)
H
CrCl
2
SO
4
(7)
(7)
3
a
ꢁ
Conditions: DMA (5 mL), glucose or fructose (1 mmol), T ¼ 120 C.
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interaction of the CrCl
3
with DMSO is strongly favorable
ꢀ1
(DG393.15K ¼ ꢀ114.56 kJ mol ) and leads to the formation of a
six-coordinated structure. From the conguration of CrCl –
3
3
DMSO in Fig. 7 we can see the Cr center is surrounded
ꢀ
completely by DMSO and Cl , impeding its combination with
the substrate and restraining the conversion of glucose. That’s
why DMSO isn’t appropriate for glucose conversion catalyzed by
CrCl
SO
forming CrCl –3HSO (DG
393.15K
3
. Similarly, in the DMA–glucose–CrCl
3
–[NMP]HSO
4
or
ꢀ
H
2
4
system (Entry 9–12), CrCl mainly combines with HSO
3
4
,
ꢀ1
¼ ꢀ78.61 kJ mol ). When the
3
4
ꢀ
1
catalyst dosage is CrCl : [NMP]HSO ¼ 7 : 7 (mol mol ), the
3
4
ꢀ
amount of HSO isn’t enough to combine with CrCl to form
4
3
more CrCl
3
–3HSO
4
, resulting in only a slight decrease of glucose
Fig. 6 Absorbance spectra of 4-nitroaniline for various Brønsted
acidic ILs in H O.
2
conversion. As the molar ratio of [NMP]HSO
4 3
and CrCl
increases, more and more Cr centers are surrounded, leading to
an obvious decrease of glucose conversion. While combination
a
3
of CrCl and cation of IL is thermodynamically unfavorable
Table 2 Calculation of H
0
2
values of different ILs in H O
ꢀ
1
29
(
DG393.15K ¼ 70.69 kJ mol ), just as described in the literature,
+
ILs
A
max
[I]/%
[IH ]/%
H
0
cations provide an efficient charge-compensating environment
for the anionic metal chloride complexes. No coordination of
the cation to metal centers is observed. So in order to simplify
the simulation, the cation of IL was not considered during the
—
1.435
0.847
0.968
1.038
100
0
—
1.15
1.31
[NMP]HSO
[NMP]Br
[NMP]Cl
4
59.02
67.46
72.33
40.98
32.54
27.67
1.41 next part of the mechanism research.
In DMA–glucose–CrCl –[NMP]Br or HBr and DMA–glucose–
CrCl –[NMP]Cl or HCl systems (Entry 13–18), interactions of
1
a
+
3
Indicator: 4-nitroaniline; [I] and [IH ]: the unprotonated and
+
protonated forms of the indicator; H
pK
0
¼ pK
a
(I)aq + log([I]/[IH ]),
3
ꢀ
a
(I)aq ¼ 0.99. Solvent: deionized water; [IL] ¼ 50 mmol L ; [4- CrCl3 with glucose, DMA, and anion of ILs (Br, Cl) were
ꢀ1
ꢁ
nitroaniline] ¼ 0.1 mmol L ; T ¼ 20 C.
compared. Just as Binder and Raines said, a halogen particu-
larly bromine can combine with chromium facilely to form
different complexes. The DFT calculations indicate that the ve-
the most thermodynamically favorable. In other words, the
reaction of glucose can be catalyzed by CrCl and CrCl –DMA in
3 3
DMA. While in the DMSO–CrCl –glucose system (Entry 5–8), the
3+
coordinated structures are the dominant Cr -containing
complexes aer addition of Brønsted acid. What’s more, the
3
3 3
resulting CrCl –2Cl and CrCl –2Br species adopt a hemispheric
3
+
geometry (Fig. 7), exposing the Cr center. As a result, this kind
of ve-coordinated structure can still catalyze glucose. But it’s
a
ꢀ
ꢀ
Table 3 Possible species of CrCl
3
in different systems before glucose still unknown whether Br and Cl improve the activity of CrCl
3
transformation
ꢀ
1 b
Entry Solvent Ligand
Species of CrCl
3
DG393.15K (kJ mol
)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
DMA
DMA
DMA
DMA
DMSO
DMSO
DMSO
DMSO
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
CrCl
CrCl
CrCl
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
–DMA
ꢀ34.00
ꢀ31.72
ꢀ16.00
ꢀ26.62
ꢀ58.34
ꢀ96.19
ꢀ114.56
ꢀ35.25
70.69
–2DMA
–3DMA
–glucose
–DMSO
–2DMSO
–3DMSO
–glucose
Glucose CrCl
DMSO
DMSO
DMSO
CrCl
CrCl
CrCl
Glucose CrCl
c
c
NMP
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
CrCl
–NMP
ꢀ
ꢀ
ꢀ
0
1
2
3
4
5
6
7
8
HSO
HSO
HSO
4
–HSO
4
ꢀ31.12
ꢀ38.13
ꢀ78.61
ꢀ67.8
4
4
–2HSO
–3HSO
–Br
–2Br
–3Br
–Cl
4
4
ꢀ
Br
ꢀ
Br
ꢀ93.6
ꢀ
Br
ꢀ81.4
ꢀ
ꢀ
Cl
Cl
ꢀ50.4
–2Cl
–3Cl
ꢀ55.2
ꢀ
Cl
ꢀ27.0
a
Optimizations and frequencies were calculated at the B3LYP/6-
3
11G(d,p) level using the polarizable continuum model (PCM) for
b
4ꢀ
ꢀ
DMA or DMSO. Dc G393.15K ¼ Gspecies/393.15k ꢀ GCrCl /393.15k ꢀ nGDMA or
Fig. 7 Combinations of CrCl
and Br .
3
and DMA, glucose, DMSO, HSO , Cl ,
3
ꢀ
DMSO (n ¼ 1, 2, 3). Cation of IL.
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for glucose isomerization. Herein a computational method was complexes in DMA were obtained (see Section S3 of ESI†). The
used to simulate the process over CrCl , CrCl –DMA, CrCl –2Br corresponding free energy diagrams at 393.15 K are shown in
3
3
3
and CrCl –2Cl, which were the dominant species of CrCl . What Fig. 8.
3
3
is illustrated is that the four complexes represent the DMA–
As is shown in Fig. 8, the free energy barrier for CrCl
3
ꢀ1
glucose–CrCl
glucose–CrCl
3
, DMA–glucose–CrCl
3
–[NMP]Br or HBr, DMA– combining with glucose directly equals ꢀ26.6 kJ mol . When
ꢀ ꢀ
3
–[NMP]Cl or HCl systems, respectively.
CrCl
3
is coordinated by any other group especially Br or Cl , it
31,42
According to the previous literature,
a mechanism for the takes a higher free energy barrier for the complex to attach to
3
+
Cr complex-catalyzed isomerization of glucose to fructose is glucose. But based on the previous discussion, a negative effect
proposed as shown in Scheme 1. The reaction is initiated by the of solvent or other groups is inevitable. So free energy diagrams
3
+
direct coordination of glucose with the Lewis acidic Cr center of CrCl –DMA, CrCl –2Br, CrCl –2Cl catalyzed process are
3
3
3
and deprotonation of the O H moiety of the carbohydrate compared (Fig. 8b). The process of H shiing from C2 to C1 is
1
(
intermediate 2). The subsequent protonation of O opens the considered as the rate-controlling step in glucose isomeriza-
5
28
2
glucopyranose ring and O H is deprotonated at the same time tion. So the free energy barriers for this step are mainly dis-
(
intermediate 4). What happens next is an H-shi from C2 to C1 cussed. The activation free energy barriers for H-shi decrease
ꢀ
1
which is the rate-determining step (TS-H-shi). Fructose is in the order of CrCl
3
–DMA (53.0 kJ mol ) > CrCl
3
–2Br (47.9 kJ
–2Cl (41.7 kJ mol ). This means CrCl –2Br,
–2Cl promote migration of H slightly. However, the overall
ꢀ
1
ꢀ1
formed by protonation of intermediate 6 at the anionic O
and closure of the furanose ring. Based on the mechanism, CrCl
optimized structures of reaction intermediates and transition- barriers increase by about 20 kJ mol . That is to say, Br and
1
site mol ) > CrCl
3
3
3
ꢀ
1
ꢀ
ꢀ
3+
state of glucose isomerization to fructose over different Cl can replace DMA to combine with Cr in DMA, but they
3+
3+
3+
Scheme 1 Mechanism of glucose isomerization to fructose catalyzed by Cr complex in DMA ([Cr ] represents different complexes of Cr
and anions or DMA).
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glucose isomerization. So only Brønsted acid works in fructose
dehydration to HMF. What is said above can be treated as a
guide to choose solvent and cocatalysts for a Lewis acid or as
guidance for the synthesis of a Lewis acid complex catalyst for
cellulosic biomass conversion.
Acknowledgements
The authors are grateful to the National Natural Science Foun-
dation of China (21206057), the Natural Science Foundation of
Jiangsu Province, China (BK2012118) and (BK2012547), and
MOE & SAFEA for the 111 Project (B13025) for nancial support.
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