FULL PAPER
DOI: 10.1002/chem.201003645
Molecular Aspects of Glucose Dehydration by Chromium Chlorides
in Ionic Liquids
Yanmei Zhang, Evgeny A. Pidko, and Emiel J. M. Hensen*[a]
Abstract: A combined experimental
and computational study of the ionicliquid-mediated dehydration of glucose
and fructose by CrII and CrIII chlorides
has been performed. The ability of
chromium to selectively dehydrate glucose
to
5-hydroxymethylfurfural
(HMF) in the ionic liquid 1-ethyl-3methyl imidazolium chloride does not
depend on the oxidation state of chromium. Nevertheless, CrIII exhibits
higher activity and selectivity to HMF
than CrII. Anhydrous CrCl2 and
CrCl3·6 H2O readily catalyze glucose
dehydration with HMF yields of 60 and
72 %, respectively, after 3 h. Anhydrous
CrCl3 has a lower activity, because it
only slowly dissolves in the reaction
mixture. The transformation of glucose
to HMF involves the formation of fructose as an intermediate. The exceptional catalytic performance of the chromium catalysts is explained by their
unique ability to catalyze glucose to
fructose isomerization and fructose to
HMF dehydration with high selectivity.
Side reactions leading to humins by
means of condensation reactions take
predominantly place during fructose
dehydration. The higher HMF selectivKeywords: biomass · chromium ·
density functional calculations · homogeneous catalysis · ionic liquids ·
reaction mechanisms
Introduction
Concerns about global warming and the depletion of fossil
resources have led to the exploration of renewable lignocellulosic biomass as an alternative feedstock to replace conventional crude oil for the production of fuels and chemicals.[1, 2] Amongst others, the efficient use of biomass requires new catalytic routes for the conversion of carbohydrates towards platform molecules with a wide range of
downstream applications.[3] 5-Hydroxymethylfurfural (HMF)
is recognized as a promising platform for the production of
fuels and chemicals.[4]
Numerous studies have been devoted to the catalytic conversion of fructose to HMF[5–13] with typically high and
sometimes nearly quantitative yields.[12] Among the various
catalysts, Brønsted[5, 6] and Lewis acids[8–13] are effective in
such different reaction media as ionic liquids,[6–8] organic sol[a] Dr. Y. Zhang,+ Dr. E. A. Pidko,+ Prof. Dr. E. J. M. Hensen
Inorganic Materials Chemistry
Schuit Institute of Catalysis
Eindhoven University of Technology
P.O. Box 513, NL-5600 MB, Eindhoven (The Netherlands)
Fax: (+ 31) 40-245-5054
E-mail: [email protected]
[+] Both authors contributed equally to the manuscript
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003645.
Chem. Eur. J. 2011, 17, 5281 – 5288
ity for CrIII is tentatively explained by
the higher activity in fructose dehydration compared to CrII. This limits the
concentration of intermediates that are
involved in bimolecular condensation
reactions. Model DFT calculations indicate a substantially lower activation
barrier for glucose isomerization by
CrIII compared to CrII. Qualitatively,
glucose isomerization follows a similar
mechanism for CrII and CrIII. The
mechanism involves ring opening of dglucopyranose coordinated to a single
Cr ion, followed by a transient self-organization of catalytic chromium complexes that promotes the rate-determining hydrogen-shift step.
vents,[9, 13] and biphasic systems.[5] It has been coined that this
reaction mainly occurs through the cyclic fructofuranose
pathway.[14] Side reactions include isomerization, condensation, rehydration, reversion, fragmentation and/or additional
dehydration reactions to form by-products that lower the
HMF yield.[9, 14] These side reactions are particularly problematic, when water is the solvent with much lower HMF
yields than in other solvents.[5, 15] Careful selection of the reaction medium and the catalyst can lead to substantial improvement of the selectivity.[11–13]
Compared to fructose it is much more difficult to dehydrate glucose to HMF. Selective conversion of glucose is desirable, because it is the most abundant monosaccharide in
lignocellulosic biomass. Typically, very low HMF selectivities
are reported for the conversion of glucose by using Brønsted
and Lewis acid catalysts.[18] Zhao et al. were the first to
report significant HMF yields of nearly 70 % by use of
CrCl2
in
an
1-ethyl-3-methylimidazolium
chloride
(EMImCl) ionic liquid at a temperature of 100 8C (3 h).[11]
The vast majority of works reporting successful catalytic
transformation of glucose to HMF concern ionic-liquidmediated catalysis by chromium.[11, 12, 17] Binder et al. have
shown the possibility to achieve an HMF yield of 80 % from
glucose at 100 8C (6 h) in N,N-dimethylaceramide containing
LiBr in the presence of CrCl3 and/or CrBr3.[13] Recently, the
use of microwave irradiation for glucose dehydration has
been demonstrated.[17] An HMF yield of 67 % was achieved
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5281
at 120 8C (5 min) by CrCl3 in 1-butyl-3-methylimidazolium
chloride (BMImCl). Besides chromium, other Lewis acids
such as lanthanide and tin chlorides have been reported as
useful catalysts, however with lower HMF yields compared
to chromium salts.[18]
The influence of the oxidation state of the metal center
on the reactivity in glucose dehydration has not been systematically investigated. In the original work by Zhao et al.
the glucose conversion and HMF yield were lower when
CrCl3 was used instead of CrCl2.[11] However, subsequent
studies suggest that there are only minor differences in the
catalytic activity of bivalent and trivalent chromium
salts.[12, 13, 17] This last finding is particularly surprising, taking
into account the substantial differences in the coordination
properties and Lewis acidity of CrII and CrIII.[19] We have
earlier reported on the formation of specific coordination
complexes between the sugar reactant and the metal cation
as a prerequisite for selective dehydration of glucose to
HMF.[20] The crucial step is the isomerization of glucose to
fructose (Scheme 1) followed by dehydration of fructose.
The isomerization step involves a number of anionic sugar
intermediates that are stabilized by interacting with the
chromium cation. Experimental observations combined with
computational modeling showed that the difficult hydrogentransfer step from the C2 to the C1 carbon is facilitated by a
binuclear Cr2 + complex. The self-organization of the catalytic chromium species at this step is driven by the more efficient stabilization of the negative charge of the intermediate
by the interaction with more than one Cr2 + Lewis acid.[20] A
Scheme 1. Generalized mechanism for the ionic liquid-mediated isomerization of glucose to fructose as a first step towards the selective dehydration of glucose to HMF.[20]
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similar mechanism involving a 1,2-hydride shift in glucose
isomerization by Sn-Beta has been recently reported by the
group of Davis.[21] The subsequent dehydration of fructose
to HMF is also an acid-catalyzed process. Therefore, an increase of the Lewis acidity of the catalytic metal centers
should improve the conversion of glucose to HMF. These
considerations indicate that CrIII will have a much higher activity than CrII. Herein we report the results of a systematic
comparison of CrII and CrIII chlorides in ionic liquids as catalysts for glucose dehydration by a combination of experimental and computational methods.
Results and Discussion
Catalytic sugar conversion: Figure 1 shows the conversion of
glucose and yields of fructose and HMF as a function of the
reaction time for anhydrous CrCl2 and CrCl3 and
CrCl3·6 H2O. There are marked differences in the kinetic behavior of these three catalysts (Figure 1a). For CrCl2, an ini-
Figure 1. Time evolution of a) glucose conversion, b) intermediate fructose yield, and c) HMF yield in the presence of CrCl2, CrCl3, and
CrCl3·6 H2O in EMImCl at 100 8C.
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Chem. Eur. J. 2011, 17, 5281 – 5288
Homogeneous Catalysis
FULL PAPER
tial rapid increase of the conversion during the first 20 min
is followed by a slower reaction rate. Nearly all reactant is
converted after 3 h. The initial reaction rate of CrCl3·6 H2O
is substantially higher. A conversion of 40 % is already
reached within 5 min. The final conversion in both cases is
very similar. The result is quite different when anhydrous
CrCl3 is the catalyst. An induction period of approximately
20 min is noted, followed by a slow increase of the glucose
conversion. Also in this case the conversion after 3 h is close
to 90 %.
All three catalysts show the intermediate formation of
fructose (Figure 1b). The highest fructose yield is detected
for anhydrous CrCl2. The amount of fructose detected in the
presence of CrCl3 is much lower and this is most evident for
anhydrous CrCl3. Nonetheless, fructose is formed faster with
CrCl3.6 H2O than with CrCl2, which suggests that the former
catalyst is more active in the isomerization of glucose. Concomitantly, the lower intermediate fructose yields for Cr3 +
indicate a higher rate of fructose dehydration compared to
Cr2 + . In accord with these suppositions, we find higher intermediate fructose yields at 90 8C for CrCl3·6 H2O (Figure 2).
Figure 2. Glucose conversion, intermediate fructose yield, and HMF yield
profiles in the presence of CrCl3·6 H2O in EMImCl at 90 8C.
It is well established that the conversion of fructose to
HMF proceeds through a number of dehydration steps.[9]
For CrCl2 the HMF yield follows a sigmoid curve, which is
in agreement with the notion of a two step mechanism involving glucose isomerization and fructose dehydration.[20a]
Such a behavior is not observed for CrCl3·6 H2O, which is
either due to much more rapid dehydration of the fructose
intermediate or to an altogether different mechanism. The
results in Figure 2 suggest that the former is the more reasonable explanation. Similar to CrCl3·6 H2O the trend of the
HMF yield for the anhydrous CrCl3 is the same as that of
the glucose conversion. This confirms the proposition of
high fructose dehydration activity of Cr3 + . The final HMF
yields are 72, 62, and 60 % for CrIIICl3·6 H2O, CrCl3, and
CrCl2, respectively. The side products are mainly humins as
follows from the darkening of the reaction mixture and the
absence of other products such as levulinic acid in the product mixture.
To support the differences in the activity in fructose dehydration between Cr2 + and Cr3 + as an explanation for the
Chem. Eur. J. 2011, 17, 5281 – 5288
Figure 3. Fructose dehydration to HMF by CrCl2 and CrCl3·6 H2O in
EMImCl at 80 8C.
lower fructose yield of Cr2 + , we also investigated the kinetics for fructose dehydration (Figure 3). These data confirm
the higher rate of fructose dehydration for Cr3 + . It is worth
noting the considerable difference in HMF yield. Compared
to the final yield of 60 % for Cr2 + , complete fructose conversion goes together with an HMF yield of 80 % for Cr3 + . The
lower the reaction rate of fructose, the lower the final HMF
yield. A tentative explanation for this behavior is that an intermediate in the dehydration of fructose to HMF undergoes condensation side reactions. Indeed, the nonselective
reaction products are mainly humins. Because these reACHTUNGREactions should be of higher order than the dehydration reACHTUNGREaction itself, a lower reaction rate implies a higher concentration of intermediates and consequently a lower selectivity
to the desired HMF product. Such considerations can also
be applied to explain the different HMF yields for the glucose dehydration experiments.
The use of anhydrous CrCl3 gives very different results
compared to CrCl2 and CrCl3·6 H2O. The reason turns out to
be the insolubility of anhydrous CrCl3 in the ionic liquid.
When CrCl3 was mixed with the EMImCl ionic liquid at
100 8C for 3 h, the amount of dissolved Cr in the ionic liquid
determined by elemental analysis (ICP-OES) was negligible.
Our initial hypothesis was that glucose dehydration is catalyzed by solid CrCl3. However, careful analysis of the Cr
content in the reaction mixture as a function of the reaction
time (Figure 4) showed that the reaction rate correlates well
with the concentration of Cr in the ionic liquid solvent.
Thus, the induction period observed for glucose dehydration
(Figure 1) is due to the slow dissolution of anhydrous CrCl3.
Figure 4. Glucose dehydration by anhydrous CrCl3 and the liquid phase
chromium concentration in EMImCl at 100 8C.
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J. M. Hensen et al.
The difference in solubility of the anhydrous and the hexahydrate forms of CrCl3·x H2O (x = 0 or 6) in EMImCl are
due to their solid-state structure. The CrCl3 crystal structure
is layered with each layer being held together by a network
of Cl-bridged octahedral CrCl6 (rACHTUNGRE(Cr-Cl) = 2.329 and
2.365 ) units.[22] It is reasonable to assume that the mechanism of dissolution in EMImCl involves the introduction of
additional Cl of the ionic liquid in the coordination sphere
of Cr, as finally octahedral Cr complexes are obtained (vide
infra). This process is expected to be difficult for anhydrous
CrCl3, because it involves a substantial perturbation of the
perfect octahedral environment of the Cr ions. In this case
there is not a substantial energy benefit from the substitution of a bridging Cl ligand by a terminal one. In
CrCl3·6 H2O, the CrIII centers are present as isolated hexaACHTUNGREaquochromium cations (rACHTUNGRE(Cr-OH2) = 1.949 ), interconnected through a hydrogen-bonding network realized between
the charge-compensating Cl anions and water ligands.[23]
This hydrogen-bonding network readily decomposes in the
presence of the highly polar EMImCl solvent.
These results raise the question of which compound is responsible for the dissolution of CrCl3 in the ionic liquid
during glucose conversion, as we furthermore found that
CrCl3 does not dissolve in EMImCl in the absence of glucose (Figure 5). Water is an obvious candidate. An experiment involving the addition of water to a suspension of
CrCl3 in EMImCl did not lead to dissolution of the chromium salt. We further tested a series of model compounds representing amongst others possible intermediates and products of glucose dehydration to HMF (Figure 5). Fructose,
HMF, and glyceraldehyde lead to the nearly complete dissolution of CrCl3 upon stirring in EMIMCl at 100 8C for 3 h.
Other compounds such as sorbitol, gluconic acid, and
methyl-a-d-glucopyranoside do not dissolve CrCl3 under
similar conditions. Glyceraldehyde, the simplest of all
sugars, dissolves CrCl3, which suggests that the open form of
the glucose is responsible for CrCl3 dissolution. Indeed,
when methyl-a-d-glucopyranoside was used, no CrCl3 dissolution was observed. Thus, the carbonyl moiety is necessary
for CrCl3 dissolution.
In summary, glucose dehydration is catalyzed by homogeneous Cr2 + and Cr3 + complexes in EMImCl. The selective
conversion of glucose to HMF proceeds via a fructose intermediate that is rapidly converted to HMF. The HMF selectivity is determined by the selectivity of fructose dehydration. The kinetic data show that Cr3 + is more active than
Cr2 + . Not only is Cr3 + more active for the first isomerization
step, but also for the subsequent dehydration of fructose as
confirmed by separate kinetic experiments for fructose dehydration. The higher HMF yield appears to be related to
the higher rate of fructose dehydration. Tentatively, this is
understood in terms of lower concentrations of intermediates between fructose and HMF, which limits bimolecular
condensation reactions that lead to humins. The higher catalytic activity of Cr3 + is due to the increased Lewis acidity
compared to Cr2 + .
Structural properties of the active complexes and the reaction mechanism: We have previously demonstrated that
monomeric distorted tetrahedral [CrCl4]2 complexes are
the reactive species in the CrCl2-catalyzed glucose dehydration in dialkylimidazolium chloride ionic liquid.[20] The coordination of these complexes to glucose leads to ring opening. The crucial step is the self-organization of such a Cr–
chloride–glucose complex with a second [CrCl4]2 ion into a
binuclear Cr complex that stabilizes the intermediates and
transition state for glucose isomerization.[20] The coordination properties of trivalent chromium are strongly different
from those of CrII.[19] This may imply a different mechanism
of the catalytic dehydration of Cr3 + compared to our earlier
proposal for Cr2 + .
Structural information about the CrIII coordination complexes formed in the ionic liquid medium was obtained by
extended X-ray absorption fine structure (EXAFS) measurements at the Cr K-edge. Table 1 lists the EXAFS fit paTable 1. EXAFS fit parameters of the CrCl3·6 H2O/EMImCl system.
Backscatter
N[a]
CrCl3·6 H2O/EMImCl
Cr Cl
5.8
glucose/CrCl3·6 H2O/EMImCl,
Cr Cl
4.1
Cr O
1.0
glucose/CrCl3·6 H2O/EMImCl,
Cr Cl
4.1
Cr O
0.9
Figure 5. Amount of CrCl3 dissolved in EMImCl after heating at 100 8C
for 3 h in the presence of several additives.
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R [][b]
Ds2[][c]
DE0 [eV][d]
2.35
0 min
2.35
2.05
180 min
2.33
2.03
0.016
4.7
0.012
0.000
4.8
0.011
0.000
3.7
[a] Coordination number (N) 20 %. [b] Coordination distance (R)
0.02 . [c] Debye-Waller factor (Ds2) 10 %. [d] Inner potential (DE0).
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Homogeneous Catalysis
FULL PAPER
rameters for CrCl3·6H2O in EMImCl ionic liquid at 80 8C
before and after addition of glucose. Upon dissolution the
six water ligands are readily replaced by the chlorine anions
of the ionic liquid solvent. In contrast to the tetrahedral coordination of Cr2 + , Cr3 + is octahedrally coordinated by six
chlorine anions at a distance of 2.35 . The replacement of
part of these chlorine anions by oxygen upon addition of
glucose at 80 8C evidences the coordination of glucose to the
Cr3 + center. Compared to CrCl2 that is inactive for glucose
conversion at this temperature, Cr3 + shows notable activity.
Consequently, the structural analysis does not necessarily
correspond to the initial molecular complex between
CrCl63 and glucose. Nevertheless, the data confirm the
rapid complexation of glucose with the anionic chromium
chloride complex necessary for the high selectivity towards
HMF.[20b]
The results of model DFT calculations further support
this supposition. The optimized geometry of the initial
CrCl63 complex charge-compensated by three 1,3-dimethyl
imidazolium cations (MMIm + ) is shown in Figure 6a. An
catalytic reaction was found to decrease its activation barrier.[20a]
To establish whether a similar mechanism is relevant for
CrIII containing complexes, we compared reaction paths of
mononuclear and binuclear Cr2 + and Cr3 + complexes. A
direct comparison starts from d-a-glucose complex with a
single Cr center. For the computation of the binuclear mechanism, the second Cr center was initially present in the form
of the stable chloride complex hydrogen bonded to glucose.
The free energy diagrams for CrII and CrIII catalysts are displayed in Figure 7a and b, respectively.
Figure 6. DFT-optimized structures of molecular models representing a)
CrCl63 and b) coordination complex of a-d-glucopyranose to [CrCl4]
species charge-compensated by an appropriate number of model
MMIm + cations.
almost perfect octahedral Cl environment is established in
this structure with an average Cr Cl distance of 2.39 . The
small deviation of this bond distance from the experimental
value is due to the non-uniform stabilization of the anionic
complex with the finite number of solvating [MMIm] + ions
assumed in the calculations. The subsequent replacement of
Cl ligands in the [CrCl6]3 complex by hydroxyl groups of
glucose is energetically favorable. Similar to the bivalent
chromium catalyst,[20a] the estimated free energy change
(DG373 K) associated with the substitution of two Cl ligands
by the O1H and O2H hydroxyl groups of glucose to the
CrIII center (Figure 6b) is equal to 12 kJ mol 1.
The coordination of glucose to the Lewis acidic metal
center forms the precursor for its subsequent selective transformation. Glucose to fructose isomerization involves the
following three reaction steps: 1) ring opening, 2) hydrogen
transfer between C2 and C1, and 3) ring closure (Scheme 1).
In the case of CrCl2, the transient formation of binuclear
CrII complexes at the rate-determining step (step 2) of the
Chem. Eur. J. 2011, 17, 5281 – 5288
Figure 7. DFT-computed free energy diagrams for glucose isomerization
catalyzed by mono- and binuclear a) Cr2 + and b) Cr3 + complexes in a
model MMImCl ionic liquid.
Initially, glucose coordinates to chromium by its O1 and
O2 hydroxyl groups. This coordination mode has previously
been shown to be the precursor for selective glucose isomerization.[11, 13, 20] It facilitates the opening of the glucopyranose
ring through proton transfer from O1 to O6. The mobile
chlorine anions of the ionic liquid act as basic mediators. In-
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J. M. Hensen et al.
dependent of the Cr oxidation state, the formation of the
open form of glucose is a facile process. Nevertheless, the
calculations indicate that ring opening is more easy for CrIII.
The mononuclear Cr complexes are preferred at this stage
over their binuclear forms. The isomerization of glucose to
fructose proceeds through a 1,2-H shift of the open form of
glucose (Scheme 1) and requires the deprotonation of the
O2-H group. At this step the formation of binuclear complexes (Figure 8) becomes favorable for CrII and CrIII. The
Figure 8. a) A simplified reaction scheme for the hydrogen-shift taking
place during the glucose isomerization and the corresponding DFT-optimized binuclear b) CrII and c) CrIII complexes with the deprotonated
sugar. DG values are given in kJ mol 1. TS = transition state.
energy differences between the mono- and binuclear structures are smaller for CrIII than for CrII (Figure 7). A considerable difference is noted for the relative energies of the
transition state and the resulting O1-deprotonated fructose
intermediate. Indeed, the activation energy in the presence
of two CrIII centers is lower by 24 kJ mol 1, whereas the reaction free energy of the hydrogen-shift step decreases by
40 kJ mol 1. Subsequent protonation of the deprotonated
fructose intermediate results in the open form of fructose
coordinated to CrIII. At this stage the stability of the binuclear structure is strongly disfavored over the mononuclear
structure because of the repulsions between the bridging
OH group and the Cl ligands surrounding the Cr centers.
Consequently, the transient binuclear complexes formed at
the rate-determining step decompose.
A comparison of the reaction energy diagrams for CrII
and CrIII catalysts provides an explanation for the higher activity of trivalent Cr. The lower barriers for CrIII are due to
its increased Lewis acidity (higher positive charge) com-
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pared to CrII, which results in a more efficient stabilization
of the anionic sugar intermediates formed during the glucose isomerization. The estimated overall activation barrier
for the CrIII-catalyzed glucose isomerization equals
66 kJ mol 1. This compares favorably to the barrier of
93 kJ mol 1 for CrII and is in qualitative agreement with the
experimental differences.
Besides the Lewis acidity of the metal center, the high
density and sufficient basicity of the anions of the ionic
liquid solvent are crucial for glucose dehydration to HMF.
The chlorine ions not only act as basic mediators promoting
the protonation/deprotonation reaction steps required for
the isomerization and dehydration reactions, but also saturate the coordination sphere of the catalytic Cr centers.
Their binding with the metal cation should be of a moderate
strength to allow facile ligand exchange and the formation
of specific coordination complexes with the sugar substrate.[20b] Thus a compromise in the basicity and the coordination ability of the anions of the ionic liquid solvent is necessary for efficient and selective glucose dehydration. Its importance for the catalytic reactivity of CrCl3·6 H2O catalyst
in glucose dehydration was investigated (see the Supporting
Information). Only in the presence of anionic groups with
moderate basicity (Cl ), selective conversion was observed.
In the presence of strongly coordinating acetate groups, the
selectivity is much lower. The use of weakly coordinating
and non-basic anions such as PF6 and BF4 also results in
very poor catalytic performance.
Thus, despite the different coordination properties of bivalent and trivalent chromium, the mechanism for the isomerization of glucose to fructose is very similar. The enhanced Lewis acidity of the cationic center with a higher
formal charge is key in explaining the higher catalytic reACHTUNGREactivity of CrIII compared to CrII. Initially, CrII and CrIII are
present as mononuclear square-planar [CrCl4]2 and octahedral [CrCl6]3 species, respectively. These anions form coordination complexes with glucose. For both oxidation states
of Cr it is beneficial to coordinate a further Cr complex to
the deprotonated glucose intermediate, because it facilitates
the H transfer required for glucose to fructose isomerization. The transient formation of binuclear chromium complexes provides a link to the xylose isomerase enzyme that
contains a binuclear metal active site. Further transformations of the fructose intermediate to HMF involve mononuclear catalytic chromium species.
Conclusion
The unique ability of Cr to catalyze the dehydration of glucose to HMF does not critically depend on the oxidation
state of the metal. CrIII is much more active than CrII in the
isomerization of glucose to fructose as well as in the subsequent fructose dehydration. A suitable CrIII salt that readily
dissolves in the ionic liquid is important.
Selective glucose dehydration to HMF proceeds via fructose as an intermediate. The overall HMF selectivity is de-
2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Homogeneous Catalysis
FULL PAPER
termined by the selectivity of the dehydration of fructose.
Tentatively, the higher HMF selectivity observed for CrIII is
explained by the higher rate of fructose dehydration, which
limits the concentration of intermediates that are prone to
bimolecular condensation reactions. Both Cr2 + and Cr3 +
form coordination complexes with glucose, which subsequently undergo isomerization to fructose. The isomerization involves the transient formation of binuclear chromium
complexes. The more efficient stabilization of the anionic reaction intermediates by the stronger Lewis acidic Cr3 + centers lowers the barrier for the hydrogen shift compared to
Cr2 + centers.
Experimental Section
Chemicals: d-Glucose, d-fructose, 5-hydroxymethylfurfural (HMF),
CrCl2, CrCl3, CrCl3·6 H2O, CrACHTUNGRE(NO3)3·9 H2O, and Cr3ACHTUNGRE(OAc)7(OH)2 were
purchased from Sigma–Aldrich. 1-Ethyl-3-methylimidazolium chloride
(EMImCl) was obtained from Merck. 1-Butyl-3-methylimidazolium chloride (BMImCl) was from Alfa-Asar. 1-Ethyl-3-methylimidazolium acetate (EMImOAc), 1-ethyl-3-methylimidazolium hexafluorophosphate
(EMImPF6), 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4),
1-butyl-3-methylimidazolium bistriflate imide (BMImTf2N), and 1-butyl3-methylimidazolium trifluoromethylsulfonate (BMImTrif) were all obtained from Sigma–Aldrich.
Reactions and analysis: The catalytic activity experiments were performed in 4 mL batch glass vials. The reaction vials were heated in an oil
bath placed on top of a magnetic stirrer. In a standard experiment,
EMIMCl (500 mg) was mixed with sugar (glucose or fructose; 50 mg).
The catalyst (6 mol %) was added to the vial. After closing the vial, the
reactor was placed in the preheated oil bath. The reaction was quenched
by placing the vial in an ethylene glycol bath at 0 8C. Analytes were diluted with water (2 mL) for HPLC analysis. The extracted samples were analyzed by a Shimadzu HPLC system equipped with UV and ELSD detectors. Hexoses were detected by an ELSD detector with a Prevail Carbohydrate ES (Grace) column. The mobile phase (1 mL min 1) was acetonitrile/water (83:17 v/v) and the column temperature 50 8C. HMF was detected by a UV detector (320 nm) with a Pathfinder PS C18 reversed
phase column. The mobile phase (0.6 mL min 1) was methanol/water
(20:80 v/v) and the column temperature 30 8C.
Glucose dehydration catalyzed by unhydrous CrCl3 was studied in
EMIMCl at 100 8C. In a standard experiment, anhydrous CrCl3 (6 %;
2.6 mg), EMIMCl (500 mg) and glucose (50 mg) were mixed in a vial.
After closing the vials, the reactors were put into a preheated oil bath.
The heating time were ranging from 5 min to 3 h. The reaction was
quenched by placing the vial in an ethylene glycol bath at 0 8C. Analytes
were diluted with water (2 mL). The mixtures were filtrated to remove
the insoluble solids and the filtrates were analyzed by HPLC and ICP.
For the dissolution experiments, other chemicals like HMF or fructose
was added together with the ionic liquid and catalysts. The following
treatments are same to the reaction above.
XAS experiments: X-ray absorption spectroscopy (XAS) experiments
were performed in small home built cells that contain the liquid and
which can be heated up to 200 8C. The liquid was placed between Kapton
windows. XAS spectra were collected in fluorescence mode at the Cr K
edge at the Dutch Belgium Beamline (DUBBLE) of the European Synchrotron Radiation Facility. A SiACHTUNGRE(111) monochromator was used for
these experiments. EXAFS analysis was then performed with
EXCURV98[24] on k3-weighted unfiltered raw data using the curved wave
theory. Phase shifts were derived from ab initio calculations using HedinLundqvist exchange potentials and Von Barth ground states as implemented in EXCURV98. Energy calibrations were carried out with Cr
and Cu foils. The amplitude reduction factor S20 associated with central
atom shake-up and shake-off effects was set at 0.87. In a typical experi-
Chem. Eur. J. 2011, 17, 5281 – 5288
ment, EMImCl (500 mg) was mixed with metal chloride (8 mg) and glucose (50 mg) at 60 8C in a nitrogen-flushed glovebox. A portion of this
mixture (3 mL) was transferred into the XAS cell and closed under inert
atmosphere. The cells were then transferred to the beamline and EXAFS
spectra were recorded of the liquid after heating to 80 8C.
DFT calculations: Density functional theory (DFT) with the PBE0 (also
denoted as PBE1PBE and PBEh)[25] hybrid exchange–correlation functional was used for the quantum chemical calculations. Prior benchmark
studies have shown the high accuracy of this method among a set of
hybrid exchange-correlation functionals for the description of a wide
range of systems[26] such as transition-metal-catalyzed reactions[27] and
magnetic systems.[28] Full geometry optimizations and saddle-point
searches were performed within Gaussian 03.[29] For geometry optimization and frequency analysis the full electron 6-31 + G(d) basis set was
used for Cr, Cl and O atoms. The C, N and H atoms were treated with
the 6-31G(d) basis set. Such a basis set combination allowed accurate calculation of extended molecular models considered in this study.[20] The
Cr2 + and Cr3 + complexes were in high spin-state (S = 4/2 and S = 8/2 for
mononuclear and binuclear Cr2 + complexes, respectively, and S = 3/2 and
S = 6/2 for mononuclear and binuclear Cr3 + complexes, respectively).
The total charge of all molecular models was 0.
The nature of the stationary points was evaluated from the harmonic
modes analytically computed using a complete Hessian matrix for all
models. No imaginary frequencies were found for the optimized structures of the reaction intermediates, confirming that these correspond to
local minima on the potential energy surface. All transition states exhibited a single imaginary frequency, corresponding to the eigenvector along
the reaction path. The assignment of the transition state structure to a
particular reaction path was tested by perturbing the structure along the
reaction path eigenvector in the directions of the product and the reagent
followed by geometry optimization.
Zero point (EZPE), finite temperature (HCORR) and entropic (TS) energy
contributions were computed using the results of the normal-mode analysis within the ideal gas approximation at a pressure of 1 atm and temperature of and 373 K. To reduce potential errors associated with the nonuniform stabilization of the MMIm + ions at the outer part of the molecular models, only the contribution of the MeCln species and the carbohydrate molecule and its derivatives were considered in calculations of the
thermodynamic properties. The contributions of the atoms of the chargecompensating MMIm + ions were removed from the Hessian matrix for
all the reaction intermediates and transition states in this case. The
values of DG8 were calculated for these conditions.
Acknowledgements
This research has been performed within the framework of the CatchBio
program. The authors gratefully acknowledge the support of the Smart
Mix Program of the Netherlands Ministry of Economic Affairs and the
Netherlands Ministry of Education, Culture and Science. NWO-NCF and
NWO-Dubble are acknowledged for access to computing and XAS facilities, respectively.
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Received: December 17, 2010
Published online: April 12, 2011
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