Conversion of mono- and disaccharides to ethyl levulinate and ethyl pyranoside with sulfonic acid-functionalized ionic liquids

Article abstract of DOI:10.1002/cssc.201100137

Value-added chemicals from sugars: Sulfonic acid-functionalized ionic liquids are attractive and promising catalyst for the conversion of sugars to ethyl levulinate and ethyl-D-glucopyranoside in ethanol. These task-specific ionic liquids can be recovered and reused in at least three cycles in the conversion of fructose to ethyl levulinate without any loss of activity. Copyright

Full text of DOI:10.1002/cssc.201100137

DOI: 10.1002/cssc.201100137
Conversion of Mono- and Disaccharides to Ethyl Levulinate and Ethyl
Pyranoside with Sulfonic Acid-Functionalized Ionic Liquids
Shunmugavel Saravanamurugan, Olivier Nguyen Van Buu, and Anders Riisager*[a]
Today most organic chemicals are produced by catalytic trans-
formations of fossil resources such as oil, coal, and natural gas.
Within a few decades, the availability of these fossil resources
is projected to decrease, which makes the use of alternative
carbonaceous resources as feedstock imperative.[1–3] Carbohy-
drates are abundant and inexpensive naturally available carbo-
naceous resources. Because carbohydrates are renewable and
carbon-neutral, finding feasible ways to convert them into
useful chemicals, such as 5-hydroxymethylfurfural (HMF), lactic
acid, levulinic acid, and others, has become increasingly impor-
tant.[4–6]
Ionic liquids (organic salts with melting point below 100 8C)
are attractive alternatives to common organic solvents owing
to their negligible vapor pressure, relatively high thermal sta-
bility, and remarkable catalyst and solvent properties.[14–16] The
solubility of mono-/di-/polysaccharides in some ionic liquids is
much higher than in common organic solvents.[17] This has led
to their use as solvent/catalyst for producing bioplatform
chemicals from carbohydrates. Zhao et al. initially reported
that chromium chloride immobilized in imidazolium chloride
ionic liquids gave a yield of 70% HMF from glucose.[6a] Since
then, numerous other ionic liquid catalyst systems for trans-
forming monosaccharides into HMF have been investigat-
ed.[18,19] Most of these studies found high yields of HMF but
low (or no) yields of levulinic acid, suggesting that the acid
strength of the ionic liquids is not sufficient to rehydrate HMF
and form levulinic acid.
Levulinic acid in particular has been recognized as an impor-
tant bioderived platform chemical that may provide a starting
point for the production of chemicals and fuels.[7] Levulinic
acid is also useful as a solvent, food flavoring agent, plasticizer,
resin intermediate and building block for, for example, tetrahy-
drofuran and succinic acid.[8] Traditionally, the production of
levulinic acid involves treatment of carbohydrates with aque-
ous mineral acid (H2SO4 and HCl) at atmospheric pressure at
1008C.[9] This method usually yields about 40% of levulinic
acid. The yield of levulinic acid may be improved to 60–70%
by continuous flow conditions at higher temperatures and
pressures using H2SO4 as catalyst.[10] However, a major draw-
back in this process is tedious work-up during the separation
stages.
To enhance the acid strength of ionic liquids, acidic function-
alities can be introduced in either the anion or the cation.[17,18]
These “task-specific ionic liquids” have demonstrated their po-
tential to replace traditional mineral acids as catalyst and sol-
vent.[20,21] Specifically, sulfonic acid-functionalized imidazolium-
and phosphonium-based ionic liquids have performed as ex-
cellent Brønsted-acidic catalyst systems for Fischer esterifica-
tion, alcohol dehydrodimerization, and pinacol-benzopinacole
rearrangement, being recycled and used in several runs with-
out any significant loss of activity.[21] Hence, ionic liquids
having strong Brønsted acid sites may possibly dehydrate fruc-
tose to form HMF followed by the formation of 5-(ethoxyme-
thyl)furan-2-carbaldehyde (HMF-ether) and subsequently rehy-
drate to form ethyl levulinate. Predominant formation of HMF-
ether from fructose has previously been demonstrated with
Amberlite-15 resin catalyst in ethanol at 1008C.[22]
Acidic ion-exchange resins have also been used as catalysts
for the transformation of sucrose to levulinic acid. The major
disadvantages of these systems are the moderate yields of lev-
ulinic acid (about 25%) and maximum operation temperatures
of around 1508C because of the thermal instability of the resin
catalyst.[11] Y-type zeolites have also been found to give moder-
ate levulinic acid yields of about 40% (and minor amounts of
HMF) when investigated as catalysts for the dehydration of
glucose and fructose at temperatures between 110 and
1608C.[12] In contrast, a Fe-pillared montmorillonite catalyst was
found to be very active and able to convert glucose quantita-
tively, though with low selectivity of 20% to levulinic acid.[13]
Instead a much higher amount of formic acid as well as a sig-
nificant amount of coke was observed in this study.[13] To cir-
cumvent the drawback of thermal instability and to improve
the yield of levulinate, we were inspired to explore the use of
ionic liquids as catalysts for these reactions.
In the present study, we report the catalytic transformation
of the sugars fructose, glucose, and sucrose to ethyl levulinate
(Scheme 1) with different sulfonic acid-functionalized ionic liq-
uids (SO3H-ILs) as catalysts, in the presence of ethanol as reac-
tant and solvent.
Initial experiments were performed with imidazolium-, pyri-
dinium- and ammonium-based SO3H-ILs containing hydrogen-
sulfate as anion (Table 1). In all these reactions ethyl levulinate
was found to be the predominant product and yields of 68, 70
and 74% were obtained with the ionic liquids [BMIm-SO3H]-
[HSO4], [BPyr-SO3H][HSO4], and [NEt3B-SO3H][HSO4], respective-
ly, with full fructose conversion. To evaluate the influence of
the ionic liquid anion on the formation of ethyl levulinate,
[a] Dr. S. Saravanamurugan, Dr. O. Nguyen Van Buu, Prof. A. Riisager
Centre for Catalysis and Sustainable Chemistry
Department of Chemistry, Technical University of Denmark
2800 Kgs. Lyngby (Denmark)
[NTf2]ꢀ, [OMs]ꢀ and [OTf]ꢀ anions were introduced into the
,
sulfonic acid-functionalized imidazolium-based ionic liquids. As
shown in Table 1, these ionic liquids gave good yield to ethyl
levulinate with practically quantitative conversion of fructose,
also. Among the examined ionic liquid catalysts, [BMIm-SO3H]-
Fax: (+45)45883136
E-mail: ar@kemi.dtu.dk
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100137.
ChemSusChem 2011, 4, 723 – 726
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723

crose with 97% conversion. This clearly indicated
that only fructose moieties were converted to ethyl
levulinate, and no significant isomerization of glucose
to fructose occurred which otherwise would have in-
creased the yield of ethyl levulinate. Instead, glucose
was found to react readily with ethanol under the ap-
plied acidic conditions to form ethyl-d-glucopyrano-
side (EDGP) (Scheme 1) resulting in EDGP yields of 25
to 32% from sucrose. The preferential transformation
of glucose to EDGP instead of isomerization to fruc-
tose was confirmed when using glucose as substrate.
Here only 6 to 13% yields of ethyl levulinate was ob-
tained with 94% conversion of the substrate, where-
as the yields of EDGP were between 50 and 62%.
Similarly, the disaccharide cellobiose (consisting of
two b(1!4) bonded glucose units) gave only 2% of
ethyl levulinate with 95% conversion but 63% of
EDGP, corresponding to a selectivity of 86%.
Alkyl pyranosides such as EDGP are important non-
ionic surfactants with good biodegradability and po-
tential applications in cosmetics, food emulsifiers,
and pharmaceutical dispersing agents.[24,25] In previ-
ous reports, alkyl pyranosides were formed from glu-
cose in the presence of alcohol and H-zeolite cata-
lysts.[5a,26] H-zeolites contain mostly strong Brønsted
acid sites, which are believed to be responsible for
the catalytic formation. In this study the presence of
strong acid groups, such as SO3H, was found to be
necessary for both the dehydration reaction and the
cyclization reaction, since only 13% conversion and
essentially no ethyl levulinate or EDGP product were
formed when using the less acidic ionic liquid
[HNEt3][HSO4] as catalyst in the conversion of su-
crose.
To understand the reactivity of the intermediates,
HMF and levulinic acid were used as substrates in-
Scheme 1. A plausible pathway for the formation of ethyl levulinate from sugars.
Table 1. Catalytic transformation of fructose to ethyl levulinate.[a]
Table 2. Catalytic transformation of glucose, sucrose, and cellobiose to
ethyl levulinate and ethyl pyranoside.[a]
Ionic liquid[b]
Conversion[c]
[%]
Yield[d]
[%]
Ionic liquid[b]
Substrate
Conv.[c]
[%]
Yield[d]
[%]
Yield[e]
[%]
[BMIm-SO3H][HSO4]
[BPyr-SO3H][HSO4]
[NEt3B-SO3H][HSO4]
[BMIm-SO3H][NTf2]
[BMIm-SO3H][OMs]
[BMIm-SO3H][OTf]
>99
>99
>99
>99
>99
>99
68
70
74
77
67
69
[BMIm-SO3H][HSO4]
[BPyr-SO3H][HSO4]
[NEt3B-SO3H][HSO4]
Glucose
Sucrose
Glucose
Sucrose
Glucose
Sucrose
Cellobiose
Glucose
Sucrose
Glucose
Sucrose
Glucose
Sucrose
Sucrose
94
97
94
97
94
97
95
94
97
94
97
94
97
13
8
41
13
43
6
41
2
8
43
3
40
8
42
0
54
29
50
27
57
30
63
55
25
62
32
57
28
3
[a,b] See Experimental Section for details. [c] Conversion based on fruc-
tose. [d] Yield of ethyl levulinate.
[BMIm-SO3H][NTf2]
[BMIm-SO3H][OMs]
[BMIm-SO3H][OTf]
[HNEt3][HSO4]
[NTf2] gave a higher yield of ethyl levulinate (77%) implying
that the formation of ethyl levulinate followed the order of the
acid strength of the ionic liquid.[23]
The study was further extended to the sugars glucose and
sucrose, and the results are presented in Table 2. All of the ex-
amined SO3H-ILs yielded 40 to 43% of ethyl levulinate from su-
[a,b] See Experimental Section for details. [c] Conversion based on glu-
cose, fructose, or cellobiose. [d] Yield of ethyl levulinate. [e] Yield of ethyl-
d-glucopyranoside (EDGP).
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ChemSusChem 2011, 4, 723 – 726

stead of sugars under identical reaction conditions. After 24 h
of reaction with [NEt3B-SO3H][HSO4] as catalyst, 100% HMF was
converted and yields of 85% ethyl levulinate and less than 1%
HMF-ether were observed. No other products were found in
significant amounts (except insoluble humins and formic acid).
With levulinic acid as substrate, the yield of ethyl levulinate
was >99% after only 30 min reaction, clearly indicating that
levulinic acid was easily converted into ethyl levulinate under
these conditions whereas HMF partly transformed into unde-
sired products, that is, humins. Moreover, when water was
used as solvent, 40% levulinic acid and 26% HMF were ob-
tained after 24 h with 95% conversion of fructose, indicating
that rehydration of HMF was relatively slow in water compared
to HMF-ether in ethanol. Consequently, the presence of alco-
holic medium was important to enhance the yield of levuli-
nate.
Further investigations showed that the formation of ethyl
levulinate increased from a yield of 28 to 49% when the
amount of ionic liquid was increased four times in experiments
with 25 wt% fructose (6% of HMF-ether formed). In contrast,
the yield of ethyl levulinate decreased significantly from 77 to
34% (43% of HMF-ether) when four times less ionic liquid was
used in reaction with 6.3 wt% fructose. These experimental re-
sults confirm that the number of available SO3H groups influ-
ences the catalyzed rehydration of all HMF-ether to ethyl levu-
linate: without enough groups undesired products will form.
The ionic liquid [NEt3B-SO3H][HSO4] was further tested for re-
cyclability in three consecutive reaction runs with 6.3 wt%
fructose, as shown in Figure 2. In the first run, the yield of
The influence of fructose concentration on the yield of ethyl
levulinate was also investigated under similar reaction condi-
tions (Figure 1). When the initial fructose concentration was in-
Figure 2. Recycle experiments with 6.3 wt% fructose solution (fructo-
se=380.2 mg, [NEt3B-SO3H][HSO4]=0.15 mmol, naphthalene=43 mg, etha-
nol=5.6 g, 1408C, 24 h).
ethyl levulinate was 72% with more than 99% conversion of
fructose. After the first run, the remaining ethanol was evapo-
rated and the reaction mixture treated with a mixture of 5 mL
dichloromethane and 5 mL distilled water. The aqueous phase
was removed and collected and the organic phase re-extracted
with water. This allowed recovery of the ionic liquid after evap-
orating of the water under reduced pressure. When the recov-
ered ionic liquid was reused a second time a slightly increased
ethyl levulinate yield of 74% was obtained, again with more
than 99% conversion of fructose. This yield and degree of con-
version was also found after testing in a third cycle.
Figure 1. Influence of fructose concentration on the yield of ethyl levulinate
(fructose=270.2 mg, [NEt3B-SO3H][HSO4]=0.11 mmol, naphthalene=30 mg,
ethanol=4.0 g, 1408C, 24 h).
creased, the yield of ethyl levulinate decreased while the yield
of HMF-ether increased. Concomitantly, the conversion of fruc-
tose remained more than 99%. Thus, at lower fructose concen-
tration (6.3 wt%) the HMF-ether was completely converted to
ethyl levulinate (73%) with more than 99% conversion of fruc-
tose, whereas the yield of ethyl levulinate decreased to 65%
with significant amount of HMF-ether (8%) also being formed
at concentration corresponding to 10 wt%. Hence, the ionic
liquid ([NEt3B-SO3H][HSO4]) was able to catalyze the dehydra-
tion of fructose and the alkylation of HMF to form HMF-ether
but not to rehydrate all HMF-ether to form ethyl levulinate
under the examined conditions. Possibly, this difference in re-
activity can be related to formation of larger amounts of
humins at higher initial fructose concentration. Accordingly, a
marked decrease in yield of ethyl levulinate (46, 34, and 28%)
was further observed at 15, 20, and 25 wt% fructose, respec-
tively. At the same time, the HMF-ether yield concurrently in-
creased to become the predominant product (32%) at high
fructose concentration.
In summary, we have shown that sulfonic acid-functionalized
ionic liquids are prominent catalysts for the successive dehy-
dration of fructose to HMF followed by etherification to HMF-
ether and rehydration to form ethyl levulinate. Ionic liquids
based on the [NTf2]ꢀ anion gave slightly higher yield of ethyl
levulinate (77%) compared to other ionic liquids, thus suggest-
ing that the reaction progression is correlated with the acid
strength of the ionic liquid. The disaccharide sucrose was
found to yield 36% of ethyl levulinate along with 33% of an-
other useful product, EDGP. Interestingly, glucose was convert-
ed into EDGP (63%) with a lower yield of ethyl levulinate.
Hence, under strong Brønsted-acidic conditions, fructose pref-
erentially dehydrated to form HMF whereas glucose reacted
with ethanol to form ethyl pyranoside. Recycle studies demon-
strated that the sulfonic acid derived ionic liquids can be
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Experimental Section
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0.11 mmol of SO3H-IL, 30 mg of naphthalene and 4 g of ethanol
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Recycle study: In the first run, 380.2 mg of fructose, 0.15 mmol of
([NEt3B-SO3H][HSO4]), 43 mg of naphthalene, and 5.6 g of ethanol
were charged to an ace pressure tube and heated under stirring to
1408C for 24 h. After the first run, a second and third run was per-
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Abbreviations for ionic liquids: 1-methyl-3-(4-sulfobutyl)imidazoli-
um hydrogensulfate ([BMIm-SO3H][HSO4]), 1-(4-sulfobutyl)pyridini-
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(4-sulfobutyl)imidazolium
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([BMIm-SO3H][NTf2]), 1-methyl-3-(4-sulfobutyl)imidazolium metha-
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Acknowledgements
We thank the Danish Council for Independent Research-Technolo-
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Published online on May 20, 2011
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