Dehydration of fructose to 5-hydroxymethylfurfural by rare earth metal trifluoromethanesulfonates in organic solvents

Article abstract of DOI:10.1016/j.carres.2011.03.009

The catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF) was investigated by using various rare earth metal trifluoromethanesulfonates, that is, Yb(OTf)₃, Sc(OTf)₃, Ho(OTf)₃, Sm(OTf) ₃, Nd(OTf)₃ as catalysts in DMSO. It is found that the catalytic activity increases with decreasing ionic radius of rare earth metal cations. Among the examined catalysts, Sc(OTf)₃ exhibits the highest catalytic activity. Fructose conversion of 100% and a HMF yield of 83.3% are obtained at 120 °C after 2 h by using Sc(OTf)₃ as the catalyst. Moreover, the catalytic dehydration of fructose was also carried out in different solvents, for example, DMA, 1,4-dioxane, and a mixture of PEG-400 and water. The results show that among the solvents DMSO is the most efficient in promoting the dehydration of fructose to HMF, and no rehydration byproducts such as levulinic acid and formic acid are detected.

Full text of DOI:10.1016/j.carres.2011.03.009

Carbohydrate Research 346 (2011) 982–985
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Note
Dehydration of fructose to 5-hydroxymethylfurfural by rare earth
metal trifluoromethanesulfonates in organic solvents
⇑
Fenfen Wang, Ai-Wu Shi, Xiao-Xia Qin, Chun-Ling Liu, Wen-Sheng Dong
Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University), Ministry of Education, School of Chemistry and Materials Science, Xi’an 710062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF) was investigated by using var-
Received 14 January 2011
Received in revised form 25 February 2011
Accepted 6 March 2011
ious rare earth metal trifluoromethanesulfonates, that is, Yb(OTf)
as catalysts in DMSO. It is found that the catalytic activity increases with decreasing ionic radius of rare
earth metal cations. Among the examined catalysts, Sc(OTf) exhibits the highest catalytic activity. Fruc-
tose conversion of 100% and a HMF yield of 83.3% are obtained at 120 °C after 2 h by using Sc(OTf) as the
3 3 3 3 3
, Sc(OTf) , Ho(OTf) , Sm(OTf) , Nd(OTf)
3
Available online 10 March 2011
3
catalyst. Moreover, the catalytic dehydration of fructose was also carried out in different solvents, for
example, DMA, 1,4-dioxane, and a mixture of PEG-400 and water. The results show that among the sol-
vents DMSO is the most efficient in promoting the dehydration of fructose to HMF, and no rehydration
byproducts such as levulinic acid and formic acid are detected.
Keywords:
Fructose
5
-Hydroxymethylfurfural
Rare earth trifluoromethanesulfonates
Dehydration
Ó 2011 Elsevier Ltd. All rights reserved.
From the viewpoint of increasing environmental consciousness
and sustainable concerns, developing sustainable and renewable
biomass resources is an efficient method to solve fuel crises and
environmental pollution. In addition, with consideration of down-
stream chemical processing, converting degradable biomass re-
sources into valuable intermediates and polymeric materials is
possess some drawbacks because of relatively lower fructose con-
versions and longer reaction times. Recently, Lewis acid catalysts,
for example, metal chlorides have received much attention due to
better stability, wider solubility, and higher reaction rates.4
However, the catalysts are less environment friendly and can cause
equipment corrosion.
,18–21
1–4
beneficial for both science and technology.
5-Hydroxymethyl-
3
Rare earth metal triflates (RE(OTf) ), as new types of Lewis acid
furfural (HMF) is a versatile biomass-derived platform compound,
which can be applied to the synthesis of precursors of pharmaceu-
ticals, furanose-based polymers, and macrocycles, particularly for
the synthesis of dialdehydes, glycols, ethers, and other organic
catalysts, have been of great interest because of their unique prop-
erties. One of the most fascinating features is that they are soluble
and stable not only in aqueous systems but also in many organic
solvents. Furthermore, rare earth metal triflates are less corrosive
and lower pollution than conventional mineral acids and are re-
5
–8
intermediates that can lead to numerous chemical products.
2
7,28
The synthesis of HMF through acid-catalyzed dehydration of
monosaccharide has been known for many years. In this respect,
many researchers have widely studied the synthesis of HMF from
hexose in the past decades. Various types of acid catalysts have
been employed in the dehydration reaction, such as mineral acids,
for example, sulfuric, hydrochloric, phosphoric acid, and organic
garded as environmentally-friendly catalysts.
Rare earth tri-
flate catalysts have been successfully employed in Diels–Alder,
allylation, Friedel–Crafts, and Mukaiyama aldol reactions among
2
2–28
others.
In the present work, the dehydration of fructose to HMF cata-
lyzed by various rare earth metal triflates was investigated. The
influence of various process parameters, for example, the solvent,
reaction temperature, reaction time, catalyst dosage, and initial
concentration of fructose on the conversion of fructose were inves-
tigated. The aim was to find more efficient catalysts for conversion
of fructose into HMF.
The conversion of fructose into HMF was carried out in DMSO
with scandium and lanthanide rare earth metal triflates at 90
and 120 °C for 2 h, respectively. The results are summarized in
Figure 1. Generally, HMF easily suffers from further rehydration
to form levulinic acid and formic acid in the presence of acidic cat-
alysts. However, in the present study, surprisingly, no levulinic
acid and formic acid were detected by using rare earth metal
9
–11
acids, for example, citric acid, maleic acid, acetic acid, etc.
In
spite of simple process, easy operation, low investment, and
improving the yield of HMF to a certain extent by using mineral
acids as catalysts, they suffer from some limitations in consider-
ation of equipment corrosion, recycling, and difficulties in separa-
tion. Solid acid catalysts, for example, ion exchange resin,
H-zeolite, metal sulfated, zirconium phosphate, niobic acid, and
niobium phosphate overcome some restrictions of mineral acids
1
2–17
mentioned above.
Nevertheless, solid acid catalysts also
⇑
Corresponding author. Tel./fax: +86(0)29 85303657.
E-mail address: wsdong@snnu.edu.cn (W.-S. Dong).
0
008-6215/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2011.03.009

F. Wang et al. / Carbohydrate Research 346 (2011) 982–985
983
tendency was also observed in a previous study. In that study, Ish-
ida et al. found that catalytic activities of lanthanide(III) chlorides
for the dehydration of D-fructose in aqueous solution increase with
a
Fructose conversion
HMF yield
21
1
00
decreasing cation radius. It was proposed that the structure of the
lanthanide–saccharide complex is the key to this lanthanide(III)-
catalyzed dehydration of saccharides. The lanthanide(III)-saccha-
ride electrostatic interaction becomes stronger as the ionic radius
decreases and the surface charge increases.
8
0
0
0
0
6
4
2
The results in Figure 1 reveal that Sc(OTf)
catalyst for the dehydration of fructose among these rare earth me-
tal triflates. Therefore, in the following studies Sc(OTf) was chosen
3
is the most effective
3
as the catalyst to investigate the influence of process parameters
on the dehydration of fructose.
2
9,30
DMA, 1,4-dioxane32, and the mixture of PEG-400
31
DMSO,
0
and water have been used as solvents in the dehydration of fruc-
tose. Hence, in the present work, the dehydration of fructose by
Sc(OTf) Yb(OTf) Ho(OTf) Sm(OTf) Nd(OTf)
3
3
3
3
3
3
using Sc(OTf) catalyst in these different solvents was studied.
Catalysts
The reaction was carried at 120 °C for 2 h, the results are provided
in Figure 2. The yield of HMF reaches 83.3% at 100% fructose con-
version in DMSO. In the aprotic high boiling point solvent DMA
b 100
1
1
1
0
0
.2
.1
.0
.9
.8
Ionic radius
(
bp 165 °C), the conversion of fructose was 98.4% and the yield of
HMF was 49.8% by using Sc(OTf) catalyst. The reaction was also
studied in 1,4-dioxane in consideration of lower boiling point
101.1 °C) and easier product separation; a fructose conversion of
6.2% with 16.3% HMF yield was obtained. Under the same reac-
8
6
4
2
0
0
0
0
3
(
8
tion condition, the reaction was performed in the mixture of
PEG-400 and water because of larger viscosity and co-solvent
3
3
property of PEG-400, only 37.0% fructose conversion with 12.9%
HMF yield was obtained. We also investigated the dehydration
reaction of fructose in water; it gave a 38.5% yield of HMF, together
with many byproducts. Although it has been pointed out that
DMSO is difficult to separate from the products, it is an efficient
solvent for promoting the dehydration of fructose to HMF, as it effi-
ciently inhibits the formation of byproducts such as levulinic acid
0
Sc(OTf) Yb(OTf) Ho(OTf) Sm(OTf) Nd(OTf)
3
3
3
3
3
Catalysts
2
,7,34
and formic acid.
Figure 1. Dehydration of fructose to HMF catalyzed by various rare earth metal
triflates at different temperatures: (a) 90 °C and (b) 120 °C.
Figure 3 depicts the effect of reaction temperature on fructose
conversion and HMF yield. From Figure 3, it can be seen that the
yield of HMF increases initially with elevating reaction tempera-
ture, and then gradually decreases. When the reaction is conducted
at 90 °C for 2 h, the color of the solution is light yellow, and the
triflates as catalysts. At 90 °C, the conversion of fructose is 95%
with 66.4% HMF yield using Sc(OTf)
Yb(OTf) is inferior to that of Sc(OTf) , but it is appreciably better
than those of Ho(OTf) , Sm(OTf)3, and Nd(OTf) . The conversion
of fructose and yield of HMF on Yb(OTf) are 93.9% and 62.4%,
respectively. For Ho(OTf) and Sm(OTf) , the conversion of fructose
is 93.5% and 92.6%, and the yield of HMF is 61.7% and 59.8%,
respectively. However, for the Nd(OTf) catalyst, only 89.0% fruc-
3
. The catalytic activity of
3
fructose conversion is 95% with 66.4% HMF yield on Sc(OTf) .
3
3
When the reaction temperature reaches 120 °C, which affords a
complete conversion of fructose, the HMF yield increases to
83.3%. However, the color of the reaction solution changes from
light yellow to brown. Further increasing the reaction temperature
from 120 to 130 °C, HMF yield decreases from 83.3% to 77.8%, and
3
3
3
3
3
3
tose conversion and 55.9% HMF yield are obtained. When the reac-
tion temperature increases to 120 °C under identical other
conditions, fructose is converted completely, HMF yields with
100
Fructose conversion
HMF yield
Sc(OTf)
0.2%, 78.1%, 73.0%, and 63.5%, respectively.
Interestingly, the cation radius of the rare earth metal triflates
3 3 3 3
, Yb(OTf) , Ho(OTf) , Sm(OTf)3, and Nd(OTf) are 83.3%,
8
8
6
0
0
significantly affects the fructose conversion and HMF yield. The
yield of HMF increases with decreasing metallic ionic radius of
3
+
the rare earth metal triflates. For example, Sc (r
i
0.81 Å) and
3
+
Yb (r 0.86 Å) have smaller ionic radius, but they show higher cat-
alytic activity in the reaction. For the larger cations such as Sm (r
i
40
3
+
i
3
+
0
.96 Å) and Nd (r
i
1.00 Å), the conversion of fructose and yield of
20
HMF are lower. This is mainly because the rare earth metallic ions
have high affinity for oxygen atoms, and they can coordinate with
fructose in the reaction mixture. The smaller metallic cation radius
of the rare earth metal triflates, the stronger binding ability of rare
earth metallic ions with the oxygen atoms of fructose. The stronger
interaction between metal cations with smaller ionic radius and
fructose molecules results in higher catalytic activity. This
0
DMA
DMSO
1,4-dioxanePEG-400 and Water
Solvents
Figure 2. Dehydration of fructose to HMF catalyzed by Sc(OTf)
3
in different
solvents.

9
84
F. Wang et al. / Carbohydrate Research 346 (2011) 982–985
1
00
9
8
7
6
5
0
0
0
0
0
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
Fructose conversion
HMF yield
90
100
110
120
130
0.002
0.004
0.006
0.008
0.010
o
Temperature
C
Catalyst dosage (g)
Figure 3. Effect of reaction temperature on fructose conversion and HMF yield
reaction conditions: fructose 0.04 g, DMSO 2 g, wfructose/wSc(OTf)3 = 10, 2 h).
Figure 5. Effect of catalyst dosage on HMF yield (reaction conditions: fructose
0.04 g, DMSO 2 g, 120 °C, 2 h).
(
Figure 5 shows the influence of catalyst dosage on fructose con-
version and HMF yield. By increasing the amount of catalyst from
the color of the solution changes into brown black. This may be due
to the fact that higher temperature leads to polymerization of HMF
or cross-polymerization between fructose and HMF, bringing about
0
.002 to 0.004 g at 120 °C for 2 h, HMF yield increases from 79.0 to
numerous polymers and humin matter.26 Generally, the polymers
83.3%, whereas fructose conversion remains 100%. Further increas-
ing the catalyst dosage to 0.006 g does not result in an increase of
HMF yield. In contrast, it leads to a dramatic reduction to a 73.8%
yield; with 0.01 g catalyst dosage, only a 61.9% yield of HMF is ob-
tained. Because no levulinic acid, formic acid, and other byproducts
are detected in the products, we believe that excess catalyst may
promote the occurrence of polymerization between HMF mole-
cules. Hence, an appropriate amount of catalyst is crucial for
attaining higher HMF yield.
In this section, we investigated the influence of fructose concen-
tration on the dehydration reaction at 120 °C for 2 h. In all the
cases, fructose is converted completely. As shown in Figure 6,
when the initial concentration of fructose is 1 wt %, the HMF yield
is 66.8% because of isomerization of fructose. For 2 wt % fructose
concentration, it gives the maximum HMF yield of 83.3%. From
the Figure 6, it can be seen that the HMF yield decreases gradually
with increasing initial fructose concentration when fructose con-
centration exceeds 2 wt %. Taking 5 wt % concentration of fructose
for instance, a lower HMF yield of 76.4% is obtained. When initial
fructose concentration increases from 5 wt % to 10 wt %, the yield
of HMF decreases to 74.7%. This is probably because self-polymer-
ization of HMF or cross-polymerization between fructose and HMF
would occur easily at higher initial concentration of fructose, giv-
and humins are black, and as the reaction proceeds, the reaction
solution gradually becomes dark because of increasing amount of
polymers and humin matter. The results show that the optimum
temperature is 120 °C for the dehydration of fructose to HMF.
The results of dehydration of fructose to HMF at various reac-
tion times are shown in Figure 4. At 120 °C for 40 min, the conver-
sion of fructose reaches 89.6%, the HMF yield is 66.4%, and the
glucose yield is 1.2%, which is formed via isomerization of fructose.
When prolonging the reaction time to 70 min, fructose conversion
reaches 100%, and HMF yield increases from 66.4% to 78.2%, but the
glucose yield decreases to 0.8%. The HMF yield reaches the maxi-
mum value when the reaction time is 2 h, and the glucose disap-
pears completely. The results suggest that with prolonging the
reaction time from 70 min to 2 h the glucose derived from isomer-
ization of fructose may further dehydrate to form HMF. Neverthe-
less, when the reaction time is longer than 2 h, the yield of HMF
slightly drops owing to the formation of undesired side products.
By analyzing reaction products, surprisingly, no rehydration
byproducts such as levulinic acid and formic acid were detected,
suggesting that the decline in the amount of HMF yield is probably
due to self-polymerization of HMF molecules.
1
00
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
90
80
70
Fructose conversion
HMF yield
6
5
0
0
1%
2%
3%
5%
10%
40
60
80
100
120
140
160
Time (min)
Fructose concentration (%)
Figure 4. Effect of reaction time on fructose conversion and HMF yield (reaction
conditions: fructose 0.04 g, DMSO 2 g, wfructose/wSc(OTf)3 = 10, 120 °C).
Figure 6. Effect of fructose concentration on HMF yield (reaction conditions: DMSO
2 g, Sc(OTf) 0.004 g, 120 °C, 2 h).
3

F. Wang et al. / Carbohydrate Research 346 (2011) 982–985
985
ing rise to the formation of brown black soluble polymers and
insoluble humins. To avoid forming polymer and humins, a lower
fructose concentration (2 wt %) is optimal.
In summary, varied rare earth metal triflates as Lewis acid cat-
alysts were investigated in the dehydration of fructose to HMF. It
was found that catalytic activity increases with decreasing ionic ra-
HMF yield (mol %):
Moles of carbon in product
Moles of carbon loaded as fructose
Y ¼
ꢀ 100%
ð2Þ
Acknowledgment
3
dius of rare earth metal cations. Sc(OTf) is the most promising cat-
alyst for the conversion of fructose into HMF among the rare earth
metal triflates examined. The yield of HMF was found to be closely
dependent on the reaction temperature, reaction time, dosage of
catalyst and concentration of fructose. To obtain higher yields of
HMF, higher temperatures and shorter reaction times are essential.
This work was supported by the Natural Science Foundation of
Shaanxi Province (SJ08B13).
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Fructose concentration in product
Fructose concentration in the loaded sample
X ¼ 1 ꢁ
1
ꢀ
100%
ð1Þ