Efficient microwave-assisted synthesis of 5-hydroxymethylfurfural from concentrated aqueous fructose

Article abstract of DOI:10.1016/j.carres.2009.09.036

Studies on the HCl-catalysed microwave-assisted dehydration of highly concentrated aqueous fructose (27 wt %) to 5-hydroxymethylfurfural (HMF) revealed a significant increase in the fructose conversion rate over the conventional heated systems. Water, bei

Full text of DOI:10.1016/j.carres.2009.09.036

Carbohydrate Research 344 (2009) 2568–2572
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Note
Efficient microwave-assisted synthesis of 5-hydroxymethylfurfural
from concentrated aqueous fructose
Thomas S. Hansen a, John M. Woodley b, Anders Riisager a,
*
a Centre for Catalysis and Sustainable Chemistry and Department of Chemistry, Technical University of Denmark, Building 207, DK-2800 Kgs. Lyngby, Denmark
b Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
Studies on the HCl-catalysed microwave-assisted dehydration of highly concentrated aqueous fructose
(27 wt %) to 5-hydroxymethylfurfural (HMF) revealed a significant increase in the fructose conversion
rate over the conventional heated systems. Water, being the most benign solvent and therefore ideal
for green and sustainable chemistry, normally is a poor solvent for the dehydration process resulting
in low HMF selectivities and yields. However, reaction at 200 °C with microwave irradiation with a short
reaction time of only 1 s resulted in good HMF selectivity of 63% and fructose conversion of 52%, while
prolonged irradiation for 60 s (or more) resulted in nearly full fructose conversion (95%) but lower
HMF yield (53%). Decreasing the fructose concentration significantly improved the HMF selectivity, but
possibly made the production route less attractive from an industrial point of view due to the resultant
low throughput.
Received 19 August 2009
Received in revised form 23 September
2009
Accepted 30 September 2009
Available online 2 October 2009
Keywords:
Fructose
5-Hydroxymethylfurfural
Homogeneous catalysis
Microwave synthesis
Green chemistry
Renewable resources
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
lites,14 and with the solvent acting as catalyst, for example, in ionic
liquids.15 Environmental concerns favour water as a solvent for the
In recent years, considerable effort has been put into the devel-
opment of alternatives to the fossil-based industry of today.1–5
Decreasing fossil fuel reserves call for the development of a new,
long term, environmentally friendly and sustainable chemical
source. One possibility could be provided by biomass,6 which is
abundant in most parts of the world as a product of the light-in-
duced fixation of CO2 in plants and algae via photosynthesis. A high
number of functional groups, often hydroxyl groups, render carbo-
hydrate biomass particularly suitable for the production of fine
chemicals used in, for example, pharmaceuticals, plastics, rubbers
and other polymeric materials, which typically contains functional
groups of –OH or derivatives thereof. However, the number of hy-
droxyl groups often also results in non-selective conversions of
carbohydrates, as many side reactions are possible.7,8
One interesting compound that is obtainable directly from the
acid-catalysed dehydration of hexoses is 5-hydroxymethylfurfural
(HMF).9,10 This furan-based compound could potentially be utilised
as a platform compound11 for further processing into commercially
valuable chemicals and thereby replacing a range of chemicals that
are presently obtained from fossil-based sources. HMF has been
known for more than a century and is produced by use of catalysts
such as mineral acids,10,12 acidic ion-exchange resins,13 H-zeo-
reaction, but water is unfortunately not very selective for the pro-
duction of HMF and many by-products comprising insoluble poly-
meric compounds (i.e., humins) and soluble polymers are often
formed.16 Additionally, levulinic acid (LA) and formic acid (FA)
are typically also obtained from rehydration of HMF (Scheme 1)
by a still poorly understood reaction mechanism, though Horvat
et al.17 have proposed a mechanism involving intermediates based
on 13C NMR measurements.
In order to minimise by-product formation, two-liquid phase
systems enabling extraction of HMF as it is produced have also
been developed.12,18 However, from a practical point of view these
reaction systems require large volumes of extracting solvent, thus
making them less attractive—and possibly uneconomical—on an
industrial scale. Additionally, high-boiling solvents such as
dimethylsulfoxide (DMSO) have also been utilised and shown to
be very effective in promoting the HMF formation. However, here
removal of the solvent from the product mixture has proved quite
difficult.19
Microwave-induced organic synthesis, traditionally employed
for organometallic synthesis in particular, has proven to be very
efficient in promoting the dehydration of fructose to HMF by
greatly reducing the reaction time and resulting in high selectivity
and yield.20,21 A high input of energy via the microwave irradiation
allows the reaction to overcome the energy barrier for product for-
mation much faster than by conventional heating and therefore
* Corresponding author. Tel.: +45 45252233.
E-mail address: ar@kemi.dtu.dk (A. Riisager).
0008-6215/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2009.09.036

T. S. Hansen et al. / Carbohydrate Research 344 (2009) 2568–2572
2569
purged externally with nitrogen to allow rapid cooling to 50 °C be-
fore being removed from the chamber. Finally, the vessel was fur-
ther cooled to room temperature, decapped and a sample was
taken for analysis. For comparison, dehydration of fructose was
also performed by conventional heating of a sealed pressure-stable
glass tube (Ace, pressure limit: 20 bar) charged with an aqueous
fructose solution (30 wt %, 2.7 mL, 5.3 mmol), an aqueous HCl solu-
tion (0.10 M 0.3 mL, 0.03 mmol) and a magnetic stirrer. Here the
tube with the reaction mixture was placed in a preheated oil bath
employed with a temperature controller (VWR, VT-5) for a speci-
fied time under stirring at a fixed temperature (reaction times
were measured after a stable oil bath temperature had been
reached). After the reaction, the tube was removed from the oil
bath and cooled to room temperature before a sample was taken
for analysis.
2.3. Product analysis
Samples of reaction products were filtered through a syringe fil-
ter (VWR, 0.45
chromatography (HPLC, Agilent 1200 series, Bio-Rad Aminex
0.005 M
H2SO4 mobile phase was employed as eluent at 60 °C with a flow-
rate of 0.6 mL/min. Concentrations of products were determined
from calibration curves obtained with reference samples, while
product yields (%), product selectivities (%) and fructose conver-
sions (%) were based on the initial concentration of fructose and
calculated as:
lm PTFE) prior to analysis by high pressure liquid
Scheme 1. Acid-catalysed synthesis of HMF from fructose and possible by-
products.
HPX-87H, 300 mm ꢀ 7.8 mm pre-packed column).
A
speeds up the reaction significantly.22 The aqueous, acid-catalysed
dehydration of fructose to HMF is a fairly slow process that can
take hours to reach full fructose conversion. Employing micro-
waves, however, increases the reaction rate significantly such that
full conversion is reached in a matter of minutes.20
Product conc
Initial fructose conc
In this work, we report a thorough investigation of the reaction
parameters in the HCl-catalysed, microwave-assisted, aqueous
dehydration of fructose to HMF. To facilitate process intensifica-
tion, reaction parameters were optimised with three main goals
in mind: (1) the solvent has to be water, (2) the utilisation of HCl
catalyst should be minimised and (3) an initial high fructose con-
centration should be employed. These conditions differ signifi-
cantly from those employed in recent work by Qi et al.20 who
used an acetone–water system with a heterogeneous acidic ion-ex-
change resin catalyst. Despite very good results with this system
its industrial utilisation could be hampered by generation of a
highly elevated pressure from the acetone component (bp
56 °C23). In addition, optimisation of the system was conducted
on a diluted 2 wt % fructose solution, resulting in a corresponding
low space-time-yield.
Product yield ¼
ꢀ 100%
ꢀ
ꢁ
Fructose conc
Initial fructose conc
Fructose conversion ¼ 1 ꢁ
ꢀ 100%
Product yield
Fructose conversion
Product selectivity ¼
ꢀ 100%
3. Results and discussion
3.1. Dehydration without catalyst
Dehydration of fructose to HMF is, in principle, possible without
a catalyst both by conventional heating and by microwave-assisted
heating, though a catalyst is, of course, expected to increase the
dehydration rate significantly. Initial experiments performed here
with microwave heating, at 160 °C without catalyst, yielded only
5% fructose conversion and only 1% HMF, as shown in Figure 1.
At lower temperatures, no significant fructose conversion occurred
whereas a considerable quantity of humins were detected in all
reactions above 160 °C, clearly suggesting catalyst promotion to
be a prerequisite to make the dehydration selective for HMF
production.
2. Experimental
2.1. Materials
Fructose (99%), 5-hydroxymethylfurfural (99%), levulinic acid
(98%), formic acid (99.8%) and HCl (P37%, p.a.) were purchased
from Sigma–Aldrich and were used without further purification.
Deionized water was used for the preparation of aqueous solutions.
At prolonged reaction times or sufficiently high temperatures
however, a build-up in LA and FA concentrations results in an acid-
ification of the reaction mixture leading to autocatalysis of the
dehydration process. The catalytic properties of the reaction by-
products can thereby render control experiments without a cata-
lyst misleading. This is clearly seen in Figure 1, where an increasing
amount of formed LA and FA from 170 °C to 190 °C induced a dra-
matic increase in the fructose conversion rate.
2.2. Dehydration of fructose to HMF
The microwave-assisted fructose dehydration was performed
using a microwave reactor vessel (pressure limit: 20 bar) charged
with an aqueous fructose solution (30 wt %, 2.7 mL, 5.3 mmol), an
aqueous HCl solution (0.10 M 0.3 mL, 0.03 mmol) and a magnetic
stirrer. After being sealed with a cap, the vessel containing the mix-
ture was mounted in a microwave apparatus (Biotage Emrys Crea-
tor, Personal Chemistry, effect range 90–300 W) and heated for a
specified reaction time (reaction times refers to the time the reac-
tion was held at a fixed terminal temperature) with preset power
supply under magnetic stirring. After the reaction, the vessel was
3.2. pH dependence
To optimise the fructose dehydration towards HMF formation
with respect to the required amount of catalyst, experiments were

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60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
previously published results.20 However, within the temperature
ranges that we were able to test (max. 180 °C for closed pressure
tube experiments), no conclusive change was found in the product
distribution compared to analogous reactions carried out in a
closed pressure tube using conventional oil bath heating (see Sup-
plementary data). The product distribution is more likely depen-
dent on other factors (such as solvent, temperature, etc.) that
were investigated in this study rather than the method of heating.
The primary advantage in the microwave heated system is the in-
crease of reaction rate leading to an increased space-time-yield in
potential industrial applications. In addition, very precise control
of reaction temperature is maintained in the microwave apparatus
whereas the conventional heated system has a rather slow re-
sponse. Employing this fast temperature control, the microwave
apparatus was able to quickly heat the reaction contents to
200 °C and cool the vessel down, resulting in the best yield re-
ported in a strictly aqueous media (vide infra) primarily thought
to be due to the fast and accurate temperature adjustments. In con-
trast, the conventional heated system is rather slow with poor con-
trol at elevated temperatures. Indeed, 180 °C was the maximum
temperature achievable in this setup. Thus, in this way, both con-
version rates and yields were increased significantly by employing
microwaves.
160
170
180
190
Temperature (ºC)
Figure 1. Yield of HMF (ꢀ), FA (N) and LA (j), HMF selectivity ( ) and fructose
conversion ( ) as a function of temperature in microwave-assisted dehydration of
fructose (5 min reaction time, 300 W, initial fructose concentration 27 wt %).
performed with various amounts of hydrochloric acid, that is, at
different initial pH values (Fig. 2).
When using a large excess of acid at a corresponding low pH va-
lue (1 M HCl, pH 0), at moderate temperature of 130 °C with 5 min
microwave irradiation, 99% fructose conversion was reached. Nev-
ertheless, rehydrated FA and LA by-products (as well as humins)
were formed in relatively high yields of 39% and 30%, respectively,
compared to only 28% HMF. A maximum HMF yield of 40% was ob-
tained at a higher pH value of 0.3 (0.5 M HCl) with 70% fructose
conversion, while a maximum reaction selectivity of 56% towards
HMF was reached at a pH of 0.6 (0.25 M HCl), though with a lower
fructose conversion of 63%. This clearly demonstrates that less
acidic conditions are necessary to suppress the acid-promoted
by-product formation. Hence, more benign acidic conditions (pH
2) were employed for the remaining experiments, corresponding
to 0.01 M HCl and 27 wt % fructose solution, with the presence of
only catalytic amounts of acid (0.5 mol % H+).
The influence of the initially applied microwave irradiation ef-
fect on reaction performance, that is, conversion, yield and selec-
tivity, was further studied by using varying initial microwave
powers of 90 W, 150 W and 300 W. Under the tested reaction con-
ditions, for example, reaction time of 5 min and temperature of
140 °C or 160 °C, no significant difference in either conversion or
product distribution was found, thus confirming the 90 W to be
sufficient to facilitate effective reaction progression (Figs. 3 and
4). Hence, conducting the reaction at higher power is more energy
intensive but does not increase conversion, yield or selectivity. As
shown in Figures 3 and 4, the microwave-assisted reaction was
found to give 48% fructose conversion, 28% HMF yield and 59%
HMF selectivity when performed at 160 °C for 5 min. with an initial
fructose concentration of 27 wt % (0.01 M HCl). The corresponding
reaction with conventional heating for 10 min only gave 29% fruc-
tose conversion, 12% HMF yield and 40% HMF selectivity (see Sup-
plementary data).
3.3. The effect of microwaves
The use of microwaves for the dehydration revealed, as antici-
pated, a dramatic effect on the reaction rate in accordance with
3.4. Temperature dependence
As already indicated above, the microwave-assisted acid-cata-
lysed dehydration of fructose to HMF was highly temperature
80
60
40
20
0
100
50
0
15
10
5
60
50
40
30
20
10
0
0.0
0.5
1.0
1.5
2.0
2.5
pH
0
100
150
200
250
300
Microwave power (W)
Figure 2. Yield of HMF (ꢀ), FA (N) and LA (j), HMF selectivity ( ) and fructose
conversion ( ) as a function of pH in microwave-assisted dehydration of fructose
(5 min reaction time, 90 W, 130 °C, initial fructose concentration 27 wt %). The pH
values were calculated based on the HCl concentration and does not account for the
acidity of the by-products formed during the reaction.
Figure 3. Yield of HMF (ꢀ), FA (N) and LA (j), HMF selectivity ( ) and fructose
conversion ( ) as a function of initial microwave power in dehydration of fructose
(5 min reaction time, 140 °C, 0.01 M HCl, initial fructose concentration 27 wt %).

T. S. Hansen et al. / Carbohydrate Research 344 (2009) 2568–2572
2571
50
40
30
20
10
0
70
60
50
40
30
20
10
0
60
40
20
0
100
75
50
25
100
150
200
250
300
0
100
200
300
Time (s)
Microwave power (W)
Figure 6. Yield of HMF (ꢀ), FA (N) and LA (j), HMF selectivity ( ) and fructose
conversion ( ) as a function of reaction time in microwave-assisted dehydration of
fructose (300 W, 200 °C, 0.01 M HCl, initial fructose concentration 27 wt %).
Figure 4. Yield of HMF (ꢀ), FA (N) and LA (j), HMF selectivity ( ) and fructose
conversion ( ) as a function of initial microwave power in dehydration of fructose
(5 min reaction time, 160 °C, 0.01 M HCl, initial fructose concentration 27 wt %).
able from an industrial viewpoint due to small reactor volumes
and improved product work-up.
We have investigated the dehydration reaction at 170 °C with
5 min of microwave irradiation using initial fructose concentra-
tions from 2.25 to 45 wt % (Fig. 7).
dependent (Fig. 5). Generally, high temperatures promoted the
dehydration reaction as also reported previously.24 However, at
temperatures above 160–180 °C, shorter reaction times proved
necessary to diminish by-product formation by rehydration and
successive polymerization which otherwise resulted in a signifi-
cant decrease in HMF selectivity and yield. Hence, reactions con-
ducted at 200 °C with microwave irradiation for 1 s resulted in
52% fructose conversion and HMF selectivity and yield of 63%
and 33%, respectively. By prolonging the reaction time to 60 s the
fructose conversion further increased to 95% and the HMF yield
reached an optimum of 53%, whereas irradiation for 180 s provided
nearly full fructose conversion (>99%) but only gave an HMF yield
of 37% due to the accompanying decrease in HMF selectivity
(Fig. 6).
In accordance with previously published results,20 the highest
HMF selectivity (75%) was found using the lowest fructose concen-
tration of 2.25 wt % and the highest conversion was obtained (80%)
with 45 wt % fructose. In contrast, the yields of HMF, FA and LA re-
mained almost constant, independent of the initial fructose con-
centration, suggesting that the amount of by-products not
detected by HPLC analysis (i.e., humins and soluble polymers) pre-
vail at higher fructose concentrations, as anticipated.
3.6. The effect of the reaction time
3.5. The effect of the initial fructose concentration
When the microwave-assisted dehydration was performed at a
relatively low temperature of 140 °C where by-product formation
was negligible, both fructose conversion and HMF yield increased
almost linearly with reaction time reaching 33% and 17%, respec-
tively, after 20 min of reaction (Fig. 8). Similarly, the HMF selectiv-
ity concurrently increased to 52% most likely due to reversible
dimerisation of fructose, which allowed monomeric fructose to
be continuously reformed as HMF production progressed. Analo-
By-product formation by polymerization of reaction compo-
nents is one of the main challenges in the dehydration of highly
concentrated aqueous fructose to yield HMF, and clearly the con-
centrations of this component are very important for the process.
The use of low fructose concentrations reduces polymer formation,
while high initial fructose concentrations are economically desir-
60
100
50
80
80
40
40
60
70
30
40
20
20
60
20
10
50
0
0
140
160
180
200
0
0
10
20
30
40
50
Temperature (ºC)
Fructose (wt%)
Figure 5. Yield of HMF (ꢀ), FA (N) and LA (j), HMF selectivity ( ) and fructose
conversion ( ) as a function of temperature in microwave-assisted dehydration of
fructose (5 min. reaction time, 90 W, 0.01 M HCl, initial fructose concentration
27 wt %).
Figure 7. Yield of HMF (ꢀ), FA (N) and LA (j), HMF selectivity ( ) and fructose
conversion as function of fructose concentration in microwave-assisted
dehydration of fructose (5 min. reaction time, 300 W, 170 °C, 0.01 M HCl).
(
)
a

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T. S. Hansen et al. / Carbohydrate Research 344 (2009) 2568–2572
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24
16
8
60
50
40
30
20
10
in previously reported work, but are practically relevant when pur-
suing a commercially viable process with low environmental im-
pact for HMF production. At the cutting edge of technology
development it seems likely that it is only a matter of time before
microwave-enhanced chemical production is routinely possible.
Already the equipment for batch and continuous operation is avail-
able from several companies.27 Clearly the use of microwaves
opens new opportunities in temperature control and adjustment
that are not possible by conventional heating.
Acknowledgements
We thank Professor Jan O. Jeppesen and Professor Poul Nielsen,
Department of Physics and Chemistry, University of Southern Den-
mark for use of their microwave equipment in this study. The work
was supported by The Danish National Advanced Technology
Foundation and Novozymes A/S.
0
2
4
6
8
10
12
14
16
18
20
Time (Min)
Figure 8. Yield of HMF (ꢀ), FA (N) and LA (j), HMF selectivity ( ) and fructose
conversion ( ) as a function of reaction time in microwave-assisted dehydration of
fructose (90 W, 140 °C, 0.01 M HCl, initial fructose concentration 27 wt %).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.carres.2009.09.036.
gous findings have also previously been described by, for example,
Van Dam et al.24 and Kuster.25
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