Electrogenerated acid as an efficient catalyst for the preparation of 5-hydroxymethylfurfural

Article abstract of DOI:10.1016/j.elecom.2010.05.040

A new method for the synthesis of 5-hydroxymethylfurfural (HMF) with high yields in dimethyl sulfoxide (DMSO) is presented. By using constant-current electrolysis more than 90% of sucrose or fructose was converted to HMF at room temperature in DMSO in the presence of traces of water.

Full text of DOI:10.1016/j.elecom.2010.05.040

Electrochemistry Communications 12 (2010) 1149–1153
Contents lists available at ScienceDirect
Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Electrogenerated acid as an efficient catalyst for the preparation
of 5-hydroxymethylfurfural
Tonino Caruso, Ermanno Vasca ⁎
Department of Chemistry, University of Salerno, Via Ponte don Melillo, 84084 Fisciano (SA), Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A new method for the synthesis of 5-hydroxymethylfurfural (HMF) with high yields in dimethyl sulfoxide
Received 26 April 2010
Received in revised form 26 May 2010
Accepted 26 May 2010
(
DMSO) is presented. By using constant-current electrolysis more than 90% of sucrose or fructose was
converted to HMF at room temperature in DMSO in the presence of traces of water.
©
2010 Elsevier B.V. All rights reserved.
Available online 9 June 2010
Keywords:
5-Hydroxymethylfurfural
Electrochemical synthesis
Electrogenerated acid
1
. Introduction
non conventional acidity is very effective because the EGA makes
extremely acidic the solution at the anode surface [13].
Carbohydrates represent a well established alternative to fossil
fuels as a renewable and economic source of energy and raw materials
1]. The development of an economically convenient method for
2. Experimental
[
producing 5-hydroxymethylfurfural, HMF, [1g,2] at high purity,
starting from sucrose, fructose or glucose as the substrates, gained
in recent years considerable attention. HMF is a versatile intermediate
in the synthesis of other chemical substances [3]. Unfortunately high
production cost (40–80$ per gram, depending on the manufacturer)
limits its availability and uses at an industrial scale [4]. Production of
sufficiently pure HMF requires high temperatures and acid catalysts to
dehydrate hexoses in water [5], in DMSO [4b,6], and in biphasic
water-organic solvent systems [7,8]. More recently its synthesis was
reported in ionic liquids [2a,b,d,e,9], two-phase modifier [2c,10] and
over solid acid and base catalyst system [2f]. Fructose showed to be
the only hexose giving HMF at high yields, though usually at high
temperatures, DMSO being the more suitable solvent.
Up to now using electrolysis to oxidise or to reduce carbohydrates
to more versatile compounds gave unsatisfactory results. In water,
solvent electrolysis leads to complex mixtures of products [11]. On the
other hand, in organic solvents only the electrolysis of some
monosaccharides in alcoholic-alkaline media is reported, but without
the formation of any of the expected oxidation products and no other
chemical of some synthetic utility [12]. Here we report a new high-
yield synthesis of HMF, based on the dehydration of fructose or
sucrose at room temperature by electrogenerated acid (EGA). This
Electrochemical synthesis of HMF from carbohydrates was
performed using constant-current coulometry. Each experiment
started by transferring in the electrolysis cell a Test Solution (TS)
prepared by dissolving in the chosen organic solvent a known amount
of a carbohydrate and of an inorganic salt. The latter had to be present
in TS in order to lower the iR drop across the electrodes, and it was
selected on the basis of its solubility in the particular solvent used.
Experiments were carried out in dimethyl sulfoxide, dimethylforma-
mide and acetonitrile as the solvents. Fructose, glucose, sucrose,
mannose, galactose, lactose, cellobiose and maltose were systemat-
ically investigated as the substrates.
Constant-current electrolysis was performed by using a two-
compartment electrochemical cell. Compartments were separated by
a porous glass frit diaphragm. Platinum foils (1.5 cm2 total surface
area) were used as the electrodes.
Among the carbohydrates used as substrate, only fructose and
sucrose, in DMSO, were converted into HMF with significant yields
also at room temperature. In those cases the electrochemical
procedure was performed by dissolving in 5.0 mL DMSO 1.0 mmol
4
fructose or sucrose and 0.5 mmol KClO as electrolyte (0.1 mol/L).
Constant-current electrolysis at 3.0 mA was performed on the stirred
solution, at room temperature.
Reaction progress was followed by drawing few microliter aliquots
of Test Solution at different electrolysis stages and analyzing them
by TLC and by NMR. Once the reaction was completed, reaction
⁎
Corresponding author. Tel.: +39 89 96 9588; fax: +39 89 96 9603.
E-mail address: evasca@unisa.it (E. Vasca).
3 2
mixtures were extracted with 5:1 CHCl :H O. The organic phase was
1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2010.05.040

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T. Caruso, E. Vasca / Electrochemistry Communications 12 (2010) 1149–1153
4
6
00 MHz spectrometer. NMR spectra in DMSO-d were collected at
4
0 °C in order to optimize signal resolution. All reagents and solvents
used were purchased from Sigma-Aldrich-Fluka.
3
. Results and discussion
The 1H-NMR spectra collected at different electrolysis stages in
sucrose and fructose solutions are presented in Fig. 2. In parallel with
electrolysis progress, a singlet relative to HMF appeared, with the
doublets relative to the anomeric forms of glucose, α- and β-GP [14].
Peak integration of the spectrum in Fig. 2a shows that sucrose
concentration decreases as a first-order kinetic [15].
In the same experimental conditions, fructose was the only one
among the simple monosaccharides investigated which could be
converted in HMF (Fig. 2b). Glucose and HMF were inert at room
temperature under the electrolysis conditions used. Actually, it is
reported [4b] that HMF can be oxidised to 2,5-diformylfuran, but only
after prolonged electrolysis at temperatures higher than 80 °C.
HMF formed was purified and quantified by HPLC after extraction.
Efficient extraction of HMF from the solvent mixture remains a quite
difficult task. Separation methods suggested to remove HMF from
DMSO are based on biphasic reactors in which a continuous extraction
of HMF by an organic solvent is performed [1a,7,8,10].
In the present work reaction mixtures were first extracted with 5:1
CHCl :H O [16], monitoring by HPLC the decrease of the aldehyde
3 2
Fig. 1. Sketch of the small-scale electrolytic cell in the NMR probe.
signal in the aqueous phase. It was possible to keep loss below 10%. To
do this, the organic solvent in which HMF was dissolved was
advantageously recovered while the residual mixture was analyzed
by HPLC [9c] and by UV–Vis spectroscopy, in order to determine the
reaction yield and the percent conversion. HMF was isolated with 80–
concentrated and analyzed by HPLC on a C18 column, using a 99:1 v/v
water/iso-propanol as eluent at a flow rate of 0.7 mL/min.
Further information on the reaction of these substrates were
obtained by carrying out experiments in a small-scale electrolytic cell
adapted to fit into an NMR tube, as in Fig. 1. Two 0.3 mm platinum
wires, each one melted in a glass capillary to avoid electrical short-
circuit between them, were inserted in a Teflon sheath intercalated in
the NMR tube. The cathode, inserted into a Luggin capillary, was
separated from the solution by a terminal porous glass frit. This way
all reactions could be performed directly in deuterated DMSO and
8
5% yields, which are slightly less than the ones evaluated on the basis
of UV and HPLC measurements performed directly on the crude
mixture (90–92%).
In order to increase the product yield, some factors affecting
electrosynthesis had to be optimized. The current density, the amount
of charge delivered and the electrode material were systematically
varied, by setting all parameters to be constant and optimizing one at
a time. The effect of applied current density on the product yield was
1
13
monitored by collecting H-, C- and DEPT-NMR spectra at selected
times during the electrolysis. At the end of each electrolysis
2
studied in the range 0.5–6.0 mA/cm . The highest product yield was
2
experiment, 50 μL D O were added to suppress the signals of the
2
obtained working at 2.0 mA/cm .
different hydroxyl groups and merge them in a single peak, in order to
simplify the interpretation of protonic spectra. In this manner the
number of peaks in the more informative zone of the spectrum,
between 3 and 4 ppm, was reduced.
In the course of electrolysis dimethyl sulfide, DMS, at the cathode,
and dimethyl sulfone, DMSO
relative to DMSO and DMS were revealed by NMR (Fig. 3) thus
confirming that dimethyl sulfoxide is not inert toward electrolysis
17]. DMSO oxidation occurs in the presence of residual traces of
2
, at the anode, were formed. Signals
2
[
2
.1. Characteristic resonance shifts of HMF
water (0.1% in our solvent, by Karl Fisher), according to the anodic
half-reaction (1) [17]
1H-NMR, δ ppm (400 MHz, DMSO-d
J=3.4 Hz, 1 H), 7.42 (d, J=3.4 Hz, 1 H), 9.45 (s, 1 H). C-NMR, δ ppm
400 MHz, DMSO-d ): 178.3, 163.2, 152.1, 124.8, 110.0, 56.0.
): 4.43 (s, 2 H), 6.53 (d,
6
13
þ
−
(
6
DMSO þ H O→DMSO þ 2H þ 2e
ð1Þ
2
2
2
.2. Apparatus and reagents
and the cathodic half-reaction (2):
An Agilent E3612A power supply was used in the coulometric
DMSO þ 2Hþ þ 2e →DMS þ H O:
−
ð2Þ
circuit. Current intensity was monitored with a Keithley 197 A
multimeter. Spectrophotometric measurements were performed with
a Perkin-Elmer Lambda EZ 201. A Thermo Surveyor Plus HPLC system,
equipped with a two-wavelength UV detector (209 nm for DMSO and
2
Solvent disproportion produces and consumes the same number of
+
H . However it is well known that an electrogenerated acid (EGA) is a
powerful acid-catalyst for several organic reactions. Oxidation of
traces of water leads to a high acid concentration at the anode surface
2
84 nm for HMF) was used, and the separation was carried out on a
Supelcosil™ LC-18 column (25 cm×21.2 mm L×i.d., 5-μm particle
size). 1H and C-NMR spectra were recorded on a Bruker DRX
13
[13]. Hence H electrogenerated in situ by water oxidation in DMSO is
+
Fig. 2. a) 1H-NMR spectra of sucrose solutions in DMSO-d
and 4.27 ppm correspond to the anomeric proton of sucrose (S), α-glucopyranose (α-GP) and β-glucopyranose (β-GP) respectively; b) H-NMR spectra of fructose solutions in
recorded at increasing electrolysis times, from bottom to top: 15, 30, 45, 60, and 75 min. The three doublets at 5.18, 4.91,
6
1
1
DMSO-d
6 6
recorded at increasing electrolysis times, from top to bottom: 10, 15, 30, and 45 min. H-NMR for 5-HMF: δ ppm (400 MHz, DMSO-d ): 4.43 (s, 2 H), 6.53 (d, J=3.4 Hz,
1
H), 7.42 (d, J=3.4 Hz, 1 H), and 9.45 (s, 1 H).

T. Caruso, E. Vasca / Electrochemistry Communications 12 (2010) 1149–1153
1151

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T. Caruso, E. Vasca / Electrochemistry Communications 12 (2010) 1149–1153
Fig. 3. a) 1H-NMR spectrum of DMSO-d
electrolysed at the cathode in the presence of 0.1% water. The signal at 2.066 ppm is relative to DMS; b) 13C-NMR spectrum of DMSO
6
2
electrolysed at the anode in the presence of propylene oxide, showing the signals relative to DMSO , 1,2-propane diol and propylene oxide excess.
the driving force of the process and directly catalyses fructose
dehydration to HMF (3)
tion (1). The net result is the production of hydrogen ions. The overall
reaction (4) occurs at the anode as a result of DMSO oxidation and of
the catalytic dehydration of fructose:
þ
H
þ
H
þ
−
C6H12O → C H O + 3H O
ð3Þ
6
6
6
3
2
C6H12O6 + ðCH Þ SO → C H O + ðCH Þ SO + 2H O + 2H + 2e
:
3
2
6
6
3
3
2
2
2
ð4Þ
according to the mechanism reported by Antal [18].
Water formed through reaction (3) can be directly oxidised in
competition with DMSO or can sustain the electrochemical reac-
By interrupting the current delivery HMF formation is inhibited.
Constant current (2.0 mA/cm ) must be delivered for at least 60 min
2

T. Caruso, E. Vasca / Electrochemistry Communications 12 (2010) 1149–1153
1153
(
(
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(
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3
2
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3
3
2
2
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(
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2 3
CO no reaction was observed.
[
9
2
(
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(
(
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