Task-specific ionic liquid for the depolymerisation of starch-based industrial waste into high reducing sugars

Article abstract of DOI:10.1016/j.cattod.2013.09.027

Development of a simple route for the catalytic conversion of starch-based industrial waste (potato peels) and potato starch into reducing sugars was investigated in two ionic liquids for comparison - 1-allyl-3-methylimidazolium chloride [AMIM]Cl and 1-(4-sulfobutyl)-3-methylimidazolium chloride [SBMIM]Cl. Over a two hour period, a 20 wt% solution containing up to 43% and 98% of reducing sugars at low temperature in aqueous [SBMIM]Cl was achieved for the starch-based waste and the potato starch, respectively. In addition, the use of microwave and low frequency ultrasound to perform the depolymerisation of the raw starch-based material was explored and compared with conventional heating processes.

Full text of DOI:10.1016/j.cattod.2013.09.027

Catalysis Today xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Task-specific ionic liquid for the depolymerisation of starch-based
industrial waste into high reducing sugars
Audrey Hernoux-Villièrea,b,∗, Jean-Marc Lévêqueb, Johanna Kärkkäinenc,
Nicolas Papaiconomoub, Marja Lajunenc, Ulla Lassia,c
a Kokkola University Consortium Chydenius, Talonpojankatu 2B, 67100 Kokkola, Finland
b Laboratoire de Chimie Moléculaire et Environnement, Université de Savoie, 73376 Le Bourget du Lac Cedex, France
c University of Oulu, Department of Chemistry, P.O. Box 3000, FI-90014 Oulu, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
Development of a simple route for the catalytic conversion of starch-based industrial waste (potato peels)
and potato starch into reducing sugars was investigated in two ionic liquids for comparison – 1-allyl-3-
methylimidazolium chloride [AMIM]Cl and 1-(4-sulfobutyl)-3-methylimidazolium chloride [SBMIM]Cl.
Over a two hour period, a 20 wt% solution containing up to 43% and 98% of reducing sugars at low temper-
ature in aqueous [SBMIM]Cl was achieved for the starch-based waste and the potato starch, respectively.
In addition, the use of microwave and low frequency ultrasound to perform the depolymerisation of the
raw starch-based material was explored and compared with conventional heating processes.
© 2013 Elsevier B.V. All rights reserved.
Received 10 January 2013
Received in revised form 24 July 2013
Accepted 16 September 2013
Keywords:
Task-specific ionic liquids
Carbohydrates
Reducing sugars
Microwaves
Ultrasounds
Hydrolysis
1. Introduction
long reaction time [3,4]. However, starch molecules are not prone
to accept water dissolution, notably due to the strong intra and
A growing concern in environmental sustainability in our soci-
ety has become an important aspect for both ecosystem health
and economic development. The intensive consumption of fossil
fuels that will eventually run out renders renewable resources as
an attractive proposition. Some by-products can be considered as
sustainable energy for the synthesis of chemicals [1]. Currently, a
Finnish company, which produces pre-cooked vacuum potatoes,
generates several tonnes of waste from potato peels daily. In our
previous study [2], a weight percentage of sugars were performed
on by a total hydrolysis of the by-product, which is mainly com-
posed of glucose (80.2%), mannose (4.9%) and galactose (3.2%). More
than 88% can be subsequently considered as the total sugar poten-
tial. This by-product is mainly composed of starch, the principal
constituent of potatoes. Starch is basely composed of two macro-
molecules, amylose and amylopectin, trapped into granules. Its
depolymerisation into reducing sugars is mainly performed under
concentrated strong acidic conditions and/or high temperature, for
intermolecular hydrogen bonds. These latters can be generally bro-
ken down under high temperature, shear and acidic conditions,
yielding both free macromolecules [5]. The depolymerisation pro-
cess in a water medium is therefore of a heterogeneous nature
and suffers some inevitable limitations (existence of diffusion lay-
ers, limitation of the mass transfer, lack of efficient mixing, etc.),
whereas homogeneous media will certainly bring a higher reac-
tivity [6,7]. One possibility for the dissolution of starch is to use
ionic liquids [8]. Known as salt with a melting point below 100 ◦C,
ionic liquids possess attracted properties as new generation of
solvents, negligible vapour pressure, wide liquid ranges (up to
400 ◦C) and the ability to dissolve carbohydrate [9]. Dissolution
of carbohydrates up to 20-wt% in ionic liquids has been reported
previously [10]. In 2006, Remsing et al. investigated the solvation
of cellulose in an imidazolium-based ionic liquid bearing a chlo-
ride counter-anion [11]. Due to their high nucleophilic capacity,
chloride ions are enabled to interact with the hydroxyl protons
of carbohydrates and to break down the hydrogen-bonding net-
work to promote dissolution. In our experiments, the first selected
ionic liquid was 1-allyl-3-methylimidazolium chloride [AMIM]Cl,
which has an excellent ability to dissolve carbohydrates [12] and
depolymerise them in the presence of solid catalysts [13] or acid
∗
Corresponding author at: Kokkola University Consortium Chydenius, Talonpo-
jankatu 2B, 67100 Kokkola, Finland. Tel.: +358 442619929.
E-mail addresses: audrey.hernoux@chydenius.fi, audrey.villiere@gmail.com
(A. Hernoux-Villière).
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.09.027
Please cite this article in press as: A. Hernoux-Villière, et al., Task-specific ionic liquid for the depolymerisation of starch-based industrial waste
into high reducing sugars, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.09.027

2
A. Hernoux-Villière et al. / Catalysis Today xxx (2013) xxx–xxx
was synthesised according to the procedure published by Zhang
et al. [27] with minor modifications. A typical procedure is as
follows: in an adapted round-bottom flask flushed with argon
(10 min), allylchloride (50 mL, 610 mmol) was added dropwise over
1-methylimidazole placed into a water/ice bath (due to exothermic
reaction) to achieve 1:1.25 proportions. Afterwards, the solution
was stirred for 18 h at 55 ◦C. The ionic liquid was washed several
times with ethyl acetate (3 × 40 mL) and cyclohexane (3 × 40 mL).
In order to obtain a clean ionic liquid, activated charcoal and
methanol (gradient grade – 50 mL) were stirred with the ionic
liquid for 90 min and then filtered on Celite® [28]. The ionic liq-
uid was then dried under a vacuum line and a water-content
of 0.1 wt% was measured by Karl Fisher coulometric titration
(Metrohm 831KF coulometer) using Hydranal 34843 Coulomat AG-
H (Fluka) as titrant. The synthesis of the [SBMIM]Cl was also based
on the literature [29] with minor modifications; the detailed pro-
tocol was as follows: 1,4-butane sultone (200 mmol) was added
dropwise to 1-methylimidazole whilst being stirred in a 250 mL
round-bottom flask, flushed with argon for 10 min beforehand. The
solution was then heated to 70 ◦C for 1 h and the resulting solid
was then cooled down, crushed and washed several times with
toluene and cyclohexane. The zwitterion obtained was dried in a
vacuum oven for 12 h (yield > 98%) followed by adding droplets
of hydrochloric acid 37% to the zwitterion in stoichiometric pro-
portions. The solution was stirred and heated at 70 ◦C for 2 h. The
resulting mixture was washed with toluene (3 × 20 mL) and cyclo-
hexane (3 × 20 mL) before being cleaned with activated charcoal
in methanol (gradient grade – 30 mL) to obtain a clear solution.
Ionic liquids are clear compounds, and a more or less yellowish
result from traces of compounds originating from the reagents
[30]. The solvents were then evaporated with a rotary evapora-
tor and a yellowish ionic liquid was formed in the inner layer of
the pear-bottom flask. The ionic liquid was dried again in a vac-
uum oven for 12 h at 70 ◦C. NMR and FTIR were performed on
both ionic liquids (see Section 2.3) whilst a low frequency (24 kHz)
ultrasound bath (Kerry Pulsatron) and a Prolabo Synthewave S402
(electric power 600 W) microwave were employed for depolymeri-
sation.
(A)
(B)
Fig. 1. (A) 1-Allyl-3-methylimidazolium chloride [AMIM]Cl and (B) 1-(4-
sulfobutyl)-3-methylimidazolium chloride (Brønsted-acidic ionic liquid)
[SBMIM]Cl.
[14]. Brønsted acidic ionic liquids (BAILs) possess simultaneously
a proton acidity with the Brønsted function and properties of
ionic liquids – non-volatile, recyclable [7,15,16]. A wide range
of moieties can be classified in the Brønsted framework: min-
eral acids, sulfonates, phosphonates, and carboxylic acids. Johnson
et al. [17] published a detailed review about fundamentals of
BAILs and their use in various organic reactions with different
location of the Brønsted acid function (anion or/and cation). The
strength of the acidity depends on the position of the acidic func-
tion; COOH or SO3H function on cation possess strong intrinsic
acidity [17]. SO3H-functionalised ionic liquids are strong Brønsted
acids [6,15,18] and possess great potential as dual catalyst/solvent
system and non-volatile acidic materials [19]. 1-(4-Sulfobutyl)-3-
methylimidazolium chloride [SBMIM]Cl possesses Brønsted-acidic
sulfonic group on the cation to play the role of both solvent and
catalyst. The chloride anion was preserved to enable the primary
target, i.e. the solubilisation of the solute [20].
Both ionic liquids (see Fig. 1 for structures and abbreviations)
are already well known in literature as they have been previously
employed mainly for the dissolution and hydrolysis of cellulose
into reducing sugars [7,21,22,23], and to the best of our knowl-
edge, no studies have been performed on native potato starch and
particularly on a real industrial waste material.
The main goal of this research was therefore to dissolve and
to depolymerise natural starch-based raw materials into reduc-
ing sugars in ionic liquids. To overcome the viscosity of these
ionic liquids, we decided to explore the effect of low frequency
ultrasound and microwave irradiation. Whereas the rapid heat-
ing of microwave irradiation can decrease the viscosity and thus
enhance mass transfer, low frequency ultrasound, through their
strong mechanical effects (harsh mixing, local heating, mass trans-
fer, etc.) may help to stir in an efficient way the ionic liquids phase
[24,25].
2.2. Dissolution and depolymerisation methods
A 10 or 20 wt% solution of starch in an ionic liquid medium was
stirred or irradiated for 120 min at several temperatures (60–90 ◦C).
The three previously stated materials were then added to the
heated solution to ease dissolution. A conical vial of 5 mL with a
dedicated triangle magnet was employed for the mechanical stir-
ring reactions. A 10 mL round-bottom flask and a 20 mL tube flask
were used for the ultrasound and microwave irradiations, respec-
tively. For the former, the indirect mode of irradiation, i.e. use of
an ultrasonic bath, is justified by the acidity of the selected TSIL,
whereas the direct mode of irradiation would suggest the use of
an ultrasonic probe directly immersed in the solution. This would
exposure the probe to corrosion and other damages. To prevent
this, an ultrasonic bath filled with water was selected in which the
reactor is immersed. A minimum of 10% (w/w) of distilled water
was added to the reaction with [SBMIM]Cl in order to dissociate
the sulfonic acid function. Afterwards, the sample was dissolved
into 10 mL of distilled water and neutralised with crushed pellets
of sodium hydroxide. The opaque solutions were then centrifuged
at 3000 tr min−1 for 10 min or filtered on a filtering funnel with
5–13 ␮m porosity. The solid phase was recovered, dried under
vacuum and weighted to determine the mass balance, while the
percentage of total reducing sugars contained in the liquid phase
was determined by adapted analytical procedures (see Section 2.3).
2. Materials and methods
2.1. Materials and preparation of ionic liquids
Three different raw materials were utilised for comparison:
potato starch, wet potato sludge and dry potato sludge. Potato
starch was purchased at a local supermarket, composed solely of
starch extracted from potatoes and utilised as the reference. Wet
potato sludge is an industrial by-product composed of waste potato
peels. Half centimetre of potato containing peels was roughly
removed with a potato rotating peeler machine. The third raw
material used is dry potato sludge, which is wet potato sludge dried
under a vacuum line and ground with a mortar and pestle prior to
use. Loss on drying analyses [2] were performed on potato starch,
wet potato sludge and dry potato sludge revealing 10%, 67% and
10%, respectively. These results were confirmed by a thermogravi-
metric analysis performed in a previous study [26].
1-Methylimidazole, allylchloride, 1,4-butane sultone and sol-
vents were purchased from VWR Finland and Sigma–Aldrich France
whilst hydrochloric acid 37% (Baker analysed ACS Reagent®, VWR
Finland) was commercially available. The ionic liquid [AMIM]Cl
Please cite this article in press as: A. Hernoux-Villière, et al., Task-specific ionic liquid for the depolymerisation of starch-based industrial waste
into high reducing sugars, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.09.027

A. Hernoux-Villière et al. / Catalysis Today xxx (2013) xxx–xxx
3
Table 1
Band assignments of the ionic liquids [SBMIM]Cl and [AMIM]Cl.
Ionic liquids
[SBMIM]Cl
Band assignments
Wavelength (cm−1
)
Alkyl C H stretching
2965, 2942 and 2878
Imidazole ring stretching
Sulfone symmetric stretching R SO2 OH
1566
1175
1157
1035
850
Imidazole H
C C and H C N bending
Sulfone asymmetric stretching R SO2 OH
In-plane imidazole ring bending
Out-of-plane imidazole ring C H bending
786
[AMIM]Cl
O
H stretching – water content
Broad peak contains alkyl C H stretching
H bending – water content
Imidazole ring stretching
Imidazole H C and H
Out-of-plane imidazole ring C H bending
3385
3040–2870
1644
1561
1165
O
C
C
N bending
767
The experiments were performed in duplicates (20 wt%) or tripli-
cates (10 wt%).
3. Results and discussion
3.1. Comparison of the ionic liquids for the dissolution and
depolymerisation of starch
2.3. Analysis: total reducing sugars, FT-IR and NMR
At 80 ◦C, a 15 wt% solution of starch in [AMIM]Cl can be totally
dissolved within 40 min [34] whilst [SBMIM]Cl can also be dis-
solved up to 10 wt% of cellulose at room temperature in a shorter
time period [21]. At first, the maximal weight percentage of disso-
lution of our starch materials in both ionic liquids was determined.
The simplest matrix, i.e. potato starch, was added in 0.1 g incre-
ments to [AMIM]Cl at 80 ◦C until the dissolution was complete up to
20 wt%. The observed instantaneous dissolution renders this ionic
liquid as an attractive prospect and certainly offers a promising
future in the field of biomass valorisation. However, in parallel,
1.0 g of potato starch was added at once to a [AMIM]Cl solution
at 80 ◦C. In this case, 15 min of stirring was also needed to reach a
clear 20 wt% mixture. Xu et al. [34] showed that corn starch could
be dissolved up to 15 wt% in [AMIM]Cl within 40 min at 80 ◦C and
up to 20 wt% within 15 min at 100 ◦C under an argon atmosphere.
Although our results differ to some extent from the studies men-
tioned above, they can be explained by the water-content of the
ionic liquid, not defined in their study. It is highly probable that our
ionic liquid contained a higher amount of water than Wu et al.,
diminishing subsequently the dissolution efficiency. It is indeed
well known that water-content can disrupt the carbohydrate disso-
lution in an ionic liquid and lead to a heterogeneous medium [38].
The dissolution of potato starch in [SBMIM]Cl required a longer
time period than in [AMIM]Cl; in fact, 20 wt% potato starch in
[SBMIM]Cl did not even stir after several minutes at 80 ◦C with an
increased viscosity. Potato starch is mainly composed of amylose
and amylopectin compared to wet potato sludge and dry potato
sludge which contain some proteins, minerals and vitamins. There-
fore, the previous protocol was not applied to these raw materials.
Their total dissolution in ionic liquids was not observed proba-
bly due of the presence of these natural compounds. Wet and
dry potato sludges were added to their respective ionic liquids at
once. Both ionic liquids are attractive for the dissolution of potato
starch, however, the results about the depolymerisation were rad-
ically different. The TRS yield of pure starch reached 54% with the
Brønsted-acidic ionic liquid at 80 ◦C (Table 2, entry 3) and only
6% in the [AMIM]Cl (Table 2, entry 17). The absence of intrin-
sic acidity and additional acidic catalyst in [AMIM]Cl is certainly
the main reason of a low TRS value. However, the existence of
this small amount of TRS can be explained. Indeed, it is known
that some first and second generations imidazolium-based ionic
liquids possess a weak acidity often tied to the nature of counter-
anion, making it reasonable to reach a low 6% of depolymerisation
[39,40]. The first generation of ionic liquids possess a halide anion
The percentage of total reducing sugars (TRS) was determined
with 1% dinitrosalicylic acid reagent (DNS) according to the Miller
method [31]. A sample of 1.0 mL of the solution to be analysed
was added to 1.0 mL of DNS reagent and boiled for 5 min precisely.
Afterwards, 0.5 mL of a 40% potassium sodium tartrate solution
was poured in order to keep the colouration of the product and
cooled down in tap water to quench the oxidation reaction. Anal-
yses were performed with an UV-Spectrophotometer (Shimadzu
UV-1800) at a wavelength of 575 nm. The concentration of reducing
sugars was determined according to the standardisation performed
on glucose. The range of experimental errors was 5% for the
TRS analysis. The scan of a standard solution of 20 wt% ionic liq-
uid and water was performed between 700 and 300 nm. None of
the ionic liquids utilised absorbed at 575 nm. Therefore, the ionic
liquids were not separated from the main solution before being
analysed.
The spectroscopy of the ionic liquids were performed with a
PerkinElmer Spectrum One Fourier Transform Infrared (FTIR) Spec-
trometer combined with a PerkinElmer Universal Attenuated Total
Reflectance (ATR) Sampling Accessory, a sampling technique which
offers a direct analysis of solid and liquid samples without any
required preparation. The assignments of the bands and the corre-
sponding wavelength of the ionic liquids are summarised in Table 1
and spectra in Fig. 2. First of all, the water-content in the ionic liq-
uids can be characterised by the presence of two peaks at 3385 and
1644 cm−1, O H stretching and bending, respectively. The inten-
sity of these peaks depends on the quantity of water entrapped in
the matrix of ionic liquids. [AMIM]Cl contained more water than
[SBMIM]Cl probably because of its high hygroscopic character. The
FTIR spectra of [AMIM]Cl and that of the zwitterion utilised for the
synthesis of [SBMIM]Cl corroborated with previous literature in
[32,33], respectively.
1H NMR spectra of ILs were recorded with a Bruker DPX 200
instrument (200.13 MHz). Spectroscopic data of [AMIM]Cl were
identical to the previous literatures [14,34,35]: 1H NMR (200 MHz,
CDCl3) ı: 4.13 (3H, s), 5.04 (2H, d, JHH = 6.3 Hz), 5.40–5.51 (2H, m),
5.97–6.10 (1H, m), 7.58 (1H, t, JHH = 1.8 Hz), 7.81 (1H, t, JHH = 1.8 Hz),
10.39 (1H, s). However, a singlet at 3.38 ppm corresponds to
methanol residues from the cleaning steps. No peak was observed
around 1.56 ppm, corresponding the H2O peak in CDCl3 [36]. The
spectroscopic data of [SBMIM]Cl followed the literature [37]: 1H
NMR (200 MHz, D2O) ı: 1.72 (2H, m), 1.98 (2H, m), 2.91 (2H,
t, JHH = 7.6 Hz), 3.86 (3H, s), 4.22 (2H, t, JHH = 7.0 Hz) 7.41 (1H, t,
JHH = 1.8 Hz), 7.47 (1H, t, JHH = 1.8 Hz), 8.72 (1H, s).
Please cite this article in press as: A. Hernoux-Villière, et al., Task-specific ionic liquid for the depolymerisation of starch-based industrial waste
into high reducing sugars, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.09.027

4
A. Hernoux-Villière et al. / Catalysis Today xxx (2013) xxx–xxx
Fig. 2. FTIR spectra of [AMIM]Cl and [SBMIM]Cl after synthesis.
(i.e. 1-allyl-3-methylimidazolium chloride – [AMIM]Cl), whereas
the second generation undergo a metathesis of the halide anion
into a more water stable one (i.e. 1-allyl-3-methylimidazolium
acetate – [AMIM]OAc) [41]. TSIL are considered as part of the ‘third
generation’ of ionic liquids due to their functionalised moieties
(i.e. 1-(4-sulfobutyl)-3-methylimidazolium chloride – [SBMIM]Cl)
[42,43]. The TSIL selected possesses a Brønsted-acid and can play
the role of both the solvent and the catalyst. [SBMIM]Cl has an acidic
function for the hydrolysis, while [AMIM]Cl is a neutral ionic liquid.
3.2. Effect of temperature on the depolymerisation of potato
starch
Temperature also plays an important role in the efficiency of
depolymerisation of starch. In order to compare the results with
our previous study performed in an aqueous acidic medium [2], the
depolymerisation of the three starch-based starting materials was
performed in an ionic liquid medium ranging between 60 and 90 ◦C.
Temperature has an effect on the viscosity of the ionic liquids by
Table 2
Yields of reducing sugars of the depolymerisation of the three starting materials (PS for potato starch, WPS for wet potato sludge and DPS for dry potato sludge).
Experiment
Raw materials
Techniques
Wt%e
Temperature (◦C)
YieldTRS (%)
1
2
3
4
5
6
7
8
[SBMIM]Cl–PS
[SBMIM]Cl–PS
[SBMIM]Cl–PS
[SBMIM]Cl–PS
[SBMIM]Cl–WPS
[SBMIM]Cl–DPS
[SBMIM]Cl–PS
[SBMIM]Cl–WPS
[SBMIM]Cl–DPS
[SBMIM]Cl–PS
[SBMIM]Cl–WPS
[SBMIM]Cl–DPS
[SBMIM]Cl–PS
[SBMIM]Cl–WPS
[SBMIM]Cl–DPS
[AMIM]Cl–PS
Mech. stir.a
Mech. stir.a
Mech. stir.a
Mech. stir.a
Mech. stir.a
Mech. stir.a
Microwaveb
Microwaveb
Microwaveb
US-LFc
10
60
70
80
90
80
80
60
60
60
60
60
60
80
80
80
60
80
80
60
60
60
6
10
54
22
32
78
61
19
67
9
10
10
10
10
10
10
10
10
10
10
10
20
20
20
10
10
20
3
9
10
11
12
13
14
15
16
17
18
19d
20d
21d
US-LFc
5
15
6
US-LFc
Mech. stir.a
Mech. stir.a
Mech. stir.a
Mech. stir.a
Mech. stir.a
Mech. stir.a
Mech. stir.a
Mech. stir.a
Mech. stir.a
4
11
12
6
6
36
9
[AMIM]Cl–PS
[AMIM]Cl–PS
H2SO4 3M–PS
H2SO4 3M–WPS
H2SO4 3M–DPS
3
3
29
a
Mechanical stirring with hot plate stirrer.
60 min of irradiation.
Ultrasound low frequency (24 kHz ultrasonic bath).
Previous research [4].
b
c
d
e
Weight percentage of starting material/ionic liquid.
Please cite this article in press as: A. Hernoux-Villière, et al., Task-specific ionic liquid for the depolymerisation of starch-based industrial waste
into high reducing sugars, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.09.027

A. Hernoux-Villière et al. / Catalysis Today xxx (2013) xxx–xxx
5
Fig. 3. Effect of dissolution temperature on the depolymerisation of the three start-
ing materials in 10 wt% [SBMIM]Cl for 120 min under mechanical stirring.
Fig. 4. Yields of the total reducing sugars obtained by the depolymerisation tech-
niques of the three starting materials in 10/12 wt% [SBMIM]Cl at 60 ◦C for 120 min
(60 min for microwave).
decreasing it [44]. The use of an ionic liquid allows work to be con-
ducted at higher operating temperatures than those used in aque-
ous sulphuric acid. Indeed, in the latter, the starch easily undergoes
gelatinisation at around 65 ◦C, making any further transformation
difficult. Reactions were performed in 20% (w/w) of water on a
10 wt% solution of all three starch-based materials in [SBMIM]Cl.
Temperature effect on the depolymerisation is shown in Fig. 3. It has
been previously shown that [SBMIM]Cl possess a higher ability to
dissolve cellulose than neutral ionic liquids at 100 ◦C [21]. Whatever
the nature and composition of the starch-based material, the high-
est TRS yield was obtained at 80 ◦C. 54% of potato starch was con-
verted into reducing sugars at 80 ◦C (Table 2, entry 3), which corrob-
orates well with the results obtained by Amarasekara and Owereh
[21] on the hydrolysis of cellulose with an identical ionic liquid.
The depolymerisation under microwave irradiation offered the
highest TRS content within 60 min regardless of the starting mate-
rial. Due to their strong polar character, ionic liquids are a very
suitable medium for microwave irradiation. This is confirmed
by the fact that for potato starch, a temperature of 60 ◦C was
high enough to generate engaging amounts of reducing sugars.
However, the brown aspect of the solution after microwave irra-
diation of the two other starting materials could be explained
by the caramelisation reaction. Caramelisation of short-chain or
monomeric sugars is known as the Maillard reaction. This was also
observed by Lajunen et al. [45] for the depolymerisation of bar-
ley starch in imidazolium-based ionic liquids under microwave
irradiation. An appropriate Plexiglas helix-ended rod was intro-
duced into the microwave reactor to limit the effect of thermal
gradient and local hot spots, but this remained inefficient and
could not attenuate caramelisation. However, the combination of
rapid heating in an ionic liquid medium increased the yield of
reducing sugars whilst reducing the reaction time. A set temper-
ature can be reached in a really short time through consecutive
rotation of the ionic molecules. This renders the combination of
microwave heating/ionic liquid as very attractive. For all raw mate-
rials, the total reducing sugars reached 3–10-fold under microwave
irradiation than with conventional heating in similar conditions.
Microwave technology has previously been employed for the con-
version of cellulose into reducing sugars or 5-HMF in ionic liquids
[46,47,48,49] or for the production of furfural from sugars with
Brønsted-acidic ionic liquids [50]; no reports exist for starch in
ionic liquids conditions. The conversion of cellulose into reducing
sugars reached 48% in only 8 min of irradiation with a HY zeo-
lite catalyst at 180 ◦C [46]. In this study, 61% of potato starch was
converted into reducing sugars under microwave irradiation with
the Brønsted-acidic ionic liquid, and only 4% using conventional
heating.
3.3. Comparison of mechanical stirring with microwave and low
frequency ultrasound irradiations
Microwave and ultrasound irradiations may enhance the
hydrolysis of carbohydrates into sugars due to their own specific
effects. With microwave irradiation, a reaction media is heated
from the inner to the outer layer and can reduce the reaction time
from hours to minutes [2]. Low frequency ultrasound irradiation
generates shock waves, which allow an efficient stirring of the reac-
tion medium and increase the total reducing sugar content [2].
Unfortunately, due to the utilisation of the ultrasound bath, the
hydrolysis could not be performed at the optimum temperature
(80 ◦C) determined in the previous section with the Brønsted-acidic
ionic liquid. Filled with distilled water, maintaining such a high
and constant temperature without changing some key parameters
of the ultrasound is particularly difficult. At 80 ◦C, a parasite phe-
nomenon called ‘vaporous cavitation’ can appear and dramatically
decreases the efficiency of acoustic cavitation. Natural bubbles of
vaporous water appear, displaying a much higher diameter than
cavitation bubbles. The latter can undergo coalescence with the
former, leading to a dramatic decrease or even the suppression of
the necessary mechanical effects brought up by the collapse of cav-
itation bubbles, expected to contribute to depolymerisation. The
raw materials were thus irradiated for 120 min at 60 ◦C with a
synthesis microwave and a low frequency ultrasonic bath for com-
parison with conventional stirring; results are shown in Fig. 4. Even
if the temperature has been decreased, a high loss of efficiency can
be observed with this indirect mode irradiation; the energy being
dissipated in the water bath and only 9%, 5% and 15% of reducing
sugars of potato starch, wet potato sludge and dry potato sludge,
respectively, were reached. An ultrasonic bath may not be powerful
enough to allow the mixing of a highly heterogeneous and viscous
system that would require the use of an ultrasonic probe, directly
immersed in the solution for a direct irradiative mode. Our previ-
ous research performed using a sulphuric acid medium provided
similar results [2].
3.4. Effect of water-content for the depolymerisation of starch in
[SBMIM]Cl
Ionic liquids display natural high viscosities (i.e. the
viscosity of 1-butyl-3-methylimidazolium iodine [BMIM]I, 1-
butyl-3-methylimidazolium tetrafluoroborate [BMIM]BF4, and
1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
[BMIM]NTf2 are 400, 280 and 50 MPa s, respectively [51,52]); the
addition of a certain weight percentage of starch-based material
renders the solution even more viscous, making it difficult to
stir reacting solution. As a typical example, a 20 wt% of raw
material/ionic liquid series was performed with 10% (w/w) of
water added at 80 ◦C for 120 min. The low TRS yield of 6%, 4% and
11% for potato starch, wet potato sludge and dry potato sludge,
respectively, is observed due to the mass transfer limitations
Please cite this article in press as: A. Hernoux-Villière, et al., Task-specific ionic liquid for the depolymerisation of starch-based industrial waste
into high reducing sugars, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.09.027

6
A. Hernoux-Villière et al. / Catalysis Today xxx (2013) xxx–xxx
Acknowledgements
The authors would like to acknowledge the Academy of Finland
(Project No. 124331), the French Rhône-Alpes Region for their
financial support and the Fortum Foundation for awarding a Grant
to A. Hernoux. Furthermore, the authors would like to thank Mr.
Jaakko Pulkkinen for the preparation and optimisation of the ionic
liquids. UBIOCHEM Cost Action is gratefully acknowledged.
References
[1] J.H. Clark, F. Deswarte, Introduction to Chemicals from Biomass, John Wiley &
Sons, 2011.
[2] A. Hernoux, J.-M. Lévêque, U. Lassi, S. Molina-Boisseau, M.-F. Marais, Conversion
of a non-water soluble potato starch waste into reducing sugars under non-
conventional technologies, Carbohydrate Polymers 92 (February (2)) (2013)
2065–2074.
[3] A.L. Morehouse, R.C. Malzahn, J.T. Day, Hydrolysis of Starch, US366336916 A,
May 1972.
Fig. 5. Yields of total reducing sugars produced during the depolymerisation of
potato starch and wet potato sludge in aqueous [SBMIM]Cl (20-wt% solution–0.50 g
of ionic liquid, 0.10 g of dry raw materials). Solutions were stirred for 120 min at
80 ◦C.
[4] V. Myllymaki, R. Aksela, Depolymerization Method, US 2009/0032015 A127,
September 2006.
[5] J. Singh, L. Kaur, O.J. McCarthy, Factors influencing the physico-chemical, mor-
phological, thermal and rheological properties of some chemically modified
starches for food applications—a review, Food Hydrocolloids 21 (January (1))
(2007) 1–22.
[6] J. Shen, H. Wang, H. Liu, Y. Sun, Z. Liu, Brønsted acidic ionic liquids as dual
catalyst and solvent for environmentally friendly synthesis of chalcone, Journal
of Molecular Catalysis A: Chemical 280 (February (1–2)) (2008) 24–28.
[7] A.R. Hajipour, F. Rafiee, Acidic bronsted ionic liquids, Organic Preparations and
Procedures International 42 (4) (2010) 285–362.
with high weight percentage. In addition, the presence of water
is required to dissociate the sulfonic acid group to enable the
acidic depolymerisation of the starch molecules. We subsequently
studied the impact of the added water on the depolymerisation
rate of 2 of the 3 starch materials used in this study, the native
potato starch and the wet potato sludge. The increase of weight
percentage of the ratio between raw material/ionic liquid and the
water content are important factors for scaling up. As shown in
Fig. 5, the optimum weight percentage of H2O was 33% for wet
potato sludge to reach 43% of reducing sugars whilst 45% (w/w)
of H2O was necessary for a total hydrolysis of potato starch. One
molecule of water is consumed for every broken glycosidic bond of
starch; therefore water is required for the hydrolysis. The results
confirm that water can improve the hydrolysis reaction of starch
into sugars in a Brønsted-acidic ionic liquid, which corroborates
with previous reports [53]. The authors suggested that aqueous
Brønsted-acidic ionic liquids promoted the attack of the glycosidic
bonds of cellulose for its conversion into ␣-glucose. A total hydrol-
ysis of potato starch was achieved probably because ␣-glycosidic
bonds are easily cleaved compare to ␤-glycosidic bonds (cellulose).
The aqueous ionic liquid was able to dissolve potato starch, whilst
the key to the hydrolysis of the Brønsted-acidic function is in the
form of a superacid and may be considered as a simple hydrolysis.
[8] K. Wilpiszewska, T. Spychaj, Ionic liquids: media for starch dissolution, plas-
ticization and modification, Carbohydrate Polymers 86 (August (2)) (2011)
424–428.
[9] C. Chiappe, D. Pieraccini, Ionic liquids: solvent properties and organic reactivity,
Journal of Physical Organic Chemistry 18 (4) (2005) 275–297.
[10] M.E. Zakrzewska, E. Bogel-Łukasik, R. Bogel-Łukasik, Solubility of carbohydrates
in ionic liquids, Energy & Fuels 24 (February (2)) (2010) 737–745.
[11] R.C. Remsing, R.P. Swatloski, R.D. Rogers, G. Moyna, Mechanism of cellulose dis-
solution in the ionic liquid 1-n-butyl-3-methylimidazolium chloride: a 13C and
35/37Cl NMR relaxation study on model systems, Chemical Communications
(March (12)) (2006) 1271–1273.
[12] Q. Ren, J. Wu, J. Zhang, J. He, M. Guo, Synthesis of 1-ally1 3-methylimidazolium-
based room-temperature ionic liquid and preliminary study of its dissolving
cellulose, Acta Polymerica Sinica (3) (2003) 448–451.
[13] F. Guo, Z. Fang, C.C. Xu, R.L. Smith Jr., Solid acid mediated hydrolysis of biomass
for producing biofuels, Progress in Energy and Combustion Science 38 (October
(5)) (2012) 672–690.
[14] K. Lappalainen, J. Kärkkäinen, J. Panula-Perälä, M. Lajunen, K. Lappalainen, J.
Kärkkäinen, J. Panula-Perälä, M. Lajunen, Preparation of water-soluble starch
oligomers from variable starch species in 1-allyl-3-methylimidazolium chlo-
ride, Starch 64 (April (4)) (2012) 263–271.
[15] A.C. Cole, J.L. Jensen, I. Ntai, K.L.T. Tran, K.J. Weaver, D.C. Forbes, J.H. Davis, Novel
brønsted acidic ionic liquids and their use as dual solvent − catalysts, Journal
of the American Chemical Society 124 (May (21)) (2002) 5962–5963.
[16] R. Sugimura, K. Qiao, D. Tomida, C. Yokoyama, Immobilization of acidic ionic
liquids by copolymerization with styrene and their catalytic use for acetal
formation, Catalysis Communications 8 (May (5)) (2007) 770–772.
[17] K.E. Johnson, R.M. Pagni, J. Bartmess, Brønsted acids in ionic liquids: fundamen-
tals, organic reactions, and comparisons, Monatshefte für Chemie/Chemical
Monthly 138 (11) (2007) 1077–1101.
[18] X. Liu, H. Ma, Y. Wu, C. Wang, M. Yang, P. Yan, U. Welz-Biermann, Esterification
of glycerol with acetic acid using double SO3H-functionalized ionic liquids as
recoverable catalysts, Green Chemistry vol. 13 (March (3)) (2011) 697–701.
[19] L. Myles, N. Gathergood, S.J. Connon, The catalytic versatility of low toxicity
dialkyltriazolium salts: in situ modification facilitates diametrically opposed
catalysis modes in one pot, Chemical Communications 49 (May (46)) (2013)
5316–5318.
[20] C. Hardacre, J.D. Holbrey, M. Nieuwenhuyzen, T.G.A. Youngs, Structure and sol-
vation in ionic liquids, Accounts of Chemical Research 40 (November (11))
(2007) 1146–1155.
4. Conclusion
In this study, we optimised the parameters to dissolve and
depolymerise a starch-based industrial waste in ionic liquids.
[AMIM]Cl appeared to be more suitable for the dissolution of potato
starch due to the imidazolium ring and the chloride anion. How-
ever, [SBMIM]Cl dissolved potato starch and depolymerised the
starting materials into reducing sugars in one step in an aque-
ous Brønsted-acidic medium. [SBMIM]Cl played the role of dual
solvent/catalyst and followed the requirements of the sustain-
able chemistry. Temperature acted as a relevant factor for the
depolymerisation of starch in conventional heating. The yield of
reducing sugars under the optimum conditions (conventional heat-
ing in aqueous [SBMIM]Cl–33% (w/w) of H2O, a solution of 20 wt%,
120 min of stirring at 80 ◦C), reached 43% for a complex wet
matrix–wet potato sludge. Overall, water disrupts the dissolution
process of carbohydrate in ionic liquids, but the method described
herein generated the greatest yield of reducing sugars. The addition
of water overcomes the high viscosity of a 20 wt% solution. Finally,
microwaves only appear to reduce the reaction time by reaching
the required temperature in a short time period.
[21] A.S. Amarasekara, O.S. Owereh, Hydrolysis and decomposition of cellulose in
bronsted acidic ionic liquids under mild conditions, Industrial and Engineering
Chemistry Research 48 (22) (2009) 10152–10155.
[22] A.S. Amarasekara, B. Wiredu, Degradation of cellulose in dilute aqueous solu-
tions of acidic ionic liquid 1-(1-propylsulfonic)-3-methylimidazolium chloride,
and p-toluenesulfonic acid at moderate temperatures and pressures, Industrial
and Engineering Chemistry Research 50 (21) (2011) 12276–12280.
[23] A.S. Amarasekara, B. Wiredu, Brönsted acidic ionic liquid 1-(1-propylsulfonic)-
3-methylimidazolium-chloride catalyzed hydrolysisof d-cellobiose in aqueous
medium, International Journal of Carbohydrate Chemistry 2012 (2012) 1–6.
[24] T.J. Mason, J.P. Lorimer, Applied Sonochemistry: The Uses of Power Ultrasound
in Chemistry and Processing, Wiley-VCH, 2002.
Please cite this article in press as: A. Hernoux-Villière, et al., Task-specific ionic liquid for the depolymerisation of starch-based industrial waste
into high reducing sugars, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.09.027

A. Hernoux-Villière et al. / Catalysis Today xxx (2013) xxx–xxx
7
[25] A. Loupy, Microwaves in Organic Synthesis, vol. 1, second, completely revised
and enlarged ed., Wiley-CH, 2006.
[26] A. Hernoux-Villière, Catalytic Depolymerisation of Starch-Based Industrial
Waste: Use of Non-Conventional Activation Methods and Novel Reaction
Media, University of Oulu – Université de Savoie, 2013.
[27] H. Zhang, J. Wu, J. Zhang, J. He, 1-Allyl-3-methylimidazolium chloride room
temperature ionic liquid: a new and powerful nonderivatizing solvent for cel-
lulose, Macromolecules 38 (October (20)) (2005) 8272–8277.
[40] K.R. Seddon, A. Stark, M.J. Torres, et al., Influence of chloride, water, and organic
solvents on the physical properties of ionic liquids, Pure and Applied Chemistry
72 (12) (2000) 2275–2287.
[41] J.S. Wilkes, P. Wasserscheid, T. Welton, Introduction, in: P. Wasserscheid, T.
Welton (Eds.), Ionic Liquids in Synthesis, Wiley-VCH Verlag GmbH & Co. KGaA,
2008, pp. 1–6.
[42] J.H. Davis, Task-specific ionic liquids, Chemistry Letters 33 (9) (2004)
1072–1077.
[28] N. Srivastava, M. Shukla, S. Saha, An unusal effect of charcoal on the purification
of alkylimidazolium iodide room temperature ionic liquids, Indian Journal of
Chemistry. Section A, Inorganic, Bio-Inorganic, Physical, Theoretical & Analyt-
ical Chemistry 49 (5) (2010) 757–761.
[29] J. Gui, X. Cong, D. Liu, X. Zhang, Z. Hu, Z. Sun, Novel brønsted acidic ionic liquid
as efficient and reusable catalyst system for esterification, Catalysis Communi-
cations 5 (September (9)) (2004) 473–477.
[30] C.M. Gordon, M.J. Muldoon, M. Wagner, C. Hilgers, J.H. Davis, P. Wasserscheid,
Synthesis and purification, in: P. Wasserscheid, T. Welton (Eds.), Ionic Liquids
in Synthesis, Wiley-VCH Verlag GmbH & Co. KGaA, 2008, pp. 7–55.
[31] G.L. Miller, Use of dinitrosalicylic acid reagent for determination of reducing
sugar, Analytical Chemistry 31 (March (3)) (1959) 426–428.
[32] S. Ramesh, R. Shanti, E. Morris, Plasticizing effect of 1-allyl-3-
methylimidazolium chloride in cellulose acetate based polymer electrolytes,
Carbohydrate Polymers 87 (March (4)) (2012) 2624–2629.
[33] T. Peppel, M. Köckerling, Imidazolium-based zwitterionic butane-1-sulfonates:
synthesis and properties of 4-(1-(2-cyanoethyl)imidazolium)butane-1-
sulfonate and crystal structures of 4-(1-alkylimidazolium)butane-1-sulfonates
(alkyl = methyl, ethyl, propyl), Zeitschrift für anorganische und allgemeine
Chemie 637 (7–8) (2011) 870–874.
[34] Q. Xu, J.F. Kennedy, L. Liu, An ionic liquid as reaction media in the ring opening
graft polymerization of [epsilon]-caprolactone onto starch granules, Carbohy-
drate Polymers 72 (April (1)) (2008) 113–121.
[43] R. Giernoth, Task-specific ionic liquids, Angewandte Chemie International Edi-
tion 49 (16) (2010) 2834–2839.
[44] H. Weingärtner, Understanding ionic liquids at the molecular level: facts,
problems, and controversies, Angewandte Chemie International Edition 47 (4)
(2008) 654–670.
[45] K. Lappalainen, J. Kärkkäinen, M. Lajunen, Dissolution and depolymerization of
barley starch in selected ionic liquids, Carbohydrate Polymers 93 (March (1))
(2013) 89–94.
[46] Z. Zhang, Z.K. Zhao, Solid acid and microwave-assisted hydrolysis of cellulose
in ionic liquid, Carbohydrate Research 344 (October (15)) (2009) 2069–2072.
[47] Z. Zhang, Z.K. Zhao, Microwave-assisted conversion of lignocellulosic biomass
into furans in ionic liquid, Bioresource Technology 101 (February (3)) (2010)
1111–1114.
[48] C. Li, Z. Zhang, Z.K. Zhao, Direct conversion of glucose and cellulose to
5-hydroxymethylfurfural in ionic liquid under microwave irradiation, Tetra-
hedron Letters 50 (September (38)) (2009) 5403–5405.
[49] X. Qi, M. Watanabe, T.M. Aida, R.L. Smith, Fast transformation of glucose and
di-/polysaccharides into 5-hydroxymethylfurfural by microwave heating in an
ionic liquid/catalyst system, ChemSusChem 3 (9) (2010) 1071–1077.
[50] J.C. Serrano-Ruiz, J.M.C. Perez, M. Francavilla, C. Menendez, A.B. Garcia, A.A.R.
REYES, R. Luque, E.J. Garcia-Suarez, Efficient microwave-assisted production of
furfural from C5 sugars in aqueous media catalysed by Brönsted acidic ionic
liquids, Catalysis Science & Technology 2 (2012) 1828–1832.
[35] W. Ning, Z. Xingxiang, L. Haihui, H. Benqiao, 1-Allyl-3-methylimidazolium
chloride plasticized-corn starch as solid biopolymer electrolytes, Carbohydrate
Polymers 76 (3) (2009) 482–484.
[36] H.E. Gottlieb, V. Kotlyar, A. Nudelman, et al., NMR chemical shifts of common
laboratory solvents as trace impurities, Journal of Organic Chemistry 62 (21)
(1997) 7512–7515.
[37] F. Tao, H. Song, L. Chou, Catalytic conversion of cellulose to chemicals in ionic
liquid, Carbohydrate Research 346 (January (1)) (2011) 58–63.
[38] M. Mazza, D.-A. Catana, C. Vaca-Garcia, C. Cecutti, Influence of water on the dis-
solution of cellulose in selected ionic liquids, Cellulose 16 (2) (2009) 207–215.
[39] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis,
Chemical Reviews 99 (1999) 2071–2084.
[51] K.-S. Kim, D. Demberelnyamba, B.-K. Shin, S.-H. Yeon, S. Choi, J.-H. Cha, H. Lee, C.-
S. Lee, J.-J. Shim, Surface tension and viscosity of 1-butyl-3-methylimidazolium
iodide and 1-butyl-3-methylimidazolium tetrafluoroborate, and solubility of
lithium bromide+1-butyl-3-methylimidazolium bromide in water, Korean
Journal of Chemical Engineering 23 (January (1)) (2006) 113–116.
[52] J.N. Canongia Lopes, M.F. Costa Gomes, P. Husson, A.A.H. Pádua, L.P.N. Rebelo, S.
Sarraute, M. Tariq, Polarity, viscosity, and ionic conductivity of liquid mixtures
containing [C4C1im][Ntf2] and a molecular component, Journal of Physical
Chemistry B 115 (May (19)) (2011) 6088–6099.
[53] Z.-D. Ding, J.-C. Shi, J.-J. Xiao, W.-X. Gu, C.-G. Zheng, H.-J. Wang, Catalytic con-
version of cellulose to 5-hydroxymethyl furfural using acidic ionic liquids and
co-catalyst, Carbohydrate Polymers 90 (October (2)) (2012) 792–798.
Please cite this article in press as: A. Hernoux-Villière, et al., Task-specific ionic liquid for the depolymerisation of starch-based industrial waste
into high reducing sugars, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.09.027