Extraction of polysaccharides from bran with phosphonate or phosphinate-derived ionic liquids under short mixing time and low temperature

Article abstract of DOI:10.1039/c003976d

A novel series of 1-alkyl-3-methylimidazolium phosphonate and phosphinate-type ionic liquids is able to extract polysaccharides from bran, which is a renewable biomass. In particular, 1-ethyl-3-methylimidazolium phosphinate is a novel low viscosity polar ionic liquid with which one can extract a series of polysaccharides from bran without heating. The physico-chemical properties of a series of these salts are also determined, in a study of the ability of these ionic liquids to extract cellulose and other polysaccharides from bran. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c003976d

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PAPER
www.rsc.org/greenchem | Green Chemistry
Extraction of polysaccharides from bran with phosphonate or
phosphinate-derived ionic liquids under short mixing time and low
temperature†
Mitsuru Abe, Yukinobu Fukaya and Hiroyuki Ohno
Received 8th March 2010, Accepted 11th May 2010
First published as an Advance Article on the web 10th June 2010
DOI: 10.1039/c003976d
A novel series of 1-alkyl-3-methylimidazolium phosphonate and phosphinate-type ionic liquids is
able to extract polysaccharides from bran, which is a renewable biomass. In particular,
1-ethyl-3-methylimidazolium phosphinate is a novel low viscosity polar ionic liquid with which
one can extract a series of polysaccharides from bran without heating. The physico-chemical
properties of a series of these salts are also determined, in a study of the ability of these ionic
liquids to extract cellulose and other polysaccharides from bran.
energy, involving the resulting sugars, into electric energy.
Introduction
Biofuel cells based on ordinary molecular liquids, including
water, have drawbacks that include the volatility of solvents,
short lifetime of enzymes, and even algae growth.11,12 Although
ILs overcome these problems, they present others such as high
viscosity. ILs are needed that facilitate these three steps.
In the present paper we focus on the first step, the extrac-
tion of polysaccharides from biomass under mild conditions.
In a pioneering work, Rogers et al. found that 1-n-butyl-
3-methylimdiazolium chloride dissolves cellulose.13,14 Unfortu-
nately, chloride salts have high melting temperatures and high
viscosities. Energy is consequently needed in both the pre-
treatment and main processes that use these chloride salts. A
new class of room temperature ILs is desirable that can dissolve
and isolate polysaccharides, including cellulose, from biomass
with minimum energy.
Polar ILs with low melting points and relatively low viscosities
have recently been synthesized that might be capable of over-
coming the drawbacks of chloride salts. For instance, a series
of alkylimidazolium carboxylates, such as acetate and formate
salts, have been synthesized as room temperature ILs having
relatively low viscosity.15,16 Rogers et al. recently reported that
some acetate salts dissolve cellulose from wood biomass in high
concentrations.16 These works all suggest that carboxylate salt-
type ILs are good solvents for a series of polysaccharides. These
carboxylates are not particularly thermally stable, however,
which limits their usage. Our previous paper17 reported the
preparation of a series of alkylimidazolium cations coupled
with alkylsulfates, alkylsulfonates, and alkylphosphates. Of
these, 1-ethyl-3-methylimidazolium methylphosphonate was a
good candidate for cellulose processing. This IL is thermally
stable, and dissolves micro-crystalline cellulose under mild
conditions. Since there is a wide variety of commercially
available phosphonate-derived acid esters and alkylhalides, it
is a good idea to combine diverse dialkylimidazolium cations
with phosphonate-derived materials as anions in the search
for polar ILs. Below, we set out our investigations of the
extraction of polysaccharides from biomass using a series
of dialkylimidazolium salts with phosphonate-derived anions,
Energy production from renewable biomass is a new and
important undertaking. In view of the perpetual necessity for
food, it is out of the question to treat foods as a starting material
for energy production. Processes involving inedible biomass
should be developed for that purpose. Use of immediately
available plant biomass such as woods, hulls, straw and so on
is a plausible strategy for providing energy. The polysaccharides
in these categories of biomass are insoluble in water, so that a
mixture of organic solvent/water/inorganic salt has generally
been used together with heating to break the highly ordered
crystalline structure, which is stabilized by multiple hydrogen
bonds. Moreover, since polysaccharides exist as supramolecular
complexes in nature, it is almost impossible to dissolve them at
the molecular level or to extract them from biomass under mild
conditions.1 In any process to generate energy from the biomass,
the minimum energy should be used to get the process going.
At present, the energy necessary is greater than the total energy
obtained from the polysaccharides.2 This led us to seek solvents
for extracting cellulose under mild conditions.
In recent years, ionic liquids (ILs),3–5 which are low melting
point organic salts designed to be liquid below 100 ◦C or
(preferably) at room temperature, have been proposed as solvents
for biopolymers such as DNA, RNA and proteins.6–8 Some
ILs are expected to act as solvents for polysaccharides.9,10 The
processing of cellulose and other polysaccharides in ILs is a
step toward feasible and economic energy conversion systems
such as biofuel cells. Three steps are involved. The first step is
the extraction and dissolution of polysaccharides from biomass.
The second step is hydrolysis of polysaccharides into mono-, di-,
or oligo-saccharides. The third step is conversion of chemical
Department of Biotechnology, Tokyo University of Agriculture and
Technology, 2-24-16 Naka-cho, Koganei, Tokyo, 184-8588, Japan.
E-mail: ohnoh@cc.tuat.ac.jp; Fax: +81 42 388 7024
† Electronic supplementary information (ESI) available: 1H-NMR spec-
tra of ionic liquid before and after extraction. 13C-NMR, IR, and TGA
charts for the extracted materials. See DOI: 10.1039/c003976d
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paying due attention to the requirements for efficient biomass
processing under mild conditions, preferably without heating.
We also determined the physico-chemical properties of a series
of these salts, to consider their ability to extract polysaccharides,
especially at low temperatures and with short mixing times.
POCH3), 3.91 (3H, s, NCH3), 4.92 (2H, t, J = Hz, NCH2CH),
=
5.32 (2H, dd, J = 13.7, 6.0 Hz, CH CH2), 6.07 (1H, dq, J =
=
22.4, 5.6 Hz, CH2CH CH2), 6.82 (1H, d, J = 559.6 Hz, PH),
7.87 (2H, d, J = 8.7 Hz, NCHCHN), 10.69 (1H, s, NCHN).
13C-NMR (100 MHz; CDCl3; Me4Si) dC = 35.63 (NCH3), 49.41
=
(POCH3), 50.56 (NCH2CH), 119.94 (CH CH2), 122.34, 123.34
=
(NCHCHN), 131.99 (CH2CH CH2), 137.29 (NCHN).
Experimental
1-n-Propyl-3-methylimidazolium methylphosphonate (3). 1H-
NMR (400 MHz; CDCl3; Me4Si) dH = 0.95 (3H, t, J =
7.2 Hz, CH2CH3), 1.75 (2H, m, CH2CH2CH2), 3.22 (3H, d,
J = 11.4 Hz, POCH3), 3.82 (3H, s, NCH3), 4.33 (2H, t, J =
7.2 Hz, NCH2CH2), 6.78 (1H, d, J = 554.1 Hz, PH), 7.33
(2H, d, J = 31.0 Hz, NCHCHN), 10.40 (1H, s, NCHN).
13C-NMR (100 MHz; CDCl3; Me4Si) dC = 13.13 (CH2CH3),
29.34 (NCH2CH2CH3), 35.49 (NCH3), 48.10 (NCH2CH2), 49.47
(POCH3), 123.78 (NCHCHN), 123.99 (NCHCHN), 137.30
(NCHN).
Materials
1-Methylimidazole was purchased from Wako Pure Chemical
Co. Ltd. 1-Ethylimidazole, 1-n-butylimidazole, dimethyl phos-
phite, diisopropyl phosphite, diethyl phosphite, dibutyl phos-
phite and ethylbromide were purchased from Tokyo Chemical
Industry, Co. Ltd. 1-Methylimidazole and 1-ethylimidazole
were dried over KOH and distilled before use. The dimethyl
phosphite was dried over K2CO3 then CaCl2, and was distilled
before use. Ethylbromide, allylbromide, n-propylbromide and
n-butylbromide were all distilled before use. THF was purified
before use with a solvent purifying system. All other commer-
cially available chemicals were used as received.
1-n-Butyl-3-methylimidazolium methylphosphonate (4). 1H-
NMR (400 MHz; CDCl3; Me4Si) dH = 0.88 (3H, t, J = 7.1 Hz,
CH2CH3), 1.16 (2H, m, J = 15.0, 7.6 Hz, CH2CH2CH3), 1.75-
1.81 (2H, br m, CH2CH2CH2), 3.26 (3H, d, J = 11.0 Hz,
POCH3), 3.80 (3H, s, NCH3), 4.11 (2H, t, J = 7.3 Hz,
NCH2CH2), 6.55 (1H, d, J = 559.2 Hz, PH), 7.35 (2H,
d, J = 32.1 Hz, NCHCHN), 10.35 (1H, s, NCHN). 13C-
NMR (100 MHz; CDCl3; Me4Si) dC = 13.21 (CH2CH3),
18.73 (CH2CH2CH3), 31.40 (NCH2CH2CH2), 35.54 (NCH3),
48.31 (NCH2CH2), 49.32 (POCH3), 122.29 (NCHCHN), 123.57
(NCHCHN), 137.18 (NCHN).
Synthesis of ILs
We prepared 1-ethyl-3-methylimidazolium phosphinate as fol-
lows. 1-Ethyl-3-methylimidazolium bromide was dissolved into
Milli Q water, and the resulting solution was passed through
a column filled with anion exchange resin (Amberlite IR-78)
to give an aqueous solution of 1-ethyl-3-methylimidazolium hy-
droxide. This solution was roughly concentrated by evaporation.
Phosphinic acid aqueous solution was added to the aqueous
solution of 1-ethyl-3-methylimidazolium hydroxide and the re-
sulting solution was stirred for 2 h at room temperature. After re-
moval of water by evaporation, the residual liquid was repeatedly
washed with excess amounts of anhydrous diethyl ether. The re-
sultingliquidwas fully mixed with dichloromethane (100ml) and
then passed through a short column filled with active alumina
(20 ml). The dichloromethane was removed by evaporation and
the resulting liquid was dried in vacuo at 80 ◦C for 24 h, yielding
1-ethyl-3-methylimidazolium phosphinate as a colorless liquid.
1-Ethyl-3-methylimidazolium i-propylphosphonate and 1-ethyl-
3-methylimidazolium n-butylphosphonate were prepared using
the same procedures with the corresponding alkyl phosphites.
Other ILs were prepared via the procedures used in our previous
study.17 The chemical structure of 1-ethyl-3-methylimidazolium
phosphinate was confirmed by 1H-, and 13C-NMR spectra.
These spectra were observed using a JEOL ECX-400.
1-Ethyl-3-methylimidazolium ethylphosphonate (5). 1H-
NMR (400 MHz; CDCl3; Me4Si) dH = 1.07 (3H, t, J = 7.1 Hz,
OCH2CH3), 1.38 (3H, t, J = 7.3 Hz, NCH2CH3), 3.72 (3H,
d, J = 11.9 Hz, POCH2CH3), 3.89 (3H, s, NCH3), 4.25 (2H,
q, J = 7.3 Hz, NCH2CH3), 6.73 (1H, d, J = 592 Hz, PH),
7.29 (2H, d, J = 11.4 Hz, NCHCHN), 10.42 (1H, s, NCHN).
13C-NMR (100 MHz; CDCl3; Me4Si) dC = 15.43 (NCH2CH3),
16.71 (OCH2CH3), 36.21 (NCH3), 44.84 (NCH2CH3), 58.85
(OCH2CH3), 121.14 (NCHCHN), 123.09 (NCHCHN), 138.79
(NCHN).
1-Ethyl-3-methylimidazolium i-propylphosphonate (6). 1H-
NMR (400 MHz; CDCl3; Me4Si) dH
=
1.05 (6H, d,
J
= 6.4 Hz, OCH(CH3)2), 1.40 (3H, t, J = 7.3 Hz,
NCH2CH3), 3.91 (3H, s, NCH3), 4.22 (2H, q, J = 7.5 Hz,
NCH2CH3), 4.34 (1H, m, OCH(CH3)2), 6.80 (1H, d, J =
592 Hz, PH), 7.25 (2H, d, J = 10.5 Hz, NCHCHN), 10.53
(1H, s, NCHN). 13C-NMR (100 MHz; CDCl3; Me4Si) dC
=
1H- and 13C-NMR data of ILs
15.45 (NCH2CH3), 24.33 (OCH(CH3)2), 36.24 (NCH3), 44.85
(NCH2CH3), 66.41 (OCH(CH3)2), 121.08 (NCHCHN), 123.03
(NCHCHN), 139.02 (NCHN).
1-Ethyl-3-methylimidazolium methylphosphonate (1). 1H-
NMR (400 MHz; CDCl3; Me4Si) dH = 1.58 (3H, t, J = 7.3 Hz,
NCH2CH3), 3.55 (3H, d, J = 11.9 Hz, POCH3), 4.06 (3H, s,
NCH3), 4.36 (2H, q, J = 7.3 Hz, NCH2CH3), 6.92 (1H, d, J =
588.5 Hz, PH), 7.58 (2H, d, J = 11.3 Hz, NCHCHN), 10.66
(1H, s, NCHN). 13C-NMR (100 MHz; CDCl3; Me4Si) dC = 15.22
(NCH2CH3), 35.83 (NCH3), 45.22 (NCHCH3), 50.05 (POCH3),
121.35 (NCHCHN), 123.17 (NCHCHN), 138.40 (NCHN).
1-Ethyl-3-methylimidazolium n-butylphosphonate (7). 1H-
NMR (400 MHz; CDCl3; Me4Si) dH
7.5 Hz, CH2CH2CH2CH3), 1.21 (2H, m,
14.9, 7.3 Hz, CH2CH2CH2CH3), 1.42 (5H, m, J = 17.2,
4.4 Hz, CH2CH2CH2CH3, NCH3), 3.67 (2H, t,
7.0 Hz, POCH2), 3.88 (3H, s, NCH3), 4.21 (2H, q,
=
0.70 (3H, t,
J
=
J
=
J
=
1-Allyl-3-methylimidazolium methylphosphonate (2). 1H-
NMR (400 MHz; CDCl3; Me4Si) dH = 3.24 (3H, d, J = 11.4 Hz,
J
= 7.3 Hz, NCH2CH3), 6.78 (1H, d, J = 592 Hz,
PH), 7.24 (2H, d, J = 10.1, 1.8 Hz, NCHCHN), 10.48
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(1H, s, NCHN). 13C-NMR (100 MHz; CDCl3; Me4Si)
dC = 13.71 (CH2CH2CH2CH3), 15.43 (NCH2CH3), 18.98
(CH2CH2CH2CH3), 32.98 (CH2CH2CH2CH3), 36.21 (NCH3),
44.87 (NCH2CH3), 63.03 (OCH2CH2), 121.09 (NCHCHN),
123.03 (NCHCHN), 138.93 (NCHN).
Results and discussion
Effect of cation structure on the extraction of polysaccharides
from bran
We first studied the effect of the side chain that is attached to
the N-position of the imidazolium ring on the extraction of
polysaccharides from bran. Bran is the hard shell of wheat and
comprises a complex of polysaccharides and lignin, making it
a scarcely soluble material. In the extraction of polysaccharides
from bran, the hydrophobic parts of the IL should match
the lignin in inducing swelling and increasing the amount
of polysaccharides extracted. We chose four cations having
different alkyl side chain lengths in order to study the effect
of the lignin affinity in increasing the amount of polysaccharide
extracted. As shown in Chart 1, we prepared and determined
the properties of 1-ethyl-3-methyl-imidazolium (1), 1-allyl-3-
methylimidazolium (2), 1-n-propyl-3-methylimidazolium (3),
and 1-n-butyl-3-methyl-imidazolium (4) salts.
1-Ethyl-3-methylimidazolium phosphinate (8). 1H-NMR
(400 MHz; CDCl3; Me4Si) dH = 1.30 (3H, t, J = 7.4 Hz,
NCH2CH3), 3.81 (3H, s, NCH3), 4.11 (2H, q, J = 7.4 Hz,
NCH2CH3), 7.19 (2H, s, J = 384 Hz, PH2), 7.26 (2H,
d, J = 6.9 Hz, NCHCHN), 10.20 (1H, s, NCHN). 13C-
NMR (100 MHz; CDCl3; Me4Si) dC = 15.43 (NCH2CH3),
36.27 (NCH3), 44.97 (NCH2CH3), 121.13 (NCHCHN), 123.03
(NCHCHN), 138.69 (NCHN).
Physicochemical property measurements
Physico-chemical properties of these ILs were measured using
the procedures described in our previous study.17 The amount of
water contamination was measured by Karl Fischer coulomet-
ric titration (Kyoto Electronics MKC-510N). The differential
scanning calorimetry (DSC) measurements were made using a
DSC-6200 (SEIKO Inst◦ruments) at a scanning rate for heating
and for cooling of 2.5 C min-1 in the range of temperatures
from -100 to 100 ◦C. The thermogravimetric analysis (TGA)
was performed using a SEIKO TG/DTA 220 instrument with
a heating rate of 10 ◦C min-1 from 25 to 500 ◦C under nitrogen
gas. Visible spectra were observed using a Shimadzu UV 2450.
Viscosities were measured using a Brookfield DV-I+ viscometer
from 25 to 90 ◦C under dry nitrogen gas. Hydrogen bonding
ability was quantified via the Kamlet–Taft parameters.17
Chart
phonate.
1
Chemical structure of dialkylimidazolium methylphos-
All the ILs prepared here were obtained as liquids at room
temperature ILs (RTILs). Table 1 shows melting temperature
(Tm), glass transition temperature (Tg), and decomposition
temperature (Tdec) of these four ILs. Every IL has one Tg around
-80 ◦C, and no crystallization was detected while when these
ILs were stored at -20 ◦C for more than three months. It means
that every IL stably exists in a supercooled state, because of
their asymmetric structure of both the anion and cations. The
thermogravimetric analysis (TGA) showed that all these ILs
were stable up to 250 ◦C.
Extraction of polysaccharides from bran
It is necessary to design ILs with relatively strong hydrogen
bond basicity to dissolve polysaccharides. Strong hydrogen
bond basicity is effective in weakening the hydrogen-bonding
network of the polymer chains.18,19 The Kamlet–Taft parameters
(a: hydrogen bond acidity, b: hydrogen bond basicity, p*:
dipolarity/polarizability) are commonly used to specify the
hydrogen-bonding ability of the ILs as well as their polarity.17,20
Table 1 also lists the Kamlet–Taft parameters of these four
ILs. These ILs are characterized by b values greater than those
for conventional ILs. It is generally believed that the b value
depends mainly on the anion species, but also on the cation
structure.20 In considering the b value, we have already reported
Each IL (1.90 g) was mixed with bran (0.10 g, powder with
particle diameter 150 50 mm) and stirred gently at 50 ◦C or
at 25 ◦C. After stirring, the insoluble part was filtered off to
give the sample solution (A g). Excess ethanol was added to the
resulting IL solutions with stirring. After repeated washing with
hot ethanol to remove residual IL, the regenerated precipitate
was collected by filtration (B g). The degree of extraction was
calculated from the following eqn (1).
B/(A− B)×1.9
(1)
Degree of extraction (%) =
×100
0.1
Table 1 Physico-chemical properties of ILs examined in this study
Kamlet–Taft parameters at 25 ◦
C
ILs
Tg a/◦C
Tm/◦C
Tdec b/◦C
a
b
p*
1
2
3
4
-86
-82
-79
-77
—
—
—
—
278
256
277
277
0.52
0.51
0.54
0.52
1.00
0.99
1.00
1.02
1.06
1.06
1.02
1.01
a Temperature at the signal peak. b Temperature at 10% weight loss.
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that a methylphosphonate anion generates strong hydrogen
bond basicity. The series of methylphosphonate salts prepared
here also had stronger hydrogen bond basicity than conventional
ILs. The p* value of the present ILs decreased with increasing
alkyl side chain length on the cation. This fact suggests that the
p* value reflects the degree of delocalization of charge on the
cation.
We next determined the viscosity of these ILs. In the extraction
of polysaccharides from bran under mild conditions in short
periods of time, the dispersibility of the bran powder is
important. Fig. 1 shows the viscosity of the ILs prepared here.
The viscosity of these ILs depends on the cation structure. At
25 ◦C the viscosity of these salts is in the order: 1 (107 cP) < 2
(123 cP) < 3 (219 cP) < 4 (287 cP). It is generally accepted that
cation size affects the viscosity of the resulting ILs.
The resulting precipitate was collected by filtration and washed
repeatedly with ethanol to remove residual ILs.
To confirm the IL stability, we measured NMR spectra of the
IL 1 after extraction experiments. From the NMR spectra (see
ESI, Fig. S1†), methylphosphonate salt was confirmed not to be
hydrolyzed in the extraction process of polysaccharides. Next,
we have conducted characterization of the extracted materials
by FT-IR, NMR, and TGA measurements. These experiments
clarified that the extracted materials are the mixture of cellulose,
hemicellulose, and residual starch. These spectra are attached in
the ESI (Fig. S2–4).
The extent of extraction of polysaccharides from bran in these
four ILs depends on the cation structure. Fig. 3 shows the degree
of extraction of polysaccharides from bran in the present series
◦
of methylphosphonate salts at 50 C. The extent of extraction
of polysaccharides after 120 min is shown in this ordering: 4
(20%) < 3 (30%) < 2 (35%) < 1 (39%). Moreover, these ILs
dissolved polysaccharides at 50 ◦C within 10 min.
Fig. 1 Viscosity of ionic liquids evaluated here.
These physicochemical properties clearly indicate that the ILs
prepared here include potential solvents for cellulose and other
polysaccharides. Previously, the extraction of polysaccharides
from plant biomass using ILs necessitated at least 24 h of
stirring at temperatures above 100 ◦C14,16 To reduce energy loss,
extraction should be taken more rapidly at 50 ◦C or below.
To each IL (2.85 g), bran (0.15 g) was mixed (Fig. 2-1) and
Fig. 3 Degree of extraction for bran with a series of ILs.
Hydrogen bond basicity is the most important property
influencing the solubility of cellulose and other polysaccharides
in ILs. The degree of extraction using these four ILs was however
independent from their b value. Furthermore, there is no relation
between the hydrophobicity of the cations and the degree of
extraction. According to Fig. 1 and 3, lower viscosity ILs
extracted more amount of polysaccharides. This may be due
to better dispersion state of bran in less viscous ILs.
◦
stirred at 50 C. The bran in the ILs was dispersed, and some
polysaccharides were extracted within 10 min at 50 ◦C (Fig. 2-
2). We have confirmed that these ILs dissolve major component
polysaccharides of bran such as cellulose, hemicellulose, and
residual starch. The insoluble portion is expected to comprise
lignin and its complexes. After stirring of the sample solution,
the insoluble part was filtered off (Fig. 2-3), and excess ethanol
was added to the IL solutions with gentle stirring (Fig. 2-4).
Effect of anion structure on the extraction of polysaccharides
from bran
To determine the principal factor governing the degree of
extraction and find ILs better suited for extraction, we modified
the anion structure. As described above, among the phospho-
nate salts the 1-ethyl-3-methylimidazolium salt (1) has good
capacity to extract polysaccharides. As shown in Chart 2, four
1-ethyl-3-methylimidazolium salts, containing methylphospho-
nate (1), ethylphosphonate (5), i-propylphosphonate (6) or n-
butylphosphonate (7) anions have been studied in order to
Fig. 2 Extraction of polysaccharides from bran. 1: Before heating, 2:
after heating for 10 min, 3: filtrate, 4: after addition of ethanol to the
filtrate.
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Table 2 Physico-chemical properties of ILs examined in this study
Kamlet–Taft parameters at 25 ◦
C
ILs
Tg a/◦C
Tm/◦C
Tdec b/◦C
a
b
p*
1
5
6
7
-86
-79
-71
-77
—
—
—
—
278
266
256
259
0.52
0.55
0.55
0.56
1.00
1.02
1.03
1.06
1.06
1.02
1.00
0.97
a Temperature at the signal peak. b Temperature at 10% weight loss.
Chart 2 Chemical structure of alkylphosphonate salts.
investigate the effect on the degree of extraction of hydrogen
bonding ability, viscosity and ion size and characteristics.
These ILs were also obtained as RTILs. Table 2 summarizes
the physicochemical properties of these ILs. They have a glass
transition temperature at around -80 ◦C. They remained liquid
state even after storage at -20 ◦C for a month, indicating a highly
stable supercooling state. Also, TGA analysis revealed that these
ILs were stable up to 250 ◦C.
Table 2 also summarizes the Kamlet–Taft parameters for this
series of ILs. These ILs have been designed to show relatively
strong hydrogen bonding characteristics, especially the b value.
The b value of ILs is in general affected mainly by the nature of
the anion species.17,19,20 The b values of these ILs were similar,
but increased slightly in the order: 1 (b = 1.00) < 5 (b = 1.02) <
6 (b = 1.03) < 7 (b = 1.06). Since the series of anions studied
Fig. 4 Viscosity of ionic liquids evaluated here.
-
here has the same unit structure ([(R)(H)PO2 ]), the differing b
values of these ILs should be explicable by the difference in the
electron-releasing ability of the alkyl side chain. The p* values of
the ILs decreased with an increasing number of carbon atoms in
the alkyl side chain of the anion. The difference in the p* values
should therefore reflect the extent of delocalization of charge on
the anion. The proton at the 2-position of the imidazolium ring
contributes to the hydrogen bond acidity.21,22 The ILs studied
here have the same 1-ethyl-3-methylimidazolium cation, but the
a value was affected slightly by the anion structure. This should
also show in the value of p*, since the value of a depends on
p*.20
Fig. 4 shows the viscosity of these ILs. The viscosity of
these ILs clearly depends on their anion structure. This is
expected to be due to the bulkiness of the alkoxy group on
the anion. In preparing low-viscosity ILs, it is important to use
alkylphosphonate with a shorter alkyl chain.
Fig. 5 Degree of extraction of polysaccharides from bran with a series
of ILs as a function of treatment time at 50 ◦C.
was the highest for 7 and the lowest for 1. In spite of this, 1
successfully extracted more polysaccharide than 7. We infer that
for a series of IL having the b value within a certain range,
the amount of polysaccharide extracted from bran in a limited
time varies depending on the viscosity more strongly than other
parameters. Of these ILs, 7 has the largest alkyl side chain, but
7 has better capacity to extract polysaccharides than 6. The
properties and interaction force derived from size of the ion are
therefore concluded not to be the primary determinants of the
degree of extraction.
Fig. 5 shows the degree of extraction of polysaccharides from
◦
bran by this series of phosphonate-derived salts at 50 C. The
degree of extraction was influenced by the anion structure of the
IL, and the value after two hours mixing is the following order:
6 (16%) < 7 (19%) < 5 (30%) < 1 (39%).
The solubility of polysaccharides depends mainly on the
b value of the ILs. However, the degree of extraction of
polysaccharides from bran in this experiment did not directly
depend on the b value. Among these four ILs, the b value
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Table 3 Physico-chemical properties of phosphate-derived ILs
Kamlet–Taft parameters at 25 ◦
C
Ionic Liquid
Tg/◦C
Tm a/◦C
Tdec b/◦C
a
b
p*
8
—
17
260
0.52
0.97
1.09
a Temperature at the signal peak. b Temperature at 10% weight loss.
The degree of extraction of polysaccharides is closely related
to the viscosity as seen in Fig. 4. The IL with lowest viscosity had
good capacity to extract polysaccharides in short times under
mild conditions. Fig. 6 shows the relation between the extent of
polysaccharide extraction (2 h after) and IL viscosity. It can be
seen that low-viscosity ILs are able to extract a large amount of
polysaccharides at 50 ◦C.
Chart 3 Chemical structure of 1-ethyl-3-methylimidazolium phosphi-
nate (8).
and DSC analysis showed that Tm = 17 ◦C for 8. This IL
◦
was crystallized by keeping it at -20 C for several days. TGA
measurement revealed that 8 was stable up to 250 ◦C. The
Kamlet–Taft polarity parameters of 8 are also shown in Table 3.
IL 8 exhibits relatively strong hydrogen bonding basicity.
Compared to the alkylphosphonate series salts (1.00 < b < 1.06;
see Table 1), the b value of IL 8 is a little low at 0.97. We have
reported previously that 1-allyl-3-methylimidazolium formate
has essentially similar hydrogen-bonding basicity (0.99), and
that it successfully dissolved cellulose in high concentrations.15
We believe IL 8 has polarity sufficient to extract polysaccharides
from bran.
Fig. 7 shows the viscosity of both 8 and 1. IL 8 has the lowest
viscosity of all the ILs considered in this paper. The viscosity of
8 is approximately 60% of that of 1. As a result of lower viscosity
and strong hydrogen bonding properties of 8, this IL is expected
to be a good solvent for polysaccharides.
Fig. 6 Relationship between degree of extraction of polysaccharides
and inverse viscosity of a series of ILs at 50 ◦C.
For polar ILs with similar b values, the degree of extraction
over limited time depends strongly on their viscosity. ILs 1 and
6 have similar b values, of 1.00 and 1.03 for 1 and 6, respectively,
6 (h = 65 cP at 50 ◦C) needs 120 min stirring at 50 ◦C to extract
◦
ca. 15% of polysaccharides from bran, 6 (h = 65 cP at 50 C)
necessitates 2 h stirring at 50 ◦C, whereas 1 (h = 31 cP at 50 ◦C)
needs only 10 min under the same condition. The solubility of
cellulose in these ILs should be independent from their viscosity,
but solubilization of celluloses is faster in less viscous ILs. The
bran dispersed more easily in lower viscosity ILs, so that greater
solubility was observed in these under mild conditions and over
short times.
Design of a new IL and extraction of polysaccharides without
heating
Fig. 7 Temperature dependence of viscosity of the IL 8 and 1.
For ILs with adequate polarity, those with low viscosity have
good ability to extract polysaccharides in short times. We have
therefore successfully designed a novel IL having high polarity
and low viscosity. Imidazolium cations with short alkyl side
chains generally give low viscosity ILs, as we confirmed with a
short alkyl side chain of phosphonate-derived anions.
Fig. 8 shows the experimentally determined extent of ex-
traction of polysaccharides from bran using 8. As expected,
8 extracted polysaccharides better than 1. As shown in Fig. 8,
IL 8 extracted 42% of polysaccharides from bran, whereas 1
extracted only 39% at 50 ◦C with gentle stirring for 2 h.
In our previous paper, we successfully dissolved cellulose
using a series of alkylphosphonate derivative salts.17 IL 1 has
been found to dissolve a certain amount of pure cellulose
The IL, 1-ethyl-3-methylimidazolium phosphinate, hereafter
8 (Chart 3), has been newly designed. It was obtained as RTILs,
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Acknowledgements
This study was supported by a Grant-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science
and Technology of Japan (Nos. 21225007 and 17073005). This
study was also supported by the Nissan Science Foundation (No.
320142).
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Conclusions
We report that biomass processing is readily performed with
certain phosphonate-derived salts at low temperature and short
mixing times. Although the b value is held to be important in
the dissolution of polysaccharides, when the ILs have sufficient
polarity, we found that the viscosity of the ILs dominates
the ability to extract polysaccharides over limited time. We
accordingly designed a low viscosity and highly polar IL, 1-ethyl-
3-methylimidazolium phosphinate (8). This novel IL rapidly
extracted cellulose and other polysaccharides from bran under
mild conditions.
1280 | Green Chem., 2010, 12, 1274–1280
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The Royal Society of Chemistry 2010
©