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Cite this: Green Chem., 2012, 14, 304
www.rsc.org/greenchem COMMUNICATION
Ionic liquids from renewable biomaterials: synthesis, characterization and
application in the pretreatment of biomass
Qiu-Ping Liu,a Xue-Dan Hou,a Ning Li*b and Min-Hua Zong*b
Received 9th September 2011, Accepted 21st November 2011
DOI: 10.1039/c2gc16128a
A series of room temperature ionic liquids (ILs), in which
cholinium acts as the cation and amino acids as the anions,
were prepared via a simple and green chemical route, and
characterized. Most of the ILs dissolved lignin efficiently
and selectively (with solubilities of 140220 mg of lignin perg of IL). The solubility of xylan in these ILs (which ranged
from
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Fig. 1 DSC and TGA curves for several representative [Ch][AA] ILs.
acid-based ILs reported previously.15 Nevertheless, they are
thermally stable enough to meet the criterion of the minimum
Td of 100C for chiral ILs proposed by Wasserscheid.19 Also,
the Td showed a clear dependence on the anion structures. Of
the ILs tested, [Ch][Gly] gave the lowest Td (150C). Similarly,
elongation of the side chain and introduction of a hydroxyl or
carboxylic acid group resulted in higher Td values, whilst the
introduction of a phenyl ring exerted no effect on the Td (Table
1, entry 11) since [Ch][Phe] has almost the same Td as [Ch][Ala]
(160 vs. 159 C). The Td of [Ch][Pro], [Ch][Phe], [Ch][Gly] and
[Ch][Thr] are consistent with the previous results, but [Ch][His]
synthesized in the present work is much more stable than thatreported by Moriel (171 vs. 128 C).8,9 Andthe other basic amino
acid-containing ILs have comparable Td (163165C).
Viscosity is one of the most important factors affecting the
applications of ILs, especially as the solvents for catalysis and
extraction etc. As a result, great efforts have been made to
design and synthesize ILs with low viscosity.2022 The viscosities
of the ILs synthesized during this work are in the range of
1215640 mPas at 25 C (Table 1). An increase in the size of
the anion generally resulted in a higher viscosity, which might
be ascribed to stronger van der Waals and/or hydrogen bond
interactions.10 For example, [Ch][Gly], with the anion being the
simplest amino acid, displayed the lowest viscosity (121 mPas).
Table 1 Properties of the [Ch][AA] ILs
Entry ILs Tg/Ca Td/
Cb Viscosity/mPasc [a]20D
d
1 [Ch][Gly] -61 150 121 2 [Ch][Ala] -56 159 163 +1.003 [Ch][Ser] -55 182 402 -4.684 [Ch][Thr] -39 172 454 -1.335 [Ch][Val] -74 177 372 +8.39
6 [Ch][Leu] -47 175 476 -1.217 [Ch][Ile] -47 175 480 +4.098 [Ch][Met] -61 178 330 -2.339 [Ch][Phe] -60 160 520 -9.6210 [Ch][Trp] -12 174 5640 -13.9911 [Ch][Pro] -44 163 500 -65.4912 [Ch][Asp] -22 202 2060 -31.6013 [Ch][Glu] -18 202 2308 -10.1914 [Ch][Asn] -14 187 1903 -15.6215 [Ch][Gln] -40 203 2589 -5.2516 [Ch][Lys] -48 165 460 +4.5017 [Ch][His] -40 171 980 -9.7418 [Ch][Arg] -10 163 1002 +8.33
a Glass transition temperatures (Tg) were determined by DSC with aheating rate of 10 C min-1, after cooling samples to -70 C undernitrogen. b Decomposition temperatures (Td) were measured by TGAwith a heating rate of 5 C min-1 under nitrogen. c At 25 C. d Solutionin CH3OH (c = 2).
In addition to the molecular size of the anion, the introduction
of extra carboxylic acid or amide group substantially increased
theviscosity of theILs (Table 1, entries 1215), possiblyowing to
strong hydrogen bond interactions. For instance, high viscosities
(19032589 mPas) were recorded for [Ch][Asp], [Ch][Glu] and
their amide analogs. The ILs with basic amino acids as the
anions displayed relatively high viscosities (9801002 mPas),
with theexception of [Ch][Lys] (460 mPas). Surprisingly, among
the eighteen [Ch][AA] ILs, [Ch][Trp] was the most viscous (5640
mPas). The rheological behaviors of the ILs including shear
rate and temperature dependence of the viscosity were studied
(Fig. 1S, available as supplementary materials). The apparent
viscosity of highly viscous ILs was substantially reduced with
the increase of shear rate and temperature, whereas the apparent
viscosity of less viscous ILs like [Ch][Gly] was independent on
shear rate and temperature. High viscosity ILs, such as [Ch][Trp],
and [Ch][Asp], showed shear thinning property, suggesting the
formation of cross-linked hydrogen bond network in the ILs
or aggregation of ions with different forms, such as anion
stacking and tail aggregation.23 The increase of shear rate
and temperature disrupted the ions aggregation and cross-
linked network, thus leading to the reduction of the viscosity.24
In addition, larger anion size and stronger intermolecularforces such as van der Waals, hydrogen bond, and p-stacking
interactions also might contribute to the high viscosity.
The optical rotations, [a]20D
, were measured in methanol
solutions (Table 1). The optical rotations of some ILs were
different from those of amino acids they contained. A similar
phenomenon was reported previously by Allen et al.25
Cellulose and lignin are, respectively, the first and second
most abundant renewable organic polymers on earth; and
combined with hemicellulose, they constitute the structural
components of plants.26 Recently, cellulose and hemicellulose
have received increasing interest as feedstocks for the production
of biofuel and many platform chemicals, to combat the gradual
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Table 2 The solubility of lignin, xylan and cellulose in [Ch][AA] ILs at90 C
Entry ILsLignin(mg g-1)
Xylan(mg g-1)
Cellulose(mg g-1)
pH(5 mM)
1 [Ch][Gly] 220 76
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sincethey are synthesizedfrom environmentally friendly starting
materials. Studies to investigate these properties are ongoing in
our laboratory.
This study was supported by the National Natural Science
Foundation of China (20876059, 20906032 and 21072065), and
the Major State Basic Research Development Program 973
(2010CB732201). We are grateful to Prof. Thomas J. Smith
in Sheffield Hallam University for the helpful suggestions andhelp in language improvement, and to Prof. Jinzhu Chen in
GuangzhouInstitute of EnergyConversion for thehelp in NMR
signal assignment.
Notes and references
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