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Green Chemistry Dynamic Article Links

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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