Y. Zhang et al. / Carbohydrate Polymers 117 (2015) 666–672
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1-allyl-3-methylimidazolium formate ([Amim][HCOO]) and
1-allyl-3-ethylimidazolium formate ([Aeim][HCOO]) had high
cellulose solubility under mild condition. Among these ILs,
[Amim][HCOO] was the best cellulose solvent, solubility of cellu-
lose in this IL increased from about 11 wt% at 60 ◦C to 22 wt% at
solubilization capacity of the ILs for cellulose was also signifi-
cantly enhanced (Chen, Liu, Li, Zhang, & Deng, 2011; Tang, Baker,
Ravula, Jonesc, & Zhao, 2012; Xu, Wang, & Wang, 2010; Zhao,
Greiner, & Leitner, 2012). Encouraged by these results and the inter-
esting work mentioned above for the dissolution of cellulose in
[Amim][HCOO], we decided to study the structure–property rela-
tionship of allyl imidazolium carboxalates in room temperature
dissolution of cellulose. The contribution of this type of knowledge
is undoubtedly helpful for the future design of novel and superior
cellulose solvent systems.
(Zhang et al., 2005). Briefly, 1-methylimidazole and allyl chloride
were added, at a molar ratio of 1:1.25, into a round-bottomed flask
8 h at 55 ◦C with stirring. The product [Amim]Cl was obtained by
sure. Then, in order to obtain [Amim][OH], an aqueous solution of
[Amim]Cl was allowed to pass through a column filled with anion
exchange resin (Ambersep IRA 900-OH) (Fukaya et al., 2006; Xu
et al., 2010). Finally, the [Amim][OH] aqueous solution was neu-
tralized with an equimolar quantity of formic acid. After removing
water by evaporation under reduced pressure, the viscous liquid
[Amim][HCOO] was thoroughly washed with diethyl ether, and
dried under vacuum for 48 h at 70 ◦C in the presence of P2O5. The
other ILs were prepared by a similar process as described for the
preparation of [Amim][HCOO].
2.3. Measurements of 1H NMR and impurity content of the ILs
1H NMR spectra of the ILs were collected at room tempera-
ture from a Bruker Avance-400 NMR spectrometer operating at
400.13 MHz. The solutions used for the 1H NMR experiments were
prepared by adding a given amount of IL into a 5 mm NMR tube,
ILs were reported in the supporting information.
For this purpose,
a series of 1-allyl-3-methylimidazolium
carboxylate ILs: [Amim][HCOO], [Amim][CH3COO], [Amim]
[CH3CH2COO], [Amim][CH3CH2CH2COO], [Amim][HOCH2COO],
[Amim][CH3CHOHCOO] and [Amim][C6H5COO] were synthesized
and characterized in this work. Thermal properties and viscosities
of these ILs were determined, and solubility of cellulose in the
resulting ILs was investigated as a function of temperature. The
effects of anionic structure of the ILs on the viscosities, thermal
properties and cellulose solubilities were evaluated. In order to
understand the high solubility of cellulose in these ILs, solva-
tochromic UV/vis probe technique was used to study hydrogen
bond accepting ability of the ILs toward protons of the hydroxyl
groups of cellulose. Meanwhile, [Amim][CH3COO] was chosen as
a representative IL to investigate the interactions of the ILs with
cellulose in the mixture of cellulose + [Amim][CH3COO] by 13C
NMR measurements. Some important information was provided
for the rational design of novel IL solvents of cellulose.
Since impurities existing in the ILs, such as water and Cl−,
would greatly affect their physicochemical properties (Jacquemin,
Husson, Padua, & Majer, 2006; Seddon, Stark, & Torres, 2000),
the water and Cl− contents in the investigated ILs were deter-
a PCl-1 chloride-selective electrode (Shanghai Precision & Scien-
tific Instrument Co., Ltd). It was found that 329–558 ppm of water
and 291–532 ppm of chloride were remained in the ILs reported
here. Zhao et al. (2012) reported that for the dissolution of cellu-
lose in [C2mim][CH3COO], water content of lower than 2500 ppm
and chloride content of lower than 1192 ppm did not affect cellu-
lose solubility. This suggests that the ILs prepared in this work are
qualified for the cellulose solubility measurements.
2. Experimental
2.1. Materials
2.4. Determination of thermal properties and viscosities of the ILs
1-Methylimidazole (>99%) was purchased from Shanghai
Chem. Co. Ltd. Anion exchange resin (Ambersep 900-OH) and
4-nitroaniline (98.0%) were purchased from Alfa Aesar. N,N-
Diethyl-4-nitroaniline (97.0%) was obtained from Beijing Chem. Co.
Allyl chloride (>98%), butyric acid (>98%), benzoic acid (>99.5%) and
glycollic acid (>98%) were the products of Aladdin Chem. Co. Ltd.
Formic acid (>88.0%), acetic acid (>99.5%), propanoic acid (>99%)
and lactic acid (85.0–90.0%) were purchased from Tianjin Kermel
Chem. Co. Ltd. Sodium dicyanamide (>98%) and methanol (>99.8%)
were obtained from Nanjing Chemlin Chem. Co. Ltd. and Sinopharm
Chem. Co. Ltd., respectively. Deuterated dimethyl sulfoxide (DMSO-
d6) was purchased from Qingdao Weibo Tenglong Technol. Co. Ltd.
These reagents were used without further purification.
Glass transition temperatures of the ILs were determined on
a differential scanning calorimeter (Netzsch DSC 204 F1). Each
IL sample was sealed in aluminum pans and heated at a rate of
5 ◦C min−1 under N2 atmosphere in the temperature range from
−130 ◦C to 30 ◦C. Thermogravimetric analysis (TGA) was recorded
on a thermal analyzer (Netzsch STA 449C) using alumina crucibles
with 10 wt% mass loss at a heating rate of 10 ◦C min−1 under N2
atmosphere, and all the IL samples were heated from room tem-
perature to 600 ◦C.
Viscosities of the ILs were measured by a suspended level Ubbe-
lohde viscometer, which was placed in a water thermostat (Schott,
(Schott, Germany), and the temperature uncertainty was 0.01 K.
The viscometer was calibrated with known viscosities of pure
water. Viscosities for water at different temperatures were taken
from literature (James, Mulcahy, & Steel, 1984). Flow time mea-
surements were performed by a Schott AVS310 photoelectric time
unit with a resolution of 0.01 s. The estimated uncertainty of the
experimental viscosity was lower than 2.0%. Viscosities of the ILs
were given by the following equation
a viscosity-average
Aesar, and used directly after being dried at 60 ◦C under vacuum for
10 h. The degree of polymerization of cellulose was determined by
using an Ubbelohde viscometer in cupriethylenediamine hydrox-
ide solutions (Xu et al., 2010).
2.2. Synthesis of the ILs
ꢀ
ꢁ
Ct − K
1-Allyl-3-methylimidazolium chloride ([Amim]Cl) was pre-
pared and purified by using the procedure reported in literature
=
(2.1)
t