212
A. Xu et al. / Journal of Molecular Liquids 197 (2014) 211–214
Deuterated DMSO (DMSO-d6) used for NMR samples was purchased
from Qingdao Weibo Tenglong Technol. Co., Ltd. The above materials
were used as received.
[C4mim][CH3CH2COO] is the most efficient for the dissolution of
cellulose. Generally, solubility of the cellulose in these ILs decreases
in the order: [C4mim][CH3CH2COO] N [C4mim][CH3COO] N [C4mim]
[CH3(CH2)2COO] N [C4mim][HCOO]. Moreover, it is interesting to
find that replacement of H in [HCOO]− anion of [C4mim][HCOO] by the
electron-donating groups such as CH3, CH2CH3, or (CH2)2CH3 leads to
the increased cellulose solubility. This may be ascribed to the electron-
donating nature of CH3, CH2CH3 and (CH2)2CH3 groups, which increases
the ability of the hydrogen bond formation of [XCOO]− (X = CH3,
CH2CH3, or (CH2)2CH3) with the hydroxyl protons of cellulose, and
thus the capacity of the ILs to dissolve cellulose. This is exactly contrary
to the fact that the replacement H in carboxylate anion by electron-
withdrawing groups like OH, SH or NH2 results in decreased cellulose
solubility [12]. Therefore, to design the ILs with efficient dissolution
capacity for cellulose, the electron-donating rather than electron-
withdrawing groups in carboxylate anion would be a rational choice.
It was reported that the 1H NMR chemical shift (δ) of the proton in
the 2-position of the imidazolium ring can be used as an indicator of
the capacity of an IL to dissolve cellulose, and the larger the δ value of
the IL is, the stronger the capacity of the IL to dissolve cellulose is
[12,20,21]. To determine the capacity of the ILs to dissolve cellulose,
the δs of the ILs relative to TMS in DMSO-d6 at a concentration of
1.0 mol kg− 1 were therefore determined and presented in
Table 1. Based on the δs in Table 1, it can be reasonably concluded
that the capacity of the ILs to dissolve cellulose follows the order: [C4mim]
[CH3CH2COO] N [C4mim][CH3COO] N [C4mim][CH3(CH2)2COO] N [C4mim]
[HCOO]. This trend is precisely in consistent with that of cellulose solubility
in the investigated ILs (See Table 1). Therefore, it is easy to under-
stand why the solubility of the cellulose in these ILsdecreases in
the order: [C4mim][CH3CH2COO] N [C4mim][CH3COO] N [C4mim]
[CH3(CH2)2COO] N [C4mim][HCOO].
2.2. Synthesis of ILs
[C4mim][HCOO], [C4mim][CH3COO], [C4mim][CH3CH2COO] and
[C4mim][CH3(CH2)2COO] were synthesized and purified by using the
procedure described in the literature [13,14]. Firstly, an aqueous solu-
tion of [C4mim]Br was allowed to pass through a column filled with
anion exchange resin to obtain [C4mim][OH]. In this process, the reac-
tion was monitored by aqueous AgNO3. Once the substitution of bro-
mide for hydroxide was completed, no precipitation of AgBr could be
found. Then, the aqueous [C4mim][OH] solution was neutralized with
an equal molar quantity of carboxylic acid. After removing water by
evaporation under reduced pressure, the viscous liquid carboxylate ILs
was thoroughly washed with diethyl ether, and finally dried under vac-
uum for 72 h at 70 °C. The 1H NMR spectra of the carboxylate ILs were
determined on a Bruker Avance-400 NMR spectrometer operating at
400.13 MHz, and they were found to be in good agreement with those
reported in the literature [13,14].
2.3. Dissolution of cellulose in ILs
Dried cellulose was added into a 20 mL colorimetric tube which
contained 2.0 g of dried IL, and the tube was sealed with parafilm. The
mixture was heated and stirred at 70 °C in an oil bath (DF-101S, Gongyi
Yingyu Instrument Factory). Additional cellulose was added until the
solution became optically clear under polarization microscope (Nanjing
Jiangnan Novel Optics Co., Ltd.). When cellulose became saturated,
judged by the fact that cellulose could not be dissolved further within
2 h, its solubility (expressed by gram per 100 g of IL) at 70 °C could be
calculated from the amount of the solvent and cellulose added.
3.2. Structure and properties of the regenerated cellulose
The regenerated cellulose from [C4mim][CH3CH2COO] was charac-
terized by SEM, TGA, and FTIR spectroscopy. SEM images of the regener-
ated cellulose films from the investigated ILs are shown in Fig. 1. It can
be seen that the free surface of the dried films display homogeneous
structures, indicating a dense architecture, in which no undissolved
original cellulose can be detected. The structure of the regenerated cel-
lulose is quite similar to that reported by Wang et al. [12] and Zhang
et al. [22], but different from the porous structure regenerated from
aqueous NaOH/thiourea solution [23].
FTIR spectra of the original and the regenerated cellulose materials
from the four ILs are shown in Fig. 2. It can be seen that all the spectra
are quite similar and no new peaks are observed in the regenerated
sample. This indicates that no chemical reaction takes place during the
dissolution and regeneration processes of the cellulose. The absorption
band at 1427 cm−1 in the regenerated cellulose was assigned to the
CH2 scissoring vibration. This band was weakened and shifted to a
lower wavenumber compared to the peak at 1431 cm−1 for the original
cellulose, suggesting the destruction of an intra-molecular hydrogen
bond involving O6 [22]. A new shoulder at 990 cm−1 was observed in
the regenerated cellulose, which could be assigned to the C\O
stretching vibration in the amorphous region [24]. The O\H vibration
in the regenerated cellulose shifts to a higher wavenumber
(3427 cm−1), indicating the breaking of hydrogen bonds to some extent
[25,26]. The absorption bands in the range of 1164–1061 cm−1 are
assigned to the C\O\C stretching of the original cellulose [27].
The presence of such bands in the absorption of the regenerated
cellulose suggests that the macromolecular structure of cellulose
is not destructed after regeneration of the cellulose.
2.4. Characterization of the regenerated cellulose
Fourier transform infrared (FTIR) spectra were recorded on a
Necolet Nexus spectrometer with KBr pellets. A total of 16 scans were
taken for each sample at a resolution of 2 cm−1. Scanning electron mi-
crographs were taken with a JEOL JSM-6390LV scanning electron micro-
scope (SEM). The regenerated cellulose films in the dry state were
frozen in liquid nitrogen, immediately snapped, and then dried under
vacuum. The free surface (side in direct contact with the coagulant) of
the films were sputtered with gold, and then photographed. Thermo-
gravimetric analysis (TGA) was carried out with a NETZSCH STA 449 C
thermal analyzer using alumina crucibles. The measurements were car-
ried out under flowing N2 at a heating rate of 10 °C min−1
.
3. Results and discussion
3.1. Influence of the alkyl chain length in carboxylate anion on the solubility
of cellulose
The solubility values of MCC in the ILs at 70 °C are shown in
Table 1. It can be seen that the alkyl chain length in carboxylate anion
markedly affects the solubility of cellulose. Among the investigated ILs,
Table 1
Solubility of cellulose (wt.%) in the carboxylate ILs at 70 °C and 1H NMR chemical shift (δ)
of the carboxylate ILs in DMSO-d6 at a concentration of 1.0 mol kg−1
.
IL
Solubility (wt.%)
δ (ppm)
TGA curves for both the original cellulose and the regenerated cellu-
lose from the four ILs are shown in Fig. 3. It is noted that TGA curves of
the original cellulose and the regenerated cellulose were nearly overlap-
ped all but the regenerated cellulose from [C4mim][CH3CH2COO].
[C4mim][HCOO]
[C4mim][CH3COO]
[C4mim][CH3CH2COO]
[C4mim][CH3(CH2)2COO]
12.5
15.5
17.5
14.0
9.989
10.361
10.405
10.311
Xu, Airong
