Investigations about dissolution of cellulose in the 1-allyl-3- alkylimidazolium chloride ionic liquids

Article abstract of DOI:10.1016/j.carbpol.2011.08.026

In this work, the 1-allyl-3-alkylimidazolium chloride ionic liquids were synthesized and characterized by increasing carbon atoms (n ≤ 6) of alkyl chains on a cationic 3-imidazole ring. The results indicated that 1-allyl-3-alkylimidazolium chloride with asymmetrical structure on the two sides of a cationic 3-imidazole ring (i.e., n = 1, 2, 6) exhibited alkalinity and lower thermal stabilities, and showed better solubility to the cellulose samples at 60-120 °C than those with symmetrical structures (n = 3, 4). The cellulose samples treated by 20% (w/w) ethylenediamine solution showed better solubility in 1-allyl-3-ethyl, hexyl-imidazolium chloride ionic liquids than that treated with 20% (w/w) NaOH solution at 5 °C for 72 h. XRD and TG analysis indicated that 0 0 2 plane apparent crystallite size as well as thermal stability of the regenerated cellulose samples from the ionic liquids decreased significantly compared with the untreated cellulose samples.

Full text of DOI:10.1016/j.carbpol.2011.08.026

Carbohydrate Polymers 87 (2012) 1058–1064
Contents lists available at ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
Investigations about dissolution of cellulose in the 1-allyl-3-alkylimidazolium
chloride ionic liquids
De-tao Liu∗, Kun-feng Xia, Wei-hua Cai, Ren-dang Yang, Li-qin Wang, Bin Wang
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, the 1-allyl-3-alkylimidazolium chloride ionic liquids were synthesized and characterized
by increasing carbon atoms (n ≤ 6) of alkyl chains on a cationic 3-imidazole ring. The results indicated
that 1-allyl-3-alkylimidazolium chloride with asymmetrical structure on the two sides of a cationic 3-
imidazole ring (i.e., n = 1, 2, 6) exhibited alkalinity and lower thermal stabilities, and showed better
solubility to the cellulose samples at 60–120 ◦C than those with symmetrical structures (n = 3, 4). The
cellulose samples treated by 20% (w/w) ethylenediamine solution showed better solubility in 1-allyl-3-
ethyl, hexyl-imidazolium chloride ionic liquids than that treated with 20% (w/w) NaOH solution at 5 ◦C
for 72 h. XRD and TG analysis indicated that 0 0 2 plane apparent crystallite size as well as thermal sta-
bility of the regenerated cellulose samples from the ionic liquids decreased significantly compared with
the untreated cellulose samples.
Received 28 October 2010
Received in revised form 5 August 2011
Accepted 12 August 2011
Available online 31 August 2011
Keywords:
1-Allyl-3-alkylimidazolium chloride ionic
liquids
Carbon atom
Cellulose dissolution
Regenerated cellulose
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
1-allyl-3-methylimidazolium chloride (AMIMCl), thus offering a
new ionic liquid for dissolving cellulose.
The growing concern about environmental pollution from the
manufacture of synthetic polymers, biodegradable plastics, and
biocompatible composites from natural resources has become an
important technology of next-generation polymers. Cellulose is a
linear homopolymer of ␤(1 → 4)-linked d-glucopyranose (Glc) unit
that can aggregate to form a highly ordered structure due to its
chemical constitution and spatial conformation (Klemm, Philipp,
Heinze, Heinze, & Wagenknecht, 1998). The three hydroxyl groups,
C6–O6H, C3–O3H, and C2–O2H, in each Glc can form intermolecu-
lar hydrogen bonds (Oh et al., 2005). The highly ordered structure
of cellulose is the reason for its desirable mechanical proper-
ties but is also the barrier to its dissolution in common solvents.
However, in the last few years, several suitable solvents for dissolv-
ing cellulose have been found, e.g., N-methylmorpholine N-oxide
(NMMO) (Doganand & Hilmioglu, 2009), LiCl/DMAc (Nattakan,
Takashi, & Ton, 2009), and NaOH/urea (Song, Zhang, Gan, Zhou, &
Zhang, 2010). Recently, ionic liquids have been found to be suit-
able green media for dissolving cellulose by destroying hydrogen
bonds (Cao et al., 2009). Almost all ionic liquids consist of neg-
ative ions (i.e., Cl−, Br−, CH3CHOO−, NO3−, and BF4−, CF3COO−)
and positive ions (i.e., alkyl-imidazolium) (Pinkert et al., 2010). Wu
and his group (Ren, Wu, Zhang, He, & Guo, 2003; Wu et al., 2004;
Zhang, Wu, Zhang, & He, 2005) developed a novel ionic liquid,
The previous studies reported that the molecular structure of
ions in ionic liquids plays an important role in dissolving cellu-
lose (Swatloski, Spear, Holbrey, & Rogers, 2002). In this study,
1-allyl-3-methylimidazolium chloride ionic liquid (AMIMCl),
1-allyl-3-ethylimidazolium chloride ionic liquid (AEIMCl), 1-
allyl-3-propylimidazolium chloride ionic liquid (APIMCl), 1-
allyl-3-butylimidazolium chloride ionic liquid (ABIMCl), and
1-allyl-3-hexylimidazolium chloride ionic liquid (AHIMCl), which
are derivatives of increasing carbon atoms (n ≤ 6) in the alkyl chains
on a cationic 3-imidazole ring, were synthesized and investigated
for their effects on the dissolution of cellulose.
2. Material and methods
2.1. Materials
Microcrystalline celluloses (MCC) (column chromatography
grade; 99.99% purity) were purchased from Shanghai Hengxin
Chemical Reagent Co., Ltd., Shanghai, China. Bleached Kraft soft-
wood pulp (Pulp) was kindly supplied by Guangdong Eagle Force
Paper Co., Ltd., Guangzhou, China. A total of five imidazole deriva-
tives (i.e., N-methyl, ethyl, propyl, butyl, and hexyl-imidazole;
analytical reagent; 98% purity) were purchased from Shanghai
Aoke Industrial Co., Ltd., Shanghai, China. The above five imida-
zloe derivatives are purified under reduced pressure distillation
before the syntheses. Allyl chloride (RS-Aldrich; analytical reagent
grade; 98% pure) with a specific gravity of 0.935 kg/m3 was
∗
Corresponding author. Tel.: +86 18665625205; fax: +86 02087111072.
E-mail addresses: dtliu@scut.edu.cn, liudetao2003@126.com (D.-t. Liu).
0144-8617/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbpol.2011.08.026

D.-t. Liu et al. / Carbohydrate Polymers 87 (2012) 1058–1064
1059
purchased from Sigma–Aldrich (Shanghai) Trading Co., Ltd., Shang-
hai, China. The fresh deionized water was prepared through a
QHY-LO-6000 model lab water purification system equipped with
8040 Dow’s reverse osmosis membranes. This water purification
system is made in Guangdong Huankai Microbial Sci & Co., Ltd.,
Guangzhou, China.
(–N–CH2–); 3.64 (–CH2–) (a), 2.57 (–CH2–) (b), 1.09 (–CH2–) (c),
0.86 (–CH2–) (d); 0.62 (–CH3).
2.4. Dissolution of cellulose
The pulp, Pulp–NaOH, Pulp–EDA, and MCC–NaOH were used as
the cellulose samples to study their dissolution in AAIMCl ionic
liquids under argon protection. The dissolution rate of the above-
mentioned cellulose samples in the ionic liquids was investigated at
60–100 ◦C as a result of dissolution time. In the dissolving process, a
certain amount of cellulose sample was added into the ionic liquids.
After the emergence of a blackburst in the cellulose solution was
determined on a JPL-1350 polarization microscope (Guangzhou
LISS Optical Instrument Ltd., Guangzhou, China), the succeeding
amount of cellulose sample was added again into the dissolved cel-
lulose solution. The above experimental procedure was repeated
for several cyclic numbers. The solubility of cellulose correspond-
ing to dissolution times in ionic liquids for each cyclic number was
recorded. The solubility and dissolution rate of the cellulose sam-
ples in the above five species of ionic liquids were calculated as
shown in Eqs. (1)–(3):
2.2. Pretreatment of MCC and pulp
A certain amount of the pulp was dispersed in water using
a high-shear ZBJ-1 Model fiber kneader (Changchun Paper Test
Depots Co., China), soaked in water for 24 h, and diluted to 5% (w/w)
consistency by magnetic stirring. Subsequently, the pulp slurry was
treated respectively with 20% (w/w) aqueous NaOH and 20% (w/w)
ethylenediamine (EDA) solution. Both the above two treatments
were performed at 5 ◦C for 72 h. The MCC were prepared at a 5%
(w/w) consistency by magnetic stirring and treated with aqueous
20% (w/w) NaOH solution at 5 ◦C for 72 h. After the treatments, the
pulp slurry and MCC were washed with water until the washings
were of pH 7. After filtration by a 60 mL G3 sand core bush fun-
nel with vacuum assistance, the treated MCC samples were dried
in DZF-6210 vacuum drier (Shanghai Permanent Science and Tech-
nology Co., Shanghai, China) at 80 ◦C for 24 h. From this process, the
cellulose samples, i.e., pulp, pulp samples treated with aqueous 20%
(w/w) NaOH solution (Pulp–NaOH) and in aqueous 20% (w/w) EDA
solution (Pulp–EDA), and MCC treated in aqueous 20% (w/w) NaOH
solution (MCC–NaOH), were obtained. The mean degree of poly-
merization of the pulp, Pulp–NaOH, Pulp–EDA, and MCC–NaOH was
measured by the viscosity method (GB, 1986), and was respectively
826, 705, 801, and 141.
n
ꢀ
T(n) =
(tn)
(1)
1
n
ꢁ
ꢂ
ꢀ
Gn
GILs
S(n) =
V(n) =
× 100%
(2)
(3)
1
ꢁ
ꢂ
Gn
× 100%
GILs × tn
where n is the cyclic number of dissolving cellulose samples in the
ionic liquids (n = 1, 2, 3, 4, . . ., n); tn is the dissolution time (min)
required for dissolving completely the mass (g) of the cellulose
samples; (Gn) in the ionic liquids at the n cyclic number; GILs is
the mass (g) of the ionic liquids; T is the dissolution time required
for dissolving completely the cellulose sample in the ionic liquids
(%, w/w) as a function of cyclic numbers; S, V is respectively the
solubility (%, w/w), dissolution rate (%/min) of the cellulose sample
in the ionic liquids as a function of dissolution time.
2.3. Synthesis and characterization of 1-allyl-3-alkylimidazolium
chloride ionic liquids
The general synthesis procedure of AMIMCl was performed
according to the method by the reports (Ren et al., 2003). To N-
methylimidazole in a 500 mL glass-lined reactor, approximately
1:1.2 molar ratio of allyl chloride was added dropwise under argon
gas atmosphere at room temperature. After completely adding the
allyl chloride, the reaction mixture was stirred magnetically with
reflux at 55 ◦C for about 10 h. After removing the residual allyl
chloride under reduced pressure, the resulting liquid was repeat-
edly washed with an excess amount of ether to eliminate the
residual N-methylimidazole. The resulting ionic liquids was ana-
lyzed for pH value and zeta potential under PHS-3D pH equipment
(Shanghai Jingke Equipment Co., Ltd., Shanghai, China), and was
subsequently dried in vacuum drier at 80 ◦C for 72 h. Thus, AMIMCl
was obtained. The derivatives AEIMCl, APIMCl, ABIMCl, and AHIMCl
were synthesized according to the above method. The density of
AMIMCl, AEIMCl, APIMCl, ABIMCl, and AHIMCl was measured by
density bottle method (GB, 1985), and was 1.2096, 1.1735, 1.1375,
1.1288, and 1.2211 g/mL, respectively. The 1H NMR spectra of the 1-
allyl-3-alkylimidazolium chloride were obtained using an AVANCE
Digital 400 MHz NMR (Bruker Company, Germany) showing the
following peaks: 1H NMR (400 MHz, DMSO), AMIMCl, ı = 9.60
(–N–CH–N–); 7.85 (–N–CH–CH–N–); 5.98 ( CH–); 5.28 ( CH2);
4.89 (CH2–N–); 3.87 (–N–CH3). AEIMCl, ı = 9.86 (–N–CH–N–); 8.08,
7.95 (–N–CH–CH–N–); 6.09 ( CH–); 5.37 ( CH2); 4.99 (–CH2–N),
3.56 (–N–CH2–); 1.45 (–CH3). APIMCl, ı = 9.80 (–N–CH2–N–);
Different cellulose samples were investigated in terms of solu-
bility in AAIMCl ionic liquids. The solubility of the pulp, Pulp–NaOH,
Pulp–EDA, and MCC–NaOH samples were compared in each ionic
liquid at 100 ◦C under argon gas protection. The effects of the disso-
lution temperatures (i.e., 60 ◦C, 100 ◦C, and 120 ◦C) on the solubility
of the MCC–NaOH samples in ionic liquids were also studied.
2.5. Preparation of the regenerated cellulose
After complete dissolution of the cellulose samples in the ionic
liquids, the resulting cellulose solution was washed with ∼20 mL
absolute ethyl alcohol 3–5 times to regenerate the cellulose sam-
ples. Subsequently, an excess amount of deionized water was used
to wash the regenerated cellulose. The regenerated cellulose sam-
ples were vacuum freeze-dried under ModulyoD-230 Freeze Dryer
(Thermo Savant, USA) at −50 ◦C for 24 h.
2.6. Scanning electron microscopy analysis
The two regenerated MCC samples, one obtained by vacuum
freeze drying and the other by air drying method, were fixed to
a metal–base specimen holder using double-sided sticky tape. The
samples were then coated with gold using a vacuum sputter-coater.
Afterwards, the fracture surfaces were observed using an S-3700N
scanning electron microscope (Hitachi, Ltd., Japan).
8.05, 7.95 (–N–CH–CH–N–); 6.10
(–CH2–N–), 3.41 (–N–CH2–); 1.84 (–CH2–) (a); 0.86 (–CH3). ABIMCl,
ı = 9.76 (–N–CH–N–); 8.01, 7.91 (–N–CH–CH–N–); 6.08 ( CH–) 5.34
( CH–); 5.35 ( CH2); 5.00
(
CH2); 4.97 (–CH2–N–), 3.47 (–N–CH2–); 1.82 (–CH2–) (a), 1.27
(–CH2–) (b); 0.90 (–CH3). AHIMCl, ı = 9.72 (–N–CH–N–); 8.02, 7.96
(–N–CH–CH–N–); 6.09 ( CH–); 5.36 ( CH2); 5.00 (–CH2–N–), 3.96

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D.-t. Liu et al. / Carbohydrate Polymers 87 (2012) 1058–1064
Fig. 1. Dissolution of cellulose in AHIMCl in the starting stage (A); initial partial dissolving stage (B); later dissolving stage (C); and completely dissolved stage (D).
2.7. X-ray diffraction (XRD) analysis
3. Results and discussion
The crystal structures of the MCC, MCC–NaOH, and the regen-
erated MCC–NaOH samples treated with AMIMCl and AHIMCl at
100 ◦C were studied using an XRD analyzer (Rigaku D/max-III X-ray
diffractometer) set at 40 kV and 30 mA. Wide-angle X-ray inten-
sities were collected for 2ꢀ, ranging from 4◦ to 60◦, with a step
scanning rate of 8◦/min and step increment of 0.04◦.
The 0 0 2 plane apparent crystallite size of the cellulose samples
was calculated by the Scherrer formula (Cao & Tan, 2002), described
in Eq. (4) as follows:
3.1. Characteristics of 1-allyl-3-alkylimidazolium chloride ionic
liquids
Based on the currently developed AMIMCl (Wu et al., 2004;
Zhang et al., 2005), AMIMCl and the series of its derivatives, AEIMCl,
APIMCl, ABIMCl, and AHIMCl, were successfully synthesized in
this study. The newly developed AEIMCl and AHIMCl in this study
exhibited reasonable capability to dissolve cellulose (e.g., in MCC
and pulp), as observed by polarization microscopy. Fig. 1A and B
shows that dissolution of the cellulose samples commenced due
to the dispersion of numerous cellulose fibers in the ionic liquids.
Eventually, the cellulose fibers disintegrated and finally disap-
peared, while the blackburst structure emerged on polarization
microscope (as shown in Fig. 1D).
In the synthesis route (see Fig. 2), the electrophilic attack of
the 2-allyl cation on alkylimidazolium formed the bigger alkylim-
idazolium cation, which underwent further nucleophilic reaction
with chloride anion. This finding indicates that the symmetry on
the two sides of the cationic 3-imidazole ring was destroyed by
the increase in alkyl chain length, which also affected the elec-
tron cloud distribution in the cationic 3-alkylimidazolium ring.
Fig. 3 shows that AMIMCl and AHIMCl, which contain micro-
moisture, exhibited alkalescent environment with a pH value
of 8.82 and 9.72, respectively, whereas AEIMCl, APIMCl and
ABIMCl were nearly neutral. Asymmetry in the distributions of
the groups on both sides of the imidazolium cation probably
distorted the electron cloud distribution, which facilitated the
Kꢁ
D =
(4)
ˇ cos ꢀ
where K is the Scherrer constant (0.89); D is the 0 0 2 plane
apparent crystallite size (nm); ꢁ is the wavelength of the X-ray
(0.154056 nm); ˇ is the full width at half maximum (rad); and ꢀ is
the diffraction angle.
2.8. Thermogravimetric analysis (TGA)
TGA was used to study the thermal stability of the 1-allyl-3-
alkylimidazolium chloride ionic liquids by Q500 TGA instrument
(TA instruments, USA), the MCC, MCC–NaOH, and the regenerated
MCC–NaOH samples treated with AMIMCl and AHIMCl at 100 ◦C.
Weight loss was recorded in the range of room temperature to
500 ◦C, with a heating rate of 10 ◦C min−1
.
Fig. 2. Synthesis route and chemical structures of 1-allyl-3-alkylimidazolium chloride ionic liquids.

D.-t. Liu et al. / Carbohydrate Polymers 87 (2012) 1058–1064
1061
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
740 min (Fig. 4A). AEIMCl showed relatively higher capacity to dis-
solve the cellulose samples than AMIMCl and AHIMCl at 100 ◦C.
The solubility of MCC–NaOH samples in AMIMCl, AEIMCl and
AHIMCl initially increased rapidly with dissolution time and
then increased slowly with further dissolution time. Accord-
ingly, the dissolution rate increased quickly in the initial stage
(≤300 min) and subsequently slowed with further dissolution
time (Fig. 4B).
Fig. 4C illustrates the affects of the dissolution temperature on
the solubility of the MCC–NaOH samples in the ionic liquids. The
AMIMCl, AEIMCl, APIMCl, ABIMCl, and AHIMCl almost did not dis-
solve the cellulose samples when the dissolution temperature was
lower than 60 ◦C. This finding agrees with the present ionic liquid
of AMIMCl (Zhang et al., 2005). The time required for dissolving
completely 1% cellulose of the MCC–NaOH samples at 60 ◦C for
AMIMCl, AEIMCl, and AHIMCl was about 419, 685, and 422 min,
respectively.
At a low dissolution temperature, AMIMCl and AHIMCl had
nearly the same solubility, showing easier dissolution of cellulose
than AEIMCl. APIMCl and ABIMCl did not dissolve the cellulose of
MCC–NaOH samples at 60 ◦C regardless of the duration (Fig. 4D).
Increasing the dissolution temperature to 120 ◦C resulted in a
much faster dissolution of 1% cellulose of MCC–NaOH sample.
ABIMCl was also found to dissolve the cellulose of MCC–NaOH com-
pletely at 120 ◦C by 718 min. This result suggests that increasing
the dissolution temperature improved the solubility of cellulose
in 1-allyl-3-alkylimidazolium chloride ionic liquids. The result also
shows that cellulose dissolved more easily in AEIMCl at a higher
dissolution temperature but not at a low dissolution temperature.
20
0
-20
-40
-60
-80
-100
-120
-140
-160
-180
0
1
2
3
4
5
6
7
Carbon atoms of alkyl chains on a cationic 3-imidazole ring in ILs (n)
Fig. 3. Dependence of carbon atoms (n ≤ 6) of alkyl chains on the cationic 3-
imidazole ring on the pH and zeta potential of ionic liquids.
interaction of chlorine atoms with H+ from H2O. The differences
in stability in ionic liquids were evaluated from the zeta poten-
tials. The zeta potentials of AMIMCl and AHIMCl were much
higher than 100 mV, whereas AEIMCl, APIMCl, and ABIMCl had
zeta potentials lower than 20 mV. These findings imply that
asymmetrical distributions of groups on both sides of the imi-
dazolium cation benefit the systematic stability of the ionic
liquids.
3.2. Dissolution of cellulose
The cellulose of the MCC–NaOH samples at 100 ◦C completely
dissolved in AMIMCl, AEIMCl, and AHIMCl, whereas the sol-
ubility of APIMCl and ABIMCl was lower than 0.42% within
Fig. 4. Comparisons of solubility (A) and dissolution rate (B) of the MCC–NaOH samples in 1-allyl-3-alkylimidazolium chloride ionic liquids at 100 ◦C; effects of dissolution
temperature (C) on the dissolution time of 1% MCC–NaOH samples in AMIMCl, AEIMCl and AHIMCl; comparison (D) of the solubility of pulp, Pulp–NaOH, Pulp–EDA, and
MCC–NaOH samples in AEIMCl and AHIMCl at 100 ◦C.

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D.-t. Liu et al. / Carbohydrate Polymers 87 (2012) 1058–1064
Fig. 5. Mechanism of processing cellulose with 1-allyl-3-alkylimidazolium chloride ionic liquids.
Fig. 6. XRD patterns (A) and crystallite sizes (B) of the MCC (a), MCC–NaOH (b), and the regenerated MCC–NaOH samples from AMIMCl (c) and AHIMCl (d).
Fig. 7. Temperature dependencies of weight loss for 1-allyl-3-alkylimidazolium chloride ionic liquids, the MCC, MCC–NaOH, and the regenerated MCC–NaOH samples in a
nitrogen atmosphere.

D.-t. Liu et al. / Carbohydrate Polymers 87 (2012) 1058–1064
1063
Fig. 8. Image of 1-allyl-3-alkylimidazolium chloride (A), the regenerated cellulose samples (B), and SEM image of the regenerated cellulose samples dried by vacuum
freeze-dry method (C) (×1000) and air-dry method (D) (×1000), respectively.
In contrast to this finding, cellulose exhibited better solubility in
AHIMCl at a lower temperature of 60 ◦C.
3.3. XRD analysis
The pretreatment of cellulose by aqueous 20% (w/w) NaOH
or EDA solution can change the physical structure of the rigid
crystalline region of cellulose, probably contributing to the eas-
ier dissolution of cellulose in ionic liquids (Pérez & Samain, 2010;
Wang, Keshwani, Redding, & Cheng, 2010). Fig. 4D shows that
compared with fibrous pulp, the MCC–NaOH samples had higher
solubility in AEIMCl at 100 ◦C. This probably resulted from the
attack of these reagents on cellulose, which could have led to
the increase of cation and chloride anions of the ionic liquid in
accessibility to the crystalline or amorphous regions of the cel-
lulose sample. This facilitates the attack on the hydrogen bonds
within or across cellulose chains. Remarkably, the pulp sam-
ples without any pretreatment also dissolved well in AEIMCl and
AHIMCl. The AHIMCl had significantly better dissolving efficiency
in the pulp samples than AEIMCl (Fig. 4D). The Pulp–EDA sam-
ples showed significantly higher solubility in AEIMCl and AHIMCl
than the Pulp–NaOH samples. The better dissolving efficiency of the
Pulp–EDA samples by AHIMCl and compared with that of AEIMCl
was observed.
Fig. 5 shows the mechanism of cellulose processing by 1-allyl-
3-alkylimidazolium chloride ionic liquids. The synthesized ionic
liquids have special chemical structures, including a small chloride
anion and another large molecule of 1-allyl-3-alkylimidazolium
cation. Similar to other ionic liquids (e.g., BMIMCl and EMIMAc)
(Lee, Doherty, Linhardt, & Dordick, 2009; Swatloski et al., 2002),
both anions and cations are considered involved in the dissolu-
tion process (Pinkert, Kenneth, Marsh, & Mark, 2009). The oxygen
and hydrogen atoms of the cellulose form electron donor–electron
acceptor complexes with the charged species of the ionic liquids.
1-Allyl-3-alkylimidazolium cation can easily connect to oxygen in
the inter- and intramolecular H-bonds between cellulose chains,
whereas the chloride anions connect to the hydrogen atom. This
process breaks the inter- and intramolecular H-bonds of cellulose
chains, leading to the dissolution of the cellulose in ionic liquids
(Pinkert et al., 2009; Zhang et al., 2005).
XRD has been used extensively for the investigation of the
super molecular order (crystalline) of cellulose and their deriva-
tives (Liu et al., 2008). Fig. 6A shows the XRD curves of the
MCC, MCC–NaOH, and the regenerated MCC–NaOH samples from
AMIMCl and AHIMCl. The distinct diffracted intensity of the MCC
at 2ꢀ values was less intense and shifted to a lower 2ꢀ value
of 21.91◦ after the treatment of 20% aqueous NaOH solution at
5 ◦C for 72 h. These results indicate that the crystal form of cel-
lulose (I) of the MCC samples changed to cellulose (II) of the
MCC–NaOH sample, showing the decrease in the crystallographic
index of the MCC. The more distinct decrease in diffracted inten-
sity was observed from the regenerated MCC–NaOH samples from
AMIMCl and AHIMCl, where the diffraction angles of 14.91◦, 16.45◦,
20.58◦, and 34.65◦ nearly disappeared. Only weak diffracted inten-
sities of the regenerated MCC–NaOH samples from AMIMCl and
AHIMCl were respectively observed at 2ꢀ value of 21.38◦ and
21.87◦. Fig. 6B shows that the 0 0 2 plane apparent crystallite size
of the MCC, MCC–NaOH, and the regenerated MCC–NaOH samples
from AMIMCl and AHIMCl was respectively 6.2466–2.0088, 0.5338,
and 0.5226 nm. This suggests that unlike in the treatment of aque-
ous NaOH, the 1-allyl-3-alkylimidazolium chloride ionic liquids
severely damaged the crystal structure of cellulose.
3.4. TGA analysis
The TGA curves are shown in Fig. 7 for the AAIMCl ionic liquids
and the regenerated cellulose samples. In the thermal degradation
process of the ionic liquids, their starting decomposition tempera-
ture nearly occurred at 195 ◦C, which is relatively lower than that
of 300–400 ◦C for most ionic liquids (e.g. [BMIM]PF6, [EMIM]BF4)
(Pinkert et al., 2009; Ngo, Compte, Hargens, & McEwen, 2000).
This finding indicates the relatively poor thermal stability of 1-
allyl-3-alkylimidazoliumchlorideionicliquids. Theweightlossesof
AMIMCl, AEIMCl, APIMCl, ABIMCl, and AHIMCl in an N2 atmosphere
occurring in the range of room temperature (30 ◦C) to 192.5 ◦C

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D.-t. Liu et al. / Carbohydrate Polymers 87 (2012) 1058–1064
varied from 9.41%, 2.83%, 8.43%, 2.86%, and 16.95%, respectively,
due to the elimination of smaller residual molecules. The great-
est decomposition temperature (Td) of the abovementioned ionic
liquids varied from 274.3 ◦C to 283.1 ◦C. The combined weight
loss from 192.5 ◦C to Td was 63.16%, 56.06%, 58.27%, 58.9%, and
59.98%, respectively, for AMIMCl, AEIMCl, APIMCl, ABIMCl, and
AHIMCl (Fig. 7A). In the rapid decomposition process from 192.5 ◦C
to Td, the anions decompose through dealkylation, whereas the
cations primarily undergo alkyl migration and elimination reac-
tions (Baranyai, Deacon, MacFarlane, Pringle, & Scott, 2004; Hao,
Peng, Hu, Li, & Zhai, 2010). Based on the above data, AMIMCl and
AHIMCl had relatively poor thermal stability due to the lower over-
all symmetry of the cation asymmetry structures than AEIMCl,
APIMCl, and ABIMCl.
The regenerated MCC–NaOH samples from AMIMCl and AHIMCl
at 100 ◦C had weight losses of 29.03% and 12.85%, respectively, at
a range of room temperature to 150 ◦C. The weight losses of the
regenerated MCC–NaOH samples were higher than 5.86% of the
MCC (Fig. 7B). The regenerated MCC–NaOH samples from AMIMCl
and AHIMCl had lower decomposition temperatures (Td) of 257.7
and 274.4 ◦C, respectively, than that of the MCC and MCC–NaOH
samples. This finding indicates that the damage in the crystal struc-
ture and the part breakage of cellulose chains resulted in the poor
thermal stability of cellulose samples.
Acknowledgements
We gratefully acknowledge the financial support provided by
the National Natural Science Foundation of China (No. 31000285),
the Fundamental Research Funds for the Central Universities, South
China University of Technology, PR China (No. 2009ZM0106), and
the Pulp & Paper Engineering State Key Laboratory, South China
University of Technology, PR China (No. 200914).
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4. Conclusions
In this study, 1-allyl-3-alkylimidazolium chloride ionic liquids
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