Theoretical and experimental investigation on dissolution and regeneration of cellulose in ionic liquid

Article abstract of DOI:10.1016/j.carbpol.2012.01.080

Density functional theory calculations and atoms in molecules theory were performed to investigate the mechanism of cellulose dissolution and regeneration in 1-ethyl-3-methylimidazolium acetate ([emim]Ac), and (1,4)-dimethoxy-β- D-glucose (Glc) was chosen as the model for cellulose. The theoretical results show that the interaction of [emim]Ac with Glc is stronger than that of Glc with Glc. Further studies indicate that the anion acetate of [emim]Ac forms strong H-bonds with hydroxyl groups of Glc. It is also observed that the H-bonds between [emim]Ac and Glc are weakened or even destroyed by the addition of water. In addition, both the original and regenerated cellulose samples were characterized with FT-IR, XRD, TGA and SEM. The experimental results prove that cellulose can be readily reconstituted from the [emim]Ac-based cellulose solution by the addition of water and the crystalline structure of cellulose is converted to cellulose II from cellulose I in the original cellulose.

Full text of DOI:10.1016/j.carbpol.2012.01.080

Carbohydrate Polymers 89 (2012) 7–16
Contents lists available at SciVerse ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
Theoretical and experimental investigation on dissolution and regeneration
of cellulose in ionic liquid
∗
∗
Zhen-Dong Ding, Zhen Chi, Wen-Xiu Gu , Sheng-Ming Gu, Jian-Hua Liu, Hai-Jun Wang
School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Density functional theory calculations and atoms in molecules theory were performed to investi-
gate the mechanism of cellulose dissolution and regeneration in 1-ethyl-3-methylimidazolium acetate
Received 16 December 2011
Received in revised form 24 January 2012
Accepted 26 January 2012
Available online 3 February 2012
([emim]Ac), and (1,4)-dimethoxy-␤-D-glucose (Glc) was chosen as the model for cellulose. The theoret-
ical results show that the interaction of [emim]Ac with Glc is stronger than that of Glc with Glc. Further
studies indicate that the anion acetate of [emim]Ac forms strong H-bonds with hydroxyl groups of Glc.
It is also observed that the H-bonds between [emim]Ac and Glc are weakened or even destroyed by the
addition of water. In addition, both the original and regenerated cellulose samples were characterized
with FT-IR, XRD, TGA and SEM. The experimental results prove that cellulose can be readily reconstituted
from the [emim]Ac-based cellulose solution by the addition of water and the crystalline structure of
cellulose is converted to cellulose II from cellulose I in the original cellulose.
Keywords:
Cellulose
Ionic liquid
Dissolution
Regeneration
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
industrialized for manufacturing regenerated cellulose. However,
it has some disadvantages such as the demand for high tempera-
ture to dissolve and high cost (Tan et al., 2009). Therefore, better
cellulose solvents are key to taking greater advantage of cellulose.
Ionic liquids (ILs) have been used for many aspects, ranging
from media for organic synthesis and catalysts to lubricants, and
more recently have generated much academic interest as replace-
ments for environmentally damaging volatile organic solvents
(Berg, Deetlefs, Seddon, Shim, & Thompson, 2005). The appearance
of ILs provides a broader scope for the development of cellu-
lose chemistry. ILs have been proven as the efficient solvents
for cellulose dissolution because of their unique properties, such
as low melting points, negligible vapor pressure, nonvolatile and
inflammable as well as wide electrochemical windows (Huddleston
et al., 2001). It has been found that cellulose can be dissolved
without derivatization in high concentrations using ILs. Such as 1-
allyl-3-methylimidazoliumchloride, 1-allyl-3-methylimidazolium
formate, 1-butyl-3-methylimidazolium chloride and so on (Fu,
Mazza, & Tamaki, 2010). Furthermore, the components of cation
and anion in the ILs can be fine-tuned to adjust the physicochem-
ical properties, which affect their performance in the interaction
with the cellulose.
Cellulose is the most abundant biorenewable material, with a
long and well-established technological base (Kirk-Othmer, 1993),
and has a highly crystalline polymer of D-an-hydroglucopyranose
units joined together in long chains by ␤-1,4-glycosidic bonds
(
Li & Zhao, 2007). The derived products of cellulose have many
important applications in the fiber, paper, membrane, poly-
mer and paint industries. The growing willingness to develop
new cellulosic materials results from the fact that cellulose
is a renewable resource. Although many of the technologies
currently used in cellulose processing are decidedly nongreen
(
Johnson et al., 1985), it is practically difficult to dissolve cel-
lulose in most common organic solvents, because of their stiff
molecules and closed chain packing via numerous intermolecu-
lar and intramolecular H-bonds (Zhang, Wu, Zhang, & He, 2005).
Recently, various types of solvents have been used for dissolv-
ing of cellulose. For example, dimethylacetamide/lithium chloride,
N O -N, N-dimethylformamide, LiClO ·3H O, LiSCN·2H O and N-
2
4
4
2
2
methyl-morpholine-N-oxide/H O system (Li & Zhao, 2007; Song,
2
Wang, & Xing, 2009; Tan et al., 2009; Zhang et al., 2005). However,
these solvents have some limitations such as volatility, toxic-
ity, difficulty for solvent recovery and instability in application.
NMR experiments, DFT calculations and molecular mechan-
ics simulations have provided mechanistic insight into cellulose
dissolution in IL. Zhang et al. (2005) speculated that the possible dis-
solution mechanism of cellulose in 1-allyl-3-methylimidazolium
The N-methylmorpholine-N-oxide/H O system is the only one
2
−
chloride is that Cl ions associated with the cellulose hydroxyl
∗ Corresponding authors.
E-mail addresses: wenxiugu@gmail.com (W.-X. Gu), wanghj329@hotmail.com
proton, and the free cations complex with the cellulose hydroxyl
oxygen, which disrupted H-bonds in cellulose and led to the
(
H.-J. Wang).
0
144-8617/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbpol.2012.01.080

8
Z.-D. Ding et al. / Carbohydrate Polymers 89 (2012) 7–16
was confirmed by 1H NMR (400 MHz, D O): ı = 1.46–1.52 (t, 3 H;
dissolution of cellulose. Remsing, Swatloski, Rogers, and Moyna
2006) demonstrated that the solution of cellulose by 1-n-butyl-
-methylimidazolium chloride involves H-bonds between the
2
H
−
(
3
CH ), ı = 1.89–1.94 (t, 3 H; CH3 COO ), ıH = 3.89 (t, 3H; N CH ),
3
H
3
ıH = 4.18–4.26 (t, 2H; CH ), ıH = 7.4 (s, 1H; Ring-CH), ıH = 7.5 (s, 1H;
2
carbohydrate hydroxyl and chloride ions. Youngs, Hardacre, and
Holbrey (2007) indicated that dominant contributions to the
sugar–ionic liquid interaction energy come from favorable H-bond
interactions between hydroxyls and chlorides. Guo, Zhang, Duan,
and Liu (2010) applied density functional theory calculations to
investigate the interactions of the cellulose molecule with several
anions, and found that the strength of interactions of anions
with cellulose follows the order: acetate anion > alkylphosphate
anion > tetrafluoroborate anion > hexafluorophosphate anion.
Janesko applied dispersion-corrected density functional theory
to study the differences between ionic liquids’ interactions with
cellulose vs. lignin, and emphasized that cellulose preferentially
Ring-CH), ıH = 8.7 (s, 1H; Ring-CH).
2.4. Cellulose dissolution and regeneration experiments
The dissolution of microcrystalline cellulose (MCC) was carried
out in a round bottom flask that was heated in the oil-bath with
vigorous magnetic stirring in the open atmosphere. 0.50 g of MCC
was added to 10 g of [emim]Ac in a 25 mL beaker (Sun et al., 2009).
When the color of the solution turns amber, MCC was completely
dissolved. Finally, a transparent solution with about 5% polymer
concentration was obtained. Cellulose was reconstituted from the
resulting clear, amber-colored, viscous liquor as flocs by rapid mix-
ing with distilled water. The powdery off-white solids were then
filtered, washed with distilled water and dried in a vacuum oven at
−
interacts with Cl (Janesko, 2011). Up to now, there are no DFT
calculations and AIM theory analyses concerning the dissolution
and regeneration mechanism of cellulose in [emim]Ac. In this
paper, Glc was chosen as the model of cellulose to investigate the
interaction between [emim]Ac and cellulose at the molecular level.
In addition, the reconstitution of cellulose from [emim]Ac-based
cellulose solution by the addition of water has been demonstrated
by theoretical and experimental research. The regenerated cel-
lulose was characterized by FT-IR, XRD, TGA and SEM compared
with the original cellulose. We hope that these efforts will result
in better understanding of the dissolution of cellulose in ILs.
◦
60 C prior to their use in FT-IR, XRD, TGA and SEM experiments. The
recovery of [emim]Ac was accomplished by evaporating to remove
water.
2.5. Characterization of the original and regenerated cellulose
The original and regenerated cellulose were characterized by FT-
IR using a Perkin Elmer Spectrum 100 FT-IR spectrometer equipped
with an attenuated total reflectance (ATR) cell with 32 scans at
2
. Materials and methods
−1
2
cm resolution.
The original and regenerated cellulose samples were also char-
2
.1. Materials
acterized by wide-angle X-ray diffraction patterns. The X-ray
diffraction patterns with Cu K␣ radiation (ꢀ = 1.5406 A˚ ) at 40 kV
N-methylimidazole was obtained from Changzhou Chemical
◦
and 30 mA were recorded in the range of 2ꢁ = 5–40 with an X-ray
Factory and further purified by distillation. Bromoethane and ethyl
acetate were distilled and then were stored over molecular sieves in
tightly sealed glass bottles, respectively. Other chemicals (AR) were
commercially available and were used without further purification.
diffraction diffractometer (D/MAX-2500, Rigaku Denki, Japan).
Differences in morphology before and after [emim]Ac treatment
were analyzed by scanning electron microscopy (SEM). SEM images
were taken at 2000× magnification using a Hitachi S-4800 field
emission SEM instrument operated at 2 kV accelerating voltage.
The thermal decomposition curves of the original and regener-
ated cellulose were determined using a precisa TGA/DSC 1/1100 SF
thermogravimetric analyzer. Each sample was analyzed in a plat-
inum pan with nitrogen as the purge gas. In all experiments, the
2
.2. Quantum chemical calculations methods
All energy minimization calculations were performed with
Gaussian 03, and the AIM calculations were performed with
the AIM2000 package (Frisch et al., 2003). The hybrid Becke
3
◦
temperature was increased from 25 to 600 C at a constant rate of
–Lee–Yang–Parr (B3LYP) exchange-correlation function with the
-31+G* basis set was employed to perform the geometry opti-
◦
−1
2
0 C min .
6
mizations in this work (Bader, 1990; Lee, Yang, & Parr, 1988).
The B3LYP/6-31+G* level represents an excellent trade-off between
accuracy and computer requirements, particularly suitable for the
investigation of ionic liquid and cellulose system (Guo, Zhang, &
Liu, 2010).
3
3
3
. Results and discussion
.1. Theoretical study of dissolution and regeneration of cellulose
.1.1. Geometries optimization
Conformational searches were performed as follows. A series of
2.3. Preparation of [emim]Ac
geometric conformations of [emim]Ac, Glc, Glc–Glc, [emim]Ac–Glc
emim]Ac–nH O (n = 1, 2, 3) and [emim]Ac–Glc–nH O (n = 1, 2)
[
[
emim]Ac was prepared following the reported procedures
2
2
were originally optimized at B3LYP/6-31+G* level. Geometries
optimization used tight convergence criteria and no restriction
on symmetries was imposed on the initial geometric conforma-
tions, therefore the geometries optimization for the saddle points
occurred with all degrees of freedom. Followed by frequency cal-
culations to verify the reasonability of the optimized structures
at the corresponding level, vibration frequency analysis on these
optimized structures gave no imaginary frequencies, suggesting
that they are true minimum energy structures. The representative
optimized configurations of [emim]Ac, Glc, Glc–Glc, [emim]Ac–Glc
(
Ellis, 1996; Yang, Zhang, Wang, & Tong, 2006). [emim]Br was
synthesized by refluxing the N-methylimidazole with excess bro-
moethane at 50 C for 48 h. The excess bromoethane was removed
◦
by evaporation and the crude product was recrystallized twice by
ethyl acetate. The resulting precipitate was brownish and was dried
in vacuum for 24 h under reduced pressure. Then 0.2 mol [emim]Br
and 0.1 mol Pb(Ac) ·3H O were separately dissolved in solutions of
2
2
methanol–water (4:1 v/v). Upon mixing the two solutions, a precip-
itate of PbBr was produced, which was removed by filtration after
2
◦
cooling at −20 C overnight. The solvent was removed from the fil-
[
emim]Ac–nH O (n = 1, 2, 3) and [emim]Ac–Glc–nH O (n = 1, 2)
trate by rotary evaporation, and the product [emim]Ac was gotted
2
2
◦
were chosen and listed in Figs. 1–3.
by drying in vacuum for 2 days at 80 C. The structure of [emim]Ac

Z.-D. Ding et al. / Carbohydrate Polymers 89 (2012) 7–16
9
3
.1.2. Intermolecular interaction and AIM analysis
The optimized configurations were used to characterize the
The interaction energy is corrected by BSSE (basis set superpo-
sition error) and ZPE (the zero point energy). Thus, the corrected
interaction energy ꢂEZPE+BSSE can be calculated from Eq. (2):
preferred interaction sites, when we studied the interaction ener-
gies of Glc–Glc, [emim]Ac–Glc [emim]Ac–nH O (n = 1, 2, 3) and
2
ꢂE
= ꢂE + ꢂEBSSE + ꢂEZPE
(2)
[
emim]Ac–Glc–nH O (n = 1, 2). The interaction energy ꢂE can be
ZPE+BSSE
2
calculated by the use of Eq. (1):
where ꢂEBSSE is the correction of BSSE, and ꢂEZPE is the correction
of ZPE.
In order to obtain more information about the intermolecu-
lar interaction, the AIM theory was used to analyze the bonding
characteristic (Bader, 1998). The AIM theory, based on a topolog-
ꢂE(kJ mol−1 = 2625.5 [EAB (a.u.) − (EA (a.u.) + EB (a.u.))]
(1)
)
where EAB is the total energy of the system and (EA + EB) is the sum
of the energy of the pure compositions.
2
ical analysis of electron density (ꢃc) and laplacian (ꢀ ꢃc) (Frisch
Fig. 1. The optimized configurations for [emim]Ac (A), Glc (A1), Glc-Glc (A2–4) and [emim]Ac-Glc (B1–3) calculated at the B3LYP/6-31+G* level. H-bonds are indicated by
dotted line, and bond lengths are in angstrom ( A˚ ).

1
0
Z.-D. Ding et al. / Carbohydrate Polymers 89 (2012) 7–16
Fig. 1. (Continued.)
et al., 2003), provides a universally applicable tool for the classifi-
cation of the bonding interactions that take place in any molecular
system, even inside a supermolecule. The ꢃc is used to describe
the strength of a bond, with stronger bond associated with larger
The H···O distances vary from 1.70 to 2.68 A˚ , which are longer than
the corresponding covalent bond distance and shorter than the sum
of the van der waals H···O distance (Bondi, 1964), indicating that
+
H-bonds have been formed. The three H-bonds between [enim]
2
−
ꢃc value. The ꢀ ꢃc describes the characteristic of the bond. As
and Ac have corresponding bond lengths and angles of O22
H
:
10
2
2
◦
ꢀ
ꢃc < 0, it is named as the covalent bond. As ꢀ ꢃc > 0, it refers to
2.68 A˚ ; O22 H13: 1.70 A˚ ; O
H15:2.07 A˚ ; ∠C1 H10 O22: 138.78 ;
23
◦
◦
a closed-shell interaction (Solimannejad, Alkorta, & Elguero, 2007)
and characteristic of ionic bond, H-bond, or van der Waals inter-
action. In this paper, we are only concerned with the values of ꢃc
∠C H13 O22: 166.23 ; ∠C H15
O : 161.89 ). The results also
4
7
23
show that the intensity of the H-bond between anion and C H is
4
stronger than the other H-bonds. Because the ꢃc is a measure of
the intensity of the interaction, stronger H-bonds have greater ꢃc
values and the corresponding bond length is shorter.
2
and ꢀ ꢃc for intermolecular H-bonds. There are a set of criteria
2
for ꢃc and ꢀ ꢃc proposed at bond critical points (BCPs) for the con-
ventional H-bonds. Both parameters for closed-shell interactions as
H-bonds are positive within the following ranges: 0.002–0.035 a.u.
for the electron density and 0.024–0.139 a.u. for its Laplacian (Parr
The intermolecular interaction energies and AIM analysis results
2
of Glc–Glc are listed in Table 2. The ꢃ and ꢀ ꢃ at BCPs of H-bonds
c
c
fall within 0.002–0.035 a.u. and 0.024–0.139 a.u., respectively. It is
observed that Glc connects with each other by H-bonds. The con-
figuration A4 has the biggest absolute value of interaction energy
(37.93 kJ/mol), which manifests that the cellulose will be more
favorable to interact with each other in this way. In addition, the
number of O H. . .O type H-bond of A4 is more than that of A2–3, it
&
Yang, 1994).
The AIM analysis results of [emim]Ac are listed in Table 1. For
2
most H-bonds considered here, the ꢃc and ꢀ ꢃc values lie in the
relative proposed ranges. Therefore, for the observed [emim]Ac,
2
ꢃc and ꢀ ꢃc at BCPs of H-bonds fall within 0.002–0.051 a.u. and
0.010–0.148 a.u., respectively. It can be concluded that the interac-
tions between cation and anion which are marked by the dotted line
are all closed shell system (van der Waals or H-bond interactions).
Table 2
The interaction energies ꢂEBSSE+ZPE (kJ/mol) and properties of the electron density
of bond critical point for the interaction of Glc–Glc (a.u).
Table 1
Conformer
Bond
␳BCP
ꢂ2␳BCP
ꢂEBSSE+ZPE
+
Properties of the electron density of bond critical point for the interaction of [emim]
with Ac (a.u).
−
A2
A3
O2 H. . .O4
O2 H. . .O6
O6 H. . .O1
0.0319
0.0343
0.0253
0.1029
0.1073
0.0835
−22.68
−34.57
2
Bond
ꢃBCP
ꢄ ꢃBCP
C5 H. . .O1
C4 H. . .O2
C3 H. . .O2
0.0217
0.0508
0.0058
0.0681
0.1477
0.0239
A4
O6 H. . .O1
O2 H. . .O6
O6 H. . .O3
0.0242
0.0198
0.0206
0.0826
0.0698
0.0675
−37.93

Z.-D. Ding et al. / Carbohydrate Polymers 89 (2012) 7–16
11
Table 3
seems that it can account for the interaction energy of A4 is bigger
than that of A2–3
The interaction energies ꢂEBSSE+ZPE (kJ/mol) and properties of the electron density
of bond critical point for the intermolecular interaction of [emim]Ac–Glc (a.u).
.
The intermolecular interaction energies and AIM analysis results
of [emim]Ac–Glc are shown in Table 3. It can be found the absolute
values of interaction energies of configurations of [emim]Ac–Glc is
in an order of B > B > B . It is clear that [emim]Ac mainly inter-
2
Conformer
B1
Bond
ꢃBCP
ꢄ ꢃBCP
ꢂEBSSE+ZPE
C6 H. . .O2
C1 H. . .O1
C5 H. . .O1
0.0059
0.0136
0.0062
0.0257
0.0441
0.0216
−52.14
3
1
2
acts with Glc through the cationic part, the interaction energy
is smaller than [emim]Ac mainly interacts with Glc through the
anionic part. This implies that the anion plays a major role in
the interaction with Glc (Liu, Sale, Holmes, Simmons, & Singh,
B2
B3
C1 H. . .O3
0.0085
0.0310
−10.07
−93.72
C2 H. . .O2
O2 H. . .O1
C5 H. . .O1
C3 H. . .O3
0.0067
0.0451
0.0048
0.0032
0.0248
0.1474
0.0181
0.0125
2
010; Remsing et al., 2008), while the cation probably plays a sec-
13
ondary role. In agreement with the C NMR relaxation time data
Remsing et al., 2006), the cations only weakly interact with the
(
sugar. The results are also consistent with the previous reported
that the dissolution of cellulose in the ILs should be a result of
the joint interactions of anions and cations with cellulose (Guo
Fig. 2. The optimized configurations for [emim]Ac–nH2O (C1–9; n = 1, 2, 3) calculated at the B3LYP/6-31+G* level. H-bonds are indicated by dotted line, and bond lengths are
in angstrom ( A˚ ).

1
2
Z.-D. Ding et al. / Carbohydrate Polymers 89 (2012) 7–16
Fig. 2. (Continued.)
et al., 2010a; Youngs et al., 2007; Zhang et al., 2005). The abso-
lute values of interaction energies (93.72 and 52.14 kJ/mol) of B3
and B1 are bigger than that of Glc–Glc (37.93 kJ/mol). This means
that cellulose is easier to combine with [emim]Ac, that is to say
Youngs et al. (2007) also indicted that the dominant contributions
to the sugar-ionic liquid interaction energy come from favorable
H-bond interactions. This confirmed that [emim]Ac appears to be
the most effective solvent, which can dissolve cellulose through H-
bonds between hydroxyl functions of cellulose and acetate anions.
The results are consistent with the previous reported that the
cellulose-acetate anions form strong interaction (Guo et al., 2010b)
and glucose–ionic liquid interactions in the liquid are H-bonds
between acetate and sugar hydroxyl groups (Youngs et al., 2011).
Most of the ILs can absorb water from the atmosphere (Visser,
Swatloski, Reichert, Griffin, & Rogers, 2000), such as [emim]Ac.
Water molecules tend to interact with the ILs, this hydrophilic
property would sometimes affect their physical properties, the
[
emim]Ac can dissolve cellulose. For all H-bonds considered in this
2
study, the ꢃc and ꢀ ꢃc values lie in the relative proposed ranges.
Therefore, for the observed [emim]Ac–Glc, ꢃc and ꢀ ꢃc at BCPs of
2
H bonds fall within 0.002–0.050 a.u. and 0.010–0.148 a.u., respec-
tively. It can be concluded that the interactions between [emim]Ac
and Glc which are marked by the dotted line are all closed shell
system (H-bonds interaction). It was clear that [emim]Ac connects
with cellulose by H-bond, which is in agreement with the previous
reported conclusion (Swatloski, Spear, Holbrey, & Rogers, 2002).

Z.-D. Ding et al. / Carbohydrate Polymers 89 (2012) 7–16
13
Fig. 3. The optimized configurations for [emim]Ac–Glc–nH2O (D1–6; n = 1, 2) calculated at the B3LYP/6-31+G* level. H-bonds are indicated by dotted line, and bond lengths
are in angstrom ( A˚ ).
rates and selectivity of reactions (Anthony, Maginn, & Brennecke,
998; Seddon & Stark, 2002). The quantum chemical (QM) calcu-
energies of [emim]Ac–H O are shown in Fig. 2 and Table 4, respec-
2
1
tively. It is clear that when one water molecule mainly interacts
with the [emim]Ac through the cationic part, using conforma-
tion C2 as reference, the absolute value of interaction energy is
5.24 kJ/mol. However, when water molecule mainly interacts with
the [emim]Ac through the anion part, using the complex C3 as ref-
erence, the absolute value of interaction energy is 75.05 kJ/mol.
This implies that the anion plays a major role in the interaction
of [emim]Ac with water molecule (Cammarata, Kazarian, Salter, &
Welton, 2001), while the cation probably play a secondary role. The
results are agreement with the previous reported that the strengths
of the interactions follow the trend anion–H O > cation–H O (Wang
lations are valuable technique for giving insight at the molecular
level into the interactions between water molecules and ILs. For
example, Li et al. had applied QM calculations to investigate the
interaction between water molecules and ionic liquids based on the
−
−
−
−
imidazolium cation with the anions Cl , Br , BF , and PF6 (Wang,
4
Li, & Han, 2006). At our QM level, the optimized geometries sug-
gested that there are three types of conformations: [emim]Ac–H O,
2
[
emim]Ac–2H O and [emim]Ac–3H O. A set of possible confor-
2
2
mations of [emim]Ac–nH O (n = 1, 2, 3) have been examined by
2
DFT calculations to characterize the interactions between H O
2
2
2
and [emim]Ac. Nine optimized geometries and the interaction
et al., 2006).

1
4
Z.-D. Ding et al. / Carbohydrate Polymers 89 (2012) 7–16
Table 4
Table 5
The interaction energies ꢂEBSSE+ZPE (kJ/mol) and properties of the electron density
of critical point for the intermolecular interaction of [emim] Ac–nH2O (n = 1, 2, 3;
a.u).
The interaction energies ꢂEBSSE (kJ/mol) and properties of the electron density of
bond critical point for the intermolecular interaction of [emim]Ac–Glc–nH2O (n = 1,
2; a.u).
2
2
Conformer
C1
Bond
ꢃBCP
ꢄ ꢃBCP
ꢄEBSSE+ZPE
Conformer
Bond
ꢃBCP
ꢄ ꢃBCP
ꢂEBSSE
C5 H. . .O1
O1 H. . .O1
0.0172
0.0413
0.0551
0.1430
−49.41
D1
D2
D3
−14.28
−24.92
−14.29
C4 H. . .O2
0.0069
0.0246
C2
C3
C1 H. . .O1
0.0152
0.0533
−5.24
D4
O2 H. . .O1
C5 H. . .O2
0.0330
0.0087
0.1215
0.0297
−43.13
C3 H. . .O1
O1 H. . .O2
0.0234
0.0524
0.0751
0.1371
−75.05
D5
D6
−10.59
−17.33
C4
C5
C4 H. . .O2
O2 H. . .O1
O1 H. . .O2
0.0149
0.0392
0.0382
0.0499
0.1262
0.1345
−94.76
C3 H. . .O4
0.0371
0.0110
changed by the addition of water. To investigate this problem,
the optimized configuration B3 of [emim]Ac–Glc was taken as the
original free fragments to study the influence of water molecules
on the interaction of [emim]Ac–Glc. There are many different
ways for water molecules interacting with [emim]Ac–Glc system,
and a series of initial geometries were obtained by considering
that water molecules were placed near hydroxyl of cellulose. The
initial geometries were fully optimized, without imaginary vibra-
C3 H. . .O1
C4 H. . .O1
C5 H. . .O2
O2 H. . .O1
O1 H. . .O2
0.0140
0.0234
0.0138
0.0428
0.0543
0.0464
0.0790
0.0458
0.1367
0.1646
−141.54
C6
C7
C6 H. . .O1
O1 H. . .O1
C3 H. . .O2
O2 H. . .O2
0.0075
0.0339
0.0182
0.0339
0.0281
0.1208
0.0589
0.1208
−86.95
tional frequencies and six optimized geometries named D –D6 are
1
C4 H. . .O3
O2 H. . .O1
O1 H. . .O2
C4 H. . .O1
C5 H. . .O2
C3 H. . .O3
0.0143
0.0531
0.0407
0.0174
0.0174
0.0161
0.0467
0.1639
0.1264
0.0504
0.0504
0.0506
−170.62
obtained as shown in Fig. 3. Subsequently, water molecules were
removed from the optimized geometries, then, the intermolecular
interaction energies and the AIM analysis were carried out between
[emim]Ac and Glc. The results are presented in Table 5. It shows
that the addition of water molecules can change the binding site
of [emim]Ac and Glc, the geometric configurations would also be
changed. As the concentration of water increases, the changes of
their conformations become obvious, and the interaction energies
of [emim]Ac–Glc tend to decrease. When two water molecules
were added into the [emim]Ac–Glc system, the range of the abso-
lute values of interaction energies of [emim]Ac–Glc is from 10.59 to
C8
C9
O1 H. . .O2
O3 H. . .O2
C3 H. . .O1
C4 H. . .O1
C3 H. . .O2
O2 H. . .O2
0.0396
0.0359
0.0176
0.0082
0.0111
0.0294
0.1211
0.1133
0.0584
0.0299
0.0384
0.0922
−145.42
−166.18
O1 H. . .O2
O3 H. . .O2
O2 H. . .O1
C5 H. . .O2
C3 H. . .O1
C3 H. . .O3
C4 H. . .O3
0.0386
0.0356
0.0397
0.0080
0.0112
0.0192
0.0069
0.1172
0.1186
0.1277
0.0339
0.0388
0.0615
0.0259
4
3.13 kJ/mol, with a decrease about 50–80 kJ/mol. It was observed
that the H-bonds between [emim]Ac and Glc are weakened or even
destroyed when the water molecules were added to the system,
the H-bonds of Glc–Glc connected again, resulting in the precipita-
tion of cellulose (Swatloski et al., 2002). The fact not only implies
that [emim]Ac prefers to form H-bonds with water, but also can
explain the phenomenon of precipitation of the cellulose when
water molecules were added to the [emim]Ac and cellulose system.
About the interaction between two water molecules and
emim]Ac, two water molecules with per pair of ions form a sym-
[
metrical structure. The configuration C5 has the biggest absolute
value of interaction energy (141.54 kJ/mol), it is bigger than that of
3.2. Experimental study of original and regenerated cellulose
[
emim]Ac–Glc (93.72 kJ/mol). When three water molecules inter-
In order to verify the theoretical calculation results are correct,
we designed the related FT-IR, XRD, TGA and SEM experiments.
FT-IR spectra of original and regenerated cellulose obtained are
very similar. For the regenerated cellulose, the absorption band of
act with the [emim]Ac, the biggest absolute value of interaction
energy is 170.62 kJ/mol. It shows that the number of H-bond and
the interaction energies between [emim]Ac and water molecules
−
1
tend to increase due to the introduction of H O in [emim]Ac. More-
the CH2 bend vibration at 1367 cm , weaken and shifts to a lower
2
−
1
over, the number of O H. . .O type H-bond also tends to increase. It
is relatively well known that O H. . .O type H-bond is far stronger
than the C H. . .O type H-bond. When there are two or more water
molecules interact with the [emim]Ac, the interaction energies are
bigger than that of [emim]Ac–Glc. This implies that [emim]Ac pri-
ors to interact with water molecules and results in the precipitation
of cellulose. Therefore, the water content is one of the important
interference factors for the solubility of cellulose. The AIM results
and interaction energies of [emim]Ac–nH O (n = 1, 2, 3) obtained
were shown in Table 4. For all H-bonds considered in this study, the
ꢃc and ꢀ ꢃc values lie in the relative proposed ranges. [emim]Ac
prefers to form H-bonds with water molecules, as the concentration
of water increases, the number of the H-bonds tends to increase.
In this work, impact of water molecules on [emim]Ac–Glc has
also been investigated. [emim]Ac and cellulose can form a solution
system through H-bonds with each other. However, the bind-
ing sites and the interaction energies of [emim]Ac–Glc will be
wavenumber, compared to the 1379 cm peak for the original cel-
lulose. Characteristic for the O H vibrations of cellulose, a broad
−
1
vibration band locates in the range of 3350–3450 cm . Its shape is
broader than that of free hydroxyls, which is believed to be caused
by the association of the cellulose chains through H-bonds. The
absorption band of the O H vibration of the original cellulose is
3380 cm−1. In contrast, the band of the O H vibration of the regen-
−
1
erated cellulose shifted to a higher frequency (3437 cm ). This is
a hint for splitting hydrogen bonds to some extent (Kataoka and
Kondo, 1998; Zhang et al., 2005; Zhou et al., 2001). But the spec-
trum of regeneration cellulose shows new peaks in the range of
2
2
−
1
2000–2500 cm . It can speculate that the chain length might be
degraded in the case of dissolutions of cellulose in [emim]Ac. It
is highly probable that there is a conversion from cellulose I to
cellulose II.
The wide-angle X-ray diffraction curves of original and regener-
ated cellulose are obtained. Clearly, the diffraction curve of original

Z.-D. Ding et al. / Carbohydrate Polymers 89 (2012) 7–16
15
cellulose is typical cellulose I structure. It has strong crystalline
peaks at 14.98 , 16.56 and 22.68 corresponding to the (1 1 0),
Appendix A. Supplementary data
◦
◦
◦
(
3
1 1 0), and (0 0 2) planes of crystals, and weak crystalline peaks at
4.56 to the (0 0 4) plane (Oh et al., 2005). However, the diffraction
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.carbpol.2012.01.080.
◦
curve of regenerated cellulose is typical cellulose II structure by the
◦
◦
presence of the broad crystalline peak at around 12.36 and 20.42
Cao & Tan, 2005). These results indicate that the transformation
(
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