Cation does matter: How cationic structure affects the dissolution of cellulose in ionic liquids

Article abstract of DOI:10.1039/c3gc41733f

As a kind of promising solvents, ionic liquids (ILs) have been used to dissolve cellulose and great progress has been made in recent years. However, the dissolution mechanism, especially the role of cations of ILs in the dissolution of cellulose, is still in debate. In this work, 13 kinds of ILs with a fixed anion [CH₃COO]^- but varied cationic backbones and alkyl chains have been prepared and characterized. The solubilities of cellulose in these ILs were measured at different temperatures. This allowed us to systematically study the effect of cationic structures on the cellulose dissolution at a given temperature. In order to investigate the dissolution mechanism, Kamlet-Taft parameters of these ILs in the temperature range from 25 to 65 °C and ¹³C NMR spectra of 1-benzyl-3-methylimidazolium acetate ([phC₁mim][CH₃COO]) + cellulose systems at 90 °C were also determined. It was found that acidic protons on the heterocyclic rings of the cations are essential for the dissolution of cellulose in the ILs, but the van der Waals interaction of cation with cellulose is not important. These protons may form C-H...O hydrogen bonds with hydroxyl and ether oxygen of cellulose to increase cellulose solubility. Cations of the ILs may also decrease cellulose solubility by strong interaction with anions or steric hindrance effect of large size group in their alkyl chains. These interactions together with strong O-H...O hydrogen bonds between the anion and hydroxyl protons of cellulose resulted in the disruption of the inter- and intra-molecular hydrogen bonds and thus effective dissolution of cellulose.

Full text of DOI:10.1039/c3gc41733f

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Page 1 of 28
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Cation does matter：How cationic structure affects the dissolution of
cellulose in ionic liquids
a,c
c
b
Benlian Lu , Airong Xu , Jianji Wang*
a
b
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000,
P. R. China
Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals,
Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of
Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R.
China
c
School of Chemical Engineering and Pharmaceutics, Henan University of Science and
Technology, Luoyang, Henan 471003, P. R. China
As a kind of promising solvents, ionic liquids (ILs) have been used to dissolve cellulose and great
progress has been made in recent years. However, the dissolution mechanism, especially the role
of cations of ILs in the dissolution of cellulose, is still in debate. In this work, 13 kinds of ILs
-
with a fixed anion [CH
3
COO] but varied cationic backbones and alkyl chains have been prepared
and characterized. Solubilities of cellulose in these ILs were measured at different temperatures.
This allows us to systematically study the effect of cationic structures on the cellulose dissolution
at a given temperature. In order to investigate the dissolution mechanism, Kamlet-Taft parameters
1
3
of these ILs in the temperature range from 25 to 65°C and C NMR spectra of
1
-benzyl-3-methylimidazolium acetate ([phC mim][CH COO]) + cellulose systems at 90°C were
1
3
also determined. It was found that acidic protons on the heterocyclic rings of the cations are
essential for the dissolution of cellulose in the ILs, but the van der Waals interaction of cation
…
with cellulose is not important. These protons may form C-H O hydrogen bonds with hydroxyl
and ether oxygen of cellulose to increase cellulose solubility. Cations of the ILs may also
decrease cellulose solubility by strong interaction with anions or steric hindrance effect of large
…
size group in their alkyl chains. These interactions together with strong O-H O hydrogen bonds
between the anion and hydroxyl protons of cellulose resulted in the disruption of the inter- and
1

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intra-molecular hydrogen bonds and thus effective dissolution of cellulose.
Introduction
In recent years, a great effort has been made to search for alternative and sustainable energy
sources because of the rapidly growing demand for energy and high speed depletion of fossil
fuels. As the most abundant biopolymer on the earth, cellulose is regarded as one of the most
1
important biorenewable resource for fuels, chemicals and materials. However, it is very difficult
to convert cellulose into biofuels because cellulose is recalcitrant toward dissolution due to its
2
highly ordered structure. Conventional cellulose dissolution processes require harsh conditions
and use uncommon and expensive organic solvents which usually have the drawbacks of toxicity,
3
-5
volatility and difficulty in solvent recovery.
As a new class of greener solvents, ionic liquids (ILs) bring a bright prospect for cellulose
dissolution. It has been found that some ILs can dissolve cellulose under relatively mild
6
-13
conditions and the ILs can be easily recycled. Unlike traditional organic solvents, ILs have
many unique properties such as negligible vapor pressure, non-flammability, wide liquid range,
high chemical and thermal stability, and strong dissolution ability for various organic and
1
4,15
inorganic materials.
Moreover, physical and chemical properties of ILs can be easily
modulated by designing structures of their cations and/or anions. Provided that the roles of the
cations and anions in the dissolving process can be understood clearly, it is possible to design
highly efficient ILs for cellulose dissolution by coupling suitable cations and anions. Therefore,
understanding the dissolving process of cellulose in ILs is imperative for the rational design of
highly efficient cellulose solvents.
Up to now, the dissolution mechanism of cellulose in ILs is still under debate. Most of the
researchers suggest that anions of ILs play a predominant part in the dissolution by forming
hydrogen-bonds with hydroxyl protons of cellulose, and the stronger the hydrogen bond basicity
1
6
of the IL, the stronger the ability of the IL to dissolve cellulose. As far as the role of the IL
cation in the dissolution of cellulose is concerned, contradictory results have been reported. For
1
7,18
example, Rogers et al.
used NMR spectroscopy to study the mechanism of carbohydrates
dissolution in
1-n-butyl-3-methylimidazolium
chloride
([C mim]Cl),
4
2

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mim]Cl), and 1-ethyl-3-methylimidazolium acetate
1
-allyl-3-methylimidazolium chloride ([C=C
2
1
3
35/37
(
[C
2
mim][CH
3
COO]). It was shown that C and
Cl relaxation rates of acetate and chloride
anions had a strong dependence on carbohydrate content. Conversely, there were only slight
variations in the relaxation rates of carbon nuclei of the IL cations. Thus they concluded that the
interactions between the IL anions and the carbohydrates are the main driving force in the
dissolution, while the interactions between the IL cations and the solutes are nonspecific. By
1
9,20
using neutron scattering, NMR and molecular dynamics simulations, Youngs et al.
investigated β-D-glucose dissolved in [C mim][CH COO] and 1,3-dimethylimidazolium chloride
[C mim]Cl). They also suggested that the principal interactions in the liquids are the
2
3
(
1
hydrogen-bonding interactions between anions of the ILs and hydroxyl groups of glucose, and the
cations have only a minor van der Waals interaction with the sugar. In the investigation of
dissolution mechanism of cellobiose in [C
2
mim][CH COO] by NMR spectroscopy under various
3
21
solute concentrations and temperatures, Zhang and coworkers found that both anion and cation
of the IL formed hydrogen bonds with the hydroxyl groups of cellobiose. The acetate anions
prefer to form hydrogen bonds with hydroxyl protons in cellobiose, while protons of the cations,
especially the most acidic H2 proton, interacted directly with the oxygen atoms of hydroxyls in
cellobiose. When all the hydroxyls of cellobiose were acetylated, the interactions between
cellobiose octa-acetate and the IL become very weak, and the acetylated cellobiose was almost
insoluble in this IL even at 100°C. This indicated that hydrogen bonding of anion and cation with
cellobiose was the major driving force for cellobiose dissolution in this system. However, based
2
2
on the analysis for the interactions involved in cellulose dissolution process, Lindman et al.
proposed that hydrophobic interactions between cations of ILs and cellulose (both are
amphiphilic) are the predominant factor.
Actually, literature survey reveals that almost all the previous investigations were focused on
the anionic action of ILs in cellulose dissolution, and very few studies have been conducted to
investigate the cationic effect of ILs. In this work, 13 kinds of ILs have been prepared which have
-
a fixed anion [CH
3
COO] but varied cationic backbones and alkyl chains such as
+
+
1
1
-butyl-3-methylimidazolium [C
4
mim] , 1-methoxyethyl-3-methylimidazolium [C
1
OC mim] ,
2
+
-hydroxyethyl-3-methylimidazolium
[C
2
OHmim] ,
1-butyl-2,3-dimethylimidazolium
3

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N-ethyl-N-methylmorpholium
+
+
[
[
[
[
C
4
2
dmim] ,
1-benzyl-3-methylimidazolium
N-allyl-N-methylmorpholium
N-butyl-N-methylpiperidium
[phC
1
mim] ,
+
+
C
mmor] ,
[C=C
2
mmor] ,
N-allyl-N-methylpiperidium
+
+
C=C
2
mpip] ,
[C
4
mpip] ,
N-butyl-N-methylpyrrolidinium
+
+
+
C
4
mpyr] , 1-butyl-3-ethylbenzimidazolium [C
4
ebim] , 1,3-diethylbenzimidazolium [C
2
ebim] ,
+
and 1-butyl-3-ethylbenzotriazolium [C
4
ebt] . Solubilities of microcrystalline cellulose (MCC) in
these ILs were determined at different temperatures, and the effect of cationic structure on
cellulose solubility was examined. In order to obtain new insight into the possible dissolution
mechanism, Kamlet-Taft parameters for some of the ILs in the temperature range from 25 to
1
3
6
5
°C and
mim][CH
C
NMR spectra for [phC
1
mim][CH COO] and cellulose in
3
[
phC
1
3
COO]+cellulose mixtures at 90°C were also measured experimentally. The
results have been discussed from hydrogen bond donor ability, hydrogen bond acceptor ability,
1
3
polarizability of the ILs, and C NMR chemical shifts of the representative IL and cellulose.
Results and discussion
Chemical structures of cations and anion of the ILs prepared in the present work were shown in
1
Scheme 1. All the ILs were characterized by H NMR. Melting point (T
m
) and glass transition
temperature (T
g
) of these ILs were determined by differential scanning calorimetry (DSC). Their
thermal decomposition temperatures (T
d
) were determined by thermogravimetric analysis (TGA).
It was shown that the prepared ILs are all room temperature ILs and have reasonable thermal
stability. The detailed spectral and thermal property data were given in Table S1 of Electronic
Supplementary Information (ESI).
It has been reported that impurity contents in ILs may have a significant effect on their
2
3
physical and chemical properties. Considering the fact that in the synthesis process of our ILs,
the main impurities would be water, halides and un-reacted organic starting materials such as
1
-metheylimidazole, we determined the contents of these impurities in the ILs, and the detailed
data were given in Table S2 (ESI). It can be seen that water content in these ILs was in the range
−
1
of 300 - 600 ppm, chloride content varied from 0.0082 to 0.015 molkg , and bromide content fell
−
1
−1
between 0.00028 molkg and 0.0030 molkg . However, the content of the un-reacted
1
-methylimidazole was found to be smaller than the detection limit of gas chromatograph. These
4

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results suggest that the quality of the ILs prepared in this work is reliable to investigate cellulose
dissolution.
Cationic structure dependence of cellulose solubility
Table 1 shows the solubility data of microcrystalline cellulose in the studied ILs as a function of
temperature. The cellulose used in the present work has a viscosity-average polymerization
degree of 229. It was found that our solubility data of cellulose in [C
4
mim][CH COO] were in
3
8
good agreement with the values reported by Xu et al., although they used the unit of wt% to
24
express cellulose solubility. Vitz et al. also determined the solubility of cellulose in
mim][CH COO] at 100°C, but their value (24 g/mol) was significantly smaller than ours (54
[
C
4
3
g/mol). The most probable reason is that the cellulose (Alfa Aesar) we used is the same (same
8
source and same polymerization degree) as that used by Xu et al., however, different cellulose
2
4
(
Avicel PH-101, Fluka) was used by Vitz et al. To the best of our knowledge, no solubility data
of cellulose in the other ILs investigated in this work has been reported in literature.
It is evident from Table 1 that cationic backbone of the ILs plays a crucial role in the cellulose
dissolution. The imidazolium-based ILs (1-5) can readily dissolve cellulose with high solubility,
some of the ILs with the cations of saturated heterocyclic ring (6-8) only have a certain extent of
ability to dissolve cellulose at higher temperature, but all the other ILs (9-13) fail to dissolve
cellulose even at the temperature of 120°C. For example, although [C
mpip][CH COO] (9) and [C mpyr][CH COO] (10) have the same cationic side chain,
solubility of cellulose in these ILs decreased dramatically with their cationic backbones. At
0°C, solubility of cellulose in [C mim][CH COO] is as high as 49 g/mol, whereas that in
mpip][CH COO] and [C mpyr][CH COO] is negligibly small. This finding may be
understood from the fact that compared with the saturated piperidium and pyrrolidinium cations
4
mim][CH COO] (1),
3
[
C
4
3
4
3
9
4
3
[
C
4
3
4
3
+
+
+
(
(
[C
4
mpip] and [C
4
mpyr] ), the conjugated imidazolium cations [C
4
mim] have acidic protons
H2, H4 and H5). Thus it is favorable for the more acidic proton to form hydrogen bonds with
oxygen atoms of cellulose. This property of acidic proton in cationic backbones has been
confirmed by comparing the solubility of cellulose in [C dmim][CH COO] (4) with that in
mim][CH COO] (1). It was found that the solubility of cellulose in [C
4
3
[
C
4
3
4
dmim][CH COO] was
3
+
significantly decreased owing to the replacement of acidic H2 in [C
4
mim] cation by a methyl
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unit −CH
3
. For example, solubility of cellulose in [C
4
mim][CH
COO]. Furthermore, the disability of
COO] (12) and [C ebt][CH COO] (13) for the
3
COO] is 25 g/mol at 50°C, but
only 5 g/mol is observed in [C
4
dmim][CH
3
[
C
4
ebim][CH
3
COO] (11), [C
2
ebim][CH
3
4
3
dissolution of cellulose may be interpreted from the absence of acidic proton in the cationic
backbone or the big steric hindrance in the bulky benzimidazolium and benzotriazolium cations
which makes the cations difficult to access the oxygen atoms of cellulose.
It is also noted that nature of the functional groups bonded to the N-position in cationic
backbone obviously affects the solubility of cellulose. For instance, replacement of −C
4
9
H in
+
[
C
4
mim] by −CH
2
CH
2
OCH
3
and −CH
2
CH
2
OH results in a decrease in cellulose solubility (see
and -OH groups, which
reduces the acidity of protons in cationic heterocyclic rings. As evidenced by α and β parameters
Table 1). This may be ascribed to the donating electron nature of -OCH
3
2
5
and density functional calculations of Deng and coworkers, hydroxyl proton of −CH
2
CH OH
2
may form hydrogen bond with oxygen of the anion, leading to the competition with the formation
of hydrogen bond between anion and hydroxyl proton of cellulose. On the other hand,
+
replacement of −C
4
H
9
in [C
4
mim] by −CH
2
C
6
5
H also leads to a decrease in cellulose solubility.
This suggests that van der Waals interactions of ILs cations with cellulose are not important for
the cellulose dissolution. The insolubility of cellulose in [C ebim][CH COO] (11),
ebim][CH COO] (12) and [C ebt][CH COO] (13) strongly supports this suggestion. In
addition, we found that at higher temperature (say 100°C), both [C=C mmor][CH COO] (7) and
C=C mpip][CH COO] (8) can dissolve cellulose, but the solubility in [C=C mmor][CH COO] is
much higher than that in [C=C mpip][CH COO]. This might suggest that morpholium cation may
4
3
[
C
2
3
4
3
2
3
[
2
3
2
3
2
3
have a stronger interaction with cellulose than piperidium cation although they all have saturated
six-membered ring.
Based on the above analysis, it is reasonable to state that acidic protons in cationic backbones
are indispensable for the dissolution of cellulose in ILs. Among the cations investigated here,
imidazolium cation is the best for the dissolution of cellulose, in which acidic proton is involved
in hydrogen bonding interactions between the cations and cellulose. However, the van der Waals
interactions of ILs cations with cellulose are not important for the cellulose dissolution. For the
rational design of efficient IL solvents for cellulose, cationic backbone with strong acidic protons
6

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but without high electronegativity atoms (say oxygen atom) and bulky steric hindrance groups in
the side chain should give top priority.
Effects of hydrogen bond acidity, hydrogen bond basicity and dipolarity / polarizability of
the ILs on the solubility of cellulose
In order to study the microscopic mechanism for the cationic structure effect in cellulose
dissolution, hydrogen bond acidity, hydrogen bond basicity and dipolarity / polarizability for 6
kinds of ILs which can efficiently dissolve cellulose, were determined in the temperature range
from 25 to 65°C, and the results were included in Table 2. Unfortunately, we are unable to
determine these parameters for the other ILs because they are generally not liquids within the
investigated temperature range.
It is known that the most common method to describe the hydrogen bond acidity, hydrogen
2
6
bond basicity and dipolarity / polarizability of ILs is the Kamlet-Taft equation:
XYZ = XYZ
0
+ sπ* + aα + bβ
(1)
where XYZ stands for a reaction rate or equilibrium constant, or a position or intensity of spectral
absorption, XYZ is a constant, α, β and π* represent hydrogen bond acidity, hydrogen bond
0
basicity and dipolarity / polarizability of ILs, respectively, and a, b and s are solvent-independent
correlation coefficients. It can be seen from Table 2 that the temperature dependence of α
parameter is much stronger than β and π* parameters because of the strong thermochromism of
2
7
the standard betaine dye in solution. With increasing temperature, the β and π* parameters keep
invariable, whereas a good linear relationship exists between α parameter and the reverse of
temperature (Figs S1−S3, ESI). Thus, values of α parameter at other temperatures can be
calculated from this linear relationship.
To investigate the correlation between cellulose solubility and Kamlet-Taft parameters of the
ILs, we used natural logarithm of the solubility (lnS) as XYZ in the Kamlet-Taft linear solvation
2
8
free energy relationship (equation 1) and performed a multivariate linear regression to
determine the coefficients for the variables {α, β, π*}. Using p-value as a test of significance, a
maximum p-value (p<0.05) was set to indicate it. When a multivariate linear regression was
7

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performed at 90°C using all variables {α, β, π*}, we obtain:
lnS = 3.82(0.32) + 1.91(0.17)* α + 1.97(0.13)* β−3.19(0.26)* π*
(2)
The italic numbers in the parentheses present the p-values for each coefficient. It can be seen that
no coefficient proved significant. Then, we performed a regression using each pair of variables in
the three pairs {α, β}, {α, π*} and {β, π*}, and the following equations were found:
lnS = -0.48(0.68) + 3.21(0.0054)* α + 2.21(0.10)* β
lnS = 7.26(0.18) + 1.13(0.44)* α −4.16(0.30)* π*
lnS = 9.41(0.011) + 1.54(0.26)* β−7.12(0.0094)* π*
(3)
(4)
(5)
Again there is not a relationship contained all significant coefficients, so we must test all three
variables {α}, {β} and {π*} individually. Under these circumstances, we have:
lnS =2.00(0.0020) + 2.97(0.0089)* α
lnS = 2.64(0.52) + 0.60(0.87) * β
(6)
(7)
(8)
lnS = 10.81(0.0016) −6.89(0.0062)* π*
It is clear that the relationship presented by equation (6) contains all significant coefficients.
This indicates that the solubility of cellulose depends on {α}. As far as π* parameter is concerned,
because the range of its values is quite narrow (see Table 2), it is not safe to state that lnS has a
linear correlation with π* parameter, although the relationship (8) also contains all significant
coefficients. Moreover, the relationship (7) does not contain any significant coefficients,
indicating that β parameter is an insignificant variable in the present work. Similar results have
been obtained at other temperatures under study.
It has been known that α parameter can be used as a measure of hydrogen bond donor ability
2
9
(
hydrogen bond acidity) of ILs and depends on the cationic structure. A larger α parameter value
indicates a stronger hydrogen bond acidity. It was noted from equation (6) that lnS has a linear
correlation with α parameter, and cellulose solubility increases with the increase of α parameter
of the ILs. Fig. 1 shows such a linear correlation at 90°C, and the same trend was observed at the
other temperatures (Figs S4-S8, ESI). Considering the donor and acidity characteristics, this
suggests that cations of the ILs may affect the cellulose solubility by forming hydrogen bonds
with hydroxyl or ether oxygen of cellulose. The stronger the hydrogen bond acidity of IL cation,
the stronger of its ability to form hydrogen bond with cellulose. This confirms the analysis of
8

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experimental results discussed in the previous section.
The β parameter has been established as a measure of the hydrogen bond accepting ability
3
0–32
(
hydrogen bond basicity) of ILs.
To our knowledge, β parameters for most of the ILs
investigated in the present work are not available in the literature. Therefore, we determined these
parameters and the results were given in Table 2 as a function of temperature. It can be seen from
the above linear regression analysis (eq. 7) that there has no correlation between solubility of
cellulose and β parameter of the ILs at a given temperature. Considering the fact that these ILs
-
have the same anion [CH
3
COO] , this suggests that the hydrogen bond accepting ability of these
ILs is essentially controlled by their anion, and the ability of their cations to accept acidic protons
for the formation of hydrogen bonds is very weak. The possible reason for the unusual behavior
observed for [C
2
OHmim][CH
3
COO] is that the terminal hydroxyl proton in the alkyl chain of this
IL forms hydrogen bond with the anion, lowering the hydrogen bond accepting ability of the
2
5,33
anion for the hydroxyl protons of cellulose.
Similar result has been reported for
3
4
[
C
2
OHmim][(CH PO ] by Ohno and his coworkers. Therefore, it is easy to understand why
3
)
2
4
solubility of cellulose varies significantly with β values of the ILswhich have the same cation but
8
different anions.
The π* value of the ILs derives from two cumulative effects: ion pairing strength and
2
9
polarizability of the cation and anion. A large π* value means strong interaction or strong
association between cation and anion in the IL. It seems likely from equation (8) and Tables 1-2
that cellulose solubility roughly decreases with the increase of π* parameter of the ILs at a given
temperature although we are not sure if the variation is linear because of the reason mentioned
above. This suggests that capacity of the ILs to dissolve cellulose decreases with the enhanced
interaction between cation and anion of the ILs because the stronger cation-anion interactions in
the ILs would decrease the self-diffusion ability of the ions, thus decreasing the possibility of
hydrogen bond interaction of cellulose with anions and/or cations. On the other hand, for the ILs
with the same anion, the increase in the delocalization degree of positive charges around the
cationic ring will result in the increase of polarizability of the cation and the decrease of Coulomb
interaction strength between cation and anion of the ILs, leading to the decrease of π* values.
Thus cations with highly delocalized positive charge would be preferred in the design of ILs for
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efficient dissolution of cellulose.
From the above discussion, it is clear that for the investigated ILs, β parameter is mainly
dependent on anionic structure, whereas α parameter is cation dependence. Cations can affect
solubility of cellulose in the ILs in two ways: (i) acidic proton in the cationic heterocyclic rings
forms hydrogen bonds directly with oxygen of cellulose, resulting in the increase of cellulose
solubility; (ii), polarizability of the cation and its ion pairing strength with anion affect solubility
of cellulose indirectly by influencing the ability of the IL anion to form hydrogen bonds with
cellulose, high polarizability and weak ionic association would benefit the dissolution of cellulose
in the ILs.
Interaction between cellulose and [phC
1
mim][CH COO] and the possible dissolution
3
mechanism
1
3
In order to further investigate the dissolution mechanism of cellulose in the ILs, C NMR
measurements of [phC mim][CH COO] and cellulose were carried out in the solutions of
phC mim][CH COO] + cellulose at 90°C, and the NMR data were given in Table 3 and Table 4,
respectively. For the sake of easy understanding, the schematic structure and numbering of C
atoms in [phC mim][CH COO] were shown in Fig. 2.
1
3
[
1
3
1
3
It is noted from Table 3 that with the increase of cellulose concentration, the signal of C2 atom
of the imidazolium ring moves upfield (a decrease of chemical shift), whereas that of the
carboxyl C15 atom downfield (an increase of chemical shift) significantly. This indicates that in
the cellulose dissolution, the acidic H2 proton in imidazolium cation most probably interacts with
the oxygen atoms in cellulose through hydrogen bonding, which leads to an increase of the
electron cloud density of the C2, and thus an upfield chemical shift. Simultaneously, as a
−
hydrogen bond acceptor, the oxygen atom in [CH
3
COO] of the IL forms hydrogen bond with the
hydroxyl proton of cellulose. This results in the decrease of the electron cloud density of the C15,
thus its chemical shift moves downfield. Moreover, it can be seen that the downfield shift value
of C15 was greater than that of C2, indicating that the carboxyl oxygen atom has stronger
interaction with cellulose than H2 proton. In addition, the observable but small upfield shift of
C14 may be due to the redistribution of the electron cloud density around the carboxyl group after
the interaction between carboxyl oxygen and hydroxyl proton. However, little changes have been
1
0

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found for the chemical shifts of the other carbon atoms (C4-C13) in the imidazoium cation. This
reveals that the interactions of the oxygen in cellulose with the C4 and C5 protons on the
imidazolium ring are very weak, which is in agreement with the weak acidity of these protons.
Also, this is an indicative for the very weak interaction between cellulose and the benzene unit in
the IL cation.
Moreover, it can be seen from Table 4 that the increase of cellulose concentration results in the
upfield shifts for C3 and C6 atoms, but downfield shifts for C1, C2 and C5 atoms in cellulose. It
is generally accepted that the interaction network of cellulose consists of three components:
intrachain, interchain and intersheet interactions, which contribute to the recalcitrance of
3
5,36
…
…
cellulose to chemicals or enzymes.
O(3)-H O(5) and O(2)-H O(6) hydrogen bonds were
…
formed between neighboring residues of the same chain (intrachain), while O(6)-H O(3)
hydrogen bonds were formed at different chains in the same sheet (interchain) (Fig. 3). At the
…
same time, numerous C-H O contacts and van der Waals interactions connect the residues on
37
neighboring sheets (intersheet). In the dissolution process of cellulose in [phC
1
mim][CH COO],
3
-
the hydrogen bonding interactions of the oxygen on the carboxyl group of [CH
3
COO] with the
proton of the C3-OH and C6-OH units in cellulose disrupt the inter- and intra-chain hydrogen
bonds in cellulose, and thus resulting in an increase of the electron cloud density of the C3 and
1
3
C6 in cellulose and an upfield shift of their C signals. Meanwhile the acidic protons on the
imidazolium cations, especially the H2 proton, interact with the C2-OH and the ether oxygen O,
…
and disrupt the intersheet C-H O bonds in cellulose, which leads to the decrease in the electron
1
3
cloud density of the C1, C2 and C5 atoms in cellulose. Consequently, the C NMR signals of
these carbon atoms shift downfield. These results are consistent with the conclusions drawn from
3
8,39
molecular dynamic (MD) simulation.
From the above analysis, it is evident that both the cation and anion of the IL play important
roles in the cellulose dissolution. The anion of the IL forms hydrogen bonds with the hydroxyl
proton of cellulose, while the acidic protons on the imidazolium cation contact with the hydroxyl
…
oxygen and ether oxygen in cellulose through weak hydrogen bond C-H O interactions. The
11

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hydrogen bonding interactions of the hydroxyl protons of cellulose with anion of the IL would be
much stronger than those of the hydroxyl oxygen and ether oxygen of cellulose with protons on
the IL cations. However, the contribution from cations of the ILs is indispensible although the
role of their anion is predominant in the cellulose dissolution process.
Conclusions
-
In this work, 13 kinds of ILs which have the same [CH
3
COO] anion but varied cationic
backbones and substituent groups have been prepared and characterized to investigate how the
cations of the ILs affect cellulose dissolution. It was found that cations of the ILs significantly
…
affect cellulose dissolution. Acidic protons of their cations may form C-H O hydrogen bonds
with hydroxyl oxygen and ether oxygen of cellulose to increase cellulose solubility. These cations
may also decrease cellulose solubility by strong association with anions, existence of high
electronegativity atoms (like oxygen and/or nitrogen) in the cationic backbone or large sized
group (like phenyl group) in the alkyl chain. These factors decrease the ability of anion and
cation to form hydrogen bonds with cellulose by anionic competition or steric hindrance. Thus
the dissolution of cellulose was determined by both the anion and the cation of the ILs.
Depending on the structures of ILs, sometimes the anions predominate and sometimes the cations
do. For example, if there is a big steric hindrance group in the alkyl chain and/or there is not an
acidic proton in the cationic backbone, it is very difficult for the ILs to dissolve cellulose.
Therefore, to design ILs with efficient dissolution capacity for cellulose, anions with strong
hydrogen bond basicity and cations with acidic protons but without high electronegativity atoms
(
like oxygen and/or nitrogen) in the cationic backbone and large sized group in the alkyl chain
would be a rational choice. The above findings provide new insight into the role of cationic
structureofILs in cellulose dissolution, and would be useful for the rational design of ILs used in
the highly efficient dissolution of cellulose. Nonetheless, duo to the narrow range of π* values of
the ILs investigated in this work, the correlations of cellulose solubility with π* and {α, π*}
cannot be determined definitely. More data are needed to confirm these correlations.
Experimental
1
2

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Materials
1
-Methylimidazole (99%) was purchased from Shanghai Chem. Co., and 1,2-dimethylimidazole
(
(
98%), N-methylmorpholine (99%), benzimidazole (99%), bromoethane (98%), 1-chlorobutane
99%), 2-chloroethyl methyl ether (98%), 4-nitroaniline (97.0%), microcrystalline cellulose and
anion exchange resin (Ambersep 900 (OH), Amberlite IRA-400 (Cl)) were from Alfa Aesar.
-Methylpiperidine (97%), 1-methylpyrrollidine (98%), 1H-benzotriazole (99%), allyl chloride
98%), benzyl chloride (AR) and tetrabutyl ammonium bromide (98%) were obtained from
1
(
Aladdin Chem. Co. Ltd. 2,6-Dichloro-4-(2,4,6-triphenyl-1-pyridinio) phenolate (≥97.0%) were
purchased from Sigma-Aldrich. 2-Chloroethanol (AR) and acetic acid (>99.5%) were obtained
from Shanghai Shiji Chem. Reagent Co. Ltd. N,N-diethyl-4-nitroaniline (97.0%) and deuterated
DMSO (DMSO-d ) were from Nanjing Chemlin Chem. Industrial Co. Ltd. and Qingdao Weibo
6
Tenglong Technol. Co. Ltd., respectively. Among the above materials, 1-methylimidazole was
distilled, microcrystalline cellulose was dried under vacuum for 12h at 70°C, 4-nitroaniline was
recrystallized from methanol and then dried under vacuum for 12h at 40°C before use, and the
others were used as received. The viscosity-average polymerization degree of 229 was
determined for the cellulose by Ubbelohde viscometer in cupriethylenediamine hydroxide
solutions.
Synthesis of the ILs
The imidazolium-based ILs [C
OHmim][CH COO] and [C
corresponding chloralkanes by using the procedures described in the literature.
mpip][CH COO], [C=C mpip][CH COO] and [C mpyr][CH COO] were also prepared by a
similar way.
4
mim][CH
3
COO], [phC
1
mim][CH
3
COO], [C
1
OC
2
mim][CH COO],
3
[
C
2
3
4
dmim][CH
3
COO] were synthesized from 1-methylimidazole and
8
[
C
4
3
2
3
4
3
4
0
[
C
2
mmor][CH COO] was synthesized by fellowing the procedure reported by Yeon. In a
3
typical procedure, 1-bromoethane (0.22 mol) was added dropwise to a round-bottom flask
containing N-methyl morpholine (0.2 mol) and acetonitrile (100 ml) at room temperature, and the
mixture was stirred for 72h to produce a pale yellow solid. This solid was then recrystallized with
acetone at room temperature to obtain [C
2
mmor][Br] (white solid). Aqueous [C mmor][OH]
2
1
3

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solution was obtained by passing an aqueous [C
anion exchange resin (Ambersep 900 (OH)). Then the aqueous [C
neutralized with equal mole of acitic acid. Water in the solution was removed by rotary
evaporation and [C mmor][CH COO] was then dried under vacuum for 72h at 60°C.
C=C mmor][CH COO] was prepared by the same procedure.
2
mmor][Br] solution through a column filled with
2
mmor][OH] solution was
2
3
[
2
3
[
C
4
ebim][CH
3
COO], [C
2
ebim][CH
3
COO] and [C
4
ebt][CH
3
COO] were synthesized by the
following steps: (i) the synthesis of 1-ethylbenzimidazole or 1-Butylbenzotriazole; (ii) the
4
1,42
synthesis of [C
4
ebim][Cl], [C
COO], [C ebim][CH
COO], benzimidazole (bim, 0.3mol) and tetrabutyl ammonium bromide (2%
2
ebim][Br] and [C
4
ebt][Br];
and (iii) the synthesis of
[
C
4
ebim][CH
3
2
3
COO] and [C ebt][CH
4
3
COO]. For example, for the synthesis
of [C
4
ebim][CH
3
mole percent of bim) were dissolved in an aqueous solution of 30% sodium hydroxide (100ml)
under vigorous stirring, then bromoethane (0.3mol) was added dropwise at room temperature.
After the dropping was finished, the mixture was stirred at 50°C for 24h until two phases were
formed. Then the upper organic phase was separated and washed with deionized water and ethyl
acetate. The remaining ethyl acetate was removed by rotary evaporation to give
1
1
-ethylbenzimidazole. Then 1-ethylbenzimidazole and 1-chlorobutane were mixed (molar ratio
:1.05) in a round bottom flask, and stirred at 70°C for 72h. The gray solid product [C ebim][Cl]
4
was collected and washed repeatedly with ethyl acetate and then dried under vacuum at 60°C for
4
8h. Finally, the aqueous [C
4
ebim][CH
3
COO] solution was prepared by passing an aqueous
-
[
C
4
mim][Cl] solution through a column filled with CH
exchanging Amberlite IRA-400(Cl) with aqueous CH
solution by rotary evaporation, the product [C ebim][CH
COO] and [C ebt][CH
3
COO anion exchange resin (obtained by
3
COONa solution). Removing water in the
4
3
COO] was collected and dried under
COO] were prepared by similar
vacuum at 60°C for 24h. [C
2
ebim][CH
3
4
3
procedures.
Characterization of the ILs
1
All the ILs prepared in this work were characterized by H NMR spectroscopy at room
temperature on a Bruker Avance-400 NMR spectrometer. Melting temperature (T ) and glass
transition temperature (T ) of the ILs were determined using a differential scanning calorimeter
Netzsch DSC 204F1) at a cooling and heating rate of 10°C min under dry N
m
g
-
1
(
2
atmosphere. The
1
4

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thermal decomposition temperatures (T ) of the ILs were measured by using a Netzsch STA 449c
d
thermal gravimetric analyzer (TGA). All samples were heated up to 600°C in aluminium pans at
-
1
a heating rate of 10°C min under nitrogen atmosphere. The temperatures reported from TGA
data were the onset temperatures, which were determined from the step tangent.
Determination of impurity contents in the ILs
2
3
Based on the procedure described by Seddon and his co-workers, chloride and bromide contents
were determined by means of a PCl-1 chloride-selective electrode and a PBr-1 bromide-selective
electrode (both, Shanghai Precision & Scientific Instrument Co., Ltd.), respectively. To this end,
a 232 saturated calomel reference electrode and a PHS-25 digital pH meter (both, Shanghai
Precision & Scientific Instrument Co., Ltd.) were used. The measurements were conducted at
2
5°C, and calibration curves were obtained from aqueous solutions of the corresponding
precursor halide ionic liquids. Triplicate measurements were performed and the reproducibility
was within 2%.
Water contents in the ILs were measured by coulometric Karl-Fischer titration with a
Metrohm 851 moisture titrator. 1-Methylimidazole is also a possible impurity in the ILs, its
content was determined by gas chromatograph using the standard addition method. Analysis was
performed on an Agilent 7890A gas chromatograph equipped with a PONA column and a FID
4
3
detector, and the procedure reported in the literature was closely followed.
Dissolution of cellulose in the ILs
Dried microcrystalline cellulose (0.5wt% of the IL) was added into a dried IL (2.0g) which was
sealed in a colorimetric tube. The mixture was then heated and stirred at a given temperature in
an oil bath (DF-101S, Gongyi Yingyu Instrument Factory). After the solution became optically
clear under a polarization microscope (Nanjing Jiangnan Novel Optics Co. Ltd.), additional
cellulose was added. When the cellulose could not be dissolved further within 2h, it could be
judged as becoming saturated. Solubility of cellulose at the given temperature could be calculated
from the amount of solvent and cellulose added. The solubility value of cellulose in each IL was
measured at the temperature range from 40°C to 120°C with 10°C intervals. According to the
1
6
recommendation by Rogers et al, the solubility data of cellulose were reported in terms of mass
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-1
of cellulose dissolved in per mole of IL (g.mol ).
Measurements of the Kamlet-Taft parameters for the ILs
The Kamlet-Taft parameters were determined by following the procedure reported by Ohno et
4
4
al.
To
a
given amount of IL,
a
concentrated solution of 4-nitroaniline,
N,N-diethyl-4-nitroaniline or 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio) phenolate (Reichardt's
dye 33) in dry methanol was added. The methanol was then removed under vacuum at 40°C for
6
h. This solution was added into a quartz cell with 2mm light-path length, and the cell was
capped and sealed. Temperature of the sample cell was controlled by water circulation. Using a
TU-1810 ultraviolet-visible spectrophotometer, the wavelength at the maximum absorption (λmax
)
was determined and used to calculate the α, β and π* values by the following equations:
-4
ν
(dye) =1/(λmax (dye) ×10 )
(9)
E
T
(30) =0.9986×(28592/λmax (Reichardt’s dye 33))-8.6878
(10)
(11)
(12)
(13)
π* = 0.314×(27.52-ν（N,N-diethyl-4-nitroaniline）
)
α= 0.0649 E (30)-2.03-0.72π*
T
β = (1.035ν（N,N-diethyl-4-nitroaniline）+2.64-ν（4-nitroaniline）)/2.80
1
3
Measurements of C NMR spectra of the [phC
1
mim][CH
3
COO] + cellulose mixtures
1
3
Samples used for the C NMR experiments were prepared by adding different amounts of MCC
into solution of [phC mim][CH COO] + DMSO-d (15 wt% of the IL) under stirring at 40°C.
After the cellulose was dissolved completely, the sample was transferred into a 5mm NMR tube.
1
3
6
1
3
Measurements of C NMR spectra were acquired on a Bruker DMX 300 spectrometer at 90°C.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No.
2
1133009 and 21273062) and the National Basic Research Program of China
No.2011CB211702). The authors are grateful to Professor Tom Welton, Imperial College,
London, for his valuable suggestions.
(
1
6

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Table 1 Solubilities of microcrystalline cellulose in the ILs at different temperatures
Solubility (g/mol)
Entry
IL
40°C 50°C 60°C 70°C 80°C 90°C 100°C 110°C 120°C
1
2
3
4
5
6
7
8
9
[C
[C
[C
[C
4
1
2
4
mim][CH
3
COO]
23
8
a
25
17
1
5
1
a
26
21
18
18
14
1
31
23
22
24
17
3
36
29
26
27
20
6
49
39
30
29
22
18
17
10
2
54
44
33
32
24
21
21
14
3
56
49
34
35
30
28
24
16
4
58
56
34
37
34
31
28
19
7
OC
2
mim][CH
3
COO]
COO]
COO]
mim][CH COO]
mmor][CH COO]
mmor][CH COO]
mpip][CH COO]
COO]
OHmim][CH
dmim][CH
3
3
a
[phC
[C
1
3
a
2
3
a
[C=C
2
2
3
a
1
a
2
4
12
a
[C=C
3
a
a
a
[C
[C
[C
[C
[C
4
4
4
2
4
mpip][CH
3
a
a
a
a
1
1
1
1
1
0
1
2
3
mpyr][CH
3
COO]
COO]
COO]
a
a
a
a
a
a
1
2
3
ebim][CH
ebim][CH
3
a
a
a
a
a
a
a
a
<1
<1
<1
3
a
a
a
a
a
a
a
a
ebt][CH
3
COO]
a
a
a
a
a
a
a
a
a
Insoluble at given temperature.
2
0

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Table 2 Values of α, β and π* parameters for the ILs at different temperatures
α, β and π*
IL
a
2
5°C
35°C
0.61
1.14
1.02
0.50
1.10
1.06
0.43
1.05
1.09
0.35
1.10
1.17
0.25
1.12
45°C
0.61
1.14
1.02
0.50
1.10
1.06
0.42
1.05
1.09
0.35
1.10
1.17
0.25
1.12
55°C
0.60
1.14
1.02
0.50
1.106
1.06
0.41
1.05
1.09
0.35
1.10
1.17
0.24
1.12
65°C
90°C
0.59
1.14
1.02
0.48
1.10
1.06
0.39
1.05
1.09
0.34
1.10
1.17
0.23
1.12
1.15
α
β
0.61
1.14
1.02
0.51
1.10
1.06
0.44
1.05
1.09
0.36
1.10
1.17
0.25
1.12
0.60
1.14
1.02
0.49
1.10
1.06
0.41
1.05
1.09
0.34
1.10
1.17
0.24
1.12
1.15
0.54
0.97
1.06
[
[
[
[
[
C
4
1
3
mim][CH COO]
π*
α
C
OC
2
mim][CH
3
COO]
β
π*
α
phC
1
mim][CH
3
COO]
β
π*
α
C
2
mmor][CH
3
COO]
β
π*
α
C=C
2
mmor][CH
3
COO]
β
π*
α
1.15
b
1.15
b
1.15
b
1.15
b
c
0.54
b
b
c
b
b
b
b
b
b
c
[
C
2
OHmim][CH
3
COO]
β
0.97
c
π*
1.06
a
b
Extrapolated to 90°C; can’t be measured; data determined at 65°C.
2
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13
Table 3 The C NMR chemical shifts (δ/ppm relative to DMSO) of [phC
1
mim][CH
3
COO] in the solution of
[
phC
1
3
mim][CH COO] + cellulose at 90°C
δ/ppm
wt % MCC
C2
C4
C5
C6
C7
C8
C9,13
C10,11,12
128.417
128.454
128.506
C14
C15
0
5
8
138.059
137.958
137.821
122.264
122.258
122.225
123.825
123.842
123.861
35.201
35.245
35.296
51.632
51.686
51.780
134.969
134.922
134.826
128.146
128.167
128.195
25.412
25.351
25.240
174.670
174.847
175.069
2
2

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1
3
Table 4 The C NMR chemical shifts (δ/ppm relative to DMSO) of cellulose in the solution of
phC mim][CH COO] + cellulose at 90°C
[
1
3
δ/ppm
wt % MCC
C1
C2
C3
C4
C5
C6
5
8
102.662
102.725
0.063
73.839
74.029
0.190
75.167
75.124
-0.043
79.131
79.122
-0.009
75.845
75.870
0.025
60.767
60.421
-0.346
Δδ
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5
4
4
O
5
6
3
N
1
N
3
R1
+
N
1
2
H C
3
2
R2
H C
R
3
1
-5
6,7
+
+
1
2
3
4
5
:R =C H ,
R =H [C mim]
2 4
1
4
9
6:R=C H
[C mmor]
2
2
5
+
+
:R =CH CH OCH , R =H [C OC mim]
1
2
2
3
2
1
2
7:R=CH CH:CH [C=C mmor]
2 2 2
+
:R =CH CH OH, R =H [C OHmim]
1
2
2
2
2
+
:R =C H ,
R =CH [C dmim]
1
4
9
2
3
4
+
:R =CH C H ,
R =H
[phC mim]
1
2
6
5
2
1
4
5
3
4
6
3
5
+
N
2
+
2
N
1
R
1
H C
CH (CH ) CH
2 2 2 3
3
H C
3
8
,9
10
+
+
8
9
:R=CH CH:CH [C=C mpip]
:R=C H9
[C mpyr]
4
2
2
2
+
[C mpip]
4
4
CH (CH ) CH
2
2 2
3
7
1
7
R
2
1
8
8
N
N
6
5
6
5
N
2
N
3
9
3
N
9
3
4
4
CH CH3
2
CH CH
2
3
1
1,12
1
+
[C4ebt]+
1
1
1:R=C H9 [C ebim]
4
4
+
2:R=C H5 [C ebim]
2
2
_
O
O
CH3
C
Scheme 1 Schematic structures of cations and anion of the ILs studied in the present work.
2
4

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Fig. 1 The linear correlation between solubility of cellulose and α parameter of the ILs at 90°C.
2
5

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4
5
1
4
9
1
0
N
3
N
1
1
5
8
11
6
7
-
O
2
O
1
3 12
1 3
Fig. 2 The schematic structure and numbering of [phC mim][CH COO].
2
6

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H
O
H
6
HO 3
4
O
HO
OH
O
OH
5
O
2
O
4
O
1
O
1
O
O
2
5
O
3
O
H
OH
HO
OH
O
H
HO
6
H
H
6
O
HO
4
3
HO
O
OH
O
OH
O
5
O
2
O
4
O
1
O
1
O
O
2
5
3
O
H
OH
HO
OH
O
HO
6
H
Fig. 3 The schematic structure and numbering of cellulose.
2
7

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Entry for the table of contents
Cation does matter：How cationic structure affects the
dissolution of cellulose in ionic liquids
a,c
c
b,c
Benlian Lu , Airong Xu , Jianji Wang*
It’s found that cations of ILs significantly affect cellulose solubility by their acidic
protons, steric hindrance and interaction with anions.