Journal of Bioresources and Bioproducts. 2016, 1(1): 58-63
Peer-Reviewed
REVIEW PAPER
Dissociation of intra/inter-molecular hydrogen bonds of cellulose molecules in the
dissolution process: a mini review
Xingya Kanga,b, Shigenori Kugaa, Limei Wanga,c, Min Wua*, Yong Huanga*
a) Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,29 Zhongguancun East Road, Haidian District, Beijing 100190,
China
b) University of Chinese Academy of Sciences, Beijing 100039, China
c) College of Chemistry and Chemical Engineering, Dezhou University, Dezhou 253023, China
*Corresponding authors: [email protected]; [email protected]
ABSTRACT
Cellulose is abundant in nature, with the advantages of low-cost, biodegradable and biocompatible, low density and high strength. However,
the development and application of cellulose has been lagging behind its potential due to its unique properties. Cellulose has a large quantity
of hydroxyl groups which can easily form hydrogen bond networks. The huge hydrogen bond network makes it extremely difficult to dissolve
or melt cellulose, thus limiting the effective use of cellulose resources. To dissolve cellulose, the key is to break the hydrogen bonds. This
article sums up recent studies on the dissociation or breakage of the intramolecular and intermolecular hydrogen bonds in the dissolution of
cellulose.
Keywords: Cellulose; Dissolution; Hydrogen bonds; Solvent; Ionic liquid
1. INTRODUCTION
As the most abundant renewable polymer resource in
nature, cellulose has gained huge interest under the pressure
of energy crisis and environment pollution. Thus cellulose is
of vast importance for sustainable economy and
development in the advantage of wide distribution, low-cost,
biodegradability and biocompatibility, low density and high
strength. About 100-150 millions of tons of cellulose is
synthesized annually by plants via photosynthesis; however,
only 2 million tons of cellulose is effectively used in the
textile and papermaking industries.1 The large majority of
cellulose is degraded or burned away in nature. One
difficulty in utilization of cellulose is its insolubility in
common solvents and lack of thermoplasticity. These
features severely restrict the development and application of
cellulose.
In the point of molecular structure, cellulose is a linear
polymer in the form of aligned β(1,4)-D-glucan molecule
linked by 1,4-glucoside bonds. There are alcoholic hydroxyl
groups on carbon 2 (secondary), carbon 3 (secondary) and
carbon 6 (primary) in each glucose unit. So there are masses
of hydroxyl groups of strong activity in cellulose, which are
easy to form huge hydrogen bond networks: intramolecular
hydrogen bonds of O(2)H... O(6’), O(3’)H... O(5), and
intermolecular hydrogen bonds O(6’’)H ... O(3), as shown in
Figure 1.2 Then cellulose molecules are closely packed
through hydrogen bonds and other interactions, showing
high degree of structural regularity and crystallinity, coupled
with the complex structure of crystalline and non-crystalline
regions, which makes it difficult to dissolve and melt, thus
seriously limiting the effective use of cellulose resources.
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To achieve the goal of best use of cellulose in terms of
science and technology, the key aspects lie in realization of
destruction and reconstruction of inter- and intramolecular
hydrogen bonds, which lay the foundation for cellulose
dissolution and processing. Thus the development of new
cellulose solvent is of vital importance.
There are numerous works about cellulose solvent,
including derivatizing and non-derivatizing solvents. Here
we emphasize the non-derivatizing solvents, in which no
chemical modifications occur in cellulose. Several common
cellulose solvents are briefly summarized, such as Nmethylmorpholine oxide (N-NMMO), lithium chloride/N,Ndimethyl acetamide (LiCl/DMAc), ionic liquids, and
aqueous alkali/urea(thiourea) system. All of these are able to
dissociate and break intra- and intermolecular hydrogen
bonds of solid cellulose, widening the horizon of new
cellulose materials.
Fig. 1 Intra/inter-molecular hydrogen bonds of cellulose. Adapted
from Ref. [2].
2. N-METHYLMORPHOLINE OXIDE (N-NMMO)
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Journal of Bioresources and Bioproducts. 2016, 1(1): 58-63
NMMO can dissolve many kinds of cellulose and more
than 99% of the solvent can be recycled because of its low
vapor pressure; thus it is called a green solvent for cellulose.
Moreover, NMMO has been successfully used in industrial
fiber-making to develop a green fiber-Lyocell.3-6
Cellulose can dissolve in the NMMO system directly
without any chemical reaction and derivatization.7
Dissociation or breakage of intra/inter-molecular hydrogen
bonds of cellulose accounts for its dissolution. NMMO is an
aliphatic cyclic tertiary amine oxide with a strong polar
group N→O with high electron density around the oxygen
atom. Hydroxyl groups of cellulose are attacked to form new
hydrogen bonds of Cell-OH…O←N along with the breakage
of original hydrogen bonds of cellulose. This complexation
occurs firstly in the amorphous region of cellulose, then
excess NMMO gradually penetrates into the crystalline
regions, and cellulose dissolve eventually as shown in Figure
2.
Peer-Reviewed
is believed to be as shown in Figure 3. The Li-O coordination
bond is easily formed between carbonyl oxygen of DMAc
with lone pair electrons with strong electronegativity and Li
atom with unoccupied molecular orbital. Then large cation
polymer Li+(DMAc)x cationic complex is formed. On the
other hand, anion Cl- can form hydrogen bonds with
hydrogen of hydroxyl groups of cellulose leading to the
breakage of hydrogen bonds within cellulose. At last, an
equilibrium is attained between large cationic complex
Li+(DMAc)x and anion Cl- accumulated along the cellulose
chains. In this system, cellulose molecules are forcibly
separated due to charge-charge exclusion and expansion
effects, by which the solvent will penetrate into crystalline
regions of cellulose to destroy the association among
cellulose molecules as shown in Figure 3(a).15 Another
hypothesis states that direct interaction exists between the
Li+ and oxygen atoms in cellulose. When the hydrogen bond
is formed between hydroxyl protons and the Cl − anions, the
Li+ cations are further solvated by DMAc molecules and
accompany the hydrogen-bonded Cl− anions to meet the
electric balance, as shown in Figure 3(b).16-17
Fig. 2 Dissolution of cellulose in NMMO solution.
NMMO is thought to be in the form of several hydrates
with different water contents, which greatly affect solubility
of cellulose Melting point of anhydrous NMMO is 172oC,
that of monohydrate (NMMO·H2O) is 74oC, and that of 2.5
hydrate (NMMO·2.5 H2O) is 36oC.8 The more water, the
weaker its cellulose solubility. When the water content is
below 16 wt %, NMMO/H2O system can dissolve cellulose
easily. In theory, anhydrous NMMO should has the best
solubility for cellulose, but the high melting point of
anhydrous NMMO can cause severe degradation of cellulose,
resulting in poor quality of the regenerated cellulose.
Actually, the monohydrate NMMO·H2O (13.3% w/w H2O)
is a good solvent for cellulose. However, when the water
content exceed 17 wt %, the system is unable to dissolve
cellulose; thus NMMO·2.5H2O (28 w/w H2O) is a poor
solvent. This is because both water and cellulose molecule
can form hydrogen bond with NMMO, and water hinders
formation of hydrogen bonds between NMMO and
cellulose.9-12 Breaking hydrogen bonds and forming new
strong NMMO−cellulose hydrogen bonds has also been
proved to promote the dissolution of cellulose by explicit allatom molecular dynamics simulations.13
3.
LITHIUM
CHLORIDE/N,
ACETAMIDE (LICL/DMAC)
N-DIMETHYL
LiCl/DMAc system dissolves cellulose directly without
any chemical reaction and derivatization.14 The mechanism
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Fig. 3 Dissolution of cellulose in LiCl/DMAc solution. a: Reprinted
with permission from Ref. [15], copyright ©1985 American
Chemical Society; b: Reprinted with permission from Ref. [17],
copyright ©2014 American Chemical Society.
Pretreatment of cellulose is needed in order to achieve
better dissolution. Solvent-exchange activation method is
commonly used to destroy the fine structure of cellulose to
weak the hydrogen bonds among cellulose molecules. It was
proved by SAXS that solvent-exchange activation can
significantly alter aggregation state of cellulose microfibers
to make cellulose molecules more accessible. Meanwhile,
solvent-exchange activation can also enhance molecular
chain motions of cellulose thereby promoting dissolution of
cellulose without changing the crystalline structure of
cellulose. However, mechanical milling process that directly
destroying the crystalline structure of cellulose has little
effect on solubility of cellulose in this solvent.18-19
Both DMAc and LiCl are highly hygroscopic, so moisture
contents greatly affect dissolution of cellulose, thus strict
control of water content is required. That is because
Li+(DMAc)x formed first and then interacts with hydroxyl
groups of cellulose, leading to the solubilizing efficiency of
this solvent system. Unfortunately, preferential solvation of
LiCl by water can break down the complex, so the system
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Journal of Bioresources and Bioproducts. 2016, 1(1): 58-63
lose the ability to dissolve cellulose. It's worth mentioning
that the ratio between water and LiCl has to be below 2:1,
otherwise the formation of either direct or mixed solvent
complex between LiCl/water or LiCl/water/DMAc can lead
to incomplete dissolution of cellulose.20
Inspired by the strong electronegativity of DMAc in
LiCl/DMAc system, dimethyl sulfoxide (DMSO), N,Ndimethylformamide
(DMF)
and
1,3-dimethyl-2imidazolidinone (DMI) and other strong polar solvents may
take place of DMAc to dissolve cellulose.21-22
4. IONIC LIQUIDS
Ionic liquids are a large class of new green nonderivatizing solvents for cellulose, which are composed of
organic cations and organic/inorganic anions. It is in liquid
state at room temperature, so it is also known as lowtemperature molten salt. Ionic liquids have awakened
interests in the last decade for its unique physicochemical
properties, such as commercial availability, strong polarity,
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non-volatility, non-flammability, thermal and chemical
stability, and designability of cation or anion, good solubility
for many substances. The ionic liquids based on imidazole
salt have been recently proposed as a common solvent for
cellulose, such as 1-allyl-3-methylimidazolium chloride
([Amim]Cl),
1-butyl-3-methylimidazolium
chloride
([Bmim]Cl),
1-ethyl-3-methylimidazolium
chloride
([Emim]Cl), and 1-butyl-3-methylimidazolium acetate
([Bmim]Ac),
1-ethyl-3-methylimidazolium
acetate
([Emim]Ac).23
1-butyl-3-methylimidazolium chloride ([Bmim]Cl) was
first proposed to directly dissolve cellulose without
activation pretreatment by Swatloski.24 And during the
dissolution process, the anion is the electron donor while
cation is the electron acceptor. Then the anion interacts with
hydroxyl proton and the cation combines with hydroxyl
oxygen of cellulose, thereby destroying extensive
intramolecular and intermolecular hydrogen bonds of
cellulose, and eventually dissolving cellulose,25 as shown in
Figure 4.26
Fig. 4 Dissolution of cellulose in ionic liquids. Adapted from Ref. [26].
Cellulose appears in the similar conformation as β-(1-4)
glucan oligomers in ionic liquid, then it is assumed that
cellulose is disordered in solution, suggesting that
intramolecular and intermolecular hydrogen bonds are
destroyed.27 Furthermore, it was demonstrated by 13C and
35/37
Cl NMR relaxation measurements that hydrogenbonding between the carbohydrate hydroxyl protons and the
IL chloride ions forms in a 1:1 stoichiometry.28 Conventional
and variable-temperature NMR spectroscopy also confirmed
that the hydrogen bonds between hydroxyls and the acetate
anion and imidazolium cation of EmimAc is the major force
for cellulose dissolution in the ionic liquid. It was also found
that there should be one anion or cation to form hydrogen
bonds with two hydroxyl groups simultaneously; in other
words, the stoichiometric ratio of EmimAc:hydroxyl may be
between 3:4 and 1:1 in the primary solvation shell groups.29
The structures of anion and cation of Ionic liquid play a
vital role on solubility of cellulose. The ability to accept
hydrogen bonds of anion determines the ability to dissolve
cellulose of ionic liquids, which is characterized by the
chemical shift values of the proton in the 2-position of the
imidazolium ring. And the solubility of cellulose increases
almost linearly with increasing hydrogen bond accepting
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ability of anions in the ionic liquids.30 The alkyl chain length
in anion and cation also markedly affect cellulose solubility.
The longer the alkyl chain of anion, the weaker solubility of
cellulose, resulting from the reducing of the effective
concentration of the anion. The longer the alkyl chain in
cation, the weaker the solubility of cellulose due to steric
hindrance.29,31 In a word, the stronger the ability of forming
hydrogen bonds between ionic liquid and cellulose, the
stronger the ability destroying the original cellulose
hydrogen bonds, the better ability to dissolve cellulose.
Water in the ionic liquid has adverse effects on solubility
of cellulose, resulting from competitive hydrogen-bonding to
cellulose.24 Ionic liquids containing anion Cl- show good
ability to dissolve cellulose, but cellulose is not dissolved in
the aqueous solution of NaCl. That is because no hydration
exists in ionic liquids or molten states. The strong ion-dipole
interaction between anion Cl- and cellulose hydroxyl proton
is able to break hydrogen bonds of cellulose to dissolve
cellulose. On the contrary, the interaction between hydrated
anion Cl- and cellulose hydroxyl proton is weak dipoledipole interaction, which fails to break the intense hydrogen
bonds.32
Although ionic liquids are highly useful in dissolving
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Journal of Bioresources and Bioproducts. 2016, 1(1): 58-63
cellulose, they have also inherent shortcomings, such as high
cost and high viscosity, which can be improved by adding
co-solvent to some extent. When ionic liquid is mixed with
co-solvent, not only the viscosity is reduced but also the
mixture containing just a small molar fraction of ionic liquid
can dissolve cellulose. DMSO, DMF, NMP, and DMI are
available for this purpose.33
Suitable co-solvents should be strongly dipolar, relatively
basic aprotic solvents and in an appropriate amount to obtain
miscibility. Otherwise, cellulose will precipitate instead of
dissolving. Especially, when DMSO is used as co-solvent,
the mixture can dissolve cellulose much faster at low
temperatures.34 This can be illustrated as: firstly, DMSO
facilitates mass transport by decreasing the solvent viscosity.
Secondly, the specific interactions between cations and
anions or between the ionic liquid and the polymer remain
unchanged.35 The aprotic solvents can partially break down
the ionic associations of ILs without preferential solvation,
while protic solvents favorably interact with anions, resulting
the weaker interaction between anions and cellulose,
reducing cellulose solubility.36-37
5. AQUEOUS ALKALI/UREA SYSTEM
A series of green aqueous solvent systems based on
alkaline-water (NaOH/urea, LiOH/urea, and NaOH/thiourea)
developed by Zhang Lina group can rapidly dissolve
cellulose at low temperature, which is the fastest solvent
system up to now.38-41
The dissolution is carried out at low temperature, which is
the biggest difference from other systems, and is considered
to be driven by dynamic self-assembly of cellulose-solvent
complex. In the case of precooled -12 oC 7 wt % NaOH/12
wt% urea aqueous solvent, large and stable hydrogen bond
network structure is formed between cellulose and solvent
components at low temperature. In detail, the NaOH
“hydrate” is easily attracted to cellulose chains through the
formation of new hydrogen-bonded networks and breaks
hydrogen-bonded networks, while urea does not directly
interact with cellulose but is capable of bonding with NaOH
by lone pair of electrons of oxygen atom. Then NaOHcellulose hydrates are covered with urea to form a new
channel inclusion complex, which prevents aggregation of
cellulose molecules. In this way, cellulose molecules can
disperse in the aqueous solution to form a transparent
solution. The channel inclusion complex is most stable at
near the freezing point, but is unstable at room temperature.
On the other hand, at low temperature, exchange between
hydrogen bonds becomes slow, which is helpful for small
solvent molecules to destroy the hydrogen bonds among
cellulose. Consequently, interaction between the solvent
molecules and cellulose is established as inclusion
complexes.42-48 The dissolution of cellulose in 4.5 wt %
LiOH/15 wt % urea solution is shown in Figure 5.49
Strong evidence states that temperature-induced
conformational changes should be responsible for the low
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temperature dissolution. Namely, the O-CH2-CH2-O
segments around the C-C bond can change their
conformation as a function of temperature. Thus, the lower
the temperature is, the more polar the cellulose is.
Accordingly, interactions between polar cellulose and polar
solvent are strengthed and promotes dissolution of
cellulose.51-52
Fig. 5 Cellulose- alkali-urea inclusion complex formed in the
alkali-urea solution of cellulose. Adapted from Ref. [49].
Both anions, cations and small molecules have
contribution to the cellulose dissolution in alkali-based
system. The OH- anion is crucial to dissolving of cellulose,
because it is a strong hydrogen bond acceptor, capable of
forming hydrogen bonds with hydroxyl groups of cellulose
by breaking intramolecular and intermolecular hydrogen
bonds. More hydroxyl groups of cellulose will contact with
OH- anions with the increase of NaOH concentration,
reducing the amount of exposed hydroxyl groups of cellulose
and preventing aggregation of cellulose, thus improving the
solubility of celluloe and enhancing the stability of the
solution. LiOH can take place of NaOH to dissolve cellulose
with stronger ability at -12 oC in the case of 4.5 wt % LiOH
/ 15 wt % urea aqueous solution. That is because LiOH
hydrates can easily combine with hydroxyl groups of
cellulose at low temperature to form a new network of
hydrogen bonds.52 But cellulose is not dissolved in the KOHurea system. Cellulose-dissolving ability depends on the
stability of the hydrated cation and the number of free Hbonded water. The interaction between cellulose and alkali
ions is the same but has a different intensity in order of LiOH
＞NaOH＞KOH resulting from the different ionic radiuses.
Li+ and Na+ have smaller ionic radius and can combine with
water molecules more tightly to form two hydration shells,
namely the tight first hydration and loose second hydration.
However, K+ has a larger ion radius, which can only form
loose hydration. The bound water can easily exchange with
water in the bulk and there is more free water in KOH
solutions, which is unfavorable to form complex with
cellulose, leading to lack of solubility for cellulose.32 Thus
the cations play a vital role in forming stable complex with
cellulose to form a stable solution in the alkali-urea system.
Besides urea and thiourea, zinc oxide or polyethylene
glycol (PEG) can also be added to the alkali system, working
in different ways. When ZnO is added to aqueous NaOH, it
forms Zn(OH)42−, which can form strong hydrogen bonds
with cellulose. In addition, the charged complex Zn(OH) 42−cellulose will make the cellulose chains repel each other and
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Journal of Bioresources and Bioproducts. 2016, 1(1): 58-63
promotes dissolution of cellulose.53 In the system of 1.0 wt %
poly(ethylene glycol) (PEG) /9.0 wt % NaOH at -15 oC,
oxygen atom of PEG act as hydrogen bond acceptor instead
of urea to form complex with cellulose, leading to cellulose
dissolution. The PEG molecules bind more tightly with
cellulose than urea, leading to a more stable solution.54
However, recent studies argue that urea, thiourea and PEG
play a similar role in dissolution of cellulose;47 the
interaction between them and cellulose is weak van der
Waals forces distributing in the hydrophobic regions of
cellulose, which can reduce the mutual interaction of
cellulose molecules and prevents aggregation. Thus a better
solubility of cellulose and a more stable cellulose solution is
obtained.
6. SUMMARY
The huge networks of hydrogen bonds of cellulose restrict
its dissolution in solvents. This paper reviews the strategies
to break the hydrogen bonds of cellulose in 4 major solvent
systems, i.e N-NMMO, LiCl/DMAc, ionic liquid and
NaOH/urea. There is still a need to develop a more efficient
process to dissolve cellulose for wider applications. The key
is to effectively break the intramolecular and intermolecular
hydrogen bonds of cellulose.
ACKNOWLEDGEMENTS
This study was supported by the National Natural Science
Foundation of China (No.51373191, 51172247, 51472253)
and the Chinese Academy of Sciences Visiting
Professorships.
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