ORIGINAL PAPER

Viscosity reduction of cellulose + 1-butyl-3-methylimidazolium acetate in the presence of CO2

Masayuki Iguchi • Kazuhiro Kasuya •

Yoshiyuki Sato • Taku M. Aida •

Masaru Watanabe • Richard L. Smith Jr.

Received: 30 November 2012 / Accepted: 9 February 2013

� Springer Science+Business Media Dordrecht 2013

Abstract Viscosities of microcrystalline cellulose ?

1-butyl-3-methylimidazolium acetate ([bmIm][Ac])

solutions (0.6–1.2 wt%) in contact with CO2 were

measured at 312 K with a resonant vibrational viscom-

eter. At 4 MPa and 312 K, the CO2 could reduce the

viscosity of 1.2 wt% cellulose ? [bmIm][Ac] solution

by about 80 %, whereas N2 at the same conditions gave

less than a 10 % reduction in viscosity. The viscosity-

averaged degree of polymerization and IR spectrum

showed that cellulose did not decompose during exper-

iments and that [bmIm][Ac] acted as a non-derivatizing

solvent during the dissolution and viscosity reduction

process. Further, although CO2 does react with [bmI-

m][Ac] to form 1-butyl-3-methylimidazolium-2-carbox-

ylate, the reaction seems to be reversible and it does not

affect the cellulose. Thus, [bmIm][Ac] with CO2

provides an effective solvent for cellulose and the solvent

system can probably be recycled or reused.

Keywords Ionic liquid �Microcrystalline cellulose �Carbon dioxide � Viscosity reduction

Introduction

The solvent, N-methylmorpholine-N-oxide monohy-

drate (NMMO), is presently used in commercial

applications for processing cellulose (Klemm et al.

2005). Although NMMO has good properties for

dissolving cellulose, side reactions occur that affect

both quality of the products and NMMO recycle.

Ionic liquids are attractive solvents for processing

cellulose because they are generally considered to be

non-derivatizing solvents for cellulose and they have

good thermal stability with many functionalities

(Gericke et al. 2012; Heinze et al. 2005; Iguchi et al.

2013; Pinkert et al. 2009). On the other hand, there are

some challenges for using ionic liquids as a solvent for

cellulose that include (1) possible reaction with

cellulose (Pinkert et al. 2009; Iguchi et al. 2013), (2)

strong adsorption of water (Vitz et al. 2009), (3)

high viscosity of cellulose ? ionic liquid solutions

(Gericke et al. 2009), (4) unknown environmental fate

of ionic liquids and (5) recovery and purification for

reuse of the ionic liquid (Gericke et al. 2012). The

viscosity of cellulose ? ionic liquid solution increases

Electronic supplementary material The online version ofthis article (doi:10.1007/s10570-013-9884-8) containssupplementary material, which is available to authorized users.

M. Iguchi � Y. Sato � T. M. Aida � M. Watanabe �R. L. Smith Jr. (&)

Research Center of Supercritical Fluid Technology,

Graduate School of Engineering, Tohoku University,

Aramaki Aza Aoba 6-6-11, Aoba-ku,

Sendai 980-8579, Japan

e-mail: smith@scf.che.tohoku.ac.jp

K. Kasuya � T. M. Aida � R. L. Smith Jr.

Graduate School of Environmental Studies,

Tohoku University, Aramaki Aza Aoba 6-6-11,

Aoba-ku, Sendai 980-8579, Japan

123

Cellulose

DOI 10.1007/s10570-013-9884-8

during the dissolution of cellulose in ionic liquids, so

that mass transfer of reactants in the solvent becomes

slow (Cruz et al. 2012; Qi et al. 2011), and even for

simple monosaccharides, reaction does not occur

without viscosity reduction (Qi et al. 2010).

The viscosity of cellulose ? ionic liquid solutions

is about 5 times larger than that of NMMO solutions

(Kosan et al. 2008). Therefore, practical applications

for processing cellulose with ionic liquids will

depend on methods that can reduce the viscosities

of the solutions without causing its degradation and

without requiring complicated recovery or recycle

steps.

One method for reducing viscosities of cellu-

lose ? ionic liquid solution is through the synthesis

of new ionic liquids that have low viscosities (Fukaya

et al. 2008; Zavrel et al. 2009). Viscosities of some

ionic liquids at 303 K are 1-ethyl-3-methylimidazo-

lium acetate ([emIm][Ac]), 105 mPa�s and 1-ethyl-3-

methylimidazolium dimethylphosphate, 265 mPa�s(Freire et al. 2011). However, even for these low

viscosity ionic liquids, their mixtures with cellulose

may still have viscosities that are too high compared

with those of NMMO.

One method for reducing the viscosity of cellulose

solutions is through the addition of an organic solvent

(Gericke et al. 2011; Rinaldi 2011). An organic

solvent can be used as a viscosity reducing agent for

cellulose ? ionic liquid solutions and these are gen-

erally medium boiling compounds such as pyridine

(Tb: 390 K) and dimethylsulfoxide (Tb: 460 K).

Although the addition of an organic solvent may be

convenient for some applications, separation and

recycle of the solvent can have a large environmental

impact and can require considerable energy.

Carbon dioxide (CO2) is a gas at atmospheric

pressure and it dissolves into many ionic liquids,

whereas most ionic liquids do not dissolve into the CO2

phase (Shariati et al. 2005). Dissolved CO2 causes a

reduction in viscosity for ionic liquids (Ahosseini et al.

2009), and especially for those that contain an acetate

anion, CO2 solubility is high due to the formation of a

complex or possible reaction between CO2 and the

ionic liquid (Carvalho et al. 2009; Shiflett et al. 2008,

2012; Yokozeki et al. 2008). Since CO2 is a gas, it

is more easily separated from a cellulose-ionic

liquid solution and therefore, from the point of

view of the environment and separation energy, CO2

could be attractive as a viscosity reducing agent for

cellulose ? ionic liquid solutions, since it can be

removed from the ionic liquid phase by simple

depressurization.

Cellulose readily dissolves into 1-butyl-3-methyl-

imidazolium acetate ([bmIm][Ac]) compared with

other ionic liquids such as 1-butyl-3-methylimidazo-

lium chloride and 1-butyl-3-methylimidazolium dim-

ethylphosphate, and this is attributed to the ion

mobility of the acetate ion (Cruz et al. 2012). In this

work, [bmIm][Ac] was chosen because it readily

dissolves cellulose (Cruz et al. 2012) and CO2 ? ionic

liquid solubility data are available (Carvalho et al.

2009; Shiflett et al. 2008). Adding CO2 to [emIm][Ac]

seems to promote the dissolution of cellulose into

[emIm][Ac] (FitzPatrick et al. 2012). However, the

reason why CO2 enhances the solubility of cellulose in

that ionic liquid is not clear. For example, confirmation

that the [emIm][Ac] and CO2 mixture act as a non-

derivatizing solvent during the dissolution of cellulose

has not been shown in the literature. Dissolved CO2 in

the cellulose ? ionic liquid solution could promote

decomposition of dissolved cellulose since CO2 pro-

motes the deprotonation of the ionic liquid at the C2

position through a carboxylation reaction of [bmIm]

ion (Besnard et al. 2012a, b; Cabaco et al. 2012;

Maginn 2005). In this work, the objective was to

determine the viscosity of the cellulose ? [bmIm][Ac]

solutions in the presence of CO2 as a possible viscosity

reducing agent, and to confirm that the conditions for

cellulose dissolution are non-derivatizing. It is shown

that low pressures of CO2 applied to cellulose ? ionic

liquid solutions greatly reduces the viscosity of the

cellulose solutions, and that the degree of polymeriza-

tion of cellulose does not change during dissolution

within the experimental error. Further, it is shown that

the carboxylation reaction does not seem to affect the

cellulose dissolution and that the reaction seems to be

reversible so that the ionic liquid can be recycled or

reused.

Materials and methods

Materials

Microcrystalline cellulose (Asahi Kasei) was sieved to

have a particle size less than 75 lm. Water content of

the cellulose was 4.6 ± 0.4 wt% as determined by

weight loss according to JIS K 0068:2001. The degree

Cellulose

123

of polymerization and the intrinsic viscosity in

cupriethylenediamine solution of microcrystalline

cellulose were 310 and 130 mL/g, respectively, as

determined by viscosity measurement using an Ost-

wald viscometer according to ISO 5351:2010 using the

parameters of Marx-Figini (Marx-Figini 1978). The

microcrystalline cellulose was confirmed to be cellu-

lose I by X-ray diffraction analysis. Amorphous

cellulose used in comparing cellulose precipitated

from the ionic liquid was prepared by ball milling

microcrystalline cellulose in a jar with alumina balls

(d = 10 mm) for 1 d using a rotor apparatus (AV-1,

Asahi Rika; 110 rpm)

Ionic liquid, 1-butyl-3-methylimidazolium acetate,

[bmIm][Ac] ([95 %, Sigma-Aldrich) was dried in

vacuum at 340–350 K over 20 h before each exper-

iment. Water content in the dried [bmIm][Ac] was

measured with Karl-Fisher coulometric titration

(Kyoto Electronics Manufacturing, MKC-501) before

each experiment. For all experiments, [bmIm][Ac]

contained below 4,000 ppm of water.

Carbon dioxide, CO2 ([99.5 %, Taiyou Nissan)

and nitrogen, N2 ([99.999 %, Taiyou Nissan) were

passed through a silica gel packed column to remove

water before use. Water was used for the regeneration

of cellulose from ionic liquid and had a conductivity of

less than 6 lS�m-1. Dimethylsulfoxide, DMSO

(99.9 %, Wako Pure Chemical Industries) was used

as a viscosity reducing agent for the cellu-

lose ? [bmIm][Ac] solutions in some analytical pro-

cedures. A solution of 1.0 M cupriethylenediamine,

CED (GFS chemicals) was used for the intrinsic

viscosity measurements. Potassium bromide, KBr

(Wako Pure Chemical Industries) was used as a

dilution agent for the IR measurements. Mixtures of

[bmIm][Ac] and microcrystalline cellulose were

prepared by mixing samples with KBr powder with

an agate mortar to examine the contamination of ionic

liquids in the regenerated cellulose with the IR

measurements. In the NMR measurements used to

monitor the carboxylate reaction of [bmIm][Ac], both

deuterated DMSO-d6 (99.9 %, Sigma-Aldrich) and

deuterated water, D2O (99.9 %, Acros) were used.

Apparatus

Figure 1 shows a schematic diagram of the apparatus.

The measurement vessel (ca. 130 mL) was equipped

with a resonant vibrational viscometer (XL7-902-

HT2-E104, Hydramotion) that is described in a section

below. Solutions were mixed with ca. 200 rpm by a

magnetic stirrer (HERACLES-16G, Koike Precision

Instrument). The vessel was connected to a vacuum

line and a pressurized gas line. Pressure of the gases,

CO2 or N2 was controlled to within 0.1 MPa by a

regulator and was measured with an online pressure

gauge (DPI282, Druck). Temperature was controlled

to within 1 K and measured by a platinum resistance

thermometer (Pt100, Netsushin) in the wall of the

vessel.

Principles of the resonant vibrational viscometer

are described briefly. The resonant viscometer vibrates

at a high frequency (ca. 1,200 Hz) and measures the

dynamic viscosity of the fluid. The drag force of a

viscous fluid on the sensor causes an energy loss that is

sensed as the dampening of the oscillation, which is

called the loss factor (LF). In this work, each cycle of

vibration was measured during the experiment. In the

data reduction, the value of LF was correlated with the

product of viscosity and density of a fluid, and this loss

factor was calibrated with certified reference fluids, JS

100, JS 500 and JS 1000 at temperatures from 298 to

313 K. All certified reference fluids were purchased

from Nippon Grease. The uncertainty of the viscosity

values from the calibration is estimated to be less than

6 %, however, some values may approach 10 %

(Online Resources, Table S1). Considering the

T

Phη

CO2

N2

(a)

(b)(c)

(m)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k) (k)

(l) (n)

Fig. 1 Schematic diagram of the viscosity measurement

apparatus: (a) viscosity sensor, (b) viscometer, (c) readout,

(d) heater, (e) vessel, (f) stirring bar, (g) stirring bar controller,

(h) vacuum pump, (i) relief valve, (j) drying column,

(k) regulator, (l) compressed gas cylinder, (m) pressure sensor

and (n) temperature probe

Cellulose

123

variation of temperature and pressure of solutions in

contact with CO2, the uncertainty of the reported

measurements is estimated to be 10 %.

Methods

Viscosity measurement of [bmIm][Ac] solutions

The apparatus (Fig. 1) was evacuated at 312 K for

several hours prior to the measurements to remove any

residual water or cleaning solvents. The [bmIm][Ac]

(30–35 g) was loaded into the vessel from an inlet port at

the top of the vessel under a flowing N2 gas stream. After

the temperature stabilized at 312 K, the viscosity was

measured and then the vessel was heated up to 330 K

while stirring. After several hours passed at 330 K, the

vessel was cooled down to 312 K and the viscosity was

measured again to confirm reproducibility. The vessel

was then placed under vacuum for 1 h, and then the

system was pressurized to 4 MPa with CO2. Measure-

ments were made during the dissolution process. The

pressure was decreased gradually (ca. 0.2 MPa/h) at

312 K as controlled by a back pressure regulator

(BP-2080-M, JASCO) while stirring (Fig. 1). When

the pressure returned to atmospheric pressure, water

content, purity, viscosity and density of the [bmIm][Ac]

were measured with Karl-Fisher titration, NMR spec-

troscopy and a Stabinger viscometer, respectively. The

[bmIm][Ac] after contact with CO2 was subjected to

vacuum (ca. 5 Pa) at 333 K overnight, and was then

analyzed with NMR spectroscopy and other techniques.

Viscosity and density measurements of [bmIm][Ac]

at atmospheric pressure

The viscosity and density at atmospheric pressure were

measured with a Stabinger viscometer (SVM3000,

Anton Paar). Measured density was corrected for

viscosity effects. Measurements were made at temper-

atures from 293.15 to 373.15 K. Temperature was

controlled to within 0.005 K and the stabilities of the

viscosity and density values were judged by the change

of viscosity and density being within 0.07 % and

0.3 kg�m-3 over a period of 60 s, respectively. Mea-

surement cells were dried by flowing N2 gas at

333.15 K for 20 min prior to the loading of the sample.

Then, the sample was loaded and 2 mL of sample was

used for wetting the surfaces in the viscometer. After

completion of the measurements at 373.15 K, 1 mL of

sample was added and the measurement was started

from 373.15 to 293.15 K. This cycle was repeated

twice. The water content in the sample after each run

was measured with Karl-Fisher titration. When the

second cycle was finished, the sample was analyzed

with NMR spectroscopy. The increase in water content

was always below 1,000 ppm and that is attributed to

handling in the measurements. The NMR spectra of the

measured samples showed that no contamination

occurred during the measurements. Reproducibilites

of the viscosity and density measurements at atmo-

spheric pressure were within an uncertainty of 0.6 and

0.1 %, respectively.

Viscosity measurement of cellulose ? [bmIm][Ac]

solutions

Cellulose was loaded into the vessel (Fig. 1), and was

dried at 323 K in vacuum overnight. After drying of the

cellulose, temperature in the vessel was set to 353 K.

The N2 gas was used when the pressure was brought

back to atmospheric pressure. While allowing N2 to flow

and while stirring, a known weight of [bmIm][Ac] was

loaded into the vessel from an inlet port. The solution of

cellulose ? [bmIm][Ac] was prepared under a N2

atmosphere at 353 K. To dissolve cellulose into

[bmIm][Ac] efficiently, a temperature of 353 K was

found to be appropriate according to the required time

(2–3 h). When the measured value with the viscometer

became stable, dissolution of cellulose in the ionic liquid

was complete and the vessel was cooled down to 312 K

and a vacuum (\100 Pa) was applied for 1 h. Once the

vessel reached vacuum at 312 K, CO2 was added to

achieve a pressure of 4 MPa. For some experiments, N2

was used instead of vacuum to investigate the effect of

an inert gas on the procedure. The decompression rate

was ca. 0.2 MPa/h as controlled by the back pressure

regulator (Fig. 1). After completion of the measure-

ments, the cellulose ? [bmIm][Ac] solutions were

analyzed with NMR spectroscopy.

Regeneration of cellulose from [bmIm][Ac] solutions

At the end of each experimental run, cellulose was

recovered from solution and analyzed by adding

30 mL of DMSO to the solutions while stirring. When

the solution became clear, 240 mL of water was added

to the solution gradually while stirring. Precipitated

samples were separated from the solution by suction

Cellulose

123

filtration using 0.2 lm PTFE membranes (Millipore).

After several washings with water, the regenerated

cellulose was dried in vacuum at 323 K until a

constant mass was obtained. The filtrate was subjected

to vacuum at 323 K overnight for removing water, and

was then analyzed with NMR spectroscopy.

Characterization

Confirmation for uptake of water in [bmIm][Ac]

Water was controlled because water can act as a

viscosity reducing agent for the ionic liquid (Le et al.

2012). Before the experiments, the uptake of water in

[bmIm][Ac] during the experiments was measured

with Karl-Fisher titration. The water concentration in

[bmIm][Ac] changed from 3,300 ± 200 to 4,100 ±

300 ppm after 3 d at 312 K in contact with CO2. The

increase in water content of [bmIm][Ac] can be

attributed to the uptake of water during loading into

the vessel and that of water in CO2.

Viscosity and density of [bmIm][Ac] samples

Viscosities (g) and densities (q) measured with the

Stabinger viscometer were correlated with the follow-

ing equations:

g ¼ A0 exp A1=ðA2 þ TÞð Þ ð1Þq ¼ B0= B1 þ Tð Þ ð2Þ

where T is temperature in K, and A0, A1, A2, B0 and B1

are adjustable parameters. The adjustable parameters

were determined by minimizing the following objec-

tive function (O.F.):

O:F: ¼ 1

N

XN

i¼1

Xcal;i=Xexp;i � 1� �

ð3Þ

where X refers to the viscosity or density, N is the number

of data points and the subscripts of cal and exp refer to the

calculated value and measured value, respectively.

Viscosity of cellulose ? [bmIm][Ac] solutions

Since densities of the cellulose ? [bmIm][Ac] solu-

tion were not measured in this work due to difficulties

in handling the viscous fluids, densities of the

solutions (qIL?cellulose) were estimated as follows:

qILþcellulose ¼Mcellulose þMIL

MIL=qIL

ð4Þ

where Mcellulose and MIL are the mass of cellulose and

[bmIm][Ac], respectively, and qIL is the density of

pure [bmIm][Ac] at a given temperature and at

0.1 MPa. Using the calculated densities of the cellu-

lose ? [bmIm][Ac] solutions (qIL?cellulose) of Eq. 4,

the viscosities of the solutions (gIL?cellulose) were

determined according to Eq. 5:

gILþcellulose ¼ðg � qÞ

qILþcellulose

ð5Þ

where g�q is the product of viscosity and density that

was obtained from the measured loss factor of the

apparatus (Fig. 1).

The relative viscosity (grel) at atmospheric pressure

in the presence of N2 at a given temperature was

defined as follows:

grel ¼gILþcellulose

gIL

ð6Þ

where gIL is the viscosity of the pure ionic liquid

measured with the resonant vibrational viscometer

(Fig. 1) at a given temperature. It is known that the

relative viscosity can be expressed by the following

empirical equation (Sescousse et al. 2010):

grel ¼ 1þ Ccel � ½g� þ kHðCcel � ½g�Þ2 þ A ðCcel � ½g�Þn

ð7Þ

where Ccel is the concentration of cellulose in solution

in g/mL, [g] is the intrinsic viscosity in mL/g, kH is the

Huggins constant, and A and n are adjustable param-

eters. The Huggins constant was assumed to be 0.5 as

proposed in the literature (Sescousse et al. 2010). The

values of [g], A and n were determined by minimizing

the objective function (Eq. 3).

Viscosity reduction ratio for [bmIm][Ac]

and cellulose ? [bmIm][Ac] solutions

The product of viscosity and density was obtained from

the measured loss factor of the apparatus (Fig. 1).

Assuming that the density did not change greatly at

0.1 MPa compared with that at a given pressure, the

viscosity reduction ratio, Dg(P, t)/g0, at a given pressure

was calculated as follows:

Cellulose

123

DgðP; tÞg0

¼ 1� gðP; tÞg0

ð8Þ

where P is pressure, t is time after contact with

pressurized CO2 or N2 and g0 is the viscosity of

[bmIm][Ac] or cellulose ? [bmIm][Ac] solution at

atmospheric pressure in the presence of N2.

Degree of polymerization of cellulose samples

The intrinsic viscosity of the regenerated cellulose in

CED solution was determined by viscosity measure-

ment using an Ostwald viscometer according to ISO

5351:2010 (Online Resources, Method section). Using

parameters of Marx-Figini (Marx-Figini 1978), the

viscosity-average degree of polymerization (DPv) of

the cellulose samples was calculated as follows:

DPv ¼ ½g�=0:42 ð9Þ

IR of cellulose samples

Infrared spectra between 4,000 and 800 cm-1 were

measured with diffuse reflectance (FT-IR 230, JAS-

CO) with a resolution of 4 cm-1, and several hundred

scans. Before the analyses, samples were dried at

323 K in vacuum overnight, and were mixed with KBr

powder using an agate mortar.

XRD of cellulose samples

The crystal structures of cellulose and regenerated

cellulose samples were determined by X-ray diffrac-

tion with Cu Ka radiation (k = 1.5406 A) at 30 kV

and 15 mA in the range of 2h = 10–40� at 0.02�interval with the fixed time method using an X-ray

diffractometer (MiniFlex, Rigaku). Samples were

measured by placing them horizontally on an alumi-

num plate.

NMR of [bmIm][Ac] and cellulose ? [bmIm][Ac]

solutions

The NMR spectra at room temperature were recorded

with a nuclear magnetic resonator (DRX500, Bruker)

operating at 500 MHz for 1H and 126 MHz for 13C,

respectively. For the case of pure [bmIm][Ac], sam-

ples were mixed with either DMSO-d6 or D2O. For the

case of cellulose ? [bmIm][Ac] solutions, samples

were mixed with DMSO-d6. Concentrations of

1-butyl-3-methylimidazolium-2-carboxylate (bmIm-

2-carboxylate) were calculated from the average ratio

of integral of the 1H peak of bmIm-2-carboxylate to

that of [bmIm][Ac] assuming that the solution con-

sisted of only [bmIm][Ac] and bmIm-2-carboxylate.

The peaks of bmIm-2-carboxylate at d = 4.2 and

at 3.8 ppm correspond to those of [bmIm][Ac] at

d = 4.1 ppm and at d = 3.7 ppm, respectively, when

D2O was used as a solvent. For DMSO-d6 as a solvent,

the peaks of bmIm-2-carboxylate at d = 8.0, 7.9, 4.5

and 4.0 ppm correspond to those of [bmIm][Ac] at

d = 7.9, 7.8, 4.2 and 3.9 ppm, respectively.

Results and discussion

Pure [bmIm][Ac] samples in contact with CO2

First, the effect of pressurized CO2 on the viscosities of

pure [bmIm][Ac] was investigated. Figure 2 shows the

time dependence of the viscosity and viscosity reduc-

tion ratio of [bmIm][Ac] in contact with CO2 at various

pressures and at 312 K. Viscosities and viscosity

reduction ratios for [bmIm][Ac] in contact with CO2 at

various pressures are given at Table 1. The viscosity of

the ionic liquid decreased gradually with time in the

presence of CO2 (Fig. 2). When the CO2 pressure was

4 MPa, the viscosity reduction ratio became 0.77 over

a time period of 20 h. The viscosity reduction ratio

decreased when the CO2 pressure was decreased in

steps (Fig. 2). At 3 MPa, the viscosity reduction ratio

became 0.70 and as the pressure was reduced to 1 MPa,

it became 0.44. After depressurizing to atmospheric

pressure, the viscosity returned to its initial value

within the uncertainty of the measurements.

To examine the carboxylate reaction of [bmI-

m][Ac] after contact with CO2 and to determine

whether the reaction is reversible or not, all samples of

[bmIm][Ac] before contact with CO2 (blank), [bmI-

m][Ac] after contact with CO2 and [bmIm][Ac] after

contact with CO2 and subjected to vacuum were

analyzed with Karl-Fisher titration, 1H NMR and 13C

NMR spectroscopy. The water content of [bmIm][Ac]

blank, [bmIm][Ac] after contact with CO2 and

[bmIm][Ac] after contact with CO2 and vacuum were

3,200 ± 300, 4,200 ± 200 and 1,300 ± 100 ppm,

respectively. Figure 3 shows a comparison among the1H NMR spectra of ionic liquids using D2O as a

Cellulose

123

solvent. The numbers in Fig. 3 represent the position

of 1H in [bmIm][Ac] (Araujo et al. 2011). New peaks

appeared in the 1H NMR spectra of [bmIm][Ac] after

contact with CO2 (Fig. 3b) and [bmIm][Ac] after

contact with CO2 and vacuum (Fig. 3c), and these

were assigned to 1-butyl-3-methylimidazolium-2-

carboxylate, bmIm-2-carboxylate (Tommasi and

Sorrentino 2006; Besnard et al. 2012a). All 13C

NMR spectra using D2O as a solvent (Online

Resources, Figure S2), and 1H and 13C NMR spectra

using DMSO-d6 as a solvent (Online Resources,

Figures S3 and S4) support the 1H NMR results. The

additional peaks are evidence that the carboxylate

reaction of [bmIm] ion occurred when it was in contact

with CO2. Viscosity reductions for [bmIm][Ac] in

contact with CO2 (Fig. 2) might be caused by either or

both the reaction of [bmIm][Ac] and the dissolubility

of CO2 into the [bmIm][Ac] phase. Due to the lack of

equilibrium constant data for this reaction, the

concentration of bmIm-2-carboxylate in [bmIm][Ac]

at 312 K under pressurized CO2 cannot be calculated.

However, using the integral of 1H peak, the concen-

tration of bmIm-2-carboxylate in [bmIm][Ac] after

contact with CO2 (Fig. 3b) and that in [bmIm][Ac]

after contact with CO2 and vacuum (Fig. 3c) can be

estimated as 25 ± 1 mol% for [bmIm][Ac] after

contact and 5 ± 1 mol% for [bmIm][Ac] after contact

with CO2 and vacuum, respectively. A decrease in the

bmIm-2-carboxylate concentration in [bmIm][Ac]

after contact with CO2 and vacuum from [bmIm][Ac]

after contact with CO2 suggests that the carboxylate

reaction is reversible (Shiflett et al. 2012).

A stable viscosity of [bmIm][Ac] in contact with a

4 MPa of CO2 at 312 K was obtained after 20 h (Fig. 2).

The long time to reach the equilibrium for viscosity of

the solution might be caused by the dissolution rate of

CO2 in [bmIm][Ac], the carboxylate reaction rate of

[bmIm] ion or both these including other factors such as

the vessel arrangement. The reaction rate for carboxyl-

ation of imidazolium-based ionic liquid has not been

reported in the literature (Gurau et al. 2011; Besnard

et al. 2012a; Shiflett et al. 2012), and so it is not known

whether the reaction is fast or slow. From the calculated

results of the spontaneous dissolution curve of CO2 in

[bmIm][Ac] based on pure diffusion (Crank 1979;

50

100

150

t /h

η /m

Pa·s

0 10 20 30 40 50

0

0.5

1

Δη/η

0

0

1

2

3

4

P /M

Pa

(a)

(b)

(c)

//

//

//

//

//

130 140

//

//

Fig. 2 Time dependence of a pressure, b viscosity, g, and

c viscosity reduction ratio, Dg/g0, for 1-butyl-3-methylimida-

zolium acetate, [bmIm][Ac], in contact with CO2 at 312 K.

Viscosity reduction ratio is defined by Eq. 8

Table 1 Viscosities, g, and viscosity reduction ratios, Dg/g0, for

cellulose ? 1-butyl-3-methylimidazolium acetate, [bmIm][Ac],

solutions in contact with CO2 or N2, and the degree of polymeri-

zation, DPv, of regenerated cellulose after contact with CO2 or N2

at different cellulose concentrations, Ccell, for [bmIm][Ac] at

various pressures at 312 K

Ccell/

wt%

Gas P/MPa g/mPa�s Dgg0

DPv of regenerated

cellulose

0 N2 0.1 157 0.00 –

CO2 1 87.9 0.44

3 46.8 0.70

4 35.8 0.77

0.64 N2 0.1 300 0.00 300

CO2 1 159 0.47

3 83.1 0.72

4 59.9 0.80

1.16 N2 0.1 386 0.00 300

CO2 1 214 0.44

3 104 0.73

4 92.9 0.76

1.24 N2 0.1 412 0.00 310

4 396 0.04

Viscosity reduction ratio is defined by Eq. 8. The DPv value of

original cellulose was 310 ± 10

Cellulose

123

Shiflett and Yokozeki 2005) under the same condi-

tions for viscosity measurement (Online Resources,

Characterization and Figure S5), the dissolution rate of

CO2 in [bmIm][Ac] is very slow. Comparing the

time dependence of the calculated dissolution of CO2

in [bmIm][Ac] and the measured viscosity of

[bmIm][Ac] ? CO2 solution shows that the dissolution

of CO2 in the measurements seems to be enhanced

0123456789

δH /ppm

(a)

(b)

(c)

2

2

2

9

9

9

7.3

7.3

7.3

6 10

4 5

4 5

6 10

4 5

6 10

12 7 8

12 7 8

12 7 8

4 5

4 5

4 5

3

9

10

1112

12

4 5

6

7

8

N N+

O

O–

Fig. 3 Comparison of 1H NMR spectra for 1-butyl-3-methyl-

imidazolium acetate, [bmIm][Ac], using D2O as a solvent under

the following conditions: a blank, [bmIm][Ac] before contact

with CO2, b [bmIm][Ac] after contact with CO2 and

c [bmIm]Ac] after contact with CO2 and subjected to vacuum.

Numbers represent the 1H position of [bmIm][Ac]. Arrowsindicate 1-butyl-3-methylimidazolium-2-carboxylate. The peak

at 4.7 ppm is the residual 1H in D2O

Cellulose

123

compared with pure diffusion. However, the effects of

the carboxylate reaction and factors such as mixing or

vessel arrangement cannot be distinguished with the

experiments performed in this research.

Physical properties were measured to confirm

whether the reaction is reversible or not. The viscos-

ities and densities of [bmIm][Ac] blank, [bmIm][Ac]

after contact with CO2 and [bmIm][Ac] after contact

with CO2 and vacuum were measured with the

Stabinger viscometer, and are given at Table 2.

Figure 4 shows a comparison between viscosities

and densities among all samples over the temperature

range from 293 to 373 K. Measured viscosities and

densities were correlated with Eqs. 1 and 2, respec-

tively, using the adjustable parameters (Online

Table 2 Experimental viscosities, g, and densities, q, of

1-butyl-3-methylimidazolium acetate, [bmIm][Ac], under the

following conditions: (blank) [bmIm][Ac] before contact with

CO2, (after contact) [bmIm][Ac] after contact with CO2 and

(after contact and vacuum) [bmIm][Ac] after contact with CO2

and subjected to vacuum

T/K g/mPa�s q/kg�m-3

Blank After contact After contact and vacuum Blank After contact After contact and vacuum

293.15 732.5 571.2 754.0 1053 1081 1057

303.15 328.4 265.5 336.6 1047 1075 1051

313.15 167.1 138.3 170.6 1041 1068 1045

323.15 94.09 79.07 95.79 1035 1061 1039

333.15 57.5 48.90 58.42 1029 1055 1033

343.15 37.57 – 38.10 1023 – 1027

353.15 25.92 – 26.26 1017 – 1021

363.15 18.70 – 18.92 1012 – 1015

373.15 14.01 – 14.16 1006 – 1010

– Could not be measured value due to instability related to the presence of gas in the sample

280 300 320 340 360 38010

50

100

500

1000

T /K

η /m

Pa·s

Fig. 4 Comparison of viscosities from 293 to 373 K at

atmospheric pressure for N2 for 1-butyl-3-methylimidazolium

acetate, [bmIm][Ac], under the following conditions: (unfilledcircles) blank, [bmIm][Ac] before contact with CO2, (filledtriangles) [bmIm][Ac] after contact with CO2 and (plus signs)

[bmIm]Ac] after contact with CO2 and subjected to vacuum.

Measured values of [bmIm][Ac] after contact with CO2 above

343 K could not be measured due to the instability of the sample

at atmospheric pressure. Lines are fitted to the measured values

of each sample using Eq. 1

100

200

400

1000

η /m

Pa·s

0.2 0.4 1 21

2

3

4

5

η rel

(a)

(b)

Ccell /wt%

Fig. 5 Plots of a viscosity of cellulose ? 1-butyl-3-methyli-

midazolium acetate, [bmIm][Ac], solutions, g, and b relative

viscosity of cellulose ? [bmIm][Ac] solutions to [bmIm][Ac],

grel, over the range of cellulose concentrations, Ccell, from 0.6 to

1.7 wt% at 312 K under N2 atmospheric pressure. Line is fitted

to data in this work using Eq. 7

Cellulose

123

Resources, Table S2). Comparison of viscosities and

densities between this work and the literature

(Almeida et al. 2012; Bogolitsyn et al. 2009; Cros-

thwaite et al. 2005; Fendt et al. 2011; Pinkert et al.

2011; Shiflett et al. 2008; Tariq et al. 2009; Xu et al.

2012) are given in the Online Resource (Figure S1).

Viscosities measured with the resonant vibrational

viscometer at 312 K under atmospheric pressure

(Table 1) were about 11 % smaller than those measured

with the Stabinger viscometer using Eq. 1. Some of the

reasons for this difference is because [bmIm][Ac] can

act as a non-Newtonian fluid (McHale et al. 2008). The

resonant vibrational viscometer measures a dynamic

viscosity at high frequency (ca. 1,200 Hz), on the other

hand, a Stabinger viscometer measures a steady viscos-

ity at moderate shear rate (ca. 200 s-1). As a result,

measured viscosities with the resonant vibrational

viscometer will be smaller than those measured with

the Stabinger viscometer. This difference of measured

viscosity of [bmIm][Ac] between resonant vibrational

viscometer and conventional viscometer has been

pointed out by McHale et al. (2008).

Measurement of viscosity for [bmIm][Ac] after

contact with CO2 above temperatures of 323.15 K

could not be obtained since the measured values were

unstable. The reason for this instability is related to the

evolution of small amounts of gas in the sample during

the measurement. Viscosities of [bmIm][Ac] after

contact with CO2 decreased and the densities

increased compared with the [bmIm][Ac] blank

(Table 2). At 313.15 K, the viscosity of [bmIm][Ac]

after contact with CO2 decreased to 17 % and the

density increased to 2.6 % compared with [bmI-

m][Ac] blank. The change in viscosity and density

of the [bmIm][Ac] after contact with CO2 might be

caused by the carboxylation reaction of [bmIm] ion as

shown in NMR spectra (Fig. 3) or by residual CO2 in

[bmIm][Ac] after contact. The viscosity and density of

[bmIm][Ac] after contact with CO2 and vacuum

agreed with those of [bmIm][Ac] blank within 2 and

0.4 %, respectively (Table 2). This agreement in

viscosity and density over the range of temperatures

between [bmIm][Ac] blank and [bmIm][Ac] after

contact with CO2 and vacuum indicates that the

carboxylate reaction is reversible.

Cellulose ? [bmIm][Ac] solutions

The viscosities of cellulose ? [emIm][Ac] solutions

have been reported by researchers who used a rotational

viscometer (Gericke et al. 2009; Sescousse et al. 2010).

In this work, the viscosities of cellulose ? [bmIm][Ac]

solution were measured with a resonant vibrational

viscometer under N2 atmosphere. Figure 5 shows the

viscosities and the relative viscosities of cellu-

lose ? [bmIm][Ac] solutions as a function of cellulose

concentration from 0.6 to 1.7 wt% at 312 K. When the

cellulose concentration was above 1 wt%, the relative

viscosity greatly increased with increasing cellulose

concentration. The slope of curve above 1 wt% was

greater than 1, which indicates that solutions above 1

wt% are in the semi-dilute regime (Kuang et al. 2008).

The intrinsic viscosity of the cellulose ? [bmIm][Ac]

solution, A and n were calculated as 88 mL/g, 0.001 and

5.7, respectively, using Eq. 7. Sescousse et al. reported

80010001200140016001800

Tra

nsm

issi

on /%

Wavenumber /cm-1

(a)

(b)

(c)

(d)

(e)

Fig. 6 IR absorption spectra between 1,800 and 800 cm-1 of

(a) regenerated cellulose from 1-butyl-3-methylimidazolium

acetate, [bmIm][Ac], at 312 K under N2 atmosphere, (b) regen-

erated cellulose from [bmIm][Ac] at 312 K in contact with CO2,

(c) microcrystalline cellulose, (d) the mixture of [bmIm][Ac]

and microcrystalline cellulose and (e) [bmIm][Ac]. (dashedline) The broad peak around 1,570 cm-1 is assigned to both the

ring stretching of the imidazolium ring in the [bmIm] ion and the

asymmetric stretching of the carbonyl group in the [Ac] ion

Cellulose

123

that the intrinsic viscosity of cellulose (DPv = 180) ?

[emIm][Ac] at 313 K was about 90 mL/g (Sescousse

et al. 2010). The intrinsic viscosity reported in this work

was close to the value of Sescousse et al. who studied

same type of cellulose (microcrystalline) and a similar

ionic liquid ([emIm][Ac]). This similarity in the fitted

parameters is evidence that the results in this work are in

accordance with the literature.

Decomposition of the dissolved cellulose in

[bmIm][Ac] was studied to understand whether

[bmIm][Ac] acts as a derivatizing solvent or a non-

derivatizing solvent. The viscosity of 1.7 wt% cellu-

lose ? [bmIm][Ac] solution was measured with the

resonant vibrational viscometer at 312 K under N2

atmospheric pressure for 7 days. The change of

viscosity after measurement from its initial value

was within the uncertainty of the measurement. The

degree of polymerization of regenerated cellulose

from [bmIm][Ac] (DPv = 310) agreed with that of the

original cellulose (DPv = 310) within the uncertainty

of the measurement. Figure 6 shows the IR absorption

spectrum of the regenerated cellulose (Fig. 6a). The

IR spectrum of the regenerated cellulose was similar to

that of the original cellulose. The regenerated cellu-

lose was not contaminated with [bmIm][Ac] from the

IR spectrum as evident from the absence of a broad

peak around 1,570 cm-1 which is assigned to both the

ring stretching of the imidazolium ring in the [bmIm]

ion and the asymmetric stretching of the carboxylate in

the [Ac] ion (Cabaco et al. 2012). The XRD pattern of

the regenerated cellulose shows that the regenerated

cellulose had high amorphous content (Online

Resources, Figure S6). This transformation from

cellulose I to amorphous cellulose is strong evidence

that complete cellulose dissolution occurs (Jiang et al.

2012). The [bmIm][Ac] did not change during disso-

lution of cellulose at 312 K for 7 days as shown in the1H and 13C NMR spectra of cellulose ? [bmIm][Ac]

solution after the experiment (Online Resources,

Figures S7 and S8). These results allow the conclusion

to be made that pure [bmIm] acts as a non-derivatizing

solvent for cellulose.

Cellulose ? [bmIm][Ac] solution in contact

with CO2

The effect of pressurized CO2 on the viscosity of

cellulose ? [bmIm][Ac] solutions was investigated.

Figure 7 shows the time dependence of the viscosity

and viscosity reduction ratios of a 0.6 wt% cellu-

lose ? [bmIm][Ac] solution in contact with CO2 at

various pressures and at 312 K. When CO2 pressure

was 4 MPa, the viscosity reduction ratio became 0.80

over a time period of 33 h. The viscosity reduction

ratio decreased when the CO2 pressure was decreased

in steps (Fig. 7). At 3 MPa, the viscosity reduction

ratio became 0.72 and as the pressure was reduced to

1 MPa, it became 0.47. After depressurizing to

atmospheric pressure, the viscosity became about 0.9

times its initial value. Once a vacuum was applied to

the solution for 1 h, the viscosity returned to its initial

value within the uncertainty of the measurement. This

indicates that the dissolved cellulose in [bmIm][Ac]

did not decompose during the experiment and supports

conclusions drawn on the analysis of the regenerated

cellulose as discussed later. The 1H NMR spectrum of

the cellulose ? [bmIm][Ac] solution after contact

with CO2 was measured using DMSO-d6 as a solvent

100

200

300

t /h

η /m

Pa·s

0 10 20 30 40 50 60

0

0.5

1

Δ η/ η

0

0

1

2

3

4

P /M

Pa

(a)

(b)

(c)

Fig. 7 Time dependence of a pressure, b viscosity, g, and

c viscosity reduction ratio, Dg/g0, for 0.6 wt% of cellulose

?1-butyl-3-methylimidazolium acetate, [bmIm][Ac], in contact

with CO2 at 312 K. Viscosity reduction ratio is defined by Eq. 8

Cellulose

123

and is shown in Fig. 8. After contact with CO2, new

peaks appeared in the 1H NMR spectrum and that are

assigned to bmIm-2-carboxylate. The 13C NMR

spectra support the 1H NMR results (Online

Resources, Figure S9). From the appearance of the

additional peaks, the carboxylate reaction of [bmIm]

ion occurred when it was contact with CO2 both in the

presence and in the absence of cellulose. Figure 8

01234567891011

δH /ppm

(a)

(b)

(c)

2

2

6

2

7.87.9

7.67.77.8

6 10

4 5

4 5

6 10

4 5

10

7 8

12 9

7 8

12 9

7 8

12 9

4 5

4 5

7.67.77.8

3

9

10

1112

12

4 5

6

7

8

N N+

O

O–

3

9

10

1112

12

4 5

6

7

8

N N+

O

O–

Fig. 8 Comparison of 1H NMR spectra for cellulose ? 1-butyl-

3-methylimidazolium acetate, [bmIm][Ac], using DMSO-d6 as a

solvent under the following conditions: a blank, [bmIm][Ac]

without cellulose, b cellulose ? [bmIm][Ac] solution after

contact with CO2 and c cellulose ? [bmIm][Ac] solution after

contact with CO2 and recovery of cellulose at 312 K. Numbers

represent the 1H position of [bmIm][Ac]. Arrows indicate 1-butyl-

3-methylimidazolium-2-carboxylate. The peaks at 2.5 and

3.8 ppm are residual 1H in DMSO-d6 and 1H in H2O, respectively

Cellulose

123

shows the 1H NMR spectrum of [bmIm][Ac] after

contact with CO2 and recovery of cellulose. Peaks for

bmIm-2-carboxylate were not observed in the 1H

NMR spectrum of [bmIm][Ac] after contact with CO2

and recovery of cellulose, and this indicates that the

carboxylate reaction is reversible.

Figure 9 and Table 1 show the CO2 pressure

dependence on the viscosity and its reduction ratio

for different concentrations of cellulose in [bmIm][Ac]

at 312 K. Regardless of the cellulose concentration, the

viscosity of cellulose ? [bmIm][Ac] solution decreased

with increasing CO2 pressure. At a CO2 pressure of

4 MPa, there was little difference in the viscosity

reduction ratio of the cellulose ? [bmIm][Ac] solution

and the pure [bmIm][Ac]. When N2 was used at the same

pressure, the viscosity of the cellulose ? [bmIm][Ac]

solution decreased by about 4 %, which is within the

uncertainty of the viscosity measurement. The cellu-

lose ? [bmIm][Ac] solution contact with CO2 caused a

large viscosity reduction compared with that of using N2

at the same conditions.

The mechanism of viscosity reduction in contact

with CO2 was investigated further. The viscosity of the

cellulose ? [bmIm][Ac] ? CO2 solution could be

lowered by (1) the decomposition of cellulose in

solution and also by (2) the dissolution of CO2 in

solution including the change of [bmIm][Ac] with

dissolved CO2. In all experiments, the DPv of regen-

erated cellulose after viscosity measurement was

examined. The DPv changes after the experiments

were within the uncertainty of the DPv measurements

(Table 1) The IR absorption spectrum of the regener-

ated cellulose was similar to those of the original

cellulose, and the regenerated cellulose was not

contaminated with [bmIm][Ac] (Fig. 6). From these

results, it can be concluded that cellulose did not

decompose under the conditions of the experiment,

and that the mixture of [bmIm][Ac] with CO2 acted as

a non-derivatizing solvent. Thus, the dissolution of

CO2 in cellulose ? [bmIm][Ac] solutions including

the reaction products of [bmIm][Ac] with CO2

probably cause the viscosity reduction of the cellu-

lose ? [bmIm][Ac] solutions in contact with CO2.

The solubility of CO2 in [bmIm][Ac] increases with

increasing CO2 pressure (Carvalho et al. 2009; Shiflett

et al. 2008). From Fig. 9, an increase in the viscosity of

the cellulose ? [bmIm][Ac] solution during depres-

surization was caused by the lower CO2 content in the

solution. The N2 is relatively insoluble in ionic liquids

compared with CO2 at the same conditions (Anthony

et al. 2005). Due to the low solubility of N2 in the

cellulose ? [bmIm][Ac] solution, the viscosity reduc-

tion of the cellulose ? [bmIm][Ac] solution under

pressurized N2 was small (Fig. 9). The viscosity

reduction obtained with CO2 will most likely enhance

mass transfer processes such as the dissolution of

cellulose in acetate anion ionic liquids.

Conclusions

Viscosities of cellulose ? [bmIm][Ac] solutions and

their contact with CO2 were measured with a resonant

vibrational viscometer. The carboxylate reaction of

[bmIm][Ac] occurred regardless of cellulose concen-

tration, and it seems to be reversible based on NMR,

viscosity and density measurements. Viscosity, DPv and

IR measurements show that the dissolved cellulose in

the [bmIm][Ac] did not decompose at 312 K in contact

50

100

500

P /MPa

Δ η/ η

0η

/mPa

·s

0 1 2 3 4

0

0.5

1

(a)

(b)

Fig. 9 Pressure dependence of a viscosity, g, and b viscosity

reduction ratio, Dg/g0, for cellulose ? 1-butyl-3-methylimida-

zolium acetate, [bmIm][Ac], solutions at 312 K. Viscosity

reduction ratio is defined by Eq. 8. Filled symbol indicates

solutions in contact with N2. Unfilled symbols indicate solutions

in contact with CO2. Symbols: (unfilled triangles) 1.2 wt%

cellulose ? [bmIm][Ac] solution, (unfilled circles) 0.6 wt%

cellulose ? [bmIm][Ac] solution and (unfilled squares) pure

[bmIm][Ac]

Cellulose

123

with CO2. These findings show that CO2 can be used as a

viscosity reducing agent for the cellulose ? ionic liquid

solution and that [bmIm][Ac] with CO2 acts as a non-

derivatizing solvent for cellulose at 312 K.

Acknowledgements The authors thank Mr. Syunsuke Kaya-

mori for his help in the NMR measurements. The authors wish to

acknowledge the Global Educational Center of Excellence

(GCOE) and the partial financial support of the Ministry of

Education, Science, Sports and Culture for support of this

research.

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