1

Study on the absorbability, regeneration characteristics and thermal

stability of ionic liquids for VOCs removal1

Wenlong Wanga, Xiaoling Ma a, Sue Grimes*b, Hongfei Caib, Meng Zhanga

a National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University,

Jingshi Road, No. 17923, Jinan, Shandong 250061, PR China

b Department of Civil and Environmental Engineering, Imperial College London, SW7 2AZ UK

Abstract: A novel method of removal of volatile organic compounds (VOCs) using

the ionic liquid [Bmim][NTf2] as an absorbent is developed as a contribution to

dealing with recent severe smog incidents in China. The effects of concentration,

temperature and flow rates on the ability of [Bmim][NTf2] to absorb VOCs were

studied using toluene as a model volatile organic pollutant. The potential of the use of

[Bmim][NTf2] as an absorbent for VOCs is shown by the solubility of toluene in the

ionic liquid; the absorptivity of the ionic liquid for toluene; and the fact that absorbed

toluene can be removed easily from [Bmim][NTf2], permitting recycle of the ionic

liquid in multiple reuse phases. The solubility of toluene in [Bmim][NTf2] is 61.5 %

at 20 °C and atmospheric pressure; the highest absorptivity of [Bmim][NTf2] for

toluene is 98.3%, achieved at a toluene concentration of 300 ppm and a flow rate of

50 mL min-1 at 20 °C; and the absorptivity of the ionic liquid is >94% over a wide

range of conditions. The ionic liquid can be recovered and recycled in the absorption

process at least five times, reducing the reagent cost in the VOCs removal process.

Keywords: Ionic liquids; Volatile organic compounds; Absorption; Thermal stability;

Regeneration

1. Introduction

The escalating quantities of volatile organic compounds (VOCs) generated from

industrial operations in China have not been controlled effectively, in spite of the fact

that a target has been set in China, to achieve 25.9 Tg by 2020 [1]. VOCs are the

primary precursors of fog and haze [2, 3], which create severe smog conditions and

affect the health of over 800 million people in east China. Currently, the technologies

Corresponding author. Wenlong Wang, Address: National Engineering Laboratory for Coal-fired

Pollutants Emission Reduction, Shandong University, Jingshi Road, No. 17923, Jinan, Shandong

250061, PR China. Tel: +86 531 88399372; fax: +86 531 88395877. E-mail: wwenlong@sdu.edu.cn Sue Grimes, Address: Department of Civil & Environmental Engineering, Imperial College London

SW7 2AZ UK Tel: +44 0207 584 5866; Email: s.grimes@imperial.ac.uk

mailto:wwenlong@sdu.edu.cnmailto:s.grimes@imperial.ac.uk

2

used to control VOCs emissions can be classified under two categories: (i) destructive

methods, such as chemical oxidation and biological oxidation [4-6] and (ii) recovery

processes that make use of absorption/adsorption, condensation and membrane

separation methods [7-9]. Compared to the destructive methods, the absorption

methods have attracted considerable attention because of their simplicity, safety,

recyclability and low cost [10-13]. The absorbent used in these technologies plays a

critical role and absorbents with a high absorption capacity and good recyclability are

preferred for VOCs removal. The types of organic solvents that have been used for

VOCs removal include a vegetable oil-water emulsion [14], a silicone oil-water

emulsion [15, 16], di(2-ethylhexyl) adipate and diisobutyl phthalate [17]. Most of the

studies on VOCs removal have been carried out on batch and semi-batch systems

without the use of a solvent recovery process. The ability to regenerate the solvents

for recycle and further use as an absorbent is, however, an important requirement to

aid the economics of solvent use in VOCs removal processes, and absorbents with a

low volatility, high thermal stability, and recyclability are urgently needed for VOCs

removal.

Ionic liquids (ILs) are environmentally friendly stable solvents, normally composed

of organic cations and either organic or inorganic anions with extremely low volatility,

and have played an important role in, for example, catalysis, solvent extraction and

organic synthesis [18-20], and in the removal of organic pollutants in environmental

control applications. As task-specific solvents, ILs exhibit excellent solubility for a

wide range of organic compounds [21]. Miquel et al. [22], have for example,

performed a systematic thermodynamic analysis of a sample of 14 representative

VOCs, using Conductor-like Screening Model for Real Solvents (COSMO-RS)

methodology, to establish a classification for VOCs with respect to their solubility

behavior. Quijano et al. [23] have confirmed the high affinity of ILs for three model

VOCs (dimethyl sulfide, dimethyl disulfide and toluene), using dimensionless

partition coefficients of the VOCs in [Bmim][PF6] (v=0.209 kg m-1 s-1, ρ=1360 kg m-3)

and [Bmim][NTf2] (v=0.040 kg m-1 s-1, ρ=1430 kg m-3), and showed that the results

were comparable to those obtained with VOCs in typical liquid solvents. Although

VOCs have been shown to have good solubility in some ILs, the application of these

absorbents for VOCs removal has received little attention. A particular advantage of

the use of ILs in absorbent methodologies is that in many applications they have been

shown to be easily recycled for multiple reuse in a number of processes [24, 25].

3

We now report for the first time on the use of the ionic liquid

1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [Bmim][NTf2], as a

recoverable and reusable solvent for the capture and removal of VOCs using toluene

as a model VOC.

2. Experiment and methods

2.1 Materials

Toluene, a typical aromatic hydrocarbon, was selected as the representative VOC

because of its wide industry applications and hazardous impacts on the environment

and human health [26].

The ionic liquid used was the hydrophobic solvent 1-butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide, [Bmim][NTf2], the molecular structure of which is

shown in Fig. 1. This IL has a relatively low viscosity and a purity of >99%, and was

purchased from the Shanghai Chengjie Reagent Co., Ltd. All chemicals, including

toluene and ethanol, used in this study were of analytical grade and were not further

purified.

Fig. 1 Molecular structure of [Bmim][NTf2]

2.2 VOC solubility and absorption

2.2.1 VOC solubility test

The solubility of toluene in the IL was measured by the cloud point method and

refers to the ratio of the maximum dissolved toluene to the mass of the IL [27, 28].

Toluene (0.5 mL) was pipetted into a test tube containing a known weight of IL and

the test tube was left in a magnetic stirring vessel for 30 s. This step was repeated

until the mixture became cloudy. With additional stirring, the cloudiness disappeared,

after which, further aliquots of toluene were added, dropwise to the IL, to the point at

which, the mixture, on standing for 10 min, remained cloudy. This indicated the point

at which toluene over-saturation in the IL had occurred. Finally the mixture was

stirred until it became clear again, indicating an equilibrium state, and the amount of

toluene dissolved was measured by the increased weight of the mixture from that of

the IL alone.

4

2.2.2 VOC absorption test

The absorption experiment was designed to evaluate the absorptivity of the IL for

toluene. The model exhaust gas was prepared by injecting an amount of liquid toluene

into a 5 L Tedlar sample bag and air was also injected. Each model gas sample was

left to stand at 20 °C for 3 hours (toluene was completely vaporized and mixed with

air). The IL was placed in a 30 mL bubbling bottle with a porous glass sand core at the

bottom of the gas pipe so that the gas could be effectively dispersed as small bubbles.

The model gas was extracted from the sample bag via a vacuum pump into the

bubbling bottle, to pass through the IL. The purified gas subsequently entered an

absorption bottle filled with anhydrous ethanol to remove any remaining toluene. A

schematic of the experimental procedure is shown in Fig. 2. During the experiment,

the weight of the IL was 20 g; the temperature of the IL was controlled by placing the

absorption bottle in a thermostatic bath (HH-2, Shanghai Lichen Technologies Co.,

Ltd.); the inlet gas flow rate was controlled using mass flow controllers (MFC,

Flowmethod FL-802); and the toluene concentration in the model exhaust gas was

regulated via different liquid toluene injection volumes.

Fig. 2 Schematic of the toluene absorption measurement system

(1) Compressed air bottle; (2) Tedlar sample bag; (3) Micro sampling needle; (4) Vacuum pump;

(5) Mass flow controllers; (6) Bubbling bottle; (7) Thermostatic bath; (8) Absorption bottle; (9)

Gas chromatograph

The toluene concentration in the gas was measured using an Agilent GC 7890A

chromatograph (Agilent Technologies, Co., Ltd, USA) with a DB-wax column (30 m

× 0.25 mm × 0.5 μm). Use of this method gives a reliable straight line correlation

between toluene concentration and peak areas over the range of concentration studied

in the work. The absorptivity (η, %) of toluene in the IL was calculated using Eq. (1).

%1 0 0

in

outin

C

CC

(1)

5

where Cin is the toluene concentration of the inlet gas (ppm), and Cout is the toluene

concentration of the outlet tail gas (ppm).

2.3 Recovery of the IL for multiple use as an absorbent

Recycle of the IL for multiple use as an absorbing solvent for VOC recovery is

important in the economics of using ionic liquids for this purpose [29]. In this work,

the toluene absorbed in the ionic liquid was removed by thermal distillation at 75 °C

and collected by condensation [30]. The toluene-free ionic liquid was then

characterized by Fourier Transform Infrared (FTIR) spectroscopy using a Nicolet

6700 FTIR spectrometer (Thermo Nicolet, Co., Ltd., USA) equipped with a single

reflection horizontal attenuated total reflection (ATR) accessory (Pike Technologies,

Co., Ltd., USA). The FTIR spectrum which is an average of 16 scans was compared

with that of the pure ionic liquid in the spectral range 4500 cm-1 to 600 cm-1.

2.4 Determination of the thermal stability of the IL

The thermal stability of the IL was characterized using a combined

Thermogravimetric Analyzer and Differential Scanning Calorimeter (TGA/DSC) 1

(Mettler-Toledo, Co., Ltd., Switzerland). The IL samples were weighed to 17±0.1 mg

and placed in an aluminum oxide ceramic crucible with a depth of 5 mm and a

diameter of 5 mm. The thermogravimetric analysis (TGA) experiments were

conducted from 298 K to 923 K at heating rates of 5, 10, 15 and 20 K min-1 with an

argon (99.999% purity) flow of 20 mL min-1.

The activation energy (Ea) is an important kinetic parameter to assess the thermal

stability of ILs, where a higher value of Ea means a stronger stability. The Ea of the

thermal decomposition for the IL was calculated based on the TGA results through the

model-free approaches of the Kissinger-Akahira-Sunose (KAS) method, the

Flynn-Wall-Ozawa (FWO) method, and the Starink method. The specific derivations

of Ea calculation, using the three methods, are given in Supplementary material.

These methods are derived on the assumption that the reaction rate only depends on

the temperature and conversion degree, and are widely used for kinetic parameter

determination. The values of correlation coefficient (r) and the standard deviation (SD)

for regression line can assess the applicability of these reported methods.

6

3. Results and discussion

3.1 VOC absorption performance of the IL [Bmim][NTf2]

3.1.1 Solubility analysis

The solubility test for toluene in [Bmim][NTf2] is conducted at room temperature,

and Table 1 summarizes the mass ratios and some specific observations at several key

stages during the test. The solution first becomes cloudy at a mass ratio of 53.62% and

then returns to clear again; this is a sign that the solution is approaching equilibrium.

After adding several drops of toluene, the solution does not show dense cloudiness

until the mass ratio reached 62.67%. From then on, the mass ratio is reduced due to

the volatilization of toluene. When the mass ratio is decreased to 61.45%, the solution

is clear again to give a solubility of toluene in [Bmim][NTf2] of 61.45% at room

temperature and atmospheric pressure. The high solubility of toluene in the IL shows

the strong affinity of the IL for toluene, indicating a high capacity of ILs to dissolve

VOCs.

Table 1 Solubility test of toluene

[Bmim][NTf2]

(g)

Toluene

added (g)

Mass ratio

(%)

Observation

1.4657

0.7859 53.62 Cloudy appearance disappears &

liquid transparency decreases

0.8447 57.63 Cloudy appearance disappears &

liquid transparency decreases

0.8819 60.17 Toluene bubbles occur

0.9186 62.67 Cloudy

0.9050 61.75 Cloudy

0.9007 61.45 Clear again

3.1.2 Effect of variable parameters on VOC absorptivity in the IL

The absorption performance of toluene in the IL is studied by examining the

influence of changes in the following parameters: toluene concentration (γ),

absorption temperature (Ta), and flow rate (v).

The data in Fig. 3(a) show the effects of the inlet toluene concentration on the

absorptivity at 20 °C and a flow rate of 75 mL min-1. A monotonical increase is

observed with increasing toluene concentration until a plateau of 97.1% is reached at

7

300 ppm, followed by a decline to 95.5% is at 400 ppm. In the absorption process, a

toluene film and an IL film are formed on the gas-liquid (toluene-[Bmim][NTf2])

interface based on the two-film theory. Toluene molecules must pass through the

gas-liquid interface to be dissolved in the IL phase. With the increase in the toluene

concentration, the concentration gradient of the gas-liquid phase is increased, which

improves the mass transfer driving force. This effect is beneficial for the absorption of

the toluene in the IL and enhances the absorptivity. [31].

100 200 300 400

60

65

70

75

80

85

90

95

100(a)

Abso

rpti

vit

y (%

)

Toluene concentration (ppm)

20 40 60 80

60

65

70

75

80

85

90

95

100(b)

Ab

sorp

tiv

ity（

%）

Absorption temperature (C)

50 100 150 200

60

65

70

75

80

85

90

95

100(c)

Abso

rpti

vit

y (%

)

Flow rate (mL min-1

)

Fig. 3 Effects of variable parameters on the absorptivity of toluene in [Bmim][NTf2]. (a) toluene

concentration (Ta=20 °C, v=75 mL min-1); (b) absorption temperature (γ=300 ppm, v=75 mL

min-1); (c) flow rate (γ=300 ppm, Ta=20 °C).

The relationship between the temperature and absorptivity was studied over

temperature intervals of 20 °C, from 20 °C to 80 °C. From the data in Fig. 3(b), the

absorptivity is 97.1% at 20 °C and slightly decreases with increasing temperature,

giving an absorptivity of 93.8% at 80 °C. Henry’s constant enlarges, leading to a

decrease in the solubility of toluene in the IL [32], when the temperature is raised

from 20 °C to 80 °C. Over the same range the viscosity of the IL will be reduced and

the dissolution capacity for toluene, strengthened. Unfortunately, the latter

contribution is not sufficient to offset the decrease in the removal efficiency, therefore,

8

20 °C is the optimal temperature for absorbing toluene in the IL.

The data in Fig. 3(c), show that there is a distinct downtrend for absorptivity with

the acceleration of flow rate. The maximum absorptivity of 98.3% is obtained at a

flow rate of 50 mL min-1; but increasing the flow rate to 200 mL min-1 leads to an

absorptivity decrease to 94.6%. The results indicate that absorptivity is significantly

influenced by the residence time of toluene in the IL, which is proportional to flow

rate. At a high flow rate, toluene may be blown out without fully contacting with the

IL. If the flow rate is too small, however, the volume of the absorption equipment

would have to be larger, which would raise the cost of the process. Hence, the optimal

flow rate is 50-100 mL min-1.

Based on the absorption performance, the optimal absorptivity is obtained at a

toluene concentration of 300 ppm and a flow rate of 50 mL min-1 at 20 °C. Xiao et al.

[30] employed two fluorocarbon surfactants (FSN100 and FSO100) to absorb toluene

and the saturation absorption capacities for FSN100 and FSO100 (solution

concentration of 0.1%) were 2.9 mg g-1, and 4.2 mg g-1, respectively. Zhang et al. [33]

investigated the characteristics of four different types of adsorbents for dynamic

adsorption/desorption of toluene. It is shown that microporous material NaY, (Na52

[(AlO2)52(SiO2)140]·240H2O), has the largest adsorption capacity of 0.2873 mL g-1. In

this study, the saturation capacity of [Bmim][NTf2] for toluene absorption was found

to be 135.49 mg g-1 indicating that [Bmim][NTf2] could be used as a promising

absorbent for toluene removal.

3.2 Regeneration of the IL for reuse

From an economic point of view, the ability to regenerate the IL for reuse is one

of the important factors for industrial applications, because regeneration will reduce

reagent costs in removing VOCs.

Previous studies have proved that lower temperature is favorable for gas

absorption and a higher temperature is preferred for gas desorption from the ionic

liquid phase [34]. In this study, the absorbent IL was regenerated via thermal

distillation and the absorptivity of the regenerated IL for toluene examined. The

toluene absorptivity during the five reuse cycles is high between 92% and 97%, with

only a slight decline in the absorptivity in the five cycles (Fig. 4), which indicates the

IL can be regenerated for recycle in this application in multiple reuse phases.

9

1 2 3 4 50

20

40

60

80

100

Frequency of recycling

Ab

sorp

tiv

ity

(%)

Fig. 4 Absorptivity variation of the regenerated IL (Ta=20 °C, v=75 mL min-1, γ=300 ppm)

The regenerated IL and original IL, as well as the recovered toluene and fresh

toluene, were all characterized using FTIR and the spectra obtained are in Fig. 5. The

spectra of the regenerated IL is consistent with that of the original IL (Fig. 5(a)), with

identical chemical groups present, including the imidazole ring C-H stretching

vibrations at 3157 cm-1 and 3120 cm-1, alkyl group chain substituted C-H stretching

vibrations at 2967 cm-1, 2940 cm-1 and 2880 cm-1 , the imidazole ring skeleton

vibrations at 1574 cm-1 and 1470 cm-1, O=S=O asymmetric stretching vibration and

symmetric stretching vibration at 1324 cm-1 and 1132 cm-1 respectively, S-N-S

asymmetric stretching vibration at 1051 cm-1, C-S stretching vibration at 788 cm-1 and

-CF3 asymmetric stretching vibration at 739 cm-1. No new functional groups are

detected in the regenerated IL. The FTIR spectra (Fig. 5(b)) for both the fresh and

recovered toluene are similar, including the benzene ring stretching vibrations at 692

cm-1 and 725 cm-1, the benzene ring skeleton vibrations at 1495 cm-1 and 1604 cm-1,

and the benzene ring C-H stretching vibration at 3026 cm-1.

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10

4000 3500 3000 2500 2000 1500 1000

0.0

0.2

0.4

0.6

0.8 Regenerated [Bmim][NTf2]

Abso

rban

ce

4000 3500 3000 2500 2000 1500 1000

0.0

0.2

0.4

0.6

0.8

Wavenumber (cm-1

)

Original[Bmim][NTf2]

Abso

rban

ce

(a)

4000 3500 3000 2500 2000 1500 1000

0.0

0.2

0.4

0.6

0.8

Absorb

ance

Absorb

ance

Wavenumber (cm-1

)

Recovered Toluene

4000 3500 3000 2500 2000 1500 1000

0.0

0.2

0.4

0.6

0.8

Fresh Toluene(b)

Fig. 5 FTIR spectra of original and regenerated (a) [Bmim][NTf2] and (b) the fresh and

recovered toluene

The IL can be effectively regenerated after the absorption of toluene via thermal

distillation. The unchanged characteristics and minimal loss of IL, due to its very low

viscosity, are beneficial for industrial applications compared to common volatile

organic solvents.

3.3 Thermal stability and kinetic calculations for regenerated IL

3.3.1 Thermal stability of the IL

Thermal stability is a critical attribute to judge an IL’s value as a VOC absorbent.

TGA and differential thermogravimetric (DTG) curves for the IL, [Bmim][NTf2], at

the heating rates of 5, 10, 15, and 20 K min-1 are shown in Fig. 6. In all cases the IL is

shown to be stable under 600 K after which a single well-defined mass loss step is

clearly observed.

300 400 500 600 700 800 900

0

20

40

60

80

100

120

5K min-1

10K min-1

15K min-1

20K min-1

Mas

s lo

ss p

erce

nta

ge

(%)

Temperature (K)

(a)

300 400 500 600 700 800 900

-48

-40

-32

-24

-16

-8

0

5K min-1

10Kmin-1

15K min-1

20K min-1

Der

ivat

ion

mas

s lo

ss

104

(s-1

)

Temperature (K)

(b)

Fig. 6 (a) TGA and (b) DTG curves for [Bmim][NTf2] at heating rates of 5, 10, 15, and 20 K min-1

11

Table 2 lists the specific thermal characteristic parameters in each analysis. The

onset mass loss temperature (Tonset) is used to describe the thermal stability of the IL

which is observed to be thermally stable up to 612.5 K at 5 K min-1 and 662.7 K at 20

K min-1. The thermal decomposition of the IL mainly occurs in the temperature range

of 612.5 K to 732.0 K with final residues of 2.21%, 3.93%, 6.13%, and 8.55% at each

heating rate, respectively. A two-peak structure is detected in the corresponding DTG

curves as shown in Fig. 6(b), which is ascribed to the decomposition of the cation and

anion portion of the IL at different temperatures [35]. The DTG spectra shown in Fig.

6 (b), confirm the TG results. Each IL sample in the thermal analysis balance is heated

by the heat radiation and convection, thus the higher heating rate decreases the heat

distribution in molecules and the thermal degradation starts at a higher temperature.

Further, the maximum derivation weight loss (DTGm) and the relevant temperature

(Tm) for the IL increase with the rise in the heating rate, from 1.36×10-3% K-1 at 700.7

K to 4.72×10-3% K-1 at 749.0 K. These results reveal that the IL used in this study is

thermally stable as an absorbent for VOC removal.

Table 2 Thermal characteristic parameters of ILs [Bmim][NTf2] at 5, 10, 15, and 20 K min-1

Sample β

(K min-1)

Tonset

(K)

DTGm ×103

(% K-1)

Tm

(K)

Final

residue

(%)

[Bmim][NTf2]

5 612.50 1.36 700.75 2.21

10 640.00 2.14 737.67 3.93

15 647.75 3.26 742.00 6.13

20 662.67 4.72 749.00 8.55

3.3.2 Kinetic calculations of the IL’s thermal stability

Based on the TGA results, the activation energy (Ea) is calculated using the KAS,

FWO and Starink methods at eight conversion (α) values varying from 10% to 80%,

with an increment of 10%, with the parallel regression lines, depicted in Fig. 7,

indicating that the Ea values follow either a single mechanism or multiple reaction

mechanisms that are unified for different conversions. The parallel lines show the

three methods employed in this study are appropriate, as confirmed with the data

presented in Table 3.

12

1.32 1.36 1.40 1.44 1.48 1.52 1.56

-11.7

-11.4

-11.1

-10.8

-10.5

-10.2

-9.9

ln

[/T

2]

(K-1

· s-

1 )

1/T103(K)

10%

20%

30%

40%

50%

60%

70%

80%

(a)

10%

20%

30%

40%

50%

60%

70%

80%

1.32 1.36 1.40 1.44 1.48 1.52 1.56

1.5

1.8

2.1

2.4

2.7

3.0

ln[

] (K

·s-1

)

1/T103

(K)

(b)

1.32 1.36 1.40 1.44 1.48 1.52 1.56

-11.1

-10.8

-10.5

-10.2

-9.9

-9.6

ln[

/T1

.92 ]

(K

-1·

s-1)

1/T103

(K)

10%

20%

30%

40%

50%

60%

70%

80%

(c)

Fig. 7 Regression lines for [Bmim][NTf2] based on the KAS method (a), FWO method (b), and

Starink method (c) at 5, 10, 15, and 20 K min-1

The corresponding values of the slope, the Ea value, the correlation coefficients (r)

and the standard deviation (SD) for the IL are summarized in Table 3. The values of r

and SD show the satisfying fits of the KAS, FWO and Starink methods for the

experimental data, which indicate these three methods can be used to calculate the Ea

for the thermal decomposition of the IL.

As the energy barrier, Ea is the critical energy that must be overcome to generate

a chemical reaction; a higher Ea value means a higher thermal stability. In the three

calculation processes, the Ea varies from 108.8 kJ mol-1 to 126.5 kJ mol-1. The

average Ea values for the IL calculated by the KAS, FWO and Starink methods are

separately 115.9 kJ mol-1, 121.3 kJ mol-1 and 116.3 kJ mol-1, respectively, and these

values show good coordination with each other. Efimova et al. [36], the Ea values for

three ILs ([Emim] halides (Cl, Br and I)) calculated by KAS method are 120±12 kJ

mol-1, 125±9 kJ mol-1, and 111±8 kJ mol-1, respectively, which are similar to the Ea

results calculated in this study. The high Ea value of [Bmim][NTf2] indicates it is

thermally stable and can be used as a VOC absorbent.

13

Table 3 Correlation coefficient (r), activation energy (Ea), and standard deviation (SD) for the IL

[Bmim][NTf2] calculated using the KAS method, FWO method, and Starink method

Sample α (%)

KAS method FWO method Starink method

r Ea

(kJ mol-1) SD r

Ea

(kJ mol-1) SD r

Ea

(kJ mol-1) SD

IL

10 0.99 109.5 0.04 0.99 114.6 0.04 0.99 109.9 0.04

20 0.99 116.9 0.04 0.99 121.9 0.04 0.99 117.3 0.04

30 0.99 121.6 0.04 0.99 126.5 0.04 0.99 122.0 0.04

40 0.99 121.3 0.03 0.99 126.4 0.03 0.99 121.7 0.03

50 0.99 120.3 0.06 0.99 125.6 0.06 0.99 120.7 0.06

60 0.99 116.7 0.11 0.99 122.2 0.06 0.99 117.1 0.11

70 0.97 112.2 0.15 0.98 118.0 0.15 0.97 112.6 0.15

80 0.97 108.8 0.17 0.97 114.9 0.17 0.97 109.2 0.17

Average 115.9 121.3 116.3

4. Conclusion

In this study, the solubility of toluene in [Bmim][NTf2] is shown to be 61.5 % at

20 °C; the highest absorptivity for toluene of 98.3%, was achieved with a toluene

concentration of 300 ppm and a flow rate of 50 mL min-1 at 20 °C, and the

absorptivity was >94% over a wide range of conditions. Additionally, the IL used was

shown to be capable of regeneration via thermal distillation of the dissolved toluene,

maintaining an absorptivity of >92% after five multiple reuse phases. The ability to

recycle the IL at least five times, reduces the reagent cost in the VOC removal process

and aids the economic value of the process. To confirm that IL regeneration was

possible, the thermal stability of [Bmim][NTf2] was evaluated through TGA

experiments, and the results revealed that the IL was thermally stable up to 612.5

K-662.67 K at heating rates of 5 to 20 K min-1. Finally, the Ea of the thermal

decomposition of [Bmim][NTf2] was calculated using the KAS, FWO and Starink

methods and the average Ea values were 115.9 kJ mol-1, 121.3 kJ mol-1, and 116.3 kJ

mol-1 respectively. The IL, [Bmim][NTf2], is thus shown to have the necessary

properties to suggest that it is a promising absorbent for VOC removal, using toluene

as a model VOC.

14

Acknowledgements

The authors thank the support of Shandong Science Fund for Distinguished

Young Scholars (JQ201514). Support from China Scholarship Council enabled

Professor Wenlong Wang to be an academic visitor at Imperial College of Science,

Technology and Medicine, London and to build up a research collaboration with

Professor Sue Grimes.

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