Mass Transfer Rate Enhancement for CO2 Separation by Ionic Liquids

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Mass transfer rate enhancement for CO2 separation by ionic liquids: effect of film thickness Wenlong Xie, Xiaoyan Ji, Xin Feng, and Xiaohua Lu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03339 • Publication Date (Web): 07 Dec 2015 Downloaded from http://pubs.acs.org on December 11, 2015

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Industrial & Engineering Chemistry Research

Mass transfer rate enhancement for CO2 separation by ionic liquids: effect of film thickness Wenlong Xie,† Xiaoyan Ji,‡ Xin Feng,† and Xiaohua Lu*,† †Department of Chemistry and Chemical Engineering, State Key Laboratory of MaterialsOriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, China ‡Division of Energy Science/Energy Engineering, Luleå University of Technology, 97187 Luleå, Sweden *Corresponding author: Xiaohua Lu, Tel/Fax: +86-25-83588063, Email address: [email protected]

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Abstract

Ionic liquids (ILs) are promising in CO2 separation, while the film thickness is particularly critical for gas transport in these viscous and expensive liquids. In this work, the influence of ILfilm thickness on CO2 absorption/desorption of two different IL immobilized sorbents was investigated, in which the results from the thermogravimetric analyzer were further used to estimate the scale of IL-film thickness. It is found that the IL-film in nano-scale is a prerequisite for efficient CO2 absorption/desorption; the equilibrium time can be 10-time different and the rate constant can be 100-time different for micro-scale and nano-scale IL-films. This is the first time to quantitatively reveal the influence of IL-film thickness and find out its scale for a significant rate enhancement in the CO2 absorption/desorption by IL immobilized sorbents.

KEYWORDS: carbon dioxide separation, ionic liquids, immobilization, film thickness


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1. INTRODUCTION Anthropogenic carbon dioxide (CO2) produced mainly from the combustion of fossil fuels especially coal is believed to be the main cause of the global warming.1-3 To mitigate CO2 emissions from fossil-fueled power plants, CO2 capture and storage (CCS) is an important option.2, 4, 5 In general, CO2 separation from a gas mixture needed in CCS is an energy intensive process, and it has been estimated that the CO2 capture contributes ¾ of the total cost of CCS using current CO2 separation technologies.4,

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Exploring a cost-effective CO2 separation

technology has become an urgent and world-wide research topic. Recent research has shown that ionic liquids (ILs)7,

8

can be promising alternatives to the

relatively mature amine scrubbing in post-combustion CO2 capture9 because of their unique properties such as extremely low vapor pressures, tunable physicochemical properties, etc.10-13 However, two crucial problems should be solved before the industrialization of IL-based technology in CO2 separation. One is the low gas-liquid mass transfer rate of CO2 in ILs due to the high viscosities of ILs. For instance, the equilibrium times for room-temperature ILs (RTILs) and task-specific ILs (TSILs) are more than 90 and 180 minutes,11 respectively, which makes it infeasible to use ILs as liquid absorbents for large-scale applications.11, 14 The other is the high cost of ILs. It was reported that even at a large-scale production level, the price of ILs would still be a factor of 10 to 20 times higher than the conventional solvents.12 While due to the low absorption capacity, usually a large amount of ILs is needed.15 Therefore, how to dramatically improve the CO2 absorption rate in ILs and reduce the amount of ILs needed is of great importance in the application of ILs for CO2 separation. Recently, it has been found that IL immobilization into porous solid supports16-20 could significantly enhance the mass transfer rate of CO2 in ILs and reduce the amount of ILs needed


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for CO2 separation.21 For example, the equilibrium time was shortened to less than 15 minutes19 for IL immobilized sorbents. In all the previous work, the enhancement of the mass transfer rate observed experimentally was attributed to the shortened diffusion path and the enlarged gasliquid contact area due to the much thinner film of ILs on support after immobilization.16-20, 22, 23 However, none of these reports has pointed out in which scale of the IL-film thickness should be in order to obtain a significant rate enhancement.16, 19, 22 In addition, it is desirable to study the CO2 absorption/desorption kinetics of IL immobilized sorbents in order to evaluate the performance of sorbents and design the gas-treating units. Various kinetic models have been used to fit the gas absorption/desorption dynamic behaviors in IL or amine immobilized sorbents. For example, the double exponential model was used by Li and co-workers to study the SO2 absorption in tetramethylguanidinium lactate (TMGL) IL supported on mesoporous molecular sieve MCM-41 at different temperatures.24 In Wang’s work, both double and single exponential models were used to investigate the CO2 absorption in amino acid ILs immobilized in a porous poly(methyl methacrylate) (PMMA) microsphere support.20 In Monazam’s work, a phenomenological kinetic model derived from the Weibull distribution function was used to describe the two-step CO2 capture process with immobilized amine on silica.25 Recently, Liu and co-workers used the Pseudo-first-order, Pseudo-second-order, Avrami’s

fractional-order

absorption/desorption

in

and

Fractional-order

kinetic

models

to

tetraethylenepentamine (TEPA) impregnated

study

the

industrial

CO2 grade

multiwalled carbon nanotubes (IG-MWCNTs).26 Among these models, the Avrami’s fractionalorder kinetic model developed on the basis of Avrami’s kinetic model27 has been found to be the most suitable one to describe the CO2 absorption/desorption behavior of amine immobilized


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sorbents.26,

28

However, none of these models has been used to study the effect of IL-film

thickness on the CO2 absorption/desorption in IL immobilized sorbents. The goal of this work is to study the effect of the IL-film thickness on the performance of CO2 absorption/desorption in IL immobilized sorbents. To achieve this goal, two ILs (1-aminopropyl3-methyIimidazolium

bromide

([APMIm]Br)

and

1-Butyl-3-methylimidazolium

acetate

([BMIm]Ac)) were loaded into titanium dioxide (P25) by means of simple impregnation under ambient conditions. CO2 absorption/desorption processes of the prepared sorbents with different IL loadings were detected precisely by thermogravimetric analysis (TGA), and the corresponding IL-film thickness was estimated in order to illustrate their effect on the CO2 absorption/desorption performance quantitatively and then find out the scale of IL-film thickness achieving efficient CO2 absorption/desorption processes for IL immobilized sorbents. The Avrami’s fractional-order kinetic model was used to investigate the effect of IL-film thickness on the kinetics of CO2 absorption/desorption in IL immobilized sorbents.

2. EXPERIMENTAL DETAILS [APMIm]Br and [BMIm]Ac are typical ILs with amino acid and acetate anions, respectively, and they interact chemically with CO2 to achieve high CO2 absorption capacities.29-35 These two ILs were chosen as solvents in this work. Titanium dioxide (P25) was chosen as supported materials because of its high specific surface area and porosity as well as thermal stability in the temperature range of 35 to 950 °C.36 In addition, the ILs chosen in this work are hydrophilic,29, 31 leading to good wettability on the hydrophilic surface of P25.37 2.1. Materials. [APMIm]Br (> 99 wt%) was purchased from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, China. [BMIm]Ac, with the same purity used by others31,


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32, 38, 39

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for the same purpose (purity ≥ 96.0 wt%; water ≤0.5 %), was purchased from Sigma-

Aldrich Chemical. These two materials were used as received. P25 particles were purchased from Degussa. Methanol (analytical reagent) was purchased from Lingfeng Chemical Reagent Co., China. KBr (FT-IR grade) was purchased from Sigma-Aldrich Chemical. Gaseous O2, N2 and CO2 were supplied by Nanjing Sanle Gas (99.99 % purity). The P25 particles were dried and degassed under vacuum at 80 °C for 8 hours before use. Gases were dried by P2O5 before used in TGA. 2.2. Preparation of P25-IL sorbents. [APMIm]Br was dissolved in methanol and then P25 particles were added with different weight ratios to form slurry. The formed slurry was placed in a rotary evaporator (RE-52AA, Rongsheng, China) under vacuum in a water bath at 60 °C to remove methanol and result in a physical adsorption (immobilization) of [APMIm]Br into the pores of P25 particles. The prepared sorbents were stored in a desiccator before use. The impregnation of [BMIm]Ac into P25 was prepared in the same way as that of [APMIm]Br. It was noted that the prepared sorbents were white powdery particles at a low feed ratio, and these sorbents were then used for experimental measurements. However, at higher feed ratios (about > 60 wt%), the prepared sorbents were wet and clay-like particles and could not be used any further. Therefore, the loading of the ILs studied in this work was less than 60 wt%. The exact IL loading in these prepared P25-IL sorbents was further determined using TGA (TG209-F3, ±0.0001 mg, Netzsch). Approximately 20 mg of each P25-IL sorbent was placed in a Al2O3 sample cell filled with flowing O2 (20 vol%) and N2 (80 vol%) at flow rates of 20 mL/min and 80 mL/min, respectively. The samples were then heated from 35 to 950 °C with a heating rate of 10 °C/min to remove IL from P25. The IL loading was determined accurately


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based on the weight before and after heating.22 The prepared sorbents were denoted as P25-IL (x wt%), where x represents the loading of IL ([APMIm]Br or [BMIm]Ac) in weight percentage. 2.3. Characterization and testing. The specific surface area (SBET) of P25-IL was characterized as described in the following text. The prepared P25-IL sorbents were degassed under vacuum at 80 °C for 8 hours. The adsorption/desorption isotherms of nitrogen in the P25IL were then measured at -195 °C with a Micromeritics TristarⅡ3020 analyzer (Micromeritics, USA). Based on the measured adsorption/desorption isotherms, the Brunauer-Emmett-Teller (BET) method was used to estimate the specific surface area of the prepared P25-IL sorbents. The amount of CO2 absorbed/desorbed in the prepared P25-IL sorbents was detected using TGA, which has been widely used with the same purpose.19,

20, 40, 41

In a typical test,

approximately 20 mg of the P25-IL sorbent was placed in the Al2O3 sample cell. The temperature was increased with a rate of 10 °C/min from 35 to 105 °C and kept at 105 °C until no weight loss was observed under dry N2 with a flow rate of 100 mL/min. This process was used to remove the moisture, solvent or other adsorbents away from the samples. The temperature was then cooled down with a rate of 5 °C/min to the desired absorption temperature (e.g. 35 or 50 °C), and the gas flow was switched from N2 to dry CO2 with a flow rate of 100 mL/min and maintained at the desired temperature for CO2 absorption detection. At the end of absorption, the temperature was increased to the desired desorption temperature (e.g. 80 or 50 °C), and the gas flow was switched from CO2 to N2 with a flow rate of 100 mL/min for CO2 desorption detection. The weight of the sorbent was recorded continuously. Based on the recorded weight of the sorbent during the absorption/desorption processes, the amount of absorbed/desorbed CO2 in mg CO2/mg sorbent (support+IL) was then obtained.


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The P25-IL sorbents before and after absorption/desorption experiments were characterized by X-ray diffraction (XRD) (D8 Advance, Bruker, Germany) and Fourier transform infrared (FT-IR) (Nicolet Nexus B70 FT-IR system, Nicolet, USA). Each sample was finely ground and then diluted with KBr before tableting. The FT-IR spectra were recorded at a spectral resolution of 400-4000 cm-1. 2.4. Avrami’s fractional-order kinetic model. Recently, based on Avrami’s kinetic model to simulate phase transition and crystal growth of materials,27 the Avrami’s fractional-order kinetic model has been developed and widely applied to describe the absorption of CO2 in supported liquid sorbents, such as amine-immobilized sorbents,26 and was found to be the most suitable model. In this work, Avrami’s fractional-order kinetic model26 was used to study the kinetics of CO2 absorption in P25-[APMIm]Br and P25-[BMIm]Ac sorbents. The general form of the model is described as follows. ∂ qt = k An A t n A −1 ( q e − qt ) ∂t

(1)

where qe and qt are the absorption capacities at equilibrium and at time t, respectively. kA is the Avrami kinetic constant and nA is the Avrami exponent. With the boundary conditions of qt=0 at t=0 and qt=qe at t=∞, the integrated eq 1 becomes nA

qt = qe (1 − e − ( k At ) )

(2)

3. RESULTS AND DISCUSSION To study the effect of IL-film thickness on the performance of CO2 absorption/desorption, [APMIm]Br and [BMIm]Ac were used to prepare P25-IL sorbents. The IL-film thickness was controlled by the weight percentage of IL during P25-IL preparation, and the exact IL loading was further determined using TGA. The IL-film thicknesses were estimated based on the


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characterized specific surface areas of P25-IL and the exact IL loadings by assuming a perfect distribution of IL on P25, i.e. IL was fully distributed on the pore surface of P25 particles and the specific surface area was considered as the surface area for IL distribution. The higher the IL loading, the thicker the IL-film. It is worth noting that the exact IL-film thickness in this work was not rigorously estimated since the wetting phenomena might leave discrete domains of liquid drops in macropores, and micropore openings might be blocked. These possibility will be discussed in our further work to study the effect to rigorous and exact value of IL-film thickness on the CO2 absorption/desorption in IL supported sorbents. However, the focus of this work is on the effect of the scale of the IL-film thickness on the CO2 absorption/desorption behavior in P25IL sorbents, which makes that the scale of IL-film thickness is more important than its exact thickness. While the estimation of the scale in this work is accurate enough. Meanwhile, we also tested the amount of CO2 adsorbed on P25 by physisorption, which was 0.0009 g/g P25 at 35 °C and 0.0021 g/g P25 at 50 °C. This adsorption amount is very low with relatively high fluctuation. Since the chemical absorption of CO2 by [APMIm]Br and [BMIm]Ac used in this work is much higher, the adsorption of CO2 on P25 was neglected. 3.1. Characterization of P25-IL sorbents. The P25-IL sorbents were characterized before and after absorption/desorption by XRD and FT-IR. The XRD patterns of P25 and P25[APMIm]Br (25.8 wt%) are illustrated in Figure 1. As shown in Figure 1, the diffraction patterns of P25 were not changed after loading [APMIm]Br, which indicates that the structure of P25 was kept without any variation. The intensity of the diffraction patterns of P25 was slightly changed, and this might be caused by the effect of [APMIm]Br loading.


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Figure 1. XRD patterns of P25 and P25-[APMIm]Br (25.8 wt%) before and after CO2 absorption/desorption experiment.

The FT-IR spectra of P25 and P25-[APMIm]Br (25.8 wt%) are shown in Figure 2. A broad band at 3430 cm−1 and sharp bands at 1630 cm−1 were observed for P25, which are assigned to the surface hydroxylic groups and chemisorbed water, respectively. Several additional peaks were observed in the FT-IR spectra of P25-[APMIm]Br compared to P25. The bands at 3440 cm1

and 1568 cm-1 are due to the antisymmetric stretching and bending vibrations of the N-H in the

-NH2 of [APMIm]Br, respectively. The bends at 1161 cm-1 is assigned to the peak of absorption of N-C of [APMIm]Br IL. Coupling the XRD results, we can further confirm that the immobilized IL sorbents were successfully prepared.


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Figure 2. FT-IR spectra of P25, [APMIm]Br and P25-[APMIm]Br (25.8 wt%) before and after CO2 absorption/desorption experiment.

The FT-IR spectra of P25-[APMIm]Br (25.8 wt%) before and after CO2 absorption/desorption experiment show that the sorbent remains unchanged after the CO2 absorption/desorption process. Meanwhile, the structure of P25 was well kept on P25-IL sorbent after CO2 absorption/desorption experiment as shown in Figure 1. This observation implies that the CO2 uptake in the P25-IL sorbents is reversible and P25-IL could be regenerated by exposing them to the flowing N2 at elevated temperatures.

3.2. CO2 absorption/desorption in P25-[APMIm]Br. The measured exact IL loadings in P25-[APMIm]Br were 25.8, 37.2, 40.4, 44.2 and 55.9 wt%, respectively, as listed in Table 1. The


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prepared P25-[APMIm]Br sorbents were further characterized, and their specific surface areas are listed in Table 1. The specific surface area decreases with increasing the amount of immobilized [APMIm]Br (IL loading), which is consistent with the observations for other IL immobilized sorbents.17, 22, 23

Table 1. The IL loading, specific surface area (SBET), IL-film thickness and CO2 equilibrium time for P25-[APMIm]Br Loading

a

wt%

c

IL volume

b

m2/g sorbent

cm3/g sorbent

nm

Absorption

Desorption

25.8

11.00

0.19

17.5

13

31

37.2

10.89

0.28

25.5

18

26

40.4

5.54

0.30

54.5

41

26

44.2

5.20

0.33

63.6

47

27

55.9

0.26

0.42

1.64×103

>60

>60

100

/

/

2.50×106

>>120

>60

SBET

Film thickness

Equilibrium time, min

a

The SBET of P25 is 50.72 m2/g; bDensity of [APMIm]Br is 1.3376 g/cm3; cThe time when the weight-change within 5 minutes was less than 1 % for absorption or 2 % for desorption of the overall weight-change was taken as equilibrium time.25

The IL-film thickness was estimated based on the IL loading detected by TGA and the specific surface area characterized with BET in this work as well as the density of [APMIm]Br. The estimated results of the IL-film thickness are listed in Table 1, which are almost in three regions: 10 nm-scale, 100 nm-scale and micro-scale. The CO2 absorption in P25-[APMIm]Br at 35 °C and desorption at 80 °C were detected in this work by TGA. The experimental results are depicted in Figure 3, in which the case of 100 wt%


12

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represents pure [APMIm]Br (without immobilization). To quantitatively study the IL-film thickness on the CO2 absorption/desorption process, the CO2 equilibrium time was estimated from the detected CO2 absorbed/desorbed in P25-[APMIm]Br based on the following definition, i.e. when the weight-change within 5 minutes was less than 1 % of the overall weight-change for absorption (or 2 % for desorption), the process was considered as an equilibrium process, and the corresponding time was termed as CO2 equilibrium time.25 Based on the results illustrated in Figure 3a and the estimated CO2 equilibrium times listed in Table 1, obviously, without immobilization, the CO2 absorption was very slow due to the extremely low mass transfer rate caused by the high viscosity of [APMIm]Br.29, 34 It took far more than 120 minutes to reach equilibrium (Table 1). After immobilization, the CO2 absorption process can be enhanced, which depends on the IL-film thickness. When the IL loading changes from 100 down to 55.9 wt%, the IL-film is in micro-scale level (1.64×103 nm), the CO2 absorption almost keeps the same slow rate as that in pure [APMIm]Br, and more than 60 minutes were needed to reach equilibrium. Further decrease of the IL-film thickness to 100 nmscale enhances the CO2 absorption process obviously. For example, when the IL loading is 44.2 wt%, the corresponding IL-film thickness is 63.6 nm, the CO2 absorption process is much faster compared to that with 1.64×103 nm IL-film, and it took 47 minutes to reach equilibrium. When the IL-film decreases from 100 nm-scale down to 10 nm-scale, the CO2 absorption rate was further enhanced, resulting in a process shorter than 20 minutes to reach equilibrium. As listed in Table 1, the CO2 equilibrium time for the absorption in P25-[APMIm]Br is 10-time different for micro-scale and nano-scale IL-films, thus, the scale of IL-film thickness is of great significance for CO2 mass transfer rate enhancement.


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In this work, the absorption experiments were carried out for 60 minutes. As listed in Table 1, for most of cases, the process reached equilibrium after 60 minutes (by the end of experiments). While for the cases with the loading of 55.9 wt% or pure IL, the CO2 equilibrium time is much longer than 60 minutes, which is infeasible for practical applications. In this work, for each sorbent sample, the CO2 absorption working capacity at the end of 60 minutes was used to further evaluate the process performance, which is illustrated in Figure 4. As we can see in Figure 4, the CO2 working capacity increases to a maximum and then decreases dramatically with increasing [APMIm]Br loading. This phenomenon was also observed by others for other supported IL sorbents.19,

20, 22, 23

This is because that the amount of the absorbed CO2 is

determined by two factors. One is the mass transfer rate, and the other is the CO2 absorption capacity of the IL loaded on the porous material. When the IL loading is low, the CO2 absorption capacity of IL is low. In other words, the CO2 that can be absorbed in IL is low. While for a higher loading of IL, the large amount of IL makes the liquid film thicker, leading to a very low mass transfer rate. For P25-[APMIm]Br, the maximum working capacity at the end of 60 minutes was obtained with the [APMIm]Br loading of 37.2 wt% and the corresponding IL-film thickness is about 25 nm. Compared to the CO2 absorption in P25-[APMIm]Br, the CO2 desorption in P25-[APMIm]Br was enhanced with decreasing the amount of IL even for the case of micro-scale IL-film (1.64×103 nm). However, as shown in Figure 3b and listed in Table 1, the CO2 equilibrium time for the desorption process with 1.64×103 nm IL-film (>60 min) is still too long for industrial applications. The further decrease of the IL-film thickness down to 100 nm-scale and 10 nmscale could result in an enhancement of mass transfer rate.


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Combing the results of the CO2 absorption with desorption, we can conclude that it is crucial to have an IL-film thinness in 100 nm-scale or thinner in order to provide efficient CO2 absorption/desorption processes. For P25-[APMIm]Br, the corresponding IL loading is less than 60 wt% if the IL-film thickness is in 100 nm-scale.


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Figure 3. The CO2 absorption/desorption in P25-[APMIm]Br with different film thickness of [APMIm]Br. (a) absorption at 35 °C; (b) desorption at 80 °C. : 17.5 nm; : 25.5 nm; : 54.5 nm; : 63.6 nm; : 1.64 ×103 nm; : 2.50 ×106 nm.


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Figure 4. The CO2 absorption working capacity in P25-[APMIm]Br at 35 °C and in P25[BMIm]Ac at 50 °C at the end of 60 minutes. : P25-[APMIm]Br; : P25-[BMIm]Ac.

3.3. CO2 absorption/desorption in P25-[BMIm]Ac. The measured exact IL loadings in P25[BMIm]Ac were 10.0, 19.0, 28.5, 38.2 and 58.1 wt%, respectively, as listed in Table 2. The specific surface areas of the prepared P25-[BMIm]Ac sorbents were characterized in this work, and the results are listed in Table 2. Similar to P25-[APMIm]Br studied in this work and the IL immobilized sorbents studied by others,17, 22, 23 the specific surface area decreases with increasing the amount of immobilized [BMIm]Ac. The film thickness of P25-[BMIm]Ac was then estimated as listed in Table 2. Again, the ILfilm thickness is in three regions: 10 nm-scale, 100 nm-scale and micro-scale, respectively.


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Table 2. The IL loading, specific surface area (SBET), IL-film thickness and CO2 equilibrium time for P25-[BMIm]Ac Loading

a

wt%

m2/g sorbent cm3/g sorbent

nm

Absorption

Desorption

10.0

32.52

0.10

2.9

9

21

19.0

20.31

0.18

8.9

12

32

28.5

7.39

0.27

36.7

10

31

38.2

3.72

0.36

97.5

13

20

58.1

0.20

0.55

2.81×103

44

> 60

SBET

IL volume

b

Film thickness

c

Equilibrium time, min

a

The SBET of P25 is 50.72 m2/g; bDensity of [BMIm]Ac is 1.053 g/cm3; cThe time when the weight-change within 5 minutes was less than 1 % for absorption or 2 % for desorption of the overall weight-change was taken as equilibrium time.25

The CO2 absorption/desorption process in P25-[BMIm]Ac was studied experimentally at 50 °C in this work. The experimental results are shown in Figure 5. Similar to the results of P25[APMIm]Br, when the IL-film thickness is in micro-scale, both absorption and desorption take long time to achieve equilibrium (more than 40 minutes). However, when the IL-film thickness decreases down to 100 nm-scale or thinner, the absorption/desorption processes are enhanced with much shorter equilibrium time (about 10 minutes for example). Correspondingly, the CO2 absorption working capacity at the end of 60 minutes was increased to a maximum and then decreased as shown in Figure 4, which is similar to [APMIm]Br studied in this work. The maximum working capacity at the end of 60 minutes for P25-[BMIm]Ac is the case with [BMIm]Ac loading of 38.2 wt% and the corresponding IL-film thickness is about 100 nm.


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Figure 5. The CO2 absorption/desorption in P25-[BMIm]Ac with different film thickness of [BMIm]Ac at 50 °C. (a) absorption; (b) desorption. : 2.9 nm; : 8.9 nm; : 36.7 nm; : 97.5 nm; : 2.81 ×103 nm.

3.4. Absorption/desorption kinetics. The experimental data measured in this work was fitted with the Avrami’s fractional-order kinetic model. The fitted kA are listed in Tables 3 and 4 together with the squared correlation coefficient (R2). The values of R2 show that the Avrami’s fractional order kinetic model can be used to reliably describe the CO2 absorption and adsorption behaviors in P25-[APMIm]Br and P25-[BMIm]Ac sorbents. With increasing IL loading in the IL immobilized sorbents, the IL-film thickness increases, and the rate constants for both absorption and desorption decrease. This confirms that the IL-film thickness significantly affects the kinetics of CO2 absorption/desorption in P25-[APMIm]Br and P25-[BMIm]Ac sorbents. For example, compared to the rate constant of CO2 absorption in P25-[APMIm]Br with a 10 nmscale IL-film thickness, it was around 10 times lower with 100 nm-scale IL-film thickness and around 100 times lower with micro-scale IL-film thickness.


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Table 3. Parameters of Avrami’s fractional-order kinetic model for CO2 absorption/desorption in P25-[APMIm]Br sorbents Loading

Film thickness

Absorption

Desorption

wt %

nm

kA, min-1

R2

kA, min-1

R2

25.8

17.5

1.90

0.917

3.72×10-1

0.990

37.2

25.5

2.42

0.971

2.81×10-1

0.996

40.4

54.5

3.57×10-1

0.996

4.02×10-1

0.995

44.2

63.6

2.44×10-1

0.999

3.18×10-1

0.995

55.9

1.64×103

2.46×10-1

0.749

1.54×10-2

1.000

100

2.50×106

2.95×10-2

0.991

3.64×10-4

0.999

Table 4. Parameters of Avrami’s fractional-order kinetic model for CO2 absorption/desorption in P25-[BMIm]Ac sorbents Loading

Film thickness

Absorption

Desorption

wt %

nm

kA, min-1

R2

kA, min-1

R2

10.0

2.9

1.30

0.941

2.69×10-1

0.997

19.0

8.9

9.76×10-1

0.982

2.59×10-1

0.991

28.5

36.7

9.98×10-1

0.931

3.46×10-1

0.983

38.2

97.5

8.32×10-1

0.979

4.19×10-1

0.986

58.1

2.81×103

1.11×10-1

0.998

5.67×10-2

0.998

Combined the results of P25-[APMIm]Br and P25-[BMIm]Ac sorbents, we can conclude that the IL-film thickness is a key factor to affect the efficiency of CO2 absorption/desorption processes qualified with the CO2 equilibrium time and rate constant. Based on the investigation


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in this work, the scale of IL-film thickness is of great significance for enhancing CO2 mass transfer rate, and the nano-scale IL-film is a prerequisite to make the process feasible in practical applications. In fact, the inherent mechanism of the effect IL-film thickness on the CO2 absorption/desorption behavior in IL immobilized sorbents is under investigation, and how to achieve an optimal process is also discussed.42 It is found that the distinct difference of mass transfer rate caused by different IL-film thickness is attributed to the alteration of the diffusioncontrol and reaction-control processes depending on IL-film thickness, i.e. the process with thick IL-films was diffusion-controlled and that with thin IL-films was reaction-controlled.42 This information is very important to ILs supported sorbent research and design of supported IL membranes. It should be pointed out that in the previous work by others, the effect of the IL loading on the gas absorption/desorption performance of supported IL sorbents was studied.16, 19, 22 However, no quantitative results of the IL-film thickness has been obtained and none has pointed out in which scale the IL-film thickness should be in order to enhance the efficiency of CO2 absorption/desorption process. To the best of our knowledge, this is the first time that the effect of the IL-film thickness on the absorption/desorption performance was studied quantitatively, and for the first time, it was found that the IL-film thickness should be in 100 nm-scale or thinner in order to achieve efficient processes.

4. CONCLUSION A systematic study was carried out in this work to investigate the effect of the IL-film thickness on the CO2 absorption/desorption performance of IL immobilized sorbents. Two ILs


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([APMIm]Br and [BMIm]Ac) were impregnated into P25 by means of simple impregnation under ambient conditions. TGA was used to detect the absorbed/desorbed CO2 in P25-IL with a specific thickness of IL-film estimated from the specific surface areas and the exact IL-loading. The Avrami’s fractional-order kinetic model was used to investigate the effect of IL-film thickness on the performance of CO2 absorption/desorption in the IL immobilized sorbents. The research results revealed that nano-scale IL-film is a prerequisite to ensure efficient CO2 absorption/desorption processes for IL immobilized sorbents qualified with the CO2 equilibrium time and rate constant; the scale of IL-film thickness is of great significance for enhancing CO2 mass transfer rate, the CO2 equilibrium time can be 10-time different and the rate constant can be 100-time different for the micro-scale and nano-scale IL-films. While the mass transfer rate increased with IL-film thickness decrease, the highest CO2 working capacity of the two IL studied showed with thickness around 25 and 100 nm, respectively. It is the first time to quantitatively reveal the IL-film thickness influence and find out its scale for a significant rate enhancement in the CO2 absorption/desorption by IL immobilized sorbents. This work will be beneficial to understand the gas transport prosperity in ILs and guide the application of these viscous and expensive solvents.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was financially supported by National 973 Key Basic Research Development Planning Program (2013CB733500), Major Research Projects of National Natural Science Fund (91334202), Major Research Program of National Natural Science Fund (21490584), Key Project of National Natural Science Foundation (21136004), National Natural Science Foundation (21176112) of China and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Mass Transfer Rate Enhancement for CO2 Separation by Ionic Liquids

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