Confinement Phenomenon Effect on the CO2 Absorption Working

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Confinement phenomenon on CO absorption working capacity in ionic liquids immobilized into porous solid supports Nanhua Wu, Xiaoyan Ji, Wenlong Xie, Chang Liu, Xin Feng, and Xiaohua Lu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02204 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on August 28, 2017

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Confinement phenomenon on CO2 absorption working capacity in ionic liquids immobilized into porous solid supports Nanhua Wu†,‡,§, Xiaoyan Ji‡, Wenlong Xie∥, Chang Liu†,§, Xin Feng†,§*, Xiaohua Lu†,§* †

State Key Laboratory of Materials-Oriented and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China ‡

Energy Engineering, Division of Energy Science, Luleå University of Technology, 97187 Luleå, Sweden

§

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 210009, China

∥China

Petroleum Chemicals Kunshan Company, No. 210, Kuntai Road, Kunshan 215337, China

*Corresponding authors, Xin Feng and Xiaohua Lu. Tel/Fax: +86-25-83588063. Email addresses: [email protected], [email protected]

Abstract In this work, the CO2 absorption working capacity and solubility in the ionic liquids immobilized into porous solid materials (substrates) was studied both experimentally and theoretically. The CO2 absorption working capacity in the immobilized ionic liquids was measured experimentally. It was found that the CO2 absorption working capacity and solubility increased up to 10 times compared to that in the bulk ionic liquids when the film thickness is nearly 2.5 nm in the [HMIm][NTf2] immobilized into the P25. Meanwhile, a new model was proposed to describe the Gibbs free energy of CO2 in the immobilized ionic liquids, and both macro- and micro-analyses of the CO2 solubility in the confined ionic liquids were conducted.

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The theoretical investigations reveal that the substrate has a crucial effect on the gas solubility in the ionic liquid immobilized into the substrates, and the model performance was approved with the consideration of substrate effect. Keywords: CO2 absorption working capacity; CO2 solubility; confined phenomenon; ionic liquid immobilization

1. Introduction CO2 separation plays an important role in capturing CO2 from the combustion of fossil fuels or producing transportation fuels e.g. via biomass gasification.1 However, the cost for separating CO2 is very high using available technologies.2,3 Ionic liquids (ILs) have been proposed as promising solvents for CO2 separation because of their unique high thermal stability, low volatility, high chemical stability and non-flammability, etc.4-8 More importantly, the number of ILs that can be synthesized was estimated to be nearly 1018, providing enormous scope for scientific innovation.9 A lot of research work has been carried out to develop IL for CO2 separation.10 Brennecke et al. investigated the CO2 solubility in ILs at room temperature (RTILs),4,11,12 however, the solubility at atmospheric pressure is at maximum 3 mol%, but it increases with increasing pressure.13 In order to improve the CO2 absorption capacity of ILs at low pressures, task-specific ILs have been developed, which can be classified into novel cation designing, novel anion designing, and cation and/or anion functionalizing.5,14-20 For example, Davis et al. synthesized the functional ILs, and the CO2 solubility was increased to 0.5 mol/mol IL.5 Jiang et al. synthesized tetraalkylammonium-based amino-acid ILs successfully, which can improve both reaction and mass transfer rates of CO2 in the ILs.14 However, several drawbacks still need to be solved before


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practical applications of IL-based technologies, e.g. the high viscosity leading to long equilibrium time as long as 180 min,7 the high price of ILs, and so on.21 Recently, it has been found that IL immobilization into porous solid supports could significantly enhance the mass transfer rate of CO2 in ILs22-24 and reduce the amount of ILs needed for CO2 separation.25 For example, the equilibrium time was shortened to less than 15 minutes for IL immobilized sorbents.26 Meanwhile, supported ionic liquid membranes (SILMs) that combine the advantages of both membranes and ILs for the selective separation of gases have received growing attention during recent years.27-30 The unique properties of ILs including low vapor pressures, high thermal stability and strong capillary force existing between ILs and support pores make SILMs be more competitive than conventional supported liquid membranes (SLMs). In both of these two options, the scale can be down to nano-meters, and the amount of ILs needed is small that can avoid the high cost from the synthesis of the huge amount of ILs. While the previous research work is still mainly focused on experimental measurements, and the effect of the immobilization or membranes down to small scale on the properties of ILs including gas solubility has not been well concerned. However, the research from other experiments and molecular simulations has revealed that the thermodynamic properties of the fluids (e.g. the gas solubility at equilibrium) depend on both size (or film-thickness) of the fluids and substrate contacted when the scale goes down to nano-meters (i.e. nano-effect).31-32 For example, the H2 solubility in the ethanol confined in the 4-nm pores was increased 4-5 times over the bulk solubility.31 Zhang et al. studied the CO2 solubility in the [BMIm][BF4] confined in mesoporous silica gels, and they observed that the CO2 solubility increased 1.5 times over the bulk solubility.33 The CO2 permeability in the SILMs was 4 times compared to its bulk as reported by Close et al.34 Baltus et al. found that when the membrane thickness was 250 µm, the CO2 solubility in [C2C1IM][NTf2] or [C6C1IM][NTf2] enhanced 1.6-2 3 ACS Paragon Plus Environment

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times.35 Based on the molecular dynamic simulation, Hung et al. investigated the structural and dynamical properties of the confined IL and found that the properties are different from its bulk as a function of pore size and pore loading.36 Shi et al. studied the properties of the IL confined in carbon nanotubes. It was found that the molar volume was 12-31% larger than that that in the bulk phase, and the solubility of CO2, H2 or N2 was 1.1-3 times higher.37 However, the research work is still limited, for example, only one work has been conducted on the CO2 solubility in the confined ILs. Furthermore, no theoretical model has been developed to represent this abnormal phenomenon. The goal of this work was to study the confinement phenomenon on the CO2 solubility in the ILs immobilized into porous solid supports experimentally and theoretically. To achieve this, three ILs (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIm][NTf2]), 1-butyl-3-methylimidazolium acetate ([BMIm][Ac]), 1-aminopropyl-3-methylimidazolium bromide ([APMIm]Br)) were loaded into two porous materials (substrates) by means of simple impregnation under ambient conditions. The CO2 absorption capacities of the prepared sorbents with different IL loadings were detected precisely by thermogravimetric analysis (TGA), and the CO2 solubilities in these ILs confined in the porous sorbents were obtained. A new model was developed to describe the Gibbs free energy of CO2 in the immobilized ILs with the consideration of both surface and substrate effects, and the model was further used to analyze the contribution of each part to the CO2 solubility in the confined ILs.

2. Experiment 2.1 Materials The ILs of [HMIm][NTf2] and [APMIm]Br (>99 wt %) were purchased from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, China. [BMIm]Ac, with the same purity


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used by others (purity ≥96.0 wt %; water≤0.5%), was purchased from Sigma-Aldrich Chemical. Methanol (analytical reagent) was purchased from Lingfeng Chemical Reagent Co., China. Gaseous N2, He and CO2 were supplied by Nanjing Sanle Gas (99.99 % purity). Gases were dried by P2O5 before used in the TGA. P25 was purchased from Degussa, and PMMA was from Sigma-Aldrich. The supports were dried and degassed under vacuum at 80 °C for 8 hours before use. 2.2 Preparation of immobilized IL sorbents Immobilizing ILs into the porous supports can be divided into chemical or physical method, 38,39

i.e. by chemical attachment of the ionic species to a solid support, or by physical fixation of

IL within a support.40-43 In this work, the physical immobilization method was conducted with impregnation according to the previous research work.26,43 In immobilization, the IL was dissolved in methanol and then the solid particles (substrates) 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 resulted in a physical adsorption (immobilization) of IL into the pores of substrates. The prepared sorbents were stored in a desiccator before use. The exact IL loading in the P25-IL sorbents was further determined using TGA (TG209-F3, ±0.0001 mg, Netzsch). Specifically, approximately 20 mg of each IL immobilized sorbent was placed in an Al2O3 sample cell filled with flowing N2 at a flow rate of 40 mL/min. The samples were then heated from 35 to 950 °C with a heating rate of 10 °C/min to remove IL. The IL loading was determined accurately based on the weight before and after heating. However, the [APMIm]Br loading in the PMMA-[APMIm]Br sorbents cannot be determined using TGA because PMMA will decompose at high temperatures. Therefore, the feed ratios in the


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preparation process were used as the IL loadings. Based on the results in our previous work, the immobilization ratio can be well controlled by the feed ratio in the impregnation process and the error was acceptable.43 The prepared sorbents and the loading weights for the corresponding immobilized ILs are summarized in Table 1. < Table 1. here > 2.3. Characterization and testing The physical properties of substrates were characterized as described in the following text. The substrates were degassed under vacuum at 80 °C for 8 hours. The adsorption isotherms of nitrogen (N2) were then measured at -195 °C with a Micromeritics TristarⅡ3020 analyzer (Micromeritics, USA). Based on the measured adsorption isotherms, the Brunauer-Emmett-Teller (BET) method was used to estimate the specific surface area (SBET). The amount of CO2 absorbed in the prepared immobilized IL sorbents was detected using TGA .44-49 In a typical test, approximately 20 mg of the immobilized 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 helium gas with a flow rate of 100 mL/min. This process was used to remove the solvent, moisture or other adsorbents away from the immobilized IL sorbents. The temperature was then cooled down with a rate of 5 °C/min to the desired absorption temperature, and the gas flow was switched from helium gas to the mixture of dry CO2 and helium gases with a flow rate of 100 mL/min and maintained at the desired temperature for CO2 absorption detection. The weight of the sorbent was recorded continuously. Based on the recorded weight during the absorption processes, the amount of absorbed CO2 in g-CO2/g-sorbent (support + ILs) was then obtained. It is worth noting that the


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CO2 absorption time was set as 90 minutes, which is generally enough for the immobilized IL sorbents to approach absorption equilibrium.50 The estimated uncertainty in the determined absorption capacity is 5% as a result of the measured errors on determining the absorbed CO2 mass (± 0.0001 mg, < 1 %), the gas flow rate (< 5 %), and the temperature (< ± 3 °C).

3. Theory The process of CO2 absorption in an IL was considered as the process of CO2 transferring from the gaseous phase to the liquid IL. At equilibrium, a constant K can be used to describe the overall CO2 absorption (dissolution) process in the IL, and the K links the Gibbs free energies. In this work, the IL was immobilized into the substrate instead of in the bulk phase, i.e. the IL has confinement phenomenon. The theoretical description on the CO2 solubility in the confined IL was illustrated in this section. For CO2 dissolution in an IL, the overall process can be described by the following equation: CO2 (g) → CO2 (in IL) At equilibrium, the constant K can be expressed with the Gibbs free energies as follows: K = exp(

GCO 2 ,in IL − GCO 2 ,g RT

(1)

)

where G, R, and T are the Gibbs free energy, ideal or universal gas constant, and the absolute temperature, respectively. We can rewrite equation (1) as: ln K =

GCO 2 ,in IL − GCO 2 ,g RT

(2)

In this work, it was assumed that the substrate only has an effect on the properties of the IL, and no effect on the CO2 in the vapor phase. Following this assumption, we further considered 7 ACS Paragon Plus Environment

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that the Gibbs free energy of CO2 in confined IL (GCO2, in IL) can be divided into three parts: bulk (GCO2,in IL,bulk) term, surface (GCO2,in IL,surface) term and interface (GCO2,in IL,substrate) term, i.e.

GCO2 ,in IL = GCO2 ,in IL,bulk + GCO2 ,in IL,surface + GCO2 ,in IL,substrate

(3)

As a result, the constant with respect to the IL in the bulk (Kbulk) and that with respect to the confined IL (K(δ)*) can be rewritten as equations (4) and (5) respectively.

ln K bulk =

GCO2 ,in IL,bulk − GCO2 ,g

ln K (δ )* =

RT GCO 2 ,in IL − GCO 2 ,g RT

(4)

=

GCO 2 ,in IL,bulk + GCO 2 ,in IL,surface + GCO 2 ,in IL,substrate − GCO 2 ,g RT

(5)

where δ is the IL-film thickness. Further assuming that the surface and substrate effects on the CO2 solubility in the IL were accounted by the surface of pure confined IL as well as the substrate effect on the pure confined IL, equation (5) can be further simplified as:

ln K (δ )* =

GCO 2 ,in IL,bulk + GIL-surface + GIL-substrate − GCO 2 ,g RT

(6)

The deviation between the confined and bulk constants can be expressed as:

ln K (δ ) * − ln K =

GIL-surface + GIL-substrate RT

(7)

The term of GIL-surface has the relationship with surface tension and contacted area:

GIL-surface = γ * A =

γ *Vm δ

(8)

where γ is the surface tension, A is the surface area, and Vm is the molar volume of the IL-film. The liquid film has two sides: the liquid/gas and the solid/liquid sides.51 It was assumed that the surface tension of the IL will not be affected by the substrate, i.e. the surface tension for 8 ACS Paragon Plus Environment

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solid/liquid side was the same as that for the liquid/gas side, as a result, the GIL-surface can be simplified as:

GIL-surface =

γ lg *Vm γ sl *Vm γ lg *Vm + = 2δ 2δ δ

(9)

For the IL immobilized into porous materials studied in this work, it was assumed that IL was a perfect distribution on the substrate, that is to say that the IL was fully distributed on the pore surface of the substrate. Subsequently, the IL-film thickness δ can be estimated from the characterized specific surface area of substrate-IL and the IL loading weight,

δ =

m S BET * ρ

(10)

In equation (10), m is the loading weight of the IL as listed in Table 1, SBET is the specific surface area measured experimentally, and ρ is the density of the IL. The calculated δ is listed in Table 2. Combining equation (10) with equation (9), we can get: GIL-surface = γ *

Vm m S BET * ρ

(11)

Substitute equation (11) into equation (7), we can get: ln

K (δ ) * 1 1 Vm * γ lg = (GIL-surface + GIL-substrate ) = ( + GIL-substrate ) K bulk RT RT δ

(12)

The term GIL-substrate was estimated from the experimental data measured in this work as described in the following section.

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A systematic investigation of CO2 absorption working capacity in the three ILs with two immobilized sorbents was conducted. The effects of the IL-film thickness, the type of ILs as well as the type of substrates on the CO2 absorption working capacity were studied. The CO2 solubility was then obtained from the CO2 absorption working capacity. The model considering both surface and substrate effects on the CO2 solubility in the immobilized ILs developed in this work was used to evaluate the confinement effect and the model performance was compared. The molecular parameter was further obtained based on the developed model.

4.1 Experimental study on CO2 absorption working capacity and solubility Based on the experimental measurements conducted at 25 °C for [HMIm][NTf2], 30 °C for [BMIm]Ac, 35°C for [APMIm]Br and atmospheric pressure, the CO2 absorption working capacity at 90 min was estimated according to the method described in our previous work.52 The results of CO2 absorption working capacity (m0) in P25-[HMIm][NTf2], P25-[BMIm]Ac, P25[APMIm]Br and PMMA-[APMIm]Br with different IL-film thicknesses were listed in Table 2. The corresponding SBET for estimating IL-film thickness was also listed in Table 2. The CO2 absorption working capacity in the bulk IL at the same temperature and pressure was detected as listed in Table 2 together with the CO2 solubility in the bulk IL taken from literatures for comparison.

< Table 2. here > To further illustrate the effect of IL-film thickness on the CO2 absorption working capacity, the experimental results are shown in Figure 1. It can be seen that the CO2 absorption working capacity increases with decreasing IL-film thickness. For the same substrate, P25, with different ILs (P25-[HMIm][NTf2], P25-[BMIm]Ac and P25[APMIm]Br): for [APMIm]Br, the CO2 absorption working capacity is lower than the CO2 10 ACS Paragon Plus Environment

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solubility in the bulk IL whatever the IL-film thickness decreases. As mentioned in the foregoing text, the CO2 absorption working capacity refers to the amount of CO2 absorbed at 90 minutes. The extremely high viscosity of [APMIm]Br leads to very slow absorption rate of CO2 in [APMIm]Br. Meanwhile, as reported, the viscosity will be increased sharply when CO2 is dissolved in [APMIm]Br.56 All these cause an extremely slow diffusion of CO2 in this IL, and the measured CO2 absorption capacity up to 90 minutes has not been reached the absorption equilibrium. While for other two ILs, their viscosities as listed in Table 3 are much lower than that for [APMIm]Br, the absorption equilibrium can be reached for all the cases, and the measured CO2 absorption working capacity is equal to the CO2 solubility in the immobilized ILs. The comparison with the CO2 solubility in the bulk ILs shows that the CO2 absorption working capacities in pure IL at equilibrium or in the immobilized ILs measured in this work are reliable, and the immobilization of IL increases its CO2 solubility. For example, when the [HMIm][NTf2]film thickness is 86.5 nm, the CO2 solubility is two times’ higher than that in the bulk. Further decreasing the IL-film thickness leads to nearly 10 times’ increase in the CO2 solubility, compared to the bulk solubility. This confinement effect is much higher than those obtained from the molecular simulation and membrane experiments.35,37 For [BMIm]Ac, the strongest confinement effect is 2 times when the IL-film thickness is nearly 63.5 nm. When the IL-film thickness is 97.4 nm, the CO2 solubility is 1.4 times compared to the bulk IL. Therefore, the confinement effect depends on both the thickness of IL-film and the type of ILs. For the same IL with different substrates (P25-[APMIm]Br and PMMA-[APMIm]Br): the CO2 absorption working capacity in P25-[APMIm]Br is lower than the CO2 solubility in the bulk, 11 ACS Paragon Plus Environment

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while that in PMMA-[APMIm]Br is higher. For PMMA-[APMIm]Br, when the IL-film thickness is nearly 25.6 nm, the CO2 solubility is up to 0.606 mol/mol-IL. The further decrease of the ILfilm thickness to 1 nm leads to an increase of the CO2 solubility to 0.896 mol/mol-IL. While the CO2 solubility in the bulk [APMIm]Br is only 0.5 mol/mol-IL as reported in literature.55 It should be addressed here that the CO2 absorption working capacity in pure IL ([APMIm]Br) with TGA experiment conducted in this work is lower than the CO2 solubility, which reveals again that the CO2 absorption has not reached the equilibrium up to 90 minutes due to the high viscosity. Therefore, the substrate will affect the CO2 absorption working capacity as well as the CO2 solubility (confinement effect). Above all, the IL-film thickness (size), the type of ILs as well as the substrate will affect the CO2 absorption working capacity and CO2 solubility in the immobilized ILs, and the CO2 absorption working capacity (and CO2 solubility) increases with decreasing IL-film thickness. The effect of PMMA is stronger than P25. Among the studied cases, P25-[HMIm][NTf2] shows the best performance with up to 10 times’ increase in the CO2 solubility compared to that in the IL-bulk.

< Figure 1. here >

4.2 Model development 4.2.1 Macroscopic generalization of GIL-substrate In order to study the mechanism of the confinement effect for the CO2 solubility in the immobilized ILs, a theoretical model was developed to describe this phenomenon. As shown in equation (12), the surface tension and mole volume of ILs are needed in modelling the 12 ACS Paragon Plus Environment

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confinement effect, and the experimental results of the CO2 solubility in P25-[HMIm][NTf2] and P25-[BMIm]Ac were chosen for analysis because of the obvious confinement effect as well as the availability of the properties of these ILs. For [APMIm]Br, its surface tension is unavailable, resulting in that the results in [APMIm]Br-PMMA cannot be analyzed. The surface tension and density of the ILs were taken directly from literatures57,58 as listed in Table 3.

< Table 3. here > The further analysis of GIL-substrate = RT ln

K (δ ) * Vm * γ lg − shows that there is a linear curve K bulk δ

between ((GIL-substrate)/(γlg*Vm))2 and 1/δ for the two ILs as shown in Figure 2. In the analysis, the value of K was calculated by (P/m0), where P is the pressure, m0 is CO2 absorption working capacity, and they were measured experimentally as listed in Table 2. This observation from the further analysis reveals that it is possible to generalize the substrate effect on GIL-substrate, and the term GIL-substrate is related to γlg and Vm. This is reasonable as the substrate effect will gradually diminish with increasing particle size (IL-film thickness) to the large scale, which is consistent with the experimental observations, and the term can be considered as a perturbation term compared to the Gibbs free energy in the bulk phase. Based on the results illustrated in Figure 2, the following generalized relation was obtained: GIL-substrate = − 3 *10 9

1

δ

* γ lg *Vm

(13)

The unit for the generalized constant is m-0.5.

< Figure 2. here > 4.2.2 Model development in microscope Based on the previous work on the confined metal melting point,61 the generalized interfacial Gibbs free energy has the relationship with particle size and molecular parameters. To obtain the


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interfacial Gibbs free energy for the immobilized IL-film, it is needed to ‘convert’ the particle size to IL-film thickness. It was assumed that the interfacial Gibbs free energies are the same for different shape of particles. Following this, we can obtain: 1 3 * 109 = r2 2

1

(14)

δ

If r is replaced by δ, the interfacial Gibbs free energy in microscopic can be re-written as: G IL-substrate = −0.0025

ρ IL ∗ ε IL * σ IL * M W ρ substrate * ∆ substrate * σ IL-sub * ε IL-sub

3 *10 9 2

1

δ

* γ lg *Vm

(15)

4.2.3 Model performance In order to investigate the substrate effect in these two immobilized IL-films, the proportions of GIL-substrate to (GIL-surface+ GIL-substrate) and GIL-surface to (GIL-surface+ GIL-substrate) at different IL-film thicknesses is shown in Figure 3. We can see that, the proportion of GIL-substrate decreases with increasing IL-film thickness, but it is always more than 75 % whatever the IL-film thickness is up to 2220 nm. This implies that the substrate effect is dominated, and the consideration of substrate effect is essential. Compared to the substrate effect on the metal melting point depression,61 for some cases, for example Au-C, there is no substrate effect. While for other cases, when the particle size is smaller than 10 nm, it is crucial to consider the effect of substrate. If the particle size is large, i.e. r > 10 nm, the contribution of substrate effect can be ignored. It is completely different from the case of the CO2 solubility in the immobilized IL-film.

< Figure 3. here > In order to illustrate the performance of the model developed in this work, Figure 4 compares the deviation of the CO2 solubility with the consideration of only surface effect as well as with both surface and substrate effects. It is obvious that the deviation of the case with only surface 14 ACS Paragon Plus Environment

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effect is much larger than that with both surface and substrate effects. The deviation increases with decreasing IL-film thickness. For [HMIm][NTf2], when the IL-film thickness is about 2.5 nm, the largest absolute deviation of CO2 solubility decreases from nearly 0.3 to 0.1 mol/mol IL. For [BMIm][Ac], compared to the experimental data, the deviation of modelling was up to about 40% with the only consideration of surface effect, while the deviation decreases to 18% when the substrate effect is also considered. Therefore, the substrate effect is crucial in representing the confinement effect.

< Figure 4. here > 4.3 Practical implication In order to predict the confinement effect based on equation (11), the molecular parameters with Steele potential are needed. However, these parameters are not so easy to obtain for ILs. In this case, the model developed in this work combined with other results can be used to obtain parameters. Based on the molecular simulation conducted in our group, the σIL for [HMIm][NTf2] was estimated nearly as 1.75 nm.62 σIL-sub can be calculated with Lorentz-Berthelot mixing rule:63

ε IL-substrate = ε IL-IL ∗ ε substrate-substrate

(16)

σ IL-IL + σ substrate -substrate

(17)

σ IL-substrate =

2

As a result, only εIL cannot be obtained. Since the microscopic expression is equal to the macroscopic expression, i.e.: − 0.0025

ρ IL ∗ ε IL * σ IL * M W ρ substrate * ∆ substrate * σ IL-sub * ε IL-sub

3 *10 9 2

1

δ

* γ lg *Vm = − 3 *10 9

1

δ

* γ lg *Vm

(18)

As the microscopic value is equal to 2, we have:

ε IL = (800 *

ρ substrate * ∆ substrate * σ IL-sub 2 ) * ε sub ρ IL * σ IL * M W

(19)


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In this equation, ρsubstrate and △substrate were taken from the references,61 and ρIL was calculated by ρIL= (density/MW)*NA. The parameters are summarized in Table 4.

< Table 4. here > After the parameters were obtained, based on equation (15), the calculated CO2 solubility is the same as those shown in Figure 4.

5. Conclusion In this work, the CO2 absorption working capacity in immobilized ILs was investigated both experimentally and theoretically. The IL-film thickness, the type of ILs and the substrate have effect on the CO2 absorption working capacity. The substrate effect on the CO2 absorption working capacity in the confined ILs increases with decreasing IL-film thickness, and the largest confinement effect is up to 10 times for [HMIm][NTf2] on P25 compared to the bulk solubility. Moreover, the substrate effect was dominated. [HMIm][NTf2] and [BMIm]Ac with the same substrate were used to study the surface and substrate effects theoretically from macroscopic and microscopic based on the results on the confined metal melting point by developing a new model. The model performance considering both surface and substrate effects show a decrease deviation from the experimental data compared to that with the consideration of surface effect only. Based on the developed model, the parameter εIL was obtained.

AUTHOR INFORMATION Corresponding Authors *E-mails: [email protected] , [email protected] , Tel: +86-25-83588063

Notes The authors declare no competing financial interest.


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Acknowledgement Financial supports from National 973 Key Basic Research Development Planning Program (2013CB733500), National Natural Science Foundation (91334202, 21490584, 51005123, 21176112, 21428601, 21136004), the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Specialized Research Fund for the Doctoral Program of Higher Education (20133221110001), Innovation Project for College Graduates of Jiangsu Province (CXLX13_424) of China, Kempe Foundation in Sweden and SICAM Scholarship by Jiangsu National Synergetic Innovation Center for Advanced Materials are acknowledged.

References (1) Stocker, T. F.; Qin, D.; Plattner, G. K.; Tignor, M.; Allen, S. K.; Boschung, J.; Nauels, A.; Xia, Y.; Bex, V.; Midgley, P. M. Climate Change 2013: The Physical Science Basis. Intergovernmental Panel on Climate Change, Working Group I Contribution to the IPCC Fifth Assessment Report (AR5)(Cambridge Univ Press, New York) 2013. (2) Haszeldine, R. S. Carbon Capture and Storage: How Green Can Black Be? Science 2009, 325, 1647-1652. (3) House, K. Z.; Harvey, C. F.; Aziz, M. J.; Schrag, D. P. The Energy Penalty of PostCombustion CO2 Capture & Storage and Its Implications for Retrofitting the Us Installed Base. Energy Environ. Sci. 2009, 2, 193-205. (4) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Green Processing Using Ionic Liquids and CO2. Nature 1999, 399, 28-29.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

(5) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H. CO2 Capture by a Task-Specific Ionic Liquid. J. Am. Chem. Soc. 2002, 124, 926-927. (6) D’Alessandro, D. M.; Smit, B.; Long, J. R. Carbon Dioxide Capture: Prospects for New Materials. Angew. Chem., Int. Ed. 2010, 49, 6058-6082. (7) Hasib-ur-Rahman, M.; Siaj, M.; Larachi, F. Ionic Liquids for CO2 Capture-Development and Progress. Chemical Engineering and Processing: Process Intensification 2010, 49, 313-322. (8) Zhang, X.; Zhang, X.; Dong, H.; Zhao, Z.; Zhang, S.; Huang, Y. Carbon Capture with Ionic Liquids: Overview and Progress. Energy Environ. Sci. 2012, 5, 6668-6681. (9) Dong, K.; Liu, X.; Dong, H.; Zhang, X.; Zhang, S. Multiscale Studies on Ionic Liquids. Chem. Rev. 2017,117,6636-6695. (10) Sarmad, S.;Mikkola, J. P.; Ji, X. CO2 Capture with Ionic Liquids (ILs) and Deep Eutectic Solvents (DESs): A New Generation of Sorbents. ChemSusChem 2016,10,324-352. (11) Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. Why is CO2 so Soluble in Imidazolium-Based Ionic Liquids? J. Am. Chem. Soc. 2004, 126, 53005308. (12) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. Solubilities and Thermodynamic Properties of Gases in the Ionic Liquid 1-N-Butyl-3-Methylimidazolium Hexafluorophosphate. J. Phys. Chem. B 2002, 106, 7315-7320. (13) Cevasco, G.; Chiappe, C. Are Ionic Liquids a Proper Solution to Current Environmental Challenges? Green Chem.2014, 16, 2375-2385. (14) Jiang, Y. Y.; Wang, G. N.; Zhou, Z.; Wu, Y. T.; Geng, J.; Zhang, Z. B. Tetraalkylammonium Amino Acids as Functionalized Ionic Liquids of Low Viscosity. Chem. Commun. 2008, 505-507.


Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(15) Goodrich, B. F.; de la Fuente, J. C.; Gurkan, B. E.; Zadigian, D. J.; Price, E. A.; Huang, Y.; Brennecke, J. F. Experimental Measurements of Amine-Functionalized Anion-Tethered Ionic Liquids with Carbon Dioxide. Ind. Eng. Chem. Res. 2010, 50, 111-118. (16) Gutowski, K. E.; Maginn, E. J. Amine-Functionalized Task-Specific Ionic Liquids: A Mechanistic Explanation for the Dramatic Increase in Viscosity Upon Complexation with CO2 from Molecular Simulation. J. Am. Chem. Soc. 2008, 130, 14690-14704. (17) Goodrich, B. F.; de la Fuente, J. C.; Gurkan, B. E.; Lopez, Z. K.; Price, E. A.; Huang, Y.; Brennecke, J. F. Effect of Water and Temperature on Absorption of CO2 by AmineFunctionalized Anion-Tethered Ionic Liquids. J. Phys. Chem. B 2011, 115, 9140-9150. (18) Cui, G.; Wang, J.; Zhang, S. Active chemisorption sites in functionalized ionic liquids for carbon capture. Chem. Soc. Rev. 2016,15,4307-4339. (19) Zhang, Y.; Zhang, S.; Lu, X.; Zhou, Q.; Fan, W.; Zhang, X. Dual Amino-Functionalised Phosphonium Ionic Liquids for CO2 Capture. Chem. - Eur. J. 2009, 15, 3003-3011. (20) Gurkan, B. E.; de la Fuente, J. C.; Mindrup, E. M.; Ficke, L. E.; Goodrich, B. F.; Price, E. A.; Schneider, W. F.; Brennecke, J. F. Equimolar CO2 Absorption by Anion-Functionalized Ionic Liquids. J. Am. Chem. Soc. 2010, 132, 2116-2117. (21) Ramdin, M.; de Loos, T. W.; Vlugt, T. J. State-of-the-Art of CO2 Capture with Ionic Liquids. Ind. Eng. Chem. Res. 2012, 51, 8149-8177. (22) Zhang, J.; Zhang, S.; Dong, K.; Zhang, Y.; Shen, Y.; Lv, X. Supported Absorption of CO2 by Tetrabutylphosphonium Amino Acid Ionic Liquids. Chem. - Eur. J. 2006, 12, 4021-4026. (23) Ren, J.; Wu, L.; Li, B. G. Preparation and CO2 Sorption/Desorption of N-(3-Aminopropyl) Aminoethyl Tributylphosphonium Amino Acid Salt Ionic Liquids Supported into Porous Silica Particles. Ind. Eng. Chem. Res.2012, 51, 7901-7909.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

(24) Vicent-Luna, J. M.; Gutiérrez-Sevillano, J. J.; Anta, J. A.; Calero, S. Effect of RoomTemperature Ionic Liquids on CO2 Separation by a Cu-BTC Metal–Organic Framework. J. Phys. Chem. C 2013,117,20762-20768. (25) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Post-Combustion CO2 Capture Using Solid Sorbents: A Review. Ind. Eng. Chem. Res. 2012, 51, 1438-1463. (26) Wang, X.; Akhmedov, N. G.; Duan, Y.; Luebke, D.; Li, B. Immobilization of Amino Acid Ionic Liquids into Nanoporous Microspheres as Robust Sorbents for CO2 Capture. J. Mater. Chem. A 2013, 1, 2978-2982. (27) Luis, P.; Van Gerven, T.; Van der Bruggen, B. Recent Developments in Membrane-Based Technologies for CO2 Capture. Prog. Energy Combust. Sci. 2012, 38, 419-448. (28) Scovazzo, P.; Visser, A. E.; Davis Jr, J. H.; Rogers, R. D.; Koval, C. A.; DuBois, D. L.; Noble, R. D. Supported Ionic Liquid Membranes and Facilitated Ionic Liquid Membranes. ACS Symp. Ser.2002,818,69-87. (29) Noble, R. D.; Gin, D. L. Perspective on Ionic Liquids and Ionic Liquid Membranes. J. Membr. Sci. 2011, 369, 1-4. (30) Lozano, L.; Godinez, C.; De Los Rios, A.; Hernández-Fernández, F.; Sánchez-Segado, S.; Alguacil, F. J. Recent Advances in Supported Ionic Liquid Membrane Technology. J. Membr. Sci.

2011, 376, 1-14. (31) Pera-Titus, M.; Miachon, S.; Dalmon, J. A. Increased Gas Solubility in Nanoliquids: Improved Performance in Interfacial Catalytic Membrane Contactors. AIChE J. 2009, 55, 434441. (32) Pera-Titus, M.; El-Chahal, R.; Rakotovao, V.; Daniel, C.; Miachon, S.; Dalmon, J. A. Direct Volumetric Measurement of Gas Oversolubility in Nanoliquids: Beyond Henry’s Law. ChemPhysChem 2009, 10, 2082-2089. 20 ACS Paragon Plus Environment

Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(33) Zhang, J.; Zhang, Q.; Li, X.; Liu, S.; Ma, Y.; Shi, F.; Deng, Y. Nanocomposites of Ionic Liquids Confined in Mesoporous Silica Gels: Preparation, Characterization and Performance. Phys. Chem. Chem. Phys. 2010, 12, 1971-1981. (34) Close, J. J.; Farmer, K.; Moganty, S. S.; Baltus, R. E. CO2/N2 Separations Using Nanoporous Alumina-Supported Ionic Liquid Membranes: Effect of the Support on Separation Performance. J. Membr. Sci. 2012, 390, 201-210. (35) Banu, L. A.; Wang, D.; Baltus, R. E. Effect of Ionic Liquid Confinement on Gas Separation Characteristics. Energy Fuels 2013, 27, 4161-4166. (36) Rajput, N. N.; Monk, J.; Singh, R.; Hung, F. R. On the Influence of Pore Size and Pore Loading on Structural and Dynamical Heterogeneities of an Ionic Liquid Confined in a Slit Nanopore. J. Phys. Chem. C 2012, 116, 5169-5181. (37) Shi, W.; Sorescu, D. C. Molecular Simulations of CO2 and H2 Sorption into Ionic Liquid 1-N-Hexyl-3-Methylimidazolium Bis(Trifluoromethylsulfonyl)Amide ([Hmim][NTf2]) Confined in Carbon Nanotubes. J. Phys. Chem. B 2010, 114, 15029-15041. (38) Gu, Y.; Li, G. Ionic Liquids-Based Catalysis with Solids: State of the Art. Adv. Synth. Catal. 2009, 351, 817-847. (39) Andrzejewska, E.; Marcinkowska, A.; Zgrzeba, A. Ionogels-Materials Containing Immobilized Ionic Liquids. Polimery 2017, 62,344-352. (40) Zhao, X. ;Xing. H.; Li, R.; Yang, Q.; Su B.; Ren Q. Gas separation based on ionic liquids . Prog. Chem. 2011, 23, 2258-2268. (41) Sasaki, T.; Zhong, C.; Tada, M.; Iwasawa, Y. Immobilized Metal Ion-Containing Ionic Liquids: Preparation, Structure and Catalytic Performance in Kharasch Addition Reaction. Chem. Commun. 2005, 2506-2508.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

(42) Andrzejewska, E. Photoinitiated Polymerization in Ionic Liquids and Its Application. Polym. Int. 2017, 66, 366-381. (43) Zhang, Z.; Wu, L.; Dong, J.; Li, B. G.; Zhu, S. Preparation and SO2 Sorption/Desorption Behavior of an Ionic Liquid Supported on Porous Silica Particles. Ind. Eng. Chem. Res. 2009, 48, 2142-2148. (44) Liu, Z. L.; Yang, T.; Zhang, K.; Yan, C.; Pan, W. P. CO2 Adsorption Properties and Thermal Stability of Different Amine-Impregnated MCM-41 Materials. J. Fuel Chem. Technol.

2013, 41, 469-475. (45) Serna-Guerrero, R.; Sayari, A. Modeling Adsorption of CO2 on Amine-Functionalized Mesoporous Silica. 2: Kinetics and Breakthrough Curves. Chem. Eng. J. 2010, 161, 182-190. (46) Bhagiyalakshmi, M.; Yun, L. J.; Anuradha, R.; Jang, H. T. Utilization of Rice Husk Ash as Silica Source for the Synthesis of Mesoporous Silicas and Their Application to CO2 Adsorption through Tren/Tepa Grafting. J. Hazard. Mater. 2010, 175, 928-938. (47) Wang, W.; Wang, X.; Song, C.; Wei, X.; Ding, J.; Xiao, J. Sulfuric Acid Modified Bentonite as the Support of Tetraethylenepentamine for CO2 Capture. Energy Fuels 2013, 27, 1538-1546. (48) Dao, D. S.; Yamada, H.; Yogo, K. Response Surface Optimization of Impregnation of Blended Amines into Mesoporous Silica for High-Performance CO2 Capture. Energy Fuels 2015, 29, 985-992. (49) Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Z. J.; Wang, Y.; Yu, Q.; Zhu, J. H. Promoting the CO2 Adsorption in the Amine-Containing SBA-15 by Hydroxyl Group. Microporous Mesoporous Mater. 2008, 114, 74-81.


Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(50) Monazam, E. R.; Shadle, L. J.; Miller, D. C.; Pennline, H. W.; Fauth, D. J.; Hoffman, J. S.; Gray, M. L. Equilibrium and Kinetics Analysis of Carbon Dioxide Capture Using Immobilized Amine on a Mesoporous Silica. AIChE J. 2013, 59, 923-935. (51) Kaptay, G. The Gibbs Equation Versus the Kelvin and the Gibbs-Thomson Equations to Describe Nucleation and Equilibrium of Nano-Materials. J. Nanosci. Nanotechnol. 2012, 12, 2625-2633. (52) Xie, W.; Ji, X.; Feng, X.; Lu, X. Mass Transfer Rate Enhancement for CO2 Separation by Ionic Liquids: Effect of Film Thickness. Ind. Eng. Chem. Res. 2015, 55, 366-372. (53) Costa Gomes, M. F. Low-Pressure Solubility and Thermodynamics of Solvation of Carbon

Dioxide,

Ethane,

and

Hydrogen

in

1-Hexyl-3-Methylimidazolium

Bis(Trifluoromethylsulfonyl)Amide between Temperatures of 283 K and 343 K. J. Chem. Eng. Data 2007, 52, 472-475. (54) Shiflett, M. B.; Kasprzak, D. J.; Junk, C. P.; Yokozeki, A. Phase Behavior of {Carbon Dioxide + [Bmim][Ac]} Mixtures. J. Chem. Thermodyn. 2008, 40, 25-31. (55) Sumon, K. Z.; Henni, A. Ionic Liquids for CO2 Capture Using Cosmo-Rs: Effect of Structure, Properties and Molecular Interactions on Solubility and Selectivity. Fluid Phase Equilib. 2011, 310, 39-55. (56) Liu, X.; Zhou, G.; Zhang, S.; Yao, X. Molecular Dynamics Simulation of Dual AminoFunctionalized Imidazolium-Based Ionic Liquids. Fluid Phase Equilib. 2009, 284, 44-49. (57) Ahosseini, A.; Sensenich, B.; Weatherley, L. R.; Scurto, A. M. Phase Equilibrium, Volumetric, and Interfacial Properties of the Ionic Liquid, 1-Hexyl-3-Methylimidazolium Bis(Trifluoromethylsulfonyl)Amide and 1-Octene. J. Chem. Eng. Data 2010, 55, 1611-1617.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

(58) Pandit, S. A.; Rather, M. A.; Bhat, S. A.; Rather, G. M.; Bhat, M. A. Influence of the Anion on the Equilibrium and Transport Properties of 1-Butyl-3-Methylimidazolium Based Room Temperature Ionic Liquids. J. Solution Chem. 2016, 45, 1641-1658. (59) Salinas, R.; Pla-Franco, J.; Lladosa, E.; Montón, J. B. Density, Speed of Sound, Viscosity, and Excess Properties of Binary Mixtures Formed by Ethanol and Bis(Trifluorosulfonyl)ImideBased Ionic Liquids. J. Chem. Eng. Data 2015, 60, 525-540. (60) Fendt, S.; Padmanabhan, S.; Blanch, H. W.; Prausnitz, J. M. Viscosities of Acetate or Chloride-Based Ionic Liquids and Some of Their Mixtures with Water or Other Common Solvents. J. Chem. Eng. Data 2011, 56, 31-34. (61) Wu, N.; Ji, X.; An, R.; Liu,C.; Lu X. Generalized Gibbs Free Energy of Confined Nanoparticles. AIChE J. 2017, DOI: 10.1002/aic.15861. (62) Tang , Z.; Lu, L.; Dai, Z.; Xie, W.; Shi, L.;Lu, X. CO2 absorption in the ionic liquids immobilized on solid surface by molecular dynamics simulation. Langmiur Revised. (63) White, A. Intermolecular Potentials of Mixed Systems: Testing the Lorentz-Berthelot Mixing Rules with Ab Initio Calculations, DTIC Document, 2000.


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Table 1. Summary of the IL loadings of the samples Substrate

Ionic Liquid

Loadings, wt%

P25

[HMIm][NTf2]

11.4, 20.7, 39.6, 49.0

P25

[BMIm]Ac

32.6, 38.2, 45.8

P25

[APMIm]Br

25.8, 37.2, 40.4, 44.2

PMMA

[APMIm]Br

15, 25, 35


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Table 2. The CO2 absorption working capacity (m0) of P25-[HMIm][NTf2], P25-[BMIm]Ac and P25-[APMIm]Br and PMMA[APMIm]Br with different IL-film thickness and corresponding SBET P25-[HMIm][NTf2] SBET

δ

2 m /g

nm

P25-[BMIm]Ac

m0

SBET

mol/mol IL

2 m /g

sorbent

δ nm

P25-[APMIm]Br m0

SBET

mol/mol IL

2 m /g

sorbent

δ nm

PMMA-[APMIm]Br

m0

SBET

mol/mol IL

2 m /g

sorbent

δ

m0 nm

sorbent

mol/mol

IL

33.52

2.5

0.386

4.9

63.5

0.771

11

17.5

0.424

107.12

1

0.896

23.1

6.6

0.195

3.7

97.4

0.523

10.89

25.5

0.378

24.74

6.3

0.823

8.72

33.3

0.00945

0.2

2220

0.486

5.54

54.5

0.184

33.07

25.6

0.606

4.16

86.5

0.00628

5.20

63.6

0.168

a

pure

0.00306

a

pure

0.007

pure53

0.00310

pure55

0.5

b

a

a

b

pure

0.380

pure54

0.38

a

b

pure

0.0069

pure55

0.5

b

The CO2 absorption working capacity in pure IL in TG experiment in this work. b The CO2 solubility in pure IL from references.53-

55


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Table 3. Surface tension and density of ILs γlg

Vm

η

J/m2

m3/mol,104

Pa ·s

[HMIm][NTf2]

0.030657

3.2657

0.070559

[BMIm]Ac

0.043758

1.8258

0.48560


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Table 4. The molecule parameters for the two ILs and substrate ILsubstrate [HMIm][NTf2]P25 [BMIm]Ac-P25

εIL

εsubstrate

εIL-substrate

eV 0.154

eV 0.426

0.326

0.426

eV 0.256

σIL nm 1.75

σsubstrate nm 0.346

σIL-substrate nm 1.048

∆ nm 0.35

ρsub nm-3 29.6

0.373

0.909

0.346

0.627

0.35

29.6


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Figure captions Figure1. The CO2 absorption working capacity and solubility of P25-[HMIm][NTf2], P25[BMIm]Ac, P25-[APMIm]Br and [APMIm]Br-PMMA with different IL-film thickness.

Figure 2. Correlation between interfacial Gibbs free energy and size. Figure 3. The proportion of GIL-substrate to (GIL-surface+ GIL-substrate) and GIL-surface to (GIL-surface+ GILsubstrate).

■: [HMIm][NTf2]; ▲: [BMIm][Ac].

Figure 4. Model performance with both surface and substrate effects and only with surface effect. ■: [HMIm][NTf2]; ▲: [BMIm][Ac].


Langmuir

0.96

P25-[HMIm][NTf2]

0.84

P25-[BMIm][Ac] P25-[APMIm]Br PMMA-[APMIm]Br

0.72

m0,mol/mol IL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

0.60

bulk 0.5 mol/mol IL

0.48 0.36

bulk 0.38 mol/mol IL

0.24 0.12 0.00 0.1

bulk 0.0306 mol/mol IL 1

10

100

IL-film thickness (nm)

Figure 1.


1000

10000

Page 31 of 34

1.20E+018

1.00E+018

R2=0.998

(G IL-substrate/γγlg *V m )2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

8.00E+017

6.00E+017

P25+[HMIm][NTf2] P25+[BMIm][Ac]

4.00E+017

Same substrate

2.00E+017

0.00E+000 0.00E+000

2.00E+008

1/δ (m-1) Figure 2.


4.00E+008

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Proportion of GIL-substrate and GIL-surface to (GIL-substrate+GIL-surface),100%

Langmuir

Page 32 of 34

100 GIL-substrate/(GIL-substrate+GIL-surface)

95

GIL-substrate/(GIL-substrate+GIL-surface)

90

GIL-surface/(GIL-substrate+GIL-surface)

85

GIL-surface/(GIL-substrate+GIL-surface)

80 75 20 15 10 5 0 0.0

0.2

1/δ (nm-1)

Figure 3.


0.4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Deviation of CO 2 capacity (  m exp.-m cal. ),mol CO2/mol IL

Page 33 of 34

0.6

surface+substrate effect surface effect surface+substrate effect surface effect

0.5

0.4

0.3

P25+[BMIm][Ac]

0.2

0.1

P25+[HMIm][NTf2]

0.0 0.0

0.2

1/δ (nm-1)

Figure 4.


0.4

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Abstract Graphic


Page 34 of 34

Confinement Phenomenon Effect on the CO2 Absorption Working

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