CO2 Absorption in the Ionic Liquids Immobilized on Solid Surface by

Subscriber access provided by University of Sussex Library

Article

CO2 Absorption in the Ionic Liquids Immobilized on solid surface by Molecular Dynamics Simulation Ziqian Tang, Linghong Lu, Zhongyang Dai, Wenlong Xie, Lili Shi, and Xiaohua Lu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02044 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 21, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

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

CO2 Absorption in the Ionic Liquids Immobilized on solid surface by Molecular Dynamics Simulation Ziqian Tang, Linghong Lu*, Zhongyang Dai, WenlongXie, Lili Shi and Xiaohua Lu College of Chemical Engineering,State Key Laboratory of Materials-Oriented Chemical Engineering,Nanjing Tech University, Nanjing, 210009, P.R. China

* Authors to whom correspondence should be addressed: LinghongLu,Emailaddress:[email protected]

ACS Paragon Plus Environment

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

ABSTRACT Based on our previous experimental research, we studied the absorption of CO2 in the ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N] ), immobilized on TiO2 [rutile (110) ] with different thickness by molecular dynamics simulation. The effects of the properties (hydrophobicity and hydrophilicity)of solid interfaces were also studied with IL immobilized on graphite and TiO2 respectively. We studied the influence of the thickness of IL immobilized on TiO2 on the absorption of CO2 via structural and dynamical properties. The results show that the self-diffusion coefficients of IL and CO2 increase as the thickness of immobilized IL decreases. And the CO2 absorption capacity increases as the thickness of immobilized IL decreases as well. Besides, more CO2 molecules are absorbed in the region near the solid interface as the thickness of IL decreases. For IL immobilized on graphite, the self-diffusion coefficients of cations and anions are larger than that of IL immobilized on TiO2 with the same thickness. They are also larger than non-immobilized cations and anions.Besides, the CO2 absorption capacity of IL immobilized on TiO2 is the largest compared with IL immobilized on graphite and non-immobilized IL with the same thickness.From our simulation work, we try to explorethe microscopicmechanism which is unexplored by experimental work, and we found the important role of IL/solid interface for CO2 absorption in immobilized ILs.

Introduction Anthropogenic carbon dioxide (CO2) produced from the combustion of fossil fuels is believed to be one of the main causes of worldwide global warming. To mitigate the release of CO2, CO2 capture and storage (CCS) should be highlighted.1To capture CO2, ionic liquids have been proposed as possible materials due to its low volatility,high absorption performance as well as low regeneration energy consumption. Additionally, when compared with other organic solvents, ILs are more environmental-friendly.2


Page 2 of 30

Page 3 of 30

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

However, ILs are much more viscous than water, with viscosity values typically 2-3 orders of magnitude larger than that of water. This property causes many serious problems during chemical processing, which significantly slows mass transfer for gas absorption. As a result, new devices and innovative methods have to be developed to solve this serious problem. Currently, in order to remarkably enhance absorption of CO2in ILs and reduce the amount of ILs needed for CO2 separation3, supported IL membranes4 and ILs immobilization into porous solid supports 5have been developed.The available work focused on the experimental studies with the major purpose of enhancing CO2 absorption as well as the investigation of the effects of IL loadings and operational conditions (eg, pressure and temperature).6It has been found that IL immobilization into porous solid supports7-10 could significantly enhance the absorption of CO2 in ILs. And from the perspective of large-scale production, IL immobilization into porous solid supports could also reduce the amount of ILs needed for CO2 separation. The thickness of IL on solid supports is animportant factor for CO2 absorption inimmobilized IL system since it affects the masstransferrate of CO2 in ILs and determinesthe amount of ILs neededfor CO2 separation. We have alreadyresearched the effect of film thickness on the absorption of CO2 in IL immobilized on solid support experimentally. In our experiments, the effects of supported ionic liquids with different thickness immobilized on different supports were explored.11,12,

13

The results showed that the mass-transfer coefficient of the

whole CO2 absorption/desorption process in IL immobilized sorbents was different in magnitude depending on IL-film thickness on supports from microscale to nanoscale. Our analysis based on the diffusion-reaction theory revealed the inherent mechanism for the rate enhancement, that is, the CO2 mass-transfer rate correspond to diffusion-control and reaction-control processes with IL-film thicknesses. The CO2 absorption/desorption process in IL was assumed to comprise two steps: surface-reaction and diffusion.11As described inFigure 1 (a)when the thickness of IL increases the diffusion resistance ratio decreases and the reaction resistance ratio increases. Thus the mass transfer coefficient of CO2 increases by diffusion-reaction


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 4 of 30

compromise. We used the thermogravimetric analyzer toestimate the scale of IL-film thickness, reveal the influence of IL-film thicknessquantitatively and find outthat the IL-film in nanoscale is a prerequisite for efficient CO2 absorption/desorptionfor the first time.12We also conducteda systematic investigation of CO2absorption behavior on five TEPA-immobilized sorbents and found that the CO2 absorption kinetics and capacity were affected by the surface chemistry of the support.13

(a)

(b)

Figure 1. (a)Diffusion-reaction compromise of CO2 absorption in IL supported on P25(a kind of nano-sized titania);(b)Schematic diagram of diffusion layer and reaction layer of CO2 absorption in IL and the interfaces of CO2-IL-solid.

Although theeffect of IL thickness and the surface chemistry of the support on CO2 absorption have been confirmed by experiment, the microscopic mechanism is still remained to be solved. The thermogravimetric analyzer can findthat the IL-film in nanoscale is a prerequisitefor efficient CO2 absorption/desorption, but it cannot “see” what happen in nanoscale. Based onexperimental analysis we can divide the nanoscale system of CO2-IL-support into reaction layer and diffusion layer as shown in Figure 1 (b).The reaction layer is actually the gas-liquid interface, and the diffusion layer is IL layer on the support. So the problem is what’s the role of the solid-liquid interface? Does the solid-liquid interface affect diffusion-reaction compromise for the absorption of CO2? It must be important thathow the solid support, IL and CO2interact each other. We need molecular level analysis since this phenomenon occurs in nanoscale.


Page 5 of 30

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

The IL layer immobilized on support is in a confined state.In terms of confined ILs, molecular simulations were conducted to display the properties between confined ILs and bulk ILs.

14, 15

Shi et al. have found that the confined ILs exhibit high

self-diffusivity compared with bulk ILs.16 Weber et al. completed the research of ILs confined in the slit of TiO2 and studied the density profiles of ILs mainly focusing on structural properties.17 Recently, Hung et al. accomplished the work of ILs confined in different materials. And the result showed that the structure of confined ILs and self-diffusivity

are

different from

bulk

ILs

because

confined

ILs

pack

heterogeneously.18 Besides, the properties of solid surfaces, for example the hydrophobicity and hydrophilicity, influence the properties of confined ILs. Hung et al. have found that the dynamics of certain IL inside the rutile pore were 2-4 times slower than the mobilities of IL inside graphite pore. Shi et al. completed the study of ILs confined in silica slit pore. And the result showed that the amount of CO2, H2 and N2 absorbed

in the confined IL are 1.1-3 times larger than those in the bulk IL. In

terms of self-diffusivities of these gases, gases in confined ILs were 2-8 times larger than those in the bulk ILs at given temperatures.19Close et al. performed the research of CO2 absorption behavior confined in ILs to show that CO2 permeability in ILs supported in alumina nanopores could be increased a lot compared to that in bulk ILs.20 Since IL has already been immobilized on TiO2 surface to absorb CO2 experimentally by us,12the microscopic view of absorption of CO2 in ILs on TiO2is what we are interested in. TiO2 (110) surface is the most stable crystal face and it has already been well studied by performing experimental techniques such as atomic force

microscopy21

dynamics22.For

and

theoretical

ionic

methods liquid

such

as

classical

molecular

1-hexyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)amide, also known as 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and often abbreviated to [hmim][Tf2N], it is stable, has low viscosity compared to the commonly investigated ILs. Besides, it also has low water solubility and is easily purified and prepared.23Due to its distinguished properties, to investigate the absorption performance of [hmim][Tf2N] supported on


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

TiO2 (110) surface for CO2 should be significant.Since graphite surface is hydrophobic and has already been well studied by other researchers,24, 25we can make a comparison considering TiO2

surface and graphite surface as the support for IL to

investigate the effect of surface chemistry of the support for CO2 absorption in microscope. In this study, TiO2 [rutile (110)]and graphite were chosen as support materials on which IL with the same thickness was immobilized and [hmim][Tf2N] was chosen as our considered IL. For TiO2, ILs with differentthickness of 2.1 nm, 2.8 nm and 3.5 nm were immobilized on it respectively. And then we analyzed the immobilized IL both structurally and dynamicallyand try to give a micro-aspect picture for our experimental results and a microscopic interpretation for macroscopic mechanism by exploring the interaction among CO2, IL and solid surface.

Methodology Molecular dynamics simulation. The force field used for ionic liquid [hmim][Tf2N] was taken from Maginn’s26 paper. The flexible model of CO2 based on TraPPE model27was also taken from Maginn’s paper. For the force field of TiO2,it was taken from Langel’s paper.28For graphite surfaces, they were described as uncharged sheets composed of carbon atoms interacting via Lennard-Jones parameters (ε = 0.2929kJ/

mol and σ = 0.355nm ). The frameworks of the simulation and the structure of [hmim][Tf2N] are shown in Figure 2.Three TiO2 slabs and one graphite slab were constructed. For IL immobilized on TiO2, the thickness of IL is 2.1nm,2.8nm and 3.5nm. For IL immobilized on graphite, the thickness of IL is 2.8 nm. Additionally, for non-immobilized IL, the thickness of IL is 2.8 nm. For all frameworks, 190 CO2 molecules were put into the gas phase as shown in Figure 2. For IL with the thickness of 2.1 nm, 2.8 nm and 3.5 nm immobilized on TiO2, the volumes of the initial CO2 boxes are the same. And the volumes of the initial CO2 boxes are 156.06 nm3. At the same time, for IL with the thickness of 2.8 nm immobilized on TiO2 and graphite and


Page 6 of 30

Page 7 of 30

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

non-immobilized IL, the volumes of their initial CO2 boxes are also the same. The value of the volume is 114.24 nm3. Here TiO2 interface was modeled as 3 layers with the thickness of 1.228 nm in the z-direction, 3.402 nm in the x-direction and 5.126 nm in the y-direction. And for graphite interface, it was modeled as 3 layers with the thickness of 0.67 nm in the z-direction, 3.316 nm in the x-direction and 5.530 nm in the y-direction.

(a)

(c)

(b)

(d)

Figure 2.Schematic representation of the ions for (a) [hmim+][Tf2N-] Atom color code: grey, carbon; blue, nitrogen; white, hydrogen; red, oxygen; yellow, sulfur; light blue, fluorine; and typical simulation cell for (b) IL immobilized on TiO2, (c) IL immobilized on graphite, (d) Non-immobilized IL.

The numbers of IL and CO2 molecules, the thickness of IL, and cell dimensions were listed in Table1. During the process, we conducted molecular dynamics simulations by using LAMMPS29 according to the following procedures.Firstly,19.0ns MD simulations were performed to get the equilibrium configuration of the whole simulation box by using NVT (canonical) ensemble. The periodic boundary condition (PBC) was applied in three dimensions. After the equilibration run,4.0 ns production run was performed to analyzethe properties with integral step of 1.0 fs.And a cutoff smoothed from 1.0 nm to 1.2 nm was used for pair interactions.Besides, TiO2 interface and graphite interface were fixed during the whole simulation process.Room temperature of 298.0 K was chosen to investigate the properties ofthe simulation system.


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 8 of 30

Table 1. Numbers of IL and CO2, thickness of IL and Cell dimensions in each system System IL immobilized on graphite IL immobilized on TiO2 IL immobilized on TiO2 IL immobilized on TiO2 Non-immobilized IL

Number of IL 91

Number of CO2 190

Thickness of IL (nm) 2.8

Cell dimensions(nm3)

68 91 114 91

190 190 190 190

2.1 2.8 3.5 2.8

3.47*5.16*17.11 3.47*5.16*16.23/18.21 3.47*5.16*21.43 3.47*5.16*13.42

3.31*5.53*15.27

The total potential energy is given by

∑

ν (r) =

k b (r - r0 ) 2 +

bonds

+

∑

k θ (θ - θ0 )2

angles

∑

k χ [1 + cos(n 0 χ - δ 0 )]

∑

k ϕ (ϕ - ϕ0 ) 2

dihedrals

+

(1)

impropers N -1

+∑

N

∑

(4ε ij[ (

i =1 j= i +1

σ ij rij

)12 − (

σ ij rij

)6 ] +

qi q j rij

)

Lennard-Jones and Coulombic terms are used for nonbonded interactions, harmonic potentials for molecular bonds and angles and CHARMM-style for molecular dihedrals.26In terms of the intermolecular interactions between ionic liquid and graphite interface,the intermolecular interactions between ionic liquid and TiO2 interface,we have used the Lennard-Jones potential using the geometric mean for ε and the arithmetic mean forσ. Here for the Lennard-Jones interactions between unlike sites,the standard Lorentz-Berthelot combining rules were adopted.And it is calculated according to the following rules .

σij =

ε ij =

1 ( σ + σ j ) (2) 2 i

ε iε j (3)

For self-diffusion coefficient, it was calculated according to the Einstein relation

Dxy = l i m t →∞

2 2 1 < x(t ) − x( 0) + y(t ) − y( 0) > (4) 4t


Page 9 of 30

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

where Dxy is the diffusion coefficient,t is time, x(t) is the position at time t in the x-direction, x(o) is the original position in the x-direction, y(t) is the position at time t in the y-direction, y(0) is the original position in the y-direction. To obtain the averaged number density profiles,the number density was calculated along the direction perpendicular to the CO2/ILs interfaces by binning method.30And the orientation of CO2, cation and anion defined in Figure 3. For cation, its tilt angle is defined as the angle between the alkyl chain and the surface normal. And the tilt angle of anion is defined as the angle between S-CF3 vector andthe surface normal. Z(Normal)

Z(Norma

θ

θ Surface

(a)

Z(Normal)

θ

N2-C10 vector

Surface

Surface

(b)

(c)

S-CF3 vector

Figure 3. Definition of tilt angle for (a) CO2,(b) cation, (c) anion.

RESULTS AND DISCUSSION The effect of the thickness of ILs Structural propertiesIn order to study the effect of the thickness of IL on the absorption of CO2, ionic liquid with different thickness was immobilized on TiO2. As described in Table 1, we considered 2.1 nm, 2.8 nm and 3.5 nm as the thickness of IL. Figure 4 showsdensity profiles of CO2, cations and anions. As shown in Figure 4, for each thickness, there is a peak at the interface of IL/CO2 for CO2. This demonstrates that for the absorption of CO2 in IL, many CO2molecules are absorbed at the interface of IL/CO2. And this phenomenon has also been displayed by Cummings30 and Jiang31. We can assume that this peak denotes the “reaction layer”. In the region between IL/CO2 interface andIL/solid interface, the number density of CO2 is lower than in “reaction layer”, we assume that this region denotes the “diffusion layer”. There are also 3 obvious peaks of CO2, anion and cation in the region near the solid TiO2


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 10 of 30

surface.For the system with IL thickness of 3.5 nm,however,the height of the peak of CO2 near the solid TiO2 surface is obviously lower than that of other two cases. From the above analysis we can see that there are “reaction layer” and “diffusion layer” in the adsorption process, this confirms the experimental assumption of the CO2 absorption/desorption process in IL comprises two steps: surface-reaction and diffusion. The 3 peaks of CO2, anion and cation in the region near the TiO2 surfaceindicate the solid-liquidinterface plays an important role, andthe thickness influences the distribution of CO2 in immobilized IL, especially in the region near the TiO2 surface.For immobilized IL with relatively smallthickness, the solid surfaces have stronger effect on the absorption of CO2.And with the increasing thickness of ILs the CO2need toovercome greater diffusion resistance, even the attractive force of the solid surface is not enough strong to pull a CO2molecule through the “diffusion layer”.As a result, we can see the low peak of CO2 near the solid surface for the 3.5 nm thickness IL.For cations and anions, there are two obvious peaks in the region near the solid TiO2 surface especially for anions. This is due to stronger interaction between TiO2 and anions as shown in Table2.In Table2, for IL with different thickness immobilized on TiO2, the average atom interaction energies between anions and TiO2 are all larger than that between cations and TiO2.Hung32,

33

et al.

havealsoreported that solid interface influences the distribution of confined IL. It is known that anions influence the absorption of CO2 in ILin the main.34Normally, the higher peak of anions should enhance the absorption of CO2 near the solid surface, but we actually see the influence of anions to CO2 also resist by the “diffusion layer”.What needs to be explained is that the “diffusion layer” is not real diffusion layer since we perform equilibrium simulation, here we just make sure the range of this layer.


Page 11 of 30

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

Figure 4.Number density profiles of cations,anions and CO2 for IL (with different thickness) immobilized on TiO2 for (a) 2.1nm, (b) 2.8nm ,(c) 3.5nm.Cations,anions and CO2 weredepicted in green, blue and red. The left part of the dash line represents the solid surface.

Table 2. Average atom interaction energies (kJ/mol) between IL and TiO2 in each system Thickness=2.1nm

Thickness=2.8nm

Thickness=3.5nm

Anion-TiO2

-3.188

-2.382

-2.125

Cation-TiO2

-2.552

-1.908

-1.703

From the above analysis, we conclude that for IL immobilized on TiO2 surface, the thickness of IL influences the absorption of CO2. And the solid interface should play an important role in the absorption of CO2 in IL.In Table 3, the amount of CO2 absorbed in IL was counted by including and excluding the interfacial CO2. For the calculation of the solubility of CO2, the number of interfacial CO2 was excluded, and the solubility of CO2 is calculated trough divide the number of absorbed CO2 by the


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 12 of 30

number of IL. So the solubility of CO2 is 0.277,0.229 and 0.191 in IL with the thickness of 2.1 nm, 2.8nm and 3.5nm as shown in Table 3respectively. This means that the solubility of CO2increases with the thickness of ILs decreases and more CO2 molecules are absorbed in the region near TiO2 surface. This result confirms the solid interface plays an important role in the process of CO2 absorption in immobilized IL. Besides, the bulk experimental data of solubility provided by Yokozeki et al.35isbetween 0.127 and 0.271 under similar pressure and temperature condition. Compared with this data, our simulation results of the CO2 solubility (including the interfacial CO2) of IL in immobilized IL is larger than that in the bulk IL. From this perspective, the immobilized IL enhances the absorption of CO2 compared with the bulk IL. More importantly, our simulation studies correlatethe thickness of IL immobilized on TiO2with the degree of enhancement of absorption of CO2, and the distribution of CO2 in immobilized IL can be observed in molecular level.

Table 3.Numbers of IL and absorbed CO2 and the thickness of IL immobilized on TiO2 Thickness of IL (nm)

No. of absorbed CO2 (uncertainty) (include interfacial CO2)

No. of absorbed CO2 (uncertainty) (exclude interfacial CO2)

Solubility of CO2

2.1

51.2 (1.08%)

26.3 (2.16%)

0.277

2.8

57.1 (1.49%)

27.0 (2.53%)

0.229

3.5

42.4 (1%)

27.5 (1.90%)

0.191

To further explore the structural properties of IL and CO2, the tilt angle distributions were analyzed. And the tilt angles of cations, anions and CO2 were defined in Figure 3. For IL with the thickness of 2.1 nm immobilized on solid TiO2 surface, the tilt angle distributions of cations, anions and CO2were shown in Figure 5 (a) (d)and (g). For cations, their tilt angles mainly distribute around 90o,which indicates that cations tend to lie flat and parallel to the solid TiO2 surface. While as the thickness increases, the probability of thedistribution oftilt angle of 90o decreases


Page 13 of 30

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

IL thickness=2.1 nm

(a)

2.8 nm

(b)

3.5nm

(c)

80

60 (d)

(e)

(f)

r/Å 40 (g) (i)

(h)

θ/

20

o

Figure 5.Tilt angle distributions of cations, anions and CO2 for different ILs thickness (a),(b),(c) for cations; (d),(e),(f) for anions; (g),(h),(i) for CO2; (a),(d),(g) ILs thickness=2.1 nm, (b),(e), (h) ILs thickness=2.8 nm, (c),(f),(i) ILs thickness=3.5 nm (blue represents low probability, red represents high probability)

in the region near the solid TiO2 surface (see Figure 5(b),(c)).This can be explained by the interaction energies, in Table2, as the thickness of IL increases, the interaction energies between cations and TiO2decrease. For anions, what's different is thatthe distributions of their tilt angles mainly distribute from 45o to 135o when the anions are in the region near the solid TiO2 surface (Figure 5(d),(e),(f)). The expansion of rangeof tilt angles distribution means that anions tend to tilt toward the TiO2 surface.This is because the anion molecule shape is more close to sphere compared with cation. For CO2 absorbed in IL, influenced by anions the range of tilt angles distribution of CO2 in the region near the solid TiO2 surface also expands. However, at the interface of IL/CO2, the tilt angles of CO2are more dispersiveand mainly distribute from 45o to 135o. In Figure 5(i) more of them distribute around 90o at the


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

CO2/IL interface, itindicates that CO2 tends to lie flat at the interface of IL/CO2 when the thickness of IL increase.Maginn et al.36 and Cummings et al.30havealso reported that CO2 molecules tend to lie flat on ionic liquid surface. From the number density of ILs profile and the Tilt angle distributions of cations, anions, we can learn that ILs is arranged in layers on the surface of solid. From the distance of layers we can get the coarse grainedmolecular parameter of IL molecule size, which are applied to the study of confinement phenomenon on CO2 absorption in ionic liquids immobilized into porous solid supportswhich was conducted in our group.37

Diffusion propertiesWe have known by experiment that CO2 mass-transfer rate corresponds todiffusion-control and reaction-control processes. And the CO2 absorption/desorption process in IL comprise two steps: surface-reaction and diffusion. Studying diffusionin molecular levelis important for fundamental understanding of mass transfer, interfacial properties and other related properties. Although self-diffusion coefficients cannot be used directly to calculate the mass-transfer rate, it is related to mass-transfer and will enormously affect mass-transfer.Here self-diffusion coefficients from mean square displacements calculated by Einstein’s equation were obtained.Figure6 presents the MSDs of IL with different thickness and CO2absorbed in immobilized IL in the xy plane.Besides, the self-diffusion coefficients of CO2 absorbed in the ionic liquid phase with different thickness and the self-diffusion coefficients of cations and anions were obtained as well.As shown in Figure 7 (b), the self-diffusion coefficients of CO2 absorbed in IL with the thickness of 2.1 nm are the largest. As the thickness of IL immobilized on TiO2 increases, the self-diffusion coefficients of CO2 decreases.Shi and co-workers have reported that confined IL diffuses faster than bulk IL.19 Besides, Hung and co-workers have also reported similar simulation results.33 Thus, we could conclude for immobilized IL with the thickness of 2.1 nm, its confinement effect is the most obvious when comparedwith immobilized IL with the thickness of 2.8 nm and 3.5 nm.As a result,


Page 14 of 30

Page 15 of 30

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

Figure 6.Mean-squared displacements of anions, cations and CO2 in the xy plane


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

Figure 7.Self-diffusion coefficients for (a) Cations and anions, (b) CO2 absorbed in IL with different thickness.

for immobilized IL with the thickness of 2.1 nm, the self-diffusion coefficients of cations and anions are the largest compared with the thickness of 2.8 nm and 3.5 nm.Considering the solubility of CO2 is also the largest (Table 3), we could learn that good mobility of IL enhances the absorption of CO2.Experimental research also found that the shortened diffusion path and the enlarged gas-liquid contact area enhance the absorption of CO2 in IL.38We can conclude that, for the design of immobilized IL, the thickness of immobilized IL and the mobility of IL should be considered fully.

The effect of the surface properties of solid surfaces Structural properties.We have conducteda systematic experimental investigation of CO2absorption behavior on five TEPA-immobilized sorbents and found that the CO2 absorption kinetics and capacity were affected by the surface chemistry of the 13

support. Howthe properties of solid surface influence the absorption of CO2 in IL should be explored in microscopic. We chose graphite(hydrophobic) and TiO2(hydrophilic) as studied supportmaterials and the thickness of IL is 2.8 nm, the numbers of IL in different systems are 91 and presented in Table 4.As shown in Figure 8(a), for CO2 absorbed in IL immobilized on graphite, the number density inthe region near the graphite surface is smaller than that immobilized on TiO2 shown


Page 16 of 30

Page 17 of 30

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

in Figure 8 (c).In terms of the number densities of cations and anions, there are no obvious peaks for non-immobilized IL which was shown in Figure 8 (b), while for IL immobilized on graphite and TiO2, there are obvious peaks in the region near the solid interfaces. Other research work also confirmed that solid interfaces influence the distribution of IL.39,40 And for IL immobilized on TiO2, the highest peak of the number density of anions is larger than that immobilized on graphite. As shown in Table 5, theatom interaction energies between anions and TiO2 are larger than that between anions and graphite.And this helps explain why the highest peak of the

Figure 8.Number density profiles of cations,anions and CO2 for IL (with the thickness of 2.8 nm) for (a) Immobilized on graphite, (b) Non-Immobilized,(c) Immobilized on TiO2.Cations,anions and CO2 were depicted in green, blue and red. The left part of the dash line represents the solid surface.

anions immobilized on TiO2 is larger than that immobilized on graphite. Here, we compared the CO2 absorption capacity as well. And the absorption number of CO2was


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 18 of 30

calculated by including the interfacial CO2 and excluding the interfacial CO2. For the comparison with the bulk experimental data, the interfacial CO2 was excluded. As shown in Table 4, for IL immobilized on graphite, the number of CO2 absorbed in IL is 23.00 and the corresponding CO2 absorption capacity is calculated as the solubility of CO2 in immobilized IL.The bulk experimental solubility data provided by Yokozeki et al.35isbetween 0.127 and 0.271 under similar pressure and temperature. For IL immobilized on graphite surface, the CO2 solubility is 0.202. For non-immobilized IL, the number of CO2 absorbed in IL is 20.50 and its CO2 solubility is 0.184. At the same time, for IL immobilized on TiO2, the number of CO2 absorbed in IL is 26.00 and the corresponding CO2 solubility is 0.222. Above all, the absorption capacity of immobilized IL is higher than non-immobilized IL. Besides, the simulation results reveal that for IL ([hmim][Tf2N]) immobilized on hydrophilic TiO2 surface, the absorption capacity of CO2 is better than that immobilized on hydrophobic graphite surface. Table 4.Numbers of IL and absorbed CO2in each system System

No. of IL

No. of absorbed CO2 (uncertainty) (include interfacial CO2)

No. of absorbed CO2 (uncertainty) (exclude interfacial CO2)

IL immobilized on graphite

91

54.0(0.96%)

22.8(2.32%)

Non-immobilized IL

91

49.6(0.99%)

20.5(2.48%)

IL immobilized on TiO2

91

59.1(0.94%)

26.0(2.28%)

Table 5. Average atom interaction energies (kJ/mol) between solid surface and with the thickness of 2.8 nm in each system IL immobilized on

IL immobilized on

graphite

TiO2

Anion-interface

-1.389

-2.099

Cation-interface

-0.824

-1.790

In order to further explore whether the properties of solid surfaces influence the


Page 19 of 30

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

orientational distributions of cations, anions and CO2, the tilt angle distributions areshown in Figure 9. For IL immobilized on graphite, as shown in Figure 9(c), the tilt angles of cations also distribute from 45o to 135o. While for cations in the region near the graphite surface, the tilt angles of cations distribute from 0o to 180o. Besides, for cations in the region near the solid surfaces and at a certain position away from the surface, the tilt angles tend to distribute around 90o, especially for TiO2 surface. This means cations tend to lie flat in the region.For non-immobilized IL, the tilt angles of cations distribute from 0o to 180o, and the tilt angles alsotend to distribute around 90oat certain positions. For anions immobilized on graphite as shown in Figure 9 (f), the tilt angle distributions are a little different from that immobilized on TiO2 surface as shown in Figure 9 (d). The distribution of tilt angles of anions near graphite surface ismore dispersivethan that near the solid TiO2 surface. For CO2 absorbed in IL,since most CO2 molecules are absorbed at the interface of IL/CO2, the tilt angles of CO2 are more dispersive. And as shown inFigure 9 (g) and (i), for CO2 in the region near the

Figure 9.Tilt angle distributions of cations, anions,CO2 for systems immobilized on different


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

support materials with the ILs thickness of 2.8 nm (a),(b),(c) for cations; (d),(e),(f) for anions; (g),(h),(i) for CO2; (a),(d),(g) immobilized on TiO2, (b),(e),(h) non-immobilized; (c),(f),(i) immobilized on graphite. ( blue represents low probability, red represents high probability)

solid surfaces, the tilt angle distributions were different. For CO2 in the region near the solid TiO2 surface, the tilt angle distributions mainly range from 45o to 90o, while for CO2 in the region near graphite surface, the tilt angles mainly distribute around 90o. From the above analysis, we can conclude the properties of solid surfaces influence the orientation of cations, anions and CO2 only in the region near solid surface. Whether the surface is hydrophobic or hydrophilic, it has smallimpact on the orientation of ILs and CO2 molecules.

Diffusion properties.In Figure 10, the MSDs of immobilized IL, non-immobilized IL and CO2 absorbed in IL in the xy plane are presented. As shown in Figure 11 (b), the self-diffusion coefficients of CO2 absorbed in IL immobilized on graphite are the largest.We calculated the interaction energies between CO2 and the solid surface,forgraphite the value is -2.115 kJ/mol,and for TiO2 surface the value is -13.187 kJ/mol.So the interactions between CO2 and TiO2 are stronger than that between CO2 and graphite. Due to this, the self-diffusion coefficients of CO2 absorbed in IL immobilized on graphite are larger than that of CO2 absorbed in IL immobilized on TiO2. While for CO2 absorbed in non-immobilized IL, their self-diffusion coefficients are the smallest. From this phenomenon, we could conclude that the graphite surface and TiO2 surface enhance the mobility of CO2 in IL due to the confinement effect. This phenomenon has also been reported by Shi et al.19They found that the self-diffusion coefficients of CO2 absorbed in confined IL are larger than the self-diffusion coefficients of CO2 in bulk IL. As shown in Figure 11 (a), the self-diffusion coefficients of cations and anions immobilized on graphite arelarger than non-immobilized IL. Besides, graphite surface enhances the self-diffusion coefficients of confined IL compared with IL confined between TiO2 surfaces and this has been explored by Hung et al.41As shown in Table 5, the interactions between IL


Page 20 of 30

Page 21 of 30

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

and TiO2 are stronger than that between IL and graphite. This demonstrates that the strong interaction between IL and TiO2 surface weakens the mobility of IL. Besides, the self-diffusion coefficients of non-immobilized cations and anions are larger than that immobilized on TiO2 surface, but smaller than that immobilized on graphite

Figure 10.Mean-squared displacements of anions, cations and CO2 in the xy plane.


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

Figure 11.Self-diffusion coefficients for (a) Cations and anions immobilized on TiO2, graphite and non-immobilized IL, (b) CO2 absorbed in IL immobilized on TiO2, graphite and non-immobilized IL.

surface due to the confinement effect. From this point, we could conclude that TiO2 surface hinders the mobility of IL compared with the graphite surface. Shi et al.19have explored that the interactions between IL and solid interface influence the self-diffusion coefficients of IL a lot. And they found that if the interactions between IL and solid interface are not so strong, the self-diffusion coefficients of IL will be enhanced compared with bulk IL. Hung and co-workers41 have also found that the self-diffusion coefficients of IL confined inside graphite slit pore are larger than that of bulk IL. On the other hand, if the interactions between IL and solid interface becometoo strong, the self-diffusion coefficients of IL will decrease compared with bulk IL. From this point, whether the support materials enhance or hinder the mobility of IL mainlydepends on the interactions between IL and support material.

The role of interfaces of IL/solid From the simulation results we have “seen” what happen in nanoscale for CO2 absorption in ILs immobilized on solid surface. Based on above analysis we can find there are three regions of “IL/solid interface region”, “immobilized ILs bulk


Page 22 of 30

Page 23 of 30

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

phase”and “CO2/ILs interface region”as the number density profile in Figure 12.Most CO2 molecules are absorbed at the CO2/ILs interface, but there is also a peak of CO2 absorption density distribution. The reaction layer proposed in experimental work is actually the gas-liquid interface here, and the diffusion layer is IL layer on the support. We calculated the percentage of CO2 in the regionIL/solid interface, immobilized ILs bulk phase and CO2/ILs interface and presented them in the tables of Figure 12. In the left table of Figure 12, the percentage of the number of CO2 inIL/TiO2interfaceincreases with decreasing of thickness of ILs. It indicates the intensity of effect of solid surface on the CO2 absorption is getting more important with the increase of thickness of ILs. We can also learn from this table that for the cases of 2.1nm and 2.8nm, the absorption of CO2 at CO2/ILs interface do not affect by the

Figure 12. Schematic diagram of the role of interfaces of IL/solid.(a) the diffusion coefficient of CO2 for different thickness of IL and distribution of CO2 in different region; (b)the number density profile(taken from figure 4b); (c) the diffusion coefficient of CO2 on different surface and distribution of CO2 in different region; left table: the value of distribution of CO2 in different region for different thickness of IL; right table: the value of distribution of CO2 on different surface

thickness of ILs little. While the absorption of CO2 in immobilized ILs bulk phase


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 30

region decrease dramatically for the case of 2.1nm. We could conclude that if the ILs thickness is too small, the absorption in the IL/solid interface is too strong, and the absorption in the ILs bulk phase is reduced.Considering the self-diffusion coefficients data, we can infer that the increase of ILs and CO2mobility due to decreasing thickness of IL enhance the CO2 absorption and will improve its mass transfer.We need to find an optimal loading of ILs for absorption of CO2 of immobilized IL to ensure high performance and low IL consumption. In the right table of Figure 12, the percentages of the number of CO2 in different support are shown. Thepercentages of the number of CO2 in IL/TiO2interface is more than in IL/graphite. It indicates the interaction from TiO2 is stronger than from graphite. As a result, the percentage of the number of CO2 at CO2/ILs interface is less than that of graphite. The absorption of CO2 in immobilized ILs bulk phase region for TiO2 is more than that of graphite but almost equal to that of non-immobilized.The above analysis shows thatthe graphite sheet cannot enhance the absorption of IL/solid interface greatly, but it can promote the total absorption of immobilized ILs.Considering the self-diffusion coefficients data, we can findthatthe confinement of solid surface improve the mobility of CO2no matter whether the surface is hydrophilic or hydrophobic, and the attraction of hydrophilic TiO2can pull the CO2 towards the solid surface, and will improve its mass transfer. Now we can see the role of the solid-liquid interface is important from simulation results.We can also conclude thatthe solid-liquid interface affect diffusion-reaction compromise for the absorption of CO2.

CONCLUSION In summary,to explore the microscopicmechanism which is remained by experimental work,

this study reports the effect of the thickness of IL

([hmim][Tf2N]) immobilized on TiO2 and the effect of the surface properties (hydrophilicity and hydrophobicity) of solid interfaces on the absorption of CO2.The results

show

that

the

thickness

of

IL ([hmim][Tf2N])


immobilized

on

Page 25 of 30

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

TiO2influencesthe absorption of CO2 both structurally and dynamically. For the thickness of 2.1 nm, the number density of CO2 is the largest in the region near the solid TiO2 surface compared with the thickness of 2.8 nm and 3.5 nm. And the for IL with the thickness of 2.1 nm immobilized on TiO2 surface, its CO2 solubility is also the largest compared with the thickness of 2.8 nm and 3.5 nm.At the same time, the thickness influences the mobility of IL as well mainly because of the interactions between IL and TiO2 surface.The self-diffusion coefficients of cations and anions decrease as the thickness of immobilized IL increases. Besides,our simulation results show that the solubility of CO2 in immobilized IL is larger than the bulk experimental IL under certain temperature and pressure.When studying the effects of the surface properties of solid interfaces, here we mainly focused on the hydrophobicity and hydrophilicity.For the number density profiles of CO2 absorbed in IL with the thickness of 2.8 nm immobilized on TiO2 and graphite, they both have an obvious peak in the region near the solid interface. But for CO2 absorbed in IL immobilized on TiO2, the peak of the number density of CO2 is higher than CO2 absorbed in IL immobilized on graphite. Besides, the solubility of CO2 in IL immobilized on TiO2 surface is larger than that in IL immobilized on TiO2. But the solubility of CO2 in immobilized IL is larger than that in non-immobilized IL.The thickness of IL mainly affects the orientation of CO2. Whether the surface is hydrophobic or hydrophilic, it has smallimpact on the orientation of ILs and CO2 molecules.The results show that for IL with the thickness of 2.8 nm immobilized on graphite, the self-diffusion coefficients of cations and anions are the largest when compared with IL with the thickness of 2.8 nm immobilized on TiO2 and non-immobilized IL. Due to strong interactions between IL and TiO2 surface, the self-diffusion coefficients of cations and anions immobilized on TiO2 surface are smaller than that of non-immobilized IL.But both of the self-diffusion coefficients of CO2 in the IL immobilized on TiO2 and graphite are larger than that of non-immobilized IL.Above all, the thickness of IL immobilized on TiO2 and the properties of solid interfaces (hydrophilicity and hydrophobicity) play an important role in the absorption of CO2 in immobilized IL. We found the role of the solid-liquid interface and its effect on diffusion-reaction


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

compromise for the absorption of CO2 in microscopic which cannot be seen in experiment. We hope this study might inspire further experimental research on immobilized IL to improve the performance of CO2 absorption and reduce the IL consumption. AUTHORS INFORMATION Corresponding Authors * LinghongLu,Emailaddress:[email protected] Notes The authors declare no competing financial interset.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China [21676137,91 334202, and 21490584]. The authors thank Dr. Wei Cao and Shanshan Wang for their constructive suggestions of the research work of this manuscript. REFERENCES (1) Schach, M.-O.; Schneider, R.; Schramm, H.; Repke, J.-U.Techno-Economic Analysis of Postcombustion Processes for the Capture ofCarbon Dioxide from Power Plant Flue Gas. Ind. Eng. Chem. Res. 2010, 49, 2363–2370. (2) Zhang,X.; Zhang, X.; Dong, H.; Zhao, Z.; Zhang, S.; Huang, Y.Carbon capture with ionic liquids: overview andprogress. Energy Environ. Sci.2012, 5,6668-6681. (3) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Post-Combustion CO2Capture Using Solid Sorbents: A Review. Ind. Eng. Chem. Res.2012, 51, 1438–1463 (4) Ren,J.; Wu, L.; Li, B.-G.Preparation and CO2Sorption/Desorption ofN-(3-Aminopropyl)Aminoethyl Tributylphosphonium Amino Acid Salt Ionic Liquids Supported into Porous Silica Particles.Ind. Eng. Chem. Res.2012, 51, 7901-7909. (5) Noble, R. D.; Gin, D. L. Perspective on ionic liquids and ionic liquid membranes. J. Membr. Sci.2011, 369,1-4. (6) Zhang, Z.; Wu, L.; Dong, J.; Li, B.-G.; Zhu, S. Preparation and SO2 Sorption/Desorption Behavior of an Ionic Liquid Supportedon Porous Silica Particles. Ind. Eng. Chem. Res. 2009, 48, 2142–2148.


Page 26 of 30

Page 27 of 30

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

(7) 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. (8) Vicent-Luna, J. M.; Gutiérrez-Sevillano, J. J.; Anta, J. A.; Calero, S. Effect of Room-Temperature Ionic Liquids on CO2Separation by a Cu-BTC Metal–Organic Framework.J. Phys. Chem. C2013, 117, 20762-20768. (9) 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. A2013, 1, 2978-2982. (10)Wang, X.; Akhmedov, N. G.; Duan, Y.; Luebke, D.; Hopkinson, D.; Li, B. Amino acid-functionalized ionic liquid solid sorbents for post-combustion carbon capture. Appl. Mater. Interfaces2013, 5, 8670-8677. (11) Xie, W.; Ji, X.; Feng, X.; Lu, X.Mass-transfer rate enhancement for CO2separation by ionic liquids: Theoretical study on the mechanism. AIChEJ.2015, 61,4437-4444. (12) Xie, W.; Ji, X.; Feng, X.; Lu, X.Mass Transfer Rate Enhancement for CO2Separation by Ionic Liquids: Effect of Film Thickness. Ind. Eng. Chem. Res.2016, 55,366-372. (13) Xie, W.; Ji, X.; Fan, T.; Feng, X.; Lu, X. CO2Uptake Behavior of Supported Tetraethylenepentamine Sorbents. EnergyFuels 2016, 30,5083-5091. (14) Hazelbaker, E. D.; Budhathoki, S.; Katihar, A.; Shah, J. K.; Maginn, E. J.; Vasenkov, S. Combined Application of High-Field Diffusion NMR and Molecular Dynamics Simulations To Study Dynamics in a Mixture of Carbon Dioxide and an Imidazolium-Based Ionic Liquid.J. Phys. Chem. B2012, 116, 9141-9151. (15) Srivastava, R.; Singh, J. K.; Cummings, P. T. Effect of Electric Field on Water Confined in Graphite and Mica Pores.J. Phys. Chem. C2012, 116, 17594-17603. (16) Shi, W.; Sorescu, D. C. Molecular Simulations of CO2 and H2 Sorption into Ionic Liquid1-n-Hexyl-3-methylimidazoliumBis(trifluoromethylsulfonyl)amide ([hmim][Tf2N]) Confined in CarbonNanotubes. J. Phys. Chem. B 2010, 114, 15029–15041. (17) Weber, H.; Salanne, M.; Kirchner, B.Toward an Accurate Modeling of Ionic Liquid–TiO2Interfaces.J. Phys. Chem. C2015, 119,25260-25267. (18) Singh, R.; Monk, J.; Hung, F. R. Heterogeneity in the Dynamics of the Ionic Liquid [BMIM+][PF6-] Confined in a Slit Nanopore. J. Phys. Chem. C2011, 115, 16544-16554. (19) Shi, W.; Luebke, D. R.Enhanced gas absorption in the ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([hmim][Tf2N]) confined in silica slit pores: a molecular simulation study. Langmuir2013, 29, 5563-5572. (20) 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. (21) Bechstein, R.; Gonzalez, C.; Schutte, J.; Jelinek, P.; Perez, R.; Kuhnle, A. 'All-inclusive'


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

imaging of the rutile TiO2(110) surface using NC-AFM. Nanotechnology 2009, 20, 505703. (22) Prˇedota, M.;Bandura, A. V.;Cummings, P. T.; Kubicki, J. D.; Wesolowski，D. J.; Chialvo, A. A.; Machesky, M. L.Electric Double Layer at the Rutile (110) Surface. 1. Structure of Surfaces and Interfacial Water from Molecular Dynamics by Use of ab Initio Potentials. J. Phys. Chem. B 2004, 108, 12049-12060. (23) Marsh, K. N.; Brennecke, J. F.; Chirico, R. D.; Frenkel, M.; Heintz, A.; Magee, J. W.; Peters, C. J.; Rebelo, L. P. N.; Seddon, K. R. Thermodynamic and thermophysical properties of the reference ionic liquid: 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide (including mixtures). Part 1. Experimental methods and results (IUPAC Technical Report). PureAppl. Chem.2009, 81,781–790. (24) Shao, M.;Zhang, F.;Wu, G.;Fang, H.;Wang, C.;Chen, S.;Zhang, Y.;Hu, J. Ordering layers of [bmim][PF6] ionic liquid on graphite surfaces: molecular dynamics simulation.J. Chem. Phys.2008, 128,134504. (25) Merlet, C.; Salanne, M.; Rotenberg, B.; Madden, P. A. Imidazolium Ionic Liquid Interfaces with Vapor and Graphite: Interfacial Tension and Capacitance from Coarse-Grained Molecular Simulations.J. Phys. Chem. C2011, 115,16613-16618. (26) Shi, W.; Maginn, E. J.Atomistic Simulation of the Absorption of Carbon Dioxide and Water in the Ionic Liquid1-n-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]. J. Phys. Chem. B 2008, 112, 2045-2055. (27) Chen, B.; Siepmann, J. I.; Oh, K. J.; Klein, M. L.Aggregation-volume-bias Monte Carlo simulations of vapor-liquid nucleation barriers for Lennard-Jonesium. J. Chem. Phys.2001, 115, 10903-10913. (28) Köppen, S.; Langel, W. Simulation of the interface of (100) rutile with aqueous ionic solution.Surf. Sci.2006, 600, 2040-2050. (29)Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys.1995, 117,1-19. (30) Li, S.; Zhao, W.; Feng, G.; Cummings, P. T. A computational study of dicationic ionic liquids/CO2 interfaces. Langmuir2015, 31, 2447-2454. (31) Gupta, K. M.; Jiang, J. Systematic Investigation of Nitrile Based Ionic Liquids for CO2Capture: A Combination of Molecular Simulation andab InitioCalculation. J. Phys. Chem. C2014, 118, 3110-3118. (32) Rajput, N. N.; Monk, J.; Hung, F. R.Ionic Liquids Confined in a Realistic Activated Carbon Model: A Molecular Simulation Study.J. Phys. Chem. C2014, 118,1540-1553. (33) 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. C2012, 116, 5169-5181. (34)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.2003, 126,


Page 28 of 30

Page 29 of 30

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

5300-5307. (35) Shiflett, M. B.; Yokozeki, A. Solubility of CO2 in Room Temperature Ionic Liquid [hmim][Tf2N]. J. Phys. Chem. B 2007, 111, 2070-2074. (36) Perez-Blanco, M. E.; Maginn, E. J.Molecular Dynamics Simulations of CO2 at an Ionic Liquid Interface: Adsorption,Ordering, and Interfacial Crossing. J. Phys. Chem. B 2010, 114, 11827–11837. (37) Wu, N.；J, X.; Xie, W.; Liu, C.; Feng, X.; Lu, X. Confinement phenomenon on CO2 absorption working capacity in ionic liquids immobilized into porous solid supports. LangmuirSubmitted. (38) Khan, N. A.; Hasan, Z.; Jhung, S. H. Ionic liquids supported on metal-organic frameworks: remarkable adsorbents for adsorptive desulfurization.Chem. Eur. J.2014, 20,376-380. (39) Aparicio, S.; Atilhan, M. Choline-Based Ionic Liquids on Graphite Surfaces and Carbon Nanotubes Solvation: A Molecular Dynamics Study.J. Phys. Chem. C2012, 116, 12055-12065. (40) Singh, R.; Rajput, N. N.; He, X.; Monk, J.; Hung, F. R. Molecular dynamics simulations of the ionic liquid [EMIM+][TFMSI-] confined inside rutile (110) slit nanopores. Phys. Chem. Chem. Phys.2013, 15, 16090-16103. (41) Shen, Y.; He, X.; Hung, F. R. Structural and Dynamical Properties of a Deep Eutectic Solvent Confined Inside a Slit Pore.J. Phys. Chem. C2015, 119, 24489-24500.


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

For Table of Content Only


Page 30 of 30

CO2 Absorption in the Ionic Liquids Immobilized on Solid Surface by

Recommend Documents