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Competitive Adsorption of CO2 over N2 in Asphaltene Slit Nanopores Studied by Molecular Simulation Haoyang Sun, Hui Zhao, Na Qi, Kai Zhang, Qiaozhi Wang, and Ying Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02656 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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Energy & Fuels

1

Competitive Adsorption of CO2 over N2 in Asphaltene Slit

2

Nanopores Studied by Molecular Simulation

3

Haoyang Sun, Hui Zhao, Na Qi, Kai Zhang, Qiaozhi Wang and Ying Li*

4

Key Laboratory of Colloid and Interface Chemistry of State Education

5

Ministry, Shandong University, Jinan, Shandong 250100, P. R. China

6 7 8 9 10 11 12 13 14

Corresponding author:

15

Ying Li

16

Tel: (86) 0531-88362078

17

Email: [email protected]

18

1

ACS Paragon Plus Environment

Energy & Fuels

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ABSTRACT

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CO2 capture and sequestration (CCS) is recognized as one of the most promising

21

alternatives to weaken the greenhouse effect, and the nanoporous materials are

22

regarded as the promising candidates, therefore to develop new cost-effective sorbent

23

to achieve CCS is crucial. In this study, asphaltene-based slit-nanopores were used to

24

simulate capturing of CO2 from flue gas. The grand canonical Monte Carlo (GCMC)

25

and molecular dynamics (MD) simulation methods were employed to examine the

26

microscopic behaviors of CO2 and N2 in asphaltene slit-nanopores. The isosteric heat

27

of CO2 and N2 molecules adsorbed in asphaltene slit-nanopores, and the adsorption

28

energy of single molecule of CO2 and N2 adsorbed on the surface of asphaltene

29

fragments, and the self-diffusion of CO2 and N2 molecules adsorbed in asphaltene

30

slit-nanopores were examined. Strong competitive adsorption of CO2 over N2 is found

31

in a broad range of temperature and pressure, and it was found that the temperature

32

plays an important role on the competitive adsorption. This work demonstrates how

33

the competitive adsorption of CO2 over N2 happened in asphaltene slit-nanopores,

34

which not only enrich the theoretical knowledge about gases behaviors in asphaltene,

35

but also give out the feasibility that the asphaltenes or asphaltene-based materials

36

might be an interesting candidate for capturing CO2 from flue gas.

37

KEYWORDS: adsorption; flue gas; asphaltene; CO2 capture; simulation

38 39

2


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Energy & Fuels

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1. INTRODUCTION

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Facing the serious global climate change that affected by the emission of carbon

42

dioxide (CO2) into atmosphere, the CO2 capture and sequestration (CCS) has got great

43

attentions.1-6 The rapidly increasing emission of CO2 is mainly generated from the

44

combustion of fossil fuels consumed in manufacturing industries and power

45

generation plants. Flue gas is one of the main exhaust gases that constituted by CO2

46

and N2 in a ratio of 15:85,7-8 hence capturing CO2 from flue gas is one of the key

47

processes in CCS.9

48

Recently, studies about the CO2 capture from flue gas by using various sorbents

49

have been extensively investigated both in experimental and computer simulations, of

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which the theoretical and simulation studies could provide more detailed

51

micro-information about the mechanism of CO2 capture.10-12 The solid nanoporous

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materials, such as zeolites, organic frameworks (MOFs), etc., were always regarded as

53

the preferable choice with the advantages of high efficiency and easy to get

54

reusability.9, 13 However, these solid sorbents with specific crystal structures are still

55

not easy to produce, and the cost is not low. Therefore, more and more common solid

56

nanoporous materials, including the nanoporous carbons (NPCs), inorganic minerals,

57

etc., were tried to achieve the CO2 capture recently, which with the obvious advantage

58

of cost-effective.8,

59

capture is still a meaningful topic with high potential.

14-16

Developing new cost-effective sorbent to achieve the CO2

60

Asphaltenes are the common byproduct of the petroleum and coal industries,

61

which can be defined as the fraction of a carbonaceous mixture with the ill-defined 3


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polydisperse of structures and chemical composition.17-18 The “island” architecture

63

with single polycyclic aromatic hydrocarbon (PAH) was widely accepted by

64

researchers to date,19-20 and the molecular structures of some asphaltene samples with

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the architecture of island had been clearly examined by using the scanning tunnelling

66

microscopy (STM) and atomic force microscopy (AFM).21-22 The asphaltenes can be

67

classified into various types as derived from different sources, including the

68

petroleum asphaltenes (PAs), coal-derived asphaltenes (CDAs) and immature source

69

rock asphaltenes (ISAs), which have different H:C ratio but similar solubility

70

characteristics.20 In addition to the extensive studies about the natural characteristics

71

of asphaltenes, such as the density, solubility, structure, composition and the

72

aggregate properties of asphaltenes,20,

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applications of asphaltenes are relative rare.

23-28

investigation about the extended

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In this study, the competitive adsorption of CO2 over N2 in mimic asphaltene

75

slit-nanopores were investigated. The adsorption density and self-diffusion properties

76

of CO2 and N2 molecules in asphaltene slit-nanopores are examined. And the isosteric

77

heats of gases in nanopores and adsorption energy of single gas molecule on

78

asphaltene fragments surface are also investigated. Strong competitive adsorption of

79

CO2 over N2 in asphaltene slit-nanopores is found, which hints the feasibility of being

80

used to capture CO2 from flue gas. This work provides some novel feasible way by

81

using nanoporous asphaltene-based materials to capture CO2 from flue gas.

82

2. MODELS AND METHODOLOGY

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2.1 Atomistic Models 4


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Energy & Fuels

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Figure 1. (a) Model of the asphaltene slit-nanopore with two asphaltene matrices (the

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surface landscape is in blue), (b) pore size distribution of the asphaltene matrix.

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The model of asphaltene slit-nanopore with the pore width of ~25 Å was

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constituted by two asphaltene matrices (Fig. 1a), and each asphaltene matrix was

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constructed by various fragments of asphaltenes, including the petroleum asphaltenes

90

(PAs) fragments, the coal-derived asphaltenes (CDAs) fragments and the immature

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source rock asphaltenes (ISAs) fragments as described in the study of Wang et al..20

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The asphaltene matrix was constructed by using the Amorphous Cell program

93

package of Materials Studio (MS),29 with the density of 1.20 g/cm3 and H/C ratio of

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1.28. And the matrix was minimized by the process of annealing dynamics from 300

95

to 1000 K with the temperature cycle.30 The optimal configuration of asphaltene

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matrix is shown in Fig. 1b with the dimension of 35 × 35 × 26 Å3, and the matrix

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was amorphous with the pore size distribution mainly around ~2.3 Å due to the

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irregular combination of the asphaltene fragments. And according to the kinetic

99

diameters of CO2 (3.3 Å) and N2 (3.64 Å),31 few CO2 and N2 molecules can be 5


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adsorbed inside the intrinsic pores of the matrix. The whole sorbent was described by

101

the COMPASS32 force field and which was regard as rigid during the simulations. For

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gases, a three-site model was used to describe the CO233 molecule and a two-site

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model was used to describe the N234 molecule, the atomic charges and bond lengths

104

were list in table 1. Table 1. Atomic charges and bond lengths of CO2 and N2.

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Molecule

Atom

Charge (e)

C

0.576

O

-0.288

N

0

CO2

N2 106

Bond length (Å)

1.180

1.102

2.2 Simulation Details

107

The COMPASS32 force field was used to describe the entire simulation, and the

108

non-bond interactions were constituted by the van der Waals (vdW) and electrostatic

109

potentials. As described in our previous work,35 the GCMC method was employed to

110

investigate the adsorption properties of CO2 and N2 in asphaltene slit-nanopores by

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using the Sorption program package of MS. The acceptance or rejection of a trial

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move was described by the Metropolis algorithm.36 The vdW interaction was

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described by the Lennard-Jones 9-6 potential, and the electrostatic interaction was

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examined by the Coulombic term. And each equilibration and calculation process has

115

5 × 106 steps. Meantime, the self-diffusion properties of CO2 and N2 in asphaltene

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slit-nanopores were examined by the MD method by using the Forcite program

117

package of MS. The NVT ensemble with the temperature thermostat of Nosé-Hoover

6


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118

was employed to examine the MD process. And each MD simulation process had a

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total run time of 5.0 ns with a time step of 1 fs, and the last 1.0 ns was used for

120

analysis. For the single molecule of CO2 and N2 adsorbed on the surface of asphaltene

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fragments, the Dmol3 program package was used to calculate the minimum energy

122

configuration.37

123

Perdew-Burke-Ernzerhof (PBE) were employed to describe the exchange-correlation

124

interaction,38 and the density functional semicore pseudopotential (DSPP)39 method

125

with the localized double-numerical basis with a polarization (DNP) functional was

126

employed, according to our previous work about the micro-behaviors of CO2 and CH4

127

molecules in kerogen slit-nanopores.35

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3. RESULTS AND DISCUSSION

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3.1 Competitive Adsorption of CO2 over N2 in Asphaltene Slit Nanopores

The

generalized

1.5

1.2 1.1 1.0 0.9

(GGA)

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

0.24 0.21 0.18

and

(b)

273 K 298 K 323 K 348 K 373 K

0.27

0.0

0.15 0.12 0.09 0.06 0.03 0.00

0

130

approximation

0.30

Adsorption Loading (mmol/g)

1.3

gradient

(a)

273 K 298 K 323 K 348 K 373 K

1.4

Adsorption Loading (mmol/g)

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

Energy & Fuels

200

400

600

800

1000

0

200

fugacity (kPa)

400

600

800

1000

fugacity (kPa)

131

Figure 2. Adsorption isotherms of CO2 (a) and N2 (b) as binary mixtures in asphaltene

132

slit-nanopores at various temperatures.

133

The absolute adsorption isotherms of CO2 and N2 as binary mixtures in

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asphaltene slit-nanopores got by the mixture GCMC method were shown in Fig. 2. It

135

is found that the adsorption amount of CO2 and N2 increases gradually with the 7


Energy & Fuels

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increasing pressure, and the temperature has negative influences on the adsorption

137

capacity of gases in asphaltene slit-nanopores. While the capacity of CO2 uptake itself

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in asphaltene slit-nanopores is weaker comparing with that in nanoporous carbons,

139

which is mainly attributed to the effects of the functionalization of the carbon

140

fragments that enhancing the adsorption amount of CO2.8 And as it was shown in Fig.

141

2, the CO2 has stronger adsorption capacity with the adsorption amount about 10

142

times larger than N2 in asphaltene slit-nanopores at the pressure of one atmosphere,

143

which indicates that the CO2 molecules have stronger adsorption interactions with the

144

asphaltenes comparing with N2. 35

30

Selectivity (CO2/N2)

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273 298 323 348 373

25

20

K K K K K

15

10

5

0 0

100

200

300

400

500

600

700

800

900

1000 1100

fugacity (kPa)

145 146

Figure 3. Selectivity of CO2 over N2 in asphaltene slit-nanopores at various

147

temperatures; y-CO2 (bulk phase) = 0.5.

148

According to the different adsorption intensity between the CO2 and N2

149

molecules in asphaltene slit-nanopores that mentioned above, the selectivity

150

parameter (S) was employed to describe the competitive adsorption of CO2 over N2 as

151

binary mixture in asphaltene slit-nanopores, with the following equation,34

152

S=

x CO 2 / x N 2 y CO 2 / y N 2

(1)

8


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in which x is the fraction of gas component in the adsorbed phase, y is the fraction of

154

gas component in the bulk phase. As shown in Fig. 3, it can be found that the

155

competitive adsorption of CO2 over N2 in asphaltene slit-nanopores is broadly

156

occurred. And the capacities of the competitive adsorption of CO2/N2 are large at the

157

initial low pressures and decrease with the increasing pressure, it is because more

158

active sites are exist at the low pressure condition (equal to low adsorption loading)

159

and which decreases with the increasing loading, therefore further adsorbed gases

160

molecules have to adsorb and locate at the less favored sites, leading to the decrease

161

of SCO2/N2 with the enlarged pressures.34 From Fig. 3, it also can be found that the

162

temperature has obvious influences on the competitive adsorption of CO2 over N2, and

163

the SCO2/N2 decreases with the increasing temperatures.

0.010 0.009

CO2

0.008 3

Density (molecule/Å )

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

Energy & Fuels

0.007

0.1 MPa 0.5 MPa 1 MPa

(a)

N2 0.1 MPa 0.5 MPa 1 MPa

0.006 0.005 0.004 0.003 0.002 0.001 0.000 -40 -35 -30 -25 -20 -15 -10 -5

0

5

10

15

20

25

30

35

40

Z (Å)

164 165

Figure 4. (a) Adsorption density profiles of CO2 and N2 as binary mixtures in

166

asphaltene slit-nanopores at various pressures and T = 323 K, (b) the corresponding

167

snapshot at 0.5 MPa and 323 K; Z = 0 refers to the center of the pore and y-CO2 (bulk

168

phase) = 0.5.

169

The distribution of adsorption density of the binary mixed CO2 and N2 in 9


Energy & Fuels

170

asphaltene slit-nanopores at different pressures and 323 K is shown in Fig. 4, it can be

171

found that the CO2 molecules prefer to adsorb close onto the pore surface, while the

172

N2 molecules mainly adsorb at the central part of the pore, and the density of both

173

increases with the enlarged pressures. From Fig. 4b which can be seen that clearly, the

174

surface sites of asphaltene slit-nanopore are almost occupied by the CO2 molecules,

175

the N2 molecules mainly located at the central part of the pore and adsorption amount

176

of N2 is low at the condition of 0.5 MPa and 323 K. It further demonstrates that the

177

CO2 molecules have stronger adsorption interactions with the asphaltene

178

slit-nanopores comparing with N2.

179

3.2 Isosteric Heats of CO2 and N2 in Asphaltene Slit Nanopores 10

isosteric heat (kcal / mol)

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9

CO2 adsorbed in asphaltene slit-nanopores

8

N2 adsorbed in asphaltene slit-nanopores

7 6 5 4 3 2 1 0 0

200

400

600

800

1000

fugacity (kPa)

180 181

Figure 5. Isosteric heats of CO2 (black line) and N2 (red line) in asphaltene

182

slit-nanopores as the variation of adsorption pressure at T = 323 K.

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To further investigate the different adsorption intensity between CO2 and N2

184

molecules in slit-nanopores of asphaltene, the isosteric heat (Qst) was employed to

185

perform the adsorption capacity of gases in nanopores, which expressed as,29

186 187

= −

〈 〉〈 〉〈 〉 〉〈 〉 〈

(2)

where is the gas constant, T is the temperature, is the adsorption energy and 10


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Energy & Fuels

188

is the number of adsorbate. The Qst of CO2 and N2 in asphaltene slit-nanopores at

189

323 K are shown in Fig. 5, it is found that the Qst value of CO2 is much larger than N2

190

in asphaltene slit-nanopores, which indicates that the CO2 molecules have stronger

191

adsorption interactions with the asphaltene slit-nanopore than N2 molecules. Both the

192

Qst of CO2 and N2 decrease with the enlarged pressure, it is because the favorable

193

adsorption sites decrease with the increasing adsorption loading.29

194

3.3 Adsorption Energy of Single Molecule of CO2 and N2 on Asphaltene

195

Fragments

196

197 198

Figure 6. Stable adsorption configurations (side view (up) and top view (down)) of

199

CO2 (a−d) and N2 (e−h) molecule adsorbed on the surface of asphaltene fragments.

200

The detailed adsorption properties of single CO2 and N2 molecule adsorbed onto

201

the basic unit of asphaltene molecule were examined by the adsorption energy (Eads) 11


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202

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with the follow equation,40

Eads = Ea+s – Es – Ea

203

(3)

204

in which Ea is the energy of the gas species, Es is the energy of the asphaltene

205

fragment, and Ea+s is the total energy of the gas molecule adsorbed on the surface of

206

asphaltene fragment. As shown in Fig. 6, four types of the asphaltene fragments were

207

chosen to investigate the Eads between the CO2 and N2 molecule adsorbed on the

208

surface of asphaltene fragments. It is found that the Eads of CO2 is weaker than N2 on

209

each asphaltene fragment, which further demonstrate that the CO2 molecule has

210

stronger adsorption interaction with the asphaltene fragments than N2. And according

211

to the adsorption sites of CO2 and N2 molecule on the surface of asphaltene fragments,

212

it can be found that the CO2 molecules almost parallel to the surface of asphaltene

213

fragments and choose the bridge site of the C-C bond as the favorable adsorption site,

214

which is consistent with our previous work about the CO2 molecule adsorbed on the

215

surface of kerogen fragments.35 The N2 molecules also prefer to parallel to the surface

216

of asphaltene fragment with no obvious favorable adsorption sites, which mainly

217

attributed to the weak interactions between the N2 molecules and the asphaltene

218

fragments.

219

3.4 Self-diffusion of CO2 and N2 in Asphaltene Slit Nanopores

12


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8

CO2 adsorbed onto the asphaltene pore surface

7

N2 adsorbed onto the asphaltene pore surface

6 5

2

Ds (10 cm /s)

4

-4

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Energy & Fuels

3 2 1 0

273

298

323

348

373

T (K)

220 221

Figure 7. Diffusion coefficients of CO2 (black symbol) and N2 (red symbol) in

222

asphaltene slit-nanopores as a function of temperature (T) at P = 0.1 MPa.

223

The microscopic diffusion properties of CO2 and N2 molecules adsorbed inside

224

the asphaltene slit-nanopores were performed by the self-diffusion coefficients (Ds) of

225

gases that expressed as,41

226

Ds =

1 d n 2 〈 ri (t ) −ri (0) 〉 ∑ lim 6 t →∞ dt i

(4)

227

where ri(t) is the position of the center of molecule i at time t, and ri(0) is the initial

228

position. Gas molecules adsorbed close onto the pore surface were marked to

229

calculate the Ds, as shown in Fig. 7. It can be found that the Ds of CO2 is much

230

weaker than N2 in asphaltene slit-nanopores due to the stronger adsorption

231

interactions between the CO2 molecules with the asphaltene nanopore. And from Fig.

232

7, it also can be found that the temperature has obvious influences on the

233

self-diffusion of CO2 and N2 molecules, the Ds increases with the temperature

234

increasing. The differences of Ds between CO2 and N2 molecules in asphaltene

235

slit-nanopores demonstrate that the CO2 molecules adsorb more strongly inside the 13


Energy & Fuels

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236

asphaltene slit-nanopores, which further verify that the CO2 molecules have stronger

237

adsorption interactions in asphaltene slit-nanopores comparing with N2 moleucles,

238

and ensures the occurrence of the competitive adsorption of CO2 over N2 in

239

asphaltene slit-nanopores.

240

4. CONCLUSION

241

This study demonstrates the competitive adsorption of CO2 over N2 in asphaltene

242

slit-nanopores by

molecular simulations.

The

microscopic adsorption

243

self-diffusion properties of CO2 and N2 as binary mixtures in asphaltene

244

slit-nanopores are revealed. The isosteric heat and adsorption energy of CO2 and N2 in

245

asphaltene slit-nanopores and on asphaltene fragments are also examined, respectively,

246

to verify the competitive adsorption of CO2 over N2. This work is an exploration

247

about using asphaltene as a candidate capturing CO2 from flue gas, which provides a

248

promising way to achieve CCS by using the asphaltene-based nanoporous materials.

249

AUTHOR INFORMATION

250

Corresponding Author:

251

*Email: [email protected]

252

Tel: (86) 0531-88362078

253

Notes

254

The authors declare no competing financial interests.

255

ACKNOWLEDGEMENTS

256

The funding of National Science Fund of China (No. 21473103 and 61575109) are

257

gratefully acknowledged. 14


and

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in Asphaltene Slit Nanopores Studied by Mole - American Chemical

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