Carbon dioxide adsorption on a modified zeolite with sodium dodecyl sulfate surfactants: A molecular dynamics study

Journal Pre-proof Carbon dioxide adsorption on a modified zeolite with sodium dodecyl sulfate surfactants: A molecular dynamics study Minerva Valencia-Ortega, Raúl Fuentes-Azcatl, Hector Dominguez PII:

S1093-3263(19)30463-2

DOI:

https://doi.org/10.1016/j.jmgm.2019.08.003

Reference:

JMG 7426

To appear in:

Journal of Molecular Graphics and Modelling

Received Date: 19 June 2019 Revised Date:

4 August 2019

Accepted Date: 6 August 2019

Please cite this article as: M. Valencia-Ortega, Raú. Fuentes-Azcatl, H. Dominguez, Carbon dioxide adsorption on a modified zeolite with sodium dodecyl sulfate surfactants: A molecular dynamics study, Journal of Molecular Graphics and Modelling (2019), doi: https://doi.org/10.1016/j.jmgm.2019.08.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.

Carbon dioxide adsorption on a modified Zeolite with sodium dodecyl sulfate surfactants: A molecular dynamics study

Minerva Valencia-Ortega, Raul ´ Fuentes-Azcatl and Hector Dominguez1 Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma ´ de M´exico, M´exico, D.F. 04510 Abstract Molecular dynamics simulations are carried out to study adsorption of Carbon dioxide (CO2 ) in a zeolite modified with anionic surfactants at different gas concentrations. Sodium dodecyl sulfate (SDS) surfactant is used and simulations at different SDS concentrations were conducted. The results show that adsorption of the gas is influenced by the amount of SDS on the zeolite surface. In addition, gas retention inside and outside the solid is observed and it strongly depends on the free sodium ions in the zeolite. The most favorable adsorption takes place at low CO2 concentrations with few SDS molecules. Adsorption was studied in terms of density profiles and pair correlation functions and strong interactions of the CO2 molecules with the sodium ions were observed.

Keywords. adsorption; anionic surfactant; carbon dioxide; molecular dynamics; zeolite.

This work was support by DGAPA-UNAM-Mexico grant IN102017; and DGTIC-UNAM grant LANCAD-UNAM-DGTIC-238.

1

[email protected]

1 Introduction. Environmental problems caused by carbon dioxide emissions have been studied for many years, especially in capturing this gas. Nowadays, several works have been conducted to find new materials with good CO2 sorption properties such as lithium ceramics at high temperatures [1–3], activated carbons [4,5], polymeric membranes [6], amines [7–9] and molten salts [10]. Ionic liquids [11], hydrocarbon surfactants [12] and zeolites materials [13] have also been used to capture CO2 . In all of those studies it has been observed how chemical and physical properties of the materials play an important role in gas adsorption. In particular surfactant molecules have proved to be a good alternative not only for gas remediation but also for other type of contaminants [14]. In fact, modified surfaces with surfactants are good options for pollutant capture [15–17], for instance, sodium dodecyl sulfate (SDS) has been used with the MicellarEnhanced Ultrafiltration technique (MEUF) to remove phenol with good results [18]. On the other hand, such complex systems can be investigated with computational methods [14, 19]. In fact, molecular simulations have been tested to study contaminant removal using surfactants [14] on solid surfaces. In particular, sorption of CO2 on solid surfaces have been studied with Monte Carlo methods and good results with actual experiments have been obtained [20, 21]. In addition, CO2 adsorption on a modifiedgraphite surface with surfactants was recently analyzed using molecular dynamics simulations [22]. In the present work adsorption of the CO2 gas in a zeolite, with and without the presence of anionic sodium dodecyl sulfate surfactants, is investigated. We are interested whether the amphiphilic molecules might be used as agents to improve CO2 capture. Thermodynamics and structural properties were used to analyze the results. Graphs of the amount of gas adsorbed in the zeolite as function of the total CO2 molecules or number of surfactant molecules are plotted. Density profiles were also analyzed to study adsorption of the gas.

2 Computational Method and Model. Molecular dynamics simulations were carried out to study CO2 adsorption in a zeolite. We use the AU Na-Y siliceous, Na88 (SiO4 )44 , zeolite with a cubic cell of 2.474 nm in a tetrahedric array of four oxygen atoms in the vertices and one Si atom in the center. The unit cell was replicated to have a zeolite with dimensions of X = 7.422 nm, Y = 4.948 nm and Z = 2.474 nm. Then, 88 Na+ ions were randomly placed inside the solid. The zeolite Lennard-Jones parameters were obtained from the work of Wantanabe et al. [23]. For the CO2 molecule a three sites flexible model, which reproduces better experimental properties, was used [24] and for the SDS surfactant a hydrocarbon chain of 12 united carbons attached to a polar headgroup (SiO− 4 ) was used [25]. Here, the appropriate number of Na+ ions were also used in the sytem to neutralize the SDS headgroup. A first series of simulations were conducted to investigate CO2 adsorption in the zeolite without SDS surfactants. Firstly, a simulation of a bulk gas in the NPT ensemble at temperature of T = 350 K and P = 1 bar was conducted. Then, the equilibrated system was put on the top of the zeolite to start the next simulations in the NVT ensemble (left of figure 1). A second series of simulations were carried out with SDS molecules to study the effect of the amphiphilic molecules in the gas adsorption. In this case, a system composed of 18180 water molecules, with the SPC/E model [26], and SDS surfactants were placed on the zeolite in a simulation box of dimensions X = 7.422 nm, Y = 4.948 nm and Z = 6.0 nm. Then, the whole system was run in the NVT ensemble at T = 298 K until the surfactant molecules were adsorbed on the surface of the solid (right of figure 1). After, water molecules were removed and CO2 molecules, previously equilibrated as described above, were put on the top of the zeolite surface. All simulations were carried out using GROMACS-4.5 [27] software with periodic boundary conditions in X-Y directions only. The final systems were run at temperature T = 350 K in the NVT simulations using the Nos´e-Hoover thermostat with a

relaxation time of τT = 0.1 ps. Electrostatic interactions were handled with the parti˚ whereas cle mesh Ewald method and the short range interactions were cutoff at 20 A bond lengths were constrained using the Lincs algorithm. Then, simulations were performed for 25 ns after 5 ns of equilibration with a timestep of dt = 0.002 ps.

3 Results 3.1 Zeolite structure

Figure 1: Typical snapshots of the CO2 adsorbed in the zeolite without (left) and with SDS surfactants (right). The pictures are for 500 CO2 molecules and 20 SDS molecules. CO2 are in blue, zeolite in red and yellow, surfactants in pink and Na+ ions in green.

Simulations were prepared, as described above, at three different number of gas molecules, N = 500, N = 1000 and N = 2000 in simulation boxes of dimensions of X = 7.422 nm, Y = 4.948 nm and using different lenghts, Z = 14.550 nm, 25.506 nm and 47.512 nm, respectively. It is well known that ion exchange in zeolites might modify their structures and surface

Total CO2 molecules

Na+ diffusion (cm2 s−1 )

500

3.90 x 10−11

1000

1.85 x 10−9

2000

2.75 x 10−5

Table 1: Diffusion coefficients of the Na+ ions in the zeolite as function of the number of CO2 molecules.

properties [28] then, it is relevant to study the behaviour of the Na+ ions in the solid. In figure 2 the position of the ions in the zeolite is characterized by density profiles at different amount of CO2 molecules. Because of the excess of oxygen atoms in the zeolite surface, the negative charges, there is a large population of sodium ions next to the solid surface as it is observed in figures 1 and 2. From figure 2 it is observed that most of the gas molecules remain close to the zeolite surface as indicated by a high peak in the density profiles. However, as the amount of gas increases that peak becomes smaller suggesting that the free sodium ions move inside the zeolite, i.e. the CO2 zeolite interactions displace the sodium ions from the surface of the zeolite into it. The interactions of the Na+ ions with the zeolite were also analyzed in terms of pair correlation functions, g(r). The first g(r) peak indicates that the Na+ ions prefer the oxygens rather than the silica atoms (see figure 3), i.e. the ions are located close to the tetrahedral structure formed by the oxygens. The distances Na-Si and Na-O are calculated ˚ and dN a−O = 0.172 A ˚ were obfrom the g(r) plots and the values of dN a−Si = 0.226 A tained which are not very far from those reported in the crystallography database [29], dN a−Si = 0.331 and dN a−O = 0.203 given errors of 30 % and 15 % , respectively [29]. Finally, for this section, ion mobility is also studied using diffusion coefficients where it is observed that these decrease as the number of CO2 increase (see table 1).

500 CO2 molecules 1000 CO2 molecules 2000 CO2 molecules

-3

Density (kg m )

300

200 Zeolite

Gas

100

0

0

0.5

1

1.5

2 2.5 Z (nm)

3

3.5

4

Figure 2: Density profiles of the sodium ions (Na+ ) at different number of CO2 molecules. The vertical pink line represents the location of the zeolite surface.

3.2 CO2 adsorption In this section simulations at three different number of CO2 molecules, N = 500, N = 1000 and N = 2000, were conducted and distributions of the CO2 molecules in the solid were analyzed. In figure 4 density profiles of the CO2 along the normal to the zeolite surface are shown. There, it is possible to observe two adsorption regions, an “inside” region where gas molecules are in the interior of the solid and an “outside” region where gas molecules are in the exterior surface of the solid. In the figure it is noted that not all CO2 molecules are adsorbed in the zeolite, i.e some gas molecules remain in the bulk phase far away from the solid as shown in the density profiles. The percentage of gas adsorbed, n, in both regions were calculated by integration of the density profiles inside and outside the zeolite (indicated by the green line en figure 4), Rr ρ(z)dz Ni = R 0∞ n= NT ρ(z)dz 0

(1)

Ni and NT are the number of molecules in each region (indicated by the upper limit

70 60 Na-Si Na-O

g(r)

50 40 30 20 10 0

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 r (nm)

1

Figure 3: Pair correlation functions of the sodium ions with the oxygen and silicon atoms in the zeolite.

”r”) and the total number of molecules in the system, respectively.

In figure 5 the percentage of CO2 adsorbed in and out the zeolite, as function of the total gas is shown. Inside the zeolite the CO2 captured in the system is slightly higher using 1000 molecules than that for the other systems. On the other hand, it is observed that the proportion of CO2 adsorbed out the zeolite (on the surface) decreases as the amount of gas increases.

To investigate the zeolite-gas interaction, in figure 6 typical pair correlation functions of the CO2 with the zeolite are plotted. In the figure it is observed that the Na+ ions are the main interaction between the solid and the gas, i.e. higher peaks are observed for the sodium ions with the carbons and oxygens of the CO2 molecules. It is even noted that the first peak of gN a−C is higher than that of the gN a−O , however, for the

600

-3

Density (kg m )

500

Zeolite

Gas 500 molecules of CO2 1000 molecules of CO2 2000 molecules of CO2

400

300

200

100

0

0

1

2

3

4

5

6

7

8

9

10

Z (nm)

Figure 4: Density profiles of the CO2 molecules adsorbed in the zeolite. The vertical green line represents the location of the zeolite surface.

last interaction two hight peaks are observed, it might be due to the two oxygens in the CO2 molecule. On the other hand, the high peak of gN a−C could be explained in terms of the excess oxygens on the surface of the zeolite, because those atoms repel the oxygens of the CO2 molecule allowing the carbon atom of that molecule to be closer to the surface, that is, close to the sodium ions in the surface. It is also worthy mentioning that there are few gas molecules interacting with the zeolite as suggested by the small gSi−O peak at closer distance than the Na-C and Na-O pairs.

3.3 CO2 adsorption with SDS surfactants Another series of simulations were carried out with SDS surfactants on the solid surface to study if modified zeolites improve gas adsorption. The systems were prepared as described above using different number of SDS molecules. From those simulations, it is observed that SDS adsorption on the solid is produced by the headgroups (see

Percentage of Molecules

100 In Out Total

80

60

40

20

0

1000

500

1500

2000

Number of molecules of CO2 Figure 5: Percentage of CO2 adsorbed in the zeolite as function of the total number of gas molecules without SDS surfactants. Blue data are gas molecules in the solid, purple data are the gas molecules out the solid and green data are the total adsorbed CO2 molecules in the solid.

right of figure 1), however only few surfactants, of the total, were adsorbed. The number of SDS molecules adsorbed on the zeolite is shown in table 2. As a general trend it is observed that the percentage of SDS adsorbed on the solid surface decreases as the total amount of surfactant increases. The next simulations were conducted using previous configurations of SDS with CO2 at different number of gas molecules. For those simulations water was removed from the systems and also those SDS molecules that were not adsorbed on the zeolite. To study the CO2 adsorption by the surfactant modified zeolite density profiles were analyzed. In figure 7 those profiles are shown for different number of SDS and CO2 molecules. As the systems without surfactants, CO2 adsorption is depicted inside and outside the zeolite. For systems with few SDS, it is observed gas molecules very close to the external solid surface indicated by the density profile peaks next to the green

8 Na-C Si-C O-C

g(r)

6

4

2

6 Si-O Oz-O Na-O

g(r)

4

2

0

0

1

0.5

1.5

r (nm) Figure 6: Pair correlation functions of the different atoms of the CO2 with the zeolite. Top figure, Carbon (C) and Bottom figure, oxygen (O) atoms of the CO2 molecule with the oxygen (Oz), silica and sodium ions in the zeolite. The plots are for the system with 2000 CO2 molecules.

line in figures 7a and 7b. As the number of SDS increases (figures 7c and 7d), less gas molecules are close to the zeolite external surface however, in all cases it is noted a high peak. As shown in figure 8, the results of the gas adsorbed inside the solid are quite interesting. In the case of low and intermediate amount of gas (500 and 1000 molecules) and low SDS (20 molecules) the number of adsorbed CO2 molecules is slightly higher than that of the other systems with SDS (blue data in figures 8a and 8b). For high amounts of gas, the adsorption inside the zeolite increases as the SDS increases (blue data in figure 8c). When adsorption results are compared with and without SDS it is observed

400

-3

Density (kg m )

c)

500 CO2 1000 CO2 2000 CO2

a) 300

200

100

Zeolite Gas

Zeolite Gas

-3

Density (kg m )

400

d)

b)

300

200

100

Zeolite 0

0

1

2

Gas 3

Gas

Zeolite

4

5

6

7

Z (nm)

8

9

10

1

2

3

4

5

6

7

8

9

10

Z(nm)

Figure 7: Density profiles of CO2 molecules adsorbed in the zeolite for different amounts of surfactant, (a) 20 SDS, (b) 37 SDS (c) 50 SDS (d) 64 SDS, at different number of CO2 molecules. The vertical green line represents the location of the zeolite surface.

that the systems with small amount of gas the internal adsorption is improved by the presence of the surfactants (figure 8a). In the case of the outside adsorption, similar results are depicted for all the systems, i.e. gas adsorption is nearly the same regardless the number of surfactants and it is slightly higher for the systems without SDS or with few SDS molecules (20 molecules), however, it is noted that the adsorption out the solid is always higher than that in the solid. It is noted that for intermediate and high number of gas molecules the total CO2 adsorbed (in and out) in the solid decreases regardless the amount of SDS (green data in figures 8b and 8c). However, for few gas molecules a different feature is noted, the CO2 adsorption is always better with SDS than without SDS surfactants (green data in figure 8a), i.e. as a general trend the best CO2 retention occurs with few SDS and low amount of gas.

Total SDS molecules

SDS adsorbed molecules

(%)

20

20

100%

40

37

92.5%

60

50

83.3%

80

64

80.0%

Table 2: Percentage of SDS molecules adsorbed on the zeolite surface.

In figure 9 typical plots of the pair correlation function of the CO2 with the SDS and the zeolite are shown (carbon of the CO2 is used for the plots). In the figure is depicted that the strongest attraction comes from the CO2 with the sodium ions (the sodium of the zeolite) indicated by the high first peak in the g(r). It is also noted a strong attraction of the CO2 with the SDS tails as shown by the g(r), although the interaction of the CO2 with the zeolite atoms is weak. Then, the results suggest that the gas retention in the zeolite occurs mainly by the ions and the SDS tails.

3.4 Conclusions Studies of CO2 adsorption in a zeolite were investigated with and without the presence of anionic surfactant (SDS) molecules. Exhaustive molecular dynamics simulations were conducted at different gas and SDS number of molecules and it was observed gas retention inside and outside the solid. The results indicate that the principal attraction of the CO2 molecules come from the free ions in the zeolite, therefore, since most of the sodium ions are deposited close to the surface the main adsorption occurs out the zeolite, i.e. next to the surface. The simulations without surfactants show that the zeolite itself cannot adsorb all of the CO2 and adsorption does not improve with large amounts of gas molecules, in fact it decreases. Simulations with different amounts of SDS surfactants were carried out and it was observed that many SDS saturate the zeolite surface preventing much CO2 from being adsorbed by the solid. However,

100

a)

80 60 40

Percentage of CO2

20 0 100

b)

80 60 40 20 0 100

In Out Total

c)

80 60 40 20 0

0

10

20

30

40

50

60

70

Number of SDS Figure 8: CO2 adsorption, inside and outside the zeolite, as function of the amount of SDS. a) For 500, b) 1000 and c) 2000 CO2 molecules.

using few CO2 the retention of the gas is complete i.e one hundred percent of the CO2 was adsorbed by the solid. Therefore, as a general conclusion, CO2 adsorption in the zeolite is improved by the presence of the anionic surfactant if the amount of gas is low. Finally, the results show that sodium ions play an important role in the adsorption process, so it is possible that the presence of other ions could improve gas retention in the same zeolite, those simulations are currently carried out to make comparisons with these data.

Figure 9: Pair correlation functions of the carbons in the CO2 molecule with the surfactant and the zeolite sites (Si and Oz). The plots are for the system with 2000 CO2 and 20 SDS molecules.

4 Conflict of interest No conflict of interest.

5 Acknowledgments The authors acknowledge support from DGAPA-UNAM-Mexico grant IN102017 and DGTIC-UNAM grant LANCAD-UNAM-DGTIC-238 for the supercomputer facilities. We also acknowledge Alberto Lopez-Vivas, Alejandro Pompa and Cain Gonzales for technical support. RFA acknowledges postdoctoral scholarship from DGAPA-UNAM.

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of

Zeolite

Structures.

SC/framework.php?STC=FAU

http://europe.iza-structure.org/IZA-

Figure 10: Graphical Abstract.

SDS surfactant-modified-zeolites are good surfaces for CO2 adsorption Low SDS surfactant concentrations show better CO2 adsorption Molecular dynamics show that CO2 adsorption occurs by sodium ions and surfactant tails