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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Molecular Modeling of Univalent Cation Exchange in Zeolite N Vinuthaa Murthy, Monireh Khosravi, and Ian Donald R. Mackinnon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12241 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 29, 2018

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The Journal of Physical Chemistry

Molecular Modeling of Univalent Cation Exchange in Zeolite N Vinuthaa Murthya*, Monireh Khosravib, and Ian D R Mackinnonb a

School of Psychological and Clinical Sciences, Charles Darwin University, Darwin, NT 0909, Australia.

b

Institute for Future Environments and Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia

CORRESPONDING AUTHOR *Vinuthaa Murthy Yellow 1, Casuarina Campus School of Psychological and Clinical Sciences Faculty of ENGINEERING HEALTH SCIENCE & ENVIRONMENT Charles Darwin University Darwin, NT 0909 Australia Telephone: +61 415667968 email: [email protected]

Email addresses of authors: *Vinuthaa Murthy: [email protected] Monireh Khosravi: [email protected] Ian Mackinnon: [email protected]

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ABSTRACT Molecular dynamics (MD) simulations are used to investigate the hydration energy and ion exchange properties of a synthetic zeolite, Zeolite N with composition |K10(H20)8Cl2| [Al12Si12O40]. The exchange of K+ ions with univalent ions such as NH4+, Na+, Rb+ and Cs+ is investigated under a range of simulation conditions using a three dimensional membrane in an electrolyte box containing explicit water molecules. Hydration energy calculations indicate that Zeolite N prefers eight water molecules per cage which is consistent with X-ray and neutron diffraction determination of the structure. Ion density profiles and calculated self-diffusion coefficients show that univalent ion exchange by Zeolite N is selective towards NH4+ in preference to other ions. The methodology used here to simulate the uptake of ions from an electrolyte within the zeolite N membrane produces results that are consistent with experimental data and implements a low computational overhead.

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1. Introduction Zeolites are widely used in industrial processes including catalysis1-3, gas separation4-6 and ion exchange7-8. Early applications relied on systematic experiments based on knowledge of the framework structure and composition9-10 to validate or infer likely success with catalysis, separation or exchange functionality. With the advent of powerful computational tools, including molecular and atomistic modeling of complex structures, structural and compositional data are used to accurately predict zeolite behavior under specific conditions11-12 . For example, the dynamics of exchange between Na+ and Ag+ ions in synthetic Zeolite A (LTA-type) in contact with an electrolyte solution calculated with Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations conform closely with experimental data over a range of temperatures and concentration gradients13. Other simulations have successfully explored the cation selectivity in Zeolite A

14

and flow of water through Zeolite A

membranes with application of pressure for hydrophyllic and hydrophobic surfaces15. For more complex structures, DFT calculations have been used to determine the site preferences for Ca+2 and K+ in the natural zeolite clinoptilolite16. DFT methods have also been used to estimate the stability of the clinoptilolite structure after various levels of de-alumination by acid treatment17-18. In these cases, the dominant exchangeable cation within the zeolite interstices is sodium. These successes stimulated our interest in evaluating the exchange kinetics of univalent ions in the potassium-rich zeolite N. The structure of zeolite N determined by Christensen and Fjellvag19-20 is orthorhombic (Space Group I222), classified as an EDI framework structure with end-member composition |K10(H20)8Cl2| [Al12Si12O40]. Potassium is known to exchange with NH4+ and Na+ while the structure shows high selectivity towards NH4+ over other cations including divalent ions7, 21. Synthesis of this zeolite has been undertaken at both ambient and hydrothermal conditions from a range of source aluminosilicates such as kaolin22-24, montmorillonite22 meta-kaolin24-25 and zeolite23. A detailed understanding of zeolite N ion exchange properties is of interest as it shows high potential for a range of applications that require control of nitrogen-rich nutrients in the environment6, 8. 3



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MD simulation is widely recognized as a robust tool that can provide atomic level insight into the distribution, exchange and mobility of ions and water in a zeolite frame work. MD simulations have been conducted to study ion exchange dynamics by Salmas et al.13, Murad et al.26-27 and Nalaparaju et al.28 on several zeolite membranes. While Murad et al.26-27 have used explicit water molecules on NaA and Na-ETS-10 membranes, Salmas et al.13 and Nalaparaju et al.28 have used implicit solvent methods with a dielectric continuum model for ionic solutions on LTA and Na-ETS-10 membranes, respectively to reduce computational costs. In this study, we report ion exchange processes in hydrated zeolite N with explicit water molecules. We also develop a model for a slab of zeolite N containing eight unit cells with explicit representations of water within the structure and on either side of the slab. This approach allows an evaluation of hydration energy and the relative diffusion of ions and water into and out of the framework structure to the surrounding electrolyte. We use non-equilibrium MD calculations to investigate the transport of water molecules and ions within, into and out of a thin membrane of zeolite N driven by chemical potentials on either side of the framework slab.

2. Methods The initial structural parameters are from a synchrotron X-ray study of Zeolite N by Christensen and Fjellvag19. The unit cell structure shows a slight orthorhombic distortion with lattice parameters a =9.9041 Å, b = 9.8860 Å and c =13.0900 Å. The zeolite N framework is similar to edingtonite in which SiO4 and AlO4 tetrahedra form an ordered framework as shown in Figure 1. The zeolite N framework is constructed of α-cages by the sharing of eight-membered rings in which the corner sites are alternatively occupied by Si and Al atoms. This arrangement results in complete Si/Al ordering with Si/Al = 1. Ion exchange processes in hydrated zeolite N with explicit water molecules are evaluated by MD simulations with an ab initio force-field and DFT generated partial charges on all atoms.

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Figure 1: Zeolite N (2x2x2) supercell viewed along (a) [001] and (b) [110]. Yellow represent Si atoms (or silica tetrahedral), pink = Al (or alumina tetrahedral), red = oxygen, white = hydrogen, lilac = potassium and light green = chlorine.

2.1 Geometry Optimisation and Ionic Charge MD simulations are performed using Forcite in Materials Studio (MS) 8.1 and MS 201729-30. COMPASS Force Field31, which is a general ab initio force-field, is used for all geometry optimizations and MD simulations. COMPASS is a high-quality general force field that consolidates parameters for organic and inorganic materials. COMPASS force field is assigned to all atoms in the zeolite N framework, extra framework ions and water molecules. The flexible SPC water model incorporated in COMPASS is used for water molecules. Partial charges on all atoms of zeolite N are calculated by periodic DFT methods. Initially, calculations are performed with extra framework ions (e.g. K+, Cl-) and water molecules removed from the structure. With this configuration, the total charge of the unit cell framework is set to -10 electrons and geometry optimization with population analyses is obtained using the GGA/PW91 functional. This calculation includes an all-electron, double numerical basis set with d functions (DND 3.5, comparable to a 5



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Gaussian 6–31G* basis set) on all non-hydrogen atoms. The SCF convergence criterion is set at an energy change of 10-5 Hartree (Ha). The convergence criterion for optimal geometry based on the energy, force and displacement convergence, are 2x10-5 Ha, 0.004 Ha/Å and 5 x10-3 Å, respectively. Charge calculations are performed with the DMol3 program in Materials Studio (MS) 8.129-30. The geometry optimized unit cell is cleaved and capped with –OH groups on both surfaces along the (001) plane with a vacuum slab of 5 Å. The cell is optimized by DFT using the constraints given above to obtain the partial charges of O and H atoms at the surface. The Mulliken atomic charges are determined from population analysis of DMol3 calculations. The partial charges used along with the COMPASS force field types selected for all atoms in this study are given in Table 1. Table 1: Partial charge and COMPASS FF atom types used on all atoms Atom

COMPASS FF atom type

Charge Zeolite N

Si

si4z

1.54

Al

al4z

1.11

Obulk

o2z

-0.912

Osurface

o2*

-0.701

Hsurface

h1o

0.245

N in NH4+

n4+

-0.783

H in NH4+

h14

0.446

H in H2O

h1o

0.410

O in H2O

o2*

-0.820

+

K

k+

1.00

+

Na

na+

1.00

Rb+

rb+

1.00

+

cs+

1.00

Cs

2.2 Molecular Dynamics of Zeolite N membrane The hydration energy and ion exchange process parameters for zeolite N while submerged in water and/or electrolyte solution, are determined for a membrane using the following approach. A 2x2x2 supercell is created, cleaved along the (001) plane (with a vacuum slab of 20 Å on either side). Terminal 6 ACS Paragon Plus Environment

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Al and Si atoms are then capped with –OH groups. Thus, a zeolite N membrane as shown in Figure 2 and labelled as “ZM” is generated. Initially, water molecules with a density of 1g/cm3 are added to the Water Layer 1 (WL1) and Water Layer 2 (WL2) regions of the model. In subsequent calculations, the numbers of water molecules and ions are varied to create a chemical potential inside and outside of the zeolite membrane. For all atoms, calculated partial charges determined by the method described in 2.1 and listed in Table1 are used.

Figure 2: Zeolite N membrane (ZM) in water (water layers, WL1 and WL2). Yellow represent Si atoms, pink = Al, red = oxygen, white = hydrogen, lilac = potassium and light green = chlorine.

Prior to MD simulations, geometry optimization of the zeolite membrane in water is carried out. Electrostatic interactions are calculated by the Particle-Particle Particle-Mesh method (PPPM) and van der Waals forces are determined by the Ewald summation method with a cut-off distance of 12 Å and minimizations carried out by a Quasi-Newton procedure. Periodic boundary conditions are applied in three dimensions so that the simulation cell is effectively repeated infinitely in each direction. Initially, the zeolite framework is held rigid to allow the extra-framework species (ions and water molecules) to vary and to optimize to a minimum with respect to each other. These optimized structures are then used as the starting configurations for MD simulations, performed in the NVT-ensemble (constantvolume/constant-temperature) where framework and extra-framework atoms are released. Since all atoms in each system are completely free to move during these simulations, use of the constant-volume 7



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model with a fixed cell shape does not introduce significant limitations to the resulting zeolite structure, nor to the dynamics and energetics of water and ion exchange. The structure of the framework is found to be quite stable during simulations (the change in bond distances are less than 0.05 Å). The small changes to the structure of the zeolite framework during simulation are shown in Figure S1 in the supplementary material.

MD simulations performed in NVT-ensemble at 298 K use a time step of 1.0 fs for all simulations. Analysis of these simulations reveals that equilibrium values for the thermodynamic parameters are generally achieved within the first 20 ps using an Andersen thermostat. An MD simulation of 30 ps using the Andersen thermostat is initiated and then followed by 500 ps and 8 ns simulations with the Nosé-Hoover-Langevin (NHL) thermostat for different hydration states, n, of the system.

2.2.1 Hydration Energy The hydration energy, ∆UH is a measure of the preferred hydration state(s) for the zeolite framework and is defined by the equation:

∆U H (Nw) =

U H (Nw) − U (0)

Nw

(1)

where, Nw is the number of water molecules and U(Nw) and U(0) are the total potential energies of the system with Nw and zero water molecules, respectively. For the calculation of ∆UH, we have used a 2x2x2 super cell for the bulk crystal and a 2x2x2 slab for ZM. Initially, simulations of 2 ns are run with extra framework ions (e.g. K+, Cl-) originally present in the crystal structure but water molecules are removed from the structure. Water molecules are then randomly added to the center of the supercell incrementally (for both bulk and ZM) and allowed to equilibrate by running simulations of the same duration. We have calculated the hydration energy of the ZM by creating a vacuum of 15 Å on either side of the ZM slab for water molecules to flow out. Water

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molecules are added to the central section of the membrane incrementally and allowed to transfer out to the surrounding water box over 2 ns periods for each simulation.

2.2.2 Ion Exchange The zeolite N membrane consists of 80 Al and Si atoms, 128 water molecules (8 H2O /cage), 96 K+ ions and 16 Cl- ions with a charge of -80e on the framework. To create a chemical potential between the inside and outside of the membrane and to check the retention of K+ ions by the membrane (“ZM” in Figure 2), an extra 80 K+ ions (to create an initial ratio of 1:1 between the original cations and the exchanging cations) are placed inside the membrane and 40 Cl- ions are equally placed in the water column on either side of the membrane. This computational step is designated as “zeoliteN_K+/K+” in subsequent sections. Similarly, exchange of other univalent ions is simulated by randomly placing cations such as NH4+, Na+,

Rb+, or Cs+ inside ZM. These exchange calculations are designated as

zeoliteN_K+/NH4+, zeoliteN_K+/Na+, zeoliteN_K+/Rb+ and zeoliteN_K+/Cs+, respectively. The exchange of ions between the zeolite N membrane and solution are determined after 8 ns of NVT simulation. This method allows evaluation of cation preference(s) within the zeolite N membrane. NVTensemble MD simulations of 1 ns duration, including an initial 30 ps time for equilibration, are undertaken to calculate the radial distribution functions (RDF), mean square displacement (MSD), selfdiffusion coefficient(s), and concentration profiles of ions inside and outside the zeolite N membrane.

3. Results We present results from a priori calculations based on the potassium-rich end-member composition for zeolite N as determined via both X-ray and neutron diffraction studies19-20. Experimental data suggest that other partially-exchanged compositions are possible for zeolite N25 but are not considered in this work. This fundamental study focuses on the hydration behavior of zeolite N and the diffusion of ions and counter-ions in a 3D membrane. 9



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3.1 Hydration A 2x2x2 zeolite N bulk and ZM contain 16 cages of aluminosilicate framework with Si/Al = 1.0. The hydration energy (HE) of zeolite N obtained using equation (1) is calculated for different hydration levels by stepwise increase of water molecules into the model as listed in Table 2a. The HE, number of water molecules retained by the ZM, and pressure obtained from simulations undertaken with incremental additions of water molecules to the ZM are shown in Table 2b.

Table2: Hydration energy of zeolite N; a) Bulk and b) ZM a) Zeolite N Bulk Total H 2O loaded

H2O\ cage

0

0

64

4

80

b) Zeolite N membrane Total H 2O loaded

H 2O retained

H 2O \ cage

0

0

0.0

-19.41

64

64

4.0

-19.54

-2.43

5

-18.06

128

99

6.2

-15.38

-1.19

HE (kcals\mol)

HE (kcals\mol)

Pressure (GPa) -2.67

96

6

-16.73

160

129

8.1

-13.04

-0.16

112

7

-14.69

192

128

8.0

-12.49

-0.34

128

8

-13.62

256

142

8.9

-10.46

0.15

144

9

-12.10

318

156

9.8

-11.31

0.12

160

10

-9.94

352

169

10.6

-11.29

0.10

176

11

-8.02

384

174

10.9

-6.74

2.84

192

12

-6.13

A plot of calculated HE for different amounts of water molecules per cage in zeolite N is shown in Figure 3. In Figure 3(a), the HE of bulk, is denoted by the red line as the number of water molecules per cage increases. The blue line indicates the number of water molecules retained per cage (after equilibration) in the ZM versus the total number of H2O molecules added to the ZM initially (before equilibration). The HE increases linearly with the number of water molecules per cage, with a slight dip as the number of water molecules in zeolite N bulk approaches eight H2O/cage. Figure 3 (b) shows the variation of HE and pressure with the number of water molecules per cage in the ZM. 10


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Figure 3 (a): Variation of HE (∆UH(Nw)) in a bulk as a function of number of H2O/cage in the zeolite N (red) and equilibration of H2O/cage in ZM versus total number of H2O added before equilibration (blue). (b) Variation of HE and pressure in membrane as a function of the number of H2O/cage in ZM.

3.2 Ion exchange The number and percentage retention of ions and water molecules inside the zeolite N membrane are summarized in Table 3 and shown in Figures 4 and 5. The data in Table 3 show time points of the simulation at 500 ps, 1 ns, 5 ns and 8 ns. For all simulations, ion exchange is at an equilibrium condition after 1 ns, as demonstrated by the limited exchange of ions at subsequent time points (Table 3). Minor fluctuations in the number of ions and water molecules inside and outside ZM indicate that the system is in dynamic equilibrium.

As shown in Table 3, at 8 ns for the ZeoliteN_K+/K+ system, 49.4 % of the K+ ions loaded into the membrane are retained and 50.6% are released to the solution outside the membrane. Similarly, in the mixed Zeolite N_ K+/M+ systems, Na+, Rb+ and Cs+ show 62.5%, 60% and 60% retention respectively, while NH4+ ions exhibit the highest retention of 68.8%. The retention ratio is determined from the number of ions within the membrane at 8 ns simulation divided by the total number of ions in the 11



system. These values are listed in Table 3 and plotted over simulation times in Figure 4. The ratios of the exchanging ions (M+) to K+ ions inside the zeolite membrane after 8 ns are 2.0 for NH4+/K+, 1.57 for Na+/K+, 1.56 for Rb+/K+, and 1.40 for Cs+/K+.

Table 3: Number of ions in ZM with K+, NH4+, Na+, Rb+ and Cs+: before and after MD simulation. ZeoliteN_K+/K+ Time 0

Ions K

+

ZM

ZeoliteN_K+/NH4+ ZeoliteN_K+/Na+

% ret Ions

176

K

+

NH4 + 500 ps

K

+

88

50.0

K

+

NH4 1 ns

K+

86

48.9

5 ns

K

84

47.7

K

8 ns

K

87

49.4

K

+

+

NH4 +

ZM

+

NH4 +

+

K

80

Na+ 37.5

K

+ +

59

73.8

Na

34

35.4

K+

55 34

+

% ret Ions

96

36 +

K+ NH4

+

55

+

68.8

Na

35.4

+

K

+

68.8

Na

33

34.4

K

+

55

68.8

Na+

ZM

ZeoliteN_K+/Rb+

% ret Ions +

96

K

80

Rb+

40

41.7

K

+ +

52

65.0

Rb

39

40.6

K+

49 39 50

+

61.3

Rb

40.6

+

K

+

62.5

Rb

39

40.6

K

+

50

62.5

Rb+

ZM

ZeoliteN_K+/Cs+

% ret Ions

ZM

% ret

96

K

+

96

80

Cs+

80

37

+

39

40.6

+

53

66.3

38.5

K

48

60.0

Cs

36

37.5

K+

39

40.6

63.8

+

52

65.0

+

37

38.5

+

48

60.0

+

41

42.7

48

60.0

51 36 49

37.5 61.3

Cs K

Cs

37

38.5

K

48

60.0

Cs+

Ion retention ratios

2.5 Retention ratio (M+/K+)

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

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2.0 K+/K+ NH4 + / K+

1.5

Na+ / K+ Rb+ / K+ Cs+ / K+

1.0 0.5 0

2

ns4

6

8

Figure 4: Ion retention ratio compared with K+ over 8 ns MD simulations for ZM.

The number of water molecules retained within the membrane over 8 ns are plotted in Figure 5. The number of water molecules per cage in the zeoliteN_K+/K+ system fluctuates between 7.5 to 8 water molecules/cage with an average of 7.7 H2O/cage. ZeoliteN_K+/Na+ shows the highest average water 12


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content per cage of 8.3 H2O/cage. Calculations on zeoliteN_K+/Rb+ and zeoliteN_K+/Cs+ membranes show an average of 6.4 and 6.2 H2O/cage, respectively. Simulation of the exchange of ammonium ion with potassium (zeoliteN_K+/NH4+) shows that the membrane holds 7.3 H2O/cage.

H2O inside zeoliteN

120

K+/K+ K+/NH4 + K+/Na + K+/Rb+ K+/Cs+

Number of H2O

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


110

100

90

80 0

2

ns 4

6

8

Figure 5: Number of water molecules inside ZM over 8 ns of MD simulations.

3.3 Ion Distributions This computational approach to evaluation of zeolite membrane behavior allows determination of the relative distribution of ions within the framework and in the regions surrounding the framework. These tools include (a) calculated radial distribution functions (RDFs) to determine the bonding characteristics of framework and non-framework atoms and (b) measuring the positions of atoms within the membrane after exchange simulations as described above. The RDFs for exchanged ions with framework atoms O, Si and Al are shown in Figure 6(a-e). In these figures, the first peak for each framework atom represents the nearest neighbor distance of the exchanged ion(s) to the framework atoms. These nearest neighbor distances are compiled in Table 4. For comparison, Figure 6f also shows the RDF for ammonium ions in the electrolyte surrounding the membrane.

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Table 4: Calculated RDFs (Å) for univalent ions (M) with atoms in the zeolite N framework.

Ion (M)

M-O

M-Al

M-Si

NH4+

1.43

2.39

2.39

Na+

2.07

2.79

2.83

K+

2.49

3.25

3.39

+

2.65

3.47

3.45

Cs+

3.03

3.59

3.59

Rb

The proportion of ions preferentially retained within the zeolite N framework in comparison to those released into solution after 8 ns of MD simulation are determined using concentration profiles or ionic density profiles. Figure 7 shows the number distribution and the density field maps of ions along the z direction within the membrane and in the solution outside the membrane. The number of ions at specific locations within the membrane along the z axis are shown in Figure 7a-d for each simulation of univalent ion exchange.

These plots also track the number and species of ion(s) present in the

surrounding electrolyte after 8 ns of simulated exchange reaction. The relative positions of each ion inside the framework obtained after simulation of exchange reaction(s) for 8 ns are visualized through density field maps and are shown in Figures 7 f-g.

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Page 15 of 29 8

8

a) K+\K+ O-K+

6

b) K+\Na+

6

O-K+ O-Na+ Al-K+ Al-Na+

Si-K+

g( r)

4

2

0

0 0

2 r (Å) 4

8

6

8

10

0

2 r (Å) 4

8

c) K+\Rb+

6

g( r)

4

2

O-K+ O-Rb+ Al-K+ Al-Rb+

4

6

8

2

10

d) K+\Cs+ O-K+ O-Cs+ Al-K+ Al-Cs+

6

g( r)

g( r)

Al-K+

4 2

0

0 0

8

2 r (Å) 4

6

8

10

0

2 r (Å) 4

2

e) K+\NH4+

4

6

8

f) NH4+ in electrolyte

1 Ow-NH4+

0.5

2

10

1.5

g (r)

O-K+ O-NH4+ Al-K+ Al-NH4+

6

g( r)

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


0

0 0

2 r (Å) 4

6

8

10

0

5

10

15

20

r (Å)

Figure 6: (a-e) RDFs, g(r), for non-framework ions to framework atoms in zeolite N and (f) for NH4+ to Ow in the electrolyte.

15



Zeolite N_K+/M+

(f) K+

(g) M+

Number of ions

6

(a)

4

K+ 2 0 0

6

10

20

30

40

50

60

70

Number of ions

(b)

4

K+ NH4+

2 0 0

10

Number of ions

6

20

30

40

50

60

70

(c)

4

K+ Na+

2 0 0

Number of ions

6

10

20

30

40

50

60

70

(d)

4

K+ Rb+

2

0 0

6

Number of ions

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 16 of 29

10

20

30

40

50

60

70

(e) K+ Cs+

4

2

0 0

10

20

30

40

Distance (Å)

50

60

70

Figure 7. (a-e) Ion density profiles along the z direction: within ZM denoted by the vertical blue dotted lines and in electrolyte solution (on either side of the blue dotted lines) after 8 ns MD simulations. (f) and (g) Density field maps for ions in the central cages (magnification of the region denoted by the green rectangle shown in ZM) of ZM: K+ is left hand panel (f) and M+;is right hand panel (g); the relative intensity increases from red to blue. 16


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4. Discussion Experimental data on the ion exchange performance of zeolite N is documented in the patent literature25 as well as in articles describing applications such as wastewater treatment7, 21 and agronomy6, 8. These data show that the Cation Exchange Capacity (CEC) for zeolite N powders ranges between 450 and 503 meq/100g for Si:Al = 1.0 depending on synthesis conditions25. Experimental data show that the potassic form of zeolite N prefers univalent ions over divalent ions in a multi-element aqueous solution6, 25. For example, Table 5 provides examples of loading data for zeolite N25 for three ammonium concentrations in solutions with 50 mg/L Ca+2 and 20 mg/L Mg+2. Similar preference for ammonium ion is observed when zeolite N is equilibrated with solutions containing both 2,000 mg/L Na+ and 100 mg/L Ca+2 ions25.

Table 5: Experimental data on zeolite N ion exchange selectivity in mixed cation solutions25

[NH4+] in starting soln (mg/L)

NH4+ loading (meq/100g)

Ca+2 Loading (meq/100g)

Mg+2 Loading (meq/100g)

Solution 1

30

104

10

4

Solution 2

200

347

25

0

Solution 3

1000

444

18

0

For this study, we focus on univalent cation exchange in an electrolyte solution containing Cl- as the counter-ion. This format is similar to experimental data which also utilizes Cl- as the counter-ion in 200 mL aqueous solution with addition of 0.2g of zeolite N equilibrated for periods of 1–2 hours25. These exchange reactions are pH dependent as noted by Thornton et al.7 who demonstrate that maximum loading of ammonium occurs in the pH range 6–7. For this study, we assume pH ~ 7 for MD simulations.

17



Page 18 of 29

Ion exchange models for zeolite structures developed from an interest in the molecular behavior of sodic zeolites such ZK-432 and zeolite 4A33 (also identified as Na-LTA13) under osmosis or reverse osmosis conditions. The basis for modeling a zeolite membrane established in this early work, led to detailed comparison of ion exchange by divalent and univalent ions26 as well as between Ag+ and Na+ in NaLTA13. In a study of supercritical and subcritical electrolyte solutions exchanged with Na-LTA, Murad et al.26 demonstrate that for Na+ it is energetically favorable to diffuse to the outside of a membrane. Salmas et al.13 show that for Na-Ag exchange in LTA, the driving force for exchange is strongly influenced by electrolyte concentration. In this work, we invoke a chemical potential between the membrane and the electrolyte by inserting additional ions (e.g. K+ or Na+ etc) and water molecules into the middle of ZM and allow the simulation to reach an equilibrium condition over nine nanoseconds.

As noted by Salmas et al.13, an implicit or explicit water model for simulation of LTA ion exchange shows no difference in outcome. In this work we utilize an explicit water model, as it helps to determine the exact hydration state34 and the involvement of water molecules during ion exchange. The framework charge is also an important influence on ion exchange properties of aluminosilicates34. Using DFT, we calculate the partial ionic charge on framework atoms for zeolite N as shown in Table 1, and note that these values are similar to that determined by Salmas et al.13 for LTA zeolite.

4.1 Hydration Ion exchange in zeolites is predominantly in an aqueous environment and, as such, the atomic-scale dynamics of extra-framework cations should be considered in the context of a hydrated system or membrane. Water molecules within zeolite channels significantly affects the location of extraframework cations in the structure20 and consequently, the ion-exchange properties of a zeolite. For example, strongly hydrated extra-framework cations show reduced tendency to exchange structural positions with other cations. In addition, the mobility of water molecules controls the motion of the

18


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exchangeable cations and, as a result, controls the performance of ion-exchange and diffusion processes34-35.

Water has two roles in zeolites: (a) completing the coordination of available cations inside the zeolite channels which increases their mobility and (b) minimising the electrostatic repulsion of the bridging oxygen in the zeolite framework36-37. Moreover, the Al content in the zeolite framework controls the amount of adsorbed water, because by decreasing the Si/Al ratio, the hydrophilicity of zeolite increases38. Understanding the influence of water on behavior of zeolites or, in other words, the hydrophilicity of zeolites, can be enhanced by comparing the differences between dehydrated and hydrated states of a zeolite system.

As shown in Table 2 and Figure 3, incremental increases in the number of water molecules to progressively hydrated states in ZM results in an equilibrium condition by 2 ns of simulation. The plot in Figure 3a which combines the results of two different sets of simulations i.e. bulk and ZM provides significant information.

The HE curve for the bulk (red line) increases linearly with the number of

water molecules per cage, exhibiting a slight dip as the number of water molecules in the cages approaches eight H2O/cage and indicates that it is energetically favorable. This, in tandem with the results of the ZM indicated in the blue line, shows a constant number of water molecules retained per cage in the ZM over two increments of H2O molecules added to the ZM. This condition also corresponds with eight water molecules per cage for the zeolite N membrane. This outcome of eight water molecules per cage is consistent with the experimental data for zeolite N based on X-ray and neutron diffraction studies19-20. This result validates our zeolite N membrane model and the methodology developed to simulate water behavior in this material.

For the membrane system, the calculated pressure shows a similar trend as that for change in hydration energy, HE, shown in Figure 3. For example, Table 3b (Figure 3b) shows the change in HE as well as 19



Page 20 of 29

calculated pressure as water molecules are added to the membrane. In this system, pressure increases in concert with HE and when the membrane achieves a preferred number of water molecules (i.e. eight per cage), pressure also plateaus at an equilibrium level up to addition of 384 water molecules (i.e. 11 per cage). At this stage and with further addition of water, pressure increases rapidly and implies a maximum operating condition for a zeolite N membrane.

4.2 Ion Exchange Using this model for ZM with surrounding electrolyte, the exchange selectivity by univalent ions for the zeolite N framework can be readily determined. Data in Table 3 provide a clear quantitative guide at specific points in time for the retention/inclusion of ions relative to K+ in ZM. In general, NH4+ uptake is stronger during initial phases of simulation (up to 500 ps) and reaches equilibrium more rapidly than other ions. Figure 4 plots the ratio of exchanged ions to K+ within ZM for the 8 ns simulation and clearly shows that NH4+ ion is preferred by zeolite N. Other ions such as Na+, Rb+ and Cs+ will exchange but at lower levels (or rates). As anticipated, the ratio of K+ within ZM before and after simulation remains relatively constant, albeit the influence of H2O molecules is implied by the ~4% variation in ratio of K+/K+ over time. The relative retention ratios shown in Figure 4 are consistent with experimental data over a wide range of ion concentrations in aqueous solutions. These general attributes of univalent ion exchange in zeolite N are determined by local atomic bonding and interactions with the aluminosilicate framework.

Figure 5 shows the number of water molecules within ZM during univalent ion exchange reactions up to 8 ns of simulation. The relative amounts of water molecules follows the sequence Na+ > K+ > NH4+ > Rb+ > Cs+ with the highest water contents for K+/Na+ exchange. This sequence is consistent with the relative increase in ionic radius for these univalent ions. The water content in ZM with each ion exchange reaction is highly variable and ranges from the average by 3%–4% for K+, up to 8% for Rb+,

20


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with trends suggesting lower content over time for Na+ and K+ exchanges. These variations in water content are difficult to interpret albeit the influence of cation hydration spheres may be implicated.

4.3 Ion Localisation Figures 6 and 7 provide three different measures of the locality of ions after ion exchange simulations. For example, radial distribution functions (RDFs) measure the intensity of distances between specific atom pairs. For zeolite N, the relative distances between framework and non-framework atoms provides an average measure of localized bonding influences within and around structural cages and are shown in Figure 6 for each ion exchange simulation. Ion distribution plots are shown in Figure 7. These plots track the proportion of each ion along the z axis direction within ZM as well as within the electrolyte and are shown in the center panel of Figure 7. Ion density maps are shown on the left and right hand panels of Figure 7 and provide detailed density distribution(s) of K+ and M+ within the cage(s) of the zeolite structure in the membrane.

The RDFs for each of the simulated ion exchange models in Figure 6 show that the values for O–K+, Al–K+ and Si–K+ are 2.49 Å, 3.25 Å and 3.39 Å, respectively, for the calculated zeolite N structure containing additional K+ ions. As expected, these values compare well with the relative bond lengths calculated from X-ray diffraction data by Christensen and Fjellvag19. As shown in Table 4, with increased ionic size of cations exchanged into the structure, O–M+ RDF values increase.

For the ZeoliteN_K+/NH4+ simulation (Figure 6e), the first peak at 1.43 Å corresponds to the HNH4+ to OFw distance. This distance is smaller than the NH4+ hydrogen bonded to the oxygen atom in water (Ow) distance of 1.75 Å shown in Figure 6f for the electrolyte. This outcome also validates the model and approach used to simulate solid-liquid interactions. Comparison of peak heights, that is, the function g(r) in Figure 6, shows substantial variations in O–M+ and Al–M+ pairs in different simulations. The peak heights of Al–K+ , Al–Rb+ and Al–Cs+ are higher than the O–M+ peaks and increase from K+ < Rb+ 21



Page 22 of 29

< Cs+. In contrast, the peak heights for O–Na+ and O–NH4+ are higher than Al–M+ peaks. This comparison shows that significant populations of Na+ and NH4+ ions reside near the O atoms in the framework, while K+ , Rb+ and Cs+ mostly reside in the center of zeolite cages. Moreover, the higher values of g(r) for O–Na+ and O–NH4+ represent nearest neighbor distances, indicating that Na+ and NH4+ ions have stronger interactions with the zeolite N framework (FW) compared to other M+ ions. For K+, Rb+ and Cs+, the highest values for g(r) are not nearest neighbor distances. These relative RDF peak intensities indicate that a higher proportion of K+, Rb+ and Cs+ ions inside ZM show weaker interactions with FW atoms.

Ion density profiles, shown in Figure 7 for each of the simulations, provide an indication of locational preferences along the z axis direction for these univalent ions. For example, Figure 7 (center panel) shows that for all simulations, K+ ions are predominantly located in the middle of the membrane. This outcome is not unexpected given that the simulation method places K+ (and M+) at the center of ZM at time t = 0. In contrast, Na+, Cs+ and Rb+ ions tend to show higher densities closer to the end(s), or just outside, of ZM. NH4+ ions show an even distribution and density across the breadth of the membrane which suggests that these ions prefer the internal cages of ZM.

Close inspection of the individual cages, as shown in Figure 7g, shows that Na+ ions are positioned closer to framework atoms, while NH4+ ions are located at the edges and the middle of the cages. Both Rb+ and Cs+ are predominantly located in the middle of the cages. Nevertheless, the position of K+ ions after these exchange simulations (Figure 7g) shows slight variations depending on the exchangeable ion. With addition of excess K+ only, these ions seem to occupy additional sites within the cage. For Na+ exchange, the remaining K+ ion shows highest density within the center of the cage while for NH4+ exchange, the K+ ion is slightly off-center of the cage. For both Rb+ and Cs+ exchange, the remaining K+ ions are displaced from the center of the cage. These relative displacements of ions within the cages of

22


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ZM influence the rates of diffusion of ions and their propensity for moderation by the presence of water molecules.

4.4 Ion Diffusion We calculate the relative mobility of ions in the membrane and in solution using self-diffusion coefficients (D) calculated from the mean square displacements (MSD) of ions and water molecules. These calculated D values provide an estimate of the average rate of transfer of ions within the medium using our computational methods outlined in Section 2.0. A comparison of these values for each of the ion exchange simulations is presented in Table 6.

Table6: Self diffusion co-efficient (D) of ions and water molecules in the Zeolite membrane (ZM) and solution calculated from MD simulation for 9ns, at 298K. Self diffusion co-efficient, D (cm2/s) ZeoliteN Ions/W Ions inside ZM

K+/K+ K+/NH4+

K+/Na+

K+/Rb+

K+/Cs+

23

Ions in electrolyte

K+

1.51x10-9

1.07x10-5

water

1.62x10-9

2.46x10-5

K+

1.76x10-8

1.62x10-5

NH4 +

8.32x10-9

7.39x10-6

water

3.54x10-8

3.03x10-5

K+

1.13x10-10

1.76x10-5

Na +

3.33x10-12

1.27x10-5

water

1.08x10-10

2.76x10-5

K+

4.57x10-10

1.29x10-5

Rb +

3.49x10-10

1.19x10-5

water

1.13x10-9

1.80x10-5

K+

1.80x10-10

1.71x10-5

Cs +

1.08x10-10

1.44x10-5

water

5.90x10-10

2.83x10-5



The average self-diffusion coefficient for water molecules in the electrolyte is 2.58 × 10-5 cm2/s. This value is similar to the experimentally determined self-diffusion coefficient of water at 298K that ranges between 2.1 × 10-5 cm2/s and 2.7 × 10-5 cm2/s for chloride solutions39. The D values for ions in the electrolyte are in the range of 1.1 × 10-5 to 1.7 × 10-5 cm2/s (average 1.42 × 10-5 cm2/s), except for NH4+ ions. The NH4+ ions have the lowest diffusion coefficient at 7.39 × 10-6 cm2/s.

1.0E-12

D of ions inside ZM Self diffusion co-efficient (cm2/s)

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 29

1.0E-11

K+ ion 1.0E-10

1.0E-09

1.0E-08

1.0E-07

K+/K+ K+/NH4 + K+/Na + K+/Rb + K+/Cs +

Figure 8: Self diffusion co-efficient of ions and water in ZM The D value for K+ ions inside ZM are larger than all other exchanging ions evaluated in these simulations. For example, Rb+ is 1.3x, Cs+ is 1.7x, NH4+ is 2.1x and Na+ is 34x slower than K+ ions in the membrane. However, as shown in Figure 8, NH4+ ions in the Zeolite N_K+/NH4+ exchange simulation within ZM show the highest D values compared to all other exchanged ions in these simulations. Consistent with earlier data shown in Section 3.0, the value of D for Na+ ions within ZM are the lowest of all univalent ions. This outcome, as well as inferences from ion distribution profiles, confirms that zeolite N membranes show exchange selectivity for specific univalent ions in the series NH4+ > Rb+ > Cs+ > Na+. While experimental data are not available for Rb+ or Cs+ exchange with K+, these models confirm experimental data obtained for NH4+ and Na+.

24


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Finally, the high preference and selectivity of NH4+ ions in Zeolite N can be attributed to hydrogen bonding of the NH4+ ions with the oxygen atoms in the framework. The four hydrogen atoms in NH4+ ions tend to form hydrogen bonds with three oxygen atoms of the framework and one oxygen atom of the water molecule, as seen in Figure 9. On close inspection of hydrogen bond interactions of NH4+ ions during the course of the simulation, hydrogen bonds of the ions break and reform with different oxygens in the framework, frequently migrating across adjacent oxygens and water molecules. This behavior explains not only the high mobility (diffusion coefficient value) of the NH4+ ions but also the rapid exchange of K+ ions with NH4+ ions. In contrast, due to the absence of hydrogen bonding for Na+ ions, though held close to the framework by electrostatic interactions with the framework, they exhibit low mobility and low exchange rates.

Figure 9: Visualization of hydrogen bonds between NH4+ and O atoms in the frame work (at 0.1ns, 0.5ns and 1ns). Other extra framework species (K+, Cl- and H2O) not involved in hydrogen bonding interaction are hidden from view.

5. Conclusions MD calculations on potassic zeolite N demonstrate that computational modelling of hydration and of ion exchange simulates, in general, experimental outcomes for NH4+ and Na+ exchange with K+. Simulation of ion-exchange with Rb+ and Cs+ also shows that partial exchange of K+ in the zeolite N structure is likely to occur in practice. The computational method, which includes explicit water molecules in a membrane and in the electrolyte, provides time- and location-dependent data on the relative efficacy of univalent ion exchange in zeolite N. We demonstrate that zeolite N prefers K+ exchange with NH4+ ions through a high retention ratio (i.e. NH4+/K+ = 2.0) and NH4+ shows the highest value for diffusion coefficient of the univalent ions evaluated. Simulations for 8 ns show that ~70% of the K+ ions are exchanged by NH4+. Other ions, such 25



Page 26 of 29

as Na+, Rb+ and Cs+, also partially exchange but with significantly lower values for diffusion coefficients. Of significance is the very low, by a factor of 34 times, diffusion coefficient for Na+ compared with K+ determined by these simulations. This slow rate, in combination with the tendency for Na+ to locate in close proximity to framework oxygen within the zeolite cage, suggests a strong interaction with the aluminosilicate framework. Unlike with NH4+, an exchange interaction of Na+ with the framework may not be facile in the presence of H2O. Calculations of the hydrated state for zeolite N, including determination of the hydration energy, show that zeolite N achieves equilibrium when the number of water molecules approaches 8 H2O/cage. This calculated value is in excellent agreement with the experimentally determined value for zeolite N. The modelling approach described in this work offers important insight into the behavior of zeolite N ion exchange and implies useful application to other aluminosilicate zeolites for a range of multivalent cations.

6. Acknowledgements Assistance with QUT’s High Performance Computing facilities particularly from Ashley Wright and Adam Siliato, as well as discussions on computational techniques with Professor Jose Alarco, are gratefully acknowledged. This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors. Monireh Khosravi gratefully acknowledges receipt of a scholarship from the Institute for Future Environments, Queensland University of Technology.

26


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