DTF Study of Layered Double Hydroxides with Cation Exchange

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

DTF Study of Layered Double Hydroxides with Cation Exchange Capacity:(A(HO))[M +AlOH) (SO)]6HO (M = Mg, Zn and A= Na, K) +

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Pedro Ivo Rodrigues Moraes, Fernando Wypych, and Alexandre Amaral Leitão J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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The Journal of Physical Chemistry

DTF Study of Layered Double Hydroxides with Cation Exchange Capacity: 2+ (A+(H2O)6)[M2+ 6 Al3 (OH)18 (SO4 )2 ]·6H2 O (M

= Mg, Zn and A+ = Na, K) †

Pedro Ivo R. Moraes,

Fernando Wypych,

†Departamento

‡

and Alexandre A. Leitão

∗,†

de Química

Universidade Federal de Juiz de Fora Juiz de Fora, MG, CEP-36036-330, Brazil

‡Departamento

de Química

Universidade Federal do Paraná P.O. Box 19032, Curitiba, PR, CEP-81531-980, Brazil

E-mail: *[email protected] Phone: *55-32-21023310

Abstract Layered double hydroxides (LDH) equivalent to the minerals Motukoreaite and Natroglaucocerinite structures were investigated by ab initio calculations as well the capacity to exchange the cations without removing the intercalated sulphate. Due to the M2+ :M3+ molar ratio in all minerals structures, the layer domains contains three posi3+ 3+ tive charges ([M2+ 6 M3 (OH)18 ] ) which after the intercalation with two sulfate anions 3+ − build a negative charged compound ([M2+ 6 M3 (OH)18 ](SO4 )2 ) and these compounds

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are intercalated with hydrated sodium cations. Structures with dierent layered compositions (Mg/Al and Zn/Al) and with dierent intercalated cations (Na+ and K+ ) were constructed. In all cases the 3R polytype was the most stable. The thermodynamic of the cation exchange reaction was performed and the sodium cations were replaced by potassium, maintainig the intercalated sulfate. LDH are traditionally anion exchangers but the studied formulations can also exchange cations, which open new perspectives of applications.

Introduction Layered double hydroxides (LDH) or hydrotalcite-like compounds 1 are natural or synthetic layered materials whose structure is similar to Brucite (Mg(OH)2 ) and are very well known as 3+ anion exchangers. LDHs can be described by a generic chemical composition [M2+ 1−x Mx (OH)2 ]

[(An− )x/n ·yH2 O], where M2+ is a divalent cation that are partially replaced by M3+ trivalent cation, resulting an excess of positive charges, which need to be balanced by the intercalation of hydrated anions (An− )x/n ·yH2 O. The most common example of LDH minerals are Hydrotalcite (Mg6 Al2 (OH)16 CO3 ·4H2 O) and Pyroaurite (Mg6 Fe2 (OH)16 CO3 ·4.5H2 O), where in both cases the intercalated anion is carbonate, due to the solubility of carbon dioxide in water and formation of carbonate under strongly alkaline conditions, and the molar ratio between M2+ /M3+ is of 3:1. Other LDHs minerals with a M2+ :M3+ 2:1 molar ratio were also reported in the literature, like in Motukoreaite ((Na(H2 O)6 )[Mg6 Al3 (OH)18 (SO4 )2 ]·6H2 O), 2 Natroglaucocerinite ((Na(H2 O)6 )[Zn6 Al3 (OH)18 (SO4 )2 ]·6H2 O), 3 Shigaite ((Na(H2 O)6 )[Mn6 Al3 (OH)18 (SO4 )2 ]·6H2 O) 4 5 and Nikisherite (Na(H2 O)6 )[Fe2+ 6 Al3 (OH)18 (SO4 )2 ].6H2 O). In the last cases, instead of car-

bonate, the counter cation allocated in the interlayer domain is sulfate. All these compounds 2+ have a generic formulation (Na(H2 O)6 )[M2+ 6 Al3 (OH)18 (SO4 )2 ]·6H2 O; M = Mg, Zn, Mn and

Fe) or in a reduced way ([Na0.111 (H2 O)0.666 ]M+2 0.666 Al0.333 (OH)2 [SO4 ]0.222 ·0.666H2 O). The Shigaite structure, 4 the interlayer domain is composed of a trivalent anion [Na(SO4 )2 (H2 O)12 ]3− 2


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+ that compensate three edge-sharing octahedral [AlMn2+ 2 ](OH)6 ] units, where an octahedral

arrangement is formed due to the sodium cations coordinated by six water molecules. In the layer top each sulfate anions is surrounded by three sodium cations, each cation is surrounded by three sulfate anions and each sulfate is linked to one aluminum cation and the aluminum cation is coordinated octahedrally by six hydroxide anions. +3 3+ In the generic formulation [M2+ 6 M3 (OH)18 ] , the layer domains contains three positive

charges and in the case of synthetic phases, only sulfate is make available as counter ion, normally supplied by a soluble sulfate, typically Na2 SO4 . The problem is the location of the three positive charges of the layer domain by a double negative charged sulfate and 1.5 sulfate anions would be necessary. Due to the ordered positioning of the cations in the layers and consequently the local positive charge distribution, sometimes the charge of the layers cannot be directly compensated by the intercalated anions, especially with double or triple charged anions. This mismatch of the charges spatial positioning results in a net negative charge, which needs to be compensated by the intercalation of cations. To solve the problem of the layer domain neutralization, one sulfate anion and one single charged species, namely [NaSO4 ]− ·yH2 O are used and minerals like Motukoreaite, Natroglaucocerinite, Shigaite and Nikisherite are obtained. Other intercalated cations together with sulfate anions are also reported in the literature like in the Wermlandite structure ((Ca(H2 O)6 )[Mg7 Al2 (OH)18 (SO4 )2 ]·6H2 O, 6,7 but in this case the M2+ /M3+ molar ratio is of 3.5 and the intercalated cation is hydrated calcium. According to the current classication proposed by S. J. Mills and co-workers, 8 all these compounds belongs to the Wermlandite group were the basal spacing is close to 11 Å and hydrated cations occur in the presence of anions (chiey sulfate and carbonate), which are allocated between the Brucite-like layers. Another more complex structures were also reported like in Karchevskyite (Mg18 Al9 (OH)54 Sr2 (CO3 ,PO4 )9 ·6H2 O·5H2 O) 9 or synthetic mixed-valence Fe2+ ,Fe3+ -layered double hydroxide, known as green rusts (GR), a family of Fe2+ /Fe3+ LDH intercalated with hydrated anions 3



2− such as Cl− , CO2− 3 and SO4 . When GR are synthesized in the presence of NaOH, sodium

cations are intercalated between the layers to partially compensate the intercalated sulfate 3+ anion and compounds with a generic formulation (Na(H2 O)6 )[Fe2+ 6 Fe3 (SO4 )2 (OH)18 ]·6H2 O

are obtained, 10 which presents also the 3a × 3a superstructure (a=b= 9.528 Å) and consequently belongs to the Motukoreaite, Natroglaucocerinite, Shigaite and Nikisherite family. The GR are dicult to be investigated because they decompose through the contact with air into iron oxides such as magnetite (Fe3 O4 ), goethite and lepidocrocite, two FeO(OH) polymorphs. 11 In general, in the LDH having 2:1 M2+ /M3+ molar ration observed in nature, there is a √ √ ordering of the M2+ and M3+ in the structure, originating superstructure of a 3 × a 3 and/or ordering of the sodium cation in a 3a × 3a structure, which stabilizes the structure, 2,46 while in the LDH with the 3:1 M2+ /M3+ , the superstructure is of 2a × 2a type. All these mineral structures have the superstructure 3a × 3a, attributed to the ordering of sodium cations between the layers, which are bonded to sulfate anions (Shigaite: a=b= 9.512 Å; Nikisherite: a=b= 9.347 Å; Motukoreaite: a=b= 9.172 Å; Wermlandite: a=b= 9.303 Å) √ √ but in synthetic phases with similar compositions, the a 3 × a 3. superstructures were observed, attributed to the ordering of the M2+ and M3+ metals in the layers. 12 Density Functional Theory (DFT) calculations are a exceptional tool to provide reliable electronic properties, assist experimental characterization and predict properties. DFT calculations have already shown great eciency in the simulation of layered compounds. The exchange-correlation functionals together with van der Walls corrections and strongly correlated electrons corrections are archiving substantial results and agreement with experimental data. 1318 The present work aims, by means of ab initio calculations, investigate the electronic structure of the Motukoreaite and Natroglaucocerinite. The simulated 3R and 1H polytypes have the follow chemical compositions [M6 Al3 OH)18 ][Na(H2 O)6 (SO4 )2 ]·6H2 O and [M6 Al3 (OH)18 ] [K(H2 O)6 (SO4 )2 ]·6H2 O where M is Mg and Zn. For the sake of simplicity these models will 4


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be called as: M-Al-(NaSO4 ) and M-Al-(KSO4 ). It is also a goal to evaluate thermodynamically a possible cation exchange. The experimental results involving the cation exchange reactions of compounds having alkaline metals and sulfate intercalated, including the phases investigated in the present manuscript, were recently investigated 12

Computational Details All ab initio calculations were performed using the codes available in the Quantum Espresso package, 19 which implements the DFT under periodic boundary conditions with plane wave functions as a basis set. 20,21 The geometry optimizations of the structures were performed with the generalized gradient approximation (GGA/PW91). 22 The ion cores were described using ultrasoft pseudopotential, 23 and the Kohn-Sham oneelectron states were expanded in a planewave basis set with a kinetic cuto energy of 60 Ry (480 Ry for the density). A Monkhorst-Pack mesh of 2x2x2 k-point sampling was used. 24 The equilibrium cell parameters and the atomic positions of all systems were found by minimizing the total energy gradient. For each set of cell parameters, the relative ion positions were relaxed until all of the force components were lower than 0.001 Ry/Bohr. All of the molecular graphics were generated by the XCRYSDEN graphical package. 25 The Motukoreaite structures were built in this work correspond to the 3x3 3R polytype supercell and Natroglaucocerinite 1H polytype as reported by Costa et al. 13 Motukoreaite and Layered Double Hydroxides (LDHs) are compounds belonging to the same family, their crystal system, class and space group are the same. The chemical formula of the Motukoreaite and Natroglaucocerinite structures were described as [M6 Al3 (OH)18 ][Na(H2 O)6 (SO4 )2 ]·6H2 O and [M6 Al3 (OH)18 ][K(H2 O)6 (SO4 )2 ]·6H2 O where M is Mg and Zn. The calculation of the thermodynamic properties like Gibbs free energy, entropy and the enthalpy of the cationic exchange reaction and Gibbs free energy increments for the studied compounds was carried out with the vibrational modes obtained from phonon calculations

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at the Γ-point using the harmonic approximation by the density functional perturbation theory (DFPT). 26,27 The Gibbs free energy (G) can be calculated using the following equation: G = H - TS. The enthalpy and the entropy were calculated for the Motukoreaite and Natroglaucocerinite models in the solid state by the following approximations, similarly as in previous work for other materials. 2832

Hsolid (T ) = Eele + EZP E + Evib (T )

(1)

Ssolid (T ) = Svib (T )

(2)

where Eele , EZP E , Evib (T) and Svib (T) are the total electronic energy at 0 K, the zero point energy and the vibrational contributions for the enthalpy and the entropy, respectively. The contribution of the PV term for the enthalpy of the solid in the Eq. 1 was neglected due to its small contribution. The enthalpy and the entropy contributions for the ions in the gas state were calculated by

Hgas (T ) = Eele + ZP E + Evib (T ) + Etrans (T ) + Erot (T ) + RT

(3)

Sgas (p, T ) = Svib (T ) + Strans (p, T ) + Srot (T )

(4)

where Etrans (T) and Erot (T) are the translational and rotational contributions of the enthalpy. RT is equivalent to the term PV, which is necessary to obtain the enthalpy of the gases. Strans (p, T) and Srot (T) are the translational and rotational contributions of the entropy. The EZP E , Evib (T), Erot (T), Svib (T) and Srot (T) contributions for a monoatomic ion are zero. The cations were calculated in a cubic cell 20 Å. The long-range Coulomb interactions in the periodic charged species were corrected by the Makov-Payne 33 correction. (aq)

(aq)

(aq)

The contributions of H, S, and G of the aqueous cation (HC , SC , GC , respectively) were computed by the following equations:

6


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C + (g) → C + (aq)

◦(hyd)

(5)

∆EA

Thus, we considered:

= (HC + ∆HC

(aq)

= (SC + ∆SC

(aq)

= (GC + ∆GC

∆SC

∆GC (g)

(g)

(g)

◦(hyd)

(aq)

∆HC

(g)

◦(hyd)

(g)

◦(hyd)

(6)

)

(7)

)

(8)

)

(g)

where HC , SC , and GC are the calculated enthalpy, the entropy, and the Gibbs free energy ◦(hyd)

of the cations in the gas phase. ∆HC

◦(hyd)

, ∆SC

◦(hyd)

, and ∆GC

are the standard enthalpy,

entropy, and Gibbs free energy of the formation of the hydrated ion C+ from its gaseous state, respectively, which were taken from experimental measurements. 34 The cationic exchange reactions considered are the following where M is Mg and Zn.

+ [M6 Al3 (OH)18 ][N a(H2 O)6 (SO4 )2 ] · 6H2 O(s) + K(aq) →

[M6 Al3 (OH)18 ][K(H2 O)6 (SO4 )2 ] · 6H2 O(s) + N a+ (aq) (R1)

Results and discussion Structural analysis The simulated 3R structures cell parameters are shown in Table 1 and the simulated and experimental Mg-Al-(NaSO4) Motukoreaite X-ray diraction patterns are shown in Figure 1,

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for 1H structures the cell parameters are shown in Table 2. As can be seen a good agreement between the simulated and experimental values of the cell parameters and XRD patterns was obtained. Figure 2 show the optimized structures of 3R Mg-Al-(NaSO4 ), Mg-Al-(KSO4 ) and 1H Zn-Al-(NaSO4 ), Zn-Al-(KSO4 ) structures. The sulfates anion and the cation were positioned linked to the octahedral Al cation and after the structure optimization they remained in these positions.

Table 1: Cell parameters of the simulated 3R polytype of the M-Al-(NaSO4 ) and M-Al(KSO4 ) structures where M = Mg and Zn with their respective relative errors. Parameters a (Å) b (Å) c (Å)

a Mineral

data 35

Mg-Al-(NaSO4 )a 9.17 9.17 33.51

b Synthesized

materials data 12

Mg-Al-(NaSO4 )b 33.03

Mg-Al-(KSO4 )b 32.61

Zn-Al-(NaSO4 )b 33.42

Zn-Al-(KSO4 )b 34.20

Mg-Al-(NaSO4 ) 9.31 / 1.52% 9.29 / 1.31% 33.42 / 0.27-1.18%

Mg-Al-(KSO4 ) 9.29 9.31 34.11 / 4.60%

Zn-Al-(NaSO4 ) 9.33 9.36 33.27 / 0.45%

Zn-Al-(KSO4 ) 9.34 9.38 33.96 / 0.70%

Table 2: Cell parameters of the simulated 1H polytype of the M-Al-(NaSO4 ) and M-Al(KSO4 ) structures where M = Mg and Zn with their respective relative errors Parameters a (Å) b (Å) c (Å)

a Mineral

data 3

Mg-Al-(NaSO4 )b 11.01

b Synthesized

materials data 12

Mg-Al-(KSO4 )b 10.87


Zn-Al-(NaSO4 )a 9.24 9.24 11.12


Mg-Al-(NaSO4 ) 9.30 9.30 11.20 / 1.72%

Mg-Al-(KSO4 ) 9.30 9.33 11.43 / 5.15%

Zn-Al-(NaSO4 ) 9.36 / 1.30% 9.36 / 1.30% 11.16 / 0.18-0.36%

Figure 1: XRD patterns of the Motukoreaite [Mg6 Al3 (OH)18 ][Na(H2 O)6 (SO4 )2 ]·6H2 O. The experimental XRD was simulated from mineral data taken from ref. 35 The diraction patterns show good agreement between the experimental and simulated 8


Zn-Al-(KSO4 ) 9.36 9.39 11.39 / 0.09%

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structures and the polytype of the compound was maintained. The higher basal spacing for K+ ion than Na+ ion would be expected because the cationic radius and the alkaline metal is surrounded octahedrally with water molecules and also with octahedral sulfate groups but experimentally this was conrmed only for Natroglaucocerinite (Na+ = 11.14 Åand K+ = 11.40 Å) but not for Motukoreaite (Na+ = 11.01 Åand K+ = 10.87 Å), in spite of having diculties to determine the values in the last case due to the diraction peaks broadening. 12 The structures in which the layer are composed by Zn (Natroglaucocerinite) showed a higher a and b parameters than the Mg (Motukoreaite) compounds, but the structures with Mg shows higher c parameter for both polytypes. The electronic total energy shows that the most stable polytype is the 3R for both compositions. The total energy dierence between 3R and 1H polytype is 16.08 and 17.41 kJ mol−1 for Mg-Al-(NaSO4 ) and Mg-Al-(KSO4 ) respectively. For the Zn structures the dierences are 17.55 and 18.84 kJ mol−1 for Zn-Al-(NaSO4 ) and Zn-Al-(KSO4 ) respectively.

Figure 2: Optimized structure of the simulated layered double hydroxydes (a) 3R Mg-Al(NaSO4 ), (b) 3R Mg-Al-(KSO4 ), (c) 1H Zn-Al-(NaSO4 ) and (d) 1H Zn-Al-(KSO4 ). Figure 3 shows the increments of ∆G for structures. As can be noticed, the 3R polytype structures are the most stable structures in all the studied temperature range, demonstrat9



ing that the vibrational contribution to the thermodynamical potentials does not change the stability. The variation of the Gibbs free energy for each temperature is approximately constant, thus the enthalpy variation is also almost constant. The increments of ∆G show that both polytypes could be accessed experimentally, since the average dierences between the 1H and 3R polytypes curves are 11.15, 18.22, 13.11 and 16.21 kJ mol−1 for Mg-Al-(NaSO4 ), Mg-Al-(KSO4 ), Zn-Al-(NaSO4 ) and Zn-Al-(KSO4 ) respectively. It is also evidenced that the 1H polytype structures reach ∆G < 0 above 15 ◦ C while the 3R polytype Gibbs free energy is less than zero throughout studied range. Although the structure of Natroglaucocerinite was indexed as 1H polytype, 3 its polytype is still doubtful in the LDH minerals community and needs to be revaluated. 8

Figure 3: Gibbs free energy increments for the M-Al-(NaSO4 ) and M-Al-(KSO4 ) structures where M = Mg and Zn.

Thermodynamic study of the cation exchange reaction The main thermodynamic potentials for the cationic exchange reaction described by the reaction R1 are shown in Table 3. The electronic total energy rules the proposed exchange cation reaction, in other words, the higher contribution for the thermodynamic partial functions is 10


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the electronic total energy. Table 3: Thermodynamic Potentials for the Cation Exchange Reactions (M-Al-(NaSO4 ) → M-Al-(KSO4 )) of Motukoreaite and Natroglaucocerinite per alkaline cation at 298.15 K (kJ mol1 ). 3R Polytype Mg Zn 1H Polytype Mg Zn

∆G 103.08 102.74

∆H 95.09 93.71

T∆S -7.99 -9.03

109.05 105.45

96.73 94.86

-12.32 -10.59

∆ZPE ∆Eel -1.87 84.60 -1.34 82.93 -0.55 -1.20

85.93 84.22

The results shown in Table 3 indicate that the cation K+ can be thermodynamically replaced by Na+ . Although not favorable thermodynamically, experimental data 12 indicated that using one higher excess of K2 SO4 in the dispersion of Na-phase and stirring the dispersion for longer times, the exchange reaction can proceed until the completeness. The reaction rates of those processes play a major in the ion selectivity, however our study takes into account only thermodynamic potentials, the ion exchange kinetics is not regarded. The cation exchange rate is higher in reactions that are thermodynamically guided. 12 This behavior is observed for both polytypes. Potassium delivery is something interesting, because it is an important major nutrient for plant nutrition. 36 This nutrient is also crucial for structural integrity of cereals, photosynthesis and osmoregulation. 37 The values of ∆ZPE are particularly relevant in the computations of ∆H, the magnitude of this contribution is small for this reactions. The change in the layer divalent cation did not show inuence in those reactions, the thermodynamic potentials are very close.

Electronic Analysis The electronic analyses were done for the 3R polytype structures, by the fact that the dierent stacking, in this case, will not aect the electronic properties, due to the similarity between the structures and nally because the 3R polytype structures are the most stables. The charge density dierence plot was constructed for 3R polytype structures in order to 11



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study the intermolecular interactions between the layers and the interlayer region, anion, cation and the hydration water molecules and between the interlayer components. The charge density dierence plots for the Figures 4 and 5 is dened as:

∆ρlayer−interlayer (~r) = ρLDH (~r) − ρlayer (~r) − ρSO4 (~r) − ρX (~r) − ρH2 O (~r) − ρH2 O

r) surrounding X (~ (9)

The ρLDH , ρlayer , ρSO4 , ρX , ρH2 O and ρH2 O

surrounding X

are the charge density of the layer

double hydroxyde, of the layer, of the SO4 , of the X, where X is Na or K, of the interlayer water molecules and of the Na or K hydration water molecules, respectively. In the Figures 4 and 5 the red regions indicate the surplus of charge density, while the blue regions represent the depletion of charge density and the contour spacing is 0.004 electrons/Borh3 . The interactions between the anion and the hydroxyl layer are stronger than those between the water molecules and the hydroxyl layer. The ZnAl hydroxyl layer interacts stronger with the interlayer region than the MgAl hydroxyl layer. It can be seen in the Figure 4, comparing the electronic density in the hydroxyl groups. There is more electronic density transfer in ZnAl hydroxyl groups than MgAl layer, it is also showed in density of states calculation. Figure 5 shows the interaction between water molecules and anions, the stabilization of the cation by its hydration water molecules and the whole hydrogen bound network which stabilize the interlayer region and consequently the layered double hydroxide structure.

12


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Figure 4: Charge density dierence of Mg-Al-(NaSO4 ), Mg-Al-(KSO4 ) (upper line) and ZnAl-(NaSO4 ), Zn-Al-(KSO4 ) (bottom line). The contour spacing is of 0.004 electrons/Bohr3 .

Figure 5: Charge density dierence focusing on interlamellar interactions of Mg-Al-(NaSO4 ), Mg-Al-(KSO4 ) (upper line) and Zn-Al-(NaSO4 ), Zn-Al-(KSO4 ) (bottom line). The contour spacing is of 0.004 electrons/Bohr3 The density of states and the projected density of states analysis, have the main objective to identify the basicity or the acidity of the structure sites. The most basic sites are those 13



in the valence band closest to the Fermi level and the most acid sites are those closest to the Fermi level in the conduction band. Figure 6 shows the PDOS of the intercalated SO2− 4 and hexahydrated cation (Na+ and K+ ). The intercalated anions and cations have the same behavior in each structure. For Mg/Al layer composition the anion electronic density in the valence band is closer to the Fermi level than for Zn/Al layer. However for Zn/Al layer composition the most basic sites are the hydroxyl groups in the layer, thus explaining the observed dierence in the electronic density transfer between the two layers composition.

Figure 6: Density of States (DOS) and projected density of states (PDOS) of the calculated 3R layered double hydroxydes with dierent lamellar composition and intercalated cation. The Bader charges analysis was performed to verify the atomic charge distribution in the Motukoreaite structure. The charge calculated using the Bader method is a good approximation to the total electronic charge of an atom. 38 The Bader charges are shown in Table 4.

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Table 4: Bader charges of the calculated 3R polytype layer double hydroxide(e). Mg Zn Al O (layer) H (layer) SO2− 4 Na K H2 O (cation)

Mg-Al-(NaSO4 ) 1.75 3.00 -1.95 1.00 -1.59 0.89 -0.02

Zn-Al-(NaSO4 ) 1.35 3.00 -1.82 1.00 -1.59 0.89 -0.01

Mg-Al-(KSO4 ) 1.75 3.00 -1.95 1.00 -1.59 0.88 -0.02

Zn-Al-(KSO4 ) 1.35 3.00 -1.82 1.00 -1.59 0.88 -0.01

The anion charge is the same for all structures and the cations charges are similar. The main charge dierence is in the hydroxyl layer between Mg and Zn charges. This dierence agrees with the analysis made before. The hydroxyl groups are the most basic sites in the Zn/Al layer double hydroxides showed in the PDOS analysis and have greater electronic density transfer. Therefore the lower charge of the Zn atoms comparing to Mg atoms explain why this hydroxyl groups are more basic. The water molecules surrounding the cation have a slightly negative behavior, caused by the cation eletrostatic interaction.

Conclusions DFT calculations were performed to investigate the structure of the Mg-Al-(NaSO4 ), Mg-Al(KSO4 ) Zn-Al-(NaSO4 ) and Zn-Al-(KSO4 ) hydrotalcite-like compounds that have exchangeable interlayer hexahydrated monovalent alkaline cations. The simulation of the natural Motukoreaite (Mg-Al-(NaSO4 )) and Natroglaucocerinite (Zn-Al-(KSO4 )) showed a good agreement with the experimental parameters. The thermodynamic analyses indicate the polytype 3R for all four tested compositions. The main dierence in the electronic density was observed when the layer composition was changed. The hydroxyl groups in the Zn/Al layer showed a basic behavior. This is evident when we compare the Bader charges of the divalent cations.

15



As far as the authors know, this is the rst time that DFT calculations were performed using LDH as cationic exchangers. The thermodynamic properties for the exchange reaction indicated that the cations can be exchanged without removing the intercalated sulfate, as also indicated experimentally. The proposed reactions is just performed by replacing the alkaline metal in the triple charges anions between the layers [Na(SO4 )2 (H2 O)12 ]3− by other cations like K+ . The thermodynamic potentials for the exchange of Na+ by K+ show that the exchange does not occur, but the reverse reaction does, although experimentally both reactions has been performed. 12 The change in the hydroxyl layer composition did not aect these exchange reactions. The same behavior is observed with a dierent polytype. Calculations and experimental data are under way to replace both the cations and sulfate by the carbonate/bicarbonate system and will be objective of forthcoming publications.

Acknowledgement This study was nanced in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Brasil (CAPES) - Finance Code 001 (fellowship for PIRM), CNPq (research grant for AAL and process 309729/2017-3, for FW and process 303846/2014-3 and 400117/20169), FAPEMIG (CEX APQ 02191/2017), Brazilian agencies and the enterprise PETROBRAS S/A - CENPES (AAL). We also acknowledge the CENAPAD-SP computational center for the use of its facilities by performed calculations.

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DTF Study of Layered Double Hydroxides with Cation Exchange

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