Layered Double Hydroxides with the Composition Mn/Al-SO₄-A (A = Li, Na, K; Mn:Al ca. 1:1) as Cation Exchangers

Abstract

Hydrated Mn/Al-SO₄-A (A = Li, Na or K) layered double hydroxides with M²⁺:M³⁺ molar ratio close to 1:1 were synthesized by co-precipitation with increasing pH. All phases presented solvated alkali metals and sulfate as intercalated species. In the exchange reactions, when sulfate solutions of alkali metals were used, sodium could be partially replaced by potassium, and the same was true for the potassium phase, where potassium could be partially replaced by Na and Li, without removing the intercalated sulfate anions. This class of compounds, considered to be anion exchanger, can also exchange cations, which opens new perspectives for applications.

Keywords:
layered double hydroxides; exchange reactions; shigaite; cation exchangers

Introduction

Layered double hydroxides (LDHs) are materials with a generic composition [M²⁺_1-xM³⁺_x(OH)₂](A^n-)_x/n.yH₂O, where M²⁺ in the brucite-like structure is partially replaced by M³⁺, generating an excess of positive charges, which are counterbalanced by the intercalation of hydrated anions. Normally crystalline materials can be obtained with M²⁺:M³⁺ molar ratios in the range of 2 to 4.

Although different anions can be intercalated between the LDHs layers,^1,2 the intercalation of sulfate are not so frequently reported, due specially to the system complexity.^3-8 In a recent work, Khaldi et al.⁹ detected two different phases in the LDH Zn/Cr-SO₄ (Zn²⁺:Cr³⁺ = 2:1), which were dependent on the pH and washing procedure. During fast washing the basal distance of 11 Å was obtained and after extensive washing, the basal distance was reduced to 8.9 Å. The authors also observed that the 11 Å phase was rich in sulfate and contained sodium while the phase of 8.9 Å was poor in sulfate and devoid of sodium. The authors also reported that the basal parameters of these compounds were dependent on the drying process and relative air humidity, during samples manipulations.

The synthesis of Zn/Al and Zn/Cr (Zn²⁺:M³⁺ = 2:1)¹⁰ also indicated several phases, with basal distances of 11 Å (d), 9.2 Å (d, r), 8.8 Å (d), 8.7 Å (r), 8.2 Å (d) and 7.1 Å (d) when different washing and drying conditions were applied (d and r represent the phase observed during dehydration and rehydration, respectively). All the phases were attributed to several degrees of sulfate hydration and interpolytype transitions during dehydration/rehydration and the lower basal spacing was attributed to grafting of sulfate to the LDH layers.

Another work⁹ described the Zn:M³⁺-SO₄ (Zn²⁺:M³⁺ = 2:1 and M³⁺ = Al, Cr) system and explored the precipitation at acid and basic pH and also the hydrothermal treatment at 125 and 150 ºC. Again, the phase of 10.9 Å was observed at room temperature, which after the hydrothermal treatment was transformed to a phase with a basal distance of 8.8 Å. In these samples, no sodium was detected and this change was attributed to interpolytype transitions.

LDH minerals intercalated with sulfate, with the ideal formula (Na(H₂O)₆)[M²⁺ ₆Al₃(OH)₁₈(SO₄)₂].6H₂O, where M²⁺ = Mg, Zn, Mn and Fe, respectively for the minerals motukoreaite,¹¹ natroglaucocerinite,¹² shigaite^13-15 and nikisherite¹⁶ are also known. Iron green rust NaFe^II₆Fe^III₃(SO₄)₂(OH)₁₈.12H₂O was also reported to have the same structure.¹⁷ In these materials, the interlayer domains present the anion [NaSO₄.SO₄(H₂O)₁₂]^3-, where sodium cations are coordinated by six water molecules in an octahedral arrangement that compensates the cationic layer domains [M²⁺₆Al₃(OH)₁₈]³⁺. As only sulfate and alkali metal cations are available, the way to produce a neutral structure is to intercalate one sulfate anion and one sodium sulfate anion, which occurs in the mineral structures {[M²⁺₆Al₃(OH)₁₈]³⁺[(SO₄ ^2-)(NaSO₄^-]^3-}.

It is important to emphasize that most of the papers involving LDH indicated that this class of materials can exchange only anions and the present work proposes that these materials can also exchange cations, as recently proposed for LDH with M²⁺:M³⁺ molar rations of 2:1.¹⁸ To contribute to research in this area we studied the synthesis of Mn/Al-SO₄-A (A = Na, K, Li), but instead of Zn:Al molar ratios of 2:1,^12-16 common for the minerals and synthetic LDH,¹⁸ we investigated phases with molar ratios of 1:1. The aim was to investigate the exchange of the alkali metal without removing sulfate anions.

Experimental

All the chemicals were of analytical grade (more than 99% purity) and were used without any treatment. In the synthesis of Mn/Al-SO₄-Na (Na-phase), co-precipitation with increasing pH was used. To 100 mL of type-I water (Milli-Q integral purification system: 18.2 MΩ cm) solutions containing 26.151 mmol of MnSO₄.H₂O, 13.079 mmol of Al₂(SO₄)₃.16H₂O and 4.365 mmol of Na₂SO₄ were titrated with NaOH 1.5 mol L^-1 in a glass reactor operating at 90 ºC, under flow of N₂ (pH was increased from 3.28 to 9.02). The materials were maintained at 90 ºC for 120 h, centrifuged at 4000 rpm, washed several times with type-I water and dried at room temperature until constant weight.

In the synthesis of Mn/Al-SO₄-K (K-phase), 25.795 mmol of MnSO₄.H₂O, 12.895 mmol of Al₂(SO₄)₃.16H₂O and 4.304 mmol of K₂SO₄ were titrated with KOH (pH was increased from 3.45 to 8.99) and the synthesis conditions were maintained constant. In the synthesis of Mn/Al-SO₄-Li (Li-phase), 26.506 mmol of MnSO₄.H₂O, 12.760 mmol of Al₂(SO₄)₃.16H₂O and 4.366 mmol of Li₂SO₄ were titrated with LiOH (pH was increased from 3.47 to 8.98) and the synthesis conditions were maintained constant.

To exchange intercalated Na with K, an aqueous dispersion of the Na-phase was stirred with an excess of K₂SO₄ (three times the concentration of intercalated sodium) for 24 h at room temperature. To exchange intercalated K with Na and Li in K-phase, the same procedure was adopted, but using Na₂SO₄ and Li₂SO₄, respectively. To avoid contamination with carbonate, all the reactions were performed under flow of N₂ and after the reactions the materials were centrifuged at 4000 rpm, washed several times with type-I water and dried at room temperature until constant weight.

To expose the basal diffraction peaks in the X-ray diffraction (XRD) patterns, the samples were prepared before the drying process, using a drop of the last dispersion after centrifugation which was deposited on the sample holder and let to dry at room temperature. The XRD patterns were obtained using a Shimadzu XRD-6000 diffractometer with CuK_α radiation source of λ = 1.5418 Å, current of 30 mA, tension of 40 kV, dwell time of 2º min^-1 and step of 0.02 degrees.

The FTIR (Fourier transform infrared) spectra were obtained in the transmission mode in a Bio-Rad FTS 3500GX spectrometer in KBr discs. A total of 32 scans were accumulated from 400 to 4000 cm^-1, with resolution of 4 cm^-1.

To investigate the morphology and the qualitative chemical composition, scanning electron microscopy (SEM) images were acquired with an FEI Quanta 450 FEG electron microscope with AZ Tech software equipped with an X-ray energy dispersive spectroscopic analyzer using an SDD (silicon drift) detector. The samples were deposited on copper double-face tape, and after the EDS (energy dispersive X-ray spectroscopy) analysis, the samples were gold sputtered for SEM image acquisition.

The spectrometric determinations were carried out with a Thermo Scientific simultaneous axial view ICP OES (inductively coupled plasma optical emission spectrometry) spectrometer (model iCAP 6500) and the ThermoiTeVa Analyst version 1.2.0.30 program was used for data treatment. Two samples of the same composition were dissolved in a solution containing 1.0% (v/v) HNO₃, diluted when necessary and analyzed in duplicate. Average values were used in the compounds formulations.

Selected area electron diffraction (SAED) spectra were obtained with a JEOL JEM 1200 EX-II apparatus operating at 120 kV.

Results and Discussion

The X-ray diffraction patterns of Na (Figure 1a), K (Figure 1c) and Li-phase (Figure 1f) were very similar to the phase obtained for the Zn/Cr-SO₄-Na reported by Khaldi et al.,⁹ indexed as 2H structure (a = b = 5.49 Å, c = 22.06 Å).

Figure 1
XRD patterns of Na-phase (a); after Na/K exchange (b); K-phase (c); after K/Na exchange (d); after K/Li exchange (e) and Li‑phase (f) (oriented to expose basal peaks). Insert: expansion of the XRD in the range of 50 to 65° region.

Although the composition M²⁺:M³⁺ is close to 1:1, the Na-phase XRD pattern (Figure 1a) is very close to the Na-shigaite (a = b = 9.51 Å, c = 33.09 Å).^13-15

Another possibility is to index the LDH as 1H polytype, although this composition sodium was not detected,¹⁹ but according to the authors, there were two superlattice diffraction peaks in the region of 18-23º (in 2θ), which were confirmed in the SAED spectra (Figure 2) and also observed in the XRD patterns, indicating that this polytype was the most probable in the synthesized samples.

Figure 2
SAED spectra of Na (a); K (b) and Li-phase (c), indicating the lattice and superlattice spots.

The cell parameters used to index the Na-phase were a = b = 5.49 Å and c = 11.03 Å. The difference between these phases and ours is that the former had M²⁺:M³⁺ molar ratio of 2:1, while the present case had Mn:Al molar ratio of 1:1 (see ICP OES analysis, in Table 1). The basal distance of the K-phase was slightly expanded to 11.23 Å (Figure 1c) in comparison with the Na-phase, while in the Li-phase the basal distance was contracted to 10.90 Å. One main difference between the Na, K and Li-phases is the existence of three diffraction peaks in the region of 57-63º (in 2θ) for the Na-phase, while this region contains four diffraction peaks in the K and Li-phases, which indicates a probable reduction of the structural symmetry.

Table 1
Compositions of the samples obtained by ICP OES

Sample	Mn (x)a	Al (y)a	SO4 (z)a	Alkali metal (w)a	Mn/Alb
Na-phase	0.553	0.447	0.321	Na = 0.195	1.24
Na-phase/K	0.512	0.488	0.322	Na = 0.095; K = 0.061; Na + K = 0.156	1.05
K-phase	0.559	0.441	0.286	K = 0.132	1.27
K-phase/Na	0.522	0.478	0.305	K = 0.057; Na = 0.074; Na + K = 0.131	1.09
K-phase/Li	0.514	0.486	0.324	K = 0.049; Li = 0.112; Na + Li = 0.161	1.06
Li-Phase	0.531	0.469	0.277	Li = 0.084	1.13

• ^a
  x,y,z,w: according to the LDH ideal formula, Mn_xAl_y(OH)₂(SO₄)_zA_w, where A means alkali metal;
• ^b
  Mn/Al: slightly higher than 1.0 is probably due to higher degree of hydration of the Al₂(SO₄)₃·16H₂O salt used in the synthesis.

After the exchange of Na with K (Figure 1b) and K with Na or Li (Figures 1d and 1e), the basal parameters remained almost the same as those of the parent material, which would indicate an unsuccessful reaction, but probably hydrated sulfate anion double layers defined the basal distance, with only a slight influence of the hydrated alkali cations. None of the phases presented any possible contaminations or reduced basal distances, as previously reported for dehydrated sulfate intercalated samples.^10,11,20 To confirm whether the samples basal distance would vary with the Mn:Al molar ratio, samples of Na, K and Li-shigaite were synthesized with the M²⁺:M³⁺ molar ratio of 2:1, using similar conditions as used for the 1:1 phases (Figure 3).¹⁸

Figure 3
XRD patterns of Na (a); K (b) and Li-shigaite (c). The samples were oriented in the sample holder to expose basal peaks. Insert: expansion of the XRD in the region of the third basal peak.

The XRD patterns were very similar and the basal distances were close to the Mn:Al molar ratios of 1:1, within the techniques experimental error range (Mn:Al 1:1, Na = 11.03 Å, K = 11.23 Å and Li = 10.90 Å; Mn:Al 2:1, Na = 11.03 Å, K = 11.28 Å and Li = 10.88 Å). The position of the 100 peak remained the same (2θ ca. 18.65º), indicating the maintenance of the same a and b parameters.

The FTIR spectra of the 1:1 phases (Figure 4) were in agreement with the literature on sulfate intercalated in LDH^9,21 and layered hydroxide salt (LHS), like gordaite.²²

Figure 4
FTIR spectra of Na-phase (a); after Na/K exchange (b); K-phase (c); after K/Na exchange (d); after K/Li exchange (e) and Li‑phase (f).

Briefly in the Na-phase, the broad band at 3440 cm^-1 is attributed to the stretching of the O–H bonds of the LDH structure and physisorbed/intercalated water molecules. The band at 1654 cm^-1 is attributed to the water molecule bending.

The S–O bonds of sulfate were observed at 1108 cm^-1 (ν3 = shoulder at 1065, 1140 and 1190 cm^-1), 960 (ν1) and 608 cm^-1 (ν4). As the ν1 (region of 960 cm^-1) and ν4 bands (region of 600 cm^-1) were also observed and sulfate is present in a tetrahedral distorted environment,^23,24 interacting with water molecules, these two anions can vibrate in-phase or out-of-phase, inducing two or three bands for the ν3 mode, as indicated for sulfate green rusts, having almost identical environment.²⁵

The M–O and M–OH bonds were detected at 770, 534, 470 and 420 cm^-1. The only small difference between the Na-phase (Figure 4a), K-phase (Figure 4c) and Li-phase (Figure 4f) was the splitting of the band at around 960 cm^-1 in the Na and Li-phases into two bands at around 945 and 975 cm^-1. The broadening and splitting of the bands at 1100 and 960 cm^-1 can probably be attributed to sulfate anions having reduced symmetries, due to the influence of the water molecules, sodium and the positively charged layers.²⁶ After the exchange reactions (Figures 4b, 4d, 4e), the spectra were similar to the parent materials.

The SEM images of the Na-phase (Figure 5a), after Na/K exchange (Figure 5b), K-phase (Figure 5c), after K/Na exchange (Figure 5d), after K/Li exchange (Figure 5e) and Li-phase (Figure 5f) presented the typical sub-micrometer size platelet-like particles with nanometer thicknesses.

Figure 5
SEM images of the Na-phase (a); after Na/K exchange (b); K-phase (c); after K/Na exchange (d); after K/Li exchange (e) and Li phase (f).

All the other samples had a similar profile. After the exchange reactions, the crystals were smaller, probably due to the mechanical abrasion caused by the magnetic stirring. In the qualitative EDS spectra, all the major expected elements were present in the analyzed samples (not shown). The results of ICP OES indicated the compounds had composition close to the M²⁺:M³⁺ molar ratio used in the synthesis and only Na, K and Li were detected in the respective phases (Table 1).

To explain the presence of alkali metal and sulfate into the structure, the reduced composition can be formulated close to the ideal Mn_0.5Al_0.5(OH)₂(SO₄)_0.333A_0.167 (A = alkali metal), which in integral number would be Mn₃Al₃(OH)₁₂(SO₄)₂A₁. Looking to the formula, it is clear to see why the alkali metal is present in the structure since the unit [Mn₃Al₃(OH)₁₂]³⁺ needs to be neutralized and only double negative charged sulfate are present. To neutralize the charged layers one SO₄^2- and one NaSO₄^- anions are necessary. In a generic view, the structure of the compound can be written as [Mn₃Al₃(OH)₁₂]³⁺[(SO₄)₂A₁]^3-·nH₂O or [Mn₃Al₃(OH)₁₂]³⁺[(SO₄)(ASO₄)]^3-·nH₂O.

Only the Li-phase presented a lower content of lithium in comparison to the Na and K phases. The amounts of sulfate and hydroxide in the structure were obtained by difference, but the presence of O and S was detected by EDS (not shown), and OH and sulfate was also confirmed by FTIR. After the exchange reactions, about 39% of the Na was replaced by K in the Na-phase, 56% of the K was replaced by Na in the K-phase and 70% of the K was replaced by lithium in the K-phase, indicating that all the phases can exchange cations, although the first reactions were less favorable. We are currently studying longer exchange times and higher concentrations of salts in solution to try to totally substitute the intercalated cations, without removing the intercalated sulfate anions as well as the exchange of sulfate by carbonate exchanging simultaneously or preserving the alkali metal cations.

To investigate the structure of the samples, SAED patterns were obtained for some selected crystals of the Na (Figure 2a), K (Figure 2b) and Li phases (Figure 2c). As can be seen, the selected Na and K phases presented hexagonal patterns where the crystals were oriented perpendicular to the basal direction (along the [00l] axis). Both presented very close a and b cell parameters, with sharp spots with a = 3.17 Å and a(3)^1/2 × a(3)^1/2 superlattice spots with d = 5.49 Å (Figures 2a and 2b).

These superlattice spots are expected to be present for the ordering of the metals in the layers of LDH with M²⁺:M³⁺ 2:1 molar ratios, but not for M²⁺:M³⁺ 1:1. The Li-phase did not present the superlattice spots (Figure 5c), due probably to the lower content of intercalated lithium (see Table 1). As expected, almost the same a and b cell parameters were also obtained for the samples after the exchange reactions (not shown), integrity (topotactic exchange reaction).

Figure 6 shows the schematic representation of the Mn²⁺:Al³⁺ 1:1 LDH structure intercalated with SO₄^2- and NaSO₄^- (also said, intercalated with SO₄^2- and Na⁺) and the structure resulted from the partial topotactic exchange reactions of Na⁺ with Li⁺ (equation 1), when the reaction was performed in the presence of Li₂SO₄ and under N₂ atmosphere, to avoid the presence of CO₂ and consequently carbonate anions in the solution/intercalated.

Figure 6
Schematic representation of (a) 1:1 Mn²⁺:Al³⁺ LDH structure intercalated with Na+ and SO₄²⁻; (b) the structure resulted from the topotactic partial exchange reactions of Na⁺ with Li⁺.

(1) $\begin{array}{c}\left[{\mathrm{Mn}}_3{\mathrm{Al}}_3{\left(\mathrm{OH}\right)}_{12}\right]\left[{\left({\mathrm{SO}}_4\right)}_2\left.\mathrm{Na}\right)\right]+x/2{\mathrm{Li}}_2{\mathrm{SO}}_4\to \hfill \\ \left[{\mathrm{Mn}}_3{\mathrm{Al}}_3{\left(\mathrm{OH}\right)}_{12}\right]\left[{\left({\mathrm{SO}}_4\right)}_2\left.{\mathrm{Na}}_{1-x}{\mathrm{Li}}_x\right)\right]+x/2{\mathrm{Na}}_2{\mathrm{SO}}_4\hfill \end{array}$

Conclusions

Hydrated phases of Mn/Al-SO₄-A (A = Na, K, Li; Mn:Al = 1:1) were synthesized by co-precipitation with increasing pH. All the samples were evaluated by XRD, presenting patterns starting with series of basal peaks, typical of materials with layered structures. No phases other than 11 Å were detected after extensive washing and drying under air.

After the exchange of Na with K and K with Na or Li, the a and b cell parameters were maintained almost constant in all samples, and only small variations were observed in the basal parameters, indicating that even after partial substitution of the alkali metal, the double layer of hydrated sulfate anions was responsible for the basal distance.

SAED patterns of the Na and K phase presented a(3)^1/2 × a(3)^1/2 superstructure, while the Li phase presented only the basic a × b hexagonal structure projection. After the exchange reactions, SAED patterns also showed maintenance of a and b parameters, indicating the layers’ structural integrity after the reactions.

SEM and EDS indicated platelet-like particle morphology in all the samples, expected for the layered double hydroxides.

The ICP OES measurements indicated that the composition was close to the M²⁺:M³⁺ molar ratio used in the synthesis, indicating successful synthesis of Al rich LDH, with the ideal formula [Mn₃Al₃(OH)₁₂]³⁺[(SO₄)₂A₁]^3-·nH₂O or [Mn₃Al₃(OH)₁₂]³⁺ [(SO₄)(ASO₄)]^3-·nH₂O (A = Li⁺, Na⁺ or K⁺).

To confirm that the synthesized materials had similar structure as Na-shigaite (Mn:Al, 2:1) and can be stable in the presence of other alkali metals, Na, K and Li phases were synthesized and the similarities were confirmed.

Although the synthesized samples belong to the layered double hydroxide family, typical anion exchangers, the proposed reaction occurs just by replacing the alkali metal in the interlayer anions without replacing sulfate. This cation exchange capacity expands the applications of this class of compounds.

To give more information about the structure and thermodynamics properties of the exchange reactions of the minerals with the composition (A⁺(H₂O)₆)[M²⁺₆Al₃(OH)₁₈(SO₄)₂]·6H₂O (M²⁺ = Mg, Zn and A⁺ = Na, K), ab initio calculations were performed.²⁷

Acknowledgments

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil, Finance Code 001, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), projects 303846/2014-3, 400117/2016-9 and the Financiadora de Estudos e Projetos (FINEP). A. R. S., N. A. G. G. and S. C. S. thank CAPES for the PhD scholarship. Loana Mara Baika and Prof Marco Tadeu Grassi are also aknowledged for the ICP OES analysis.

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Publication Dates

• Publication in this collection
  16 Sept 2019
• Date of issue
  Sept 2019

History

• Received
  07 Jan 2019
• Accepted
  09 May 2019