Cation Exchange Reactions in Layered Double Hydroxides

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Cation exchange reactions in layered double hydroxides intercalated with sulfate and alkaline cations (A(H2O)6)[M +26Al3(OH)18(SO4)2].6H2O (M+2= Mn, Mg, Zn; A+= Li, Na, K). Anne Raquel Sotiles, Loana Mara Baika, Marco Tadeu Grassi, and Fernando WYPYCH J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11389 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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Journal of the American Chemical Society

Cation exchange reactions in layered double hydroxides intercalated with sulfate and alkaline cations (A(H2O)6)[M+26Al3(OH)18(SO4)2].6H2O (M+2 = Mn, Mg, Zn; A+ = Li, Na, K). Anne Raquel Sotiles, Loana Mara Baika, Marco Tadeu Grassi, Fernando Wypych*. Departamento de Química - Universidade Federal do Paraná. Caixa Postal 19032, CuritibaPR, CEP-81531-980, Brazil. E-mail adresses: Anne Raquel Sotiles: [email protected]; Loana Mara Baika: [email protected]; Marco Tadeu Grassi: [email protected]; Fernando Wypych*: [email protected] Abstract Layered double hydroxides (LDHs) with similar compositions to the minerals Shigaite, Natroglaucocerinite and Motukoreaite were synthesized by coprecipitation with increasing pH and characterized by several instrumental techniques. These minerals have previously been described to occur only with sodium and sulfate +2 +2 (Na(H2O)6)[M 6Al3(OH)18(SO4)2].6H2O (M = Mn, Mg and Zn). These phases were synthesized successfully along with others containing lithium and potassium. Cation exchange reactions were performed in the presence of alkaline metal sulfates and for the first time several instrumental techniques were employed to show that the cations can be totally exchanged without removing the intercalated sulfate anions. This class of compounds, traditionally considered to be anion exchangers, can also be considered cation exchangers, which opens new avenues for future scientific and industrial applications. Keywords: Layered double hydroxides; Cation exchange; Shigaite; Natroglaucocerinite; Motukoreaite. 1. Introduction Layered double hydroxides (LDHs) following the general formula M+21-n +2 are +3 xM x(OH)2(A )x/n.yH2O are based on the Brucite (Mg(OH)2) structure, where M octahedrally coordinated with hydroxide anions and the octahedra share edges, forming twodimensional layers. In LDH structures, M+2 are partially replaced by M+3 generating an excess of positive charges in the layers’ domains, which are counterbalanced by the intercalation of hydrated anions (A-n)x/n.yH2O, denominated interlayer domains. Hydrotalcite (Mg6Al2(OH)16CO3.4H2O) and Pyroaurite (Mg6Fe2(OH)16CO3.4.5H2O) are common examples of LDH minerals, where in both cases the molar ratio between M+2:M+3 is 3:1 and the intercalated anion is carbonate. LDHs can be synthesized with molar ratios between 2 and 4, while some rare minerals have M+2:M+3 molar ratio of 2:1 and are intercalated with sulfate instead carbonate, with a generic compositions +2 (Na(H2O)6)[M 6Al3(OH)18(SO4)2].6H2O. Among these are Motukoreaite (M+2 = Mg),1 Natroglaucocerinite (M+2 = Zn),2 Shigaite (M+2 = Mn)3 and Nikisherite (M+2 = Fe).4 Figure 1 shows the schematic representation of the Shigaite structure,3 where the sulfate intercalation generates a basal spacing close to 11 Å, as also observed for the other minerals of the group.1-4 In the Shigaite structure, the layer domains are composed of three edge-sharing octahedral [Mn+22Al](OH)6]+1 units, which are counterbalanced by the 1 ACS Paragon Plus Environment

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interlayer domains with composition [Na(SO4)2(H2O)12]-3, where sodium cations are coordinated by six water molecules in an octahedral arrangement.

Figure 1 Schematic representation of the Shigaite structure ([Na(H2O)6]Mn6Al3(OH)18[SO4]2.6H2O),3 viewed along the crystallographic structure axis. Each sodium cation is surrounded by three sulfate anions and each sulfate is linked to one aluminum cation, coordinated in octahedral geometry by six hydroxide anions. In this specific structure, it can be easily noted that to neutralize the layer domain [M+26Al3(OH)18]+3 only SO4-2 anions are available to occupy the interlayer domain. If one sulfate anion is used, an excess of positive changes is created in the structure ({[M+26Al3(OH)18]+3[SO4-2]}+). If two sulfate anions are used, an excess of negative charges is created ({[M+26Al3(OH)18]+3[(SO4)2]-4}-. Consequently, since alkaline metal cations are available, there are two way to produce a neutral structure is to intercalate one sulfate anion and one sodium sulfate anion, which occurs in the mineral structures {[M+26Al3(OH)18]+3[(SO4-2)(NaSO4-]-3} or the intercalation of 1.5 sulfate anions. Other intercalated cations together with sulfate anions have also been reported in the literature, such as Wermlandite, with structure ((Ca(H2O)6)[Mg7Al2(OH)18(SO4)2].6H2O5-7. According to the classification suggested by S.J. Mills and co-workers,8 all these compounds belong to the Wermlandite group. In this group, hydrated cations are intercalated in the presence of anions (especially sulfate and carbonate) and the basal spacing is close to 11 Å. Synthetic or natural sulfate green rusts (GRS04) also a similar structure to the abovementioned minerals, with compositions (Na(H2O)6)[Fe6+2Fe3+3(SO4)2(OH)18].6H2O.9,10 All have a basal distance close to 11 Å and present a 3ax3a superstructure (A = distance between the metal atoms in the layers), attributed to the ordering of the sodium cations between the layers (Fig. 1). Many attempts have been made to intercalate sulfate anions between the LDH layers, resulting in various basal distances, like those reported by De Roy and co-workers,11,12 Radha and co-workers13-15 and Liu and Yang.16 Common aspects are basal distances of ~ 8.9 Å and ~ 11 Å, with phases that are very sensitive to humidity, and in some cases with not totally explained interpolytypical transitions. It is important to emphasize that different polytypes normally present different physical and chemical properties, so interpolytypical conversion is an important topic of study. 2 ACS Paragon Plus Environment

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Some authors also have reported the presence or absence of alkaline metals between the layers together with sulfate, like those reported for the minerals,1-4 as well as grafting of sulfate anions to the layers’ surface after mild thermal treatment, generating basal distances in the range of 6.7 to 7.2 Å.12,17 Grafting of sulfate has also been observed in layered hydroxide salts like sodium Gordaite [NaZn4(OH)6(SO4)Cl.6H2O] and Theresemagnite [NaCo4(OH)6(SO4)Cl.6H2O].18-21 Khaldi et al.11 reported that when materials were rinsed with sulfate solutions of ammonium or alkaline metals, the structure’s basal spacing changed from 11 Å to 8.9 Å, suggesting that the removal of alkaline metals provokes this reduction of basal distance. Different behavior was observed when sulfate green rusts9 were washed under similar conditions, in which case the basal distances were maintained almost constant at 11 Å. As far as the author know, the only reported way to replace cations into LDH, is through the process known as diadochy, where cations present in the layers can be replaced by other cations from the solution.22-24 Based on the contradictions in the literature regarding the presence of sulfate/alkaline metals and the changes after exchange reactions, the objective of the present work was to synthesize materials having structure of Shigaite, Natroglaucocerinite and Motukoreaite, all containing sulfate anions and alkaline metals like lithium, sodium and potassium cations, and to study the ion exchange reactions. The main purpose was to show that cations can be exchanged without removing intercalated sulfate anions. 2. Materials and methods All the chemicals were of analytical grade (more than 99% of purity) and were used without any treatment. The Shigaite, Natroglaucocerinite and Motukoreaite compounds were synthesized by coprecipitation by increasing the pH. Solutions of M+2SO4 (M+2 = Mg, Zn, Mn), Al2(SO4)3 and A2SO4 (A+ = Li, Na, K) in 100 mL of Mili-Q water, with molar ratios close to 6:3:1 (Table 1) were slowly added to a solution of AOH 1.5 mol L-1 (A+ = Li, Na, K) in an automatic glass titration reactor operating at 90 °C, under N2 flow. After the precipitation, the materials were maintained at 90 °C for 120 h, centrifuged at 4000 rpm and washed several times with Milli-Q water and dried at room temperature until constant weight. Table 1 – Amounts of chemicals used and pH in each synthesis. Compound M+2SO4 Al2(SO4)3 A2SO4 M+2:M+3 Initial Final pH (mol L-1) (mol L-1) (mol L-1) (M+2:A) pH Shig. Li 26.506 6.630 2.183 2.00 (6.07) 3.47 8.98 Shig. Na 26.151 6.535 2.182 2.00 (5.99) 3.50 9.01 Shig. K 25.796 6.439 2.123 2.00 (6.17) 3.39 9.00 Natro. Li 25.108 6.281 2.092 2.00 (6.00) 3.40 9.55 Natro. Na 24.795 6.186 2.042 2.00 (6.07) 3.55 9.48 Natro. K 24.447 6.106 2.008 2.00 (6.09) 3.54 9.55 Mot. Li 31.605 7.899 2.638 2.00 (5.99) 3.37 8.98 Mot. Na 31.118 7.772 2.605 2.00 (5.97) 3.53 9.01 Mot. K 30.591 7.645 2.525 2.00 (6.06) 3.39 9.00 Shig. = Shigaite (M+2 = Mn); Natro. = Natroglaucocerinite (M+2 = Zn); Mot. = Motukoreaite (M+2 = Mg); Phases of Li, Na and K (A+ = Li, Na and K, respectively) 3 ACS Paragon Plus Environment


Using as examples the Na-Shigaite phase and the exchange of intercalated Na by K, an aqueous dispersion of the Na-Shigaite was stirred with an excess of K2SO4 (three times the concentration of intercalated sodium) for 96 hours at room temperature. To avoid contamination with carbonate, all the reactions were performed under N2 flow and after the reactions the materials were centrifuged at 4000 rpm, washed and dispersed again in a new portion of Milli-Q water and then placed for some seconds in an ultrasonic bath. This process was repeated several times. The solid materials were dried at room temperature until constant weight. For the other exchange reactions, the right combinations of different alkaline metal sulfates were used with the layered solids. The X-ray diffraction (XRD) patterns were obtained from the last samples dispersed in water after the last washing. These samples were placed in glass sample holders and after drying were gently pressed to avoid any diffraction peak displacements. The measurements were performed using a Shimadzu diffractometer (model XRD-6000) with CuKα radiation source of λ = 1.5418 Å, current of 30 mA, tension of 40 kV, dwell time of 2° min-1 and step of 0.02 degree. Fourier-transform infrared (FTIR) spectra were obtained in the transmission mode with a Bio-Rad spectrometer (model FTS 3500GX), in KBr discs using around 1% of each sample (w/w). A total of 32 scans were accumulated from 400 to 4000 cm-1, with resolution of 2 cm-1. To investigate the morphology and the qualitative chemical composition, scanning electron microscopy (SEM) images were acquired with an FEI Quanta 450 FEG electronic microscope with AZ Tech software, equipped with an X-ray energy dispersive spectroscopic analyzer using an SDD detector. The samples were deposited on copper double-face tape and after the EDS analysis the samples were gold sputtered for SEM imaging. The elements present in the samples were quantified with a Thermo Scientific simultaneous axial view ICP-OES spectrometer (model iCAP 6500), and the data were treated with the ThermoiTeVa Analyst version 1.2.0.30 program. The samples 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. The selected area electron diffraction (SAED) spectra were obtained with a JEOL JEM 1200 EX-II apparatus operating at 120 kV, using Au as standard for the calculations of the cell parameters. 3. Results and discussion. 3.1 – Synthesized phases Table 2 shows the chemical composition of all synthesized phases, according to the ICP-EOS data. Sulfate was analyzed by EDS and the data, although qualitative, are consistent with the proposed formula (see Supporting information). It can be seem that the M+2:M+3 molar ratio is very close to the value used during the synthesis, indicating quantitative precipitation, and all the samples retained not only sulfate, as indicated by the FTIR data (Fig. 3), but also the alkaline metal. For all compounds, the molar ratios are very close to the expected formula of the minerals (Ex.: for Na-Shigaite, the expected M+2:M+3:SO4:Na is of 6:3:2:1) (Table 2).3 The X-ray diffraction patterns of Shigaite (Fig. 2A), Natroglaucocerinite (Fig. 2B) and Motukoreaite-like (Fig. 2C) compounds showed a series of basal peaks, typical of layered materials. All the phases could be synthesized with lithium (Fig. 2A,B,C-a), sodium (Fig. 2A,B,C-b) and potassium (Fig. 2A,B,C-c), but some differences were observed. 4 ACS Paragon Plus Environment

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Table 2 - Compositions of the samples obtained by ICP/EOS. Sample Mn(x) Al(y) SO4(z) Alkaline metal (w) M+2:M+3:SO4:A Shig. Li 0.661 0.339 0.220 Li = 0.100 6:3.08:2.00:0.91 Shig. Na 0.664 0.336 0.218 Na = 0.100 6:3.04:1.97:0.90 Shig. K 0.663 0.335 0.216 K = 0.101 6:3.03:1.95:0.91 Sample Zn(x) Al(y) SO4(z) Alkaline metal (w) M+2:M+3:SO4:A Natro. Li 0.662 0.338 0.220 Li = 0.102 6:3.06:1.99:0.92 Natro. Na 0.666 0.334 0.217 Na = 0.100 6:3.00:1.95:0.90 Natro. K 0.667 0.333 0.217 K = 0.100 6:3.00:1.95:0.90 Sample Mg(x) Al(y) SO4(z) Alkaline metal (w) M+2:M+3:SO4:A Mot. L 0.666 0.334 0.217 Li = 0.100 6:3.00:1.95:0.90 Mot. Na 0.666 0.334 0.217 Na = 0.100 6:3.00:1.95:0.90 Mot. K 0.666 0.334 0.218 K = 0.101 6:3.00:1.96:0.91 Shig. = Shigaite; Natro. = Natroglaucocerinite; Mot. = Motukoreaite; x,y,z,w = According to the LDH ideal formula: M+2xAly(OH)2(SO4)zAw (A+ = Li, Na and K). B

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Figure 2 - XRD patterns of the Shigaite (A), Natroglaucocerinite (B) and Motukoreaite-like (C) compounds. Phases of lithium (a), sodium (b) and potassium (c). *- Minor contaminations. All the Shigaite-like phases showed the sharp basal peaks as well as the sodium and potassium of Natroglaucocerinite-like phases, indicating good structural ordering of the packed layers and long-range crystalline domains in the basal direction. In the case of the 5 ACS Paragon Plus Environment


lithium and potassium phases of Natroglaucocerinite, sharp basal peaks were also observed with minor contaminations probably attributed to co-intercalation of carbonate. In the case of Motukoreaite-like compounds, all the phases presented broad basal peaks, attributed to packing disorder and short-range crystalline domains in the basal direction. Since only the sodium structures were described as minerals,1-4 only sodium phases could be compared with the literature, in which cases the published basal distances and the experimental data presented good agreement (Table 3). Table 3 - Basal distance obtained from XRD and data from the literature (in brackets), available only for the sodium phases [1-3,8]. *- Minor contaminations. Material Basal distance (Å) Li-Shigaite 10.88 Na-Shigaite 11.03 (11.02) K-Shigaite 11.28 Li-Natroglaucocerinite 11.15; *7.62 Na-Natroglaucocerinite 11.14 (11.18) K-Natroglaucocerinite 11.40; *11.09 Li-Motukoreaite 10.96 Na-Motukoreaite 11.01 (11.17) K-Motukoreaite 10.87 Figure 3 shows the FTIR spectra of the synthesized Shigaite (A), Natroglaucocerinite (B) and Motukoreaite-like (C) compounds. A

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a  H2O

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Figure 3 - FTIR spectra of the Shigaite (A), Natroglaucocerinite (B) and Motukoreaite-like (C) compounds. Lithium (a), sodium (b) and potassium (c). 6 ACS Paragon Plus Environment

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As can be observed, the bands for all the spectra are similar due to the structural similarities. In the sodium and potassium phases of Shigaite (Fig. 3A-b,c), the main broad band attributed to the 3 asymmetrical bending in the region of 1100 cm-1 are split into two or three bands (1109, 1144 and 1192 cm-1). Since the  (region of 960 cm-1) and  bands (region of 600 cm-1) were also observed, sulfate was present in a distorted tetrahedral environment, 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.25-27 The ν3 band coalesced to a broad band for Li-Shigaite (Fig. 3A-a) and Na and K-Motukoreaite (Fig. 3C-b,c). The splitting of the ν3 originated one separate band at 1235 cm-1, observed for Li phases of Natroglaucocerinite (Fig. 3B-a) and Motukoreaite (Fig. 3C-a). The ν1 band is clearly split into two separate bands only for potassium phases of Shigaite and Motukoreaite (Fig. 3A,B-c). Also, as observed in the XRD patterns of the LiNatroglaucocerinite phase (Fig. 2B-a) and to a lesser extent in the K-phase (Fig. 2B-c), the same phase presented one extra band at 1363 cm-1. This can be attributed to the presence of carbonate, which also contributes to the broadening of the band in the region of 650-700 cm1 (Fig. 3B-a,c). The SEM images of the Li-Shigaite (Fig. 4A), Na-Shigaite (Fig. 4B) and K-Shigaite (Fig. 4C) indicated that all the compounds presented the typical micrometer platelet-like particles with the sub-micrometer thicknesses. Most of the crystals presented well defined corners with angles close to 120°, typical of hexagonal structures and preferential growth in the axb directions (perpendicular to the layer stacking), typical of layered compounds. Li-Shigaite (Fig. 4A) and Li-Natroglaucocerinite (Fig. 4D-F) had bigger particles than the other phases. As indicated by the broadening of the diffraction lines in the XRD pattern of all Motukoreaite-like phases (Fig. 1C), the particles were smaller and highly aggregated in this sample (Fig. 4G-I) in comparison to the other samples. In the qualitative EDS spectra (see Supporting information), all the expected elements were present, excluding Li that could not be detected by the technique. The SAED spectra of all Shigaite samples (Fig. 5A,B,C) and Natroglaucocerinite (Fig. 5D,E,F) had hexagonal patterns where the crystals were oriented perpendicular to the basal direction (along the [00l] axis). In Motukoreaite, although the crystallinities were lower, similar results were observed (not shown). In the SAED spectra of the Shigaite samples, the strong spots present the a = b parameters of 3.19 Å and a supercell of 5.49 Å (weak spots), representing a superstructure of a(3)0.5x a(3)0.5, close to the observed structure of sulfate green rusts reported in the literature (a = 5.36 Å and c = 11.04 Å),27 (a = 5.524 Å and c = 11.011 Å)28 and Na-Shigaite as minerals where the distances between the metals were of 3.171 Å.3 For K-Motukoreaite-like (Fig. 5D) and other samples (not shown), the same profile was observed but the a and b parameters were close to 3.08 and 3.06 Å for the Natroglaucocerinite and Motukoreaite samples, in accordance with the parameters of the respective minerals containing sodium (distances between the metals were 3.079 and 3.057 Å, respectively ).1,2 Another sulfate green rust also indicated the structure, without any superstructure (a = 3.19 Å and c = 11.07 Å).26 In fact, all Shigaite samples should present a superstructure of the 3ax3a type, as observed in the Na-Shigaite mineral,3 very close to another sulfate green rust reported in the literature (a = 9.528 Å and c = 10.968 Å),9 also common for other 7 ACS Paragon Plus Environment


homologous minerals.1,2,4,29 The expected superstructure of 3ax3a was not observed, probably due to the absence of alkaline metal ordering between the layers, so the observed superstructure was probably related to the ordering of the aluminum metals in the layers, as also indicated for the ordering of Fe+3 in the sulfate green rust with similar composition NaFe(II)6Fe(III)3(SO4)2(OH)18.12H2O, originating the a(3)0.5x a(3)0.5 superstructure.9

Figure 4 - SEM images of Shigaite: Li (A), Na (B), K (C); Natroglaucocerinite: Li (D), Na (E), K (F); Motukoreaite: Li (G), Na (H), K (I).

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Figure 5 - SAED spectra of Shigaite: Li (A), Na (B) and K (C) and Natroglaucocerinite: Li (D), Na (E) and K (F). 3.1 – Materials after the exchange reactions As indicated in the ICP-EOS analysis (Table 4), after the exchange reactions most of the alkaline metals were exchanged and the metals present in the layers remained almost constant, indicating that only the metal cations were exchanged. Figure 6A shows the XRD patterns of Li-Shigaite (a), after exchange of Li with Na (b), after exchange of Li with K (c), Na-Shigaite (d), after exchange of Na with K (e), after exchange of Na with Li (f), K-Shigaite (g) and after exchange of K with Na (h) and K by Li (i). It can be clearly seen that the basal distance of the Li-Shigaite of 10.88 Å (Fig. 6A,B-a) changed to the basal distance of the corresponding metals: Li/Na and Li/K exchanged phases (Fig. 6A,B-b and Fig. 6A,B-c) presented, respectively, basal distances of 11.01 Å and 11.27 Å, matching the synthesized Na and K phases, with respective basal distances of 11.03 Å (Fig. 6A,B-d) and 11.28 Å (Fig. 6A,B-g). The Na phase (Fig. 6A,B-d) and K phase (Fig. 6A,B-g) had respective basal distances of 11.03 Å and 11.28 Å, almost the same distances after the exchange reactions (Na/K = 11.04 Å and Na/Li = 11.02 Å; K/Na = 11.26 Å and K/Li = 11.20 Å), indicating that in spite 9 ACS Paragon Plus Environment


of the almost complete exchange reactions (Table 4), the basal distance was not influenced, being defined by the sulfate double layer (Fig. 1). Table 4 - Compositions of the samples obtained by ICP/EOS. Sample Mn(x) Al(y) SO4(z) Alkaline metal (w) Mn/Al Ex.(%) Shig. Li 0.661 0.339 0.220 Li = 0.100 1.95 Shig. Li/Na 0.664 0.336 0.223 Li = 0.009; Na = 0.100; Li+Na = 0.109 1.98 92 Shig. Li/K 0.663 0.337 0.221 Li = 0.008; K = 0.097; Li+K = 0.105 1.97 92 Shig. Na 0.664 0.336 0.218 Na = 0.100 1.98 Shig. Na/K 0.663 0.337 0.220 Na = 0.001; K = 0.102; Na+K = 0.103 1.97 99 Shig. Na/Li 0.665 0.335 0.240 Na = 0.002; Li = 0.142; Na+Li = 0.144 1.99 99 Shig. K 0.665 0.335 0.218 K = 0.101 1.98 Shig. K/Na 0.663 0.337 0.229 K = 0.001; Na = 0.120; Na+K = 0.121 1.97 99 Shig. K/Li 0.666 0.334 0.223 K = 0.001; Li = 0.110; K+Li = 0.111 1.99 99 Sample Zn(x) Al(y) SO4(z) Alkaline metal (w) Mn/Al Ex.(%) Natro. Li 0.662 0.338 0.220 Li = 0.102 1.96 Natro. Li/Na 0.667 0.333 0.218 Li = 0.024; Na = 0.079; Li+Na = 0.103 2.00 77 Natro. Li/K 0.666 0.334 0.220 Li = 0.023 ; K = 0.082 ; Li+K = 0.105 1.99 78 Natro. Na 0.666 0.334 0.217 Na = 0.100 1.99 Natro. Na/K 0.664 0.337 0.209 Na = 0.016; K = 0.063; Na+K = 0.079 1.97 80 Natro. Na/Li 0.666 0.334 0.240 Na = 0.018; Li = 0.127; Na+Li = 0.145 1.99 88 Natro. K 0.667 0.333 0.217 K = 0.100 2.00 Natro. K/Na 0.664 0.336 0.223 K = 0.002 Na = 0.107; Na+K = 0.109 1.98 98 Natro. K/Li 0.664 0.336 0.233 K = 0.014; Li = 0.115; Na+Li = 0.129 1.98 89 Sample Mg(x) Al(y) SO4(z) Alkaline metal (w) Mn/Al Ex.(%) Mot. Li 0.666 0.334 0.217 Li = 0.099 1.99 Mot. Li/Na 0.667 0.333 0.220 Li = 0.011 ; Na = 0.096; Li+Na = 0.107 2.00 88 Mot. Li/K 0.666 0.334 0.218 Li = 0.014 ; K = 0.087 ; Li+K = 0.101 1.99 86 Mot. Na 0.666 0.334 0.217 Na = 0.100 1.99 Mot. Na/K 0.662 0.338 0.212 Na = 0.000; K = 0.085; Na+K = 0.085 1.96 96 Mot. Na/Li 0.665 0.335 0.226 Na = 0.019; Li = 0.094; Na+Li = 0.113 1.99 83 Mot. K 0.666 0.334 0.218 K = 0.101 1.99 Mot. K/Na 0.664 0.336 0.229 K = 0.002; Na = 0.119; Na+K = 0.121 1.98 98 Mot. K/Li 0.665 0.335 0.219 K = 0.003 ; Li = 0.100; Na+Li = 0.103 1.99 97 Shig. = Shigaite; Natro. = Natroglaucocerinite; Mot. = Motukoreaite. x,y,z,w = According to the LDH ideal formula: M+2xAly(OH)2(SO4)zAw; A = Alkaline metal (Li, Na or K); Ex.(%) = percentage of exchange. Since the lithium, sodium and potassium phases of Shigaite have different basal distances, we still do not have an explanation why, even after a total exchange reaction of the alkaline metals and considering that all the pure phases have distinct basal distances, this parameter only changed for lithium. As the basal distances changes with drying under mild temperatures, one hypothesis is the different levels of hydration of the intercalated cations. As can be seen, unlike Li-Shigaite, despite the almost total exchange reaction (Table 4), the Li-Natroglaucocerinite phase with basal distance of 11.15 Å (Fig. 7A,B-a) after exchanging 10 ACS Paragon Plus Environment

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Li with Na (Fig. 7A,B-b) and Li with K (Fig. 7A,B-c), the basal distance changed to 11.16 Å and a mixture of phases with distances of 11.22 and 11.30 Å. After the exchange of Na (Fig. 7A,B-d) with K (Fig. 7A,B-e) and Li (Fig. 7A,B-f), the basal distances changed from 11.14 Å to 11.22 and 11.13 Å, respectively. After exchanging K (Fig. 7A,B-g) with Na (Fig. 7A,Bh) and Li (Fig. 7A,B-i), the basal distance changed from 11.40 Å to mixtures of phases having 11.17/10.78 Å and 11.06/10.78 Å, respectively. A

B

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Figure 6 - A- XRD patterns of Li-Shigaite (a), after exchange of Li with Na (b), Li with K (c), Na-Shigaite (d), after exchange of Na with K (e), Na with Li (f), K-Shigaite (g), after exchange of K with Na (h) and after exchange of Li with K (i). B- Expansion of the XRD patterns in the region of the third basal peak. A

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Figure 7 - A- XRD patterns of Li-Natroglaucocerinite (a), after exchange of Li with Na (b), Li with K (c), Na-Natroglaucocerinite (d), after exchange of Na with K (e), Na with Li (f), K-Natroglaucocerinite (g), after exchange of K with Na (h) and after exchange of K with Li (i). B- Expansion of the XRD patterns in the region of the third basal peak. The XRD patterns of Li (Fig. 8A,B-a), Na (Fig. 8A,B-d) and K-Motukoreaite (Fig. 8A,B-g), although the structure was highly damaged after the exchange reactions and almost total exchange reaction (Table 4), all phases maintained the same basal parameters. This is logical since all the pure phases had almost the same basal parameters (Table 3). Even though we did not analyze the sulfur by ICP-EOS, the EDS data (see Supporting information), although semi-quantitative, indicated that all samples had a similar proportion of sulfur, consistent with the proposed formula for all the compounds. The 11 ACS Paragon Plus Environment


presence of sulfate is also clearly seem in the FTIR spectra (Fig. 9), indicating that all the samples still retained this anion intercalated in the structure after the exchange reactions. A

Motuko_Na Motuko_K curve translate: curve translate: curve translate: curve translate: curve translate: curve translate: curve translate:

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Figure 8 - XRD patterns of Li-Motukoreaite (a), after exchange of Li with Na (b), Li with K (c), Na-Motukoreaite (d), after exchange of Na with K (e), Na with Li (f), and KMotukoreaite (g), after exchange of K with Na (h) and K with Li (i). B – Expansion of the XRD patterns in the region of the first basal peak. i

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f e d c b a

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Figure 9 - FTIR spectra of: A- Li-Shigaite (a), Li exchange with Na (b), Li with K (c), NaShigaite (d), Na exchanged with K (e), Na with Li (f), K-Shigaite (g), K exchanged with Na (h) and K with Li (i). B- Li-Natroglaucocerinite (a), Li exchanged with Na (b), Li with K (c), Na-Natroglaucocerinite (d), Na exchanged with K (e), Na with Li (f), K-Natroglaucocerinite (g), K exchanged with Na (h) and K with Li (i). C- Li-Motukoreaite (a), Li exchanged with Na (b), Li with K (c), Na-Motukoreaite (d), Na exchanged with K (e), Na with Li (f), KMotukoreaite (g), K exchanged with Na (h) and K with Li (i). 12 ACS Paragon Plus Environment

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A more detailed characterization of the spectra can be seen in Figure 3. After the exchange reactions of Shigaite (Fig. 9A), the products presented the same profile as the parent materials (Fig. 3A), indicating again that the exchange reactions only affected the alkaline metal, not the sulfate. The Natroglaucocerinite Li and K derivatives were contaminated with carbonate from the start (Fig. 9B-a,g) and the same contamination was observed after the exchange reactions (Fig. 9B-b,c,h,i). The absence of carbonate in the Na phase (Fig. 9B-d) was also observed for the exchanged phases (Fig. 9B-e,f). Some traces of contamination were also noted for the Motukoreaite phases, especially after exchange of Li with Na and K (Fig. 9C-b,c). The extra band attributed to 3 vibration of sulfate was observed at 1235 cm-1 for the Li-Natroglaucocerinite (Fig. 9B-a) and after exchange of Na with Li in Natroglaucocerinite (Fig. 9B-f) and in Li-Motukoreaite (Fig. 9C-a). The SEM images (Fig. 10) show that in the exchanged phases of all the Shigaite-like phases (Fig. 10A-C), the crystals presented rounded edges due to the effect of magnetic stirring but preserved the size and morphology, indicating that the reaction is topotactic. This effect was not seen in the and Natroglaucocerinite-like samples, where the crystals were bigger and thicker (Fig. 10E,F).

Figure 10 - SEM images of Shigate after exchange of Na with Li (A), Na with K (B), K with Na (C) and K with Li (D); Natroglaucocerinite after exchange of K with Li (E), K with Na (F), Li with Na (G), Li with K (H), Na with Li (I); Motukoreaite after exchange of Li with Na (J) and Li with K (K).



SAED measurements of all exchanged materials reproduced the spectra of the parent materials, showing the same hexagonal projection, with most of them presenting a structure and a a(3)0.5xa(3)0.5 superstructure (not shown). 4. Conclusions Layered double hydroxides homologous to the minerals Shigaite, Natroglaucocerinite and Motukoreaite were synthesized by coprecipitation with increasing pH. These minerals have previously been described to occur only with sodium and sulfate. These phases were successfully synthesized as well as others containing lithium and potassium along with sulfate. All the samples were evaluated by several instrumental techniques before and after exchange of hydrated alkaline metals with other hydrated alkaline metals. The XRD patterns presented a series of basal peaks, typical of materials with layered structures, and all the alkaline metals presented different basal parameters. The XRD patterns also showed that only the phases of Li-Natroglaucocerinite presented multiple phases, attributed to several degrees of hydration, although the SAED spectra indicated the same metal distances. ICP-EOS analysis corroborated the metal proportions used in the formula, and compositions close to (A(H2O)6)[M+26Al3(OH)18(SO4)2].6H2O (M+2 = Mn, Mg and Zn; A = Li, Na and K) where obtained for all compounds. Morphological characterizations by SEM also indicated typical platelet-like particles, with micrometer dimensions along the basal plane and submicrometric thicknesses. The FTIR spectra and EDS analysis indicated the presence of the expected elements, sulfate and the respective alkaline metals, indicating that all synthesized compounds are isostructural. SAED spectra indicated for most of the compounds a typical hexagonal structure and a a(3)0.5x a(3)0.5, typical of ordering of the metals in the layers, as expected for M+2:M+3 molar ratios of 2:1. In all compounds the majority of the alkaline metal could be replaced, where the changes in crystal morphology and sizes were not visibly detected, indicating that the reactions were topotactic. In relation to the structure, only small changes or maintenance of the basal parameters were detected after the exchange reactions, indicating that the main contributor to the basal distance is the double layer of sulfate anions. Acknowledgments This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brazil (CAPES) - Finance Code 001, CNPq (FW: 303846/2014-3, 400117/2016-9) and FINEP. ARS also thanks CAPES for the PhD scholarship and Centro de Microscopia Eletrônica – CEM/UFPR for the SEM, EDS and SAED measurements. Supporting Information EDS spectra of Shigaite, Natroglaucocerinite and Motukoreaite in the Li, Na e Ka phases. EDS spectra of Shigaite, Natroglaucocerinite and Motukoreaite in the Li, Na e Ka phases after exchange reactions. 5. References (1) Rodgers, K. A.; Chisholm, J. E.; Davis, R. J.; Nelson, C. S. Motukoreaite, a New Hydrated Carbonate, Sulfate, and Hydroxide of Magnesium and Aluminum from Auckland, New Zealand, Miner. Mag. 1977, 41, 389-90. 14 ACS Paragon Plus Environment

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(2)

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(18) Adiwidjaja, G.; Friese, K.; Klaska, K. H.; Schluter, J. The crystal structure of gordaite NaZn4(SO4)(OH)6Cl.6H2O, Zeits. Kristal. 1997, 212, 704-7. (19) Maruyama, S. A.; Krause, F.; Tavares, S. R.; Leitão, A. A.; Wypych, F. Synthesis, cation exchange and dehydration/rehydration of sodium Gordaite: NaZn4(OH)6(SO4)Cl·6H2O. Appl. Clay Sci., 2017, 146, 100-5. (20) Kasatkin, A.V.; Plasil, J.; Belakovskiy, D.I.; Marty, J. Cobaltogordaite, IMA 2014-043. CNMNC Newsletter 22, 1243, 2014; Miner. Mag. 2014, 78, 1241-8. (21) Maruyama, S. A.; Westrup, K. C. M.; Nakagaki, S.; Wypych, F. Immobilization of a cationic manganese(III) porphyrin on lithium gordaite (LiZn4(OH)6(SO4)Cl·6H2O), a layered hydroxide salt with cation exchange capacity. Appl. Clay Sci. 2017, 139, 10811. (22) Komarneni S.; Kozai N.; Roy R. Novel function for anionic clays: selective transition metal cation uptake by diadochy, J. Mat. Chem., 1998, 8, 1329-1331. (23) Park M.; Choi C. L.; Seo Y. J.; Yeo S. K.; Choi J.; Komarneni S.; Lee J. H. Reaction of Cu2+ and Pb2+ with Mg/Al layered double hydroxide. Appl. Clay Sci., 2007, 37, 143148. (24) Liang X.; Zang Y.; Xu Y.; Tan X.; Hou W.; Wang L.; Sun Y. Sorption of metal cations on layered double hydroxides, Coll. Surf. A, 2013, 433, 122-131. (25) Majzlana, J.; Alpers, C. N.; Koch, C. B.; McCleskey, R. B.; Myneni, S. C. B.; Neil, J.M. Vibrational, X-ray absorption, and Mössbauer spectra of sulfate minerals from the weathered massive sulfide deposit at Iron Mountain, California. Chem. Geol. 2011, 284, 296-305. (26) Ahmed, I. A. M.; Benning, L. G.; Kakonyi, G.; Sumoondur, A. D.; Terrill, N. J.; Shaw, S. Formation of Green Rust Sulfate: A Combined in Situ Time-Resolved X-ray Scattering and Electrochemical Study. Langmuir. 2010, 26, 6593-603. (27) Zegeye, A.; Ona-Nguema, G.; Carteret, C.; Huguet, L.; Abdelmoula, M.; Jorand, F. Formation of Hydroxysulphate Green Rust 2 as a Single Iron(II-III) Mineral in Microbial Culture, Geom. J., 2005, 22, 389-99. (28) Simon, L.; Francois, M.; Refait, P.; Renaudin, G.; Lelaurain, M.; Genin, J. M. R. Structure of the Fe(II–III) layered double hydroxysulphate green rust two from Rietveld analysis. Solid State Sci. 2003, 5, 327-34. (29) Brindley, G. W. Motukoreaite additional data and comparison with related minerals. Min. Mag. 1979, 43, 337-40. Graphical abstract


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Cation Exchange Reactions in Layered Double Hydroxides

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