Synthesis and Characterization of a [Li0+xMg2-2xAl1+x(OH)6][Cl·mH2O] Solid Solution with X = 0 - 1 at Different Temperatures

Abstract

The synthesis of a novel Li+ /Mg2+ /Al3+ containing layered double hydroxide (LDH) by using a hydrothermal synthesis route is represented in this work. The autoclaves were heated up to 100oC, 120oC, 140oC and 160oC for 10 h and 48 h with a water to solid ratio (W/S) of 15:1. The physicochemical properties of the synthesized LDHs were investigated by X-ray powder diffraction (PXRD), fourier transform infrared spectroscopy (FTIR), thermo gravimetric and differential thermal analysis (TG-DTA), inductively coupled plasma optical emission spectroscopy (ICP-OES) and scanning electron microscopy (SEM). The formation of a solid solution phase depends strongly on the composition of the reactants and the synthesis temperature. Using an exact stoichiometric ratio of Li+/Mg2+/Al3+ resulted in the synthesis of amorphous phases without producing plenty of crystalline amounts of the expected solid solutions while using higher temperatures than 140oC resulted in a formation of AlO(OH). To avoid the formation of an Al containing amorphous phase or an AlO(OH) crystalline phase, the stoichiometric ratio of Li+ was changed. The results show solid solutions with the formula [Li0+xMg2-2xAl1+x(OH)6][Cl.mH2O] with X ≥ 0.9. The lattice parameters and chemical compositions for solid solutions with different compositions were determined and the pure solid solution with the highest amount of Mg (x = 0.9) is [Li0.9Mg0.2Al1.9(OH)6] [Cl.0.50H2O] with the lattice parameters a = 5.1004(4) Å, c = 15.3512(1) Å, V = 345.844(9) Å3. For X < 0.9 two separate phases, a Mg2+ and a Li+ dominated solid solution, are coexistent.

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Niksch, A. and Pöllmann, H. (2017) Synthesis and Characterization of a [Li0+xMg2-2xAl1+x(OH)6][Cl·mH2O] Solid Solution with X = 0 - 1 at Different Temperatures. Natural Resources, 8, 445-459. doi: 10.4236/nr.2017.86029.

1. Introduction

Layered double hydroxides (LDHs) consist of alternate positively charged mixed metal hydroxide layers and negative charged interlayer anions. The stoichiometry of these materials can be formulated as [Mz+1−xM3+x(OH)2]p+[(An−)p/n·mH2O] with z = 2, M = bi- and trivalent metallic elements, A = organic or inorganic anions and m = amount of interlayer H2O depending on the temperature, relative humidity and hydration level [1] . A special case is Mz+ = Li+(z = 1) and M3+ = Al3+. The ratio between Li and Al is always 1:2 [2] while the ratio between Mz+ and M3+ (z = 2) can vary strongly [3] depending on which M2+ ion or synthesis parameters are used. These layered materials are able to intercalate negatively charged and neutral molecules or exchange the interlayer anion with organic [4] [5] [6] [7] [8] or inorganic [3] [9] anions of different sizes or charges. The [Mz+1−xM3+x(OH)2]p+ main layer remains stable and is not capable of ion exchange once it is formed.

Two well-known and described LDHs are [LiAl2(OH)6] [Cl∙mH2O] [2] [9] [10] and [Mg2Al(OH)6] [Cl∙mH2O] [11]. Both compounds are generally synthesized by a direct reaction of a LiXor MgX2(X = Cl, OH, NO3, etc) with Al(OH)3[2/4]or by a hydrothermal reaction with higher temperatures and pressures [1] .

The structure of Al(OH)3is built up of double layered sheets of hexagonally packed O atoms. Two thirds of the octahedral holes are occupied by Al atoms. Using LiX as the reaction partner leads to the formation of [LiAl2(OH)6] [X·mH2O] with Li+ cations entering the vacancies in the aluminum hydroxide layers and A entering the interlayer space [1] [9] . The structure of the resulting Li-LDH depends directly on the structure of the used aluminium hydroxide. Syntheses using gibbsite as starting material leads to Li-LDHs with hexagonal symmetry, while reactions with bayerite or nordstrandite produce LDHs with rhombohedra symmetry [1] [10] [12] . In the brucite-like structure of [Mg2Al(OH)6][X·mH2O], Mg2+ is octahedrally coordinated to six OH anions. These octahedrons share edges and form thereby a layer. Substituting Mg2+ with a trivalent ion like Al3+ leads to a positive charge which can be compensate by interlayer anions [11] . [Mg2Al(OH)6][X·mH2O] can also be rhombohedra or hexagonal [13] [14] . The pure [Mg2Al(OH)6] [X·mH2O] phase produced within this work was hexagonal (P6/m).

Almost all publications concerning the interlayer anion exchange or the synthesis and physicochemical properties use a combination of Mz+ (z = 1 or 2) + M3+ in the main layer with a variation of two different elements [3] [5] [15] - [20] . The aim of this research is to invent a novel solid solution by adding a Me2+ cation (Mg2+) into the structure of a Li-LDH. The distance between Li+ and O2− ions in a [LiAl2(OH)6] [Cl·mH2O] LDH is 2.129Å and between Al3+ and O2− 1.926Å [2] . In a [Mg2Al(OH)6][Cl·mH2O] LDH, Mg2+ and Al3+ ions occupy the same positions with the same distance of 2.013Åbetween the cations and O2− [21] . Comparing both structures and the bonding distances, it should be possible for Mg2+ ions to occupy the Al3+ and the Li+ position in the solid solution.

2. Experimental

2.1. Reagents

The starting materials for this work were LiCl (ROTH, purity ≥ 99%), MgCl2・6H2O (AppliChem ≥ 99%), AlCl3・6H2O (Serva ≥ 98%) and NaOH (Fluka ≥ 97%). XRD investigations and loss of ignition (LOI) were done with all chemicals to exclude contaminations and determine the amount of crystal water.

2.2. Measurements

APAN anlytical X’PERT³ Powder diffractometer with Pixcel detector and a Cu radiation (45 kV/40 mA) was used for the X-ray powder diffraction (XRD). The samples were prepared with back loading procedure and recorded from 5˚ - 70˚2θ with a step width of 0.017˚2θ and a irradiation time per step of 19.685 s. Thermogravimetric analysis and differential thermal analysis (TGA/DTA) for the dried samples (relative humidity (RH) 35%) were done simultaneously by the 320U from Seiko Instruments under nitrogen flow and a 2.5 K/min heating rate between 25˚C - 1000˚C. Fourier transform infrared spectra (FTIR) were recorded by an IFS 55 Equinox FTIR spectrometer from Bruker (400 - 4000 cm−1). The scanning electron microscope (SEM) pictures were taken by a JOEL 640 SEM and the chemical compositions of the samples were proven by a Horiba Ultima2 inductively coupled plasma optical emission spectroscopy (ICP-OES).

2.3. Synthesis

All mixtures of the initial components were prepared in a glove box with nitrogen atmosphere to avoid carbonatization. The synthesis were carried out in 35 ml PTFE-lined stainless-stealautoclaves [1] by adding solutions of LiCl, MgCl2∙6H2O and AlCl3·6H2Owith a W/S of 15: 1(a total of 1 g salts with 15 ml deionized/ decarbonized water) and 5M NaOH until an alkaline pH (8.5) was reached and heating it up 10 h and 48 h. A series of experiments with different temperatures, synthesis times and pH were carried out to achieve the best result for a pure solid solution. The synthesis temperature was varied between 100˚C, 120˚C, 140˚C and 160˚C and two different synthesis times (10 h and 48 h) and pH (8.5/9.5) were tested. To synthesize pure [Mg2Al(OH)6][Cl·mH2O] an exact ratio of 2 mol Mg2+: 1 mol Al3+was chosen andthe pure [LiAl2(OH)6][Cl·mH2O] was prepared by adding the Li+ and Al3+ salts in an exact 1 mol : 2 mol ratio. While the Mg containing LDH was prepared without problems, the Li LDH showed a high proportion of an amorphous phase. ICP-OES investigations stated that only 20% of Li+ was incorporated in the LDH structure leaving 80% of Li+ in the solution and the so remaining excess of Al3+ as an amorphous phase. A five times higher concentration of Li+ [10] , as required by stoichiometry for the preparation of the pure [LiAl2(OH)6][Cl·mH2O] LDH, was necessary to compensate the 80 % lack of Li+ in the solid state.

After the synthesis of pure [Mg2Al(OH)6][Cl·mH2O], the amount of Li+ was increased and the amount of Mg2+ was reduced in 10 mol% (X = 0.1) steps until 100 mol% Li ([LiAl2(OH)6][Cl·mH2O]) was reached. The products were filtered, washed with 30 ml deionized water and dried (RH 35%) until a constant mass was reached.

The mineralogical phases were determined by X-ray powder diffraction, the chemical compositions of the products by ICP-OES using the filtrate and the synthesis products dissolved in HNO3 [11] [16] .

3. Results and Discussion

3.1. Stoichiometric Composition

First experiments were carried out at 100˚C, pH 8.5, 10 h synthesis time and a W/S ratio of 15:1. Using an exact stoichiometric ratio of Li+/Mg2+/Al3+ resulted in the synthesis of a high proportion of an amorphous phase with a small amount of a crystalline solid solution. After drying at 80˚C, XRD analysis showed a recrystallized Al(OH)3 phase next to the LDH main phase.

Investigations of the filtered solutions and the dissolved products with ICP-OES stated that, independent from the Mg reactant amount, 99% - 100% of Mg2+ but only 20% of Li+ were build-in into a LDH phase. The other 80% of Li+ remained in the solution. Due to the stoichiometric reactant ratio the leftover Li+ ions in the solution are leading to leftover Al3+. These Al3+ ions formed Al(OH)3 in the basic environment. Using higher temperatures (up to 160˚C) or synthesis times (48 h) showed no positive effect for the crystallization of a pure LDH phase. Increasing pH from 8.5 to 9.5 resulted in a slightly higher amount of a crystalline phase.

3.2. Composition with Increased Li+ Content

After a five times increasement of the stoichiometric amount of Li+ (equal to the pure Li-LDH synthesis), with a resulting ratio of Li: Mg: Al = 5: 1: 1, a pure crystalline LDH phase could be achieved. ICP-OES studies stated that >99% of Mg2+ and Al3+ and the needed 20 % of the five times higher Li+ concentration were build-in into the crystalline phase.

3.2.1. PXRD Analysis

By increasing X from 0 to 1 in 0.1 mol steps in [Li0+xMg2−2xAl1+x(OH)6][Cl∙mH2O], the amount of Mg2+is decreased and replaced by Al3+ and Li+. This leads to a change of the lattice parameter a and the cell volume. Comparing the ion radii of Mg2+ (0.65 Å) with Li+ (0.60 Å) and Al3+ (0.50 Å) it is to be expected that the lattice parameter a starts to decrease with higher Li+/Al3+content [7] . A dependent change in the lattice parameter c or the basal reflections (00l) is not visible. By means of the (110)/(112) and (300)/(302) peaks, it is easily possible to distinguish the two different phases [Mg2Al(OH)6][Cl·mH2O] (P6/m) and [LiAl2(OH)6][Cl·mH2O] (P63/m). Starting with X = 0 (pure [Mg2Al(OH)6][Cl·mH2O] phase) a separation in two phases is visible between X = 0.1 and X = 0.8 (Figure 1 and Figure 2). The ˚2Θ positions of the 110/112peaks at X = 0.1 - 0.8 show a peak shift to higher ˚2Θ angles in relation to the pure [Mg2Al(OH)6][Cl·mH2O]

Figure 1. XRD pattern of the test series with X for[Li0+xMg2−2xAl1+x(OH)6][Cl·mH2O] (120˚C/10 h) showing no visible phase separation at the (002)/(004) main peaks but two coexisting phases at higher ˚2Θ angle (60 - 65).

Figure 2. XRD pattern in the range of 60˚ to 65 ˚2Θ of the test series with X for [Li0+xMg2−2xAl1+x(OH)6][Cl·mH2O]. The pattern for x = 0.1 until x = 0.8 show two different phases.

phase (Figure 2 and Figure 3) which is also visible in the lattice parameter (Table 1). This shift increases with higher Li+ reactant amounts, which indicates Mg dominated solid solutions with different Li+/Mg2+ ratios.

While the (110)/(112) peaks are completely erased for X = 0.9, the (300)/(302) peaks shifted and the solid solution hasa different lattice parameter compared to [LiAl2(OH)6][Cl·mH2O] at X = 1 [2] (Figure 2 and Figure 3, Table 1). The lattice parameter a is closeto the calculated ideal position of a solid solution. Between X = 0.1 - 0.8 the (300)/(302) peaks have nearly the same position which is shifted to lower ˚2Θ angles and the lattice parameter are also nearly constant. This indicates a stable Li dominated solid solution with a defined amount of

Figure 3. Lattice parameter a of two different phases with X for [Li0+xMg2−2xAl1+x(OH)6] [Cl·mH2O]. The black dashed line shows the theoretical lattice parameter of the solid solutions.

Table 1. Pawley fitted lattice parameter a/c for [Mg2Al(OH)6][Cl·mH2O] (X = 0), [LiAl2(OH)6][Cl·mH2O] (X = 1) and the split Li and Mg dominated solid solutions.

Mg2+ independent from the Mg2+ reactant amount. The miscibility gap for X = 0.1 - 0.8 was observed at all tested synthesis temperatures (100˚C - 160˚C) and times (10 h/48 h).

To synthesize pure solid solution phases, test series between X = 0.9 and X = 1 (in 0.02 mol steps) were conducted. XRD results show a single mineral phase with h0l peak shifts (Figure 4 and Figure 5). This peak shifts follow nearly the

Figure 4. XRD pattern of the test series with X = 0.9 - 1 for[Li0+xMg2−2xAl1+x(OH)6] [Cl·mH2O]. X was increased in 0.02 mol steps (120˚C/10 h). Due to a preferred orientation of 00l, the (100) and (105) peak is no longer visible for X = 0.96; 0.98; 1.

Figure 5. XRD pattern in the range of 60˚ to 65 ˚2Θ of the test series with series with X = 0.9 - 1 for [Li0+xMg2−2xAl1+x(OH)6][Cl·mH2O] with marked peaks. A shift for the (300) and (302) peaks is visible.

calculated shifts for the solid solutions (Figure 6). These experiments were also done at four different temperatures (100˚C, 120˚C, 140˚C, 160˚C). Although there is a shift difference depending on the temperature (Figure 6), no phase separation was observed for all investigated solid solutions (Figure 5).

The optimal results for a pure solid solution phase were achieved at 120˚C/10 h synthesis time/pH 9.5 and W/S ratio 15:1 (Figure 6/Table 2). The measured lattice parameters a differ only slightly from the calculated and the lattice parameters c are nearly constant (Table 2).

The products were fitted by Pawley fit and the space group was determined as P63/m for all pure solid solutions up to X = 0.9. Investigations of the lattice parameter a show a straight increase from ~5.08Å (X = 0) [8] to ~5.10Å (X = 0.1) as calculated (Figure 6/Table 2).

(a) (b)(c) (d)

Figure 6. Theoretical and measured lattice parameter a for the solid solutions with X = 0.1, 0.8, 0.9 - 0.98 for [Li0+xMg2−2xAl1+x(OH)6] [Cl·mH2O]and the pure Mg/Li LDH at (a) 100˚C; (b) 120˚C; (c) 140˚C; (d) 160˚C. For X = 0.1/0.8 two separated LDH phases are visible.

Table 2. Theoretical and measured/fitted lattice parameter (a) and (c) for the solid solutions with X = 0.9 - 0.98 and [LiAl2(OH)6][Cl·0.51H2O] at X = 1 (120˚C/10 h/pH 9.5/W/S 15: 1).

3.2.2. ICP-OES Analysis

To determine the chemical formula, all products were completely dissolved insuprapur 65% nitric acid and investigated with ICP-OES [11] [16] . The results were used to calculatethe LDH formulas (Table 4). These calculations also stated a maximum content of an amorphous phase of <1%. Recrystallization tests showed no Al containing phases. Synthesis temperatures higher than 140˚C led to a destabilization of the LDH phase and the formation of AlO(OH) (Figure 7). The test series with 160˚C were repeated several times producing always AlO(OH) next to the LDH. Calculations showed an Al containing amorphous phase and crystalline AlO(OH) proportion of 10 % to 90 % (Table 3). The resulting lack of Al3+ in the solid solution leads to LDH phases with a higher Mg/Al ratio than 2:1 and therefore to the formation of a LDH with higher Mg2+ amounts next to the AlO(OH) phase (Figure 6(d)).

Table 3. Proportion of the amorphous phase/AlO(OH) depending on the synthesis temperature.

Figure 7. Pawley fit of a solid solution [Li0.90Mg0.25Al1.87(OH)6] ][Cl·mH2O] synthesized at 160˚C. A phase of AlO(OH) (*) is visible next to the solid solution (#). The broadening at 40˚ and 47˚ ˚2θ(small picture) is interpreted as stacking faults [7] .

3.2.3. Thermal Analysis

The amount of interlayer water was determined by TG/DTA for [LiAl2(OH)6] [Cl·0.50H2O], [Mg2Al(OH)6][Cl·0.55H2O]and all pure solid solutions (Table 4). An example for the Li-LDH, Mg-LDH and the solid solution with the highest Mg2+ amount [Li0.9Mg0.2Al1.90(OH)6][Cl·0.51H2O] is shown in Figure 8. Comparing the solid solution with the pure Li- and Mg-LDH, there is a high similarity in mass loss and exothermal reaction. The mass loss at 75˚C - 100˚C is caused by the removal of intercalated interlayer water [2] [6] . With 4.5% for the pure Li-LDH, 4.2% for the solid solution and 4.7% for the pure Mg-LDH it corresponds with the loss of 0.50 to 0.55 water per formula unit of the LDHs. While the differential thermal analysis of the pure Li- and Mg-LDH show a single endothermic reaction at 275˚C - 325˚C, the solid solution shows two (260˚C and 320˚C). At this temperature, the LDH starts to dehydroxylate which results in the destruction of the metal hydroxide main layer [2] [6] [7] [11] . Combining Li+ and Mg2+ with Al3+ in the main layer leads to a two-step dehydroxylation.

(a)(b) (c)

Figure 8. Thermogravimetric and differential thermal analysis of (a) [LiAl2(OH)6] [Cl·0.51H2O]; (b) [Li0.9Mg0.2Al1.90(OH)6] [Cl·0.50H2O]; (c) [Mg2Al(OH)6][Cl·0.55H2O] (120˚C/10 h/pH 9.5/W/S 15: 1) show the loss of interlayer water at 75˚C - 100˚C). Temperatures above 275˚C destroy the structure of the main layer. Heating rate: 2.5 K/min.

Table 4. Calculated chemical formulas based on ICP-OES results and interlayer water of the solid solutions X for [Li0+xMg2−2xAl1+x(OH)6][Cl∙mH2O] (120°C/10h/pH 9.5/W/S 15: 1).

3.2.4. FTIR Spectroscopy

To prove purity of the products, all samples were investigated by FTIR spectroscopy (Figure 9). Although there are 10 mol% Mg2+ in the solid solution, there is only a slight difference to a pure [LiAl2(OH)6][Cl·0.51H2O] FTIR spectrum visible. All three spectra show the typical H2O/OH absorption at ~3500 cm−1 and 1630 cm−1 [5] [18] [22] and only the spectra of [Li0.9Mg0.2Al1.90(OH)6] [Cl・0.50H2O] and [Mg2Al(OH)6][Cl・0.55H2O] show an insignificant amount of carbonatization with the absorption at 1380 cm−1 [5] [18] [23] . The absorption of Al (980/720/520 cm−1) related groups is very good visible for the pure Li-LDH but not as distinct for the solid solution [11] [24] . Mg related absorptions at 415 cm−1 are only visible in the pure Mg-LDH (Table 5). The amount of Mg2+ is high enough to influence the absorption spectra but not to show a clear Mg related absorption.

3.2.5. SEM Analysis

SEM pictures (Figure 10) show flat, (pseudo-) hexagonal particles with different sizes, starting at 2 - 3 µm until nearly nanosize. These particles form cluster in the size of 200 - 600 µm.

3.2.6. Structure of the Solid Solution

Based on the assumption that Mg2+ ions can occupy the positions of Li+ and Al3+ because of the fitting bonding length [2] [21] , the ion radii [7] and the determined hexagonal P63/m space group, the structure of the pure phased solid solution should be identical with the Li-LDH (Figure 11). This is also indicated by the chemical composition with the formula [Li0.9Mg0.2Al1.90(OH)6][Cl・0.50H2O]. If Mg2+ ions could not enter one of the two octahedral positions, there would be two possibilities: they would exchange with Li+ ions only, which would reduce the amount of Li+ in the solid solution while the amount of Al3+ would not change, or they would exchange only with Al3+ ions with the opposite result. The results of this work show, that in fact Mg2+ has to be statistically distributed with 5 mol% on the Li+ and 5 mol% on the Al3+ position to provide the measured chemical formula.

Figure 9. FTIR spectrum of (a) [[LiAl2(OH)6][Cl・0.51H2O]; (b) [Li0.9Mg0.2Al1.90(OH)6] [Cl・0.50H2O]; (c) [Mg2Al(OH)6][Cl・0.55H2O] with the typical absorbed water (~3500 cm−1 and 1620 cm−1) and the metal-O and metal-OH vibrations (>1000 cm−1). Absorption at 2400 cm−1 is device related.

Table 5. Observed wavenumbers and the assignment bending.

Figure 10. SEM pictures of [Li0.9Mg0.2Al1.90(OH)6][Cl・0.50H2O] flat hexagonal particles with average crystal size of >3 µm.

Figure 11. View of the unit cell of [Li0.9Mg0.2Al1.90(OH)6]Cl・0.5H2O(based on Li-LDH structure [2] ―interlayer water excluded) with the octahedral positions of Li+ and Al3+. Both positions are occupied with 5mol% by Mg2+.

4. Conclusion

It is possible to synthesise a pure [Li0+xMg2−2xAl1+x(OH)6][Cl·mH2O] solid solution using autoclaves with temperatures of 100˚C, 120˚C and 140˚C with a maxi- mum amount of 10 mol% Mg2+ (X = 0.9). Using more Mg2+ in the reactant leads to a parallel formation of an Mg2+ dominated and a Li+ dominated solid solution. Optimal results for a pure solid solution can be achieved at 120˚C, pH 9.5, W/S15: 1, 10 h synthesis time. Changing the temperature to 160˚C provides the formation of an AlO(OH) phase. The pure solid solution with the highest Mg content is [Li0.9Mg0.2Al1.9(OH)6][Cl·0.50H2O].

Conflicts of Interest

The authors declare no conflicts of interest.

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