Synthesis and Characterization of a [ Li 0 + xMg 2 − 2 x Al 1 + x ( OH ) 6 ] [ Cl ∙ mH 2 O ] Solid Solution with X = 0-1 at Different Temperatures

The synthesis of a novel Li /Mg /Al containing layered double hydroxide (LDH) by using a hydrothermal synthesis route is represented in this work. The autoclaves were heated up to 100 ̊C, 120 ̊C, 140 ̊C and 160 ̊C 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/Mg/Al resulted in the synthesis of amorphous phases without producing plenty of crystalline amounts of the expected solid solutions while using higher temperatures than 140 ̊C 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 Mg and a Li dominated solid solution, are coexistent.


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 [M z+  1−x M 3+ x (OH) 2 ] p+ [(A n− ) p/n ·mH 2 O] with z = 2, M = bi-and trivalent metallic elements, A = organic or inorganic anions and m = amount of interlayer H 2 O depending on the temperature, relative humidity and hydration level [1].A special case is M z+ = Li + (z = 1) and M 3+ = Al 3+ .The ratio between Li and Al is always 1:2 [2] while the ratio between M z+ and M 3+ (z = 2) can vary strongly [3] depending on which M 2+ 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 [M z+  1−x M 3+ x (OH) 2 ] p+ main layer remains stable and is not capable of ion exchange once it is formed.Al(OH) 3 [2/4]or by a hydrothermal reaction with higher temperatures and pressures [1].
The structure of Al(OH) 3 is 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 [LiAl 2 (OH) 6 ] [X·mH 2 O] 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.
These octahedrons share edges and form thereby a layer.Substituting Mg 2+ with a trivalent ion like Al 3+ leads to a positive charge which can be compensate by interlayer anions [11].

Reagents
The starting materials for this work were LiCl (ROTH, purity ≥ 99%), MgCl

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, MgCl  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 HNO 3 [11] [16].

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 + /Mg 2+ /Al 3+ 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 Mg 2+ 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 Al 3+ .These Al 3+ 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.

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 Mg 2+ and Al 3+ and the needed 20 % of the five times higher Li + concentration were build-in into the crystalline phase.

PXRD Analysis
By increasing X from 0 to 1 in 0.
the amount of Mg 2+ is decreased and replaced by Al 3+ and Li + .This leads to a change of the lattice parameter a and the cell volume.Comparing the ion radii of Mg 2+ (0.65 Å) with Li + (0.60 Å) and Al 3+ (0.50 Å) it is to be expected that the lattice parameter a starts to decrease with higher Li + /Al 3+ content [7].A dependent change in the lattice parameter c or the basal reflections (00l) is not visible.By  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 + /Mg 2+ 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 (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  Mg 2+ independent from the Mg 2+ 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  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 P6 3 /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).

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 Al 3+ 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 Mg 2+ amounts next to the AlO(OH) phase (Figure 6(d)).
Table 3. Proportion of the amorphous phase/AlO(OH) depending on the synthesis temperature.

Thermal Analysis
The amount of interlayer water was determined by TG/DTA for [LiAl 2 (OH) 6 ] ]and all pure solid solutions (Table 4).
An example for the Li-LDH, Mg-LDH and the solid solution with the highest Mg 2+ amount [Li 0.9 Mg 0.   [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 Mg 2+ is high enough to influence the absorption spectra but not to show a clear Mg related absorption.

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.

Structure of the Solid Solution
Based on the assumption that Mg 2+ ions can occupy the positions of Li + and Al 3+ because of the fitting bonding length [2] [21], the ion radii [7] and the determined hexagonal P6 3 /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 [Li 0.9 Mg 0.

[Mg 2
Al(OH) 6 ][X·mH 2 O] can also be rhombohedra or hexagonal[13] [14].The pure [Mg 2 Al(OH) 6 ] [X·mH 2 O] 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 M z+ (z = 1 or 2) + M 3+ 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 Me 2+ cation (Mg 2+ ) into the structure of a Li-LDH.The distance between Li + and O 2− ions in a [LiAl 2 (OH) 6 ] [Cl·mH 2 O] LDH is 2.129Å and between Al 3+ and O 2− 1.926Å [2].In a [Mg 2 Al(OH) 6 ][Cl·mH 2 O] LDH, Mg 2+ and Al 3+ ions occupy the same positions with the same distance of 2.013Åbetween the cations and O 2− [21].Comparing both structures and the bonding distances, it should be possible for Mg 2+ ions to occupy the Al 3+ and the Li + position in the solid solution.

Mg 2+ : 1
mol Al 3+ was chosen andthe pure [LiAl 2 (OH) 6 ][Cl·mH 2 O] was prepared by adding the Li + and Al 3+ 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 Al 3+ as an amorphous phase.A five times higher concentration of Li + [10], as required by stoichiometry for the preparation of the pure [LiAl 2 (OH) 6 ][Cl·mH 2 O] LDH, was necessary to compensate the 80 % lack of Li + in the solid state.After the synthesis of pure [Mg 2 Al(OH) 6 ][Cl·mH 2 O], the amount of Li + was increased and the amount of Mg 2+ was reduced in 10 mol% (X = 0.1) steps until 100 mol% Li ([LiAl 2 (OH) 6 ][Cl·mH 2 O]) was reached.The products were filtered, washed with 30 ml deionized water and dried (RH 35%) until a constant mass was reached.

Figure 2 .
Figure 2. XRD pattern in the range of 60˚ to 65 ˚2Θ of the test series with X for [Li 0+x Mg 2−2x Al 1+x (OH) 6 ][Cl·mH 2 O].The pattern for x = 0.1 until x = 0.8 show two different phases.

Figure 3 .
Figure 3. Lattice parameter a of two different phases with X for [Li 0+x Mg 2−2x Al 1+x (OH) 6 ] [Cl·mH 2 O].The black dashed line shows the theoretical lattice parameter of the solid solutions.

Figure 5 .
Figure 5. XRD pattern in the range of 60˚ to 65 ˚2Θ of the test series with series with X = 0.9 -1 for [Li 0+x Mg 2−2x Al 1+x (OH) 6 ][Cl·mH 2 O] with marked peaks.A shift for the (300) and (302) peaks is visible.

2
Al 1.90 (OH) 6 ][Cl•0.50H 2 O].If Mg 2+ 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 Al 3+ would not change, or they would exchange only with Al 3+ ions with the opposite result.The results of this work show, that in fact Mg 2+ has to be statistically distributed with 5 mol% on the Li + and 5 mol% on the Al 3+ position to provide the measured chemical formula.
It is possible to synthesise a pure [Li 0+x Mg 2−2x Al 1+x (OH) 6 ][Cl·mH 2 O] solid solution using autoclaves with temperatures of 100˚C, 120˚C and 140˚C with a maximum amount of 10 mol% Mg 2+ (X = 0.9).Using more Mg 2+ in the reactant leads to a parallel formation of an Mg 2+ 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 [Li 0.9 Mg 0.2 Al 1.9 (OH) 6 ][Cl·0.50H 2 O]. 2