Hydration-induced interpolytype transformations in the bayerite-derived nitrate-intercalated layered double hydroxide of Li and Al
Graphical abstract
A schematic plot of the change in orientation of the nitrate ion in the interlayer upon hydration.
Introduction
The layered double hydroxides (LDHs) of Li and Al are a unique class of anionic clays with applications in shape-selective intercalation [1], environmental amelioration [2], [3], and triggered release of useful anions [4]. The basic building block of the [Li-Al] family of LDHs is the metal hydroxide layer of the composition [LiAl2(OH)6]+, in which the Li+ ions are arranged in an ordered manner with respect to the Al3+ ions (Fig. 1a). This layer has hexagonal symmetry (ao ≈ 5.1 Å) and belongs to the layer group also written as [5]. The LDH crystal is obtained by the stacking of these LDH layers one over another, mediated by the anions which are incorporated in the interlayer region for the purpose of charge neutrality.
If a single metal hydroxide layer is represented by the symbol P and its mirror image by , two stacking sequences can be envisaged (Fig. 1b, c): (i) PPP···, and (ii) ···. Both these stacking sequences yield crystals of hexagonal symmetry. The widely studied gibbsite-derived LDHs [6] adopt sequence (ii) above and crystallize in P63/mcm or P63/m space group. These structures are two-layer polytypes (c = 2 × co; co: basal spacing), in which successive metal hydroxide layers are mirror images of one another. There is until now only one instance of PPP··· stacking (sequence (i) above) reported in the literature [7].
The bayerite form of Al(OH)3 comprises a PPP··· stacking of metal hydroxide layers. In Al(OH)3, the layers have the composition [Al2▯(OH)6] (▯: octahedral cation vacancy). The question arises: Does the imbibition of Li+ into the cation vacancies of the bayerite crystal yield LDHs with the PPP··· stacking? In this paper, we investigate the imbibition of LiNO3 into the bayerite form of Al(OH)3.
Nitrate ions are of interest for several reasons:
- (i)
NO3- ion is known to intercalate into the LDH galleries in two different orientations [8]:
- (a)
with its molecular plane parallel to the metal hydroxide layer, and
- (b)
with its molecular plane inclined to the metal hydroxide layer.
- (a)
- (i)
The orientation of the NO3- ion in the interlayer gallery is determined not only by the layer charge, but also by the degree of hydration in the interlayer [9].
NO3- ion is a major inorganic pollutant in agricultural waste water [10] and there is considerable interest in its uptake and mineralization by LDH materials.
Section snippets
Synthesis
Bayerite was synthesized by ammonia precipitation following the procedure reported elsewhere [11]. The [Li-Al-NO3] LDH was prepared by imbibition of LiNO3 into bayerite. 0.5 g of bayerite was soaked in 10 mL of ~10 M LiNO3 solution and hydrothermally treated in a Teflon-lined autoclave (80 mL capacity, 140 °C, 24 h). The sample was centrifuged, washed with Type II water (specific resistance 15 MΩ cm, Millipore Academic water purification system) and dried in a hot air oven at 65 °C.
Characterization techniques
The Li
Results and discussion
The imbibition of LiNO3 into bayerite-Al(OH)3, yielded a white crystalline powder. The results of gravimetry (Al content), flame photometry (Li content) and ion chromatography (NO3- content) yielded a mole ratio of [Al]: [Li]: [NO3] = 1: 0.33: 0.37. TGA analysis (Fig. S1) showed a mass loss of ~8.4% (120 °C). Combination of these independently estimated values yields an empirical formula [Li0.33Al(OH)3(NO3)0.37]·0.55H2O. Since there is a reasonable agreement between the Li and nitrate contents
Conclusion
In conclusion, we show by experiments and computation that the nitrate ion intercalated in the Li-Al LDH undergoes a change in orientation upon hydration resulting in a reversible expansion of basal spacing. Simultaneous with this, successive metal hydroxide layers rigidly translate relative to one another by (a/3, 0, 1) leading to a 1 H () →1 M (C2/m) structure transition. It is demonstrated that in the hydrated phase, the nitrate ions in the interlayer are differently ordered. As a
Acknowledgments
The authors thank the Department of Science and Technology (DST), Government of India (GOI) (EMR/2016/002331) for financial support. SN is a recipient of a Senior Research Fellowship (NET) of the University Grants Commission, India.
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