Preparation and Properties of F-Substituted Layered Double Hydroxides (LDHs):Anion-Intercalation Material
Authors: Sudare Tomohito , Hayashi Fumitaka , Teshima Katsuya 
Published: February 2020

Introduction: There have been increasing attention to anion-intercalation behavior and materials. In lithium ion battery field, intercalation property of PF6
− into graphitic carbon was studied for a hybrid capacitor. In ion-exchange field, selective removal of toxic anionic species by anion-exchangeable layered compounds has been focused. Layered double hydroxides (LDHs) was representative of anion-exchangeable layered compounds, represented by a general formula of [M2+
1−x
M3+

x
(OH)2]
x

+[A
nx

/n
]
x

−
mH2O consisting of octahedral brucite-like host-layers (M2+/M3+: divalent and trivalent metal cations), charge-balancing anions (A
n

−) and interlayer water molecules. Their ion-exchange properties, selectivity and capacity, have been widely studied. Miyata previously reported the order of affinity of various anions to Mg-Al LDHs; OH− > F− > Cl− > Br− > NO3
− > I−. Although other researchers have tried to control the affinity by changing composition of metal cations (M2+/M3+) to control charge density of host layers, significant change in the order was hardly achieved. Looking back to intrinsic crystalline structure of LDHs, host-layers, optimal positions of interlayer anions, and water molecules is supposed to be determined with hydrogen-bonding interaction among them. In other words, the affinity of anion to LDHs should be controlled by tuning strength of hydrogen-bonding in addition to charge density of host layers. In this presentation, we tried to prepare F-substituted Mg-Al LDHs (F-LDHs), OH groups belonging to host layer of LDHs was substituted by F atoms, and anion-exchange selectivity was investigated.

Experiment: Based on traditional co-precipitation method with solution pH = 10, F-LDHs was prepared by using AlCl3, NaAlF6, and MgCl2·6H2O as a precursor. Mg/Al ratio was controlled as 2.5，3.0，3.5，and 4.0 by changing ([AlCl3]+[NaAlF6])/[MgCl2] ratio and F-substitution ratio was controlled by changing [AlCl3]/[NaAlF6] ratio. Crystal structure, morphology, chemical composition, local structure of Al and F atoms of obtained samples were analyzed by XRD, FE-SEM (EDS), XPS, solid-state NMR. Next, anion-exchanging properties of obtained F-LDHs was tested. Firstly, as a pre-treatment, LDHs samples was immersed in acidic NaCl aqueous solution for 24 hours at room temperature and interlayer “contaminated” carbonate was exchanged by chloride ions as much as possible. Seconds, by using chloride-exchanged samples, anion-exchange test was carried out with 1.2~1.5 mM NaNO3, Na2SO4, Na2HPO4, NaF, and NaI aqueous solutions for 24 h, where solid to liquid ratio was fixed to 1.0 gL-1. After 24 h, exchanged amount was analyzed by ion-chromatography.

Results and discussion: XRD patterns of all samples exhibit successful formation of LDHs without byproducts even after chloride-exchange treatment. Successful inclusion of F atoms into host-layer of LDHs was confirmed by solid-state NMR and, in all samples, 80% of interlayer anions was exchanged by chloride ions, which is confirmed by compositional analysis with EDS and XPS. XRD patterns also revealed that the basal spacing, d
003, of F-LDHs tend to decrease comparing to original LDHs, which implies that host-layers strongly interacts with interlayer anions and water molecules. In addition, F-substitution amount was found out to be limited by about 10%, LDHs can be obtained without byproducts. Next, distribution coefficient, useful indicator of the affinity of respective anions to LDHs, was calculated from the result of ion-exchange test. It clarified that the order of affinity was NO3
− > HPO4
2− > Br− > F− > SO4
2− > I−, which is totally different from previous reports. Furthermore, the affinity of NO3
− ions comparing to SO4
2− ion and F− ions was increased depending on F-substitution amount. In conference, detailed formation mechanism of F-LDHs and anion-exchanging mechanism was presented.