Extended Solid-Solubility Limit in Layered Double Hydroxides: Tuning the Anion-Adsorption Selectivity

Abstract

Unique atomic arrangements in nonthermodynamic solid solutions containing thermodynamically immiscible elements facilitate the introduction of new functionalities in various classes of materials, such as alloys, ceramics, and coordination polymers. Although high-temperature quenching has been widely used for synthesizing such materials with the aid of the surface effect and high configuration entropy, it is not applicable to materials that are unstable at high temperatures and contain only a few constituents. Thus, a new synthesis strategy is required. In this study, we demonstrated that a low-temperature topochemical reaction facilitates the extension of the conventional solid-solubility limit in layered double hydroxides (LDHs), a class of natural and synthetic lamellar compounds. Using a pristine layered oxide, Na[Ni 1−x Fe x ]O 2 (0 < x < 1.0), the solid-solubility limit of LDHs () could be extended from x = 0.33 to 0.59. Furthermore, compared to LDHs (x = 0.30), the obtained LDHs (x = 0.59) exhibited a 20-fold higher selectivity for fluoride ions than for sulfate ions. This selectivity possibly originated from a trivalent-cation clustering of Fe 3+ −Fe 3+ neighbors in the LDH hydroxide layers; this caused a steric hindrance between sulfate and chloride ions in the interlayer space while preventing hindrance between fluoride and chloride ions. We expect that this study will guide the design of unexplored nonthermodynamic solid solutions.

Understanding the role of solvents in bottom-up synthesis of multi-element hydroxides

“…5,6 However, it has been widely accepted that the solubility limit of x = M 3+ /(M 2+ + M 3+ ) is around 0.33 in LDHs. 1,2 The high-entropy design concept, i.e., creating materials by increasing congurational entropy as a driving force, has been introduced to the two-dimensional hydroxides in 2020-2022. [7][8][9][10][11][12][13][14][15][16][17][18] In high-entropy materials, four or more metal species in equimolar or near equimolar ratios are mixed together to form a single-phase crystal structure.…”

“…Layered double hydroxides (LDHs) are a class of synthetic clays that have the brucite Mg(OH) 2 -like structure, where the divalent metal cations are partially substituted by high-valence (typically trivalent) metal cations. 1–3 LDHs have the general formula [M 2+ 1− x M 3+ x (OH) 2 ] x + [A n − ] x / n · y H 2 O, where M 2+ is the divalent metal cation, M 3+ is the doping trivalent cation, A n − is the counter anion that keeps charge balance, and x is the mole fraction of the trivalent cation. 1,2 The divalent metal cations can be Mg 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ or Zn 2+ , while the trivalent metal cations can be Al 3+ , V 3+ , Cr 3+ , Mn 3+ or Fe 3+ .…”

“…1–3 LDHs have the general formula [M 2+ 1− x M 3+ x (OH) 2 ] x + [A n − ] x / n · y H 2 O, where M 2+ is the divalent metal cation, M 3+ is the doping trivalent cation, A n − is the counter anion that keeps charge balance, and x is the mole fraction of the trivalent cation. 1,2 The divalent metal cations can be Mg 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ or Zn 2+ , while the trivalent metal cations can be Al 3+ , V 3+ , Cr 3+ , Mn 3+ or Fe 3+ . 1 The unique positively charged layered structures of LDHs and their good compositional flexibility find themselves wide applications in broad areas, such as catalysts for water splitting and adsorbents for dye removal.…”

“…5,6 However, it has been widely accepted that the solubility limit of x = M 3+ /(M 2+ + M 3+ ) is around 0.33 in LDHs. 1,2…”

Evaluation of Solubility and Thermodynamic Properties of Synthetic Nickel Hydroxide Carbonate

“…Thermodynamic parameters of oxidized minerals of transition metals (copper, zinc, nickel, etc.) are important when studying minerals in the oxidation zone of deposits [1], corrosion of alloys [2], immobilization of heavy metals [3], obtaining new materials [4][5][6], etc. Nickel compounds of this kind have magnetic properties [7].…”