Halide exchange on Mg(II)-Al(III) layered double hydroxides: exploring affinities and electrostatic predictive models

Area of Science:

• Materials Science
• Inorganic Chemistry
• Nanomaterials

Background:

• Layered double hydroxides (LDH) are versatile materials with tunable properties.
• Anion exchange is a key process for modifying LDH functionalities.
• Understanding anion exchange mechanisms is crucial for material design.

Purpose of the Study:

• To investigate the anion exchange behavior of chloride in a Mg-Al layered double hydroxide (LDH) with fluoride, bromide, and iodide.
• To analyze the structural changes and exchange isotherms during chloride displacement.
• To correlate experimental findings with theoretical models and existing literature data.

Main Methods:

• Synthesis of Mg0.75Al0.25(OH)2Cl0.25·mH2O layered double hydroxide.
• Gradual anion exchange experiments with F(-), Br(-), and I(-).
• Analysis of exchange isotherms and structural characterization.
• Comparison of exchange constants with bibliographic data for analogous LDH hosts.

Main Results:

• Bromide and iodide exhibited ideal exchange behavior, following ideal exchange isotherms.
• Fluoride displayed significant deviations from ideal exchange, including phase segregation.
• A strong linear free energy correlation was observed when comparing exchange constants with literature data.
• Higher layer charge densities (increased Al(III) to Mg(II) ratio) enhanced selectivity for smaller halides.

Conclusions:

• Anion size and layer charge density significantly influence the exchange mechanism and selectivity in Mg-Al LDHs.
• Electrostatic models effectively describe the exchange free energy for both studied LDH hosts.
• The findings provide insights into designing LDHs with specific anion affinities for various applications.