Layered double hydroxides (LDHs), with a typical formula of , have a reversible Mg-(OH)-Mg and Mg-(OH)-Al layered structure with interlayer spacing occupied by carbonate ions (CO32-) and hydrated water. The octahedral crystal structure evolves to amorphous mixed solid oxide upon heat treatment at high temperatures (i.e., >300„aC). Specifically, the interlayer CO32- will decompose and release CO2 by heating to above 400„aC. At presence of CO2 and water vapor, the microporous structure of layered double hydroxides will be recovered, assigning the LDHs unique properties for membrane fabrication, particularly for application in high temperature CO2 separation.

In this work, supported layered double hydroxide membranes were synthesized on porous ƒÑ-alumina support through a process including nanoparticle fabrication and membrane formation by dip-coating. The monodispersed layered double hydroxide with different morphologies including spherical nanoparticles and nanosheets were synthesized by direct hydrothermal coprecipitation and exfoliation, respectively. Uniform thin film (~1.0 ƒÝm) of layered double hydroxide on porous ƒÑ-alumina support was further formed by dip the porous substrate into a homogeneous suspension containing monodispersed MgAl layered double hydroxide nanoparticles or exfoliated nanosheets. The dispersion of agglomerated LDH aggregates into individual LDH nanoparticles was found critical to form uniform thin film. Different techniques including X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), and nitrogen adsorption isotherm (BET) have been applied to study the membrane properties such as crystallinity, membrane morphology, and micropore size distribution. Both single gas and binary gas permeation of equimolar N2 and CO2 were carried out at elevated temperatures ranging from 200„aC to 400„aC. The membrane showed CO2 permselective with a separation factor of 1.32 and CO2 permeance of 9.76°Ñ10-8 mol.m-2.s-1.Pa-1. The permeation behavior was explained by preferable chemisorption of CO2 on layered double hydroxide and micro-, mesoporous structure of the membrane. Post-synthesis treatment for mesopore removal is recommended for enhancement of CO2 permselectivity.

Key words: Layered double hydroxide, LDH nanoparticle, exfoliated nanosheets, CO2 separation, membrane fabrication, thin film.