Atomistic simulation of nanoporous layered double hydroxide materials and their properties. II. Adsorption and diffusion.

Nanoporous layered double hydroxide (LDH) materials have wide applications, ranging from being good adsorbents for gases (particularly CO(2)) and liquid ions to membranes and catalysts. They also have applications in medicine, environmental remediation, and electrochemistry. Their general chemical composition is [M(1-x)(II)M(x)(III)(OH(-))(2)](x+)[X(nm)(m-)nH(2)O], where M represents a metallic cation (of valence II or III), and X(nm)(m-) is an m-valence inorganic, or heteropolyacid, or organic anion. We study diffusion and adsorption of CO(2) in a particular LDH with M(II)=Mg, M(III)=Al, and x approximately = 0.71, using an atomistic model developed based on energy minimization and molecular dynamics simulations, together with a modified form of the consistent-valence force field. The adsorption isotherms and self-diffusivity of CO(2) in the material are computed over a range of temperature, using molecular simulations. The computed diffusivities are within one order of magnitude of the measured ones at lower temperatures, while agreeing well with the data at high temperatures. The measured and computed adsorption isotherms agree at low loadings, but differ by about 25% at high loadings. Possible reasons for the differences between the computed properties and the experimental data are discussed, and a model for improving the accuracy of the computed properties is suggested. Also studied are the material's hydration and swelling properties. As water molecules are added to the pore space, the LDH material swells to some extent, with the hydration energy exhibiting interesting variations with the number of the water molecules added. The implications of the results are discussed.