(244f) Molecular Simulation of Adsorption in Polycrystalline Nanoporous Materials

We study adsorption of CO2 in a layered double hydroxide (LDH) materials, an important class of nanoporous materials with a wide variety of application, ranging from adsorbents for gases and liquid ions, to membranes, catalysts, and medical applications. They have a well-defined layered structure with nanometer (0.3 ~ 3 nm) interlayer separations, and contain certain important functional groups. They consist of two types of metallic cations that are accommodated by a close-packed configuration of a variety of anions in a positively-charged brucite-like layer. Water and the anions are distributed in the interlayer space for charge compensation. They often exist as aggregates of many single crystals, forming a polycrystalline structure.

It is difficult to directly characterize the polycrystalline structure by experimental methods, because the size of the intra-crystalline region is generally very small and the states, positions and orientations of its aggregated single crystals vary widely. We develop an atomistic model of such materials using energy minimization and molecular dynamics (MD) simulations. To simulate the intra-crystalline region, several single crystals of the LDHs are inserted in the simulation unit cell with arbitrary positions and orientations and the energy of the system is minimized, followed by MD simulations. X-ray diffraction and vibrational frequencies of the aggregates are computed, as are the diffusion coefficient and adsorption isotherm of CO2 in the resulting material. The results are compared with experimental data, and their implications for the use of such materials as adsorbents, catalysts and membranes are discussed.