Adsorption and Diffusion of Gases in Nano-Porous Materials

In this work, a systematic computational study directed toward developing a molecular-level understanding of gas adsorption and diffusion characteristics in nano-porous materials is presented. Two different types of porous adsorbents were studied, one crystalline and the other amorphous. Physisorption and diffusion of hydrogen in ten iso-reticular metal-organic frameworks (IRMOFs) were investigated. A set of nine adsorbents taken from a class of novel, amorphous nano-porous materials composed of spherosilicate building blocks and isolated metal sites was also studied, with attention paid to the adsorptive and diffusive behavior of hydrogen, methane, carbon dioxide and their binary mixtures. Both classes of materials were modeled to correspond to experimentally synthesized materials. While much research has targeted adsorption in IRMOFs, very little has appeared for these amorphous silicates, which contain cubic silicate building blocks: Si₈O₂₀ [spherosilicate units], cross-linked by SiCl₂O₂ [silicon chloride] bridges and decorated with either -OTiCl₃ [titanium chloride] or -OSiMe₃ [trimethylsilyl] groups. Based only on physisorption, the amorphous silicates show competitive adsorptive capacities and selectivities with other commercial gas adsorbents.

The tools employed in this dissertation were computational in nature. Adsorptive properties, such as adsorption isotherms, binding energies and selectivities, were generated from Grand Canonical Monte Carlo molecular (GCMC) simulations. Self-diffusivities and activation energies for diffusion were generated using Molecular Dynamics simulations. Adsorption isotherms are reported at temperatures of 77 K [Kelvin] and 300 K for pressures ranging up to 100 bar. The most favorable adsorption sites for all gases studied in the amorphous silicates are located in front of the faces of the spherosilicate cubes. Regardless of material, the hydrogen adsorption process is governed by entropic considerations at 300 K. At 77 K energetic considerations control hydrogen adsorption at low pressures and entropic effects dominate at high pressure. For methane and carbon dioxide at 300 K, the adsorption process is governed by energetic considerations at low pressure and by entropic (packing) constraints at high pressure. The amorphous silicates showed very high selectivity for carbon dioxide over hydrogen. The presence of titanium sitesdid not enhance physisorptive capacity or selectivity.