Methane and carbon dioxide adsorption and diffusion in amorphous,metal-decorated nanoporous silica

The adsorptive and diffusive behaviour of methane and carbon dioxide in amorphous nanoporous adsorbents composed of spherosilicate building blocks, in which isolated metal sites have been distributed, is examined. The adsorbent contains cubic silicate building blocks (spherosilicate units: Si₈O₂₀), which are cross linked by SiCl₂O₂ bridges and decorated with either –OTiCl₃ or –OSiMe₃ groups of the other cube corners. The model structures were generated to correspond to experimentally synthesised materials, matching physical properties including density, surface area and accessible volume. It is shown that both methane and carbon dioxide adsorb via physisorption only in the modelled materials. Adsorption isotherms and energies at 300 K for pressures up to 100 bar were generated via molecular simulation. The maximum gravimetric capacity of CH₄ is 16.9 wt%, occurring at 300 K and 97 bar. The maximum gravimetric capacity of CO₂ is 50.3 wt%, occurring at 300 K and 51.6 bar. The best performing adsorbent was a low-density (high accessible volume) material with no –OTiCl₃ groups. The presence of –OTiCl₃ did not enhance physisorption even on a volumetric basis, and the high molecular weight of –OTiCl₃ groups is a significant penalty on a gravimetric basis. Based on the pair correlation functions, the most favourable adsorption sites for both adsorbates are located in front of the faces of spherosilicate cubes. The self-diffusivity and activation energy for diffusion are also reported.