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 300K for pressures up to 100bar were generated via molecular simulation. The maximum gravimetric capacity of CH4 is 16.9wt%, occurring at 300K and 97bar. The maximum gravimetric capacity of CO2 is 50.3wt%, occurring at 300K and 51.6bar. 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.