(71e) Molecular Modeling of Carbon Dioxide Adsorption on Coal-like Adsorbents

Capture of carbon dioxide from fossil fuel power plants via adsorption and sequestration of carbon dioxide in unmineable coal seams are achievable near-term methods of the reducing atmospheric emissions of this greenhouse gas. To investigate the influence of surface chemical heterogeneity upon the adsorption of carbon dioxide in activated carbons and coal, isotherms were generated via grand canonical Monte Carlo simulation for CO2 adsorption in slit-shaped pores with an underlying graphitic structure and several variations of chemical heterogeneity (oxygen and hydrogen content), pore width, and surface functional group orientation. Both van der Waals and electrostatic interactions were included in the computation of the adsorbate-adsorbent potential.

It was observed that CO2 adsorption generally increased with increasing surface oxygen content, although exceptions to this trend were observed on structurally heterogeneous surfaces with holes or furrows that yield strongly adsorbing sites that preferentially bind CO2. The pressure at which pore filling with carbon dioxide occurred varied over approximately one order of magnitude for pores in the size range between 1.35 nm and 2.4 nm. Carboxylate-substituted surfaces bound CO2 more strongly than phenolic surfaces of similar acid site density. Interestingly, none of the heterogeneous pore structures investigated adsorbed carbon dioxide more strongly than a planar, homogeneous graphitic slit pore. However, the molecular model for the coal-like adsorbent assumes a rigid structure, whereas actual coals are known to swell significantly upon adsorption of CO2. Thus, the carbon dioxide isotherms calculated for rigid coal-like pores are likely to understate the true sorption capacity of coal matrices for CO2 uptake.