Adsorption of CH4 and CH4/CO2 mixtures in carbon nanotubes and disordered carbons: A molecular simulation study

Abstract

We report a comparison of the adsorption of CH4 and CO2/CH4 mixtures of different composition in three different types of nanoporous carbons including carbon nanotubes, and activated carbon fibre (ACF-15) and silicon carbide derived carbon (SiC-DC) having distinctly different disordered structures, using Monte Carlo simulation. CO2 is represented as a linear molecule, and both the united-atom and full-atom models are investigated for CH4. It is found that the united-atom model of CH4 overestimates the adsorption of CH4 in all these adsorbents compared to the 5-site model, as a consequence of the enhanced 1-site CH4-adsorbent potential energy. Moreover, the selectivities of the nanoporous carbons for CO2 relative to CH4 calculated using the 1-site CH4 model are underestimated compared to those from the 5-site model, at pressures up to 3.0MPa. However, differences in the structural disorder of porous carbon models have little impact on CO2 selectivity. Our simulations reveal that the selectivity of an adsorbent for a particular species is strongly dependant on adsorbate–adsorbate interaction effects, comprising the adsorbate–adsorbate potential interactions and an adsorbate sieving effect. As a balance between the confinement and adsorbate–adsorbate effects, it is found that increasing the concentration of CO2 in the gas phase increases the selectivity of (10, 10) CNT dramatically, while having negligible impact on the selectivities in amorphous carbons. Further, it is shown that increasing the temperature reduces the performance of all the carbons in separating CO2, and that an isolated (7, 7) CNT has the best performance for CO2/CH4 separation in comparison to the disordered nanoporous carbons investigated.