Displacement of Methane by Coadsorbed Carbon Dioxide Is Facilitated In Narrow Carbon Nanopores

By using simulation methods, we studied the adsorption of binary CO 2 –CH 4 mixtures on various CH 4 preadsorbed carbonaceous materials (e.g., triply periodic carbon minimal surfaces, slit-shaped carbon micropores, and Harris’s virtual porous carbons) at 293 K. Regardless of the different micropore geometry, two-stage mechanism of CH 4 displacement from carbon nanospaces by coadsorbed CO 2 has been proposed. In the first stage, the coadsorbed CO 2 molecules induced the enhancement of CH 4 adsorbed amount. In the second stage, the stronger affinity of CO 2 to flat/curved graphitic surfaces as well as CO 2 –CO 2 interactions cause the displacement of CH 4 molecules from carbonaceous materials. The operating conditions of CO 2 -induced cleaning of the adsorbed phase from CH 4 mixture component strongly depend on the size of the carbon micropores, but, in general, the enhanced adsorption field in narrow carbon ultramicropores facilitates the nonreactive displacement of CH 4 by coadsorbed CO 2 . This is because in narrow carbon ultramicropores the equilibrium CO 2 /CH 4 selectivity (i.e., preferential adsorption toward CO 2 ) increased significantly. The adsorption field in wider micropores (i.e., the overall surface energy) for both CO 2 and CH 4 is very similar, which decreases the preferential CO 2 adsorption. This suppresses the displacement of CH 4 by coadsorbed CO 2 and assists further adsorption of CH 4 from the bulk mixture (i.e., CO 2 /CH 4 mixing in adsorbed phase).