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<p>Experimental studies and theoretical models presently disagree on methane adsorption
energetics on carbon materials that include crystalline graphene-like structures to amorphous
materials with or without significant edge structure. However, this information is critical for the
rational design and optimization of the structure and composition of adsorbents for natural gas
storage. The delicate nature of the interactions inherent to methane physisorption, such as
dispersion interactions, polarization of both the adsorbent and the adsorbate, interplay between H-
bonding and tetrel bonding, and induced dipole/Coulomb interactions, requires computational
treatment at the highest possible level of theory while remaining non-prohibitive in terms of
computational cost. In this study, we employ the smallest reasonable computational model, a
maquette, of porous carbon surfaces with a central atomic binding site for substitution. The most
accurate predictions of the methane adsorption energetics were achieved by electron-correlated
molecular orbital theory (CCSD(T)) and hybrid density functional theory (MN15) calculations, both
employing a saturated all-electron basis set. The characteristic geometry of methane adsorption on
a carbon surface was likened to a “lander” position over the ring centers of the adsorbent. This
adsorbate/adsorbent arrangement arises due to bonding interactions of the adsorbent π-system
with the proximal H–C bonds of methane, in addition to tetrel bonding between the antibonding
orbital of the distal C–H bond and the central atom of the maquette (C, B, or N). The polarization of
the electron density as well as structural deformations in both the adsorbate and adsorbent
molecules clearly indicate a ~3 kJ mol-1 preference for methane binding on the N-substituted
maquette. In this study, the B-substituted maquette showed a comparable or lower binding energy
than the unsubstituted, pure C model, depending on the level of theory employed. The calculated
thermodynamic results indicate an unambiguous guiding strategy toward incorporating electron-
enriched substitutions (e.g., N) in carbon materials as a way to increase methane storage capacity
over electron deficient (e.g., B) modifications. The thermochemical calculation methodologies were
critically evaluated in order to establish a conceptual agreement between the experimental
isosteric heat of adsorption and the binding enthalpies/free energies from statistical
thermodynamics principles.
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Methane Adsorption on Heteroatom-Modified Maquettes of Porous Carbon Surfaces
