Anomalous isosteric enthalpy of adsorption of methane on zeolite-templated carbon.

A thermodynamic study of the enthalpy of adsorption of methane on high surface area carbonaceous materials was carried out from 238 to 526 K. The absolute quantity of adsorbed methane as a function of equilibrium pressure was determined by fitting isotherms to a generalized Langmuir-type equation. Adsorption of methane on zeolite-templated carbon, an extremely high surface area material with a periodic arrangement of narrow micropores, shows an increase in isosteric enthalpy with methane occupancy; i.e., binding energies are greater as adsorption quantity increases. The heat of adsorption rises from 14 to 15 kJ/mol at near-ambient temperature and then falls to lower values at very high loading (above a relative site occupancy of 0.7), indicating that methane/methane interactions within the adsorption layer become significant. The effect seems to be enhanced by a narrow pore-size distribution centered at 1.2 nm, approximately the width of two monolayers of methane, and reversible methane delivery increases by up to 20% over MSC-30 at temperatures and pressures near ambient.