Fundamentals of methane adsorption in microporous carbons

This paper analyzes from a fundamental point of view the adsorption of methane on microporous carbons. CH4 adsorption on a series of microporous activated carbons (AC), activated carbon fibers (ACF) and superactivated carbons (SAC), at 298 K and 4 MPa (supercritical conditions) is analyzed. These results are compared with those obtained from the adsorption of other gases (N2 at 77 K up to 0.1 MPa and CO2 at 273–298 K up to 4 MPa). A detailed analysis is made to obtain realistic values of the saturation pressure, the affinity coefficient and density of adsorbed CH4. The results show that CH4 adsorbs in narrow micropores (<0.7 nm), which are well characterized by CO2 adsorption. For samples with uniform micropore size distribution (MPSD), CH4 adsorption data fit very well the characteristic curve. Contrarily, on samples with heterogeneous MPSD, CH4 adsorption data do not fit the characteristic curve. These results point out the dependence of CH4 density with the pore size at supercritical conditions. Although CH4 adsorbs in the microporosity and fills it as a compressed gas, the adsorption data has been also related to the specific BET surface area. A linear relationship between the estimated BET areas obtained from both adsorptive (N2 and CH4) is observed. In this paper, a new molecular surface area for CH4 at 298 K has been estimated (0.38 nm2).