Where q (mmol g-1) is the saturation adsorption amount,q 1 and q 2 (mmol g-1) is the saturation capacity on site 1 and site 2 of the adsorbent, respectively. The constant k 1(kPa-1) and k 2(kPa-1) represents the affinity coefficient for site 1 and 2, and v 1 andv 2 stands for the deviations of site 1 and site 2 from an ideally homogeneous surface, respectively. The variable p(kPa) is the total gas pressure. Based on ideal adsorbed solution theory (IAST), adsorption selectivity of gas was predicted according to the following equation (eq. 2) 24,25.