FIGURE 2 (a) CO2 adsorption and desorption isotherms obtained for MIL-120Al at 195 K; (b) pore size distribution of MIL-120Al; (c) TG curve obtained for MIL-120Al; (d) comparison of the pH value of the initial solution and thermal stability of conventional CH4/N2 separation materials.
Limited by the small window diameter, the pore-size of MIL-120Al cannot be obtained using N2 as a probe (Figure S2). Thus, a carbon dioxide sorption experiment was performed at 195 K, as shown in Figure 2a. The isotherm obtained for MIL-120Al shows a typical type I behavior, indicating its microporous nature. The Brunauer–Emmett–Teller (BET) surface area and pore volume were 529 m2/g and 0.24 cm3/g, respectively. These values were slightly higher than those previously reported, which may be attributed to the sample washing step using high temperature methanol.44 The pore size distribution was evaluated using the Horvath–Kawazoe model (Figure 2b) and the calculated pore size (4.8 Å) was consistent with the results obtained from the crystal structure (5.4 × 4.7 Å).40 From a practical point of view, the pH value of the initial solution and thermal stability are two key metrics for evaluating the adsorbent. Figure 2c and Table S1 show MIL-120Al can only be synthesized in an alkaline environment (pH >7) among the conventional CH4/N2 separation materials, which means that a conventional stainless-steel reactor can be used in the industrial synthesis of this material. In addition, of TGA (Figures 2c, 2d and Table S1) shows that MIL-120Al exhibits higher thermal stability (613 K) when compared to ATC-Cu (553 K)12 and Ni(ina)2 (573 K).27