FIGURE 4 (a) CH4 and N2adsorption heat of MIL-120Al calculated using the Clausius–Clapeyron
equation; (b) a comparison of the CH4 adsorption heat
observed for some previously reported CH4-selective
materials.
The isosteric heat of adsorption (Qst) for
CH4 and N2 in the MIL-120Al were
calculated based on the single-component adsorption isotherms obtained
at 273 and 298 K using the Clausius–Clapeyron
equation.54 Figure 4a shows the Qst at
zero coverage for CH4 (20.87 kJ/mol) was higher than
that of N2 (17.83 kJ/mol), suggesting the stronger
interactions formed between the framework and CH4molecules. Interestingly, we found that upon increasing the
CH4 loading, the Qst value were smooth
lines with almost unchanged values (20.48–20.87 kJ/mol). This implies
the energetic homogeneity of the MIL-120Al surface, and a similar
phenomenon was also observed in
[Co3(C4O4)2(OH)2].1In addition, when compared with the outstanding materials reported in
the literature (see Figure 4b), MIL-120Al exhibits a low initial
Qst value for CH4, which is beneficial
to sorbent regeneration in an industrial process.