Figure 5 The proportional correlation between CO2 uptake capacity with the BET surface area of materials. The dash line is fitted by using the experimental results of MOF-74, py-MOF-74a and py-MOF-74c.
3.2.2 | Zero coverage adsorptionenthalpies
Adsorption enthalpies (Q st) were determined by Virial equation (Figure S1-S6) based on single-component isotherms at 298 K and 273 K. The zero coverage Q st of CO2 on MOF-74 and py-MOF-74 is higher than that of CH4 and N2, indicating special affinity of CO2to frameworks. The zero coverage Q st of CO2 on MOF-74 is 33.6 kJ mol-1. The zero coverage Q st shows a drop trend with the introduction of py (Table 1), since the open metal sites were partially or completely capped by pyrazine molecules. Furthermore, the lowest zero coverage Q st of CO2 appears on py-MOF-74c with ultimate pyrazine insertion. It provides an indirect proof of pyrazine positioning into the parent framework. In short, the zero coverageQ st of py-MOF-74 for CO2 is moderate, which would facilitate the effective diffusion and regeneration processes. This trait is important for practical adsorption and membrane separation.
Table 1 Zero coverage adsorption enthalpies (Qst) of CO2, N2 and CH4 on MOF-74 and py-MOF-74