Applicability of using CO₂ adsorption isotherms to determine BET surface areas of microporous materials

While nitrogen and argon isotherms at their respective boiling points are commonly used for the BET analysis of porous materials, CO₂ isotherms at 273 K have been suggested as an alternative option for materials containing very small pores in which nitrogen and argon molecules cannot access at cryogenic temperatures. However, it has been known that the BET surface areas obtained from CO₂ isotherms are not really meaningful in an absolute sense due to the strong CO₂-CO₂ interaction. In this study, CO₂ isotherms in metal-organic frameworks (MOFs) and zeolites were predicted by grand canonical Monte Carlo (GCMC) simulations and used to evaluate the BET analysis for these materials. For all the microporous materials with varied pore sizes, the BET surface areas calculated from the simulated CO₂ isotherms agreed roughly with their geometric surface areas from the crystal structures. Moreover, a clear selection of a proper BET linear region was possible for the material possessing only ultra-micropores. These results indicate that the BET surface areas determined from CO₂ isotherms at 273 K may be considered reasonable for adsorbents containing only ultra-micropores where the BET surface areas based on N₂ or Ar isotherms at cryogenic temperatures are not available.