NLDFT adsorption models for zeolite porosity analysis with particular focus on ultra-microporous zeolites using O₂ and H₂

The most often used gases for the PSD characterization of zeolites are N₂ and Ar. According to the IUPAC Technical Report (2015), Ar is recommended as more reliable than N₂, which molecules possess a significant quadrupole moment that may influence the adsorption isotherms on polar surfaces. In practice, however, using Ar as a cryogen for Ar adsorption measurements is not preferred due to its higher cost than liquid N₂. We propose using O₂ which has a much lower quadrupole moment than N₂, and its adsorption can be measured using standard liquid N₂ as a coolant. To support using O₂, we demonstrate good agreement between the PSDs calculated from O₂ and Ar adsorption. In the present work, we develop several semiempirical models based on nonlocald density functional theory (NLDFT) for the PSD characterization of several types of zeolites. We discuss the underlying difficulties in modeling gas adsorption on zeolites which adsorption potential field depends on both the pore width and the chemical structure of the zeolite. For the analysis of ultra microporous zeolites such as Chabazite and molecular sieve 5A, we apply H₂ in combination with O₂ at 77 K. H₂ molecule has a smaller diameter than O₂ and diffuses faster into ultra micropores, reducing the time of isotherm measurement. Moreover, we show that the dual gas analysis method can be used with O₂ isotherms measurements omitting low-pressure points, making the analysis faster.