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

Abstract

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.