Adsorption of CO2 Molecules in Silicalite: Structural Investigation and Isotherm Modeling Using In Situ High-Resolution Powder X‑ray Diffraction

The adsorption of carbon dioxide (CO2) in porous materials is of great importance to address current environmental issues. We propose an approach where high-resolution powder X-ray diffraction and isotherm modeling are combined to unravel the adsorption process of CO2 in the zeolite silicalite. Four main positions where the CO2 molecules locate in silicalite are identified, two in the straight channels and two in the sinusoidal channels, which imposes a maximum adsorption capacity of 16 molecules per unit cell (2.77 mmol·g–1) at 21 bar. The resulting global isotherm is successfully fitted with a Toth model, in accordance with heterogeneous adsorption and the presence of intermolecular interactions. Then, to characterize the adsorption process at the sorption site level, a parametric Rietveld refinement is implemented, where the occupancy of each site is calculated from a site-specific isotherm model. The most favored site, in the straight channels, follows a simple Langmuir model, with the homogeneous adsorption of isolated molecules. The three other sites, less favored, display a Toth adsorption behavior, in accordance with the presence of molecule–molecule interactions. This combined approach, where Rietveld refinement and adsorption modeling are intrinsically linked, is a way to gain deeper knowledge of adsorption processes in nanoporous materials.