A comparative study of CO₂ adsorption in a series of zeolites

Carbon dioxide (CO₂) adsorption in zeolites represents an essential area of research, particularly relevant to meeting environmental challenges through carbon capture technologies. This study aims to elucidate the CO₂ adsorption mechanisms at macroscopic and microscopic levels in four zeolites: two pure-silica zeolites, chabazite (Si-CHA) and SSZ-23 (Si-STT), and two aluminosilicate zeolites, faujasite (Na-FAU), and zeolite A (Na-LTA). The methodology combines volumetric adsorption measurements, in situ high-resolution X-ray powder diffraction, Monte Carlo simulations, and isotherm modeling. For each of the four systems, the global adsorption isotherms in the 0-1 bar and the 0-20 bar ranges were retrieved experimentally from volumetric data and Rietveld refinement carried out against in situ high-resolution powder diffraction patterns, respectively, and compared to simulations. In addition to this global description, we used a parametric Rietveld refinement approach to quantitatively assess the adsorption process at the level of each of the individual adsorption sites, using site-specific isotherm models in the structural model to calculate site occupancies. Favored adsorption sites in pure-silica zeolites follow a Langmuir model, with no notable interaction between adsorbed molecules. On the other hand, in the case of ideal adsorption, the aluminosilicate zeolites display a Toth behavior, with rapid uptake linked to the attractive possibility for CO₂ molecules to interact with Na cations. However, a change in the regime from Toth to Langmuir is observed in the case of non-ideal adsorption, due to the presence of remaining water molecules in the aluminosilicate zeolites. This approach, which combines adsorption modelings, Rietveld refinement, experiments, and simulations, offers new insights into the adsorption mechanism of CO₂ into zeolite frameworks, complementing the classic macroscopic description with an in-depth description at the adsorption site level.