Properties of multiple Lewis acid sites in alkali metal-exchanged chabazites probed by CO adsorption

• Published in: Microporous and mesoporous materials Vol. 374; p. 113152
• Main Authors: Halamek, Jakub, Bulánek, Roman, Rubeš, Miroslav, Bludský, Ota
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 15.06.2024
• Subjects: ab initio modeling; Adsorption; Alkali metal; Calorimetry; Carbon monoxide; Chabazite; FT-IR; Lewis acid sites; Chabazite; Adsorption; FT-IR; Calorimetry; ab initio modeling; Alkali metal; Carbon monoxide; Lewis acid sites
• ISSN: 1387-1811; 1873-3093
• DOI: 10.1016/j.micromeso.2024.113152

Carbon monoxide adsorption on alkali-metal exchanged chabazites (M-CHA, where M = Li, Na, K) was investigated across various Si/Al ratios. The study reveals significant insights into the adsorption behavior, including the persistence of cationic preferences with decreasing Si/Al ratios and the existence of multiple-center interactions involving alkali-metal cations and CO. Results show that for high-silica M-CHA zeolites, CO adsorption is effectively described by single and dual adsorption site models, with cation preferences varying by type. In low-silica zeolites, cation positions are primarily influenced by the aluminum distribution and Coulombic interactions. However, the propensity for single-site cation positions (Si/Al→∞) is preserved to a certain degree. The most noticeable example is the small difference between SIII’ occupancies (cations in 8-membered ring windows) in Na-CHA-2 and K-CHA-2 (0.80 vs. 0.85) that strongly influences the rate of diffusion of CO in the M-CHA-2 samples. While FT-IR spectra of high-silica zeolites can be accurately described using cation site stabilities, interaction energies, and CO stretching frequencies, predicting spectra of low-silica chabazites requires a statistical approach and/or molecular dynamics simulations at the DFT level. The findings demonstrate that the dynamical behavior of adsorbates changes dramatically between different alkali metal-exchanged chabazites, highlighting the complex nature of CO adsorption at multiple Lewis acid sites. [Display omitted] •CO adsorption in M-CHA (M = Li, Na, K) zeolites across various Si/Al ratios.•High-silica M-CHA: single/dual adsorption site models explain FT-IR spectra.•Low-silica M-CHA: properties of multiple sites depend on Al distribution.•Dynamics of CO adsorption differs significantly in low-silica M-CHA samples.•Theoretical models align well with experimental spectra.