Flexibility in zeolites: origin, limits, and evaluation

Abstract

Numerous pieces of evidence in the literature suggest that zeolitic materials exhibit significant intrinsic flexibility as a consequence of the spring-like behavior of Si-O and Al-O bonds and the distortion ability of Si-O-Si and Al-O-Si angles. Understanding the origin of flexibility and how it may be tuned to afford high adsorption selectivity in zeolites is a big challenge. Zeolite flexibility may be triggered by changes in temperature, pressure, or chemical composition of the framework and extra-framework compounds, as well as by the presence of guest molecules. Therefore, zeolite flexibility can be classified into three categories: (i) temperature and pressure-induced flexibility; (ii) guest-induced flexibility; and (iii) compositionally-induced flexibility. An outlook on zeolite flexibility and the challenges met during the precise experimental evaluations of zeolites will be discussed. Overcoming these challenges will provide an important tool for designing novel selective adsorbents.

Fig. 1. (a) In situ variable-temperature XRD scans of nanosized RHO zeolite from 30 to 700 °C and back to 28 °C, (b) plot of the ellipticity of eight-membered ring opening (in blue) and the lattice parameter (black). The dashed line delineates the adoption of either the acentric (squares) or centric (diamonds) space groups (reprinted with permission from ref. , copyright 2022 American Chemical Society).

Fig. 2. (a and b) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis of the nanosized RHO submitted to thermal treatment between 200 and 800 °C at different magnifications. Scale bars of 50 and 20 nm for the first and second rows, respectively. (c) Superimposed micrographs acquired at different temperatures show the volume expansion of the region highlighted by red color and denoted by t (adopted with permission from ref. , copyright 2023 American Chemical Society).

Fig. 3. Schematic illustration of temperature and pressure-induced, guest-induced, and compositionally-induced flexibility in zeolites (Mⁿ⁺ and Nⁿ⁺ are two arbitrary cations assuming smaller cationic diameter for Mⁿ⁺ compared to Nⁿ⁺).

Fig. 4. Integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) images of MFI straight channels (a) before and (b) after benzene adsorption. MFI structure flexibility is also schematically highlighted (c) before and (d) after benzene adsorption. Scale bar, 500 pm (reprinted with permission from ref. , copyright 2022 The American Association for the Advancement of Science).

Fig. 5. Schematic illustration of reversible ellipticity of RHO nanosized zeolites upon dehydration and CO₂ adsorption (adopted with permission from ref. , copyright 2020 American Chemical Society).

Fig. 6. Schematic illustration of cation gating (a) permanent relocation of Na⁺ cations, this is a cooperative mechanism by which CO₂ molecules could pass through a window site between α-cages in zeolite Na-Rho proposed by Lozinska et al. (reprinted with permission from ref. , copyright 2012 American Chemical Society). (b) Reversible Cs⁺ relocations namely trapdoor behavior in Cs-CHA proposed by Shang et al. (adopted with permission from ref. , copyright 2012 American Chemical Society).

Fig. 7. Ellipticity of zeolite RHO containing different cations (a) based on experiments and (b) based on simulations (adopted with permission from ref. and , copyright 2015 and 2016 American Chemical Society).

Fig. 8. Maximum changes in the unit cell volume of the most flexible zeolite structures presented in the literature.

Sajjad Ghojavand

Eddy Dib

Svetlana Mintova