A New Type of Acidic OH-Groups in the LTL Zeolite

Contini, Alessandro; Jendrlin, Martin; Zholobenko, Vladimir

Abstract

Acidic properties of ion-exchanged LTL zeolites have been studied using FTIR spectroscopy, complemented by X-ray powder diffraction, SEM-EDX, XRF and N2 physisorption. Infrared spectra of the ion-exchanged zeolites show the presence of two intense bands of the bridging OH-groups: a narrow band at ~3640 cm−1 that is attributed to Si(OH)Al groups freely vibrating in 12 MR and a broad, intense band at ~3250 cm−1 that is assigned to bridging OH groups forming hydrogen bond with neighbouring oxygen atoms, e.g., in six-membered rings. The former can be selectively removed by caesium or rubidium cations with up to 3 Cs+ or Rb+ per unit cell readily ion-exchanged into the LTL zeolite, replacing an equivalent number of acidic OH-groups or K+ cations within the structure. The cation migration of the larger cation, evaluated by the Rietveld refinement method, occurs mostly via the main 12 MR channels. By contrast, less than 1 Li+ or Na+ cation per unit cell can be introduced under similar conditions. Accordingly, the concentration of Si(OH)Al groups in back-exchanged NH4-K-LTL with smaller cations (Li+, Na+) does not differ considerably from the concentration of Brønsted acid sites in the original NH4-K-LTL. Lower concentrations of acid sites have been detected in the samples back-exchanged with Cs+, Rb+ and K+. In addition, the acidic properties of NH4-LTL samples have been compared with a structurally related NH4-MAZ zeolite.

Citation

Contini, A., Jendrlin, M., & Zholobenko, V. (in press). A New Type of Acidic OH-Groups in the LTL Zeolite. Crystals, 14(8), Article 745. https://doi.org/10.3390/cryst14080745

• Journal Article Type: Article
• Acceptance Date: Aug 16, 2024
• Online Publication Date: Aug 21, 2024
• Deposit Date: Sep 9, 2024
• Journal: Crystals
• Publisher: MDPI
• Peer Reviewed: Peer Reviewed
• Volume: 14
• Issue: 8
• Article Number: 745
• DOI: https://doi.org/10.3390/cryst14080745
• Keywords: acid sites, XRD, FTIR spectroscopy, zeolite LTL
• Public URL: https://keele-repository.worktribe.com/output/891865
• Publisher URL: https://www.mdpi.com/2073-4352/14/8/745