Interplay between alkali-metal cations and silanol sites in nanosized CHA zeolite and implications for CO2 adsorption

Silanols are key players in the application performance of zeolites, yet, their localization and hydrogen bonding strength need more studies. The effects of post-synthetic ion exchange on nanosized chabazite (CHA), focusing on the formation of silanols, were studied. The significant alteration of the silanols of the chabazite nanozeolite upon ion exchange and their effect on the CO₂ adsorption capacity was revealed by solid-state nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR) spectroscopy, and periodic density functional theory (DFT) calculations. Both theoretical and experimental results revealed changing the ratio of extra-framework cations in CHA zeolites changes the population of silanols; decreasing the Cs⁺/K⁺ ratio creates more silanols. Upon adsorption of CO₂, the distribution and strength of the silanols also changed with increased hydrogen bonding, thus revealing an interaction of silanols with CO₂ molecules. To the best of our knowledge, this is the first evidence of the interplay between alkali-metal cations and silanols in nanosized CHA.

Hydroxyl environments in zeolites probed by deuterium solid-state MAS NMR combined with IR spectroscopy

Deuterium (2D) solid-state MAS NMR spectroscopy is used for the first time for identifying both silanols and Brønsted acid sites in zeolites. The environments of hydroxyl groups in faujasite-type zeolites...

The role of mixed alkali metal cations on the formation of nanosized CHA zeolite from colloidal precursor suspension

A clear understanding of the crystal formation pathways of zeolites remains one of the most challenging issues to date. Here we investigate the synthesis of nanosized chabazite (CHA) zeolites using organic template-free colloidal suspensions by varying the time of aging at room temperature and the time of hydrothermal treatment at 90 °C. The role of mixed alkali metal cations (Na⁺, K⁺, Cs⁺) on the formation of CHA in the colloidal suspensions was studied. Increasing the aging time of the precursor colloidal suspension from 4 to 17 days resulted in faster crystallization of CHA nanocrystals (3 h instead of 7 h at 90 °C) to afford significantly smaller particles (60 nm vs 600 nm). During the crystallization a considerable change in the content of inorganic cations in the recovered solid material was observed to coincide with the formation of the CHA nanocrystals. The Na⁺ cations were found to direct the formation of condensed and pre-shaped aluminosilicate particles in the colloidal precursor suspensions, while K⁺ cations facilitated the formation of secondary building units (SBUs) of the CHA type framework structure such as d6r and cha cages, and the Cs⁺ cations promoted the long-range crystalline order facilitating the crystallization of stable zeolite nanocrystals.

The preparation of hierarchical SAPO-34 crystals via post-synthesis fluoride etching

SAPO-34 (CHA-type) was subjected to treatment with a mixed NH₄F–HF aqueous solution resulting in a hierarchical micro-mesoporous material with retained intrinsic properties.