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

Abstract

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.

Investigating the Physicochemical Properties of an Extra-large Pore Aluminosilicate ZEO-1

“…These findings conclude that the hydroxyl groups responsible for this spectral region originate from silanol nests actively involved in hydrogen bonding. Our group has recently reported the healing of these silanol nests in the presence of Cs for chabazite zeolite, where an increase in Cs content at the expense of potassium (K) promoted the healing of silanols . Furthermore, the persistence of these nests when ZEO-1 is activated in its protonic form suggests that they are stabilized by hydrogen bonding with the neighboring Brønsted acid sites, resulting in the lower wavenumber shift of these Brønsted acid sites.…”

CO2 Adsorption Behavior of Nanosized CHA Zeolites Synthesized in the Presence of Barium or Calcium Cations

“…The Si/Al ratio determined by 29 ) aluminum atoms respectively. [47,48] The overall broadening of all the Q 4 species and mainly the Q 4 (nAl) where n is bigger than 2 in the Ca-CHA and Ba-CHA samples compared to the Ref-CHA, suggests an additional angular distortion in the framework. This is due to the partial substitution of monovalent cations by bivalent cations that are only stable when two or more aluminum are present in their surroundings.…”

Flexibility in zeolites: origin, limits, and evaluation

“…Additionally, in situ solid-state NMR while adsorbing guest molecules can be of importance to clarify the atomic order that contributes to the guest-induced exibility. 131 Recently, Ilkaeva and co-workers developed a method to follow CO 2 adsorption using in situ solid-state NMR which shed light on different CO 2 chemisorbed species on SBA-15. 132 In situ FTIR while adsorbing guest molecules also illustrates the atomic orders especially those of the silanol sites.…”

“…132 In situ FTIR while adsorbing guest molecules also illustrates the atomic orders especially those of the silanol sites. 131,133 As a result, combining all these in situ techniques (i.e. XRD, NMR, and FTIR), while adsorbing guest molecules, can help to solve the intertwined dynamics between the zeolite framework and extra-framework cations.…”