# Selective and Reversible Cation-Gating Adsorption Behavior in Gmelinite Zeolites for Efficient CO2 Separation

## TL;DR

This paper shows how a specific type of zeolite can efficiently capture and release CO2 using a unique two-step process controlled by sodium ions.

## Contribution

The study reveals that Na+ ions in gmelinite zeolites enable reversible, stepwise CO2 adsorption, a novel mechanism for CO2 separation.

## Key findings

- Na+-type gmelinite zeolites show stepwise CO2 adsorption due to Na+ ion migration.
- Stepwise adsorption is reversible and observed even in pelletized zeolite powder.
- Only Na+ ions induce this cation-gating behavior, unlike Li+ or K+ ions.

## Abstract

Zeolites exhibiting
stepwise adsorption behavior, i.e.,
a two-step
increase in carbon dioxide (CO2) uptake, have attracted
attention in the field of zeolite research due to the potential to
recover CO2 using a small amount of energy. In this study,
a Na+-type gmelinite (GME) zeolite exhibited stepwise adsorption
behavior due to the migration of Na+ ions in the GME framework.
Gas adsorption measurements, in situ powder X-ray diffraction (PXRD)
analysis, and magic-angle spinning (MAS) nuclear magnetic resonance
(NMR) analysis revealed that Na+ serves as a gate-opening
cation that induces the migration of CO2 from straight
channels to the grain-like cages, resulting in notable stepwise adsorption.
Remarkably, repetitive CO2 adsorption measurements clarified
that this stepwise adsorption performance was reversibly induced.
Other cation-type GME zeolites such as Li+- and K+-GME zeolites exhibited type-I CO2 adsorption isotherms,
indicating only Na+ ions were capable of inducing the cation-gating
CO2 adsorption performance. Furthermore, time-resolved
PXRD analysis revealed that the migration rate of Na+ was
high under the CO2 adsorption and desorption processes.
In addition, stepwise adsorption behavior was observed even when the
GME zeolite powder was pelletized. These findings pave the way for
the development of highly efficient CO2 separation processes
that rely on zeolites with stepwise adsorption properties.