Ion-exchange dynamics at the zeolite/solution interface studied by the chemical-relaxation method

Abstract

The kinetics of the hydrolysis of surface hydroxyl groups on the aluminosilicate framework and ion exchange of NH₄⁺, CH₃NH₃⁺, C₂H₅NH₃⁺, n-C₃H₇NH₃⁺, i-C₃H₇NH₃⁺, (CH₃)₂NH₂⁺, (CH₃)₃NH⁺ and (CH₃)₄N⁺ for Na⁺ at the zeolite/solution interface have been studied by using the pressure-jump relaxation method with electric-conductivity detection. Above pH 11.5 a single relaxation was found and was attributed to the hydrolysis of surface hydroxyl groups. Below this pH value two relaxations were found in aqueous suspensions of the systems comprising zeolite 4A with NH₄⁺, CH₃NH₃⁺, C₂H₅NH₃⁺, n-C₃H₇NH₃⁺ and (CH₃)₂NH₂⁺, but no relaxation was observed in the systems involving zeolite 4A and i-C₃H₇NH₃⁺, (CH₃)₃NH⁺ and (CH₃)₄N⁺. The fast and slow relaxations observed were attributed to diffusion of the alkylammonium ion on the surface of the particles, followed by adsorption of the alkylammonium ion on a site in the zeolite cage. From the kinetic and static experimental results it was found that the affinity of zeolite 4A for exchangeable cations is limited by the available intracrystalline space in the cage with larger cations unable to diffuse into the cage because of a lack of available intracrystalline space. The difference in the values of the rate constants associated with the alkylammonium ion entering the cage is interpreted in terms of a steric factor.