Isotopic exchange kinetics of zinc ions in Zn-A zeolite

Abstract

The isotopic exchange kinetics of Zn²⁺ ions in hydrated Zn-A zeolite of composition Zn_5·55Na_0·90(AlO₂)₁₂(SiO₂)₁₂·aq, have been investigated by measuring the fractional attainment of isotopic equilibrium between a ZnCl₂ solution and a ⁶⁵Zn-labelled Zn-A zeolite (30 and 45 µm particle radii) as a function of time, in the temperature range 25–60°C. The exchange mechanism is a two-step process which has been resolved, using the Brown–Sherry–Krambeck model, into diffusion in the solid particles, with Zn²⁺ diffusivity of D= 10^{–3.97 ± 0.03} exp (–E_D/RT)m² s^–1, E_D= 67.1 ± 0.5 kJ mol^–1, and an intracrystalline first-order exchange between bound and mobile Zn²⁺ ions in the network, with a rate constant of k₂= 10^{3.05 ± 0.25} exp(–E_k/RT)s^–1, E_k= 56.5 ± 1.5 kJ mol^–1.