Layer development and growth history of polycrystalline zeolite A membranes synthesised from a clear solution

A case study is presented on the specific layer growth history of an α-Al2O3 supported NaA zeolite membrane synthesised from a clear solution. Using a defined set of synthesis parameters, the layer development over time (1.0–4.0 h) was described in terms of morphology, growth rate and elemental composition. It was shown that membrane growth proceeds along two distinct morphological pathways over the duration of synthesis – an initial layer of semicrystalline, hemisphere-shaped grains transforming into a fully crystalline layer with cubic morphology at the end of the growth process. A two-step growth rate trend was observed and could be correlated to the respective growth phases within the two underlying morphology types. The development of the hemisphere-shaped grains was associated with a period of accelerated growth during the first 2.5 h of synthesis (3.3 × 10−10 m s−1), followed by a period of slower growth for the formation of the cubic morphology (1.9 × 10−10 m s−1). Localised changes in supersaturation, combined with the possible effects of grain crowding, were offered as feasible explanations for the observed morphology and growth rate tendencies. Following the elemental make-up of the developing membrane showed a gradual decrease in the Na/Si ratio with increasing crystallisation times, which was explained by the consumption of the amorphous content in the membrane as growth proceeds. The solid phase compositions (Na/Si ratio) could however not explain the observed morphology and growth rate changes.