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 alpha-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 x 10(-10) m s(-1)), followed by a period of slower growth for the formation of the cubic morphology (1.9 x 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. (c) 2006 Elsevier Inc. All rights reserved.