Possibilities and limitations of the prediction of the Si/Al ratios of zeolites from the batch composition

Abstract

In earlier studies, it was shown that the Si/Al ratio of faujasite and some related zeolites crystallizing from batches with the composition NaAlO 2: n(Na m H 4– m SiO 4): pH 2O can be described by (A) ( Si/ Al) zeolite =1+b([ SiO 2]/[ OH −]) Solution , where [OH −] is the total alkali content and [SiO 2], the silica content of the solution phase in the batch. From Eq. (A) with some assumptions (B) ( Si/ Al) Product ≈ (b+m)n b+nm can be obtained, containing only the parameters n and m of the batch composition and the fitting parameter b from Eq. (A). For a large number of faujasite batches and for other large pore zeolites with four- and six-membered rings in the structure, a good agreement was obtained between the experimental Si/Al ratios and the values obtained from Eq. (B) using b=2. For zeolites with five-membered ring structures values of b=7–10 proved to be suitable. For high template contents, no common description can be obtained by Eq. (B). It could, however, be shown that this equation would be a good help for planning syntheses of mordenite, template-free ZSM-5 and BETA. The experiments with BETA were best described with b=14. From kinetic studies, Eq. (A) and the parameter b can be at least qualitatively explained by a model assuming that an amorphous layer is formed in an equilibrium reaction at the crystallizing surface. The formation of the final structure occurs by a reorganization of this layer under the influence of cations and templates. This reaction step is rate determining. The Si/Al ratio is determined in the attachment reaction. From an analysis of the alkalinity in the solution phase, it can be seen that, for the zeolites with four- and six-membered rings, the attachment of monomer silicate and the silico-aluminate species with about one silicate ligand can be expected. For the five-membered ring zeolites oligomer silicates and silico-aluminate complexes with two or more ligands are more probable. A borderline between the two situations can be drawn at about ( nm) 55.5/p≈[ OH]≈1 mol/l .