The mechanism of faujasite growth studied by crystallization kinetics

Abstract

The rate constants of linear growth k have been studied in crystallization experiments of faujasite in an extended range of batch compositions in dependence on the alkali, the silicate, and for a series of batches also on the aluminate content in the solution phases. The dependence of the rate constant on the concentrations in the solution phase can be fitted to a function Download full-size image or also to Download full-size image which is similar to the Langmuir-Hinshelwood equation of a bimolecular catalytic reaction assuming that the reaction step is rate determining. If the reaction product in the Langmuir-Hinshelwood theory is identified with a ring structure like, e.g. Download full-size image Functions (A1) and (A2) describe the rate of formation of this species starting with monomeric silicate and aluminate in the solution phase, which is bound to the surface and form then the mentioned ring structures and by further reorganization the structural elements of the growing zeolite. These steps seem to be rate determining. The respective relation derived from the Hou-gen-Watson theory for the attachment of silicate or aluminate as the rate-determining step does not describe the experimental data correctly. Further it is shown that a diffusion-controlled supply of silicate and aluminate is at least 2 orders of magnitude faster than the observed values and cannot be regarded as a rate-determining step.