Quimica do petroleo

An empirical model for the nucleation and growth of zeolites
Department of Chemistry, University of Manchester, Manchester, UK
Stuart W. Carr

Peter M. Budd, Graham j. Myatt, and Colin Price

Unilever Research, Port Sunlight Laboratory, Bebington, Wirral, Merseyside, UK
The rate of nucleation of zeolite crystals typically passes through a maximum and may, in many cases, be described by a function of the form: n(x) = S[x rn - (xm'lt(nrd-r,I)], where n(x) is the rate of nucleation at a time x and tn denotes the end of the primary nucleation period. Crystalline growth curves may thus be calculated, given the linear growth rate of individual crystals. The model is applied successfully to recent results for the hydrothermal synthesis of zeolite NaA in the presence of various surfactants and water soluble polymers.
Keyworde: Zeolite crystallization; nucleation; growth; mathematical model

INTRODUCTION The well-defined molecular-scale pores of zeolites make them ideal for a range of applications, including ion exchange, catalysis, and separations, l It is possible to produce materials that exhibit subtle variations in physical and chemical characteristics. However, the design of a zeolite structure "a priori" has not yet been achieved and the synthesis of new materials can be viewed as an "Edisonian type of process. ''2 Although there are some guidelines for synthesis, the development of new materials with specific properties is a time-consuming process. This largely arises because of the complexity of the zeolite synthesis mechanism. Synthesis involves a threedimensional polymerization of small molecular and oligorneric units. How these come together to produce a zeolite is poorly understood. Various groups have attempted to use spectroscopic techniques (Raman, 3 n.m.r., 4's i.r. 1) to study zeolite synthesis by looking for precursor species in the solution and solid phases of the gel. In this paper, our approach was to develop a model based on the