CHAPTER ONE

• Introduction

Zeolites are porous crystalline alumino-silicates of regular skeleton structures formed by alternating silicon-oxygen and aluminum-oxygen tetrahedrons. Although only natural zeolites were initially used, synthetic zeolites, due to their well-tailored and highly-reproducible structures, have been used extensively as ion exchangers, adsorbents, separation materials and catalyst¹.The negative charges in aluminum-oxygen tetrahedron, which are not rigidly fixed to the skeleton of zeolites, are compensated with cations, so they are capable of interchanging. Silicon-oxygen and aluminum-oxygen tetrahedrons in the zeolites of the type A, X and Y form a complex structural unit of cubooctahedron. The combination of such units forms the structure of type A, X and Y [fig 7].^. The difference between them consists in the fact that they are interconnected by means of different number of member rings (i.e., eight member rings (A), twelve member rings (X, Y). The chemical difference of zeolite is defined by the ratio of Si/Al. For zeolite A this values is in the range of 0.95-1.05^1-3. Zeolites A, X and Y are the most important ones to be used in pharmaceutical, petrochemical and detergent industries.

Zeolites with different structure are known to be obtained by synthesis ^2-7. They are either synthesized from alumino-silicate hydrogel or by conversion of clay minerals. The hydrogel can be prepared from different sources of silica and alumina, but the types of starting materials and the method of mixing determine the structure of the resulting gel. Moreover, the nature of the gel influences the rate of the subsequent crystallization, which affects the particle size distribution, and the formation of impurities⁸. The general pathway for zeolite synthesis follows a specific temperature gradient at low temperatures (<60 ^oC) where the sources of aluminum, silicon and water are placed in solution and mixed until a gel is formed⁹.