Zeolite

Background

Zeolites (Greek, zein, »to boil », lithos, « a stone ») are aluminosilicates that have welldefined porous structures. The term was originally coined in the 18th century by a Swedish mineralogist named Axel Fredrik Cronstedt who observed, upon rapidly heating a natural mineral that the stones began to dance about as the water evaporated. Using the Greek words which mean « stone that boils », he called this material as zeolite.
Strictly speaking, zeolites are defined as crystalline microporous aluminosilicates with pore structures consisting of sharing TO4 tetrahedra, where T is Si or Al. Zeolites can be described with the following empirical formula:
Mn+1/n . AlO2- . x SiO2 . yH2O
Where M – counter ion
n – counter ion valence
x – silicon/aluminum ratio
y – content of hydrate water
Owing to the well-defined pore structure, zeolites are also known as « molecular sieves ». The term molecular sieve refers to a particular property of these materials, i.e., the ability to selectively adsorb molecules based primarily on a size exclusion process. This is due to a very regular pore structure of molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a zeolite is controlled by the diameter of the pore channels.

Structure

The flexibility of the zeolite Si-O-Si bond explains the fact that about 200 structures have been determined. Indeed, there is little energetic difference (10-12 kJ/mol) between these remarkable porous silicates and higher density phases such as quartz. More than 150 zeolite types have been synthesized and 48 naturally occurring zeolites are known.The structure commission of the International Zeolite Association (IZA) provides up to date classification by framework type. Each framework is assigned a three-letter code, recognized by the IUPAC Commission on Zeolite Nomenclature.[3] According to the IZA structure commission, zeolite frameworks can be thought to consist of finite or infinite (i.e., chain- or layer-like) component units. The primary building units are single TO4 tetrahedra. The finite units which have been found to occur in tetrahedral frameworks are shown in Figure 1.1. These secondary building units (SBU), which contain up to 16 T-atoms, are derived assumption that the entire framework is made up of one type of SBU only. A unit cell always contains an integral number of SBUs. In some instances, combinations of SBUs have been encountered. However, it should be noted that the SBUs are only theoretical topological building units and should not be considered to be or equated with species that may be in the solution/gel during the crystallization of a zeolitic material.Zeolites can be also classified on grounds of their pore openings and the dimensionality of their channels. Thus, one distinguishes small pore zeolites (eightmembered-ring pores), medium pore zeolites formed by ten-membered rings, large pore zeolites with twelve-membered-ring pores and extra-large pore zeolite category. This classification simplifies comparisons in terms of adsorptive, molecular sieving and catalytic properties.

General Properties of Zeolites

As mentioned above, the presence of Al in the structure of zeolites results in the formation of anion sites within the framework. Charge neutralization may occur by either protonation or by interaction with a metal cation or a hydronium ion. Thus, both Brönsted and Lewis acidities may be present within the zeolite framework. The protonation of the Al-O-Si oxygen center can result in Brönsted acidity in the zeolites structure. Lewis acidity is typically related to the compensating metal ions and defects in the aluminosilicate framework. Brönsted acid sites in zeolites can change into Lewis acid sites through dehydroxylation on heating.Although zeolites are usually considered acid catalysts, cation substitution with Rb and Cs, as well as metal doping, creates a basic zeolite.The presence of heavy metal cations is believed to increase the negative charge on the aluminum center, which is transferred to the adjacent oxygen atom, creating a basic site.The hydrophobicity is an important characteristic of zeolites since it can have a profound influence on their chemical reactivity. Zeolites containing charges are normally hydrophilic materials that, depending on the framework Si/Al ratio, can be more or less selective adsorbents for polar or nonpolar molecules. However, silicalite-1 which is a pure silica zeolite is a highly hydrophobic material. In contrast, FAU zeolite with the Si/A1 ratio between 2 and 5 is a highly hydrophilic absorbent. It is then clear that the polarity of a given zeolite could be controlled by controlling the Si/Al ratio by direct synthesis or by postsynthesis treatments, and this, together with appropriate control of the number of silanol groups by synthesis or postsynthesis treatments, should make it possible to prepare zeolite catalysts within a wide range of surface polarities.