Zeolite

Zeolites are crystalline aluminosilicates with a 3-dimentional, open anion framework consisting of oxygen-sharing TO₄ tetrahedral, where T is Si or Al. Their framework structure contains interconnected voids that are filled with adsorbed molecules or cations. Zeolite micropore channels have very well-defined diameters so that bulky molecules will be excluded from the internal surface. The general empirical formula is :

                       

                               M_x/m  .  Al_xSi_2-xO₄  .  nH₂O

Where m is the valence of cations M, n the water content and 0 ≤ x ≤ 1 (Houssin, 2003).

Zeolites are composed of pores and corner-sharing aluminosilicate (AlO4 and SiO4)tetrahedrons, joined into 3-dimensional frameworks. The pore structure is characterized by cages approximately 12 Å in diameter, which are interlinked through channels about 8 Å in diameter, composed of rings of 12 linked tetrahedrons (Kaduk and Faber, 1995). The pores are interconnected and form long wide channels of varying sizes depending on the mineral. These channels allow the easy movement of the resident ions and molecules into and out of the structure. Zeolite have large vacant spaces or cages within and resemble honeycomb or cage like structures. The presence of aluminium results in a negative charge, which is balanced by positively charged cations (E. Polat et al., 2004).

There are two types of zeolite :

1.       Natural Zeolites

These zeolites are found in volcanic or metamorphic rocks and their growth involves geological conditions (low temperature and pressure, low pH (8-9)) and time scale (thousands of years). There are more than fourty types of these  zeolites that have been reported by different research group. Among these minerals, analcime (sometimes known as analcite), clinoptilolite, erionit, chabazite, mordenite, and philipsite are well known (Doğan, 2003).

2.       Synthesize Zeolite

                   There are more than 150 zeolites that have been syntesized. Some of the common synthetics are              

                   zeolites A, X, Y and ZSM-5. (the descriptions of these zeolites will come soon) ^v^”

There are four types of applications that zeolites are used for:

1.       Drying agents (used for drying solvents)

2.       Shape selective separations (e.g. dewaxing desel fuel)

3.       Shape selective catalysis (predominantly acid catalysis, but also redox)

4.       Selective ion exchangers (water softeners, radioactive waste treatment) (Wilkinson A. P.)

A schematic reperesentation of zeolite formation process is given in this figure.

Simplified zeolite synthesis scheme. SDA stands for structure-directing agent (Houssin, 2003).

References : Houssin, Christophe J.Y. 2003 Nanoparticles in zeolite synthesis Eindhoven : Technische 

                                            Universiteit Eindoven

          E. Polat, Mahmet K., Halil D. and A. Naci Onus, 2004 J. Fruit Ornam. Plant Res.vol. 12

                          Kaduk J. A. , Faber J 1995. Crystal structure of zeolite Y as a function of ion exchange. RIGAKU J.
                                             12: 2, 1434

                          Doğan H. 2003. Doğal ve Sentetik Zeolitler ve Uygulama Alanları, Bor Teknolojileri ve Mineraller 
                                             Grubu. TÜBİTAK Marmara Araştırma Merkezi .

         Wilkinson, Angus P., School of Chemistry and Biochemistry Georgia Institute of Technology 
                              Atlanta, GA 30332-0400