Quality synthetic zeolites have composition and properties similar to chemical individua. That is why the properties of molecular sieves SLOVSIT correspond to the general characteristics of synthetic zeolites.

Zeolite molecular sieves, synthetic or natural, are crystalline aluminosilicates of metals. They have a three-dimensional network structure. The basic structural units are tetrahedrons [SiO₄]-4 and [AlO₄]-5, where the atom of silicium and aluminum are placed centrally and at the corners of the tetrahedron are placed larger oxygen atoms. The tetrahedrons are mutrially convected with oxygen atoms that are in common for two adjacent tetrahedrons. The negative charge of the lattice is neutralized by the positive charge of the metal cations. In the basic zeolites these are usually cations of univalent and bivalent metals or their combination.

The chemical composition of these zeolites in dehydrated form is expressed by the general formula

Me_2/m . Al₂O₃ . nSiO₂

Where Me is the metal ion, mostly ion of alkaline metal Na⁺, K⁺ or alkaline earth, mostly Ca⁺², or Mg⁺² and Ba⁺² .
M is the valency of the corresponding metal and n is the mole ratios SiO₂/Al₂O₃ or n/2 = Si:Al – that is also designated as module, a significant characteristic of zeolites.

The basis structural unit of type A, X, Y zeolites are truncated suboctahedrons, known also as sodalite cells. They are formed from 24 elementar tetrahedrons [SiO₄] [Al₂O₃]. The mutual ration of tetrahedrons, i.e. modules, and the crystallographic structure characterize the type of zeolite. At the cuboctahedron surface are 6 four-membered oxygen rings, with octahedral symetry and 8 six-membered oxygen rings with tetrahedral symetry through the bridges, formed from oxygen atoms of the rings, is the mutual joining of cuboctahedrons. The mode of joining determines the crystallographic structure and is characteristic for zeolites of the type A or X and Y. In zeolite type A each cuboctahedron is bonded with six additional cuboctahedrons through four-membered oxygen bridges. The elementary cell of zeolite A contains in total the combination of 24 ions of silicium and aluminium, the ratio of which corresponds to the characteristic modul of this zeolite 0,9 – 1, and 48 ions of oxygen. Beside the cuboctahedrons, connected through four-membered oxygen bridges the so called big cavity is formed having an diameter 11,4 . 10^-10 m and volume 0,28 cm³/g, and is accessible through six openning, i.e. pores. The diameters of these pores, in case of zeolite type A, are limited by 8-membered oxygen rings and the cations located at proximity.

The crystallographic structure of zeolites X and Y is topologically equal to the natural faujasite, that is why these zeolite types are also called faujasite. Their aluminosilicate skeleton is built, like in the case of zeolite A, from cuboctahedron units, but is connected by oxygen bridges formed from 6-membered oxygen rings. Each cuboctahedron is joined with four additional cuboctahedrons in tetrahedral arrangement. The elementary cell of zeolites type X and Y is formed from eight cuboctahedrons and sixteen oxygen bridges. The adsorption space of big cavities is in the case of faujasites surrounded by 10 cuoctahedrons. These cavities are accessible through four openings, having a diameter of 7,4.10^-10 m and limited by 12 membered oxygen rings, which are characteristic for zeolites type X and Y.

Modifications

Synthetic zeolites are produced as primary products, in general as sodium type. Modification means a zeolite derived from primary types by ion-exchange, i.e. introducing other metal ions into the zeolite structure. The ion-exchange is done contacting the zeolite with a solution of the corresponding metal or by a salt combination. This exchange is impossible to bring about to a 100%. That is why even in the case of exchange of the original ion of one metal only, the final zeolite contains their combination. From the electro-neutrality principle of the elementary cell follows that the higher is the modul Si:Al, the lower is the number of cations neutralizing the negative charge of the lattice.

Zeolites type A are offerred on the market mainly in modifications with cations potassum, calcium and the basic modification with sodium cations, zeolite X in sodium and calcium form and zeolite Y in sodium form. Beside additional zeolites miwth other metals and various metal combination or decationized (metal is substituted by hydrogen cation) are produced for special adsorption and separation or catalytic processes.

Effective pore diameter

A parto of cations is located near to the 6 or 10 membered oxygen rings, that are limiting the diameter of access openings to the big cavities of basic zeolites. Cations with larger ion radius lower the passage through the pores. Introducing cations of higher valency or increase of the Si:Al module in the zeolite menas proportional decrease of cation numbers and augmenting the passage throug the pores. The pores of given zeolite type can be affected by the mentioned facts and that is why their passage is characterized by the effective diameter of pores.

The type of exchanged or prevailing cations has, in the basic zeolite but mainly in type A, a significant effect on the effective pore diameter. Potassum, having ion radius 1,33 . 10^-10 m is the largest among the cations K⁺, Na⁺, Ca⁺², forms the zeolite modification KA or more exact KnaA with smallest effective pore diameter 3.10^-10 m and therefore this zeolite is designated by the symbol 3A as well. Zeolite CaA or CaNaA or 5A represents modification of type A with largest effective pore diameter
5.10^-10 m. Through calcium has the ion radius 1,05.10^-10 m larger than sodium (0,95.10^-10 m) but in this case for the size of effective pore diameter is decisive the half of calcium ion numbers, that is substituting during the exchange the sodium ions. Zeolites of the type 3A, 4A, 5A are representing adsorbents with ideal gradation of individual dimension of access channels to the adsorption cavities, with regard of molecule separation and production of important materials.

Zeolite of the type X and Y have in sodium and calcium modifications effective pore diameter in the range 8 – 12.10^-10 m. They are from the viewpoint of molecular diameter zeolites with wide – pores where the screening effect mostly is unused but their high adsorption capacity and selectivity for polar and polarizable molecules is exploited.

Critical molecular diameter

Critical molecular diameter are from the view point of adsorption characteristic dimensions, that enable to judge the passage of adsorbates through the effective pore diameter of the corresponding type and modification of molecular sieve. For adsorbate molecules having a critical diameter smaller or comparable to the effective pore diameter, the internal adsorption surface of the zeolite is accessible. For zeolites A, X, Y that have access pores of circular cross-section it is sufficient to characterize molecules with one value of critical diameter.

Review of critical molecular diameter of important substances is in table 1.