Zeolite & Molecular Sieve Purification

Separation may be defined as a process that transforms a mixture of substances into two or more products that differ from each other in composition.

The surface of a solid represents a discontinuity of its structure. The forces acting at the surface are unsaturated. Hence, when the solid is exposed to the gas, the gas molecules will form bonds with it and become attached. This phenomenon is termed as adsorption. Adsorption, the binding of molecules or particles to a surface, must be distinguished from Absorption, the filling of pores in a solid. The binding to the solid is usually weak and reversible. The adsorption process involves nothing more than the preferential partitioning of the substances from the gaseous or liquid phase onto the surface of a solid substrate.

Adsorption, also known as adsorptive separation, can be simply defined as the concentration of a solute, which may be molecules in a gas stream or a dissolved or suspended substance in a liquid stream, on the surface of a solid.

The adsorptive separation is achieved by one of three mechanisms: steric, kinetic or equilibrium effects. The steric effect derives from the molecular sieving property of zeolites. In this case only small and properly shaped molecules can diffuse into the adsorbent, whereas other molecules are totally excluded. Kinetic separation is achieved by the virtue of the difference in diffusion rates of different molecules. A large majority of processes separate through the equilibrium adsorption of the mixture and hence are called Equilibrium separation processes.

Physical Adsorption vs. Chemisorption
The phenomenon of adsorption is essentially an attraction of adsorbate molecules to an adsorbent surface. The preferential concentration of molecules in the proximity of a surface arises because the surface forces of an adsorbent solid are unsaturated. Both repulsive and attractive forces become balanced when adsorption occurs. Adsorption is nearly always an exothermic process.

We can distinguish between two types of adsorption process depending on which of these two force types plays the bigger role in the process. Adsorption processes can be classified as either physical adsorption (van der Waals adsorption) or chemisorption (activated adsorption) depending on the type of forces between the adsorbate and the adsorbent.

Physical adsorption is caused mainly by van der Waals forces ad electrostatic forces between adsorbate molecules and the atoms, which compose the adsorbent surface.

Thus adsorbents are characterized by surface properties such as surface area and polarity. A large specific surface area is preferable for providing large adsorption capacity, but the creation of a large internal surface area in a limited volume inevitably gives rise to large numbers of small sized pores between adsorption surfaces. The size of micro pores determines the accessibility of adsorbate molecules to the internal adsorption surface, so the pore size distribution of micro pores is another important property of characterizing adsorptivity of adsorbents.

Chemisorption involves electron transfer and is essentially two-dimensional chemical reaction. In this type of adsorption, the chemistry of adsorbate is of central importance. In a particular system both types of adsorption may occur as well as intermediate types. The solids best suited to adsorption are very porous, and have very large effective surface areas.

MOLECULAR SIEVES
The discovery of molecular sieves can be traced back to 1756 when the word zeolite was first used. It came from the Greek meaning "boil" and "rock" after the observation was made that minerals lost their content of water when heated. Molecular sieves are a class of adsorbents, which can trap molecules by adsorption into their pores.

The term "molecular sieve" was originated by J.W. Mcbain to define porous solid materials, which have the property of acting as sieves on a molecular scale. Zeolites are example of naturally occurring molecular sieves.

Molecular sieve adsorbents are crystalline alumino-silicates. Their unique structure allows the water of crystallization to be removed leaving a porous crystalline structure. These pores or "cages" want to readsorb water or other molecules. Aided by strong ionic forces caused by the presence of cations such as sodium, calcium and potassium, the molecular sieve will adsorb a considerable amount of water or other fluids. If the fluid to be adsorbed is a polar compound, it can be adsorbed with high loading even at very low concentrations of the fluid.

The strong adsorptive force allows molecular sieves to remove many gas or liquid impurities at very low levels. Another feature of molecular sieve adsorbents is its ability to separate gases or liquids by molecular size. The pore or "cage" openings are of the same size as many molecules. In the case of hydrocarbon paraffins, the normal, straight-chained molecules can fit into the pores and be adsorbed while the branched chain molecules cannot enter the pores and pass by the molecular sieve adsorbents.

Molecular Sieve Zeolites
Molecular-sieve zeolites are crystalline aluminosilicates of group IA and Group IIA elements such as sodium, magnesium, potassium and calcium. Chemically, they are represented by the empirical formula:
M2/nO * Al2O3 * YSiO2 * wH2O

Where Y is 2 or greater, n is the cation valence and w represents the water contained in the voids of the zeolite. Structurally, zeolites are complex, crystalline inorganic polymers based on an infinitely extending framework of AlO4 and SiO4 tetrahedra linked to each other by sharing of oxygen ions. The fundamental unit is a tetrahedral complex consisting of a small cation, such as Si4+, in tetrahedral coordination with four oxygens (Pauling's first rule). The Al3+ ion commonly coordinates tetrahedrally as well as Octahedrally with oxygen in silicates.

This framework structure contains channels or interconnected voids that are occupied by the cations or water molecules. The cations are mobile and ordinarily undergo ion exchange. The water may be removed reversibly, generally by the application of heat, which leaves intact a crystalline host structure permeated by micropores, which may amount to 50% of the crystals, by volume.

The structural formula of zeolite is based on the crystal unit cell, the smallest unit of structure represented by:
Mx/n [(AlO2)x (SiO2)y] * wH2O

Where n is the valence of the cation M, w is the number of water molecules per unit cell, x and y are the total number of tetrahedral per unit cell, and y/x usually has values of 1-5. The cations are necessary to balance the electrical charge of the aluminum atoms, each having a net charge of -1. The water molecules can easily be removed with ease upon heat and evacuation, leaving an almost unaltered aluminosilicate skeleton.

The molecular-sieve zeolites are distinct from the other three major adsorbents in that they are crystalline and that adsorption takes place inside the crystals, the access to which is limited by the pore size. Zeolite molecular sieves have pores of uniform size (3 to 10A).

Zeolites selectively adsorb or reject molecules based on differences in molecular size, shape and other properties such as polarity. During the adsorption of various molecules, the micorpores fill and empty reversibly. Adsorption in zeolites is a matter of pore filling, and the usual surface-area concepts are not applicable.

The channels of zeolites are only a few molecular diameters in size, and overlapping potential fields form opposite walls result in a flat adsorption isotherm, which is characterized by a long horizontal section as the relative partial pressure approaches unity. The adsorption isotherms do not exhibit hysteresis.

Zeolites adsorb molecules, in particular those with permanent dipole moments, which show other interaction effects, with selectivity that, is not found in other solid adsorbents.

Separation may be based on molecular-sieve effect or may involve the preferential or selective adsorption of one molecule species over another. These separations are governed by several factors:

• The basic framework structure, or topology, of the zeolite determines the pore size and void volume.
• The exchange cations, in terms of their specific location in the structure, their population or density, their charge, and size affect the molecular-sieve behavior and adsorption selectivity of the zeolite. By changing the cation types and number, one can tailor or modify with in certain limits the selectivity of the zeolite in a given separation.
• The cations, depending on their locations, contribute electric field effects that interact with the adsorbate molecules.
• The effect of the temperature of the adsorbent is pronounced in cases involving activated diffusion.

The framework contains channels and interconnected voids, which are occupied by cation and water molecules. The cations are quite mobile and may be exchanged to varying degree by other cations. The structure can be changed depending on the cation exchange.

In crystal form, zeolites are distinct from other adsorbents in that, for each type, there is no distribution of pore size. The lattice into which the adsorbate molecules can or cannot enter is precisely uniform.

For this reason zeolites are capable of separating effectively on the basis of size and they are often known as molecular sieves.

In addition to changes to cationic structure, the Si/Al ration can be varied, thus zeolites with widely different adsorptive properties may be tailored by the appropriate choice of framework structure, cationic form, and silica -to-alumina ration in order to achieve the selectivity required for a given separation.

The ionic nature of most zeolites means that they have a high affinity for water and other polar molecules such as carbon dioxide and hydrogen sulfide. However, as the silica-to-alumina ration is increased the material can become hydrophobic. Such zeolites can be used in the removal of volatile organic compounds from air.

More than 150 synthetic zeolite types are known, the most important commercially being the synthetic types A and X, synthetic mordenite and their ion-exchange varieties. Of the 40 or so mineral zeolites the most important commercially are chabazite, faujasite and mordenite.

Applications of Zeolites

• Adsorption: Can adsorb a variety of materials in such processes as drying, purification and separation.
• Catalysis: Zeolites can shape selective catalysts on the basis of molecular diameter and are used in petroleum refining and synthetic fuels production.
• Ion Exchange:
• The largest use where they have replaced phosphates as water softening agents
• The cleaning of radioactive contaminated water with the removal of Caesium and Strontium
• Removal of ammonia and ammonium ions from waste water

Apart from the applications described above, they are also used to remove hydrogen sulfide and other sulfur compounds from natural gas and LPG liquids to meet extracting specifications. The removal of carbon dioxide from air and natural gas before cryogenic processing is routine. Molecular sieves are the premium desiccants for insulated windows. Special varieties are used for refrigerant drying.

Selection of adsorbent
The selection of proper sorbent for a given separation is a complex problem. The predominant scientific basis for sorbent selection is the equilibrium isotherm. The equilibrium isotherms of all constituents in the gas mixture, in the temperature and pressure range of operation, must be considered. As a first and possibly over simplified approximation, the pure gas isotherms may be considered additive to yield the adsorption from a mixture. Based on the isotherms, the following factors that are important to the design of the separation can be estimated:

• Capacity of sorbent, in operating temperature and pressure range
• The method of sorbent regeneration
• The length of unused bed
• The product purities