From detergent production to oil refining, zeolite plays a fundamental role in many industries. Its ion exchange and adsorption capabilities make it ideal for chemical separation processes and gas treatment. In this article, we analyze how companies are leveraging the unique properties of zeolite to enhance their products and processes.
The most relevant properties of natural zeolites are porosity, adsorption, and ion exchange.
a) Porosity
Zeolites are formed by regular and uniform channels and cavities of molecular dimensions (3 to 13 nm), which are similar to the kinetic diameters of a large number of molecules. This type of microporous structure means that zeolites have an extremely large internal surface area relative to their external surface area. The IUPAC (The International Union of Pure and Applied Chemistry) recognizes three types of pores based on their size (Sing et al. 1985). If they are larger than 50 nm, they are known as macropores; if their diameter is between 2 and 50 nm, they are mesopores; and if they are smaller than 2 nm, as is the case with zeolite pores, they are micropores.
When the distance between two surfaces is sufficiently short, the adsorption potentials add up, so that a molecule located inside the pore is attracted by the entire surface of the pore, increasing the force with which it is attracted. In other words, as the pore size decreases, the potential well becomes deeper. If the pore is sufficiently wide, the molecules will adsorb forming a monolayer at a certain distance from the surface (adsorption distance), and as the amount adsorbed increases, the adsorbate organizes into successive layers (multilayer filling) (Gregg and Sing, 1967).
b) Adsorption
The surface of solids is a unique region that is responsible for or at least conditions many of their properties. The atoms on the surface do not have their cohesive forces balanced, as is the case for atoms within the solid, which is ultimately responsible for the adsorption properties of solids. At sufficiently large distances, there is no appreciable interaction between a molecule approaching a surface, so the energy of this system is close to zero. As the molecule approaches the surface, the energy of the system begins to decrease because the cohesive forces of the surface atoms start to be balanced. In other words, the adsorption potential generates an attractive force that causes the molecule to approach the surface. When the distance between the surface and the free molecule begins to decrease, the repulsive forces (due to the proximity of the electron layers of the surface atoms with the atoms of the free molecule) become significant. Therefore, there is a distance at which the energy of the system is minimized. The high adsorption efficiency of zeolites is related to their large internal surface area. When the pore size decreases, there is a significant increase in the adsorption potential, caused by the overlapping of the potentials of the pore walls. Thus, for the same adsorbate, the interaction with the pore walls is greater the smaller the pore size, and therefore, the better the confinement of the adsorbed molecule (Garcia, M.J, 2002).
c) Ion Exchange (I.E.)
The Ion Exchange (I.E.) property has been observed in crystalline silicate minerals such as clays, feldspars, and zeolites. It is considered an intrinsic property of these minerals because it is the product of the isomorphic substitution of silicon atoms in their crystalline structure by other atoms. In the case of zeolites, this substitution occurs by tetravalent aluminum atoms, which produces a net negative charge in the structure that is compensated by cations outside it. These cations are exchangeable, hence the intrinsic property of I.E., which is also a manifestation of their microporous crystalline structure, as the dimensions of their cavities and the cations being exchanged determine the course of the process.
The I.E. behavior in zeolites depends on several factors that determine greater selectivity of zeolites to certain cations: -nature of the cations in solution, temperature, concentration of the cations in solution, anions associated with the cations in solution, solvent – water, organic solvent, zeolite structure – network topology, and network charge density.
The ion exchange capacity (I.E.C.) of a zeolite is a magnitude that measures the amount of equivalents of a cation that a mass of zeolite can retain by ion exchange. This capacity is directly related to the Al present in the zeolitic network and depends directly on its chemical composition (Breck, 1974). A high ion exchange capacity corresponds to zeolites with a low SiO2/Al2O3 ratio (Clarke, 1980). The theoretical maximum I.E.C., the number of exchangeable equivalents per mass of the unit cell, cannot always be achieved due to the existence of inaccessible exchange sites.
