Zeolites are complex and beautiful hydrated inorganic structures that can best be described as molecular sieves since they can selectively separate gases and liquids on the molecular level by virtue of inherent supramolecular cage structures. The principle chemistry of these structures finds practical application in the environmental remediation of toxic waste. Zeolites can also improve the performance of commercial products as water softeners used in water purification systems and laundry detergent. Since these structures efficiently remove volatile organics from waste, zeolites are often used as kitty litter for house pets. The petroleum industry relies on these structures for processes that facilitate the so-called “cracking” of hydrocarbons. Zeolites have also found applications as industrial catalysts for organic reactions inline with current trends toward environmentally friendly alternatives to heavy metals (green chemistry). The most accessible application of zeolites would be found in a fish tank, since zeolites remove ammonia from aquarium water to maintain homeostasis. The following article is an introduction to zeolite structures and practical applications in the modern world.
The word zeolite roughly translates to “boiling stones” since water bound in the crystal structure is released when heated through a reversible process. In structure, these compounds are alumina silicates that pair with a cation (e.g. barium, calcium, potassium, sodium, strontium etc.) in some interesting configurations. Zeolites form supramolecular cages that hold channels or cavities. These cavities can be (but not limited to) hexagon or octagon shaped cages that are capable of sequestering or excluding specific ions or compounds that pass through it. The size of the open cavity within these supramolecular cages defines the application of the specific zeolite. It has been reported that more sophisticated applications of zeolites allow for ion exchange chemistry at the surface within these cavities, with some development work focused on in situ surface modification to diversify the commercial utility of a specific product. Zeolites exist in nature as solid-state crystalline structures. It follows that these crystals can exist as polymorphs. It also follows that point-defects in crystal structure can result in deviations in the predicted performance of these natural materials. The rules of IUPAC rules for nomenclature for zeolites are complex and beyond the scope of this introduction.
Analytical methods to characterize the physical and chemical profiles of zeolites include but are not limited to: electrochemical measurements; fourier transform infrared spectroscopy (FTIR); particle size distribution analyzers (PSD); scanning electron microscopy (SEM); x-ray diffraction (XRD).
Zeolites are found naturally occurring in nature; however, these materials can be synthesized in laboratory to optimize specific physical characteristics for improved product performance. Zeolites are mined in a number of countries including: Bulgaria, United States, India, Japan, China and Italy. Current research in this field is performed at the following universities: Rice University, The University of Massachusetts (Amherst), Northwestern, the University of Bern. The environmental protection agency (EPA) also has reported on the characterization of zeolites and their applications. Zeolites have found industrial interest by such companies as: Clariant International, Englehard, Union Carbide. At present, there are a number of comprehensive textbooks on this subject.