What are Zeolites ?
Zeolites are microporous crystalline solids with well-defined structures. Generally they contain silicon, aluminium and oxygen in their framework and cations, water and/or other molecules wthin their pores. Many occur naturally as minerals, and are extensively mined in many parts of the world. Others are synthetic, and are made commercially for specific uses, or produced by research scientists trying to understand more about their chemistry.
Because of their unique porous properties, zeolites are used in a variety of applications with a global market of several milliion tonnes per annum. In the western world, major uses are in petrochemical cracking, ion-exchange (water softening and purification), and in the separation and removal of gases and solvents. Other applications are in agriculture, animal husbandry and construction. They are often also referred to as molecular sieves.
Framework Structure
A defining feature of zeolites is that their frameworks are made up of 4-connected networks of atoms. One way of thinking about this is in terms of tetrahedra, with a silicon atom in the middle and oxygen atoms at the corners. These tetrahedra can then link together by their corners (see illustration) to from a rich variety of beautiful structures. The framework structure may contain linked cages, cavities or channels, which are of the right size to allow small molecules to enter - i.e. the limiting pore sizes are roughly between 3 and 10 Å in diameter.
In all, over 130 different framework structures are now known. In addition to having silicon or aluminium as the tetrahedral atom, other compositions have also been synthesised, including the growing category of microporous aluminophosphates, known as ALPOs.
Catalysis
Zeolites have the ability to act as catalysts for chemical reactions which take place within the internal cavities. An important class of reactions is that catalysed by hydrogen-exchanged zeolites, whose framework-bound protons give rise to very high acidity. This is exploited in many organic reactions, including crude oil cracking, isomerisation and fuel synthesis. Zeolites can also serve as oxidation or reduction catalysts, often after metals have been introduced into the framework. Examples are the use of titanium ZSM-5 in the production of caprolactam, and copper zeolites in NOx decomposition.
Underpinning all these types of reaction is the unique microporous nature of zeolites, where the shape and size of a particular pore system exerts a steric influence on the reaction, controlling the access of reactants and products. Thus zeolites are often said to act as shape-selective catalysts. Increasingly, attention has focused on fine-tuning the properties of zeolite catalysts in order to carry out very specific syntheses of high-value chemicals e.g. pharmaceuticals and cosmetics.
The shape of para-xylene means that it can diffuse freely in the channels of silicalite
Adsorption and Separation
The shape-selective properties of zeolites are also the basis for their use in molecular adsorption. The ability preferentially to adsorb certain molecules, while excluding others, has opened up a wide range of molecular sieving applications. Sometimes it is simply a matter of the size and shape of pores controlling access into the zeolite. In other cases different types of molecule enter the zeolite, but some diffuse through the channels more quickly, leaving others stuck behind, as in the purification of para-xylene by silicalite.
Cation-containing zeolites are extensively used as desiccants due to their high affinity for water, and also find application in gas separation, where molecules are differentiated on the basis of their electrostatic interactions with the metal ions. Conversely, hydrophobic silica zeolites preferentially absorb organic solvents. Zeolites can thus separate molecules based on differences of size, shape and polarity.
Sodium Zeolite A, used as a water softener in detergent powder
Ion Exchange
The loosely-bound nature of extra-framework metal ions (such as in zeolite NaA, right) means that they are often readily exchanged for other types of metal when in aqueous solution. This is exploited in a major way in water softening, where alkali metals such as sodium or potassium prefer to exchange out of the zeolite, being replaced by the "hard" calcium and magnesium ions from the water. Many commercial washing powders thus contain substantial amounts of zeolite. Commercial waste water containing heavy metals, and nuclear effluents containing radioactive isotopes can also be cleaned up using such zeolites.
Zeolites and the Environment
Zeolites contribute to a cleaner, safer environment in a great number of ways. In fact nearly every application of zeolites has been driven by environmental concerns, or plays a significant role in reducing toxic waste and energy consumption.
In powder detergents, zeolites replaced harmful phosphate builders, now banned in many parts of the world because of water pollution risks. Catalysts, by definition, make a chemical process more efficient, thus saving energy and indirectly reducing pollution. Moreover, processes can be carried out in fewer steps, miminising unecessary waste and by-products. As solid acids, zeolites reduce the need for corrosive liquid acids, and as redox catalysts and sorbents, they can remove atmospheric pollutants, such as engine exahust gases and ozone-depleting CFCs. Zeolites can also be used to separate harmful organics from water, and in removing heavy metal ions, including those produced by nuclear fission, from water.
The Zeolite Group of Minerals
The zeolites are a popular group of minerals for collectors and an important group of minerals for industrial and other purposes. They combine rarity, beauty, complexity and unique crystal habits. Typically forming in the cavities, or vesicles, of volcanic rocks, zeolites are the result of very low grade metamorphism. Some form from just subtle amounts of heat and pressure and can just barely be called metamorphic while others are found in obviously metamorphic regimes. Zeolite crystals have been grown on board the space shuttle and are undergoing extensive research into their formation and unique properties.
The zeolites are framework silicates consisting of interlocking tetrahedrons of SiO4 and AlO4. In order to be a zeolite the ratio (Si +Al)/O must equal 1/2. The alumino-silicate structure is negatively charged and attracts the positive cations that reside within. Unlike most other tectosilicates, zeolites have large vacant spaces or cages in their structures that allow space for large cations such as sodium, potassium, barium and calcium and even relatively large molecules and cation groups such as water, ammonia, carbonate ions and nitrate ions. In the more useful zeolites, the spaces 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. Zeolites are characterized by their ability to lose and absorb water without damage to their crystal structures. The large channels explain the consistent low specific gravity of these minerals.
Zeolites have many useful purposes. They can perform ion exchange, filtering, odor removal, chemical sieve and gas absorption tasks. The most well known use for zeolites is in water softeners. Calcium in water can cause it to be "hard" and capable of forming scum and other problems. Zeolites charged with the much less damaging sodium ions can allow the hard water to pass through its structure and exchange the calcium for the sodium ions. This process is reversable. In a similar way zeolites can absorb ions and molecules and thus act as a filter for odor control, toxin removal and as a chemical sieve. Zeolites can have the water in their structures driven off by heat with the basic structure left intact. Then other solutions can be pushed through the structure. The zeolites can then act as a delivery system for the new fluid. This process has applications in medicine, livestock feeds and other types of research. Zeolites added to livestock feed have been shown to absorb toxins that are damaging and even fatal to the growth of the animals, while the basic structure of the zeolite is biologically neutral. Aquarium hobbyists are seeing more zeolite products in pet stores as zeolites make excellent removers of ammonia and other toxins. Most municipal water supplies are processed through zeolites before public consumption. These uses of zeolites are extremely important for industry, although synthetic zeolites are now doing the bulk of the work.
Zeolites have basically three different structural variations.
There are chain-like structures whose minerals form acicular or needle-like prismatic crystals, ie natrolite.
Sheet-like structures where the crystals are flattened platy or tabular with usually good basal cleavages, ie heulandite.
And framework structures where the crystals are more equant in dimensions, ie Chabazite.
A zeolite can be thought of in terms of a house, where the structure of the house (the doors, windows, walls and roof) is really the zeolite while the furniture and people are the water, ammonia and other molecules and ions that can pass in and out of the structure. The chain-like structures can be thought of like towers or high wire pylons. The sheet-like structures can
be thought of like large office buildings with the sheets analogous to the floors and very few walls between the floors. And the framework structures like houses with equally solid walls and floors. All these structures are still frameworks (like the true tectosilicates that zeolites are).
These variations make the zeolite group very diverse, crystal habit-wise. Otherwise zeolites are typically soft to moderately hard, light in density, transparent to translucent and have similar origins. There are about 45 natural minerals that are recognized members of the Zeolite Group. Industrially speaking, the term zeolite includes natural silicate zeolites, synthetic materials and phosphate minerals that have a zeolite like structure. The complexity of this combined group is extensive with over 120 structural variations and more are being discovered or made every year. Collecting zeolites can be very enjoyable and fulfilling.
These are the members of the Zeolite Group:
The Analcime Family:
Analcime (Hydrated Sodium Aluminum Silicate)
Pollucite (Hydrated Cesium Sodium Aluminum Silicate)
Wairakite (Hydrated Calcium Sodium Aluminum Silicate)
Bellbergite (Hydrated Potassium Barium Strontium Sodium Aluminum Silicate)
Bikitaite (Hydrated Lithium Aluminum Silicate)
Boggsite (Hydrated calcium Sodium Aluminum Silicate)
Brewsterite (Hydrated Strontium Barium Sodium Calcium Aluminum Silicate)
The Chabazite Family:
Chabazite (Hydrated Calcium Aluminum Silicate)
Willhendersonite (Hydrated Potassium Calcium Aluminum Silicate)
Cowlesite (Hydrated Calcium Aluminum Silicate)
Dachiardite (Hydrated calcium Sodium Potassium Aluminum Silicate)
Edingtonite (Hydrated Barium Calcium Aluminum Silicate)
Epistilbite (Hydrated Calcium Aluminum Silicate)
Erionite (Hydrated Sodium Potassium Calcium Aluminum Silicate)
Faujasite (Hydrated Sodium Calcium Magnesium Aluminum Silicate)
Ferrierite (Hydrated Sodium Potassium Magnesium Calcium Aluminum Silicate)
The Gismondine Family:
Amicite (Hydrated Potassium Sodium Aluminum Silicate)
Garronite (Hydrated Calcium Aluminum Silicate)
Gismondine (Hydrated Barium Calcium Aluminum Silicate)
Gobbinsite (Hydrated Sodium Potassium Calcium Aluminum Silicate)
Gmelinite (Hydrated Sodium Calcium Aluminum Silicate)
Gonnardite (Hydrated Sodium Calcium Aluminum Silicate)
Goosecreekite (Hydrated Calcium Aluminum Silicate)
The Harmotome Family:
Harmotome (Hydrated Barium Potassium Aluminum Silicate)
Phillipsite (Hydrated Potassium Sodium Calcium Aluminum Silicate)
Wellsite (Hydrated Barium Calcium Potassium Aluminum Silicate)
The Heulandite Family:
Clinoptilolite (Hydrated Sodium Potassium Calcium Aluminum Silicate)
Heulandite (Hydrated Sodium Calcium Aluminum Silicate)
Laumontite (Hydrated Calcium Aluminum Silicate)
Levyne (Hydrated Calcium Sodium Potassium Aluminum Silicate)
Mazzite (Hydrated Potassium Sodium Magnesium Calcium Aluminum Silicate)
Merlinoite (Hydrated Potassium Sodium Calcium Barium Aluminum Silicate)
Montesommaite (Hydrated Potassium Sodium Aluminum Silicate)
Mordenite (Hydrated Sodium Potassium Calcium Aluminum Silicate)
The Natrolite Family:
Mesolite (Hydrated Sodium Calcium Aluminum Silicate)
Natrolite (Hydrated Sodium Aluminum Silicate)
Scolecite (Hydrated Calcium Aluminum Silicate)
Offretite (Hydrated Calcium Potassium Magnesium Aluminum Silicate)
Paranatrolite (Hydrated Sodium Aluminum Silicate)
Paulingite (Hydrated Potassium Calcium Sodium Barium Aluminum Silicate)
Perlialite (Hydrated Potassium Sodium Calcium Strontium Aluminum Silicate)
The Stilbite Family:
Barrerite (Hydrated Sodium Potassium Calcium Aluminum Silicate)
Stilbite (Hydrated Sodium Calcium Aluminum Silicate)
Stellerite (Hydrated Calcium Aluminum Silicate)
Thomsonite (Hydrated Sodium Calcium Aluminum Silicate)
Tschernichite (Hydrated Calcium Aluminum Silicate)
Yugawaralite (Hydrated Calcium Aluminum Silicate)
Zeolites have many "cousins" or minerals that have similar cage-like framework structures or have similar properties and/or are associated with zeolites; but are not zeolites, at least as defined mineralogically. These include the phosphates: kehoeite, pahasapaite and tiptopite; and the silicates: hsianghualite, lovdarite, viseite, partheite, prehnite, roggianite, apophyllite, gyrolite, maricopaite, okenite, tacharanite and tobermorite. It is interesting to compare these minerals to the zeolites.
About Zeolite
Millions of years ago, zeolite deposits formed when volcanoes erupted enormous amounts of ash–aluminosilicates of alkaline and alkaline earths. Some of the wind borne ash settled to form thick ash beds. In some cases the ash fell into lakes and in others, water percolated through the ash beds. In all cases, the chemical reaction of volcanic ash and salt water resulted in the formation of natural zeolites.
The characteristics of a zeolite deposit are decided in it’s genesis. Small natural differences such as temperature, geographic location and ash/water properties impart a slightly different composition and therefore some unique properties to a few of the deposits. These small differences present during the formation of a zeolite deposit are the reason that each natural zeolite property has distinctly unique properties.
The alumina and silica from the ash stack into a stable, open and three dimensional honey-comb structure–there are over forty other natural zeolite structures. For example, clinoptilolite has a silica to alumina ratio of 5 to 1.
Zeolite is an amazing crystalline mineral capable of adsorbing and absorbing many different types of gases, moisture, petrochemicals, heavy metals, low-level radioactive elements and a multitude of various solutions. The channels in the zeolite provide large surface areas on which chemical reactions can take place. The cavities and channels within the crystal could occupy up to 50% of its volume. Zeolites can adsorb or absorb large amounts of materials, such as ions or gas molecules.
Zeolites are hydrated aluminosilicates characterized by high surface areas and high cation exchange capacities. Zeolite characteristics important to a wide range of applications:
Water Treatment
* Water filtration
* Heavy metal removal
* Swimming pools
Wastewater Treatment
* Ammonia in municipal sludge
* Heavy metal removal
* Septic leach fields
Agriculture
* Odor control
* Confined animal environmental control
* Livestock feed additives
Horticulture
* Nurseries, Greenhouses
* Floriculture
* Vegetables/herbs
* Foliage
* Tree and shrub transplanting
* Turf grass soil amendment
* Reclamation, revegetation, landscaping
* Silviculture (forestry, tree plantations)
* Medium for hydroponic growing
Household Products
* Household odor control
* Pet odor control
Industrial Products
* Absorbents for oil and spills
* Gas separations
Radioactive Waste
* Site remediation/decontamination
Aquaculture
* Ammonia filtration in fish hatcheries
* Biofilter media
Feed Additive :
Clinoptilolite application on the daily diet of the animals, reduction of the digestion system illnesses.Clinoptilolite addition to the feed formulation has a positive effect on the growing up of the animals.
Fertilizer Additive / Soil Conditioner :
Clinoptilolites are slow release fertilisers. Plant nutrients such as nitrogen and potassium are held by the negatively charged clinoptilolite structure, and released on demand.
Waste Water Treatmeant Media:
Zeolites contribute to a cleaner, safer environment in a great number of ways. In fact nearly every application of zeolites has been driven by environmental concerns, or plays a significant role in reducing toxic waste.
Cat Litter :
Zeolite Mineral that is a molecular sponge with high porosity and lock-up capability
to absorb free ammonia other cat odours
Pool Filtration Media :
Zeolite offers superior performance to sand and carbon filters, giving purer water and higher throughput rates with less maintenance required. Zeolite’s highly porous structure captures particulate contamination down to 4 microns in size.
Aquacultural Uses :
Zeolites can reduce ammonium and hydrogen sulphide levels in fish/prawn ponds, resulting in increased fish/prawn growth rates and population densities.
Aquarium Filtration Media :
Zeolite is an efficient ammonia remover and also provides a large surface area for nitrifying bacteria in recirculating systems. Zeolite does not only provide optical clearness of your water, but it also keeps it biologically clean.
Radioactive Waste Treatment :
Natural zeolite has a high ion exchange capacity and a particular affinity for heavy metal cations. It can absorb elements such as strontium 90, caesium 137 and other radioactive isotopes from solution, and hold them in its 3 dimensional crystal framework.
Fillers :
Zeolites are used as fillers in the manufacture of paper. These filler grades of zeolite have a large potential for utilization in the paint and plastics industries.
Gas Purification and Seperation:
Zeolites act as molecular sieves, they are used to purify or sweeten natural gases through the removal of impurities such as carbon dioxide, sulfur dioxide and water. In addition to upgrading natural gas, zeolites are used to separate oxygen and nitrogen in pressure swing adsorption columns.
Lightweight Construction Materials:
Natural zeolite can be used to prepare lightweight concrete for construction. It’s porous silicate structure makes it much lighter than sand which gives increased volume per tonne but with similar hardness and strength. Zeolite is free from clay which reduces the overall strength of the concrete, and it’s porous structure holds moisture which facilitates a more rapid curing of the concrete.
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