ZEOLITES AND THEIR EFFECTIVENESS

Background

“Zeolites” refers to a group of silicate minerals that share a similar chemical composition, mineral associations and geologic occurrences. Mineralogists describe them as hydrated aluminosilicates. This means that they contain water, aluminum and silicate molecules. The water molecule is trapped in the spaces created by the aluminum and silica molecules. Their hardness’s range between 3 ½ and 5 ½. They are light (that is, have low density) with specific gravities ranging between 2.0 and 2.4. They are typically well crystallized and occur in vugs and veins in igneous rocks such as basalt and gabbros. They form when alkaline groundwater reacts with the elements in the basaltic igneous rock. They also occur in altered volcanic tuff and certain sedimentary deposits.

Chemically, zeolites are similar to clay minerals. In clay minerals, the molecules bond together in a fashion that creates loosely connected plates. The molecules in zeolites, however, are connected in a framework structure that is characterized by spaces or pores between the molecule groups. These “micro pores” are used for a number applications in industry and agriculture.

There are approximately 50 naturally-occurring and 150 synthetic zeolites. The most common natural zeolites are natrolite, analcime, chabazite, heulandite, phillipsite, and stilbite. Natural zeolites have enough impurities to make them less useful for some specialized industrial applications, so synthetic zeolites are designed for those specific applications. Zeolites contain a number of different cations including K+, Na+, Mg2+, and Ca2+, which are the most common. These cations are not strongly bound to the zeolite molecule so they can easily be removed and replaced with other cations.

The Swedish mineralogist Axel Cronstedt created the name zeolite in 1758. He noticed that when he heated a zeolite specimen it would swell up and then crackle and pop (which happens as heat drives the water molecules out of the zeolite’s crystal structure). So he named them zeolites after the Greek verb zein which means to boil and the Greek noun lithos which means stone, literally “boiling stone.” Interestingly, the zeolite’s crystal structure is not destroyed when it is heated and the water is driven out of the crystal. This is very different than other hydrated minerals like gypsum. The water molecule in gypsum is part of the overall crystal structure, so when gypsum is heated and the water is driven off, the crystal structure is actually destroyed and the gypsum turns to powder.

Zeolites

Zeolites are a large group of natural and synthetic hydrated aluminum silicates. They are characterized by complex three-dimensional structures with large, cage like cavities that can accommodate sodium, calcium, or other cations (positively charged atoms or atomic clusters); water molecules; and even small organic molecules. Ions and molecules in the cages can be removed or exchanged without destroying the aluminosilicate framework. Zeolites find wide use as ion-exchange agents, catalysts, and molecular filters in a range of industrial processes. The word "zeolite" comes from the Greek for "boiling stone," because of the early observation that zeolites release water when heated. As their compositions are not fixed, they are examples of nonstoichiometric compounds.

Zeolite is a group of some 175 different minerals. Phillipsite, pollucite, scolecite, sodalite and stilbite are common and popular types of zeolites. They are characterized by their absorbent nature and are often used in the industrial world. Zeolites are used in heating and cooling, detergents, agriculture and in medical equipment to name just a few of it's uses. Both natural and synthetic zeolites are currently used; however, natural zeolites are more common as they are fairly easy to mine and are in abundance around the world.

Zeolites belong to a family of naturally occurring volcanic minerals with unique physical and chemical characteristics. There are over forty-eight varieties of natural zeolite minerals with similar structures and molecular makeup, each with its own particular attributes – some subtle and some more obvious.

Zeolites are most commonly secondary minerals. They are born out of volcanic lava that cools and forms into volcanic rock. The heat of the cooling volcanic rock interacts with the ground water and creates the zeolites. Some forms of zeolites are created in malfic rock, and others are formed in basaltic rock. The type of volcanic rock that the zeolite is found in affects its color, lustre and chemical makeup.

Zeolites are formed fairly close to the surface of the earth and thus can be mined in quarries or open pits. The extra rock is blasted away to reveal the Zeolite ore, which is then extracted and processed. The open pit method of mining is thought to be the safest type of mining there is for both human beings and the earth.

Occurrence

Natural zeolites occur in a number of distinct geologic environments. They occur in the holes (properly called vugs) and cracks frequently found in the iron- and magnesium-rich igneous rocks called basalt and gabbro. Significant zeolite deposits of this type are found in the United States in New Jersey. There are significant similar deposits in Canada, Tasmania and India. The flood basalts in India, for example, cover millions of square miles, much of which contains significant quantities of zeolites.

In recent years, geologists have discovered significant sedimentary sources of zeolites all over the world.

Zeolites also develop in volcanic tuffs that have been altered by salt water and alkaline water. Deposits of this type are found in Arizona, California, Idaho, Nevada, Wyoming, Texas, Utah, Oregon and New Mexico.

In addition to these extensive natural deposits, synthetic zeolites are developed and produced for many different applications.

The Structures of Zeolites

The atomic structures of zeolites are based on three-dimensional frameworks of silica and alumina tetrahedra, that is, silicon or aluminum ions surrounded by four oxygen ions in a tetrahedral configuration. Each oxygen is bonded to two adjacent silicon or aluminum ions, linking them together. Clusters of tetrahedra form boxlike polyhedral units that are further linked to build up the entire framework. In different zeolites the polyhedral units may be equidimensional, sheetlike, or chainlike. The aluminosilicate framework of a zeolite has a negative charge, which is balanced by the cations housed in the cagelike cavities. Zeolites have much more open, less dense structures than other silicates; between 20 and 50 percent of the volume of a zeolite structure is voids. Silicates such as zeolites that have three-dimensional frameworks of tetrahedra are termed tectosilicates. Besides the zeolites, other tectosilicates include quartz and feldspars.

Natural Zeolites

There are about forty-eight natural zeolites. They form in a number of relatively low temperature geologic environments. Gas pockets in basalt and other volcanic rocks may contain dramatic crystal groups of zeolites. Economically more important are the fine-grained zeolites such as clinoptilolite (Na, K)AlSi5O12 ·3H 2 O formed by the alteration of fine-grained volcanic deposits by underground water. These are mined in the western United States and Mexico. Zeolites also form in alkaline desert lake sediments, in alkaline soils in deserts, and in marine sediments. Zeolites occur in low-temperature metamorphic rocks in geologically young regions of mountain building, such as South Island, New Zealand.

Substitutes and Alternative Sources

The unique physical and chemical properties of zeolites make them valuable commodities. Natural occurrences are widespread in a variety of geologic environments. However, natural zeolites are often contaminated with ions and other minerals that can reduce their usefulness for certain applications. Fortunately, the technology

needed to synthesize zeolites is well understood and very effective. It may be possible to synthesize the zeolites needed for all of the varied applications, eliminating the possibility of running out of zeolites. No other material does what zeolites do, so they are not easily replaced with other materials.

In short, there appears to be more than abundant naturally-occurring zeolite deposits worldwide to meet industrial and agricultural demands well into the future. The ability to design and synthesize zeolites for specific needs should also provide a nearly limitless source of this special material.

Synthetic Zeolites

Although some natural zeolites occur in large amounts, they offer only a limited range of atomic structures and properties. Synthetic zeolites have a wider range of properties and larger cavities than their natural counterparts. They were first produced in the 1950s. Today more than 150 different zeolites have been made, and the annual production of synthetic zeolites exceeds 12,000 tons. Zeolites are manufactured in a number of ways; one important technique involves mixing sodium, aluminum, and silica chemicals with steam to create a gel (an amorphous, noncrystalline, water-rich solid). The gel is aged, and then heated to about 90°C (194°F). Another technique uses kaolin clay that has been heated in a furnace until it begins to melt, then chilled and ground to powder. This powder is mixed with sodium salts and water, aged, and heated. In all the synthesis methods, the zeolite produced depends on the compositions of the starting materials and the conditions of reaction, including acidity, temperature, and water pressure.

Synthesis

In another way we can say that zeolites are normally crystallized from aqueous alkaline gels containing sources of silica, alumina and cations. Crystallization can take weeks to accomplish, under pressure and at temperatures up to 200 °C. Small changes in mixture concentration, pressure and temperature can change the properties of the final material. Variation of the inorganic base can result in a range of products, and interesting effects can be achieved from the use of organic templating cations, e.g. Tetraalkylammonium cations, NR4

+. Using large organic templating cations high-alumina zeolites are more favorably formed. The use of templates is quite sophisticated, and zeolites can be tailored to certain specifications.

Zeolite structures can also be modified after synthesis, the simplest being the exchange of extra-framework species. The Si/Al ratio can also be changed by dealumination procedures which involve steaming, acid treatment and ammonium exchange. Other atoms, such as B, Ga, Fe and Ti can also be introduced into the zeolite framework.

Commercially Important Physical/Chemical Properties of Zeolite

All commercially useful zeolites owe their value to one or more of three properties: adsorption, ion exchange, and catalysis.

Adsorption

The most fundamental consideration regarding the adsorption of chemical species by zeolites is molecular sieving. Species with a kinetic diameter which makes them too large to pass through a zeolite pore are effectively "sieved." This "sieve" effect can be utilized to produce sharp separations of molecules by size and shape.

The particular affinity a species has for an internal zeolite cavity depends on electronic considerations. The strong electrostatic field within a zeolite cavity results in very strong interaction with polar molecules such as water. Non-polar molecules are also strongly adsorbed due to the polarizing power of these electric fields. Thus, excellent separations can be achieved by zeolites even when no steric hindrance occurs.

Adsorption based on molecular sieving, electrostatic fields, and polarizability are always reversible in theory and usually reversible in practice. This allows the zeolite to be reused many times, cycling between adsorption and desorption. This accounts for the considerable economic value of zeolite in adsorptive applications.

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.

Ion Exchange

Because cations are free to migrate in and out of zeolite structures, zeolites are often used to exchange their cations for those of surrounding fluids. The preference of a given zeolite among available cations can be due to ion sieving or due to a competition between the zeolite phase and aqueous phase for the cations that are present.

... Sodium zeolite A is among the world's most efficient removers of water hardness ions. This is its principal function as a detergent builder.

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.

Catalysis

Zeolites make extremely active catalysts.... Steric phenomena are very important in zeolite catalysis, and a new term, "shape selective catalysis," was coined to describe these effects. Extremely selective reactions can be made to occur over zeolites [when certain products, reactants or transition states are kept from forming within the pores because of size or shape].

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 synthesis of high-value chemicals e.g. pharmaceuticals and cosmetics.

Uses of Zeolites

The uses of zeolites derive from their special properties: They can interact with water to absorb or release ions (ion exchange); they can selectively absorb ions that fit the cavities in their structures

(molecular sieves); they can hold large molecules and help them break into smaller pieces ( catalytic cracking). Zeolites are used as water softeners, to remove calcium ions, which react with soap to form scum. The water is filtered through a sodium-bearing zeolite, which absorbs the calcium and releases sodium ions into the water. When the zeolite can absorb no more calcium, it may be recharged by flushing it with brine (a saturated sodium chloride solution), which forces out the calcium ions and replaces them with sodium. At the Hanford Nuclear Facility in Richland, Washington, radioactive strontium-90 (Sr 90 ) and cesium-137 (Cs 137 ) have been removed from radioactive waste solutions by passing them through tanks packed with the natural zeolite clinoptilolite. Zeolites have also been used to clean radioactive wastes from the Three Mile Island nuclear power plant site and elsewhere. In addition, clinoptilolite is used to clean ammonium ions (NH 4 

+ ) from sewage and agricultural wastewater.

Sulfur dioxide (SO 2 ) is a pollutant produced by burning high-sulfur coal. It is a major cause of acid rain . Natural zeolites are the most effective filters yet found for absorbing sulfur dioxide from waste gases. As efforts to improve air quality continue, zeolites can be used to help purify the gases from power plants that burn high-sulfur coal from the Ohio River Valley and other regions.

Industrial applications make use of synthetic zeolites of high purity, which have larger cavities than the natural zeolites. These larger cavities enable synthetic zeolites to absorb or hold molecules that the natural zeolites do not. Some zeolites are used as molecular sieves to remove water and ni trogen impurities from natural gas. Because of their ability to interact with organic molecules, zeolites are important in refining and purifying natural gas and petroleum chemicals. The zeolites are not affected by these processes, so they are acting as catalysts. Zeolites are used to help break down large organic molecules found in petroleum into the smaller molecules that make up gasoline, a process called catalytic cracking. Zeolites are also used in hydrogenating vegetable oils and in many other industrial processes involving organic compounds.

Remember that zeolites contain water in the spaces between the aluminum and silica molecules (tetrahedra). This water moves easily in and out of the crystal. Consequently, any molecule similar in size to water or smaller can pass through the spaces in the zeolite. Larger molecules, however, cannot. Zeolites are therefore used as sieves or filters to remove molecules of a particular size. Liquids and gases can pass through these spaces or pores. Also remember that the cations are loosely connected in the zeolite crystal structure. As liquids or gases pass through the pores, ions can be exchanged. This is called cation exchange or base exchange. These two properties are utilized in a wide variety of industrial, agricultural and home applications.

The single most common use of zeolites is in the detergent industry. Almost 1 ½ million metric tons of anhydrous zeolites (that is, zeolites without water molecules in their structure) are used as a component in the manufacture of soaps and detergents. Zeolites are regularly

used in water purification systems. “Hard” water is water that has high calcium content, usually from being in contact with limestone. Calcium deposits build up in hot water heaters, washing machines and pipes, dramatically affecting their efficiency and life expectancy. Soaps do not lather and work as effectively in hard water. Such water is therefore “softened” by passing the hard water through masses of sodium-containing zeolites, such as the mineral natrolite. As the water passes through, the zeolite releases the sodium cation, which is replaced by the calcium cation in the water. Hard water with calcium goes in, but soft water with sodium comes out. When the zeolite no longer has sodium cations in its crystal, the material can be treated with salt water, a process known as recharging. The calcium now comes out and is replaced with sodium from the salt (NaCl) and the zeolite is ready to be used again to soften water.

In the agriculture industry, the zeolite called clinoptolite is used to treat soil. It naturally releases potassium into the soil at a predictable rate. It is also possible to treat clinoptolite with ammonium ions. Over time, this treated zeolite will release nitrogen into the soil, an element that is essential for successful crop development.

In the medical field, zeolites are used as a sieve to take the air you breathe every day (which is 21% oxygen and 79% nitrogen) and filter out nitrogen and other unwanted gases to produce a gas with higher concentrations of oxygen. Frequently patients will need higher concentrations of oxygen as part of their healing regimen.

Carefully designed synthetic zeolites are used to separate specific gases from raw natural gas. For example, they are used to remove unwanted water, sulfur dioxide and carbon dioxide. Other zeolites are used to filter out very specific gases such as nitrogen, which is then bottled and used for applications such as filling tires.

Zeolites are also used in the nuclear and petrochemical industries. They are used in the concrete industry to help control the water content of concrete so that it can dry at a slower rate. Engineers have discovered that when concrete dries more slowly, the final product is stronger than a concrete that dries more quickly.

It is easy to see that the group of minerals called zeolites is unique and useful in a wide variety of situations.

Other Applications:

Refrigeration

The heat of water adsorption for zeolites is high. They also possess high adsorption capacity, undergo reversible adsorption/desorption, and are structurally stable. These properties enable zeolite to be used in solar-powered refrigerators and to store energy during off-peak periods and release it during peak periods. Zeolites can also be used in refrigeration and air cooling systems to reduce water in the air to very low concentrations, allowing very effective evaporative cooling to occur.

Environmentally Driven Market Applications

Radioactive Waste Treatment

Natural zeolites are being used to treat low and intermediate aqueous waste. Current users are British Nuclear Fuels in Great Britain, West Valley Nuclear and Date Ridge National Laboratory. Natural zeolite has been used in the clean-up at Three Mile Island and Chernobyl.

Municipal Waste Water Treatment

Certain natural zeolites have a high affinity for ammonium ions and are being used in a tertiary water treatment system at Truckee, California. Municipal effluent is treated by passing it through columns packed with a natural zeolite clinoptilolite to reduce the ammonium ion concentration to less than 2 ppm.

Pet Litter and Odor Control

Natural zeolites are uniquely effective in adsorbing ammonia and also adsorb hydrogen sulfide. These properties make natural zeolites ideal for use in pet litter to prevent emanation of irritating odors. For similar reasons, natural zeolites can be used for effective control of irritating gases in horse stalls, barns, kennels, etc.

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.