Zeolites

Last updated: November 29, 2009.

Heat a glass of water and you'll see steam rise off it sooner or later as it comes to the boil. You certainly don't expect the same thing to happen if you heat a rock—unless it's a special kind of rock called a zeolite, which traps water inside it. Back In 1756, Swedish geologist Axel Cronstedt (1722–1765)—best known as the discoverer of nickel—coined the name "zeolite" because it literally means "boiling stone"; today, the term refers to over 200 different minerals that have all kinds of interesting uses, from water softeners and cat litter to animal food and industrial catalysts. What are zeolites and how do they work? Let's take a closer look!

Photo: The power of nothingness: in zeolite crystals, like the one in this illustration, the pores in between the aluminum, silicon, and oxygen atoms are as important as the crystal structure itself. By courtesy of NASA Marshall Space Flight Center (NASA-MSFC).

What are zeolites?

Zeolites are hydrated aluminosilicate minerals. In simpler words, they're solids with a relatively open, three-dimensional crystal structure built from the elements aluminum, oxygen, and silicon, with alkali or alkaline-Earth metals (such as sodium, potassium, and magnesium) plus water molecules trapped in the gaps between them. Zeolites form with many different crystalline structures, which have large open pores (sometimes referred to as cavities) in a very regular arrangement and roughly the same size as small molecules. There are about 40 naturally occurring zeolites, forming in both volcanic and sedimentary rocks; many more artificial zeolites are specially made for specific purposes.

Photo: Zeolite crystals grown at CAMMP (Center for Advanced Microgravity Materials Processing), a NASA-sponsored Research Partnership Center. Photo courtesy of Dr. Albert Sacco and NASA Marshall Space Flight Center (NASA-MSFC).

What special properties do zeolites have?

The most interesting thing about zeolites is their open framework structure and the way it can trap other molecules inside it. This is how water molecules and alkali or alkaline-Earth metal ions (positively charged atoms with too few electrons, sometimes called cations) become a part of zeolite crystals—although they don't necessarily remain there permanently. Zeolites can exchange other positively charged ions for the metal ions originally trapped inside them (technically this is known as cation exchange) and, as Cronstedt found over 250 years ago, they can gain or lose their water molecules very easily too (this is called reversible dehydration). Zeolites have regular openings in them of fixed size, which let small molecules pass straight through but trap larger ones; that's why they're sometimes referred to as molecular sieves. These properties make zeolites useful in all sorts of ways.

What are zeolites used for?

One of the biggest everyday uses for zeolites is in water softeners and water filters. In ion-exchange water softeners, for example, hard water (rich in calcium and magnesium ions) is piped through a column filled with sodium-containing zeolites. The zeolites trap the calcium and magnesium ions and release sodium ions in their place, so the water becomes softer but richer in sodium. Many everyday laundry and dishwasher detergents contain zeolites to remove calcium and magnesium and soften water so they work more effectively.

Two other very common, everyday uses of zeolites are in odor control and pet litter; in both, the porous crystalline structure of the zeolites helps by trapping unwanted liquids and odour molecules. This simple idea, so effective in our homes, has much more important uses outside them: zeolites have proved extremely effective at removing radioactive particles from nuclear waste and cleaning up soils contaminated with toxic heavy metals. The many other uses for zeolites including concrete production, soil-conditioners, and animal food.

What are zeolite catalysts?

Another important use for zeolites is as catalysts in drug (pharmaceutical) production and in the petrochemical industry, where they're used in catalytic crackers to break large hydrocarbon molecules into gasoline, diesel, kerosene, waxes and all kinds of other byproducts of petroleum. Again, it's the porous structure of zeolites that proves important. The many pores in a zeolite's open structure are like millions of tiny test tubes where atoms and molecules become trapped and chemical reactions readily take place. Since the pores in a particular zeolite are of a fixed size and shape, zeolite catalysts can work selectively on certain molecules, which is why they're sometimes referred to as shape-selective catalysts (they can select the molecules they work on in other ways beside shape and size, however). Like all catalysts, zeolites are reusable over and over again.

Photo: Zeolite catalysts are used in catalytic crackers like this one, which turn crude oil (petroleum) into dozens of useful everyday products and chemicals. By courtesy of UD DOE/NREL (Department of Energy/National Renewable Energy Laboratory).