Zeolites
by Chris Woodford. Last updated: February 7, 2023.
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: A typical zeolite. Photo by Andrew Silver courtesy of Brigham Young University Department of Geology, Provo, Utah and
USGS Photo Library.
What are zeolites?
Zeolites are hydrated aluminosilicate minerals made from interlinked tetrahedra of
alumina (AlO4) and silica (SiO4).
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.
Artwork: 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).
There are about 40 naturally occurring zeolites, forming in both volcanic and sedimentary rocks;
according to the US Geological Survey, the most commonly mined forms include
chabazite,
clinoptilolite, and
mordenite.
Dozens more artificial, synthetic zeolites (around 150) have been designed for specific purposes,
the best known of which are zeolite A (commonly used as a laundry detergent),
zeolites X and Y (two different types of faujasites, used
for catalytic cracking), and the petroleum catalyst ZSM-5 (a branded name for pentasil-zeolite).
Chart: Where do natural zeolites come from? Estimated world mine production for 2020. China remains the leading producer, accounting for almost a third of all natural zeolites (320,000 tons), but nothing like the 2 million tons that
it had previously claimed to produce. Source: US Geological Survey: Mineral Commodity Summaries: Zeolites (Natural), January 2021. According to the USGS, total world reserves of zeolites are unknown "but are estimated to be large."
What special properties do zeolites have?
Zeolites are very stable solids that resist the kinds of environmental conditions that challenge many other
materials. High temperatures don't bother them because they have relatively high melting points (over 1000°C),
and they don't burn. They also resist high pressures, don't disssolve in water or
other inorganic solvents, and don't oxidize in the air. They're not believed to cause health
problems through, for example, skin contact or inhalation, though in fibrous form, they may
have carcinogenic (cancer-causing) effects. Since they're unreactive and based on naturally occurring minerals, they're not believed to have any harmful environmental impacts.
Although zeolites might sound incredibly boring, their stable and unreactive nature isn't what makes them useful.
Animation (above): Ion exchange in zeolites: the zeolite "cage" (gray) traps incoming ions (red and orange) and releases others (yellow) in their place.
The most interesting thing about zeolites is their open, cage-like, "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.
Unlike natural zeolites, which occur in random forms and mixed sizes, synthetic
zeolites are manufactured in very precise and uniform sizes
(typically from about 1μm to 1mm) to suit a particular application; in other words,
they're made a certain size to trap molecules of a certain (smaller) size inside them.
Although all zeolites are aluminosilicates, some contain more alumina, while
others contain more silica. Alumina-rich zeolites are attracted to polar molecules
such as water, while silica-rich zeolites work better with nonpolar molecules.
Photo: Synthetic zeolite crystals grown at CAMMP (Center for Advanced Microgravity Materials Processing),
a NASA-sponsored Research Partnership Center. The ones on the right are about 10 times bigger (25μm) than the ones on the left (2.5μm). Photo courtesy of Dr. Albert Sacco and NASA Marshall Space Flight Center (NASA-MSFC).
What are zeolites used for?
Photo: NASA has been experimenting with using zeolites for storing hydrogen that can be used
in fuel cells. The top photo shows zeolites grown on Earth; the bottom one shows similar crystals grown in microgravity in space. Photo by NASA Marshall Space Flight Center courtesy of
Internet Archive.
The cage-like structure of zeolites makes them useful in all sorts of ways.
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 odor 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. (Following the Fukushima nuclear disaster in Japan in 2011,
rice farmers spread zeolites on their fields in an attempt to trap any lingering radioactive contaminants.)
The many other uses for zeolites including concrete production, soil-conditioners, and animal
food.
Photo: Scientists at the US Department of Agriculture have found that a coating of zeolite
can help to protect the country's valuable, $10 billion-a-year alfalfa crop against soil-borne diseases.
Since the zeolite occurs naturally, this treatment counts as
organic. Photo by Deborah Samac courtesy of USDA-ARS.
What are zeolite catalysts?
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
US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).
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.
