 |
| Animation (above): Ion exchange in zeolites: the zeolite "cage" (gray) traps incoming ions (red and orange) and releases others (yellow) in their place. |
 |
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.
|
Zeolites
hydrated aluminosilicate minerals
by Chris Woodford.
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!
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.
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).
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.
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.
What are zeolites used for?
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.
It's easy to find
websites claiming all sorts of health benefits for zeolite food supplements,
even advocating them as a cure for cancer.
However, you need
to be extremely careful about believing medical information you find online:
when it comes to your health, make sure you refer to websites you know you can
trust — and preferably ones that aren't trying to sell you things.
A search of Pubmed
(the worldwide database of peer-reviewed biomedical papers run by the US
National Library of Medicine) suggests there is currently little scientific evidence
to support the idea that zeolites have positive benefits in human food
supplements, though there are some papers that suggest possible benefits in dairy cows,
poultry, goats, and mice.
In a one-page summary last updated in June 2017, the Memorial Sloan Kettering Cancer Center confirms
this: "Zeolites are also marketed as dietary supplements to treat
cancer, diarrhea, autism, herpes, and hangover, and to balance pH and remove
heavy metals in the body... [There are] Benefits of using zeolites in animal
feed... However, none of these benefits are applicable to humans... Currently,
no studies of zeolite as a cancer treatment in humans have been published."
It's also worth
noting that the US Food and Drug Administration has sent a number of warning
letters to websites making unsubstantiated therapeutic claims about zeolite
products (see references below for examples).
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.
Hello!
I'm Chris Woodford, a British science writer, and I write all the
articles on Explain that Stuff.
Books
I've
written lots of books on science and technology, including the hugely popular
how-it-works titles Cool Stuff and How it Works (How Cool Stuff Works) (with
Ben Morgan et al), Cool Stuff 2.0 (The Gadget Book) (with Jon Woodcock), and
Cool Stuff Exploded (published by Dorling Kindersley/DK), which have sold
nearly four million copies worldwide. A couple of years ago, I wrote Science: A
Visual/Children's Encyclopedia with Steve Parker, which has been garnering lots
of five star reviews on Amazon and is now in its third major edition. My latest
book, Atoms Under the Floorboards: The Surprising Science Hidden in Your Home,
is published by Bloomsbury and is available worldwide.
I also
work as a consultant and science adviser on other people's books, including
children's science titles by Robert Winston, Johnny Ball, and Richard Hammond.
Most recently, I've been helping David Macaulay with a revised edition of his
classic mammoth book, The Way Things Work Now, which was Amazon.com's number
one children's non-fiction book for 2016.
Awards
Books
I've written, edited, consulted on, or otherwise contributed to have been
translated into about 20 different languages and won or been shortlisted for
over 40 awards, including the Royal Society Young People's Book Prize (in the
UK) and the National Science Teachers Association Outstanding Science Trade
Book award (in the United States). Atoms Under the Floorboards won The American
Institute of Physics Science Writing Award for Books 2016 and was named one of
Physics World's Top 10 Physics Books of 2015.
You can
find a full list of my books, with review comments and details of the awards or
other recognitions they've received on my personal website, chriswoodford.com.
Qualifications
and experience
In case
you're interested, I have an MA in Natural Sciences from Cambridge University.
I specialized in physics (which I was very lucky to have been able to study at
the Cavendish Laboratory) and experimental psychology, but also covered chemistry,
crystallography, materials science, and math. (That's largely why the articles
on Explain that Stuff lean so heavily toward the physical sciences rather than
life-science topics, which I know less about.)
I've
also spent a lot of my time working with computers and the Internet (I worked
at IBM for about five years) and built my very first website in 1994, using an
IBM System/390 mainframe and Mosaic. I had my first magazine article published
back in 1981 (I was very young!) and I've been writing about science and
technology pretty much ever since.
 |
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).
|
 |
| 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. |
 |
| Zeolite catalysts are used in catalytic crackers like this one, which turn crude oil (petroleum) into dozens of useful everyday products and chemicals. |
No comments:
Post a Comment