United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park, NC 27711
EPA 456/F-98-004
July 1998
Air
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TECHNICAL BULLETIN
ZEOLITE
A VERSATILE
AIR POLLUTANT ADSORBER
C LEAN
A IR
T ECHNOLOGY
C ENTER
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EPA-456/F-98-004
July 1998
ZEOLITE
A VERSATILE
AIR POLLUTANT ADSORBER
Prepared by
Clean Air Technology Center (MD-12)
Information Transfer and Program Integration Division
Office of Air Quality Planning and Standards
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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DISCLAIMER
This report has been reviewed by the Information Transfer and Program
Integration Division of the Office of Air Quality Planning and Standards, U.S.
Environmental Protection Agency and approved for publication. Approval does not
signify that the contents of this report reflect the views and policies of the U.S.
Environmental Protection Agency. Mention of trade names or commercial products is
not intended to constitute endorsement or recommendation for use. Copies of this
report are available form the National Technical Information Service,
U.S. Department of Commerce, 5285 Port Royal Road, Springfield, Virginia 22161,
telephone number (800) 553-6847.
IV
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ACKNOWLEDGMENTS
This technical bulletin was made possible through the diligent and persistent efforts of
Lyndon Cox, Senior Environmental Employee with the Clean Air Technology Center (CATC).
Lyndon did an exceptional job identifying information sources, gathering relative data and
putting this bulletin together. The CATC appreciates the assistance of peer reviewers Madeleine
Strum with the Coating and Consumer Products Group, Emission Standards Division, Office of
Air Quality Planning and Standards, and Chester A. Vogel with the Emissions Characterization
and Prevention Branch, Air Pollution Prevention and Control Division, National Risk
Management Research Laboratory for their helpful and timely comments and cooperation.
In addition, the CATC thanks the individuals, companies and institutions who supplied
information on zeolite and zeolite adsorption technology used to prepare this Technical Bulletin.
Contributors are indicated in the REFERENCES section of this bulletin.
VI
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TABLE OF CONTENTS
TOPIC Page
THE PURPOSE OF THIS PAPER 1
WHAT IS ZEOLITE? 1
WHY DO WE SYNTHESIZE ZEOLITE? 1
WHAT ARE THE IMPORTANT CHARACTERISTICS OF ZEOLITE? 3
HOW CAN ZEOLITE BE USED TO CONTROL AIR POLLUTION? 3
DOES ZEOLITE HAVE USE OUTSIDE OF CONTROLLING AIR POLLUTION? 5
WHAT SHOULD WE EXPECT IN THE FUTURE? 7
WHAT ELSE DO WE NEED TO KNOW? 8
REFERENCES 9
FIGURES
1. Y Zeolite Crystal Structure 2
2. VOC Concentrator 4
3. Vacuum Regeneration Adsorber 7
4. Cascade Vacuum Separation 8
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ZEOLITE, A VERSATILE July 8,1998
AIR POLLUTANT ADSORBER
THE PURPOSES OF THIS PAPER
The purposes of this paper are to make you familiar with zeolite, what it can contribute to control
of air pollution — particularly control of Volatile Organic Compounds (VOC) — and what to
expect in the future.
WHAT IS ZEOLITE?
Zeolite is a rock composed of aluminum, silicon, and oxygen. It occurs naturally in several
regions of the world where prehistoric volcanic activity has happened near water, or the water
has been present for millennia since the eruption. In 1756, the Swedish mineralogist Baron Axel
Fredrick Cronstedt discovered zeolite. One account has his dog digging up the stone and him
naming it zeolite from a word which means "dog" in Swedish. In another account, he found that
when the zeolite (it was actually Stilbite) was heated it gave off steam. Zeolite means "boiling
stone" in Greek. Since I do not know either Swedish or Greek, you will have to judge whether
either or both of these stories are correct. We do know for sure that zeolite was discovered.
Zeolite has a natural porosity because it has a crystal structure with windows, cages, and
supercages. The natural zeolites are limited in their window size ("pore size") and are all
hydrophilic (having an affinity for water). Some synthetic zeolites are similar to an absorbent
carbon, since both can be considered hydrophobic (having an affinity for organics, with lack of or
a decreased affinity for water), and can adsorb organic vapors with molecules smaller than their
"pore size." With both carbon and zeolite you can adsorb water and organic molecules;
however, that for which it has affinity will displace the other molecules. Zeolite has a uniform
"pore size" which leads to it being referred to as a "molecular sieve," while carbons appear to
have pores leading to smaller pores leading to still smaller pores adinfmitum.
"Absorbent carbon", which is actually an adsorber, also has an affinity for water at some sites,
which decreases its capacity to adsorb organic molecules from streams containing water
molecules. This can be either true or false for zeolite depending on the type of zeolite that you
select. Generally, the higher the silicon to aluminum ratio, the more hydrophobic the zeolite.
Hydrophobic zeolite must be synthesized, for it does not occur in nature. Therefore, the race to
synthesize zeolite with specific abilities is on!
WHY DO WE SYNTHESIZE ZEOLITE?
Synthesis is important because it allows scientists and engineers to (1) predict the properties of
the zeolite, (2) make a zeolite with a larger "pore size," and (3) make hydrophobic zeolite. Early
attempts to synthesize zeolite simulated volcanic conditions. Hydrophilic zeolite was finally
synthesized in 1949 by Milton, Breck, and others under hydrothermal conditions (i.e., below
100 C.). In 1985, Bibby and Dale reported the synthesis of a pure silica zeolite. Others have
synthesized a wide range of hydrophilic and hydrophobic zeolites.
1
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Zeolite-like crystal structures have even been created by substituting other elements for either the
aluminum or the silicon in the synthesis, but the zeolite that we will discuss here is purely an
alumino-silicate.
There have been 40 natural zeolites discovered; 118 zeolites have been cataloged by the Structure
Committee of the International Zeolite Association; more than 600 zeolites have been identified,
and, if you were to include zeolite-like crystal structures using other elements, the number would
be increasing daily. One zeolite is not exactly like any other.
The structure of a zeolite crystal is based on a tetrahedron formed by four oxygen atoms joined to
a silicon atom by its four valence electrons. These tetrahedrons are joined at the oxygen
"corners" to make crystals. When aluminum is present while the crystals form, the silicon is
replaced by aluminum atoms which have only three valence electrons. The remaining valence
electron is usually provided by either hydrogen or sodium because they are commonly found in
the hydrating water. This distorts the tetrahedron and makes an "acid site" for exchange of other
ions. This also makes the structure hydrophilic. If the zeolite is formed in a mix that contains an
organic cation (i.e., a cation having a carbon backbone), the structure can be formed around the
organic cation, using it as a template. This has led to silicon-rich hydrophobic zeolites with large
enough "pore size" (or window size) to admit all but the largest petroleum molecules.
The crystal structure of a zeolite consists of windows, cages, and supercages. The windows are
how the molecules reach the cages and supercages to be adsorbed or catalyzed. The cages are the
smaller cells in the crystal structure. The supercages are cells in the crystal structure that are
larger than cages and may even contain cages. Figure 1 shows the structure of Y zeolite as seen
by red light.
light
Figure 1 — Y Zeolite Crystal Structure - Showing Windows, Cages, and Supercages
Reprinted with permission from CHEMTECH, June 1996, 26(6), pp 24-30. Copyright 1996 American Chemical
Society.
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Activation of a zeolite consists of heating or evacuating the zeolite to desorb the hydrating water
or other material from the adsorption area, just as with carbon.
The cost of hydrophobic zeolites ranges from $6.50 to $40.00 per pound, according to PQ
Corporation. Compare this with $1.40 to $2.00 per pound for carbon adsorbents, according to
Calgon Carbon. Generally, a minimum of a ton of zeolite is needed for concentration of VOC,
and this can handle up to 8,000 cfm. However, the cost of either carbon or zeolite is only a small
part of the total cost of the adsorber or concentrator.
WHAT ARE THE IMPORTANT CHARACTERISTICS OF ZEOLITE?
1. Zeolite is capable of selective ion exchange.
2. Zeolite can adsorb molecules on its large internal area, provided they can pass through the
windows. Zeolite and carbon are both the same in this respect, just differing in the adsorption
area that can be reached by a given molecule passing through their "pores."
3. Zeolite can be a solid acid catalyst. It can function as a strong acid (though it remains solid)
when the hydration has substituted a hydrogen for the additional valence electron, or
isoelectronic exchange with the aluminum occurs.
4. Zeolite can be used as a molecular sieve because it has a uniform window (or pore) size.
5. Zeolite is metastable; i.e., it is stable as long as it is at a suitable temperature and pH. Within
this range, it is unaffected by wide swings in temperature, pressure, and ionizing radiation.
Natural zeolite exists as a natural rock, and these can reach boulder size. However, synthesized
zeolite crystals are always less than a millimeter in size - because the crystals grow slowly, and
we usually want the zeolite in this century or millennium. These small grains can rapidly
transport the adsorbed molecule to the adsorption area. Air flow through a bed of zeolite powder
creates a large pressure drop. Granular zeolites have now been developed in which grains are
bonded together to create larger air flow channels, and hence have lower resistance to air flow.
HOW CAN ZEOLITE BE USED TO CONTROL AIR POLLUTION?
Hydrophobic zeolite can be made by controlling the silicon to aluminum ratio or by
dealuminizing a zeolite. Different types of zeolite have windows from 0.3 to 3.0 nanometers (3
to 30 Angstroms), but any one type has a nominally fixed window size. Changing the Si/Al ratio
can affect the size of the window. Exchanging ions can also affect the size of the window.
Hydrophobic zeolite is currently used to concentrate VOC before destruction. These
concentrators are shown schematically in Figure 2. They are essentially temperature-swing
adsorbers with the bed mounted on a rotor which gets regenerated on each rotation (two to five
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times per hour). The air-vapor flow from one vendor is axial through sectors in the rotor.
Another vendor's design has the air-vapor flow move radially inward through segments located
along the outside of the rotor.
rotation
vaP°r in ^ " air out
adsorber
air bleed
heat
incinerator
Figure 2. V O C Concentrator
The size and cost of an incinerator are related to air flow, and the air flow from the concentrator
to the incinerator is reduced by about 90%. The major cost savings comes from reducing the
flow of air through the incinerator, which reduces the heat needed to raise the temperature of the
air flow.
Because of this, the concentration of the VOC is also increased by a factor of approximately ten,
as is the heating value of the VOC. The concentrator is thus used to both reduce the size and cost
of the incinerator that is used to destroy the VOC, and to reduce the amount of fuel that must be
added to safely obtain the full destructive efficiency of the incinerator.
The VOC level after concentration must remain below one-fourth of the Lower Explosive Limit
(LEL) for safety; therefore, the input to the concentrator should be no more than about 500 parts
per million (ppm). Commonly, the inlet concentration is only a few hundred ppm. Thus, the
concentrator should be used for VOC concentrations between the levels that are economical for
the fixed bed adsorber and the condenser.
If an incinerator is used at such VOC levels without concentration, a lot of fuel must be added
because the heating value of the VOC is so small before concentration. By concentrating the
VOC, the heating value is increased and less fuel must be added. The total cost of destroying
VOC is in the range of $1,000 to $3,000 per ton, according to Munters ZEOL. The higher total
costs are for the more dilute concentrations.
Hydrophobic zeolites can be very efficient at adsorbing and desorbing VOC, as shown by the
paper "Sorption and Diffusion of VOC on Hydrophobic Zeolites" Paper 83f, 1996 Annual
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Meeting, American Institute of Chemical Engineers. Zeolites can be used over a wider
temperature range than carbon, and are generally superior to carbon as an adsorber.
Gasoline engines emit VOC in their exhaust when they are cold. Hydrophobic zeolite is used in
the catalytic converter to adsorb this VOC from the cold engine exhaust. When the engine
warms up, and the hot exhaust gas desorbs the VOC from the zeolite, the zeolite then catalyzes
the oxidation of the VOC. The zeolite is not altered by the 1,000 F temperatures that are
reached.
Diesel engines have an entirely different problem. They emit NOx which can be catalytically
reduced to nitrogen and oxygen (as in clean air) by a cerium-exchanged zeolite using either urea
or ammonia as a reducing agent.
Separation of carbon dioxide from flue gas was investigated in Korea (Industrial Engineering
Chemical Research, 1995, 34, pp. 591-598). The authors found that zeolite 13X was superior to
an "absorbent carbon" in a pressure swing adsorption system. Since carbon dioxide is a
greenhouse gas associated with global warming, this could be very important in reducing such
emissions.
Chabazite, another zeolite, has been used as a molecular sieve to remove sulfur dioxide from flue
gas. Sulfur oxides are also a product of processing some ores. Sulfur oxides are constituents of
acid rain. Reducing the emissions of them could have a profoundly beneficial effect on the
environment.
Clintoptilolite (a natural zeolite) can remove ammonia from wastewater. It has been used to
control ammonia from urine in both "kitty litter" and horse stables. It is also used to adsorb VOC
that cause odors from mildew, foot fungus, and carpets. It regenerates in sunlight and clean air.
It is noteworthy that zeolite can even be used to catalyze oxidation of hydrocarbons in the
presence of light. However, it can be very selective in these reactions according to
CHEMTECH, 1996, 26(6), 24-30 American Chemical Society.
DOES ZEOLITE HAVE USE OUTSIDE OF CONTROLLING AIR POLLUTION?
In a word, YES. In treatment of radioactive wastes, zeolite is used to remove radioactive cesium
and strontium from wastewater. Since zeolite is impervious to even high doses of radiation, it is
used to exchange and trap these ions. It is capable of removing radioactive strontium at a
concentration of 1 microgram per liter while sodium is present at 150,000 times that
concentration. The zeolite is in powder or sand-size grains which are then used as a filler in
concrete. The concrete is then cast in drums for disposal. This method of disposal is deemed
impervious to salt water leaching, and the radioactive waste can be disposed of either in a landfill
or by ocean dumping.
Zeolite has been used in water softening. Passing hard water through a bed of zeolite to
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exchange the calcium and magnesium ions prevents them from forming insoluble soaps which
we see as "soap scum." The zeolite can be regenerated by passing salt brine through it. The
chlorides of the exchanged ions are water-soluble and therefore disposal is easy. Because natural
zeolites are attacked by acidic tap water, more resistant materials are now used, although
hydrophilic zeolite was used for decades.
Akin to water softeners, and not subject to such attack because they are used only once, are
"builders" in detergents. Zeolite is used as a "builder" and replaces most of the sodium
tripolyphosphate which was formerly used and found to be environmentally harmful. Here again
the zeolite removes calcium and magnesium ions from the water. Seven hundred million tons of
zeolite per year are used this way. Powdered zeolite has no harmful effect on the environment
beyond sometimes being a nuisance dust.
The use of zeolite catalysts in "catalytic crackers" in oil refineries was dependent upon discovery
of a large window zeolite that was hydrophobic. Charles J. Plank and Edward J. Rosinski
invented such a zeolite and its synthesis. Zeolite catalysts now reduce our consumption of
petroleum imports by $8 to $16 billion dollars per year. Added to that savings is the reduced
carbon dioxide emissions from reduced energy requirements of refineries.
Researchers at the University of the West Indies are seeking to use zeolite to make gasoline from
ethanol. New Zealand already uses zeolite as a catalyst to make a large percentage of its motor
fuel from methanol, because they lack petroleum reserves.
Zeolite is used as a dryer for compressed air. Its acts as a desiccant to adsorb moisture. It can
hold up to one-fourth of its weight in water without an appreciable rise in vapor pressure.
Zeolite is also used in refrigerators to dry out the refrigerant. This prevents the formation of ice
crystals which could clog the flow passages and valves.
Slow release fertilizers use zeolite to control their release to the soil. Urea is converted to
ammonia by soil bacteria. Clintoptilolite adsorbs the ammonia, and prevents the soil bacteria
from converting it to nitrate. Subsequent slow leaching of the ammonia allows this nitrification
to occur over a period of months. Otherwise, the surge of nitrates would wash out of the soil and
pollute both groundwater and streams.
Zeolite is used to separate nitrogen from oxygen in air. The small difference in size between a
nitrogen molecule and an oxygen molecule, along with polarization and a suitable zeolite, allows
oxygen to be adsorbed selectively from air. The nitrogen is vented, and the oxygen is then
desorbed. This is used to generate oxygen for aircraft crews, industrial processes, and aquariums.
Zeolites have been synthesized as a membrane, which allows their use as a molecular sieve.
Molecular sieves let molecules through for which they have affinity, and that are smaller than
their pore (window) size. This property is being used for chemical transducers in mixtures where
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the molecular species are known. If a known diameter of molecule passes through and causes
any response from the transducer, that response can be calibrated to the concentration of the
known molecular species. Thus, we now have transducers that sense chemical concentration
without the need for chemical reaction.
Similarly, the University of Greenwich in London, England has developed a zeolite which is used
as a molecular sieve that will allow passage of water molecules — but will not allow passage of
VOC. This is a hydrophilic zeolite, for this is in the form of a thin membrane which must allow
water molecules to adsorb/desorb and thus pass through. They can grow this zeolite with a pore
size of 0.42 nanometers. They use it as a molecular sieve to remove the water from specimens
without any loss of the VOC.
WHAT SHOULD WE EXPECT IN THE FUTURE?
Now that zeolite can be formed into granules which will allow free airflow, we should see zeolite
replace carbon in fixed bed adsorbers in the U. S. This has already occurred in Europe. There is
a reason for this to happen. Carbon (or metals in or on it) catalyze oxidation of VOC, and this
has caused carbon bed fires. This would not necessarily occur in a zeolite bed, and even if a bed
fire does occur, only the VOC would burn. The zeolite also has a non-linear adsorption isotherm,
which increases adsorption at lower concentrations of VOC. This allows a removal efficiency of
99% for zeolite where carbon would have a 95% efficiency. Carbon has the efficiency advantage
only at higher concentrations. Zeolite also regenerates more completely than carbon.
vapor m_ . .
I " adsorber ^_
air out
purge gas
vacuum pump —VOC
adsorber
Figure 3. Vacuum Regeneration Adsorber
Catalysis also happens when a monomer, such as styrene, is adsorbed on carbon and desorbed by
steam. This leaves a "heel" of polymerized monomer on the carbon in a thermal-swing steam
regenerated system. Using a zeolite vacuum regenerated bed (as in Figure 3) could extend the
life of the bed, recover the stryene, and perhaps make the use of zeolite economically attractive.
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When zeolite becomes more commonly used in fixed bed adsorbers in the United States, we can
expect that selective adsorption based on molecular diameter will be used. This would be made
possible by cascading adsorbers of different zeolites, as in Figure 4. Each adsorber in the
cascade would adsorb a range of molecular diameters. By selecting the zeolite based upon type
and pore size, each VOC could be recovered with purity at least comparable to a commercial
grade — if only one VOC were recovered per cascade in the adsorber. Thus, the recovered VOC
could provide a return on investment to help pay for the operation of the adsorber system. We
understand that some VOC are changed to a different isomer by the catalytic action of some
zeolites. If that is the case with your VOC, then the plant can either sell the isomer, use the new
isomer instead of the old one, or change the type of zeolite which is used.
vapors in
adsorber 1
adsorber 2
vacuum
air out
^-
gas bleed in
— VOC recovery device
Figure 4. Cascade Vacuum Separation
WHAT ELSE DO WE NEED TO KNOW?
We need to know (1) what the effective window size is for different zeolites with different
exchange atoms, and (2) what the effective molecular diameter is for each VOC. Having this
knowledge, we can list the VOC that can pass through the windows and reach the adsorption
area. We also need to know the critical temperature and pressure for catalyzing reaction for each
VOC with each zeolite (as well as for types of carbon). The critical temperature and pressure
may actually be curves, with one value for polymerization, and another for oxidation. Using
these types of information, we can design the adsorber system to either avoid or use these
reactions.
Some of this data is already available on the Internet and in libraries. However, it is frequently
not in a form usable to persons who would design such systems. For example, available
crystallographic pore sizes are actually different from the effective window sizes for VOC
molecules. The information needs to be made more widely available in an appropriate form for
engineers to use.
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REFERENCES:
This paper is based upon information from the following sources:
(Note that the Internet address is given where the page can be obtained from the Internet.)
Chemistry Department, University of Manchester Institute of Science and Technology, Manchester, England, UK
trigger.ch.umist.ac.uk:808/zeolite/web/allchaps.htm
Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester. MA
otis.msfc.nasa.gov/MSAD/scirev/sacco.html
Moulinex Marketing, Mumbai, India
www.bjmoulinex.com/mol.htm
Philip Schmidt and David Price, Center for Energy Studies, University of Texas, Austin, TX
www.me.utexas.edu/~prep/mwr.html
California Institute of Technology, Dr. Mark Davis
www.aiche.org/docs/chapterone/zeolites.htm
University of Illinois at Chicago
www2.uic.edi/~rpeddi3/zeolites/zeolites.html
Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN
www.cems.edu/~zeolites/
Zeolyst International
zeolyst.com/faq.html
Technical Transfer Information Partnerships, U. S. Geological Survey
www.usgs.gov/tech-transfer/factsheets/94-066b.html
Laboratorie for Kristallographie ETH, Zurich, Switzerland
kristall.erdw.ethz.ch/research/zeolite_group
Research Foundation of State University of New York
imr.chem.binghamton.edu/labs/zeolite.html
Axel Brunger's Research Group, Yale University, Hew Haven, CT
www-iza-sc.csb .yale.edu/IZA-SC/Atlas
Agricultural and Biological Research Group, Katholich Universiteit, Leuven, Belgium
www.agr.kuleuven.ac.be/pers/tomr/zeostruc.htm
Department of Chemical Engineering, Cambridge University
www.cheng.cam.ac.uk/~jahl015/zeolites.html
Oxford University, Chemistry Department
www.chem.ox.ac.uk/mom/zeolite.html
Dr. Daniel Resasco, School of Chemical Engineering and Materials Science
University of Oklahoma www.ou.edu/catalysis
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Ph.D. Project of the OSPT - Delft
www.ct.utwente.nl/~ospt/minipost96/tud/wijnand.html
Pollution Online
news.pollutiononline.com/industry-news/ind02289704.html
Zeolite Membrane Passes Only Water
Filtration & Separation, October 1997, p. 814
Air and Waste Management Association:
Paper 96-RA106.05, Adsorption of VOCs on Zeolite
Paper 96-RA106.02, Microwave Regeneration of VOC Adsorbents
CHEMTECH, The American Chemical Society
acsinfo.acs.org/hotartcl/chemtech/96/jun/zeo.html
Department of Chemical Engineering, California Institute of Technology
www.che.caltech.edu/research/catalysis.html
Zeolite (a commercial page for clintoptilolite)
www.gtamart.com/mart/products/zeolite
University of the West Indies, Mona, Jamaica
wwwchem.uwimona.edu.jm: 1104/lectures/synfuel.html
Munters ZEOL, Amesbury, MA
1996 AIChE Annual Meeting, Session 83, Paper 83f, Sorption and Diffusion of Volatile Organic Compounds on
Hydrophobic Zeolites, M. Chandak and Y. S. Lin, University of Cincinnati
Zeotech, Arlov, Sweden
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