Asked Questions About CO2 & Green Chemistry
Carbon Dioxide /
|Carbon dioxide (CO2)
is a global-warming gas. Why is its use as a solvent considered "green"?|
carbon dioxide used as a solvent in chemical processes is not made “on
purpose”. It is the
by-product of other major chemical processes, such as ammonia manufacture.
Its use as a solvent in no way adds CO2
and, in fact, it slightly reduces
CO2 in the
Second, use of CO2
potentially allows replacement of organic solvent-based processes, which
contribute to direct human chemical exposure and to both air and water
pollution. Use in aqueous
processes potentially reduces water pollution.
processes can be much more energy efficient than current processes,
especially water-based processes, which require huge amounts of energy for
evaporation. The amount of CO2
released to the atmosphere can be reduced by 10-100 times the amount used in
the process due to improved energy efficiency.
Thus, using CO2
as a solvent can reduce the
greenhouse effect compared to existing processes.
|What is meant by "sustainable
A sustainable growth company is
one that creates increasing shareholder and societal value while reducing
its environmental footprint.
A company’s “environmental footprint” is determined by
the amount of depleteble raw materials and nonrenewable energy it consumes
to make its products, and the quantity of waste and emissions that are
generated in the process.
Traditionally, for a company to grow, the footprint had to get
Nowadays, green companies are looking for ways to grow while reducing
the size of the footprint.
This is sustainable growth - growth
that does not depend on consuming ever-increasing amounts of the world’s
a speech by Chad Holliday, Chairman and CEO of E. I. du Pont de Nemours and
|What is meant by "green
answer from EPA (United States Environmental Protection Agency)
dioxide is one of nature’s simplest chemicals and has been known for
centuries. Why all the sudden
The following is a
partial listing of some of the potential advantages of using carbon dioxide
as a solvent
non-toxic and non-polluting
properties (with pressure)
heat of vaporization
recovered and recycled
Why has it not been used? Generally,
it’s not a very good solvent. However,
recent advances in surfactants (materials that behave like soap) have
enabled scientists and engineers to use it as a reaction medium to make new
molecules, especially polymers. Also,
these surfactants can be used to make CO2 a good cleaning agent.
Finally, some recently developed polymers are actually soluble in CO2
and high-quality films can be made from these solutions.
Carbon dioxide has the potential to be used in dozens of applications—from
computer chips to potato chips.
|To be used as a solvent carbon dioxide must
be utilized under high pressure. Isn't this prohibitively expensive?|
Director's Letter in the Spring 2000 Kenan Center Report
|What is the critical point of a compound?|
The critical point is that point
on a phase diagram at which there is no distinction between liquid and gas.
It marks one end of the liquid/gas saturation curve. (The other end is
marked by the triple point, the only point at which solid, liquid and gas
exist simultaneously.) At this point the meniscus between liquid and gas
disappears, and a new state--a supercritical fluid--exists. For the phase
behavior of carbon dioxide click here.
Carbon dioxide is a linear molecule with one carbon and two oxygen atoms, CO2,
as shown on our FAQ logo. The black part of the model represents carbon;
and the red, oxygen. Under normal conditions it is a colorless, odorless
gas. Under pressure and at temperatures between minus 56oC and
31oC it can be converted into a liquid. Carbon dioxide is
usually shipped in metal cylinders as a liquid under pressure (about 850
pounds per square inch—psi). With greater cooling and/or more pressure, it
becomes a solid, commonly known as “dry ice”. Above 31oC
(below 80,000 psi) it becomes a “supercritical” fluid; that is, the
meniscus (the boundary separating the gas and liquid phases) disappears.
The densities of the phases become equal, and two phases become one. Under
supercritical conditions a fluid is neither gas nor liquid but has some
properties of both. Click
to see the “phase diagram” showing conditions for solid, liquid, gas
and super-critical conditions for CO2.
Where is CO2 found in nature? Carbon dioxide is a minor,
but very important, constituent of our atmosphere. It is present (2006) at
about 381 parts per million (ppm) and has been rising at about 2 ppm/yr over
the past 30 years. That is about one part in 2625. Green plants remove CO2
from the air and use it in a process called “photosynthesis” to produce
sugars. (Plants also combine sugar molecules to make starches and
presence of light and chlorophyll) CO2 + H2O → sugar
Most animals eat plants for their sugars, which they oxidize or “burn” to
produce energy. “Burning” sugar in their bodies produces carbon dioxide.
sugar + O2
→ CO2 + H2O
Plants provide food for animals; and, in a way, animals provide “food”
(CO2) for plants.
In addition to the atmosphere, CO2 is present in
water. Some CO2 simply dissolves, but most reacts with water and
is converted into carbonates, HCO3- or CO3=.
In fact, there is much more carbon dioxide in the oceans than in the
atmosphere. Many types of plants and animals use this carbonate with
naturally occurring calcium to produce shells of calcium carbonate.
Countless trillions of tons of CO2 are tied up in carbonate
rocks, reefs and ocean sediments—thousands of times the amount in the
atmosphere. This cycle of CO2 through plants and animals, the
earth, sea and atmosphere is part of the
carbon cycle, the basis of life on earth. It is one of many
delicate balances upon which life depends.
How is CO2 used today? CO2 is
great stuff! A very common use is in sodas—it’s the fizz in soda pop and
“pop rocks”. In pressurized canisters it can be used as a propellant,
for example for paint ball, pellet guns and other recreational uses. It’s
used in fire extinguishers. It can even be used to increase recovery of
petroleum. Pressurized and cooled, it can be converted into “dry ice”,
which is commonly used for shipping frozen foods, like ice cream. Dry ice
is also used in “sandblasting” to remove paint (less messy than using
real sand). And you’ve probably seen it used to make “fog” in a
movie, a play, or your favorite Halloween House of Horror!
CO2 is beginning to be used as a solvent to replace potentially
hazardous solvents. “Naturally decaffeinated” coffee has had its
caffeine removed using CO2. Many extractions besides caffeine
may be done using CO2. “Essential oils”, used in making
perfume, can also be extracted this way. Other applications may include
removing something unwanted (such as removing fat from potato chips) or
concentrating something valuable (such as recovering nutrients from soy
beans, grape skins, olive pits, etc.)
Our Science and Technology Center is looking for new ways to use CO2
that can make Planet Earth cleaner for future generations. We’re learning
to use it to replace toxic solvents in applications like dry cleaning or for
other cleaning purposes, such as cleaning computer chips during their
manufacturing process. It may be possible for the entire computer
chip-manufacturing process to be conducted using CO2. In a
DuPont factory in Fayetteville, NC, CO2 is being used in a new
process, developed by scientists at our Center, to make TEFLON. Click
here to read more.
We are seeking ways to use CO2 to make other polymers.
Carbon dioxide appears to be useful in a new branch of science called
(“Nano” refers to structures that are measured in nanometers. Small
molecules, like those in gasoline, are about one nanometer long. Your
finger nail is roughly one millimeter thick. It would take one million
nanometers to equal the thickness of your finger nail.) It might be
possible to create tiny devices by using CO2 in a process called
“self-assembly”. Possible uses for these devices include targeted
delivery of pharmaceuticals within the body and electronic components.
Scientists in our Center are working in these areas.
Water is the “universal solvent”. Why not use it? Wouldn’t that
be more “environmentally friendly” than CO2? No,
using water as a solvent is not environmentally friendly! When water is
used, pollutants often become dissolved in the water and have to be
removed. Otherwise they would contaminate groundwater and/or streams when
the used water is released. Water can also be absorbed in many processes,
as in the dying of textiles. Removal of that water requires huge amounts of
energy, which puts more greenhouse gases into the atmosphere. Pollutants
generally do not dissolve in CO2, or if they do are easily
The properties of CO2 are also beneficial in many applications.
For example, in making computer chips, spacing between the features (think
of them as “lines”) is about 100nm or about 1/1000th the
thickness of a human hair! At this tiny spacing
water may be too
viscous to penetrate into the spaces.
(Ever try to pour syrup right from the refrigerator? Could you suck it up a
thin straw? No, it is too “thick” or viscous.) If the spaces are
large enough for water to penetrate, then
surface tension of water can
cause the circuit to collapse and “short out”. (Ever do a “belly
flop”? Then you know how strong the surface tension of water is!)
Carbon dioxide is 40 times better at penetrating and at least 15 times less
likely to cause collapse due to surface tension. Also, CO2 is
“dry”. Can you imagine using water to extract fat from a potato chip
and have it remain crispy? CO2 can do that.
In processes that can benefit from the use of water,
“emulsions” of water in CO2
might be used. (Never heard of emulsions? You drink one when you drink
milk.) Tiny droplets of water can be suspended in CO2. These
droplets behave like “mini-reactors”. To make these droplets, special
“surfactants” are needed. (Never heard of a surfactant? You use
one when you wash your hands with soap or your clothes with detergent.) The
amount of water used in emulsions can be 20-100 times less than using pure
water. Much of the work of our Center is devoted to identifying surfactants
that make stable emulsions with CO2.
What is the
effect”? When sunlight hits the earth, the earth is
warmed and radiates some of that heat back towards space. Certain gases in
the atmosphere (known as “greenhouse gases”) can absorb some of that
radiation, preventing it from escaping. The earth retains more heat from
the sun than it would in the absence of these gases, making the earth about
35oK warmer than it would be otherwise. Without this greenhouse
effect, the earth would likely be a giant ice ball. Life as we know it
probably would not exist.
“sister planet”) has a surface temperature of 750o K, hot
enough to melt lead. The surface temperature is twice that of Mercury, even
though the latter is much closer to the sun. Why? Venus has a “runaway
greenhouse effect” because its dense atmosphere is 96% CO2.
This effect is estimated to raise the surface temperature of
Venus 400o K.
Is the “greenhouse effect” the same as
“global warming”? No. As
the amount of greenhouse gases (such as CO2) increases so does
their warming effect. Increasing greenhouse gases in the atmosphere can
lead to global warming.
What will be the effect of continued increase of greenhouse gases?
No one knows for sure, but likely effects include sea-level rise and
changes in weather patterns. Greenhouse gases, especially CO2,
have been rising for over a hundred years due to industrialization
deforestation. [Hyperlink from deforestation to: ] Many
scientists conclude that this increased greenhouse effect is already
causing global warming. There appears to be a link between the increase in
greenhouse gases and a gradual (1oF) increase in average global
temperature during the past century, especially in polar regions. Sea ice
in polar regions appears to be shrinking. Less ice may lead to greater
absorption of solar radiation. As the earth begins to warm another
greenhouse gas, methane or “natural gas”, which is trapped in the
oceans and boggy soil, may be released, further increasing the greenhouse
effect and accelerating global warming. The earth has undergone many cycles
of global warming and cooling over millions of years, causing significant
changes to life on the planet. But the exact nature of these changes is
unpredictable. For example, one surprising possible effect of global
warming may be
a new ice age!
To learn more about past warming/cooling cycles click
Do scientists disagree about the greenhouse effect
and global warming? No! Scientists seldom agree 100% on
anything, but the vast majority of reputable scientists agree that
increasing levels of certain atmospheric gases (like CO2)
contribute to global warming. There is essentially no disagreement that
growing use of fossil fuels on the current upward trend will lead to
increased global warming. If there is disagreement, it is regarding the
impact and timing of consequences due to this warming. Some models predict
catastrophic effects like rising sea levels within our lifetimes. (Indeed,
we can already observe substantially increased melting of polar
ice.) Some predict stronger hurricanes immediately. (That
additional heat must go somewhere, somehow.) Others say damaging
effects are farther in the future. To learn more about global warming and
If damaging effects are in the future, what’s the urgency? Many
scientific models show what is called a
“tipping point in global warming”.
(The “tipping point” is the point beyond which our attempt to avoid
catastrophe will be ineffective.) When average global temperatures
consistently rise to a certain level global warming may spiral out of
control. For example, when more polar ice melts, earth’s surface becomes
less reflective, on average, and more solar radiation will be absorbed.
This raises temperatures more, causing more polar ice to melt, etc.
Permafrost melts, releasing billions of tons of greenhouse gases into the
atmosphere accelerating global warming. To learn more about the effect of
global warming on arctic regions click
Most scientists believe that we have not yet reached that tipping point.
However, it may arrive in the not-too-distant future.