Frequently Asked Questions

Frequently Asked Questions About CO₂ & Green Chemistry

= Carbon Dioxide /
CO₂ molecule

	Carbon dioxide (CO₂) *is a global-warming gas. Why is its use as a solvent considered "green"?* First, 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 atmosphere. 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. Third, CO₂-based 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 technology"?* 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 larger. 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 finite resources. From a speech by Chad Holliday, Chairman and CEO of E. I. du Pont de Nemours and Company
	*What is meant by "green chemistry"?* An answer from EPA (United States Environmental Protection Agency)
	Carbon dioxide is one of nature’s simplest chemicals and has been known for centuries. Why all the sudden interest? The following is a partial listing of some of the potential advantages of using carbon dioxide as a solvent · very inexpensive · relatively non-toxic and non-polluting · low viscosity · low surface tension · tunable properties (with pressure) · low heat of vaporization · readily 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? see 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.
	What is carbon dioxide? 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 56^oC and 31^oC 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 31^oC (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 here 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 cellulose). (in the presence of light and chlorophyll) CO₂ + H₂O → sugar + O₂ 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 CO₂simply 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 “nanotechnology”. (“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 removed.

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 1/1000^th 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 chemicals called “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 “greenhouse 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 35^oK 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.

The planet Venus (earth’s “sister planet”) has a surface temperature of 750^o 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 400^o 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 (especially burning fossil fuels) and 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 (1^oF) 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 here.
	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 hurricanes click here.
	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 here.

Most scientists believe that we have not yet reached that tipping point. However, it may arrive in the not-too-distant future.