Science and Technology

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Carbon Dioxide: Good for Something?
Jessica Marshall, Discovery News


April 10, 2008 -- The carbon in oil and coal is used to make many useful things: fuel, plastics, paints, detergents, pharmaceuticals...the list is long. Unfortunately, most of that carbon -- especially from fuel -- ends up in the atmosphere as good-for-nothing, climate-change-inducing carbon dioxide.

But is it really good for nothing? Maybe not for long. Chemists are developing strategies to put CO2 to use making products normally derived from oil. These approaches could take a bite out of power plant CO2 emissions that would normally go into the atmosphere.

For instance, CO2 could take the place of the poisonous gas phosgene in production of certain plastics, according to findings released this week at a meeting of the American Chemical Society by Toshiyasu Sakakura of the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan.

Sakakura and colleagues developed a new catalyst that efficiently converts CO2 and methanol into a plastic precursor whose synthesis currently requires phosgene. Phosgene, which is derived from petroleum, is particularly nasty. It was used as a chemical warfare agent in World War I.

"It does not produce much waste," Sakakura said of the new process. "The waste is just water, so it is simple and clean."

Bringing New Meaning to the Word "Recycled"

Sakakura and other researchers have targeted other processes for making plastics, which are, essentially, long chains of carbon. CO2 can react with a class of chemicals called epoxides to make polycarbonate, the tough, clear plastic used in compact discs, eyeglass lenses, bulletproof windows and more.

Using CO2 in such processes is a challenge, said Thomas Müller of the CAT Catalytic Center in Aachen, Germany, who also presented at the meeting, because it is relatively inert and "low in energy." After all, carbon dioxide is what is left after the energy stored in the chemical bonds of the molecules that make up fuel has been released by combustion.

This means that reactions using CO2 require something else, like methane or an epoxide, to act as a source of energy in creating a stable, higher-energy product like plastics.

"It would be great if you could polymerize CO2 directly," said chemist Geoffrey Coates of Cornell University in Ithaca, NY, referring to the process of linking carbon atoms together. "But you would defeat the laws of nature."

These co-reactants generally come from fossil fuel sources, so these CO2-based processes generally decrease, but do not eliminate, the need for petroleum. Coates has made polymers, for instance, that are 30 to 50 percent CO2.

Down With Oil

To eliminate the petroleum source altogether, he used an epoxide derived from the oil in orange peels as a co-reactant to make plastic with CO2. Coates is working to commercialize CO2-based plastics with a range of properties, and other "environmentally benign" polymers, through a company called Novamer.

Even if fossil fuel contributions were eliminated from all of these reactions, experts agree that making plastics from CO2 generated in power plants or other industrial processes will not fix the climate change problem.

"When we keep burning as much fossil fuel as we are, it's going to be impossible for one chemical use to negate all of the CO2 that's made on a daily basis," Coates said. Production of all polymers worldwide amounted to about 260 million tons in 2005, according to Müller, while CO2 emissions added up to more than 100 times more.

"However, if you're using CO2 instead of a petrochemical source, then you are prolonging the lifetime of the petrochemical resources that we have," said Christopher Rayner, of the University of Leeds in the U.K., who is working to make formic acid, which can be used in fuel cells, from CO2 and hydrogen.

Doing As the Plants Do

Another approach to providing the energy needed to turn the carbon in CO2 into a more useful form is with electrochemical cells -- which use electricity and a catalyst to convert CO2 into carbon monoxide and, eventually, into a fuel such as methanol. Daniel Dubois of the Pacific Northwest National Laboratory, another meeting presenter, is tackling this problem.

"Part of the problem is, where do you get your electricity?" Rayner said of this method. Unless it comes from a renewable source, the electricity supply creates CO2 emissions that undo the gains in absorbing CO2 in the electrochemical process.

There is, of course, a precedent for stripping CO2 out of the atmosphere on a large scale and converting it into all sorts of useful molecules: Plants do it all day by harnessing the sun's energy through photosynthesis and using it to build their cells and tissues.

Some researchers are trying to turn CO2 back into fuels using methods similar in principle to photosynthesis -- using the energy in light to transform carbon dioxide into higher-energy molecules.

Others are trying to capitalize directly on plants' ability to convert CO2 into potentially valuable molecules, including sources of fuel. It will be a challenge to identify which compounds can economically be made this way, Müller pointed out, also noting that only one percent of the sun's energy is converted by photosynthesis into plant tissue. He hopes synthetic approaches can be more efficient.

"Of course we can learn from nature," he added, "We'd like to copy them."


http://dsc.discovery.com/news/2008/04/10/carbon-dioxide-plastic-print.html