When it comes to the issue of fossil fuels and global warming, the big culprit that everyone can point to is the simple molecule, carbon dioxide. Carbon dioxide is produced when carbon-containing compounds (such as gasoline and diesel) are burned in the presence of oxygen. Carbon dioxide, which is a gas, then floats up into our atmosphere and begins to trap heat near the planets surface – the so-called “greenhouse” effect, making carbon dioxide a greenhouse gas. While great advances are being made in energy conservation and the reduction of gas emissions, those efforts are only half of the equation. What is also needed is a way to sequester, or capture, carbon dioxide that is in the atmosphere and to store it in a condensed fashion somewhere.
Carbon dioxide is a very agreeable gas to compress, and solid carbon dioxide finds all types of wonderful uses (otherwise known as “dry ice”, it’s used widely for refridgeration). However, conventional methods of compressing carbon dioxide require a lot of energy, which makes carbon dioxide pollution a particular difficult problem to solve. A totally radical method of carbon dioxide capture is needed, one which breaks the mold of current thinking. Some chemists from South Korea believe that they have stumbled upon the beginnings of a method which could address this pressing need. They have developed a new class of materials known as metal-organic frameworks, or MOFs, and report their discovery in the journal Science.
MOFs are essentially large molecular sponges. They are composed of two different components – a metal and a carefully designed organic molecules, which assemble to produce a solid that is absolutely riddles with microscopic tunnels and pores. Indeed, the MOFs which are reported in the latest edition of Science magazine boast the largest porosity ever seen in this type of material – close to the theoretical upper limit for porosity. By having a large number of open pores, large amount of gases can enter and be compressed into small volumes. Having a large porosity is an essential property for a material to capture carbon dioxide.
As I’ve written before here on Associated Content, compounds with a large surface area often behave differently than a bulk sample of the material. Ice, for example, takes a long time to melt if it’s just sitting there in a block, but the same amount of ice powdered down to small chips will melt in no time at all. In a similiar fashion, the mixture of metals and organics in teh newly-described MOFs behave differently than a sample of metal and organic material just sitting in a flask.
The MOFs described by the South Korean chemists don’t require expensive materials to make. The two components are zinc oxide (a white powder used in cosmetics) and a simple one-ring organic compound that is the basis for plastic water bottle production. When combined in the right fashion to produce a metal-organic framework, the two materials assemble into a cavern of passages and tunnels for molecules of carbon dioxide to enter and become compressed. One gram of their new material – about the same weight as a paper clip – has more total surface area than 10 football fields. It is this extraordinary porosity which makes these compounds a true molecular “sponge”. By introducing carbon dioxide gas, the gas becomes entrapped in the tunnels. Just like a kitchen sponge can soak up many times its weight in water, so can these materials soak up large amounts of carbon dioxide. Because the material is lightweight and also extremely cheap to produce, scientists envision a future when these materials become the equivalent of molecular trashbags; after they are filled with the greenhouse gas, they are buried underground or at the bottom of the ocean, and the carbon content is removed from the atmosphere. This will no doubt have beneficial environmental effects. I for one encourage any research that will make my summers in Atlanta a little bit cooler.
http://www.sciencemag.org/cgi/content/abstract/science.1192160v1
