It is clear that no longer can we continue to spew carbon dioxide recklessly into the atmosphere; and it is also clear that eventually we will run out of fossil fuels. So, how about killing two birds with one stone? (Only figuratively, of course.) The challenge consists of capturing the carbon dioxide formed in combustion processes before it is released into the atmosphere — and then finding a way to convert it into useful compounds.
After all, carbon is the basic building block of all organic compounds, be they in gasoline, pharmaceuticals or plastics. And carbon is obviously one of the components of carbon dioxide.
There certainly is precedence for using carbon dioxide as a source of organic compounds. Just consider photosynthesis, the reaction that makes all life on Earth possible by allowing carbon dioxide to react with water under the influence of the catalyst chlorophyll, to yield oxygen and glucose.
Plants then use the carbon framework of glucose for the synthesis of their myriad chemical constituents. The complex reactions involved in photosynthesis have defied efforts at commercial replication, but a host of reactions have been developed that use carbon dioxide as a raw material for the synthesis of various organic compounds.
Research in this area is complicated by the stability of carbon dioxide. Basically, this molecule does not easily engage in chemical reactions, as is of course witnessed by its buildup in the atmosphere.
In chemistry, the usual way to coax unreactive molecules into activity is through the introduction of a catalyst, a substance that speeds up a reaction without undergoing a change itself. Chemists have long dreamed of finding the right catalyst for reactions that would allow abundant carbon dioxide to be used as a feedstock instead of non-renewable crude oil. That dream is now on the verge of becoming a reality.
Technically speaking, the capture of carbon dioxide from combustion effluent is the smaller part of the problem. Several technologies exist, with bubbling the carbon dioxide through the solution of an amine being a prime example. One reaction that carbon dioxide does engage in quite easily is with water to form carbonic acid. This doesn’t happen to a great extent, which is why carbonated water is only slightly acidic. But amines are bases that react with acids; and as the carbonic-acid concentration drops, more carbon dioxide reacts with water to replace the acids that have been mopped up by the amine.
With enough amine in solution, eventually all the carbon dioxide is absorbed. The beauty of this process is that heating the solution liberates the carbon dioxide, leaving the amine solution available to be recycled. Obviously there is an energy cost to this process, about 20 per cent of the total power output. Unfortunately, when it comes to reducing carbon emissions, there is no free lunch.
What, then, is to be done with the massive amounts of carbon dioxide that can be captured by such processes? Some can be sold to soft-drink manufacturers, but that is not a significant amount. For now, the plan is to inject the gas into geological formations several kilometres underground for safe storage. That doesn’t seem to be the best way to deal with a potentially valuable material. Of course the key word here is “potentially,” because while many research groups have found ways to convert carbon dioxide into useful organic compounds, the challenge remains to make these reactions practical on a larger scale.
Some efforts are tantalizingly close. Bayer, the company whose name is mostly associated with Aspirin, is close to finding a treatment for the headache that carbon dioxide has been causing researchers. After testing over 200 catalysts, its chemists came up with one that allows carbon dioxide to react with compounds in the epoxide family to yield polyether polycarbonate polyols (PPPs). Some may find these unpronounceable, but they are very functional. When reacted with isocyanates they form polyurethanes, one of the most useful classes of plastics. You’ll find them as foams in furniture, mattresses and insulating materials, as well as in ski boots and the soles of running shoes.
Bayer has partnered with a power plant to obtain the carbon dioxide collected from its amine scrubber which will be used to make polyurethane foam for mattresses. While the epoxides and isocyanates still have to be sourced from petroleum resources, the use of carbon dioxide results in an overall environmental benefit. The same can be said for polypropylene carbonate (PPC), a biodegradable plastic that can be made from epoxides and carbon dioxide for use in packing materials, linings for food cans and as a softener for more brittle plastics like polylactic acid.
And then there is the alluring possibility of turning carbon dioxide into ethylene glycol, a compound that has great commercial application as antifreeze and is also one of the components used to formulate polyester for water and soft-drink bottles. Liquid Light, a New Jersey company, has found a catalyst that will convert a solution of carbon dioxide to ethylene glycol as an electric current passes through it. Another possibility is to use a special molybdenum catalyst to couple ethylene with carbon dioxide to form acrylic acid, widely used to make adhesives, synthetic fibres, water treatment chemicals, Plexiglas and adsorbents in diapers.
Using carbon dioxide to make chemicals does have the double-barrel effect of reducing greenhouse-gas emissions while concurrently saving petroleum. However, the amount of petroleum used for chemical manufacture is a small fraction of that used for fuel. But waste carbon dioxide can even play a role in fuel production.
Algae are sensational little factories for oil production. Put them in a pond, supply some nutrients, pump in carbon dioxide and they’ll provide a spectacular harvest. Then place them in a closed reactor with some sugar from food-industry waste and they will plump up with oils that can be burned as fuel or converted into biodiesel. And finally, carbon dioxide can be captured and converted into sodium bicarbonate, which is just what is needed to treat the indigestion caused by comments from people who maintain that carbon dioxide emissions are not a problem.
