Scientists have considered CO2 to be a valuable source of carbon for more than 25 years, though interest in its possibilities has cycled up and down. In the past decade, several workshops have been held, culminating in reports and review articles on capturing CO2 and potential uses of the gas. And at the American Chemical Society national meeting in Chicago last month, the overarching theme of sustainability encouraged a new round of presentations covering a broad array of chemistries related to CO2 capture, sequestration, and utilization.
"There can be little doubt that CO2 currently has the highest public profile of any molecule," chemistry professor of the University of Leeds, in England, told C&EN. Rayner served as chairman of a workshop on converting CO2 into chemicals, held in July 2006 and sponsored by the Royal Society of Chemistry (RSC). He also published a review article recently on the potential of CO2 in synthetic organic chemistry (Org. Proc. Res. Dev. 2007, 11, 121).
The road to determining the future of CO2 clearly is still under construction. And chemists are planning a number of other workshops and conferences to help speed up the progress and perhaps redraw the map. One of these meetings is "," which serves as the joint CHEMRAWN XVII conference and 9th International Conference on Carbon Dioxide. It is being sponsored by and , Ontario, and will be held at the university July 8-12. CHEMRAWN, which stands for Chemical Research Applied to World Needs, is a branch of the .
The reaction can be made greener in several ways, Li pointed out. First, using water as the solvent in this case is a plus compared with using an organic solvent. Next, an expensive metal catalyst or metal oxidizing reagent is not needed, as the oxidation can be catalyzed by using an ammonium bromide salt, an amine base, and H2O2. Bromine reacts with the olefin in water to form a bromohydrin intermediate (containing bromine and hydroxyl substituents). The base subsequently deprotonates the hydroxyl group to form an alkoxide that attacks CO2 to form the cyclic carbonate. The peroxide serves to reoxidize bromide ions to complete the catalytic cycle, leaving water as the only by-product. Eghbali and Li have filed a patent on the synthesis.
"Biomass, methane, and carbon dioxide are huge renewable carbon resources for organic synthesis," commented chemistry professor of McGill University, Montreal. "But using a renewable feedstock alone doesn't mean a process is green." A number of factors still need to be met, including reducing organic solvent use, reducing the number of reactants and reaction steps, reducing energy consumption, and reducing waste.
Bulk chemicals already produced routinely from CO2 include urea to make nitrogen fertilizers, salicylic acid as a pharmaceutical ingredient, and polycarbonate-based plastics, Rayner said. Carbon dioxide also could be used more widely as a solvent, he added. For example, supercritical CO2 (the state existing at 31.0 °C and 72.8 atm) offers advantages in terms of stereochemical control, product purification, and environmental issues for synthesizing fine chemicals and pharmaceuticals, Rayner noted. Other avenues that he mentioned for using CO2 include oil and gas recovery, enhanced agricultural production, and ponds of genetically modified algae that can convert power-plant CO2 into biodiesel.
CARBON DIOXIDE is nontoxic, nonflammable, and essentially free for the taking. Those attributes make it sound like CO2 could be a great feedstock for making commodity chemicals, fuels, and materials—and it already is playing that role for a few applications. But there are a few catches. One is that CO2 is very stable, which means it takes extra effort to activate the molecule so it will react. Another reality check is that so much of the unwanted greenhouse gas is escaping into the atmosphere as a consequence of burning fossil fuels that even shunting millions of tons of it each year into making chemicals won't have much of a bearing on the gas's global warming threat.
Using CO2 directly as a chemical feedstock isn't the only option, several chemists pointed out. The gas can be converted to carbon monoxide, which is considered a more versatile starting material than CO2. Carbon monoxide can be used in a host of organic syntheses, but it's best known as a component of synthesis gas (a mixture of CO and H2), which is an important feedstock in the chemical industry for making hydrocarbons via Fischer-Tropsch reactions.