What Can We Do With Carbon Dioxide?

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.

So what's one to do with CO₂?

C&EN asked that question of a number of chemists and chemical engineers. These investigators were unanimous on the scope of the CO₂ problem. They understand that technology to capture large amounts of CO₂ and sequester it deep underground or under the seafloor probably is going to be necessary if humanity decides it wants to reduce the gas's expected effects on the planet's climate. But they also are optimistic that much good could come from a more focused research effort to find new ways to utilize CO₂ as a chemical feedstock.

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 Christopher M. Rayner 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).

Since the early 1990s, the number of papers on CO₂ has been rising "almost as fast as the atmospheric level of CO₂ itself," Rayner said. This trend reflects the growing effort to develop technologies to separate CO₂ from industrial flue gases. But part of the rise likely is due to some researchers not understanding that using CO₂ as a raw material "can only have a very limited effect on reducing the greenhouse gas's contribution to global warming, simply because of the sheer scale of emissions," he added.

The approximately 115 million metric tons of CO₂ used annually by the global chemical industry really doesn't compare to the approximately 24 billion metric tons of annual anthropogenic CO₂ emissions. Doing the math makes it obvious that CO₂ capture and sequestration will be necessary, regardless of how much CO₂ industry ends up using as a feedstock. Potentially, however, there are better ways to deal with CO₂, a growing number of researchers are saying.

"Industrial processes can in principle be developed to have a positive influence on atmospheric CO₂—we don't have to bury it all to have an impact," Rayner said. "But the products would have to be in high demand." Otherwise, large piles of unwanted polycarbonates or "pretty white mountains" of calcium carbonate akin to the White Cliffs of Dover might dot the globe, he mused.

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.

ONE PROJECT Rayner and colleagues from across the U.K. are working on involves catalytic processes for reducing CO₂ to formic acid (HCO₂H). Formic acid has potential to power fuel cells for electricity generation and automobiles and as a precursor for other fuels and commodity chemicals, including polymers, he said. Hydrogen will be needed for the conversion and would have to be sourced from elsewhere, Rayner explained. "But compared with using hydrogen alone or methanol, we believe formic acid has much greater potential," he said.

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.

Lots of opportunities exist, Rayner believes. "Chemists are uniquely positioned to make fundamentally important contributions to climate change and reduction of CO₂ levels," he said. "But an enormous challenge lies ahead."

"Using CO₂ as a raw material probably is never going to reduce atmospheric CO₂ levels or lessen the effects of CO₂ on climate change???the numbers just don't add up," cautioned chemical engineer Eric J. Beckman of the University of Pittsburgh. "But there are places where using CO₂ as a raw material could create needed products. A bonus will be if such products could be made in a green and economically feasible fashion."

Progress to that end has been slow and not terribly creative so far, Beckman continued. "Typically what we have seen is people making things that they are able to make, rather than making things that are truly useful," he observed. While much of this research is worthy of publication in top journals, it's not yet fully addressing real problems and challenges, Beckman said. "For example, most of these attempts make no mention of carbon—would more CO₂ be generated in the process than is consumed?"

Target areas should focus on using CO₂ to replace large-volume starting materials derived from petroleum and natural gas, Beckman suggested. Other possibilities include using CO₂ as a benign solvent. In both cases, using CO₂ would make sense if it worked just as well as or better than other chemicals and saved energy while lowering overall costs.

One possibility, highlighted in a poster at the ACS meeting by Qunlai Chen, a graduate student in Beckman's group, is a green process to generate hydrogen peroxide. Chen described how H₂O₂ can be made directly from O₂ and H₂ by using noble-metal catalysts in supercritical CO₂. Currently, H₂O₂ is prepared industrially by an oxidation-reduction cycle involving anthraquinone, O₂, and H₂, which requires organic solvent and generates waste.

A new alternative route to H₂O₂ is to generate it directly from H₂ and O₂ in methanol, Chen noted. But methanol is flammable and has a tendency to oxidize to form by-products. Thus the CO₂ approach has some merit as a safe way to make H₂O₂ while reducing waste and energy usage, Beckman said.

In the 1980s, there was a rush to develop supercritical fluid technologies following the success with using supercritical CO₂ to extract caffeine from coffee, Beckman explained. But this turned out to be "a solution in search of a problem," he said. The "feeble" power of supercritical CO₂ to solubilize polar and high-molecular-weight compounds made it suitable only for niche applications, he noted.

Since the early 1990s, the use of surfactants and cosolvents has empowered supercritical CO₂. For example, it is now used in DuPont's polymerization of fluorinated monomers and in dry cleaning, as an alternative to halocarbon solvents. But using CO₂ as a solvent "seems to have hit a wall," he added. "Beyond traditional food processing, we haven't been seeing much in the way of large-scale applications coming out, and funding for such work is down a ton." But there's still potential for large-scale applications to emerge, he said.

"Biomass, methane, and carbon dioxide are huge renewable carbon resources for organic synthesis," commented chemistry professor Chao-Jun Li 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.

One strategy Li envisions as having some potential in this arena is CO₂ chemistry in which water is the solvent. In Chicago, Li described a project by Nicolas Eghbali in his group to convert CO₂ and olefins into cyclic carbonates in water (Green Chem. 2007, 9, 213). This oxidative carboxylation of olefins is not exactly a new approach, he said, but there are few references to it in the chemical literature, and they all report low efficiencies.

Carbonates are useful and often greener substitutes for toxic phosgene (COCl₂) and dimethyl sulfate in a host of chemical reactions, Li said. They also serve well as solvents, especially in medicines and cosmetics, and they are electrolytes of choice in lithium-ion batteries.

The compounds are prepared readily from CO₂ and an epoxide. But preparing the epoxide is an extra step that could be avoided, Li explained during his ACS meeting presentation. "A simpler and cheaper approach would be the direct synthesis of cyclic carbonates from simple olefins instead of epoxides," he said. This synthesis also would contribute to a chemical industry goal of developing an inexpensive, direct route to carbonates that avoids COCl₂ or Cl₂ to make the epoxides, as well as eliminating wastes from purification steps.

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.

IN ANOTHER EXAMPLE of CO₂ utilization, Geoffrey W. Coates and his group at Cornell University have spent a decade developing catalysts to incorporate CO₂ into polymers. Two successes, building on work by other groups dating to the late 1960s, are β-diiminate zinc acetate and salen cobalt carboxylate complexes. These catalysts promote alternating copolymerization of various epoxides with CO₂ to make biodegradable aliphatic polycarbonates.

The work has been fruitful enough for Coates and former graduate student Scott D. Allen to start a company called Novomer, located in Ithaca, N.Y., to make specialty polymers using their catalysts. The polymers, which contain 30-50% CO₂ by weight, have gas-barrier and degradation properties that make them attractive for food packaging, foam-casting to make automotive parts, and electronics processing applications, Coates said. The polymers also can be used to replace propylene oxide segments in polyurethane foams, which would help cut costs. The foams are used for insulation and seat cushions, among other applications.

"About 150 million tons of plastics is produced globally in a year, and most of it is nonbiodegradable and from energy-intensive processes that use petroleum-based feedstocks," Coates told C&EN. "With sustainability a topic of growing importance, we are drawing upon naturally occurring monomers to synthesize biodegradable plastics."

In one example, Coates and coworkers devised a strategy to use limonene oxide derived from citrus fruit waste as a potential epoxide monomer for copolymerization with CO₂. Other research in progress involves developing a catalytic system that can use untreated CO₂ directly from industrial waste streams to make polymers.

Novomer is competing against existing polycarbonate producers, but the company believes its greener approach to reduce energy use, feedstock costs, and waste could help bring down production costs. Novomer currently is customizing materials for clients, such as Kodak, and working to scale up polymer production.

Coates's group also is exploring applications in carbon monoxide chemistry that have been highlighted in two recent journal papers and also are being considered for commercialization by Novomer. In one paper, the researchers describe the first example of converting simple epoxides and CO into a variety of succinic anhydrides (J. Am. Chem. Soc. 2007, 129, 4948). These one-pot reactions utilize an aluminum-cobalt catalyst to mediate a tandem double carbonylation of the epoxides. "The work has important implications for biodegradable polyesters, as succinic anhydrides are important feedstocks for those materials," Coates said.

A second paper summarizes work in Coates's group and other groups that utilizes homogeneous catalysts for reacting CO with heterocycles, such as epoxides, aziridines, lactones, and oxazolines (Chem. Commun. 2007, 657). Some of the possibilities the Cornell chemists have explored include ring-expansion carbonylations, which yield β-lactone monomers that can be used to make polyesters.

ON A DIFFERENT FRONT, several researchers at the Chicago ACS meeting described work on "artificial photosynthesis," which in the case of CO₂ involves designing photocatalyst systems that use solar energy to reduce the gas to hydrocarbons. Considering that the chemical industry isn't likely to put a serious dent in reducing CO₂ emissions by using the gas to make value-added chemicals, "an extremely attractive scenario would be to efficiently produce large quantities of methanol or hydrocarbon fuels directly from captured CO₂," noted David C. Grills of Brookhaven National Laboratory. "This approach has the potential not only to help alleviate global warming but, more important, to address the problem of our rapidly depleting fossil-fuel reserves."

In Chicago, Grills described research in collaboration with Brookhaven chemist Etsuko Fujita to improve the efficiency of homogeneous rhenium catalysts that photochemically reduce CO₂ to CO. Fujita has worked for several years exploring the mechanisms and kinetics of CO₂-to-CO reduction using a variety of metal catalysts. With these artificial photosystems, she has identified reaction intermediates by using transient and conventional spectroscopic techniques.

"Artificial, bioinspired systems are far less complicated and therefore easier to study than natural photosynthesis, in which sunlight, water, and CO₂ are converted into O₂ and carbohydrates," Grills told C&EN.

Like a number of other researchers, Fujita has found success by using rhenium tricarbonyl complexes to mediate CO₂ reduction. The researchers homed in on one set of catalysts bearing bipyridine (bpy) ligands, which includes (bpy)Re(CO)₃ and its dirhenium analog. But the reaction rates for CO production are slow due to the stability of CO₂, Grills pointed out. "They are nowhere near efficient enough to use in a practical application," he said.

One drag on the reaction rates may be the polar organic solvent, such as tetrahydrofuran, which is needed to dissolve the catalyst, Grills explained. Coordinating solvents such as tetrahydrofuran end up binding to the catalyst metal centers, blocking CO₂ molecules.

Grills, who has a background in working with supercritical CO₂ solvent systems, hit upon the idea to replace the organic solvent with supercritical CO₂. "Now, our solvent also is the reactant, and nothing gets in the way," he said. The concentration of CO₂ molecules in supercritical CO₂ can be up to 200 times higher than in CO₂-saturated organic solvents, Grills added, offering the potential to dramatically speed up reactions.

Grills and Fujita, together with collaborator Michael W. George of the University of Nottingham, in England, first had to design new rhenium tricarbonyl complexes with better solubility in supercritical CO₂, which is a nonpolar solvent with properties similar to hexane. This was accomplished by using bipyridine ligands bearing long alkyl chains.

Preliminary studies show promising results, Grills reported. The researchers are able to monitor CO₂ reduction and identify key reaction intermediates by nanosecond time-resolved infrared and UV-visible spectroscopy. (The apparatus for this work was developed at Nottingham and is shown on this week's cover.)

"We don't have a full story yet," Grills continued. The CO₂-to-CO reduction "is a good first step to work on," but the ultimate goal would be to develop a system that converts CO₂ directly to methanol or a similar compound that could be used as a fuel, he said. "That will be quite challenging."

IN ANOTHER EXAMPLE of attempting to chase after Mother Nature's methods of capturing CO₂, organometallic chemist Daniel L. DuBois of the Institute for Interfacial Catalysis at Pacific Northwest National Laboratory spoke about CO₂ reduction in electrochemical cells. Like that of other groups, DuBois' idea is to develop a catalyst capable of reducing CO₂ to useful fuels. In the case of methanol, the six electrons for the conversion could be supplied by a solar photovoltaic device and the six hydrogen ions could come from water, he said.

So far, DuBois and his colleagues have developed palladium catalysts with triphosphine ligands that reduce CO₂ to CO, but they use electrons that flow from a power outlet and hydrogen ions from an acidic solution (Organometallics 2006, 25, 3345). Because palladium is expensive, DuBois would like to switch to a nickel-based catalyst to make large-scale reduction of CO₂ feasible. He has his eye on engineering nickel catalysts that resemble the active site in carbon monoxide dehydrogenase enzymes. The research eventually will have to tackle hydrogenation of CO as well, which would lead to methanol via nickel hydride intermediates, he said.

DuBois is most interested in developing a set of predictive tools to design electrocatalysts that are capable of multiple electron and hydrogen-ion transfers. These tools would allow scientists to create a variety of catalysts that would make it possible to choose the best fuel for a given application, such as electricity generation or driving automobiles. Besides methanol, methane, or ethanol from CO₂, some other examples include hydrogen from water and ammonia from nitrogen, he said.

"If catalytic systems were designed to make these conversions during periods of excess energy production from solar, wind, or nuclear energy, chemical fuels could serve as a form of large-scale energy storage," DuBois observed. "In non-fossil-based systems of the future, it will be necessary to reversibly interconvert between these various types of fuels and electricity as needed using inexpensive catalysts."

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 "Greenhouse Gases: Mitigation & Utilization," which serves as the joint CHEMRAWN XVII conference and 9th International Conference on Carbon Dioxide. It is being sponsored by DuPont Canada and Queens University, Kingston, 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 International Union of Pure & Applied Chemistry.

The conference will take place six months before the Kyoto protocol commitment period begins. Signatory governments to the Kyoto treaty are committed to meeting their CO₂ emission targets within the 2008-12 period. The conference aims to address this question: How will this goal be achieved?

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