Few chemical names are better known to the public at large than carbon dioxide. CO2’s connection to climate change and humanity’s growing energy needs is a regular news theme in the scientific and general press alike; and for good reason. The rising level of atmospheric CO2 is widely regarded as one of the most pressing environmental concerns of our age. And devising technologies to capture and sequester CO2 to mitigate the problem is often cited as one of the 21st century’s most urgent technical challenges.
Ideas for meeting those challenges are plentiful. One set of proposals calls for capturing CO2 emitted from coal- and gas-fired electrical power plants and injecting it deep underground in various types of secure geological formations. Proponents of that approach include R. Stuart Haszeldine, a geoscientist at the University of Edinburgh who says that capturing and storing CO2 emitted by power plants has the potential to decrease global emissions of CO2 by some 20%. But achieving that level of reduction, he adds, requires surmounting numerous technological and commercial barriers. What’s more, it necessitates implementing carbon-capture technology on a worldwide scale.
“In the U.S. alone, CO2 emissions from just coal-fired power plants are on the order of 1,500 megatons per year,” asserts Chunshan Song, a specialist in catalysis and energy-related materials at Pennsylvania State University. Those kinds of numbers have motivated Song and other researchers in materials chemistry to tackle various aspects of the carbon-capture and sequestration problem.
“In the U.S. alone, CO2 emissions from just coal-fired power plants are on the order of 1,500 megatons per year.”
Some of the latest work in this area was presented at the recent American Chemical Society meeting in Anaheim, Calif., at a symposium sponsored by the Division of Fuel Chemistry and organized by Song together with colleagues at the National Energy Technology Laboratory (NETL) in Pittsburgh. Researchers are developing novel materials that are chemically customized to capture CO2. Their focus is on organic-inorganic hybrid materials, metal-organic framework (MOF) compounds, porous forms of carbon, and other CO2 sorbents that outperform the collection of materials now available to capture CO2.
None of these technologies is ready yet to compete with amine scrubbing, the most widely practiced method today for stripping CO2 from industrial gas streams. That technology, which is based on treating exhaust or flue gas with concentrated aqueous solutions of basic compounds such as monoethanolamine, has been available and practiced commercially for some applications for decades.
“The technology is well established, but it comes with a stiff energy penalty,” says Jeffrey R. Long, a professor and materials chemist at the University of California, Berkeley. The penalty refers to the large energy input required to regenerate the amine solution by heating it to drive off the absorbed CO2.
In large demonstration units, consisting of power plants fitted with CO2 scrubbers, the solution is regenerated by heating it to 100–150 °C for several hours. Engineering estimates show that regeneration saps some 30% of the power produced by the plant. In addition, the corrosive nature of the solution and its tendency to slowly volatilize and degrade, especially in the presence of oxygen and sulfurous flue gases, further drive up the cost and complexity of the operation.
Solid sorbents could help sidestep the corrosion problem, energy demands, and some of the other limitations of amine scrubbers. Researchers have examined many sorption materials as alternatives to traditional wet scrubbers, but finding a single front-runner isn’t simple, because performance depends on the specific chemical and physical environment from which CO2 needs to be captured.
For these reasons, researchers are studying materials for various carbon-capturing scenarios. In postcombustion capture, CO2 capture occurs after the fuel is burned, thereby separating the greenhouse gas from the exhaust stream, which consists mainly of nitrogen. Precombustion capture, meanwhile, aims to remove CO2 from the fuel before the fuel is burned. Here, a methane-rich fuel such as natural gas is converted to synthesis gas (or syngas), which consists mainly of carbon monoxide and hydrogen. That mixture is then reacted with steam at high temperature to produce more hydrogen fuel and CO2, which can then be captured. In this case, capture takes place in a hydrogen-rich environment.
Another process under investigation, oxyfuel combustion, burns coal, natural gas, or other fuels in a nitrogen-free environment with a recirculating stream that’s rich in CO2. This treatment leads to concentrated CO2 in the exhaust, which could simplify capture of the greenhouse gas.
A number of research groups have reasoned that combining tried-and-true amine chemistry with solid-state materials ought to do the capture job well. At Penn State, for example, Song, Xiaoliang Ma, Xiaoxing Wang, and coworkers have taken that approach over the past couple of years to develop a family of materials that they refer to as molecular basket sorbents—so called because of the materials’ microscopic structures.
In one study, the group infused 50% by weight of polyethylenimine (PEI) in a nanoporous silica material known as SBA-15. They showed that the polymer-silica product can take up 140 mg of CO2 per gram of sorbent at a CO2 partial pressure of 15 kilopascal. They attribute that high capacity, which is some 50% higher than that of materials the group synthesized previously, to a network of pores, channels, and interparticle gaps that endows the material with high surface area and facilitates efficient gas diffusion (J. Am. Chem. Soc., DOI: 10.1021/ja8074105).
Karen Uffalussy, a chemical engineer who collaborates with Götz Veser and others at the University of Pittsburgh, also makes hybrids of a polyimine and silica, but with a twist. She explains that the combination of nickel clusters and a silica precursor in her synthesis procedure results in nanometer-sized hollow silica spheres—nanobubbles—that are laced with angstrom-sized pores. Treating the particles with PEI distributes the polymer throughout the nanobubbles, thereby yielding a sorbent that, according to preliminary studies, can capture 125 mg of CO2 per gram of sorbent.
Details of the bubble formation mechanism are still unclear, Uffalussy says, but the structure of the products is controllable nonetheless. She aims to use that synthetic control to identify key structural parameters that affect CO2 capture by na noencapsulated amines.
If treating hollow silica spheres with one polymer featuring amine functionality works well, combining two such polymers in a hollow particle might work even better. In a recent study, Cornell University materials scientist Emmanuel P. Giannelis, together with Christopher W. Jones at Georgia Institute of Technology and coworkers, showed that such a strategy has merit. By using latex particles as microscopic templates, the team formed hollow silica capsules and loaded them with a combination of PEI and tetraethylenepentamine.
Under various test conditions in simulated flue gas, the novel particles exhibit fast capture kinetics and exceptionally high capacity—nearly 350 mg of CO2 per gram of sorbent, the team found. The team reports that the sorbent is readily regenerated at temperatures below 100 °C. They add that some of the samples described in their recently published study function with minimal loss of CO2 capacity even after 50 adsorption-desorption cycles (Energy Environ. Sci., DOI: 10.1039/c0ee00213e).
Jones’s team at Georgia Tech has also pursued another way to incorporate amines in silica supports such as SBA-15. Rather than physically impregnating the porous solid with a polymer, Jones, Praveen Bollini, Jason C. Hicks (now a chemical engineering professor at the University of Notre Dame), and coworkers do so chemically, through a one-step ring-opening polymerization of the three-membered heterocycle aziridine in the presence of SBA-15. The procedure leads to hyperbranched aminosilica adsorbents with the amino polymers covalently bound to silica.
In an early application of the in situ polymerization method, Jones’s group prepared materials that were shown to have a capacity of about 136 mg CO2 per gram of sorbent in simulated flue gas conditions (J. Am. Chem. Soc., DOI: 10.1021/ja077795v). In a follow-up study of a large number of samples prepared similarly, Jones’s group together with NETL researchers found that some of them, depending on polymer loading and other variables, adsorbed up to 242 mg CO2 per gram of sorbent (Adv. Funct. Mater., DOI: 10.1002/adfm.200901461).
Much of the carbon-capture discussion focuses on separating CO2 from flue gas, where it’s present at a concentration of roughly 10%. But about one-third of global carbon emissions are associated with “distributed sources” such as automobiles, Jones points out. That’s one reason why he and others have turned their attention to sorbents that could grab CO2 from air, where it’s diluted to just a few hundred parts per million. In addition, technology to capture CO2 from air could enable the food and oil industries and other major users of CO2 to avoid the cost of transporting CO2 by permitting them to capture it and use it on-site.
In a study published earlier this year in Environmental Science & Technology, Jones and coworkers compared CO2 uptake and regeneration of their hyperbranched aminosilica materials in tests with two types of gas mixtures, one containing 10%, the other 400 ppm CO2. They report that the sorbent’s performance is “only marginally influenced” by the large difference in CO2 concentration (Environ. Sci. Technol., DOI: 10.1021/es102797w).
MOF compounds are another class of solid sorbents widely investigated for carbon-capture applications. These compounds are crystalline materials composed of metal ions or clusters that are connected by organic linkers. Because MOFs can be tuned so broadly by tweaking various chemical handles—for example pore sizes can be adjusted by modifying the organic units—research groups such as Omar M. Yaghi’s at the University of California, Los Angeles, have been able to prepare dozens of MOFs, some of which claim record-setting CO2-uptake capacities.
Meanwhile, Zoey R. Herm, Kenji Sumida, and other members of Long’s UC Berkeley group have been designing MOFs that are tailor-made to tackle various aspects of the carbon-capture challenge. Long’s group just described a strategy for tuning MOFs to adsorb CO2 with high selectivity in the presence of hydrogen, a key requirement for precombustion CO2 capture (J. Am. Chem. Soc., DOI: 10.1021/ja111411q).
The trick, which they demonstrated by comparing the performance of MOFs known as Cu-BTTri and Mg-DOBDC with that of other high-surface-area MOFs, is to incorporate a high concentration of exposed metal cation sites. That property, Long explains, favors uptake of CO2 over hydrogen because of CO2’s enhanced po larizability.
Much the same strategy led Long’s group to prepare the first Cr(II)-based MOF, Cr3(BTC)2, which selectively adsorbs oxygen in high capacity in the presence of nitrogen (J. Am. Chem. Soc., DOI: 10.1021/ja1027925). That feature could help spur development of technology for oxyfuel combustion.
Regardless of chemical composition, sorbents will need to be affordable to be adopted widely. That requirement is driving researchers to examine low-cost starting materials, including various forms of carbon. Song’s group for example, recently showed that molecular basket compounds prepared from commercial carbon blacks and PEI have a CO2 capacity that nearly matches the group’s more expensive SBA-15 samples (Energy Fuels, DOI: 10.1021/ef101364c). Similarly, Marta Sevilla and Antonio B. Fuertes of Spain’s National Institute of Coal, in Oviedo, just reported that cellulose and sawdust can be used to make activated carbon with record-setting CO2 uptake (211 mg per gram) (Energy Environ. Sci., DOI: 10.1039/c0ee00784f).
There’s no shortage of ideas for capturing carbon. Even a weeklong session such as the one in Anaheim is hardly broad enough to touch on all of the relevant proposals. But as Edinburgh’s Haszeldine and other like-minded scientists point out, government action mandating limits to CO2 emissions has barely started. If carbon capture and storage is to play a large role in limiting climate change, he says, urgent action is required now.