While gaseous carbon dioxide has been a harmful byproduct of human industry—it is the main greenhouse gas emitted through human activities, according to the Environmental Protection Agency—it is an essential ingredient for plant life. Artificially fixing carbon to use as an energy source, by converting carbon dioxide into liquid fuel, could not only provide power but could also cut carbon dioxide emissions and therefore help reduce the effects of global warming.
Carbon dioxide emissions potentially provide 1,570 terawatt-hours worth of electricity annually—almost 400,000 times as much power as the Hoover Dam’s annual 4 billion kilowatt-hours—according to a July 23, 2013 Environmental Science & Technology Letters report titled “Harvesting Energy from CO2 Emissions.”
Utilizing carbon dioxide as an energy resource would not only provide a greener alternative to heavily polluting fossil fuels (which often contain heavy metals and smog-reactive compounds), but could provide a sort of carbon-neutral loop that could ease the release of excess carbon dioxide in the atmosphere that results from burning those fossil fuels and contributes to climate change.
Plants use carbon dioxide as the basis for producing sugars, structural building blocks from which to grow and reproduce, in a process called carbon fixation. The most prominent form of this chemical reaction, which involves sunlight and water, is photosynthesis—a process children are introduced to in elementary school. Here’s a visual refresher:
 |
| Image Credit: Wiki user Alt09kg, CC BY-SA 3.0 |
Humans, however, have found it difficult to cheaply and easily replicate the process that comes naturally to plants; carbon dioxide’s stable nature makes it difficult to cause it to react. Therefore, large amounts of energy are typically necessary to break the molecule apart. So far, artificial methods of fixing carbon dioxide have proven inefficient and costly. A common method involves removing one of carbon dioxide’s oxygen atoms and then fusing the remaining carbon monoxide molecule with a dihydrogen molecule to create methanol. However, parts of this process require heat as high as 1,000 degrees Celsius.
An effective catalyst, however, would lower the activation temperature and therefore ease the reaction and likely the prospective cost of the process.
Now researchers at the University of Pittsburgh in Pittsburgh, Penn., have found the two primary factors to determine optimum catalysts for converting carbon dioxide into liquid fuel. The study, “Screening Lewis Pair Moieties for Catalytic Hydrogenation of CO2 in Functionalized UiO-66,” was published in the journal ACS Catalysis.
“We’re trying to speed up the natural carbon cycle and make it more efficient,” Karl Johnson, the study’s principal investigator and the William Kepler Whiteford Professor in the Department of Chemical & Petroleum Engineering at the University of Pittsburgh, said in a press release. “You don’t have to waste energy on all the extra baggage it takes to grow plants, and the result is a man-made carbon cycle that produces liquid fuel.” It’s a tall order to use a first-principles approach and expect to outdo the efficiency of a system that’s been subject to millions of years of natural selection pressure, but the hope is that computer-aided simulations will help them find the tools they’re looking for much faster than the trial-and-error that physical systems must adhere to.
Johnson and co-author, postdoc Jingyun Ye, simulated the reactions of eight common catalysts. The catalysts used comprised the functional groups, or reactive components, of eight Lewis acid and base pairs.
Now, Brønsted-Lowry theory defines acids and bases by proton exchange, giving rise to the familiar “pH” notation, based on the molecular concentration of hydrogen ions (i.e. free protons) in solution. Unlike Brønsted-Lowry theory, the Lewis theory considers electron movement: Lewis bases donate electron pairs and Lewis acids accept electron pairs. While a Lewis base is by definition a Brønsted–Lowry base, able to accept protons by virtue of its excess electrons, a Lewis acid isn’t always a Brønsted–Lowry acid. This helps expand the definition of acids and better explains what’s seen experimentally.
 |
| Image Credit: The Chemogenesis Web Book |
The University of Pittsburgh team identified two key factors that characterize ideal catalysts for carbon fixing: low-energy dihydrogen adsorption—meaning an H2 molecule can stick to it with ease—and a high “hardness” to the Lewis acid portion. Hard acids and bases tend to be non-polarizable, and smaller in size than soft acids and bases, which have larger ionic radii and are highly polarizable. Technically, hardness describes the reaction mechanisms and stability of Lewis acids and bases, but as a general rule, soft acids react faster and bind more strongly with soft bases while hard acids react faster and form stronger bonds with hard bases, but don’t do so exclusively.
 |
| Elemental Lewis acids, arranged periodically and color-coded with respect to hardness Image Credit: Wiki user Tem5psu CC BY-SA 3.0 |
 |
| Elemental Lewis bases, also arranged periodically and color-coded with respect to hardness. Image Credit: Wiki user Tem5psu CC BY-SA 3.0 |
As a theorist, Johnson now plans to work with experimental researchers to better elucidate and the implications of this work and eventually realize an artificial method of utilizing carbon dioxide as a fuel source.
-Clinton Parks