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Summer 2015

Computational Design: Taking on Methane

Molecular modeling to predict new catalysts capable of repurposing methane as a liquid fuel

S. Garrett Williams

Methane flaring lights up the night sky at an oil field in Colorado. Methane is such a potent greenhouse gas that it is more environmentally conscious to burn this compound than allow it to escape into the atmosphere.

Rhodium-based catalysts investigated by the scientists at the CCHF for their methane conversion capabilities.

Methane is a potent greenhouse gas and a powerful fuel. Scientists have endeavored to utilize this molecule because of its abundance and potential as a fuel source. Methane, however, is quite stubborn. It possesses inert carbon-hydrogen bonds as well as a propensity for oxidation to an equally frustrating molecule, carbon dioxide. To overcome this challenge, catalysts that utilize transition metals have been explored for their methane conversion capabilities. Recently, scientists at the Center for Catalytic Hydrocarbon Functionalization (CCHF) using computation and theory have designed novel rhodium-based catalysts that convert methane to methanol. These catalysts have been synthesized and tested experimentally, validating this breakthrough in catalyst development.

Methane’s low carbon emission, relative to other hydrocarbons, makes it a promising fuel. However, its gaseous form makes it difficult and potentially dangerous to store as well as expensive to transport. In addition, methane is a potent greenhouse gas; approximately 21 times more efficient at capturing thermal energy than carbon dioxide. Many oil and gas companies therefore choose to flare, or set ablaze, surplus or uncaptured methane. This is a tremendous waste of energy and capital.

"More than 35 percent of the gas from the Bakken shale formation is flared unproductively with a recent estimate indicating that nearly $100 million of natural gas are flared each month in North Dakota," said T. Brent Gunnoe, Director of the CCHF.

The catalytic conversion of surplus methane to methanol at moderate temperatures and pressures is a promising solution to this conundrum. Methanol is an attractive target because it is a liquid at standard conditions making it easier to store, transport, and use. Further, methanol can be used in flex fuel vehicles, blended with gasoline, or converted to diesel fuel. It is a versatile precursor for important chemical feedstocks. Methane conversion to methanol, however, is extremely difficult. Scientists and engineers have been searching for 30 years to find a successful catalyst.

To confront this challenge, research scientists at the CCHF from the University of Virginia (Gunnoe research group) and the California Institute of Technology (research group of William Goddard III) used quantum mechanical computational studies to design a catalyst predicted to surgically cleave carbon-hydrogen bonds while holding the products together, preventing carbon dioxide formation. Trifluoroacetic acid (TFA) is subsequently formed and is easily converted to methanol. In Chemistry - A European Journal, the scientists report comparative investigation of five rhodium bis(quinolinyl)benzene compounds predicted to be good catalysts for converting methane into TFA.

Using quantum mechanics (density functional theory or DFT), the investigators evaluated their five rhodium catalysts against three distinct catalytic pathways at two different temperatures (298 K and 498 K). DFT is a computational technique that allows investigators to calculate the free energy of a molecule. Using energies determined via DFT, the scientists determined which of the complexes is most likely to perform the catalysis effectively using each of the possible mechanisms. In short, evaluating the energy of a complex as it cycles through a catalytic cycle allows researchers to identify the anticipated energy barriers.

In this research, they found that the simplest of the metal binding molecules, bis(quinolinyl) benzene, would likely perform the catalysis most effectively. While the more complex variants showed promise at specific steps in the catalytic cycle, the total energetic costs were higher overall.

Currently, the scientists of the CCHF are synthesizing the rhodium complexes to confirm their findings.

More Information

R Fu, ME O'Reilly, RJ Nielsen, WA Goddard III, and TB Gunnoe. 2015. "Rhodium Bis(quinolinyl)benzene Complexes for Methane Activation and Functionalization." Chemistry - A European Journal 21(3):1286-1293. DOI: 10.1002/chem.201405460

Acknowledgments

This work was solely supported as part of the Center for Catalytic Hydrocarbon Functionalization, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. YDE-SC0001298.

About the author(s):

Unadorned Catalyst Has Flare for Making Methanol

Calculations show best option for turning surplus methane gas into storable liquid

Currently, methane is burned off or flared at oil wells. Converting it to methanol is an attractive option as methanol is easier to store, transport, and use. Further, methanol can be used in flex fuel vehicles, blended with gasoline, or converted to diesel fuel.

Flares light up the Colorado night sky as methane gas is burnt off at oil wells. What if that methane was instead turned into an easier-to-transport liquid fuel? One that added to the energy economy? Scientists at the Center for Catalytic Hydrocarbon Functionalization, or CCHF, examined the mechanics of five catalysts that drive the methane-to-methanol conversion. The catalysts are four different takes on a simpler complex using the metal rhodium. Complicated calculations showed that the fastest catalyst, the one that most easily overcomes the natural energy barriers, is the basic, unadorned catalyst. With these results, scientists are now building the best options in the laboratory to delve into the details. The CCHF, part of the first set of 46 Energy Frontier Research Centers, was led by the University of Virginia.

More Information

R Fu, ME O'Reilly, RJ Nielsen, WA Goddard III, and TB Gunnoe. 2015. "Rhodium Bis(quinolinyl)benzene Complexes for Methane Activation and Functionalization." Chemistry - A European Journal 21(3):1286-1293. DOI: 10.1002/chem.201405460

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.