Molecular modeling of CO2 capture by an ionic liquid membrane. A typical fossil-fuel plant (top left) produces significant CO2 and other greenhouse gas emissions. A membrane based on ionic liquids may absorb such emissions. (LNNL)
Separating carbon dioxide from its polluting source - such as the flue gas from a coal-fired power plant - may soon become cleaner and more efficient thanks to a screening method developed by a Lawrence Livermore National Laboratory researcher.

The new screening method would use ionic liquids - a special type of molten salt that becomes liquid under the boiling point of water - to separate carbon dioxide from its source, making it a cleaner, more viable and stable method than what is currently available.

Currently, the few coal plants with commercial CO2 capture capability use processes based on chemical absorption with monoethanolamine (MEA), which is non-selective, corrosive, and effective only under low to moderate partial pressures of CO2. Using ionic liquids as a separation solvent has unique advantages over traditional solvents, including high chemical stability, low corrosion, almost zero vapor pressure, and many ion choices - which can be potentially optimized for CO2 solubility.

LLNL scientist Amitesh Maiti's work involved devising a computational strategy that can reliably screen any solvent, including an ionic liquid, for high CO2 capture efficiency. "By creating a computational tool that can decipher ahead of time which ionic liquids work best to separate CO2, it can be a much more efficient process when field tests are conducted,” said Maiti.

Maiti developed a quantum-chemistry-based thermodynamic approach to compute the chemical potential of a solute (CO2 in this case) in any solvent at an arbitrary dilution. He found that this result, coupled with an experimentally fitted equation-of-state data for CO2, can yield accurate solubility values in a large number of solvents, including ionic liquids.

Next, he used this method to predict new solvent classes that would possess CO2 solubility nearly two times as high as the most efficient solvents experimentally demonstrated. Maiti's hope is that the accuracy of the computational method will allow scientists to see useful trends, which could potentially lead to the discovery of practical solvents with significantly higher CO2 capture efficiency.