Ionic Liquids

Properties and Synthesis of Ionic Liquids

An ionic liquid is a salt which exists in the liquid state at room temperature. Most liquids are made up of electrically neutral molecules. Ionic liquids are made instead of ion pairs, with one or both of the ions being bulky and presenting an obstacle to crystal formation. As a result, ionic liquids have unique properties that make them desirable for many chemical applications: low vapor pressure, non-flammability, a wide liquid temperature range, a large electrochemical window, and high thermal stability. Further, it is possible, by manipulating the composition of ionic liquids, to modify the materials for specific functions. Ionic liquids offer nearly infinite capacity for this type of manipulation because of the huge variety of available structures; there are 1018 ionic liquids that can be accessed.

Gases such as CO₂ are absorbed in ILs through diverse mechanisms such as physical absorption, chemical reaction, and formation of molecular complexes. In the case of physical absorption, the CO₂ fills the available free molar volume of the IL and interacts preferentially with the anion rather than the cation (pronounced cat i on). The CO₂ solubility increases for cations with a longer alkyl side chain and decreases at elevated temperatures.

Investigations of regioisomeric ionic liquids were undertaken to give insight into the structure-property relationship of these important materials. For a set of triazolium-based ILs with a common cation, NETL researchers studied the effect of substituents and the effect of regioisomerization on thermal stability, viscosity, and the physical state associated with the position of the substituents, as well as CO₂ solubility and inductive effects.

This research underscores the importance of structural configuration and its effect on the properties of ionic liquids. It is clear from previously published data and from NETL’s own research, that the choices of cations, the substituents on the cation, and the anion can enhance or suppress the properties of ILs.

Ionic Liquids for Capturing Carbon Dioxide

Properties such as low vapor pressure, non-flammability, a wide liquid temperature range, a large electrochemical window, and high thermal stability are attractive characteristics that have promoted IL use in various applications including synthesis, as both solvents and catalysts; energy storage, as electrolytes; extraction of radioactive materials, metals, and organic liquids; polymer processing; and gas separation.

Currently, there is a high demand to reduce the amount of carbon dioxide (CO₂) released into the atmosphere to alleviate the greenhouse effect. In the United States, greenhouse gas emissions are primarily associated with fossil fuel combustion for energy production. Energy-related CO₂ emissions represented about 82% of the total U.S. anthropogenic greenhouse gas emissions in 2006. Goals put forth by the Department of Energy (DOE) require 90% CO₂ capture at an increase in the cost of electricity of less than 35% for post-combustion capture and less than 10% for pre-combustion capture. One method that could be used for CO₂ capture in the pre-combustion processes is based on the use of supported ionic liquid membranes (SILMs).

Interactions of ionic liquid with a carbon nanotube.

A quality of particular interest to NETL is the ability of tailored ionic liquids to selectively absorb gases. Ionic liquids containing structures that can function as bases can make the ionic liquid a good solvent for CO₂. Specifically, many ionic liquids have high CO₂ solubility in conjunction with low solubility for H2, N2 and CH4, or a tendency to absorb carbon dioxide but not the other gases. This feature makes them ideal for use in carbon capture technologies.

Traditionally, gas absorption, or scrubbing, is a process through which a gas stream is brought into contact with a liquid in order to dissolve a component of that gas mixture. In carbon capture, flue gas from coal burning power plants would be brought into contact with an ionic liquid. The ionic liquid would then absorb the CO₂, removing it from the gas stream, and be put through a desorption process. This process would allow the ionic liquid to be recycled back into the scrubbing cycle.

Novel Materials for CO₂ Separation

One version of an Integrated Gasified Combined Cycle plant--technology that uses a gasifier to turn coal and other carbon based fuels into synthesis gas (syngas)--gasifies coal and then employs the water-gas shift reaction to produce a high pressure process stream that is rich in CO₂ and H2. It would be advantageous to process this mixture so that it simultaneously yields a high pressure CO₂ stream for geologic sequestration and a high pressure hydrogen stream for generating electricity. Commercial separation technologies, such as membranes, pressure swing adsorption (PSA), and gas absorption columns, have the ability to produce high pressure CO₂ or high pressure H2, but not both. The goal of this work is to determine the ability of novel CO₂-philic materials to be used in a separation process that yields a high pressure hydrogen stream and a high pressure CO₂ stream.

Hollow fiber ionic liquid membranes for gas separation.

In addition to utilizing ionic liquids as conventional absorbents, it is possible to immobilize an ionic liquid on a support for use in a membrane. A membrane is a barrier that induces separation through selective permeation. In other words, membranes allow some gases to pass through them while others are excluded. Ionic liquid membranes can be effectively employed in carbon capture, acting as semi-permeable barriers that allow CO₂ to leave gas streams and retain the other components. The major challenge to large scale use of membranes in industrial applications is stability. In order to be effective, a liquid membrane must be stable; should the ionic liquid become displaced or the membranes become ruptured, the gas separation will fail. Research at NETL has led to the development of novel supports, which address stability issues, for use in combination with tailored ionic liquids.

Advantages of supported ionic liquid membranes (SILMs) include the ability to customize membranes for a variety of separations. However, the number of combinations can be staggering. Additionally, membrane technologies bring challenges such as having to survive large trans-membrane pressure differences and fabricating thin liquid layers, which require development of new techniques. NETL is overcoming these challenges by using molecular modeling and chemical informatics to narrow the candidates for practical SILMs by developing an integrated technology development approach through multi-disciplinary collaboration and using new synthesis and characterization methods to improve technology development efficiency.

Contact: David Luebke