Joan F. Brennecke¹, Brandon L. Ashfeld², Edward J. Maginn¹, William F. Schneider¹, ² and Mark A. Stadtherr¹, ¹Department of Chemical and Biomolecular Engineering, ²Department of Chemistry and Biochemistry, University of Notre Dame

This research will follow a systematic approach to the design and development of novel ionic liquids for the separation of carbon dioxide (CO₂) from pre-combustion gases with the overall goal of mitigating CO₂ emissions from gasification-based power plants. This project seeks to increase the capacity for CO₂ capture well above the level achieved thus far in conventional post-combustion IL systems.

In pre-combustion capture, coal, biomass or other fuel is processed in a gasifier and converted to a synthetic gas (syngas) consisting primarily of hydrogen and carbon monoxide (CO).  The CO can be converted to CO₂ and more hydrogen by adding water and taking advantage of the water-gas-shift reaction. Chemical impurities and CO₂ are removed from hydrogen prior to its use as a clean-burning fuel. This process has the potential of capturing and removing 90 percent of CO₂ emissions from power plants - major contributors to global warming.

This project explores the use of ionic liquids (ILs) in pre-combustion capture.  ILs are salts with low melting points, wide liquid-phase operating ranges and endless tunability. While ILs for post-combustion CO₂ capture are a popular research topic, less attention has been focused on ILs for pre-combustion systems. To our knowledge, ILs appropriate for pre-combustion CO₂ separation do not currently exist.

IL-absorbent materials have many advantages over conventional pre-combustion CO₂ capture technologies, including longer lifetimes and higher stability over a range of process conditions. Conventional pre-combustion capture relies on physical solvents, such as Rectisol (chilled methanol). However, ILs can operate at higher temperatures than Rectisol. Thus, a major advantage of ILs would likely be reduced energy consumption, since neither the absorption solvent nor the pre-combustion gases would have to be chilled. In addition, ILs require no added water as a diluent or carrier. 

ILs can also be tuned in a variety of ways, such as altering the structure and components of positive and negative ions, or by including an additive that changes the process chemistry. This flexibility offers the potential of tuning a given absorption process to specific gas conditions and compositions at individual power plants.

Approach

Absorbent materials for pre-combustion CO₂ capture must be able to (1) separate CO₂ from hydrogen; (2) take advantage of the much higher total and partial pressures of CO₂; and (3) operate at high temperatures.

The goal of this project is to design materials that can capture and release CO₂ using temperature or pressure swings. The fundamental approach will be to develop, synthesize and test pre-combustion ILs using computational property predictions, and systems analysis and life cycle analysis modeling (see Figure 1).  This model-driven development framework will lead to new materials with the desired selectivity that will enable tuning of the absorption capacity for pre-combustion CO₂ separation. 

Figure 1: New ionic liquids (ILs) will be synthesized through computational property predictions and system and lifecycle analysis modeling.

This approach should yield three novel types of ILs for pre-combustion CO₂ capture:

1. AHA (aprotic heterocyclic anion) ILs: Modifications to an existing AHA-based IL platform may lead to materials with desirable viscosity. The investigators will develop new AHA ILs that complete CO₂ separation in the desired performance range by varying the positive and negative ionic components of the platform.  The ideal absorbents for pre-combustion carbon capture should result in weak specific binding – that is, the formation of relatively weak complexes with CO₂ compared to post-combustion capture.
   
   
2. ILs featuring structural cooperativity: Researchers will develop two new classes of ILs designed to absorb cooperatively, thereby increasing their capacity for CO₂ capture while consuming less energy. One effort will focus on designing an IL with multiple, chemically coupled binding sites in which CO₂ binds to the first site through a relatively weak interaction, then activates other sites that facilitate stronger binding of additional CO₂ molecules - a technique called “structural cooperativity.”
   
   
3. ILs featuring physical cooperativity: Another approach will focus on the physical cooperativity of the IL. The formation of a CO₂-AHA IL complex is an equilibrium reaction. Under certain conditions, the formation of one complex will drive the formation of a new complex in a cooperative fashion.

Understanding and controlling these cooperative reactions will play a key role in developing a viable pre-combustion IL technology capable of capturing 90 percent of CO₂ emissions from power plants with low operating and capital costs.