CLICK ON GRAPHIC TO ENLARGE     Fig. 2. General modeling scheme used for screening solid CO₂.

One of the major downsides of using fossil fuel combustion to produce electricity is the release of carbon dioxide (CO₂) into the atmosphere. CO₂ is a greenhouse gas with potentially severe implications for global warming. The use of carbon-free renewable energy sources can help reduce the amount of CO₂ released into the atmosphere. However, the continuous increase in demand for electricity cannot be currently accommodated based exclusively on renewable sources, and so the use of fossil fuels is expected to continue for decades. Consequently, it is imperative that we develop new technologies to capture CO₂ emissions.

There is a critical need to develop new materials that can capture and release CO₂ at reasonable energy and operating costs. Traditional solvent-based systems consume too much energy, either in operation or during regeneration of the solvents. One new method being considered at NETL is based on the use of regenerable solid sorbents. In this case, sorbents such as alkaline earth metal oxides or hydroxides are used to absorb CO₂ at temperatures that typically range from about 100-300 °C. The key phenomenon in these processes is transformation of the oxide or hydroxide materials to a carbonate after CO₂ absorption. The sorbent can be regenerated, if necessary, in a subsequent step, by the reverse transformation from the carbonate phase back to the oxide or hydroxide phases. The efficiencies of these processes are highly dependent on the optimum temperature and pressure conditions at which absorption and regeneration are performed. In the case of high-performance sorbents, both of these mechanistic steps occur with the lowest possible energetic and operational costs.

The most likely candidates, from the numerous possible sorbent materials, can be selected by starting with an analysis of each material’s intrinsic atomistic structure and the transformations that occur after its interaction with CO₂. It is particularly important to identify the corresponding thermodynamic and kinetic characteristics of the sorbent material of interest. For this purpose, scientists at NETL have developed a multi-step computational methodology, based on combined use of first principles calculations and the vibrational properties of each material, to describe the thermodynamic properties of CO₂ capture reactions by solid sorbents. This methodology has been used to screen different classes of solid compounds and identify optimum candidate materials, which are then subjected to experimental testing. This novel computational method, co-authored by Dr. Yuhua Duan and Dr. Dan C. Sorescu, is described in several recent publications (Journal of Chemical Physics 2010, vol. 133, 074508 and Physical Review B 2009, vol. 79, 014301). The advantage of this method is that it identifies the thermodynamic properties of the CO₂ capture reaction as a function of temperature and pressure without any experimental input beyond crystallographic structural information of the solid phases involved. The thermodynamic information then guides experimental groups at NETL in development of highly optimized CO₂ sorbents.

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Using Ionic Liquids to Capture Carbon Dioxide

CLICK ON GRAPHIC TO ENLARGE     Fig. 3. Molecular configurations for complexes formed between CO₂ and representative anions used in ionic liquids. Based on ab initio methods, the geometry of each complex has been determined as well as the binding energy between the anion and the CO₂ molecule. The binding energies are given (in kcal/mol) relative to the most strongly-bound complex shown in the bottom left panel, the complex in which the CO₂ molecule interacts with both oxygen atoms of the acetate ion.

Another method that is being considered for CO₂ capture is based on the use of supported ionic liquid (IL) membranes. Ionic liquids are a unique class of compounds; they are organic salts that, in many ways, are similar to common inorganic table salt (NaCl). The reason these ionic salts are liquid at room temperature lies in the shape of their ions. The sodium (Na) and chlorine (Cl) that make up table salt are small, spherical ions that stack very efficiently into cubic crystal structures, so that they only melt at high temperatures. But unlike table salt, ionic liquids contain large organic ions with much more complicated shapes that cannot stack efficiently, so they cannot easily form crystalline solids except at very low temperatures. Ionic liquids have many desirable properties, such as low vapor pressure and thermal stability. They can be designed to have a wide variety of gas sorption properties, including high CO₂ solubility, which could form the basis for low-cost CO₂ separation applications.
 
Computational scientists at NETL are carrying out high-level ab initio calculations to characterize the nature of the interactions that underlie the solubility of CO₂ in ILs. Because there are so many possible ion pairings capable of forming ionic liquids, computational methods are being used to identify which ionic liquids possess the best properties for CO₂ removal. These calculations quantify the strength of the interactions between the CO₂ and a series of ions, and describe the geometry of low-energy CO₂-ion complexes, allowing us to better understand the types of interaction between CO₂ and the ions.

We have shown that gas-phase ab initio data on an anion-CO₂ pair can yield predictive information about the experimentally determined solubility of CO₂ in ILs. These studies examined the interaction of CO₂ with a series of anions including: acetate [AC], tetrafluoroborate [BF₄], formate [FOR], iminoacetic acid acetate [IAAC], isobutyrate [ISB], levulinate [LEV], nitrate [NO₃], hexafluorophosphate [PF₆], proprionate [PRO], succinamate [SUC], bis(trifluoromethylsulfonyl)imide [TF₂N], trifluoroacetate [TFA], tetrafluoroethanesulfonate [TFES], and trimethylacetate [TMA].

In a recent study that will be published in Journal of Physical Chemistry, Dr. Jan Steckel examined the interaction of CO₂ with the cations 1-butyl-3-methylimidazolium [BMIM] and 1-ethyl-3-methylimidazolium [EMIM]. Her calculations showed that these anion-CO₂ compounds have significant induction effects and charge-transfer capabilities. The results obtained so far indicate that these ionic liquids show promise for high CO₂ solubilities.

Another advantage of computational chemistry is the possibility of developing new concepts of materials or processes that can then be experimentally tested and validated. Synthesis and testing of ionic liquids (IL) containing different combinations of functional chemical groups has represented one way of developing supported ionic liquid membranes for CO₂ capture in precombustion processes. However, novel properties of a given ionic liquid also can be obtained and tuned if the liquid is placed in an environment that significantly modifies its structural bulk properties.

CLICK ON GRAPHIC TO ENLARGE     Fig.4 Representative snapshots of the molecular configurations for 10, 20, 40, and 60 pairs of [hmim][Tf₂N] ionic liquid confined in a (20,20) carbon nanotube. The (a) and (b) set of configurations are viewed perpendicular to and along the carbon nanotube axis, respectively.   Such a case is represented by confinement of ILs in nanostructured materials. The confinement in this case has a major impact on diverse types of molecular properties, ranging from modification of the melting point to enhanced solubility of absorbed gases. Thus, the confinement effect is equivalent to synthesizing a new type of IL with properties different from those of the same compound in bulk phase.

In an effort to shed insight into the new opportunities offered by ILs when confined in nanostructured materials, computational chemists at NETL have determined the structural, energetic, and transport properties of ILs confined in carbon nanotubes (CNTs). They have found that IL molecules in a CNT exhibit self-diffusivity coefficients that are roughly 10 – 100 times larger than corresponding bulk IL molecules. Additionally, the composite IL-CNT material exhibits higher sorption selectivity for CO₂/H₂ than either the individual IL or CNT systems. These new theoretical findings, which were recently published in the Journal of Physical Chemistry (W. Shi and D. C. Sorescu, J. Phys. Chem. B 2010, 114, 15029), open new avenues for experimental analysis. It is anticipated that combination of nanoporous materials and ionic liquids can lead to new classes of materials with properties superior to individual components. The theoretical scientists are eager to see if experiments validate their predictions.