273358 Development of Reaxff Force Field for Carbon Dioxide Capture with Ionic Liquids: A Combined First Principles and Classical Simulation

Thursday, November 1, 2012: 5:15 PM
301 (Convention Center )
Bo Zhang, US DOE-National Energy Technology Laboratory, Pittsburgh, PA, Adri C.T. van Duin, Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA and Karl Johnson, Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA

Ionic liquid (IL) are salts in the liquid state below 100°C. The number of ions that can form IL is well over tens of thousands. ILs are considered as promising materials for CO2 capture due to their wide liquid range, negligible vapor pressure, thermal and chemical stability, and the ability to tailor the interaction with CO2 through choosing appropriate ions. We are exploring ILs that can capture CO2 through chemical reactions and physical interactions via first principles and classical simulation techniques. Our goals are to understand the mechanism of IL and CO­­2 interaction, to develop a new force field that will capture both the physical interactions and chemical reactions, and to calculate the thermodynamic and transport properties of bulk and confined IL with CO­2 via large-scale simulations. These simulations will be used to accelerate the design of new ILs that have tailored interactions and acceptable viscosities for pre-combustion CO2 capture processes. We use the ReaxFF formalism for describing the physical and chemical interactions in the IL-CO2 system. ReaxFF has been proven to be a very successful formalism for modeling chemical as well as physical interactions with an empirical approach. [P(C44][Gly] is chosen as a model IL to elucidate the interactions between ILs and CO2. Two possible interaction sites have been identified with first principles molecular dynamics (MD) simulations. One is the –NH2 center and the other one is the –CO2- center, both on the [Gly] anion. The calculated binding energy and CO2 angle distribution indicate that the interaction with the amine center is a chemical type interaction and the interaction with carboxylic group appears to be a complicated weak chemical interaction. The reaction between CO2 and –NH2 has been identified as a two-step reaction mechanism. The first step involves a proton transfer with barrier height of about 1 kcal/mol, and the second step is mainly a libration motion, also leading to a proton transfer, with barrier height around 2.3 kcal/mol. The reaction pathways along with other data, including bond-energies, angle-energies, torsion angle-energies have been compiled into a training set to parameterize the ReaxFF force field. The force field is optimized against the calculated structure-energy relationships. With the optimized force field, the model IL [P(C4)4][Gly] interacting with CO2 has been studied through large scale MD simulations. The thermodynamic and transport properties have been computed.

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