(772f) Development of Reaxff Force Field for Carbon Dioxide Capture with Ionic Liquids: A Combined First Principles and Classical Simulation

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(C₄)­₄][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.