Subject Number : S-19-MS-0011
Support Type : Collaborative research
Proposal Title (English) : A Density functional theory insight towards the design of ionic liquids for CO2
capture and conversion
Username (English) : Karan Bobuatong, Jitlada Kodchasee
Affiliation (English) : Rajamangala University of Technology Thanyaburi, Thailand

1. Summary
Ionic liquids (ILs) are promising green solvents that have attracted attention from the technological communities for CO2 capture from various waste gases. Thus, CO2 interaction energies over a set of 250 guanidinium and imidazolium based ILs were predicted using the DFT in order to figure out the key factors that affect the absorption of CO2. The results indicated that anions in ILs play an important role for the adsorption. To improve the absorption capability, the calculations on a series of the functionalized anions with various electron donating groups were performed. The ether group (-OR) was found to have a significant improvement towards the absorption ability of CO2 over guanidinium and imidazolium based ILs. On the other hand, functionalized cations doesn’t affect the absorption energies for CO2 absorption on guanidinium and imidazolium based ILs
2. Computational method
All DFT calculations were performed using the Gaussian 09 suite of programs. [1] DFT calculations using the SMD continuum solvation model in combination with the M06-2X density functional and the 6-31++G(d,p) basis set were performed. The geometries of the. The minimum energy for the reactant and product will also be confirmed using positive vibrational frequencies.
3. Results ad Discussion
As a first approximation, CO2 capture at the molecule level could be related with the strength of the interactions between the ions and the CO2 molecule. In this work, the interaction strength has been mainly analyzed based on adsorption energy. Prior to analysis of CO2 capture by selected ILs, anion/cationCO2 and ionic pairs were also briefly assessed. Such information could be useful to rationalize the behavior of ILCO2 systems. Figure 1. shows computed adsorption energies for CO2 on guanidinium and imidazolium based ILs. In general, the selected cations provide similar absorption energies, whose values lie between -31.70 kJmol1 ([BMPyr]+) and -42.92 kJmol1 ([CH]+). In concordance with Damas’s work, [2] the binding energy for the imizadolium family decreases upon alkyl side chain elongation. In fact, from [EMIM]+ to [HMIM]+, adsoption energy varies by only 1.83 kJmol1. However, larger alkyl side chains such as [OMIM]+ and [HdMIM]+ lead to a slight increase in adsorption energies upon chain elongation. AnionCO2 adsorption energies are, in general, larger than CationCO2 adsorption energies, with values varying between 41.19 kJmol1 ([NTf2]) and 123.37 kJmol1 ([H2PO4]). Thus, absorption energies of those ones based on dialkyl phosphate slightly decreases ( 4.19 kJmol1 ) upon alkyl chain elongation. The alkyl chain absence in [H2PO4]  leads to adsorption values of 29.12 kJmol1 which is greater than that of [Et2PO4]. Similar adsorption behaviors are noted for sulfate-based ions, wherein the presence of an ethyl chain leads to a decresing of 16.48 kJmol1. Adsorption of CO2 with halides yielding of 22.18 kJmol1 (in average).
In the future, chemical reaction kinetic would be carried out in order to elucidate the possible reaction mechanisms between Anions and CO2 using M06-2X/6-31++G(d,p)-PCM (Tetrahydrofuran) level at 300 K. Furthermore, the orbital interaction between the two reactants would be studied via natural bond orbital analysis. The key factor determining the transition states would also be explored with the NBO analysis of the transition states.

Figure 1. Computed absorption energies of selected CationCO2 (up) and AnionCO2 (bottom) systems.

4. References
[1] Frisch, M. J. et. al. Gaussian 09, Revision E.01, Gaussian, Inc., Wallingford CT, 2016.
[2] Damas, G. B.; Dias, A. B. A.; Costa, L. T. J. Phys. Chem. B, 2014, 118, 9046–9064.

5. Publication/Presentation
(1) J. Kodchasee, K. Bobuating, M. Ehara, to be submitted.
6. Patent N/A