April 10-14, 2016
Polymeric ionic liquids (PILs) are promising CO2 adsorption materials for that they combine the unique properties of ionic liquids with the flexibility and properties of macromolecular architectures. Previous studies have demonstrated that a carbonated-functionalized quaternary ammonium based PIL could adsorb CO2 from atmosphere when dry and release CO2 when wet, which constituted a moisture swing cycle, which provides an economical approach for air capture. Moreover, the performance of the PIL (such as CO2 affinity, reaction kinetics, swing size and hydrophobicity), could be improved owing to its fine tunability. To find out its optimal structure, molecular simulation was conducted to investigate the effects of phenyl, distance between cations, substituents and anions, on the properties of this PIL.
In this work, quantum chemistry calculations were performed at the B3LYP/6-311++G** level of theory to obtain the geometrical parameters after interaction between cation-anion, ion-CO2, and ion-H2O. The reaction kinetics of CO2 adsorption for PILs with different structures are analyzed based on the calculated results. At last, the influence of backbone on hydrophobicity was discussed. The molecular structure investigated in this work is relatively large, and ONIOM method was adopted to save the computational cost, which is shown in Fig. 1.
The results indicate that phenyl plays an important role in CO2 adsorption reaction. The large π bond of phenyl could attract lots of positive charges from quaternary ammonium, which would decrease the interactions between anions and cations. Besides, the phenyl could increase the mobility of the cations by elongating their arms, which would lead to the decrease of activation energy of proton transfer process. The distance between cations shows a great influence on CO2 adsorption reaction (see Fig. 2), and can be tuned by change its degree of crosslinking and monomer of its backbone. A distance of 8.5 Å could be the best choice for the relatively low activation energy and relatively high free energy change. The substituent on the phenyl could affect the charge distribution as well as the interactions between anions and cations. More importantly, we can tune the hydrophobicity for application in different environments by choosing different substituents. Those groups, which could form H-bonds with hydrated water, could enhance the hydrophilia greatly. The anions that could form strong H-bonds with hydrated water and whose products after adsorption show different H-bond distribution, have the potential of moisture swing. The swing size was determined by the degree of difference of water affinity between reactant and product. Carbonate anion has been proved the best choice for its large swing size and strong affinity of CO2. In a word, the optimal structure of this PIL should be carbonated-functionalized, have phenyl in its arms and keep the distance between cations around 8.5 Å.
Kun Ge and Tao Wang, "Optimal molecular design of poly (ionic liquids) for CO2 capture from the atmosphere" in "CO2 Summit II: Technologies and Opportunities", Holly Krutka, Tri-State Generation & Transmission Association Inc. Frank Zhu, UOP/Honeywell Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/co2_summit2/36