Thermodynamics of ionic liquid solvents in gas purification and exfoliation mechanisms: molecular dynamics simulation and Monte Carlo calculations
Demand for green sources of energy is increasing due to the critical need to decrease greenhouse gas emissions. This research involved different approaches for reducing CO2 emission to the atmosphere. In the first study, the exfoliation of bismuth telluride (Bi2Te3) as a well-know thermoelectric (TE) material was investigated. In the literature, it has been experimentally and computationally proven that producing a thinner layer of Bi2Te3 increases the “figure of merit” by reducing the thermal conductivity and enhancing the electrical conductivity. A liquid-phase exfoliation technique is one of the potential approaches to exfoliate Bi2Te3. In my simulation work, different types of imidazolium-based ionic liquids (ILs) were screened to find the most efficient exfoliant, by first considering the value of the solid surface energy and surface tension of the applied liquids. We found that [Tf2N-]-based ionic liquids are relatively effective at enhancing the exfoliation, and this performance can be correlated to the unique molecular-level solvation structures developed at the Bi2Te3 surfaces. In the second study, I modeled CO2 separation during typical pre-combustion and post-combustion condition using a novel IL + polymer membrane material. This work was inspired by recent experimental findings from the Bara group at UA. The new class of materials was generated by adding ionic liquid molecules to the backbone of polymers while using (pyromellitic dianhydride) PMDA as an organic ligand. For the first time, these polymers, “ionic polyimides” (i-IPs), were computationally investigated as a potential membrane for CO2 separation. The presence of the IL significantly displaces the CO2 molecules from the ligand nitrogen sites in the neat i-IP to the imidazolium rings in the i-IP + IL composite. These molecular details can provide critical information for the experimental design of highly selective i-IP materials, as well as provide additional guidance for the interpretation of simulated adsorption systems. It is found that the 50% IL addition can increase CO2/CH4 selectivity by 16% in [BF4-]-based and by 36% in [PF6-]-based structures. While the [BF4-]-based system shows higher CO2/CH4 selectivity, the [Tf2N-]-based system shows higher CO2/N2 gas separation performance. These findings are exemplified by high gas solubility of [PF6-]-based structures, which also compensate to a correlated larger theoretical surface area.