Selective gas adsorption of gas-water mixtures in nanoscale systems and its application to gas separation

Gas separation;Molecular Dynamics;Nanoscale System

Lee, Joonho

关键词: Gas separation;    Molecular Dynamics;    Nanoscale System;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/78783/LEE-DISSERTATION-2015.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Interfacial structure and transport dynamics of individual water and gases in nano-devices has been investigated extensively since anomalous molecular structures near the nanoscale surface can affect the slip behavior of transport in channels and the hydrophobic interaction of apolar objects in solutions. However, even though solvation of gases in water at atmospheric pressure is a common phenomenon and gases dissolved in water can significantly influence the molecular behavior of water, gas distribution in the channels or near surfaces in the presence of gas-water mixtures has not been understood in detail. Given the technological importance of gas-water mixtures, we investigate the equilibrium transport and interfacial structure of gas-water mixtures on nanoscale structure using molecular simulation. For nanoscale channels and surfaces, we use carbon nanotubes (CNTs) and graphene because the remarkably smooth structures of CNT and graphene make them promising materials for various applications relying on fast transport of water and gases. We first study about various gas-water mixtures, such as CO2-water, O2-water and H2-water mixtures, through CNTs by performing molecular dynamics simulations. For all the three CO2-water, O2-water and H2-water mixtures, we observed adsorption of gases into the CNTs. The gas molecules formed single-file chains in the (10,0) CNT and once the nanotube is filled with single-file gas chains they prevented entry of water into the nanotube. The single-file diffusion of gas molecules in the nanotube in the case of gas-water mixtures is lower compared to the single-file diffusion of gases in gas-only simulations, except in the case of H2-only which does not form a single-file chain. These results suggest that under equilibrium conditions in the presence of gases, water molecules will be blocked from CNTs, and gases from gas-water mixtures will be selectively adsorbed by CNTs.Furthermore, to investigate the physical origin and quantitative understanding of gas enrichment near surfaces, we performed molecular dynamics simulations for gas-water mixtures near graphene surfaces with different surface parameters and develop a mechanistic understanding of gas enrichment by performing thermodynamic analysis. Although the gas-wall interaction is considered to be an important force giving rise to gas enrichment near surfaces, we show that the force exerted by water on the gas molecules, referred to as the solvent-induced one, can be quite significant in gas enrichment. The decomposition of the solvent-induced potential into its entropic and enthalpic components reveals that the entropic component near the surface is favorable to gas enrichment, whereas the enthalpic component is unfavorable. The significance of the solvent-induced potential on gas enrichment can depend on the type of the gas-water mixture and the surface. Finally, based on investigation of gas-water mixtures near nanoscale structures, we developed the idea for new gas separation procedures using a liquid membrane model with porous graphene membranes and water. By introducing a water slab between a gas-mixture and the graphene membrane, we show that the gas-mixture can be separated. By considering various gas-mixtures we show that the separation ratio follows the water-solubility ratio of the gas molecules in the mixture. We also demonstrate the water-solubility-driven separation of gas-mixtures using a carbon nanotube, but we show that the graphene membrane provides higher selectivity ratio because of its single-atom thickness.We also studied the CO2 separation of membrane contactor model using the porous graphene and an ionic liquid (IL) instead of water in order to enhance the selectivity. The 0.99 nm diameter graphene pore and 4% reduction of the IL loading under 1bar pressure drives dramatic selectivity of CO2 separation. By calculating the density distribution, we showed that the 0.99 nm diameter graphene pore induces strong layering even near the pore, which is not observed in 2.22 nm diameter pore cases. This strong layering prevents O2 molecules from diffusing into the IL. It also decreases the flux of CO2 molecules passing through the pores but complete blocking does not happen to CO2 molecules. Void analysis for different pores and different IL loadings shows that O2 solvation needs enough size of void and CO2 need smaller size of void comparable to their molecular sizes. 0.99 nm graphene pore and 4% reduction of the IL loading under 1bar pressure gives enough size void for CO2 and not enough for O2, which provides ideal condition to CO2 and O2 separation.