Molecular Simulation Study of the Performance of Supported Ionic Liquid Phase Materials for the Separation of Carbon Dioxide from Methane and Hydrogen
S Budhathoki and JK Shah and EJ Maginn, INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH, 56, 6775-6784 (2017).
Molecular dynamics and Gibbs ensemble Monte Carlo simulations were used to compute the self-diffusion coefficients and solubilities of CO2, CH4, and H-2 in model membranes consisting of slit pores with diameters of 2 and 5 nm. Solubility selectivities, diffusion selectivities, and permselectivities of CO2 for binary gas mixtures of CO2/CH4 and CO2/H-2 were also computed. The calculations were repeated for the same pores filled with the ionic liquid (IL) 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C(4)mim(+)Tf2N(-)) and for bulk IL. The bulk IL system was used as a model for a supported ionic liquid membrane separator having large pores, while The confined IL systems were used: to assess whether, extreme nanoconfinement, of ILs has an effect on permselectivity. Permselectivities were about factor of 10 higher in all the IL systems compared to the empty nanopores. Nanoconfinement tends to increase the solubility and decrease the diffusivity of all gases. relative to the bulk IL. The bulk IL has significantly higher solubility selectivity for CO2 over CH4 and H-2, relative to the empty pores, and,nanoconfinement of the,IL further increases solubility selectivity a modest amount. Diffusion selectivites in the nanoconfined IL for CO2 over CH4 are slightly enhanced relative to bulk IL but are slightly smaller for CO2 over H-2:The net result is that nanoconfinement of the IL is predicted to slightly increase permselectivity for CO, over CH4 but has :little effect on the permselectivity of CO2 over H-2 when compared to bulk IL. Although the IL leads to significantly enhanced permselectivities of CO2 compared to the empty nanopores, gas diffusivities are more than 2 orders of magnitude smaller in the IL when compared to the empty nanopores. This suggests that while the use of this IL in a supported ionic liquid membrane separator will lead,to enhanced selectivities, the overall permeation rate may be reduced relative to a conventional membrane if diffusion in the pores is rate limiting.
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