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Ind. Eng. Chem. Res. 2009, 48, 3702
Comments on the Origins of N2/CO2 Selectivity of Gas Separation Membranes Parveen Kumar, Sangil Kim, Junichi Ida, and Vadim V. Guliants* Department of Chemical and Materials Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0012 In a recent letter to the editor submitted in response to our study published in this journal,1 Favre et al.2 noted that “no theoretical basis for a nitrogen selective permeation process through a membrane material, be it dense or porous, has been reported”. They further argued, on the basis of their experimental data for the transient single-component permeation of dry CO2 through a dense polyetherimide (PEI) membrane, that reverse N2/CO2 selectivity may be expected only under transient conditions or explained by some multicomponent effects and/ or subtle coupling phenomena, e.g. by the role of water in the reverse selectivity. The observed reverse selectivity was explained in our study by enhanced N2/CO2 diffusion selectivity owing to strong interactions of CO2 with the polymeric amine in the presence of water vapor1 during week-long CO2/N2 permeation experiments, which strongly suggested steady-state permeation conditions. Water may play a very important role in reversing the selectivity of these membranes in the CO2-N2 separation. The unusual N2/CO2 selectivity may be explained by strong association between CO2 and H2O adsorbed in the PEI membrane component resulting in slower CO2 permeation than that of N2 which interacts weakly with both PEI and H2O. CO2 indeed forms various complexes with water, which are more stable than the N2-H2O complexes (the interaction energies of -6.5 to -33.4 kJ/mol3,4 vs -1.5 to -3.5 kJ/mol5) according to the results of both experimental and theoretical (B3P DFT and MP2) studies. Moreover, experimental evidence suggests that CO2 diffuses more slowly than N2 in bulk water6,7 and its diffusion may be further hindered in confined environment of PEI/MCM* To whom correspondence should be addressed. E-mail:
[email protected].
48 membranes. However, there is a clear need for further experimental and theoretical studies to confirm cooperative permeation behavior of CO2 and H2O and establish the mechanism of the unusual N2/CO2 selectivity for the PEI-modified MCM-48 membranes. We hope that this discussion will stimulate further fundamental studies of this interesting permeation behavior in order to advance its understanding to a point where such coupling phenomena may be rationally exploited to accomplish important and currently challenging molecular separations. Literature Cited (1) Kumar, P.; Kim, S.; Ida, J.; Guliants, V. V. PolyethyleneimineModified MCM-48 Membranes: Effect of Water Vapor and Feed Concentration on N2/CO2 Selectivity. Ind. Eng. Chem. Res. 2008, 47, 201. (2) Favre, E.; Roizard, D.; Bounaceur, R.; Koros, W. CO2/N2 Reverse Selective Gas Separation Membranes: Technological Opportunities and Scientific Challenges. Ind. Eng. Chem. Res. 2009, 48, 3700-3701. (3) Altmann, J. A.; Ford, T. A. Ab initio calculations of some weakly bound dimers and complexes: II. The complexes of carbon dioxide with water and hydrogen sulphide. J. Mol. Struct.: THEOCHEM 2007, 818, 85. (4) Danten, Y.; Tassaing, T.; Besnard, M. Ab Initio Investigation of Vibrational Spectra of Water-(CO2)n Complexes (n ) 1, 2). J. Phys. Chem. A 2005, 109, 3250. (5) Coussan, S.; Loutellier, A.; Perchard, J. P.; Racine, S.; Bouteiller, Y. Matrix isolation infrared spectroscopy and DFT calculations of complexes between water and nitrogen. J. Mol. Struct. 1998, 471, 37. (6) Tamimi, A.; Rinker, E. B.; Sandall, O. C. Diffusion Coefficients for Hydrogen Sulfide, Carbon Dioxide, and Nitrous Oxide in Water over the Temperature Range 293-368 K. J. Chem. Eng. Data 1994, 39, 330. (7) Verhallen, P. T. H. M.; Oomen, L. J. P.; v. d. Elsen, A. J. J. M.; Kruger, A. J. The diffusion coefficients of helium, hydrogen, oxygen and nitrogen in water determined from the permeability of a stagnant liquid layer in the quasi-steady state. Chem. Eng. Sci. 1984, 39, 1535.
IE9001487
10.1021/ie9001487 CCC: $40.75 2009 American Chemical Society Published on Web 03/03/2009
