Effect of Ionic Liquid Confinement on Gas Separation Characteristics Laila A. Banu, Dong Wang, and Ruth E. Baltus* Department of Chemical and Biomolecular Engineering, Clarkson University, Potsdam, New York 13699-5705, United States * S Supporting Information ABSTRACT: Work in our laboratory has focused for a number of years on examining the potential of room-temperature ionic liquids for post-combustion carbon capture processes. Results from studies of carbon dioxide solubility, diffusivity, and permeation across supported ionic liquid membranes have raised questions about the impact of a solid interface on the properties of ionic liquids. In this paper, we report results from measurements of carbon dioxide uptake into ionic liquids confined within a ceramic nanoporous film and compare carbon dioxide solubility and diffusivity to values measured with bulk phase ionic liquids. Results show that both solubility and diffusivity are enhanced in confined ionic liquids when compared to values observed in unconfined liquids. These observations have implications for gas separation processes involving supported ionic liquid membranes. 1. INTRODUCTION Room-temperature ionic liquids (RTILs) are salts consisting of a bulky cation and an inorganic anion with melting points below 100 °C. The large cation size allows for delocalization and screening of charges, resulting in a reduction in the lattice energy and, thereby, the melting or glass transition temper- ature. In recent years, there has been increased interest in the potential of ionic liquids for a variety of applications, such as chemical synthesis, catalysis, electrochemical processes, and gas separations. 1-5 Because ionic liquids have no measurable vapor pressure, solvent losses can be minimized when these materials are used in a separation process or in electrochemical devices. Ionic liquids can solubilize a wide range of compounds and have properties that can be tailored by appropriate choice of anion and cation. Ongoing work in our laboratory has focused on examining the potential of ionic liquids for carbon dioxide capture processes. 6-11 Many ionic liquids have an imidazolium-based cation with different alkyl side chains. In this paper, the acronym C n C m im is used to represent an imidazolium ring with alkyl chains of length n and m carbons on the nitrogen atoms. The RTILs used in this work have bis(trifluoromethanesulfonyl)imide as the anion, which is given the acronym Tf 2 N in this paper. Many applications of ionic liquids involve the contact of these unique fluids with solid surfaces, often in the form of porous solid supports. For example, a number of investigators have examined supported ionic liquid phase catalyst systems where a transition-metal catalyst is dissolved in a thin ionic liquid film that is supported on a solid surface by covalent attachment or physisorption. 12-18 Others, including our group, have examined supported ionic liquid membranes (SILMs) and polymeric ionic liquid membranes for gas separations. 7,11,19-37 While interest in various aspects of ionic liquid chemistry has grown significantly in the past 10 years, efforts to examine the interfacial properties of ionic liquids have arisen more recently, with sometimes contradictory observations. Using atomic force microscopy (AFM), Bovio et al. 38 found evidence of solid-like layering in thin films of the ionic liquid C 4 C 1 imTf 2 N on mica, amorphous silica, and oxidized Si (110) surfaces. These organized structures were observed in films up to 50 nm thick. A number of investigators have examined confinement of ionic liquids in porous oxide networks (“ionogels”) prepared using a sol-gel process conducted in the ionic liquid phase. Bideau et al. 39 measured relaxation times using 1 H nuclear magnetic resonance (NMR) of ionic liquids confined in silica ionogels with pore diameters of 12 and 15 nm and found liquid-like behavior at temperatures below the bulk crystal- lization temperature. Ne ́ ouze et al. 40 used differential scanning calorimetry (DSC) and 1 H NMR to examine the properties of ionic liquids confined in ionogels that were modified with hydrophobic methyl groups. While relaxation times indicated ionic liquid behavior intermediate between liquid and solid, measured conductivity values were characteristic of bulk liquid. The effect of the ionic liquid structure on interfacial organization on mica, graphite, silica, and gold surfaces was examined by Hayes et al. 41 using AFM. It was found that ionic liquids that form organized structures in bulk solution, with polar and nonpolar domains, show strong organization near surfaces. Gö bel and co-workers 42,43 characterized functionalized silica monoliths and examined properties of ionic liquids that filled these porous structures, with pore sizes ranging from 2.5 to 30 nm. Infrared (IR) spectra, small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and NMR did not indicate significant structural difference between bulk and confined ionic liquid. However, DSC measurements indicated that the monolith surface affected the phase transition characteristics of these liquids. Kanakubo et al. 44 observed pore-size- dependent melting point depression for a number of different ionic liquids confined in controlled pore glasses with pore sizes ranging from 2 to 15 nm. The sensitivity of melting point Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: December 10, 2012 Revised: February 8, 2013 Published: February 11, 2013 Article pubs.acs.org/EF © 2013 American Chemical Society 4161 dx.doi.org/10.1021/ef302038e | Energy Fuels 2013, 27, 4161-4166