A Review on Ionic Liquid Gas Separation Membranes

Abstract

Ionic liquids have attracted the attention of the industry and research community as versatile solvents with unique properties, such as ionic conductivity, low volatility, high solubility of gases and vapors, thermal stability, and the possibility to combine anions and cations to yield an almost endless list of different structures. These features open perspectives for numerous applications, such as the reaction medium for chemical synthesis, electrolytes for batteries, solvent for gas sorption processes, and also membranes for gas separation. In the search for better-performing membrane materials and membranes for gas and vapor separation, ionic liquids have been investigated extensively in the last decade and a half. This review gives a complete overview of the main developments in the field of ionic liquid membranes since their first introduction. It covers all different materials, membrane types, their preparation, pure and mixed gas transport properties, and examples of potential gas separation applications. Special systems will also be discussed, including facilitated transport membranes and mixed matrix membranes. The main strengths and weaknesses of the different membrane types will be discussed, subdividing them into supported ionic liquid membranes (SILMs), poly(ionic liquids) or polymerized ionic liquids (PILs), polymer/ionic liquid blends (physically or chemically cross-linked 'ion-gels'), and PIL/IL blends. Since membrane processes are advancing as an energy-efficient alternative to traditional separation processes, having shown promising results for complex new separation challenges like carbon capture as well, they may be the key to developing a more sustainable future society. In this light, this review presents the state-of-the-art of ionic liquid membranes, to analyze their potential in the gas separation processes of the future.

Keywords: gas separation; ion gel membrane; ionic liquid; ionic liquid blends; polymerized ionic liquids; transport properties.

Figure 1

Overview of some of the most common anions and cations used in ion ionic liquid membranes.

Figure 2

Schematic summary of the membrane characteristics and preparation techniques employing ionic liquid (IL)-based materials in membrane-based gas separation.

Figure 3

Schematic representation of the possible types of supported ionic liquid membranes. Green = ionic liquid, grey = porous support of gel former, orange = silicone.

Figure 4

Schematic overview of the different architectures of polymeric ionic liquids (PILs), based on polycations. Blue spheres: cationic moieties; green spheres, anions. Analogous structures are possible with polyanions.

Figure 5

Poly(ionic liquids) synthesis via polymerization of task-specific IL monomers (top-left) and post-modification of polymeric precursors (bottom-left) yields similar polymeric structures based on their ionic composition: Cationic (top-center), anionic (bottom-center), polyzwitterions (top-right), and polyampholytes (bottom-right). Ions (small spheres) or ionic moieties (large spheres) are indicated with the positive (blue +, cation) or negative (green -, anion) symbol.

Figure 6

Major approaches for post-synthetic modification of polymers to produce ‘top-down’ polyelectrolytes include (a) ionization, (b) quaternization, (c) sulphation, and (d) cross-linking. XDC abbreviates p-Xylylene dichloride.

Figure 7

Robeson plots for CO₂/CH₄ (left) and CO₂/N₂ (right) for PILs (Table 5) with corresponding upper bounds from 1991 [238], 2008 [239], and 2019 [240]. (1 Barrer = 10⁻¹⁰ cm³_STP cm cm⁻² s^-1 cmHg⁻¹ = 3.348 × 10⁻¹⁶ mol m m⁻² s⁻¹ Pa⁻¹).

Figure 8

Thermally reversible ion gel based on an ABA triblock copolymer. The blue block is soluble in the ionic liquid, indicated as (+)(−), and the red block is insoluble at room temperature and becomes soluble upon heating. Reprinted from Ref. [252] with permission from AAAS.

Figure 9

(A) Correlation of the selectivity and the ionic liquid content and (B) correlation of the permeability and Young’s modulus for p(VDF-HFP)/[EMIM][Tf₂N] membranes with IL content ranging from 0% to 80%. 1 Barrer = 10⁻¹⁰ cm³_STP cm cm⁻² s⁻¹ cmHg⁻¹ = 3.348 × 10⁻¹⁶ mol m m⁻² s⁻¹ Pa⁻¹. Reprinted from [256]. Copyright (2012), with permission from Elsevier.

Figure 10

(A) Network structure for stoichiometry 3:1, assuming 100% conversion, and (B) CO₂/N₂ Robeson plot (right) with data for epoxy-amine ion gels tested with single gases (▲) [145] and with humidified mixed-gas feeds (□) [188], where the ratios 3:1 and 3:2 refer to the mole ratio of the epoxy monomer and the amine monomer. 1 Barrer = 10⁻¹⁰ cm³_STP cm cm⁻² s⁻¹ cmHg⁻¹ = 3.348 × 10⁻¹⁶ mol m m⁻² s⁻¹ Pa⁻¹. Robeson plot reprinted from [188]. Copyright (2015), with permission from Elsevier.