(396n) Fundamental Investigation On CO2 Permeation Mechanism of Amino Acid Ionic Liquid-Based Membranes

Prevention of global warming is a critical issue related to the conservation of the global environment. Carbon dioxide is one of the major contributors to the greenhouse effect. The CO₂ separation technologies have been researched in the world for prevention of global warming. In recent years, membrane separation has attracted attention because it has advantages including operational simplicity, high energy efficiency, simple process equipment and low capital cost. It is necessary to fabricate CO₂ selective membrane with high CO₂ permeability and selectivity for commercial viability. Many membranes for selective CO₂ separation have been developed during the last several decades. Especially, it is well known that a facilitated transport membrane (FTM) has high CO₂ permselectivity [1, 2]. FTMs show very high performance on CO₂ transportation because FTMs contain CO₂ carriers in membrane that react with CO₂. However, in the case of traditional FTMs, CO₂ carriers in FTMs cannot react with CO₂ without water. This restriction limits the field of practical applications of CO₂ selective FTMs. In our previous work, we have reported that amino acid ionic liquid (AAIL) based FTMs overcame this problem [3, 4]. Ionic liquids (ILs) are molten salts with unusually low melting point which have attractive properties such as negligibly small vapor pressure, tunable chemical and physical properties and so on. AAILs are one of the ionic liquids which have amino acid in their structure [5]. It has been reported that AAILs have high CO₂ absorption capacity because the amine group in the molecules react with CO₂[6-9].

We reported that AAILs in membranes worked as CO₂ carriers and diffusion mediums. AAIL-FTMs showed extremely high CO₂ permselectivity under various relative humidity at 373 K [3]. In addition, it was found that AAILs-based membranes facilitated CO₂ transport under dry conditions, whereas conventional FTMs cannot work in low humidity environments. On the other hand, contrary to great performance at elevated temperature, AAIL-FTMs showed critically low CO₂ permselectivity around room temperature under dry condition. To improve the CO₂ permselectivity around room temperature under dry condition, investigation on CO₂permeation mechanism of AAIL-FTMs were conducted.

The CO₂ permeation mechanism in AAIL can be divided into 3 steps. The first step is the formation of an AAIL-CO₂ complex via chemical reaction between amine group of an amino acid in AAILs and CO₂at the feed side of the membrane. Subsequently, diffusion of the complex across the membrane occurs. Finally, the complex is dissociated at the permeate side of the membrane. It can be presumed that the diffusion of the complex is the rate-determining step, since generally the reaction rate is much faster than the diffusion rate in the liquid. Therefore, diffusivity is the key factor to control the CO₂ permeability of the AAIL-FTMs. That is to say, lowering the viscosity is promising to improve CO₂ permeability of the AAIL-FTMs. Regarding the viscosities of AAILs after CO₂ absorption, Goodrich et al. reported considerable phenomena [7]. For the primary AAILs, outstanding raise of viscosity was observed after they absorbed CO₂. On the other hand, the increase of the viscosity of the prolinate AAIL after CO₂ absorption was little. These results implied that the viscosity of AAIL-CO₂ complex would be a main factor to determine the CO₂ permeabilities of the AAIL-FTMs. In fact, in our previous work, we also reported the AAIL-FTMs containing the AAIL with prolinate anions showed higher CO₂ permselectivity than those with glycinate anions [4]. This would be due to the lower viscosity of proline based AAIL after CO₂ absorption. Glycine and proline have different amine group. Glycine has a primary amine, while proline has a secondary amine. From this point of view, understanding the differences of the CO₂ permselectivity as well as the viscosity among AAIL with different amine groups would be important to improve the CO₂ permselectivity of AAIL-FTMs around room temperature under dry condition. Among all the amino acids of the AAILs used in our previous research, only proline has secondary amine group [3, 4]. In this study, to investigate the affect of amine groups on the CO₂ permeability of AAIL-FTMs, AAILs with 3 types of amine group (primary, secondary and tertiary amine) were synthesized. The change of viscosities before and after CO₂ absorption of the newly synthesized AAILs was examined to understand the CO₂permeation mechanism of AAIL-FTMs.

In this work, viscosities and CO₂ permeabilities of a series of tetrabuthylphosphonium ionic liquids with amino acid as anions, [P₄₄₄₄][AA] (AA= glycine (Gly), N-methylglycine (mGly) and N,N-demethylglycine (dmGly)), were measured. We investigated the effect of the viscosities of the AAIL-CO₂ complexes on the CO₂ permeation properties of the AAIL-FTMs. The viscosity measurement of the AAILs before and after CO₂ absorption indicated that the viscosities of the CO₂-saturated AAILs were higher than those of the pure AAILs. In addition, the viscosities of [P₄₄₄₄][mGly]-CO₂ complex was lower than those of [P₄₄₄₄][Gly]-CO₂ complex at elevated temperature. Regarding CO₂ permeability of the AAIL-FTMs containing [P₄₄₄₄][Gly] and [P₄₄₄₄][mGly], the CO₂ permeabilities of [P₄₄₄₄][mGly] based FTM were higher than those of [P₄₄₄₄][Gly] based FTM at elevated temperature. The trends of [P₄₄₄₄][mGly] were same to proline based AAIL. On the other hand, viscosity enhancement of [P₄₄₄₄][Gly] and [P₄₄₄₄][mGly] were significant around room temperature. The phenomena would be attributed to formation of intermolecular hydrogen bonding among the AAIL-CO₂ complexes. In our examination, the viscosities of [P₄₄₄₄][Gly]-CO₂ complex were higher than that of [P₄₄₄₄][mGly]-CO₂ complex above 320 K, and vice versa below 320 K. If the viscosity of AAILs-CO₂ complex strongly affected to the CO₂ permeabirity of AAIL-FTM, the CO₂ permeabilities of [P₄₄₄₄][Gly]-FTM and [P₄₄₄₄][mGly]-FTM would be reversed at 320 K. As it was expected, the temperature dependence of CO₂ permeabilities revealed that the CO₂ permeability of [P₄₄₄₄][Gly]-FTM was higher than [P₄₄₄₄][mGly]-FTM below 320 K and vice versa above 320 K. From these result, it was strongly suggested that the diffusivity of the AAIL-CO₂ complex in AAIL-FTMs determines the CO₂ permeability of AAIL-FTMs. In addition, it was suggested that the increase of the viscosities of AAILs via CO₂ absorption would be coursed by the formation of hydrogen bonding between the formed AAIL-CO₂ complexes. In conclusion, it can be said that viscosity of AAIL-CO₂ is the most important factor that controls the CO₂permeability of AAIL-based facilitated transport membrane.

This research was supported by JST-ALCA.

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