(226c) Ionic Liquid-Based Gel Membranes with Tough Double-Network for CO2 Separation

Concerns on CO₂ emissions from coal-fired power plants are rapidly increasing because of their major contribution to global warming and climate change. Hence, the development of an economically viable CO₂ capture technology is inevitable. Recently, ionic liquids (ILs) have been proposed as attractive media for CO₂ separation membranes owing to their distinct properties such as high CO₂ absorption capacity, negligible vapor pressure, huge chemical diversity, and high thermal stability. Therefore, the use of ILs as a CO₂ separation medium in a membrane has been extensively investigated. As a type of IL-based CO₂ separation membrane, IL-based gel membranes have both good CO₂ solubility selectivity and high CO₂ permeability, and they have attracted considerable attention. Among several kinds of IL-based gel membranes, the IL-based gels with polymer networks and polymer/ILs composites have been investigated as a material for CO₂ separation membranes. However, because conventional IL-based gel membranes had high polymer network composition to give enough mechanical strength, the CO₂ permeability as well as the CO₂ diffusivity in the gel was limited. In other words, the CO₂ permeability could be improved by decreasing the network content of the gel layer; the CO₂ permeability of the IL-based gel membrane could be increased by increasing the IL content in the gel. Thus, it is important to overcome the trade-off between the mechanical strength and solute diffusivity in a gel without increasing the network composition. One promising way to overcome this trade-off is to utilize tough gels.

Recently, we successfully developed a novel IL-based gel membrane composed of 80 wt% of room temperature ILs and a specific inorganic/organic composite network, termed inorganic/organic double-network ion gel membrane.¹⁾ Although normal IL-based gel membranes show a trade-off relationship in the mechanical strength and gas permeability, our developed double-network ion gel membrane having low composition of loosely cross-linked organic polymer network indicated both high mechanical strength and high gas permeability. The prepared DN ion gel with ca. 80 wt% of IL showed an extraordinarily high mechanical strength with a compressive fracture stress of more than 25 MPa. Owing to the high mechanical strength, the DN ion gel membrane exhibited excellent pressure resistance. The CO₂ permeability and N₂ barrier property of the DN ion gel membrane were also good, and were comparable to those of the supported IL membrane containing the same ILs. Our prepared DN ion gel membranes have the potential to overcome the trade-off limitation with respect to the mechanical strength of the ion gel and the solute diffusivity in the gel.

Here, we present a novel ion gel membrane with an extraordinarily high mechanical strength and high CO₂ permeability. The ion gel membranes consisted of an inorganic/organic composite network and a large amount of RTILs up to 80 wt%. The developed ion gel membranes can be prepared via a single-pot preparation process. For the synthesis of the inorganic/organic composite ion gel membrane, the inorganic component was formed by thermally induced polycondensation of tetraethylorthosilicate (TEOS), and the polymer network was formed by UV-initiated free-radical polymerization of N,N-dimethylacrylamide (DMAAm). The TEOS polycondensation resulted in the formation of silica nanoparticles, which are inter-connected to form a three-dimensional (3D) silica particle network in the IL. The silica particle network could be formed by weak physical interactions such as hydrogen bonding and van der Waals attraction between the silica nanoparticles in the IL. Since the fragile silica particle networks acted as the sacrificial bond, it could dissipate the loaded energy by destruction during the application of a force. On the other hand, polydimethylacrylamine (PDMAAm) was selected as the polymer network because of the good compatibility with several RTILs. After the formation of silica nanoparticles in the IL, a ductile PDMAAm network was formed to make an inorganic/organic composite network. The inorganic/organic composite network can be synthesized in the same IL pot, which allows the preparation of freely shapeable ion gel, including a film shape. In addition, the mechanical strength of the inorganic/organic ion gel can be varied by controlling not only the cross-linking density of the organic network but also the inorganic/organic network composition. As mentioned above, the mechanical strength of conventional ion gels could be controlled by the network content and the cross-linking density of the network, although both of them have the trade-off limitation between the mechanical strength and solute diffusivity in the gel. Here, we propose an inorganic/organic composite ion gel membrane, which can overcome the trade-off limitation. Without changing the IL content in the inorganic/organic composite ion gel membranes, the mechanical strength could be controlled by changing the cross-linking degree of the polymer network and the composition of inorganic/organic networks.

The mechanical strength of the composite ion gel membrane could be increased by increasing the inorganic network composition as well as decreasing the cross-linking degree of the organic polymer network. Increasing of the inorganic network composition, in other words the decrease of the organic network composition, directly enhanced the energy dissipated by the internal rupture of the inorganic networks. Decreasing the cross-linking degree of the polymer network allowed the ion gel membrane to elongate higher degree, which caused additional internal rupture of the inorganic network during further elongation and increased the energy dissipation. As the results, the ion gel membrane having low composition of loosely cross-linked polymer network showed high mechanical strength. On the other hand, because the decrease of the content and cross-linking degree of the polymer network enhanced the diffusivity of the gases absorbed in the ion gel membrane, the ion gel membrane having low composition of loosely cross-linked polymer network showed high CO₂ permeability. The composite ion gel membrane with optimized network composition and 80 wt% ionic liquid sustained about 1200 barrer of CO₂ permeability and 25 of CO₂/N₂ selectivity for more than 300 h at 50 ^oC under humid condition.

In this presentation, we demonstrate the good pressure resistance, IL holding property, and excellent CO₂ permeability of the fabricated composite ion gel membrane with the optimized inorganic/organic composite network.

Reference:

1) E. Kamio T. Yasui Y. Iida J. P. Gong H. Matsuyama, Inorganic/Organic Double‐Network Gels Containing Ionic Liquids, Advanced Materials, 29, 1704118, 2017