Amino Acid Ionic Liquids-based Ion Gel Membranes with Superior Pressure Resistance for CO2 Capture Application
Farhad Moghadamü§, Eiji Kamioü§, Ayumi Yoshizumiü§ and Hideto Matsuyamaü§,*
ü§ Center for Membrane and Film Technology, Department of Chemical Science and Engineering, Kobe University
* e-mail address: firstname.lastname@example.org, tel.: +81-78-803-6180, fax: +81-78-803-6180
Amino acid ionic liquids (AAILs)-based membranes have been recognized as an attractive alternative to conventional facilitated transport membranes (FTMs) due to unique properties of AAILs, such as the high CO2 absorption capacity, selective reactivity with CO2, negligible vapor pressure, high thermal stability, and tunable chemical structure. The AAILs-based gel membranes as well as supported ionic liquid membranes previously developed in our group exhibited good CO2 permeability and CO2/N2 selectivity, but the poor stability under pressurized conditions is a major obstacle to their practical application [1,2].
Here we present a new class of AAILs-based ion gel membranes with not only superior CO2 permeability and CO2/N2 selectivity but also excellent stability under pressurized conditions. The developed AAIL-based gel membranes consisted of two specific independent interpenetrating polymer networks, so-called doubel-network (DN) . The DN was composed of two asymmetric polymer networks of which the first network was a rigid, brittle, and tightly cross-linked polyelectrolyte and the second network was a soft, ductile and loosely cross-linked polymer with good compatibility with AAILs. As the solvent of the DN gels, AAILs composed of phosphonium type cation and prolinate anion were used because of their excellent CO2 transport properties . The mechanical strength of the fabricated AAILs-based DN gel membranes were tested by compressive and tensile stress-strain measurements, and the DN gels composed of more than 80wt% AAILs showed more than 25 MPa of compressive and about 0.7 MPa of tensile fracture stresses.
According to the CO2 permeation performances of the AAIL-based DN gel membranes, the DN gels membrane with 85 wt% of AAILs showed remarkable CO2 permeability of more than 5000 barrer and high CO2/N2 selectivity of more than 170 (at 373 K and CO2 partial pressure 10 kPa). From the trend of the CO2 and N2 permeabilities of the DN gel membranes at different CO2 partial pressures, we confirmed that the permeation mechanism of CO2 and N2 were facilitated transport and solution–diffusion mechanisms, respectively. In addition, the pressure resistance of the DN ion gel membranes containing 85 wt% of AAILs was investigated by increasing the feed side pressure up to 500 kPa and the stable performance of membrane were confirmed (Fig.1). Moreover, the DN ion gel membranes exhibited good durability; the CO2 permeation performance remained stable more than 5 days at 500 kPa.
Regarding the good mechanical strength of AAIL-based DN gels, it was possible to prepare thin membranes. We fabricated AAIL-based DN gel membranes with different thicknesses and evaluated the gas permeation mechanisms. From the obtained results, it was found that CO2 and N2 permeances showed reverse proportional relationship to the DN gel membrane thickness, which clearly demonstrated that the rate-controlling step of CO2 and N2 permeations across the DN gel membranes were intra-membrane diffusion. In the present stage, the AAIL-based DN gel membrane with the thinnest thickness (58 mm) showed the CO2 permeance of ca. 120 GPU and CO2/N2 selectivity of ca. 100 under the conditions of T= 373 K and 10 kPa of the CO2 partial pressure. However, as the rate-controlling step of the CO2 permeation is intra-membrane diffusion, we could increase the CO2 permeance by fabricating much thinner AAIL-based DN gel membranes.
Fig.1. Pressure resistance behavior of AAILs-based PAMPS/PDMAAm DN ion gel membranes (Experimental conditions: T = 373 K, Psweep = atmospheric pressure, PCO2,feed = 10 kPa)
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