AIST and Daicel develop separation membrane for direct air capture of CO₂ using mixture of two ionic liquids with distinct roles

A research group led by Senior Researcher Yuki Kohno, Mitsuhiro Kanakubo, and Group Leader Takashi Makino of the Research Institute for Chemical Process Technology at the National Institute of Advanced Industrial Science and Technology (AIST), in collaboration with Daicel Corporation (Daicel), has successfully developed a membrane for the highly selective separation and recovery of dilute CO₂, such as CO₂ in the atmosphere.

Carbon recycling technology, which separates and recovers CO₂ emitted into the atmosphere and reuses it as a raw material for synthesizing various carbon compounds as carbon resources, is attracting attention to help achieve carbon neutrality by 2050. In recent years, technologies using chemical absorption and chemical adsorption have been demonstrated in Europe and the United States, but their disadvantage is that they consume a large amount of heat to recover the CO₂ absorbed or adsorbed in the separation material.

The research group focused on the membrane separation method, a CO₂ separation technology that, in principle, does not require thermal energy. CO₂ separation and recovery using chemically reactive membrane materials requires rapid progression of CO₂ absorption into and desorption from the membrane. As such, the researchers achieved faster absorption and desorption by mixing two kinds of ionic liquids with distinct roles. They developed a mixture of ionic liquids that chemically react with CO₂ and ionic liquids that solvate with compounds formed by the chemical reaction (ionic liquid mixture). They produced an ionic liquid membrane by impregnating a porous material with this ionic liquid mixture. A model gas mixture of CO₂ and N₂ (CO₂ concentration: 0.04%) was supplied upstream of the membrane, and He was supplied downstream as the sweep gas.

The CO₂/N₂ selectivity reached about 200 times higher than that of conventional membranes in tests using 0.04% CO₂, which is comparable to CO₂ in the air. Furthermore, by developing an ionic liquid with an optimized molecular structure of anions, they succeeded in developing an ionic liquid membrane with a CO₂/N₂ selectivity of more than 10,000 times, and eventually reduced CO₂ to about 70%. This is the highest class of performance as a material for separation membranes for DAC (separation and recovery technology to reduce CO₂), and its CO₂/N₂ selectivity is equivalent to about 500 times that of conventional polymer membranes that show equivalent CO₂ permeability.

"In the future, we will develop manufacturing and modularization technologies for ionic liquid membranes to apply DAC technology using mixed ionic liquid membranes," explained Makino, leader of the research group. "To achieve carbon neutrality, it is also essential to separate and recover CO₂ not only from the atmosphere, but also from a variety of CO₂ sources. The concept of mixed ionic liquid membranes is not limited to atmospheric CO₂ separation and recovery. We would like to develop the separation membranes for various sources of CO₂ emissions."

Journal Information
Publication: ACS Omega
Title: Ionic Liquid Mixtures for Direct Air Capture: High CO₂ Permeation Driven by Superior CO₂ Absorption with Lower Absolute Enthalpy
DOI: 10.1021/acsomega.2c04756