Unveiling a new technique for preparing ionic liquid-based membranes for mixture separation

Separating mixtures into their constituent substances is essential in many fields. For example, mixture separation plays a key role in the petrochemical industry, as well as in chemical purification and synthesis plants. Moreover, separating mixtures is also important from a sustainability perspective. By selectively separating compounds from mixtures, we can recover and reutilize useful substances while capturing harmful gases from the output of industrial processes.

Recently, ionic liquids (ILs), which are molten salts consisting of an organic cation and either an organic or inorganic anion, have attracted significant attention from materials scientists. These compounds offer unique and promising properties for membrane-based mixture separation. More specifically, by coating a supporting porous membrane with carefully chosen ILs, it is possible to selectively extract specific gases from a mixture by tuning the membrane’s affinity to those gases.

Despite the potential of membranes with immobilized ILs for mixture separation, manufacturing them remains somewhat complex. In past studies, researchers first prepared siloxane compounds with an IL-type group via liquid-phase reactions, and then coated this material onto a nanoporous membrane using a well-known technique called sol-gel technique. This multistep process can be tedious, time-consuming, and somewhat inflexible.

To tackle these issues and make IL-based membranes easier to produce, a research team from Japan, led by Associate Professor Yuichiro Hirota from Nagoya Institute of Technology, has come up with an innovative solution. They developed a simpler, versatile, and straightforward method to produce IL-immobilized membranes through gas-phase reactions. This work was published in the Journal of Membrane Science. Professor Shunsuke Tanaka from Kansai University was also part of this research group.

The proposed strategy begins with dip-coating nanoporous aluminum oxide tubes onto a solution containing polymerized (3-chloropropyl)diethoxy(methyl)silane (ClPDMS). This forms a thin polymeric membrane with exposed chloropropyl groups on the nanoporous tubes’ surface.

Then, the researchers employed a technique dubbed vapor-phase transport (VPT) treatment, in which the ClPDMS membranes are placed in a closed vessel and exposed to 1-methylimidazole vapor at a controlled temperature. This treatment transforms nearly all the chloropropyl groups into an imidazolium-type IL structure with chlorine anions and imidazolium cations. Afterwards, simply immersing the membrane-coated tubes into an aqueous HN(SO₂CF₃)₂ solution is enough to exchange anions from chlorine to (CF₃SO₂)₂N⁻.

To prove the effectiveness of the VPT strategy, the researchers thoroughly characterized the resulting membranes using X-ray photoelectron spectroscopy, scanning electron microscopy, and AgCl precipitation reactions. They also conducted permeability and permselectivity tests to measure how well different membranes could extract gases such as H₂, H₂O, and toluene from mixtures.

“Our paper is the first known instance of fabricating separation membranes using VPT and anion exchange in ionic liquid-based materials and evaluating the membranes’ performance in terms of permeation and separation,” says Hirota. Adding further, he says, “The developed techniques exhibit excellent potential for preparing various IL-immobilized siloxane membranes.”

Overall, this study presents a convenient methodology to prepare tailored membranes for mixture separation. Making such membranes more versatile and accessible will likely increase their presence in industrial applications, which could be of great value towards sustainability.

“With membrane-based separation technology, processes for synthesizing the various products and fuels that surround us could save energy and thus help solve environmental problems such as global warming,” comments Hirota. “This will contribute to our collective goal of achieving carbon neutrality by 2050.”