Designing a Liquid That Can Swallow Gas

Have you ever driven by an oil refinery or a really large power plant and wondered, “Gee, there’s a lot of carbon dioxide and other bad stuff entering the air?” The pollutants exiting factories and power plants have profound effects on the atmosphere, from reducing the amount of ozone to creating huge quantities of smog in places like Los Angeles.  It would be incredibly beneficial to the environment and to public health if there were a way to capture pollutants before they are released into the air.

Enter Professor Stuart James, of Queen’s University Belfast, in Northern Ireland. Professor James and his colleagues have developed “porous liquids,” a revolutionary technology to limit the pollutants entering the atmosphere.

The “porous liquids” are only useful if they combine the ability to trap gas molecules with a low viscosity. The gas would need to be absorbed by the liquid and prevented from escaping, and the liquid would have to flow quickly, to enable the constant trapping of gases.

Professor James got the idea to synthesize these “porous liquids” after observing the properties of zeolites, which are porous solids used to manufacture plastics and petroleum. These zeolites contain many pores, but only exist in the solid phase. Professor James wanted to combine the porous nature of these zeolites with the continuous flow of a liquid in order to capture a steady stream of unwanted chemicals.


Comparison of different cage-molecules.

Mastalerz 2015, Nature

To synthesize  the porous liquid, the researchers first designed the shape of a molecule in the solid state, and made sure that the molecular structure included gaps and holes. Substances with this structure are dubbed “cage molecules,” because their function is to trap gases inside and prevent them from leaving. This cage molecule was designed so that the solvent it was dissolved in could not enter the cage, but a smaller molecule of gas could.

After synthesizing the compound, which consisted of the “cage molecules” dissolved in crown ether, the researchers recorded the ability of the cage molecule to absorb a gas that ran through the liquid. They compared the amount of gas absorbed with what a normal liquid without pores, like water, could absorb. When they tested both the ability of the porous liquid and a normal liquid to absorb methane gas, the porous liquid was able to absorb eight times as much methane as the normal liquid.

How can this be applied to solving the problem of increased greenhouse gases? Since the porous liquid can trap large quantities of gases, power plants and oil refineries can take advantage of this technology. In the plant, the waste gases, like methane, are separated and sent up a column to be released into the atmosphere. If instead, the plant were to run these gases through a solution containing Professor James’ cage molecules, the plant would be able to cut back its emissions of methane by almost a factor of eight.

While the researchers did test methane, the trapping capability of “porous liquids” could extend to other, small-molecule gases such as carbon dioxide. Each different species of gas would require a uniquely sized cage-molecule in order to properly trap the gas.

If the plant were to run pollutants through a solution containing cage-molecules, the plant would be able to cut back its emissions of methane by almost a factor of eight.

More research still needs to be done on porous liquids in order to find the optimal structure of the “cage-molecules” and determine the best way to store or dispose of the liquids, but this promising technology has the ability to efficiently and inexpensively limit our carbon footprint.


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