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Porous Liquid Promises Potent Pluses

Feb. 1, 2016
Unique structure dissolves greater amount of gas and may permit selective separation

A liquid whose molecules contain permanent holes can absorb much more gas than conventional liquids and may enable new or improved chemical processes, report researchers at Queen’s University, Belfast, U.K.

“…We have managed to demonstrate a new principle — that by creating holes in liquids we can dramatically increase the amount of gas they can dissolve. These remarkable properties suggest interesting applications in the long term,” notes Stuart James, a professor in the School of Chemistry and Chemical Engineering at the university.

Special Structure

Figure 1. Rigid cage-shaped molecule enables absorption of much more gas than is possible with conventional liquids. Source: Queen’s University.

“What we have done is to design a special liquid from the ‘bottom up’ — we designed the shapes of the molecules that make up the liquid so that the liquid could not fill up all the space. Because of the empty holes we then had in the liquid, we found that it was able to dissolve unusually large amounts of gas,” he explains. The free-flowing liquid has permanent porosity.
A group at Queen’s led the effort, which also involved researchers from the University of Liverpool, U.K. as well as schools in Argentina, France and Germany. More details appear in a recent article in Nature.

The team produced the porous liquid by dissolving rigid organic cage molecules in a solvent that is too large to enter the pores (Figure 1). Because the pores in the cages remain empty, they can absorb gases. The fractional void volume due to the cages — 0.7% of the total volume in one porous liquid — can profoundly affect the liquid’s gas solubility. For instance, the researchers found the porous liquid provided an eight-fold increase in the solubility of methane.

The fractional void volume is small compared to the typical pore volumes in solids, admits James. “This is not necessarily a problem since the applications will probably be complementary to those of porous solids… They [porous liquids] will be able to find applications which are not possible for porous solids such as in continuous separation processes or as solvents for chemical processes.”

The porous liquids may improve processes by enabling use of lower pressures. “In processes in which gases need to dissolve in a liquid, often high pressures have to be applied to get enough gas to dissolve. Since porous liquids dissolve larger amounts of gas than conventional liquids, high pressures may not be needed, potentially reducing the cost and hazard of such processes,” he explains.

The porous liquids also may serve for gas separation, he believes. “The pore size and shape in porous liquids can potentially be tuned to match a given type of gas molecule, and so make the porous liquid dissolve one particular gas very well. It should therefore be possible to design porous liquids to selectively dissolve one gas from a mixture of gases. Since porous liquids can be pumped easily through pipes (unlike porous solids), it should then be possible to set up a cyclic separation system.”

Recovering gas dissolved in the pores involves increasing the liquid’s temperature or decreasing its pressure, just as for conventional liquids.

The researchers hope to create liquids with greater pore volume, tailor porous liquids to specific applications, and scale-up production of the liquids. “Each of the above is important, but also general principles of demonstrating size- and shape-selective dissolution — ‘molecular sieving’ — achieving in liquids what zeolites, etc., do in the solid state,” says James.

“The ones we have prepared to date are not indefinitely stable, especially in the presence of water or high temperatures — creating chemically and thermally robust porous liquids is also important (and comes under tailoring them to specific applications) in the move toward applications,” he adds.

“We have yet to find the limits of what is possible. There are some general characteristics which we will need to bear in mind — such as volatility (i.e., evaporation), which does not occur with solids and could be a problem in some applications, but I also think there will be ways around this.”

It likely will take ten years of research before actual applications occur, James says.

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