One of the biggest unsolved challenges in industrial separations is molecular separations in organic liquid systems. From the oil and gas industry, refineries, right through to pharmaceutical manufacturing, there are very few membranes available with high permeability and selectivity that are stable in organic solvents. An efficient separation membrane that can operate in the presence of organic solvents is desirable to reduce the high energy requirements of gas and liquid separations.

Researchers from Imperial College London have now combined two ideas to enhance the microporosity of a membrane and also ensure its stability in organic solvents. First, the researchers used interfacial polymerization to form a highly cross-linked polymer nanofilm that is only 20 nm thick. Secondly, they chose rigid contorted monomers in order to control the free volume of the pore size in the resulting film.

According to their recent publication in Nature Materials, the polyarylate nanofilm composite membranes that they developed showed two orders of magnitude higher solvent permeance than membranes fabricated with nanofilms made from non-contorted planar monomers. The new membranes were tested successfully both for gas separation as well as for organic solvent nanofiltration.

Andrew Livingston, who leads the team that conducted the research at the Department of Chemical Engineering of ICL, uses a culinary example to explain the use of contorted monomers. "Imagine taking a stick of spaghetti, chopping it up into small pieces and then try to reassemble a network from those spaghetti sticks," Livingston says. "Because of their shape, the pieces of spaghetti can pack quite tightly. But if you repeat the procedure with pieces of fusilli, the cork screw type of pasta, because they have a contorted structure, you can't make such a tight film from it."

The first step in the process was the synthesis of the polymers by means of interfacial polymerization. In this method, polymerization occurs at an interface between two immiscible liquids, an organic solvent containing one monomer and an aqueous solution containing a second monomer. Rapid reaction between the two monomers at the organic-water interface results in the formation of a polymer membrane.

Here is where the researchers confronted one of the first challenges. "Initially, we found we couldn't access the contorted monomers," Livingston says. "So instead of making a polyamide, which is a normal interfacial polymerization reaction, we had to make a polyester-in this case, polyarylate."

Two different contorted aromatic phenols were chosen to react with trimesoyl chloride dissolved in hexane, while two non-contorted phenols were used as controls. Getting the contorted monomers to react was another key point. "Not all of the systems we tried were reactive. We solved this by using a basic solution for the phenols (sodium hydroxide aqueous solution with pH 13) instead of water, to strip the hydrogen off the phenolic materials," Livingston says.

After synthesizing the polymers, making a very thin nanofilm was another challenging issue: "When you start making these nanofilms to thicknesses of less than a micron, 100 nm or less, they can become difficult to handle," Livingston says.

"This is a major breakthrough in the area of membrane fabrication. The team of Professor Livingston has very cleverly combined polymers of intrinsic microporosity with interfacial polymerization, to produce very thin, highly microporous membranes," says Ryan Lively, assistant professor at Georgia Institute of Technology, who works on molecular separations for organic solvents and gases and is not affiliated with this research. "Usually, interfacial polymerization results in membrane layers that are approximately 100 nm, but in this case the researchers were able to create membrane layers, that are approximately 20 nm thin and still have them defect-free." And this is an impressive advance, according to Lively.

Michael Guiver from Tianjin University in China, who has also worked with microporous polymer structures (PIMs) for gas separations and was not involved in the study, says, "In membrane separations the key performance parameters are permeance and selectivity. In this study, researchers used microporous polymer structures which they prepared in situ on the surface of the support membrane, to achieve high permeance. They also went a step further, to engineer ultrathin selective films on the surface of the membranes, by interfacial polymerization. This innovative approach helped them to achieve very high product flux and good selectivity for a number of organic separations," he says.

The team is planning next steps in several different directions. "We have only [investigated] polyarylate nanofilms, but there is a wider range of monomers that we still need to explore," Livingston says. The research team is also planning to examine the durability of these membranes over time. "Because they are highly networked, we expect that they will not undergo a great deal of ageing and that they should be quite stable. So we want to pursue monomer systems that give us good stability." Above all, the researchers are interested in producing these membranes in industrially usable modules and on a larger scale.

Read the abstract in Nature Materials.