Cyclodextrins line up for better filtration membranes

By lining up molecular pores within ultrathin films, researchers have created highly selective separation membranes that can purify cannabidiol (CBD) more efficiently than commercial rivals (Nature 2022, DOI: 10.1038/s41586-022-05032-1).

Polymer membranes potentially offer a more sustainable way to separate chemical mixtures, compared with energy-intensive distillation or expensive chromatography. During organic solvent nanofiltration (OSN), for example, membranes can act as molecular sieves because their tiny pores block dissolved molecules while allowing solvent molecules to pass through.

In many conventional membranes, these pores are voids in a mesh of interlinking polymer chains. But it’s difficult to precisely control the size of the pores, reducing their selectivity for particular molecules. Prolonged exposure to organic solvents can also disrupt the pores, which degrades the membranes’ performance.

To produce OSN membranes with robust, tunable pores, chemists have previously created films containing intrinsically porous macrocycles, such as barrel-shaped cyclodextrin molecules. “The problem is that, up to now, there has really been no good ways of aligning the macrocycles within the polymer film” so that they form a continuous channel through the membrane, says Andrew Livingston, a chemical engineer at Queen Mary University of London.

To help these macrocycles line up, Livingston’s team made cyclodextrins that carry amine groups around one rim and hydroxyl groups around the other. The researchers suspended the molecules at an interface between water and hexane, with the hydroxyl groups pointing downward into water and the amine groups pointing upward into hexane.

They then linked the cyclodextrins by polymerizing their amine groups with an acyl chloride. This produced sturdy films just 3.5 nm thick that contained pores, each consisting of three aligned cyclodextrins which the team thinks are held in place by hydrogen bonds between adjacent macrocycles.

“They’ve nicely differentiated the upper rim from the lower rim, dictating how these macrocycles will sit,” says Niveen M. Khashab of King Abdullah University of Science and Technology, who develops OSN membranes and was not involved in the research. “It gives you this very well-defined pore.”

The process produced films with an area greater than 600 cm², which the researchers mounted on a porous polymer support to make them suitable for practical use. Medium-sized cyclodextrins offer a pore size of about 0.6 nm, while larger or smaller cyclodextrins can selectively filter molecules that differ in size by as little as 0.2 nm.

As a proof of principle, the researchers used two membranes with different-sized pores to separate cannabidiol from other molecules in ethanol, which served as a proxy for hemp oil. CBD is increasingly used as a treatment for anxiety and depression. The first membrane removed big molecules like chlorophyll, while the second membrane trapped CBD itself, allowing smaller molecules like limonene and ethanol to pass through. The enriched mixture captured by this second stage was sent back to the start of the process to increase the concentration of CBD over multiple cycles.

After 7 days of continuous filtration, the system achieved CBD concentrations three times as high as commercial polyimide OSN membranes (DuraMem) achieved in the same time. Ethanol also flowed through the macrocycle membrane roughly 10 times as fast as it flowed through commercial membranes, which Livingston says could enable more cost-effective separations.

Increasing the selectivity of a membrane generally decreases the rate that other molecules can pass through it, Khashab adds, so it’s impressive that the macrocycle membrane offers improvements on both fronts. “It’s very important work,” she says. “I believe that using macrocycles is really the future of sustainable separations.”