Faster Membrane Gas Separations

Pulling gas mixtures apart is an important industrial process, used to separate the components of air, for example. It is generally carried out via cryogenic distillation, a costly, energy-intensive method. Membrane-based gas separations, in contrast, use much less energy. A few are already practiced commercially. But the frustrating trade-off between membrane permeability and selectivity for one component of a gas mixture has generally meant that membrane-based separations move at a sluggish pace.

European chemists have now shown how to sidestep that problem by making permeable polymers with rigid molecular chains that lock in the material’s intrinsic microporosity (Science, DOI: 10.1126/science.1228032). The research team—which includes Mariolino Carta, Neil B. McKeown, and coworkers at Cardiff University, in Wales, as well as scientists at the University of Calabria, in Italy—made the new polymers by exploiting a little-used reaction first reported 125 years ago. The resulting membranes were selective and let gases diffuse through them quickly.

The difficulty in separating gases with permeable microporous polymers arises from the flexibility of the polymer chains. In contrast to zeolites and other molecular sieves with fixed structures, polymer chains have available to them a range of motions that causes them to pack in ways that block pores and crowd would-be void spaces.

For that reason, McKeown’s group used a rigid bridged bicyclic amine known as Tröger’s base (TB), which made its debut in 1887, to prepare monomers for polymerization. The team then added dimethoxymethane in trifluoroacetic acid at room temperature, triggering a polymerization reaction.

One of the polymers, PIM-EA-TB, which combines the rigid TB monomers with rigid ethanoanthracene (EA) units, has greater microporosity and void volume than a comparison group of microporous polymers. The new polymer has an internal surface area greater than 1,000 m²/g. It is soluble and readily cast as robust films and membranes from solution.

The team conducted a series of gas separation tests and found that membranes made from PIM-EA-TB work well as molecular sieves. The membranes selectively let through smaller gas molecules, such as hydrogen and oxygen, while keeping out larger molecules, such as nitrogen and methane. For example, the team reports that the small difference between the so-called kinetic dia­meters of molecular oxygen and nitrogen—just 5%—results in a substantially higher throughput of the smaller molecule (O₂).

Commenting on the work, Michael D. Guiver of the Canadian National Research Council in Ottawa, Ontario, notes that methods for making highly permeable and selective membrane materials may help address important challenges such as large-scale separations of gas streams containing CO₂(DOI: 10.1126/science.1232714).