Separating the wheat from the chaff – molecular sorting machines

How can aromatic compounds be separated from so-called aliphatic compounds efficiently without having to rely on energy-intensive processes? In the scientific journal Angewandte Chemie (Applied Chemistry), chemists from Heinrich Heine University Düsseldorf (HHU) present an innovative molecular sieve made of partially fluorinated macrocycles that can separate these compounds selectively.

Aromatic compounds – substances with flat, ring-shaped structures made up of carbon atoms – play an important role in organic chemistry. Among other things, they serve as solvents or feedstock for many plastics and are also used in fuels. One well-known example is benzene (C₆H₆).

Its aliphatic counterparts, such as cyclohexane (C₆H₁₂), are also ring-shaped. However, by contrast with aromatics, they are flexible and thus form a zigzag-like, three-dimensional structure. Separating aromatics from other organic compounds – in particular aliphatic hydrocarbons – is a major challenge, yet often necessary. For example, cyclohexane is produced by hydrogenation of benzene, resulting in a mixture of both substances. Separation processes used to date require a significant amount of energy, as the physical properties of the compounds, such as boiling points and vapour pressure, are virtually identical.

The research team headed by HHU chemist Dr Bernd M. Schmidt (Functional Supramolecular Systems Research Group) and the research group headed by Professor Dr Christoph Janiak (Chair for Nanoporous and Nanoscale Materials) have together developed a “supramolecular sorting machine”, which can realise the separation in a different way. It comprises electron-deficient, fluorinated macrocycles with a rectangular structure called “squareimines”, which predominantly adsorb aromatic molecules.

Schmidt: “In the squareimines, small, three-dimensional molecules accumulate in the solid body in such a way that the connection of the cavities creates a network of pores.” This ultimately results in a network of many tubes arranged in parallel next to each other, each of which has a diameter of less than one nanometre. “This porous structure, acting as a ‘supramolecular sponge’, can trap small molecules such as gases or volatile organic compounds,” Schmidt continues.

The researchers optimised the adsorption capability of their material through the targeted, controlled linking of the structures. Tobias Pausch, PhD student in the research group headed by Dr Schmidt and lead author of the study, which has now been published in “Angewandte Chemie”: “The squareimine NDI²F⁴² has a strong affinity for aromatic compounds such as benzene and toluene, while ignoring their aliphatic counterparts.” The chemists are already measuring high selectivities of up to 97:3 for benzene over cyclohexane and 93:7 for toluene over methylcyclohexane in initial tests. “This means that almost exclusively aromatic compounds are adsorbed into the crystalline, supramolecular sponge, while the aliphatic compounds are left behind,” says Pausch in summary.

Schmidt outlines further aspects: “The identified squareimines offer great potential for molecular separation. This is due not only to their favourable structure, but also to their diversity, making it possible to produce tailored sorters for highly specific compounds. They are also easy to produce, making them a promising platform for new, innovative and lightweight adsorber materials.”

Original publication:

Tobias Pausch, Samanta Clopot, Dustin N. Jordan, Oliver Weingart, Christoph Janiak and Bernd M. Schmidt, Fluorinated Squareimines for Molecular Sieving of Aromatic over Aliphatic Compounds, Angewandte Chemie International Edition. 2024, e202418877.
DOI: 10.1002/anie.202418877