Halogenated alkanes are high-value chemical feedstocks in synthetic chemistry and petrochemical industry [1, 2]. Therefore, the separation of halogenated alkane isomers is one of the necessary chemical processes in the petrochemical industry. However, the isomers normally have close boiling points due to the same molecular weight and similar chemical structure, which makes it difficult to separate the isomers with high purity by traditional methods such as distillation. Consequently, it is of great significance for petrochemical industry to find an efficient and energy-saving scheme to separate halogenated alkanes isomers.

Recently, macrocyclic supramolecular systems are widely used in many fields such as biomedicine [3], optical materials [4, 5], electronic devices [6, 7]. Especially, with the good preference in host-guest recognition, macrocyclic compounds show high selectivity on binding guest molecules, which attracts a great attention on their derived adsorbents. As a new generation of macrocyclic compounds, pillararenes also show excellent potential in adsorption selection [8].

More recently, Ying-Wei Yang and coworkers from Jilin University developed an adaptive macrocycle-based supramolecular-level adsorption material based on a leggero pillar[5]arene derivative BrP[5]L, which could be used for the separation of 1-/2-bromoalkyl isomers with outstanding selectivity (Fig. 1). Significantly, upon complexation with bromoalkane, these skeleton-trimmed leggero pillar[5]arenes will undergo a guest-induced amorphous-to-crystalline transformation in the solid state. This work innovatively proved that the adsorption material without porosity and crystallinity can also be used for molecular adsorption and separation, which is enlightening to chemical products adsorption and separation areas.

In this work, the original BrP[5]L was an active solid with amorphous and non-porous features, which had the ability to adsorb 1-bromopropane and 1-bromobutane from equal volume of 1-/2-position isomers with 98.1% and 99.0% purity, respectively (Fig. 2A). Within the adsorption process, the solid was transformed from the original amorphous to the guest-loading crystalline structure, the author obtained single crystals of BrP[5]L loaded with haloalkane to further characterize the structure (Fig. 3). Compared with original BrP[5]L, the PXRD and TGA patterns after absorption changed noticeably. As shown in PXRD pattern, the obvious sharp peaks appeared after the adsorption of haloalkane, indicating that the complex gained crystallinity (Figs. 2B and C). Different from the BrP[5]L, the TGA profile of the host-guest complex showed a significant drop before 200 ℃, which should be attributed to the release of adsorbed halogenated alkanes (Figs. 2D-F). In addition, the material could be completely regenerated by simply heating and reused, and its adsorption capacity and selectivity to 1-bromoalkane hardly reduced after be recycled for several times.

To discover the mechanism, they combined the single crystal structure with theoretical calculation and TG-DSC measurement. The results showed that one 1-bromoalkane was stably included in the cavity center of BrP[5]L through multiple intermolecular C—H···π, C—H···O, C—H···Br interactions. Different from the complex structure of 1-bromoalkane and BrP[5]L, the host-guest interaction region of BrP[5]L and 2-bromoalkane was mainly concentrated at the edge of pillarene cavity, and one BrP[5]L could complex two 2-bromoalkane molecules through weak host-guest interactions. Therefore, the adsorption selectivity was mainly due to the perfect host-guest matching of size and shape between BrP[5]L and 1-bromoalkane, leading to higher thermal stability compared with the corresponding 2-bromoalkane complex. Furthermore, the authors also demonstrated that the excellent adsorption selectivity of BrP[5]L benefited from the intrinsic free-rotation phenylene subunit on its backbone by control experiments using the corresponding traditional pillararenes.

In summary, this contribution presented a family of skeleton-trimmed pillararene derivatives, leggero pillar[n]arene which could be applied for the separation of bromoalkane isomers with outstanding selectivity and further offered a novel strategy for designing highly guest-selective pillararenes to enhance the structural adaptability. This work was expected to not only offer a high-efficiency strategy for haloalkane isomers separation but also render new enlightenments for the exploration of the practical applications based on supramolecular-level materials, showing new perspectives in nonporous/porous materials, crystal engineering, separation science, and many other related basic scientific research fields. We believed that macrocycle-based supramolecular-level materials may have potential utility for the applications of chemical industry, even though they may have no obvious porosity and crystallinity. Consequently, it showed a new research prospect for the important chemical field of petrochemical product separation and we were convinced that the work provided a superior perspective for the practical industrial application of supramolecular chemistry.