AsiaChem | Chemistry in Japan | December 2021 Volume 2 Issue 1

www.asiachem.news December 2021 | 79 We used the alkane-shape selective complexation to separate linear and breached alkanes (Figure 13).42 Figure 13 Representation of the method to obtain highly pure isooctane from a mixture of isooctane and n-heptane using crystalline pillar[6]arene as the adsorbent. Reproduced with permission from reference.42 Separation of linear and branched alkanes is an important technique because gasoline is mainly a mixture of the branched alkane—isooctane and linear alkane—n-heptane. Gasoline with a high isooctane ratio gives a good performance, therefore separation of isooctane and n-heptane is an important target. However, the boiling point of isooctane (99°C) is almost the same as that of n-heptane (98°C), therefore the separation of these alkanes by distillation is difficult, and new separation techniques are needed. We investigated the use of pillar[6]arene crystals for the isolation of isooctane from a mixture of isooctane and n-heptane. When pillar[6] arene crystals were exposed to vapor mixtures containing 5% isooctane, the ratio of isooctane in the crystals was 95%. Exposure of the crystals to vapor mixtures containing >17% isooctane increased the ratio of isooctane in the crystals to >99%. The included isooctane could be stored in air at 25°C for 1 month, therefore the crystals can be used for storage of volatile isooctane. The release of high-purity isooctane (>99%) was achieved by heating the host–guest complex crystals at 110°C for 12 h. The crystals did not contain isooctane after heating, and could therefore be reused for gasoline quality improvement. Conclusions Research in the field of pillar[n]arene chemistry started in 2008 from our accidental discovery during phenolic polymer synthesis. The fundamental properties of pillar[n]arenes, e.g., their host–guest properties and planar chirality, were then discovered. In addition, position-selective functionalizations such as mono-, di- and per-functionalization procedures were developed. Uncovering these fundamental properties and the development of functionalization procedures triggered construction of various functional molecules at the single molecule level. Functional materials can be produced by installing functional groups on both rims of pillar[n]arenes. On the basis of their host–guest properties, pillar[n] arene-based interlocked molecules such as rotaxanes, catenanes, and polyrotaxanes were prepared. The direction of research into pillar[n] arene chemistry then turned from the single molecule level to supramolecular architectures and to supramolecular assemblies. Pillar[n]arenes have simple regular polygonal structures, therefore pillar[n]arenes are useful building blocks for creating various supramolecular assemblies with ordered structures. 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