Amorphous Molecular Organic Solids for Gas Adsorption

摘要

There has been intense interest in developing new porous materials for the storage and separation of important gases such as carbon dioxide and hydrogen. In this regard, crystalline materials such as zeolites, metal-organic frameworks (MOFs) or coordination polymers (CPs), and cova-lent organic frameworks have been the subject of significant research. These classes of compounds generally possess nanochannel structures suitable for trapping guests and have concomitantly large surface areas. Amorphous materials such as activated carbon, carbon nanotubes, and cross-linked organic polymers are also being investigated. Amorphous materials are typically difficult to characterize and display lower degrees of'gas adsorption, but their availability and scalability renders them industrially applicable. In the gas sorption arena, molecular organic solids have been largely neglected because of the close packing. Voids larger than 25 A~3 are rarely seen in molecular organic crystals, but some exceptions have been reported. One such example is the low-density phase of p-tert-butylcalix[4]arene (TBC4) that adopts a dimeric capsule-like arrangement. This phase is obtained by sublimation under reduced pressure, and the capsule exists with a slightly offset head-to-head arrangement of tBu groups, and combination of the two cavities results in the generation of a void space of approximately 235 A~3. Remarkably, it has been shown that various gases can penetrate crystalline low-density TBC4, coming to rest in the void space present within the capsule.