Tunable nanochannels enable selective gas adsorption

Typical gas separation nanomaterials such as zeolites and microporous carbon have rigid pores, making it difficult to control gas diffusion within the material. Now, researchers led by Kyoto University’s Susumu Kitagawa have designed a nanoporous soft crystal that can selectively adsorb and store various gas molecules (Science 2019, DOI: 10.1126/science.aar6833). The material consists of a copper-based porous coordination polymer with metal centers linked by butterfly-shaped ligands made from iso­phthalic acid and phenothiazine 5,5-dioxide (OPTz). This structure forms an array of nanocages connected by diffusion channels. At low temperatures, the entrances to the nanocages are so small that gas molecules can’t fit through. But with increased temperature, the wing tips of the OPTz units flip up, enlarging the nanocage entrances and allowing the gas molecules to move from one nanocage to another. By varying the temperature, researchers can control the size of the cage entrances so that they selectively adsorb certain gases depending on their size. The team used the material to sieve oxygen from an oxygen-argon mixture and separate ethylene from ethane. And because the researchers can shut down gas diffusion by lowering the temperature, pre-adsorbed gas molecules can be locked into the material at ambient temperatures, providing potential long-lasting gas storage, the researchers say.