Optimized acetylene/carbon dioxide sorption in a dynamic porous crystal.

The low compression limit of acetylene (C(2)H(2)) and its similarity to carbon dioxide (CO(2)) challenge the development of novel adsorbents. We illustrate in this report that the unique static and dynamic pore characteristics of a metal azolate framework, [Cu(etz)](n) (MAF-2, Hetz = 3,5-diethyl-1,2,4-triazole), can combine to show extraordinary C(2)H(2)/CO(2) sorption behaviors, which have been elucidated by a combination of gas sorption measurements and single-crystal structure analyses of the sorption complexes of both C(2)H(2) and CO(2). As demonstrated by single-crystal X-ray crystallography, C(2)H(2)/CO(2) hexamers are confined inside the nanocages of MAF-2 in different configurations. The subtle difference between C(2)H(2) and CO(2) is magnified by consequent framework dynamics, which produce sigmoid isotherms that are optimized for practical adsorptive applications. Large C(2)H(2) uptake (70 cm(3) g(-1)) and high C(2)H(2)/CO(2) uptake ratio (3.7) at 298 K, 1 atm as well as facile gas desorption are revealed. Since the C(2)H(2) uptake at 298 K, 1 atm is far from saturation (119 cm(3) g(-1)), MAF-2 permits a usable C(2)H(2) storage capacity 20 times higher than its volume or 40 times higher than that of a gas cylinder working between practical limits of 1.0-1.5 atm.