Optimized Acetylene/Carbon Dioxide Sorption in a Dynamic Porous Crystal

The low compression limit of acetylene (C₂H₂) and its similarity to carbon dioxide (CO₂) 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 C₂H₂/CO₂ sorption behaviors, which have been elucidated by a combination of gas sorption measurements and single-crystal structure analyses of the sorption complexes of both C₂H₂ and CO₂. As demonstrated by single-crystal X-ray crystallography, C₂H₂/CO₂ hexamers are confined inside the nanocages of MAF-2 in different configurations. The subtle difference between C₂H₂ and CO₂ is magnified by consequent framework dynamics, which produce sigmoid isotherms that are optimized for practical adsorptive applications. Large C₂H₂ uptake (70 cm³ g⁻¹) and high C₂H₂/CO₂ uptake ratio (3.7) at 298 K, 1 atm as well as facile gas desorption are revealed. Since the C₂H₂ uptake at 298 K, 1 atm is far from saturation (119 cm³ g⁻¹), MAF-2 permits a usable C₂H₂ 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.