Ethane/Ethylene Separations in Flexible Diamondoid Coordination Networks via an Ethane-Induced Gate-Opening Mechanism

Abstract

Separating ethane (C₂H₆) from ethylene (C₂H₄) is an essential and energy-intensive process in the chemical industry. Here, we report two flexible diamondoid coordination networks, <b>X-dia-1-Ni</b> and <b>X-dia-1-Ni</b>_<b>0.89</b><b>Co</b>_<b>0.11</b>, that exhibit gate-opening between narrow-pore (NP) and large-pore (LP) phases for C₂H₆, but not for C₂H₄. <b>X-dia-1-Ni</b>_<b>0.89</b><b>Co</b>_<b>0.11</b> thereby exhibited a type F-IV isotherm at 273 K with no C₂H₆ uptake and a high uptake (111 cm³ g^-1, 1 atm) for the NP and LP phases, respectively. Conversely, the LP phase exhibited a low uptake of C₂H₄ (12.2 cm³ g^-1). This C₂H₆/C₂H₄ uptake ratio of 9.1 for <b>X-dia-1-Ni</b>_<b>0.89</b><b>Co</b>_<b>0.11</b> far surpassed those of previously reported physisorbents, many of which are C₂H₄-selective. <i>In situ</i> variable-pressure X-ray diffraction and modeling studies provided insight into the abrupt C₂H₆-induced structural NP to LP transformation. The promise of pure gas isotherms and, more generally, flexible coordination networks for gas separations was validated by dynamic breakthrough studies, which afforded high-purity (99.9%) C₂H₄ in one step.