Hydrogen-bonded metal-nucleobase frameworks for highly selective capture of ethane/propane from methane and methane/nitrogen separation

The separation of light hydrocarbons, including C2H6 and C3H8, is essential to natural gas upgrading. Meanwhile, N2 removal from CH4 is also crucial to concentrating low-quality coalbed methane, but the adsorption process is challenging because of the close kinetic diameter. This work reports two hydrogen-bonded metal-nucleobase frameworks (HOF-ZJU-201 and HOF-ZJU-202) capable of efficiently separating C3H8/CH4, C2H6/CH4, and CH4/N2. Due to strong affinity for C3H8 and C2H6, the low-pressure capacity for C3H8 (5 kPa) and C2H6 (10 kPa) of HOF-ZJU-201a exceeds most adsorbents. The ideal adsorbed solution theory (IAST) selectivity of C3H8/CH4 and C2H6/CH4 is 119 and 45 at ambient conditions. According to density functional theory calculations, surface polarization environments formed by electron-rich anions and electron-deficient purine heterocyclic rings contribute to the selective capture of C3H8 and C2H6 with greater polarizability. Furthermore, the high CH4 adsorption capacity (1.73 mmol/g for HOF-ZJU-201a and 1.50 mmol/g for HOF-ZJU-202a at 298 K and 1.0 bar) and excellent CH4/N2 selectivity (6.0 for HOF-ZJU-201 at 298 K), as well as dynamic breakthrough experiments of binary CH4/N2 gas mixture implied their efficacy in the concentration of low-quality coalbed methane.