Microwave synthesis and characterization of MOF-74 (M = Ni, Mg) for gas separation

• Comparison between hydrothermal and microwave synthesis of MOF-74 (Ni, Mg).
• Microwave synthesis produced MOFs with smaller particle size and larger BET.
• Comprehensive characterization of all four samples with various analytical methods.
• Adsorption equilibrium and kinetics of seven small gas molecules at 298 K.
• Calculation of adsorptive separation selectivities for different pairs of gas mixtures.

Isostructural MOF-74 (M = Ni, Mg) were successfully synthesized with both hydrothermal method (1 and 3) and microwave-assisted method (2 and 4). These MOF-74 samples were characterized with scanning electron microscopy for crystal structure, powder X-ray diffraction for phase structure, and nitrogen adsorption for pore textural properties. The experimental results showed that MOF-74 samples synthesized by the microwave-assisted method had a smaller particle size with relatively more uniform particle size distribution. The microwave effects also helped to produce a larger specific surface area and micropore volume, with a similar median pore diameter. Adsorption equilibrium and kinetics of various gases (CO2, CH4, N2, C2H4, C2H6, C3H6 and C3H8) on these MOF-74 samples were determined at 298 K and gas pressures up to 1 bar. Adsorption equilibrium selectivity (α), combined equilibrium and kinetic selectivity (β), and adsorbent selection parameter for pressure swing adsorption processes (S) were estimated. The relatively high values of adsorption selectivity indicates the potential to separate CO2/CH4, CO2/N2, C2H4/C2H6, C3H6/C3H8 and C3H6/C2H4 pairs in a vacuum swing adsorption process using the MOF-74 as adsorbent. The microwave-assisted method was found to improve MOF-74 with a larger adsorption capacity and somewhat higher selectivity for gas separation.

Adsorption isotherms of CO2 on four MOF-74 adsorbents at 298 K (1, magenta; 2, blue; 3, red; 4, black; dash line, Langmuir equation; solid line, dual-site Sips equation; dot line, single-site Sips equation).Figure optionsDownload as PowerPoint slide