Tuning the composition of highly stable mixed-metal MOFs by microwave-assisted hydrothermal method for ultra-high selective and simultaneous capture of CO2 and H2S

In the present study, water-stable bimetallic M−Cu−BTC−II (M=Mg, Fe, Ni, Co, Zn, Mn, and Zr; II=isopropanol and imidazole) was synthesized via the microwave-assisted hydrothermal method without the use of any hazardous organic solvents, and the material was used as an adsorbent for CO2 and H2S gases under ambient conditions, i.e., 25 °C and 1 atm. While all adsorbents possessed the capability to adsorb both CO2 and H2S gases under ambient conditions, Ni-Cu-BTC-II was the best adsorbent in terms of adsorption capacity, i.e., 5.91 and 5.84 mmol g−1 for CO2 and H2S, respectively, which is higher than that of most of the previously reported materials, and stable for at least 10 cycles for CO2 adsorption. The high adsorption capacity of Ni-Cu-BTC-II is primarily attributed to the high specific surface area (1877 m2 g−1), large pore volume (0.607 cm3 g−1), appropriate sizes of the tetrahedral (5.1 Å) and square channel (9.0 Å) cages among M−Cu−BTC−II. The mechanistic study revealed that the process of absorbing CO2 and H2S gases begins with electrostatic interactions, which play an important role in the absorption of CO2 and H2S. For the H2S adsorption process, in addition to electrostatic interactions, the chemical bonding between M ions (Cu2+ and Ni2+) and S atoms in H2S also significantly contributes to the absorption capacity of H2S.