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.