Microwave-assisted synthesis of bimetallic NiCo-MOF-74 with enhanced open metal site for efficient CO₂ capture

Metal–organic frameworks (MOFs) containing two different inorganic metal nodes (known as bimetallic MOFs) could exhibit enhanced CO₂ adsorption compared to their monometallic counterparts. Herein, a series of bimetallic NiCo-MOF-74 synthesized by microwave-assisted method were investigated for CO₂ adsorption. It was revealed that narrow micropore channel with open metal site (OMS) of the bimetallic NiCo-MOF-74 influence CO₂ binding affinity and CO₂/N₂ adsorption. The CO₂ uptake of Ni₁Co₁-MOF-74 at 0 ​℃ and 1 ​bar (100 ​kPa) was 8.30 ​mmol ​g⁻¹ which is higher than those of Ni-MOF-74 (3.99 ​mmol ​g⁻¹), Ni₆Co₁-MOF-74 (3.62 ​mmol ​g⁻¹), Ni₁Co₆-MOF-74 (6.40 ​mmol ​g⁻¹) and Co-MOF-74 (5.03 ​mmol ​g⁻¹). While this could be related to the high specific surface area of Ni₁Co₁-MOF-74, Ni₁CO₂-MOF-74 with relatively low specific surface areas still shows good CO₂ adsorption capacity up to 5.70 ​mmol/g, which is higher than those of adsorbents Ni-MOF-74, Ni₆Co₁-MOF-74 and Co-MOF-74, indicating that adsorption performance mainly relies on coordinated metals. Ni₁Co₁-MOF-74 showed remarkable recyclability performance, ranking selectivity of CO₂/N₂ reach up to 34, and suitable isosteric heat (31–23 ​kJ ​mol⁻¹), manifesting a great probability for industrial CO₂ capture. As revealed, incorporated Ni²⁺/Co²⁺ nodes within Ni₁Co₁-MOF-74, which are acting as active and open sites for CO₂ capture, led to the synergetic effects comprising of micropores as well as dense dual-metal sites.