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 ​°C 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.