Bimetallic metal-organic frameworks (MOFs) synthesized using the spray method for tunable CO2 adsorption

Abstract

A microdroplet-based spray process was applied for the fast and facile synthesis of a range of bimetallic metal–organic frameworks (MOFs), including Cu-TMA(Fe), Cu-TMA(Co), Cu-TMA(Mg), Cu-TMA(Al), and Cu-TPA(Fe) (TMA: trimesic acid; TPA: terephthalic acid). By forming M-O (M: metal, O: oxygen) bonds with the ligands, the secondary metal sites were incorporated into the framework partially substituting the original Cu sites in the parent MOF (i.e., Cu-TMA), which induced changes in surface areas, pore structures, and gas sorption properties. In particular, the bimetallic MOFs exhibited slightly higher surface areas than the parent MOF, which was attributable to the expansion of the unit cells, specifically because of the elongated M-O bonds. Meanwhile, the pore volumes of the MOFs showed a positive correlation with the atomic radii of the metal sites, suggesting that the metal substitution is an effective method to adjust the pore structures of the MOFs. Additionally, diverse gas sorption performances were observed among the bimetallic MOFs and the parent MOF, which was mainly associated with the different electrostatic interaction between the gas molecules and the frameworks induced by the incorporation of various secondary metal sites. Specifically, metal sites with larger electronegativity have a higher impact on the properties of the adsorbed CO2, such as CO bond length and OCO bond angle, leading to more asymmetric geometry and polarization of the adsorbed CO2 molecules. As a result, the gas sorption capacity, selectivity, and isosteric heat of adsorption vary with various secondary metal sites inside the framework. The results from this work offer an alternative method for the rapid synthesis of bimetallic MOFs and add new aspects to the fundamental understanding of gas sorption using bimetallic MOFs.