Controlled synthesis of multivariate metal-organic frameworks

Abstract

Metal-organic frameworks (MOFs) are robust extended structures built from an organic link and inorganic joint components. MOFs are used as rigid platforms for physical interactions such as gas storage and adsorption, as well as chemical reactions such as catalysis and post-synthetic modification. Most of MOFs reported to date are synthesized using a single type of links and inorganic joint units. Nevertheless, MOF structures incorporating more than one type of linkers (multivariate, MTV-MOFs) and more than one type of metal cations (solid solution MOFs) are rare, and their synthesis and characterization are challenging. The few reported multivariate and mixed metal MOFs exhibited an enhancement in their physical and chemical properties. Here, we outline a strategy for synthesizing MTV-MOFs in a controlled manner by using a reaction-diffusion framework (RDF) at room temperature. Our synthesis system is based on the separation of the reactants into two parts, inner and outer electrolytes. Initially, 1,3,5-benzene tricarboxylic acid (BTC) is dissolved in the gel matrix, and then the outer solution of the copper salt is added on the top of the gel matrix and allowed to diffuse through the inner electrolyte forming thereby the MOF-199 crystals. Based on this initial success, other organic linkers incorporating different functional groups, such as Isophthalic acid, 5-hydroxyisophthalic acid, 5-aminoisophthalic acid, 5-cyanoisophthalic acid and 5-sulfoisophthalic acid, have been mixed in different ratios within the gel matrix leading to the formation of multivariate MOF-199 (MTV-MOF-199). The resulting MTV-MOFs are fully characterized using powder X-ray diffraction (PXRD), Brunauer, Emmett and Teller (BET) analysis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). We have found that several linkers can be incorporated within the same MOF-199 topology in a controlled manner. Thus, different functional groups can be implemented within the framework, leading to controlled interactions between the MOF structure and the guest molecules. Furthermore, the newly synthesized MTV-MOFs are used to remove organic pollutants from water, and their performance is optimized by changing the ratio of the different functional groups in the hetero-MOFs structures.