Metal-carboxylate MOFs

SBU Secondary building unit in the context of coordination polymer network synthesis (reticular chemistry) refers to the geometry of metal coordination cluster fragments units as defined by the points of extension (such as the carboxylate C atoms in most carboxylate MOFs). 

The previous structural and sorption studies of MOFs are all based on the discrete Zn40(C02)6 SBU. Recent development has demonstrated that rod-shaped metal-carboxylate SBUs can also give rise to a variety of stable solid-state architectures and permanent porosity. Three illustrative examples are presented below. 

On the basis of similar method, MOFs can be prepared by precipitation. In this case, the synthesis reaction equilibrium is shifted by the use of a base in order to accelerate the formation of metal carboxylate (or imidazolate). Despite the use of additional basic chemical and the subsequent formation of nitrate or chloride amine salts, this method has the advantages of obtaining materials in shorter time and at room temperature. 

The metal-containing centers in MOFs can be either a mononuclear metal ion or a cluster unit, which are assembled by two or more metal ions. The metal cluster center commonly refers to as SBUs, which offer a prospective avenue toward the design and construction of MOFs [22], The formation of the SUBs can be controlled through the selection of suitable synthetic conditions, to some extent. Until now, more than hundred kinds of geometries of SBUs have been reported, among them, most are transition-metal carboxylate assemblies produced during the formation of MOFs under given conditions (Fig. 3.3). 

Another noticeable example of an MOF constructed from a polytopic carboxylic acid and the binuclear metal-carboxylate paddlewheel-like cluster MBB, equivalent to a square planar SBU, is the one commonly referred to as HKUST-1 MOF, with the general formula [Cu3(TMA)2 (H20)3] where TMA denotes trimesic acid (1,3,5-benzene-tricarboxylic acid) (Figure 13, ). The MBB is defined by four carboxylate ions coordinating two Cu(II) ions in a paddlewheel-like structure with four points of extension defining a square-planar SBU. The authors reported the capability of exchanging the axial water molecules coordinated to the Cu(II) paddlewheel, postsynthetically, with pyridine molecules, opening the door for further investigations toward the viability of preferential sorption of small molecules and/or catalysis on coordinatively unsaturated metal centers. 

The final target in this area is to develop an industrial process based on the use of MOFs as heterogeneous catalyst. Since one of the major criticisms against the use of MOFs in catalysis is their lack of structural stability under reaction conditions, the implementation of an industrial process based on a MOF as catalyst will serve to prove the superiority of these materials with respect to other possible alternatives, showing that this strong criticism can be overcome. The most likely areas in which MOFs can become catalyst of commercial processes are those in which currently transition metal carboxylates, dissolved either in solution or in a nonporous material, are currently used to promote aerobic oxidations or any type of coupling. In this type of transformations, MOFs could replace advantageously the metal carboxylates since their structure and presumably the nature of the active sites should be, in principle, not that different. 

Photoluminescence may be quenched in the presence of heavy metals, and in these examples, Hg(II) or Hg(0) uptake can be monitored by emission spectra as the fluorescence of the Zn-carboxylate or Zr-carboxylate MOFs is quenched in the presence of mercury. This suggests potential applications for these materials in monitoring the presence of mercury contamination and in its removal. 

Figure 4.1 Representation of the structure of MOF-5 (a) six carboxylic acid groups from terephthalic acid are connected to four Zn" cations to form a metal-organic elementary zinc carboxylate cluster (only...

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